Influence of Ancillary Ligands in Dye-Sensitized Solar Cells

Review pubs.acs.org/CR

Influence of Ancillary Ligands in Dye-Sensitized Solar Cells Babak Pashaei,† Hashem Shahroosvand,*,† Michael Graetzel,‡ and Mohammad Khaja Nazeeruddin*,§ †

Chemistry Department, University of Zanjan, Zanjan, Iran Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland § Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland ‡

ABSTRACT: Dye-sensitized solar cells (DSSCs) have motivated many researchers to develop various sensitizers with tailored properties involving anchoring and ancillary ligands. Ancillary ligands carry favorable light-harvesting abilities and are therefore crucial in determining the overall power conversion efficiencies. The use of ancillary ligands having aliphatic chains and/or π-extended aromatic units decreases charge recombination and permits the collection of a large fraction of sunlight. This review aims to provide insight into the relationship between ancillary ligand structure and DSSC properties, which can further guide the function-oriented design and synthesis of different sensitizers for DSSCs. This review outlines how the new and rapidly expanding class of chelating ancillary ligands bearing 2,2′-bipyridyl, 1,10-phenanthroline, carbene, dipyridylamine, pyridyl-benzimidazole, pyridyl-azolate, and other aromatic ligands provides a conduit for potentially enhancing the performance and stability of DSSCs. Finally, these classes of Ru polypyridyl complexes have gained increasing interest for feasible large-scale commercialization of DSSCs due to their more favorable lightharvesting abilities and long-term thermal and chemical stabilities compared with other conventional sensitizers. Therefore, the main idea is to inspire readers to explore new avenues in the design of new sensitizers for DSSCs based on different ancillary ligands.

CONTENTS 1. Introduction 2. Ruthenium Photosensitizers Based on 4,4′-Substituted 2,2′-Bipyridyl Ancillary Ligands 2.1. Bipyridyls Containing Aliphatic Groups 2.2. Bipyridyls Containing Aromatic Groups 2.3. Bipyridyls Containing both Aliphatic and Aromatic Groups 3. Ruthenium Photosensitizers Based on 1,10-Phenanthroline Ancillary Ligands 3.1. 1,10-Phenanthroline 3.2. 1,10-Phenanthroline-Containing Imidazole Groups 4. Ruthenium Photosensitizers Based on Cyclometalation 4.1. Cyclometalates Containing Phenylpyridyl and Benzothiazole Ligands 4.2. Other Cyclometalates 5. Ruthenium Photosensitizers Based on Bis(4,4′dicarboxylic acid-2,2′-bipyridyl) 5.1. Ancillary Ligands with Strongly ElectronDonating Bidentate S, O, or N 5.2. Ancillary Ligands Containing Aromatic Groups 5.3. Ancillary Ligands Containing Monodentate Groups

© 2016 American Chemical Society

6. Ruthenium Photosensitizers Based on Dipyridylamine Ligands 7. Multinuclear Ruthenium Photosensitizers 8. Ruthenium Photosensitizers Based on PyridylBenzimidazole 9. Ruthenium Photosensitizers with Two Functionalized Pyridyl Azolate Ancillary Ligands 10. Ruthenium Photosensitizers Based on Tridentate Ancillary Ligands 11. Other Ruthenium Photosensitizers 12. Conclusion Author Information Corresponding Authors Notes Biographies Acknowledgments References

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1. INTRODUCTION Climate change is the biggest global threat of the 21st century, influencing disease patterns, food, and water sources (Figure 1).1 Reducing carbon emissions will help to mitigate these effects; meanwhile, there are economic, health, and social opportunities

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DOI: 10.1021/acs.chemrev.5b00621 Chem. Rev. 2016, 116, 9485−9564

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useful fuels or via direct conversion into electrical power.5,6 Solar cells, which are also called photovoltaic (PV) devices, convert sunlight to electricity. Dye-sensitized solar cells (DSSCs) are an efficient type of PV device that has attracted the attention of many researchers because of their low fabrication costs, high efficiency, ability to work in low light, and mechanical robustness.7 DSSCs consist of a mesoporous layer of TiO2 nanoparticles, an anchored dye as the sensitizer to harvest light photons and convert them to an electric current, a redox mediator [often iodide/triiodide (I−/I3−)], and a counter electrode (CE). The sensitizer dye is considered the most fundamental and important component dictating the performance of the DSSCs.8−12 Among various types of sensitizers, ruthenium-based dyes are widely used due to their broad metal-to-ligand charge transfer (MLCT) absorption bands and suitable ground- and excitedstate potentials. An effective sensitizer plays an important role in favorable electron injection into the conduction band (CB) of a semiconductor. Some of the requirements for an ideal sensitizer include8−10,13−18 (i) a broad absorption spectrum, preferably covering sunlight from the visible to the near-infrared; (ii) a high molar extinction coefficient (ε) of the absorption spectrum in the visible region; (iii) efficient electron injection from the lowest unoccupied molecular orbital (LUMO) to the CB before the deactivation process (from LUMO), such as the emission of light or heat to the highest occupied molecular orbital (HOMO); (iv) chemical and thermal stability in the ground and oxidized states to endure the harsh cycling environment and to cycle at least 108

Figure 1. Billiard ball model: the impacts of climate change directly on three of the goals. Reprinted with permission from ref 4. Copyright 2014 Overseas Development Institute.

in low-carbon development pathways. Decentralized low-carbon energy, such as solar and wind, can provide electricity for the 70% of sub-Saharan Africans who currently have no access. Growth in off-grid solar has given 2.5 million households in Kenya access to energy.2,3 Among various energy resources, sunlight is a reliable energy source and is of critical importance. Our planet receives a huge amount of energy from sunlight (4.3 × 1020 J in 1 h); therefore, solar energy is considered a potential “green energy” that can be exploited in the future either through its transformation into

Figure 2. Schematic depicting the working principle of dye-sensitized solar cells (DSSCs). Steps in the main charge-transport process are shown by Arabic numerals, along with the four components of a champion dye indicated by uppercase Roman numerals. The steps highlighted in this review are shown as green and red lines for forward and backward processes, respectively: (step 1) Photoexcitation of dye (II); (step −1) decay process of dye (II*) from excited state to ground state; (step 2) injection of excited-state electron; (step 3) regeneration of the oxidized sensitizer by an electron donor in the electrolyte; (step 4) collection of the electron by the counter electrode (CE) from the external circuit and catalysis of the reduction of I3− ions; and finally, charge recombination (CR) of the TiO2 electrons to either an oxidized dye (step 5) or an oxidized redox mediator (step 6). 9486


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potential of I−/I3− and the lower limit of the CB in TiO2, respectively.43 The use of aliphatic long chains leads to a slow recombination process by preventing water-induced desorption of the dye from the TiO2 surface44 as well as suppressing molecular aggregation on the surface, etc.45−52 All these factors are presumed to improve the overall performance of DSSCs through ancillary ligand substitutions. The working principle of DSSCs is shown schematically in Figure 2. Excitation of the dye (II) grafted onto the electrode surface produces the excited dye (II*) (step 1).53 The excited dye injects an electron into the CB of the semiconductor from a normal distribution of donor levels (step 2) and is therefore oxidized. This leads to charge separation at the interface. Regeneration of the oxidized dye occurs via an electron donor (I−) acting as a relay electrolyte (step 3). The injected electrons diffuse through the semiconductor network to arrive at the back contact, and with the application of an external load, they arrive at the CE to perform useful work. Reduction of I3− at the CE regenerates I− (step 4), and the solar cell is therefore regenerative (green lines in Figure 2). As shown in Figure 2, the voltage generated under illumination is the difference between the redox potential of the electrolyte (Eredox) and the quasi-Fermi level (EF) of the mesoporous TiO2 layer.54 The nonradiative decay of the excited dye molecules (step −1) as well as charge recombination of the TiO2 electrons to either an oxidized dye (step 5) or an oxidized redox mediator (step 6) are shown with red lines in Figure 2.55,56 In recent years, many polypyridyl ligands have been synthesized in which one or both of the pyridine rings are functionalized with aliphatic, aromatic, or combined aromatic and aliphatic moieties.57−67 As seen in Figure 3, there has been near-exponential growth in the number of publications dealing with DSSCs, and this review concentrates on the use of different ancillary ligands for engineering new photosensitizers in DSSCs. A number of reviews have appeared detailing the general aspects of DSSC performance. These include the development of photoanodes, sensitizers, and electrolyte components. In this

times, which is approximately equal to a 20-year duration of continuous irradiation;19−21 and (v) compatibility among the energy levels of the sensitizer, the conduction band of the semiconductor, and the electrolyte redox couple, which is important for efficient charge transport. The excited-state energy level of the dye must be sufficiently higher (by ∼150−200 mV) than the CB of the semiconductor to be able to inject electrons into TiO2,22 and the ground state must be sufficiently higher (by ∼200−300 mV) compared to the electrolyte redox couple to regenerate the sensitizer and prevent the electron recombination process from the CB to the oxidized sensitizer.22 For optimum absorption of sunlight, the HOMO− LUMO gap should be approximately 1.5 eV.5,23,24 Sensitizers have been introduced by the general structure MLY(X)2, which is composed of four parts (Figure 2). (i) The metal center (M), which should be Ru(II) or Os(II), presents a t2g6 electronic configuration in the ground state. The redox properties of Ru(II/III) are suitable for convenient electron regeneration. (ii) Immobilization of the adsorbing dye molecules onto the surface of semiconducting metal oxide nanoparticles, which collectively act as the working electrode of a DSSC, is usually achieved by the addition of an “anchoring group”.25 The benchmark anchoring ligand is 4,4′-dicarboxylic acid-2,2′bipyridyl (dcbpy), which anchors efficiently onto the TiO2 surface and enhances the light-harvesting efficiency, leading to superior results. However, the results obtained with ruthenium complexes having non-carboxylic acid anchoring groups, such as phosphonic acid, sulfonic acid, and catechol, are lower than those for carboxylic acid groups, due to insignificant overlap of the ligand orbitals with the TiO2 conduction band.9,26−31 (iii) The donor ligand (X), such as isothiocyanate (-NCS), is used to modulate the HOMO levels of the metal.32,33 However, the replacement of NCS with other competing ligands in the electrolyte, such as electrolyte additives [e.g., 4-tert-butylpyridine (4-TBP) and I−], is the main obstacle to enhancing the device’s power conversion efficiency (PCE or η).34−36 It has been reported37 that N719 ([Ru(dcbpy)2(NCS)2]·2TBA, where TBA = tetrabutylammonium) or other NCS-based Ru polypyridyl sensitizers (for example, Z907)38 lose an NCS ligand at elevated temperatures (80−100 °C) through the replacement of electrolyte components such as 4-TBP. This important limitation has been solved by employing different ancillary ligands and will be explained in later sections. A blue shift of approximately 30 nm in the optical absorption spectrum of N719-4-TBP compared to that of N719 causes a decrease in the efficiency of approximately 50%.39 Other reports replacing NCS with similar monodentate ligands, such as cyanide, phenylcyanamide, or halides, have always yielded much lower energy conversion efficiencies. To resolve these deficiencies, many scientists have examined the replacement of NCS with other chelating bidentate, tridentate, and cyclometalated ligands. Among all alternative candidates for replacing NCS, cyclometalated ligands have shown the most appeal for this substitution.40 The corresponding reports are explained in section 4. (iv) Most modifications in the design of a sensitizer dye for DSSCs involve choosing an appropriate ancillary ligand.41 In this regard, several approaches have been successfully attempted to enhance the light-harvesting abilities of ancillary ligands.42 They include extending π-conjugation through the incorporation of aromatic rings into different positions of the ancillary ligands, for improving ε along with the red shift of the absorption spectrum, and introducing electrondonating/accepting groups in the ancillary ligand, to suitably match the HOMO and LUMO of the dye with the redox

Figure 3. Number of publications annually involving DSSCs, showing near-exponential growth in recent years in the interest in DSSCs (data collected from Web of Science, August 9, 2015). 9487


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Table 1. Molecular Structures of Ruthenium Photosensitizers with Various Bpy-based Ancillary Ligands, Showing the Aliphatic Moieties and Photovoltaic Properties of Their Corresponding DSSCsa

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Table 1. continued

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Table 1. continued

a

The structure contains only one substituent for 4, and another is CH3. bOne of the H atoms of carboxylic acid in the general formula is replaced with Na. cBoth H atoms of carboxylic acids in the general formula are replaced with N(C4H9)4. dThe structure contains only one substituent for 4, and another is C7H15. eThe structure contains only one substituent for 4, and another is H.

2. RUTHENIUM PHOTOSENSITIZERS BASED ON 4,4′-SUBSTITUTED 2,2′-BIPYRIDYL ANCILLARY LIGANDS

review, we will focus on ancillary ligands and the correlation between ancillary ligand substitution and DSSC properties to

2.1. Bipyridyls Containing Aliphatic Groups

highlight the importance of ancillary ligands. The motivation for

In this section, the photovoltaic consequences of the presence of aliphatic groups on the 4,4′-positions of 2,2′-bipyridyl (bpy) are explained (Table 1). Figure 4 summarizes the performance of a DSSC containing aliphatic groups on bpy, leading to a decrease in recombination. Heteroleptic amphiphilic complexes are an interesting class of sensitizers for DSSC applications.68 A new series of amphiphilic

this review stems from the urgent need for molecular engineering of panchromatic sensitizers, which convert visible and nearinfrared radiation into electricity. 9490


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between the conversion efficiencies of the devices with polymer and liquid gel electrolytes. A set of efficient amphiphilic ruthenium photosensitizers (7− 10) based on hydrogenated alkyl, cholesteryl, and perfluorinated chains were developed by Lagref et al.70 A decrease in the LUMO energy level was found due to the existence of ester groups. The presence of amide groups on bpy also caused this reduction. Meanwhile, research on ruthenium sensitizers with amphiphilic structures and their applications in DSSCs led to the synthesis and full characterization of complexes containing amidefunctionalized ligands and alkylhydroxy ligands, 11−14.71 These dyes exhibited a lower ε value of around 15% for the MLCT band compared to N3 dye [Ru(dcbpy)2(NCS)2]. Complex 13 shows the best performance among these dyes, with an η value of close to 9%. Moreover, amphiphilic heteroleptic Ru(II) complexes (3−5 and 7) substituted with alkyl chains of various lengths at the 4,4′ positions on one of the bpy ligands displayed remarkable stability under both thermal stress and light soaking.72 The ε value of the π−π* transitions of 4,4′-dialkyl-2,2′-bpy in complexes 3−5 and 7 is approximately 35−40% higher than the ε value of the dcbpy transitions. In addition, due to the presence of 4,4′-nonyl-2,2′bpy in 4, which is a stronger electron donator compared to dcbpy, a blue shift of 280 cm−1 and a lower ε value were observed relative to N3 dye. Under the same conditions, increasing the length of the chains slightly increased the short-circuit photocurrent density (Isc) of the investigated complexes. Consequently, in this class, the best efficiency (8.6%) and IPCE (90%) were achieved by dye 3.72 In continuing, with the aim of protecting the dye layer against the ingression of water from the electrolyte and, hence, enhancing the stability of the device, hydrophobic dyes were examined under identical cell fabrication and measurement conditions.73 Alkyl chains function as an electrical insulating barrier layer between the sensitizer dye and the hole-transport medium, thereby reducing the interfacial charge recombination (CR) losses and increasing the opencircuit potential (Voc) as well as Isc. Hence, the Voc of the DSSCs decreases with alkyl chain length [C13 (N621) ≈ C9 (Z907) > C6 (N820) ≈ C1 (KD1)],72 which is consistent with the efficiency trend along with improved device stability under thermal stress and light soaking. The Isc and Voc values increase considerably with chain length (Figure 6b). Because of increased Voc, conversion efficiency of the C13 cell at 1 sun is superior to that of the C9 cell.74 However, the C18 dye (6) deviates from the series for the following possible reasons: (a) decrease in the regeneration reaction with increasing chain length (in particular, dye 6 shows a 700-fold lower regeneration rate constant);72 (b) slower CR between electrolyte and injected electron;73 and (c) faster recombination rate between dye and injected electron.68 These drawbacks were reflected in the significantly reduced device performance of dye 6. Furthermore, inhomogeneous dye loading on the TiO2 surface, due to folding of the long C18 chains, combined with incomplete swelling of the long C18 chains reduces the recombination blocking effect of the alkyl chain spacer.74 A rational design of charge-transfer sensitizer 15 (K51) with an ion-coordinating ancillary ligand was synthesized and investigated, in which the hydrophobic alkyl chains in Z907 were replaced with ion-coordinating tri(ethylene glycol) methyl ether groups.75 Interestingly, the coordination of lithium to the backbone of this complex caused a striking improvement in photovoltage and performance, with a voltage of nearly 900 mV regularly achieved and efficiencies that improved from 3.2% to

Figure 4. Pictorial representation showing reduced recombination between semiconductor electrons and electrolyte with a 4,4′-substituted 2,2′-bpy hydrophobic layer.

ruthenium polypyridyl photosensitizers, 1−3, in solar cells based on nanocrystalline TiO2 films resulted in incident photon to current conversion efficiencies (IPCE) of approximately 80%. These novel hydrophobic complexes demonstrated excellent stability toward water-induced desorption over a period of 50 days. An interesting amphiphilic polypyridyl ruthenium dye, 4 (Z907), combined with a photochemically stable fluorine polymer and poly(vinylidenefluoride-co-hexafluoropropylene) was investigated.69 The DSSC based on this conjugated system showed an η value of over 6% in full sunlight [air mass (AM) 1.5, 100 mW·cm−2] and resulted in extraordinarily stable cell performance under both conditions of thermal stress at 80 °C and soaking with light (Figure 5). In addition, after 1000 h of light soaking at 55 °C, the cells covered with an ultravioletabsorbing polymer film maintained 94% of their initial performance. Ultramicroelectrode voltammetric measurements illustrated that the I−/I3− couple can freely carry out charge transport in the polymer gel. However, there was no difference

Figure 5. Details of device parameter variations for cells with a polymer gel electrolyte during accelerated aging at 80 °C. Reprinted with permission from ref 69. Copyright 2003 Nature Publishing Group. 9491


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coordinating ruthenium sensitizer dyes in quasi-solid state TiO2 solar cells is discussed as follows: White et al.77 developed a new heteroleptic Ru dye, 17 (RC730), containing crown-ether moieties on the 4,4′ positions of the bpy ligand to trap Li+ ions assembled with gel polymer electrolyte. UV−vis electronic absorption spectra of RC730 showed a blue shift in the wavelength, which was attributed to the electron-withdrawing effect of the crown ether moieties.77 To probe the properties of these ion-coordinating dyes, DSSCs were assembled with a gel polymer electrolyte based on two different cations, Li+ and Na+. When lithium was used in the electrolyte, the solar cells prepared with RC730 provided higher photocurrent and photovoltage values, such that the η value approached 2%. The reason for this behavior may relate to the aptitude of the ion-coordinating RC730 dye in trapping Li+ ions, which minimizes the CB edge shift. However, if the trapped species are located near the TiO2 surface, the effect of trapping Li+ ions in the dye structure may have also increased the recombination losses. Moreover, there are several factors explaining the higher photocurrent density values observed for devices based on N719 dye compared to RC730. These factors include different dye structures, a decrease in the available cations for charge compensation, an increase in local accumulation of the electron-accepting species, and the geometry of the TiO2 surface.77 When RC730 dye is used along with a polymer electrolyte and Li+, the maximum IPCE efficiency is 31%, which is almost half the value for N719 dye under identical conditions. A bpy ligand containing two tert-butyl side groups was used to synthesize amphiphilic polypyridyl Ru(II) complex 18 (K005).78 The two MLCT absorption bands of the K005 dye showed a red shift of approximately 10 nm relative to those of Z907, and the ε value was almost the same. Due to the donor influence of tertbutyl-bpy on the Ru metal, a difference of 0.125 V in the oxidation potentials of K005 and N3 was observed. Furthermore, Sahin et al.79 reported a new amphiphilic heteroleptic Ru(II) complex, 19 (CS9), containing a swallowtail bpy ligand as an effective protection barrier for back electron transfer from the semiconductor CB to the electrolyte. It is believed that the swallowtail aliphatic chain of CS9 prevents CR, as it showed better spacer properties compared to single chains. The maximum IPCE and η values achieved for this dye were 60% and 5.68%, respectively. These values correspond to the cells prepared from a dimethylformamide (DMF) solution, whereas the obtained efficiency value was 5.27% in the acetonitrile/tertbutyl alcohol (1:1) solution of the CS9 dye. This difference in the efficiencies strongly originates from the different solubilities of CS9 in these solvents, such that lower solubility results in higher CR. Figure 7 shows a one-electron redox process for Ru(II/III) and two redox half-waves for the dcbpy and 4,4′-bis(dihexylmethyl)-2,2′-bpy ligands. Furthermore, the same research group synthesized new heteroleptic Ru(II) dyes (20− 23) with bpy and pyridine ligands along with amphiphilic structures substituted with these ligands. The influence of these ancillary ligands on electron back-transfer into the adsorbed Rudye molecules on the TiO2 surface was investigated.89,80 The obtained results illustrated that branching of the side groups had no important effect on the absorption and electrochemical properties.80 By the surface photovoltage (SPV) method, it was demonstrated that the strong influence of oxygen-containing side groups on the modulated SPV amplitude of the dye sensitized the ultrathin nanoporous TiO2 layers in a gas atmosphere. Thus, different states may have appeared on the TiO2 surface via the coordination of oxygen from the -C−O−C- groups.89

Figure 6. (a) Incident photon to current conversion efficiencies (IPCE) for dyes with different hydrocarbon chain lengths. Values for the C9 dye device are the highest, followed by those of C13, C6, C1, and C18, successively. (b) I−V characteristics under 1 sun illumination (AM 1.5 global, 100 W·cm−2) for the devices made with these dyes. Reprinted with permission from ref 74. Copyright 2005 American Chemical Society.

3.8% for the lithium-coordinating sensitizer relative to a non-ion coordinating analogue. The light absorption behavior of K51 and Z907 was similar, both in solution and when absorbed on TiO2. When lithium ions were added to the liquid electrolyte with concentrations equal to that of the dye adsorbed on the surface of TiO2, the Z907 dye exhibited a decrease in Voc and an increase in Isc, and a counterbalance of the parameters resulted in the same efficiency. In contrast, in the same situation, the K51 dye showed Li+ “ion-trapping” functionality and inhibited adsorption of Li on the TiO2 surface, and it showed potential invariance in the TiO2 CB, with a striking improvement in Isc and Voc values. At higher Li+ concentrations, excess Li+ was adsorbed on the TiO2 surface, resulting in a slight drop in the Voc value. However, the Isc value increased as the TiO2 surface adapted to a total positive charge. This caused a greater local concentration of I−, leading to an increased dye regeneration rate.75 To increase the stability of K51 during the accelerated aging tests, because of the hydrophilic nature of the ligand, K51 was modified to K68 (16) by introducing heptyl chains at the end of the ethylene oxide chains.76 This substitution caused the K68 dye to show better thermal stability resulting from an increase in hydrophobicity. For this reason, after 1 month of accelerated testing at 80 °C in the dark, the photoelectric conversion efficiency retained more than 94% of its initial strength. Similar to the K51 dye (7.6%), K68 also produced a much higher η value than the Z907 dye (7.0%) with the same electrolyte.76 For K68, it is possible that, in combination with the ion-coordinating ability, the extra heptyl chains also aided in the overthrow of CR. In addition, the Isc and Voc factors for K68 in the presence of Li+ ion were very close to those for K51.76 How this relates to ion9492


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With the aim of reducing electron recombination in the solar cells, a novel ruthenium complex, 29 (RG1), bearing butylamideterminated poly(amidoamine) dendrons was successfully synthesized and investigated.83 Under AM 1.5 simulated sunlight illumination, solar cells of RG1 were sensitized with (RG1-TBP) and without (RG1-N) 4-tert-butylpyridine. Interestingly, observations showed that these prepared cells exhibited the same performance characteristics. Moreover, the results demonstrated that the RG1 dye bearing dendritic structures acted not only as a dye but also as an additive in the electrolyte due to the better capability of the dendrons to delay CR. This is mainly a result of the large steric structures of dendrons and the formation of more compact layers of RG1 via H-bonds from the amide groups in the dendrons, thus improving the Voc. Meanwhile, the interactions between the nitrogen atoms contained in poly(amidoamine) dendrons (PAMAM) with ions were also expected to enhance Voc. A cross-linkable ruthenium complex, 30 (Ru−C), was designed and synthesized in which the aliphatic side chains carry the allyl groups as cross-linking ligands on one of the bpy ancillary ligands for the prevention of leakage and evaporation.84 This was the first report of the cross-linkable Ru dye having ever been applied to DSSCs. The average η value of the DSSC with cross-linked Ru−C was 5.1%, whereas the corresponding value with the cross-linked Ru−C-co-MAA remained at 5.18% (where MAA is methylacrylic acid). It is proposed that the incorporation of MAA to cross-link with Ru−C led to a higher current density but slightly lower Voc and fill factor, leading to a slightly increased η value. The polymerized MAA units that contain carboxylic acid were able to attract I−/I3− redox couples in the gelled electrolyte system. Therefore, they enhanced the dye regeneration, thereby increasing the photocurrent. For the time-course change test of the power efficiency in storage, both DSSCs showed an initial increase in their η values and then nearly maintained these values for a month.84 Furthermore, the previous group also synthesized Ru(dcbpy)(4-nonyl-2,2′-bpy)(NCS)2, denoted as RuC9 (31), by tethering a single alkyl chain to compare the properties of this dye with those of the standard Z907 dye, which tethers two similar alkyl chains.85 On the basis of the fact that RuC9 has only one C9 side chain, it can be concluded that its deposition on the TiO2 surface does not interfere with the adsorption of dyes on its neighboring spots, as apparent in the Z907 dyes. The RuC9 dyes tend to aggregate to form vesicles in the acetonitrile/tert-butanol cosolvent as a result of the amphiphilic structure, whereas the Z907 dyes aggregate to form lamellae. Surprisingly, the DSSC with the RuC9 dye presented higher Isc and IPCE values than those with Z907, which has been attributed to the dye’s higher ε value along with its higher adsorption onto the mesoporous TiO2. On the other hand, the Voc, fill factor (FF) and η values of the DSSC with the Z907 dye were higher than those with the RuC9 dye, probably due to the fact that the greater number of alkyl chains in Z907 formed a molecular layer with a higher hydrophobicity, reducing CR with the electrolyte.85 An N3-type Ru heteroleptic complex, 32 (AK1), having a bpy ligand modified with two lipoic acid units for binding to platinum, was investigated as a sensitizer in DSSCs. The results showed that AK1 can generate a photocurrent on TiO2.86 Cyclic voltammogram studies on a sensitized Pt wire showed that the dye also binds to Pt and that it is electronically coupled to Pt. An in situ nucleophilic aromatic substitution reaction has been used in the synthesis of a new Ru(II) sensitizer complex, 33, for use in DSSCs. This dye displayed improved light harvesting in

Figure 7. (a) Cyclic voltammograms of CS9 () and Z907 (---). (b) Five consecutive voltammograms of CS9. (c) Differential of the CS9 voltammogram measured in DMF solutions with a scan speed of 250 mV·s−1. Reprinted with permission from ref 79. Copyright 2008 Elsevier.

Synthesis and characterization of amphiphilic heteroleptic Ru(II) complex 24 (S8) containing amphiphilic bpy ligands along with C8 and carboxylate groups was developed and compared with that of commercially available N3 as a reference dye.81 The Voc of the cells including amphiphilic chains of a hydrophobic nature was higher than that of N3, but in the amphiphilic S8 dye, the Voc was found to be lower than that of cells using the N3 dye. Nonetheless, with the same TiO2 electrode and electrolyte, an η value of 5.36% was attained for S8, which was higher than the corresponding value in the N3 dye (5.02%). In S8, the aliphatic chains acted as effective electrondonor groups and prevented CR of the electron to the electrolyte from the TiO2 CB. Additionally, similar aliphatic chains in bpy were used as ancillary ligands in the synthesis of the novel Ru(II) sensitizer 25 (A597),44 in which the amide moieties caused a red shift in MLCT absorbance and reduced the solubility. Moreover, the long-chain alkyl groups prevented π-stacking of the complexes and inhibited water-induced desorption of the dye from the TiO2 surface. Although the A597 dye displayed redox behavior together with promising absorption in solution under standard conditions, its η value (7.25%) was lower than that of Z907 (8.32%). This can be attributed to the faster CR and/or the poor light harvesting of the dye when attached onto the TiO2 surface. The first amphiphilic heteroleptic Ru complexes [26 (CT4), 27 (CT7), and 28 (CT8)] containing fluorous bis-ponytailed2,2′-bpy ancillary ligands were reported by Lu et al.82 The hydrophobic polyfluorinated chains in these new complexes led not only to an affinity of the dye for TiO2 but also overpowered dye leaching. Additionally, the fluorous chains of the bpy ancillary ligand have additional adsorption capabilities toward the TiO2 surface, causing CT7 and CT8 to have much higher dye packing densities compared with the normal long aliphatic chains on Z907. Consequently, DSSCs based on these dyes showed surprising efficiencies analogous to that of the standard N719, and they outperformed the Z907 dye due to the presence of hydrophobic fluorous chains. The conversion efficiencies of CT7 and CT8 were close to that of N719 and ∼9% higher than that of Z907 with a nonfluorous bis-ponytailed bpy ancillary ligand. Notably, both of them had a higher IPCE than the standard N719.82 9493


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Table 2. Molecular Structures of Ruthenium Photosensitizers with Various Bpy-Based Ancillary Ligands Bearing an Aromatic Moiety and Photovoltaic Properties of Their Corresponding DSSCsa

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Table 2. continued

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Table 2. continued

a The structure contains only one substituent for 4, and another is CH3. bOne of the H atoms of carboxylic acid in the general formula is replaced with N(C4H9)4. cThe structure contains only one substituent for 4, and another is COOH.

the 400−450 nm region compared with N719 and produced cells with a PCE of 3.18%.87 Finally, highly efficient benzodifurans based on ruthenium sensitizers 34 and 35 (M43 and M44) were prepared and compared with the N719 and Z907 dyes by use of dip dyeing and flow dyeing methods in large area testing cells.88 MLCT transition absorptions for the M43 and M44 bands were redshifted compared to those for Z907 and N719. The fabricated DSSCs with the M43, M44, N719 and Z907 dyes for the two dyeing methods showed the same trend, with the order M44 > M43 > N719 > Z907 for current−voltage characteristics and IPCE curves. Nonetheless, the flowing process provided better photovoltaic parameters than the dipping process. 2.2. Bipyridyls Containing Aromatic Groups

In this section, the antenna effects of aromatic groups for enhancing light harvesting will be explored (Table 2, Figure 8). Yao et al.90 were the first researchers who synthesized a standard dye, N3, in which two -COOH groups were replaced by electron-donating -CH2N(CH3)(C6H5) groups to give 36 (mN3). The ε value for N3 is 20−30% higher than that for mN3, which is a disadvantage for light harvesting. The maximum IPCE and η values for this dye were 56.2% and 4.6%, respectively.

Figure 8. General graphic of bpy-based dyes containing aromatic moieties and their roles as antennae for light harvesting.

Subsequently, dye 37 (Z910), which has a higher ε, broader spectral response, and cell η value greater than 10%, was synthesized, and the complex was shown to be stable for 9498


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Figure 9. (a) Cyclic voltammograms of Ru-TPD-NCS, Ru-TPA-NCS, and N719 dyes, showing reversible oxidation processes. (b) HOMO levels of RuTPD-NCS, Ru-TPA-NCS, and N719 dyes and spiro (HC). (c) UV−vis absorption spectra of N719, Ru-TPA-NCS, and Ru-TPD-NCS dye solutions. (d) I−V characteristics of solid-state dye-sensitized TiO2 solar-cell devices prepared with N719, Ru-TPA-NCS, and Ru-TPD-NCS dyes. Reprinted with permission from ref 96. Copyright 2007 Wiley−VCH.

80 °C. In addition, a novel heteroleptic Ru complex, 40, containing 4,4′-bis(p-methoxystyryl)-2,2′-bpy as the ancillary ligand (coded as K-73) was synthesized and investigated through UV−vis absorption, emission spectroscopy, excited-state lifetime, and spectroelectrochemical measurements.93 The introduction of a methoxy group extended the conjugation of the bpy donor ligand, significantly increasing its solar light-harvesting capacity and ε. In this study, the ε value for K-73 was approximately 33% higher than that for Z907, which does not have an extended conjugation system. Due to the difference in the number of protons between the K-19 and K-73 dyes, the ε values of these two dyes were different in the excited lifetimes. Finally, dye K-73 gave a conversion of 9% with a nonvolatile “robust” electrolyte. By replacing methyl in K-73 with tert-butyl, dye 41 (K77) was reported as a newly designed and high-ε sensitizer along with a volatile electrolyte [Z675 = 1.0 M 1propyl-3-methylimidazolium iodide (PMII), 0.03 M iodine, 0.1 M guanidinium thiocyanate (GuNCS), and 0.5 M tertbutylpyridine in acetonitrile and valeronitrile (3:1 v/v)].94 The IPCE of this device at its maximum value was 90% at 550 nm, and it exhibited a photoelectrical conversion efficiency of more than 10.5%. K77 showed a significant improvement in efficiency compared to the K19 dye. Using the K77 sensitizer in combination with a newly formulated nonvolatile organicsolvent-based electrolyte [Z674 = 1.0 M PMII, 0.15 M iodine, 0.1 M GuNCS, and 0.5 M 1-butyl-1H-benzimidazole (NBB) in 3-methoxypropionitrile (MPN)] resulted in highly efficient DSSCs (up to 9.5%) that exhibited unprecedented long-term stability (1000 h) under both light soaking and thermal stressing.94 Moreover, the absorption band at 426.5 nm relating to the π−π* transitions of 4,4′-bis{2-[4-(N,N′-diphenylamino)phenyl]ethenyl}-2,2′-bpy of K77-7 (42) had a red shift of 80.5 nm relative to K77 (346 nm).95 Furthermore, when the ratio of

DSSCs.46 The ε value for this newly developed dye is remarkably higher than the corresponding values for Z907 and N719. Consequently, increasing ε improves the photovoltaic performance of DSSCs. Meanwhile, the maximum IPCE of Z910 reached 87% at 520 nm. To control the charge separation and recombination dynamics at the hole and TiO2 nanostructure interfaces, an arylaminebased secondary electron donor group, namely, a “supersensitizer”, was incorporated into the supramolecular complex 38 (N845), which retarded the interfacial charge recombination dynamics while retaining the efficient light-induced charge separation characteristics.91 The N845 dye exhibited recombination dynamics 3 orders of magnitude slower than that of the N719 dye, indicating that interfacial electron transfer is critical for determining the overall recombination dynamics. In addition, increasing the distance between the hole and TiO2 surface results in a long-lived charge-separated pair, which is particularly striking for applications in DSSCs.91 Furthermore, an essential requirement for the design and development of new molecules for DSSC applications is the creation of directionality in the excited state of the sensitizer, that is, efficient electron transfer from the excited dye to the semiconductor CB.92 Considering this important point, Nazeeruddin et al.92 demonstrated the functionalization of a ruthenium complex, 39 (N945), with 4,4′-di[2-(3,6-dimethoxyphenyl)ethenyl]-2,2′-bpy to increase ε of the dye and also to adjust the LUMO level of the ligand to provide directionality in the excited state. This monoanionic dye showed a red shift of 830 cm−1 absorption in comparison with Z910, and the ε value follows the order N945H > Z910 > N3. The photovoltaic data for the N945 sensitizer displayed an η value of 9.6% under standard AM 1.5 sunlight and revealed excellent photochemical and thermal stabilities. The efficiency remained 95% of the initial value after 35 days of thermal stress at 9499


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Another interesting investigation was reported that used 4,4′bis[(E)-2-(3,4-ethylenedioxythien-2-yl)vinyl]-2,2′-bpy containing conjugated electron-rich heteroaromatic rings as donor end substituents (49).100 This sensitizer indicated a higher ε value for the MLCT band and a 0.22 V cathodic shift of E1/2Ru(III/II) compared to the N3 dye. This evidence reflects the influence of the new electron-rich ligand that destabilizes HOMO orbitals. A new strongly electron-donating ligand, 4,4′-bis[p-diethylamino]-α-styryl]-2,2′-bpy, along with the corresponding ruthenium complex, 50 (HRS-2), was designed and synthesized with extremely high ε values, which were comparable to those of organic dyes.101 The ε and absorption maximum values of the high-energy MLCT band for HRS-2 showed 100% and 400% increases, respectively, compared to corresponding values of the N3 dye. Furthermore, a red shift of 10 nm in the low-energy MLCT band, which is attributed to the presence of an extension of π-conjugation of the ligand, brings the LUMO and HOMO levels closer. In addition to the ε value and frontier orbital energy levels, the size and structure of the sensitizer also play important roles in the performance of the dye.102 For example, two novel hybrid sensitizers incorporating an organic antenna into a ruthenium complex, 51 (JK-55) and 52 (JK-56), were reported in which a huge enhancement in the ε value was observed, yielding an IPCE of 83% and a PCE of 9.16%. JK-55 adsorbed less onto the surface of TiO2 due to the two bulky organic groups, which raised a large unoccupied area on the TiO2 surface, resulting in a higher dark current, and also displayed an inferior electron lifetime relative to JK-56. The η value decreased in the order JK-56 > N719 > JK55.102 Figure 10 illustrates (a) electron diffusion coefficient De

K77-7 and chenodeoxycholic acid (CDCA) was the same at 300 μM, the DSSC efficiency based on the K77-7 dye increased, presumably due to suppression of back electron transfer from the CB or the surface state of TiO2 to I3− in the electrolyte. To introduce bpy donor antenna groups with extended πelectron delocalization, ligands such as triphenylamine (TPA) and tetraphenylbenzidine (TPD) were connected to bpy via a conjugated vinyl spacer to synthesize Ru-TPA-NCS (43) and Ru-TPD-NCS (44).96 It should be noted that TPA and TPD groups in the dyes ensure a polarity match between the highly polar Ru dye and the relatively nonpolar hole−conductor (HC), consequently improving interfacial wetting and contact, which is crucial for good performance. Additionally, the higher absorption arising from extended conjugation led to efficient light harvesting, which in turn resulted in a higher Isc. Figure 9a shows that the oxidation potentials obtained varied considerably and were dependent on the donor antenna group. It is also evident from Figure 9b that both Ru-TPA-NCS and Ru-TPDNCS had HOMO values between those of N719 and HC [spiroOMeTAD = N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi(9H-fluorene)-2,2′,7,7′-tetramine]. The HOMO distribution over the antenna groups increased the spatial separation between the dye cation center and electrons injected into the TiO2, hence retarding the CR dynamics. Faster regeneration of the dye is a consequence of multistep chargetransfer cascades taking place, which is very similar to photosynthetic cascade processes.96 Subsequently, Ru complexes similar to Ru-TPD-NCS and Ru-TPA-NCS were synthesized without the NCS groups.97 These were abbreviated as Ru-TPD and Ru-TPA. TPD can improve the shift of the dye cation center further away from the Ru(II) core to the hole transport moiety because it is more delocalized and electron-rich than TPA. The donor-antenna dyes containing -NCS had better molar extinction values and longer absorption than the dyes without NCS ligands.97 Furthermore, a highly efficient donor−acceptor ruthenium complex, 45 (IJ-1), similar to Ru-TPA-NCS with additional methyl groups, was synthesized that, when attached on the surface of TiO2 films, displayed an η value of 10.3% and an IPCE value of 87%.98 The high PCE has been attributed to the highly efficient visible light-harvesting capacity of the methyl substituted onto the TPA donor antenna dye. Moreover, similar to Ru-TPA-NCS, this dye exhibited enhanced ε and cell η values in comparison with N820. A representative example using bpy-containing aromatic ancillary groups was also demonstrated by Giribabu et al.,99 who synthesized two efficient heteroleptic Ru(II) sensitizers, 46 (HRD-1) and 47 (HRD-2), based on the extended πconjugation concept including amphiphilic ligands such as 3,5di-tert-butylphenyl and 2,4,6-trimethylphenyl groups, respectively. The ε value for HRD-1 was higher than those for dyes HRD-2 and K77. The cell devices using these new Ru(II) sensitizers displayed excellent stability (1000 h) under both thermal stressing (aging at 80 °C) and light soaking. Additionally, in an attempt to develop an extended π-conjugated Ru(II) complex, the same group59 synthesized a complex similar to HRD-1 in which tert-butyl was placed at the para position of the phenyl group to give 4,4′-bis(4-tert-butylstyryl)-2,2′-bpy as the ancillary ligand. Compared to N719, the H112 dye (48) displayed improved molar absorptivity and a reversible oneelectron oxidation process and also provided a comparable solar light-to-electricity conversion efficiency.

Figure 10. (a) De and (b) τe values in photoelectrodes adsorbing different dyes (N719, JK-55, and JK-56). Reprinted with permission from ref 102. Copyright 2008 Royal Society of Chemistry. 9500


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and (b) lifetime τe values versus Isc in the photoanodes adsorbing dyes N719, JK-55, and JK-56. The difference in τe values was a result of the different amounts of dyes absorbed on TiO2. It is believed that electron recombination occurred more significantly in the photoelectrodes adsorbing the dyes with more bulky structures, owing to the relatively large TiO2 surface area unoccupied by dye molecules. Synthesis of a Ru-complex dye (53), named E322, with a (diphenylamino)styryl-thiophene group was published in 2009 by Nonomura et al.103 with the aim to improve ε and show broad absorption in the visible−near-infrared light region. Despite indicating a broader absorption spectrum and a higher ε, the E322-sensitized cell showed an IPCE value of 0.45% at 540 nm and an efficiency of 0.12% in comparison with N719. The low efficiency was attributed to energy levels between the LUMO potential of the dye and the CB of TiO2, which was not consistent, and also to aggregation adsorption of the dye. However, the efficiency value increased to 1.83% upon exposure of the cell to simulated sunlight containing UV irradiation under short-circuit conditions, due to the removal of aggregated dye molecules from the surface of the electrode. Thus, the conduction band of TiO2 shifted toward a more positive potential.103 One of the main hurdles to overcome before DSSC technology is placed on the market is the long-term stability of the devices, which relies on the judicious combination of a highly stable dye with a robust electrolyte.104 Hence, a dendritic terthiophene-functionalized ruthenium sensitizer, 54, was examined in which the ancillary ligand allowed for the extension of π-conjugation, therefore enhancing light absorption of the complex.104 The observed red shift of the MLCT band and the increase in ε were a result of destabilization of the LUMO level of the corresponding Ru complex due to incorporation of the [2,2′;3,2″]terthien-5-yl unit in the new bpy ligand. Additionally, an interesting photovoltaic performance with a maximum PCE of 7.4% was measured in air at 1.5 global sunlight irradiation and in combination with an I−/I3−-based electrolyte. Furthermore, remarkable stability of the photovoltaic performance was demonstrated during a light soaking test with a duration of more than 1000 h. Strikingly, the efficiency of the electric power generation under light soaking, with simulated AM 1.5 G full solar irradiation, increased during the first 12 days, leading to augmentation of all photovoltaic performance parameters. The initial increase of cell efficiency during light soaking was explained by the diminution of charge transfer resistance at the Z646 electrolyte (1.0 M PMII, 30 mM I2, 0.5 M NBB, and 0.1 M GuNCS in MPN)/CE interface and by augmentation of the energetic barrier for electron recombination with I3−.104 A series of new heteroleptic Ru(II) (dcbpy) (bpy donorantenna ligand) (NCS)2 dyes, 55−58, were developed to increase the ε value of the sensitizers by use of different donorantenna moieties attached to bpy via a vinylene spacer for DSSC applications, which marks a remarkable advance.105 All of these dyes showed higher ε values compared to the standard N719 dye over a broad range of UV−vis spectra. Increasing the size of the conjugated π-system of the donor-antenna functionality enhances the photocurrent density [with the exception of RuDTBT-NCS (55)]. However, Ru-DTBT-NCS has an extraordinary performance, revealing that the absorption behavior is only one of many features necessary for good overall performance. Additionally, to increase the concentration of Li+ ions at the interface, “‘ion-binding”’ groups, such as oligo(ethylene oxide) in Ru-TPA-EO-NCS (57) dye, can coordinate with the Li+ ions,

thus blocking them from adhering to the TiO2 surface, resulting in the highest Voc obtained for this dye. On the basis of the remarkable features of the ketene thioacetal group, including an electron-donating group more easily prepared and more thermally stable than dialkylamines, a strategy was adopted for designing an amphiphilic rutheniumbased sensitizing dye, T18 (59).106 Compared to the N719 dye, T18 had a larger MLCT absorption coefficient and was also redshifted by 18 nm, which can be attributed to an increase in the electronic transition dipole moment upon the introduction of a ketene thioacetal moiety and the extended conjugation of the ancillary ligand. Additionally, results showed that T18 surpassed the performance of N719 in a cobalt-based electrolyte. Meanwhile, in DSSCs incorporating T18, the combination process rates increased between the injected electrons in the semiconductor and the iodine-based electrolyte. To further improve the light-harvesting capacity of sensitizers with a conjugated segment and a carbazole hole-transporting moiety, a series of ruthenium sensitizers, 60−62, were reported.107 Due to the presence of two N-phenylcarbazoles attached to the ancillary ligand, both HMP-11 (61) and HMP-12 (62) displayed a red shift in the maximum absorption and a higher ε of the MLCT band compared to Rut-B1 (60). HMP-11 has one more N-phenylcarbazole donor attached to the ancillary ligand compared to Rut-B1, which demonstrated a positive effect on the device performance. In addition, the Voc of HMP-12 was higher than that of HMP-11. The ancillary ligand of HMP-12 was modified with methoxy substituents at the 3- and 6-positions of the carbazole moieties, which caused retardation of CR between the injected electron and the electrolyte. Kim et al.108 reported a novel ruthenium bpy sensitizer, 63 (JJ12), incorporating the highly conjugated benzo[1,9]quinolizino(acridin-2-yl)vinyl-2,2-bpy ligand and its excellent photovoltaic performances in DSSCs. Owing to increased π-conjugation in the ancillary ligand and the donor capability of benzo[1,9]quinolizinoacridine, the low-energy MLCT band of JJ-12 was 10 nm red-shifted relative to that of N3. A solar-cell device, based on JJ-12 in combination with a volatile electrolyte, gave a remarkable efficiency of 8.34%. In a valuable attempt to explore the structure−activity relationships in their photophysical and electrochemical behaviors and in their performance in DSSCs, a motivating series of heteroleptic Ru(II) complexes, 64−68,109 were prepared with 4,4′-di(p-X-phenyl)-2,2′-bpy as the ancillary ligand in which the substituent X is CN (64), F (65), H (66), OMe (67), or NMe2 (68). It is interesting to note that 64−68 showed higher ε values than N3 over the entire studied region, owing to the conjugation effect of the p-X-phenyl substituents (Figure 11). As X changes from the strongly electron-donating NMe2 group (68) to the strongly electron-withdrawing CN group (64), a systematic bathochromic shift occurs. Density functional theory (DFT) calculations evidently showed that the HOMO/LUMO levels can be systematically tuned by varying X from the strongly electron-donating NMe2 to the strongly electron-withdrawing CN while maintaining the basic electronic structure of N3. Additionally, by increasing the electrondonating power of X, efficiencies increase and reach a maximum for the 67-based device, probably because of different lightharvesting efficiencies in the dyes adsorbed on the TiO2 surface. Transient spectroscopic investigations on the completed devices demonstrated that CR between photoinjected electrons and dye• + decreases in the order 64 > N3 > 65 > 66 > 67; meanwhile, the reaction rates for electron transfer from electrolyte to dye• + 9501


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optimization of their electronic and optical properties by selective substitution at one of the 4,4′-positions of a single bpy ligand with π-excessive heteroaromatic groups.112 The results show that they are effectively able to introduce three carboxylic anchoring units into the dye and achieve enhanced dye light harvesting, demonstrating the design concept. As a drawback of this type of dye, the synthesis leads to a mixture of dye isomers that are rather tedious to separate. Two new heteroleptic ruthenium complexes, coded as KW1 (76) and KW2 (77) and featuring triphenylamine electrondonating antennas, were discovered to have broad absorption and efficient light-harvesting properties compared to the reference dye Z907.113 Photovoltaic devices using these sensitizers showed an efficiency approximately 20% higher than that of the Z907-based reference device, which was attributed to a largely improved Isc. Additionally, these devices exhibited good durability during accelerated tests (60 °C for 1000 h in a solar simulator, 100 mW·cm−2). Another interesting investigation was reported by Wang et al.114 using Ru(II) photosensitizers, 78−80, containing electronrich diarylamino-functionalized 2,2′-bpy. The electron-rich diarylamino groups were expected to extend π-electron delocalization and also facilitate charge transfer. Additionally, the dimethylamino (78), methoxy (79), and methyl (80) units on diarylamino can increase electron density of the ligands and solubility of the photosensitizers. A maximum IPCE of 80.6% was obtained for dye-methyl, which corresponded to an efficiency of 5.68% under standard air mass (AM) 1.5 sunlight (versus N719 at 6.76%). Moreover, four novel Ru(II) bpy complexes, MH12−15, were designed and synthesized in which MH12 (81) and MH13 (82) contained a labile isothiocyanate (-NCS) ligand. In the MH14 (241) and MH15 (242) complexes, the -NCS ligand was substituted by 2-(4-trifluoromethylphenyl)pyridine cyclometalated ligand, while the same stilbazole-based bithiophene and terthiophene ancillary ligands were maintained.115 Incorporation of a cyclometalated ligand in MH14 and MH15 produced a considerable red shift of 24 and 28 nm in the low-energy MLCT band compared to those of the noncyclometalated analogues MH12 and MH13, which can be attributed to destabilization of the HOMO t2g of Ru(II). Furthermore, MH12−15 had remarkably high ε values due to the strong electron-donating ligand and the extended conjugation of thiophene units coupled with one cyclometalation center.

Figure 11. Electronic absorption spectra for DSSCs with 64−68 and N3. Adapted with permission from ref 109. Copyright 2011 American Chemical Society.

remained fairly constant, independent of the dyes.109 Therefore, the close relationship between device performance and CR kinetics indicated that slow CR is the main contributor to the efficient solar cell behavior of the 67-based device, whereas the poor behavior of the 64-based device was likely a result of relatively fast CR. In addition, the poor solar-cell behavior of the 68-based device was associated with the particular chemical nature of the Me2N substituent, which increased CR between the oxidized electrolyte and the injected electrons in addition to quenching the excited-state dye. Amphiphilic ancillary ligands based on methoxynaphthalene, pyrene, N-benzylcarbazole, dibenzofuran, and benzothiophene and their corresponding complexes, 69−73, having different conjugation lengths and electron-donation strengths, were designed and synthesized to study the influence of these ligands with the aim of producing panchromatic properties and absorbance of energetic photons across a wide range of the solar spectrum, especially from 400 to 920 nm.110,111 A comparison of the photovoltaic performance of two of these dyes indicates that 70 (MH11) was significantly more effective than 69 (MH06). Furthermore, the intensity-modulated photovoltage spectroscopic results show that TBA-substituted complexes of N719 and MH11-TBA had longer electron lifetimes than MH11 and MH06, due to formation of a thicker, more compact layer of TBA on the TiO2 surface. Electrochemical impedance spectroscopy (EIS) studies also revealed that electron recombination resistance (RCR) decreases in the order N719 > MH11-TBA > MH11 > MH06, as the incorporation of a TBA unit into the molecular structure of the dyes significantly reduces charge recombination at the electrolyte/dye/TiO2 interfaces.110 In addition, the electrochemical data indicated that the efficiency of electron injection and charge separation from the excited state into the CB of TiO2 decreases in the order 71 (MH08) > 72 (MH09) > 73 (MH10) because of the increased negative free energy of MH08 > MH09 > MH10. The obtained results show that incorporation of N-benzylcarbazole (MH08) attained more efficient electron donation, better hole transport, and a longer conjugation length of the ancillary ligand than dibenzofuran (MH09) or benzothiophene (MH10) analogues.106 To exploit the “three anchoring sites” concept, a class of Ru(II) dyes, 74 and 75, based on mixed bpy ligands were reported to preserve the optimal anchoring mode of the prototypical N719 sensitizer by three carboxylic groups while allowing for tunable

2.3. Bipyridyls Containing both Aliphatic and Aromatic Groups

The presence of both aliphatic and aromatic moieties on the bpy ancillary ligand makes it an ideal candidate for engineering “panchromatic sensitizers” to increase light harvesting and decrease the recombination process (Table 3). These effects will be introduced in this section, as shown in Figure 12. The development of sensitizers with improved ε values, preserving the appropriate stability under thermal stress and light soaking, is absolutely essential.48 A sensitizer with a high ε value was published by Wang et al.48 for stable DSSCs, coded K19 (83) and containing 4,4′-bis(p-hexyloxystyryl)-2,2′-bpy as the ancillary ligand. Under the same conditions provided, this dye achieved higher conversion efficiencies compared to the N719 and Z907 sensitizers. In addition, studies showed that the styryl unit attached to the bpy ligand remains intact after prolonged visible light soaking. The calculated excited-state redox potential 9502


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Table 3. Molecular Structures of Ruthenium Photosensitizers with Various Bpy-Based Ancillary Ligands Bearing both Aliphatic and Aromatic Moieties and Photovoltaic Properties of Their Corresponding DSSCsa

9503


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Table 3. continued

9504


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Table 3. continued

9505


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Table 3. continued

9506


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Table 3. continued

9507


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Table 3. continued

9508


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Table 3. continued

9509


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Table 3. continued

9510


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Table 3. continued

a

The structure contains only one substituent for 4, and another is CH3. bOne of the H atoms of carboxylic acid in the general formula is replaced with N(C4H9)4. cOne of the H atoms of carboxylic acid in the general formula is replaced with Na. dThe structure contains only one substituent for 4, and another is 4-carboxylphenyl.

ing TEOME group, was developed.116 The influence of Li+ ions in the electrolyte on photovoltaic parameters of K60 dye was similar to that of K51 dye. The DSSC device containing K60 sensitizer displayed excellent stability and sustained more than 93% of the initial photovoltaic performance under continuous thermal stress at 80 °C or light soaking at 60 °C for 1000 h.116 A substantial improvement in photovoltaic performance was achieved by use of a novel hydrophobic Ru complex, 85, containing the 4,4′-di(hexylthienylvinyl)-2,2′-bpy ligand.117 Compared to N719, the maximum absorption wavelength for HRS-1 (85) was red-shifted by 10 nm, and the lowest energy of the MLCT band increased by 33%. Unlike N719, this dye demonstrated long-term stability of a DSSC under prolonged thermal stress and light illumination. Surprisingly, in the case of sensitizer HRS-1, IPCE values of more than 80% were observed in ranges up to 575 nm, with a maximum at around 540 nm. For an ancillary ligand with thiophene, the introduction of alkyl chains into position 3 (or 4) showed very high solubility compared with attachment of the alkyl group at position 2 (or 5). On this basis, two novel ruthenium sensitizers, 86 and 87, were synthesized by extending the π-conjugation of 3,4- or 3alkylthiophene-substituted bpy ligands.118 The low-energy MLCT transition absorption peak at 546 nm for SY-04 (86) and SY-05 (87) was red-shifted by 15 nm. The higher ε values for both SY-04 and SY-05 dyes, compared to N3 and HRS-1, are attributed to extension of π-conjugation and better solubility of SY-04 and SY-05 arising from hexyl groups in the β (or γ) position of the thiophene ring. The latter also causes an increase in adsorption of the two dyes compared with the N3 dye. In addition, the difference in oxidation potential between SY-04 and

Figure 12. General graphical of bpy-based dyes containing both aliphatic and aromatic moieties for retarding CR and light harvesting, respectively, in DSSCs.

of K19 is more negative than that of the TiO2 CB edge, which provides a thermodynamic driving force for electron injection. In the following scenario, K51 was introduced with both advantages and disadvantages: the two disadvantages of K51 are its high solubility in organic solvents and its low ε, which are both caused by the hydrophilic triethylene oxide methyl ether (TEOME) groups. To solve its solubility problem, synthesis of the K60 dye (84), comprising a 4,4′-bis{2-[4-(1,4,7,10tetraoxyundecyl)phenyl]ethenyl}-2,2′-bpy ancillary ligand incorporating both extended π-conjugation and the ion-coordinat9511


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Figure 13. Molecular orbital energy diagram of N3 and TG6. The HOMO−LUMO gaps are reported in electronvolts, and the lowest time-dependent (TD) DFT excitation energies are in nanometers (data in parentheses). The isodensity plots shown are for HOMO − 3, HOMO − 1, HOMO, LUMO, and LUMO + 1 of TG6. Reprinted with permission from ref 52.Copyright 2008 Royal Society of Chemistry.

to D20 (5.38%) was higher than those of the other dyes and was comparable to η of 6.32% for a N3/TiO2 solar cell. In contrast, the low cell η value of D23 is attributed to the low absorption of D23 on TiO2 due to the presence of a bulky porphyrin group. A publication by Matar et al.52 introduced the TG6 (98) dye, containing a hexasulfanylstyryl-modified bpy group as the ancillary ligand, which comprised a high absorption coefficient and extended absorption in the visible region of the solar spectrum due to extended π-conjugation. The presence of a hexyl group, in addition to reducing CR processes, protected the dye from water-induced desorption from the TiO 2 surface, improving the long-term stability of the DSSC. Due to the presence of a sulfur atom in the aliphatic chain, the HOMO− LUMO energy gap of TG6 (2.50 eV) was lower than that of N3 (2.60 eV) by 0.1 eV, as determined by DFT calculations (Figure 13). Furthermore, Eox* of the LUMO level of TG6 was 0.13 V more negative than that of N719, which is a prerequisite for fast electron injection into TiO2.52 Moreover, the investigation of TG6 as a sensitizer with ZnO-based DSSCs resulted in an efficiency of 5.3%.121 To gain further insight into dyes with high ε, 99 (DCSC13) was synthesized with a donor ligand, 4,4′-bis{4-[2,5-bis(3methylbutoxy)styryl]-2,5-bis(3-methylbutoxy)styryl}-2,2′-bpy, wherein the substituted methoxy groups tune the LUMO level of the ligand to provide directionality in the excited state.122 The investigations showed that DCSC13 had low adsorption onto the TiO2 film due to its larger size compared to that of the N820 sensitizer. In addition, the substituted di{2-[3,6-bis(dimethylbutoxy)]ethenyl} groups prevent I3− in the electrolyte from recombination with e− (TiO2). This phenomenon caused a reduction in interfacial recombination and an increase in Voc, compared to the N820 sensitizer.122 Although DCSC13 exhibited 40% less absorption onto TiO2, both the Voc and η values of this dye were higher than those of N820.

N3 is attributed to electron donation of thiophene groups in the ancillary ligands. Under the same cell fabrication conditions, the SY-04-based DSSC showed a 27% higher η value than that of the N3-based DSSC. In continuing efforts to improve solar energy conversion efficiency, 4- and 4,4′-oligophenylenevinylene-functionalized Ru(II)-bpy dyes, consisting of both styryl for extending the πconjugated backbone and N-butylamino moieties as electrondonating groups, were reported (88−93).119 The presence of extended π-conjugation systems in D5 (88) and D6 (89) dyes caused them to exhibit better light absorption in comparison with N3 dye. The Isc and cell η values decreased in the order D6 > D5 > N3. In contrast to the increase in Isc of these dyes, the Voc values were the same compared with N3 dye, which is probably due to its inability to form hydrophobic layers around TiO2, thus not reducing back electron transfer from TiO2 to the electrolyte. In addition, four new Ru(II)-bpy dyes were synthesized containing N,N-dimethylaniline moieties at the end of their π-conjugated backbones, thereby expanding π conjugation. Contrary to expectations, the Isc and efficiencies of cells using D13−D16 (90−93) were less than half those of N3. A reason for the low Isc may be attributed to their fast nonradiative decay from an excited state to the ground state in the presence of π-conjugated backbones with N,N-dimethylaniline moieties at the end, as a result of distortion of their Ru(II) complexes immobilized on the TiO2 surface. Another reason for the lower Isc could be the absence of middle phenyl groups in the long chains of these sensitizers caused by aggregation on the TiO2 surfaces, which is detrimental to electron injection quantum yield. Four efficient new sensitizers, D20−D23 (94−97), containing chromophores such as cyanobiphenyl or porphyrin as light harvesters, were synthesized as photosensitizers for DSSCs.120 All of these dyes indicate a broad MLCT band at around 520− 530 nm. However, among these dyes, the η value corresponding 9512


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Figure 14. (a) Photocurrent density−voltage characteristic curves of various DSSC devices based on the CYC-B11 dye measured in the dark (curves A and C) and under illumination of AM 1.5 G full sunlight intensity (100 mW·cm−2) (curves B and D). (b) IPCE as a function of wavelength. Reprinted with permission from ref 123. Copyright 2009 American Chemical Society.

volatile electrolyte and a solid-state hole-transporting material, respectively. The IPCE spectrum (Figure 14b) shows a plateau of over 80% from 460 to 685 nm, with a maximum of 95% at 580 nm. Additionally, heteroleptic ruthenium complexes SJW-B18 (104) and CYC-B11H (105), similar to CYC-B11, were specially designed to study the compatibility between ruthenium dye and cobalt electrolyte.66 The SJW-B18-sensitized device had a higher efficiency (7.30%) than the CYC B11H-based cell (6.65%), mainly due to higher Voc (0.829 vs 0.784 V). On the basis of EIS and intensity-modulated photocurrent/photovoltage spectroscopic (IMPS/IMVS) data, this better performance of SJW-B18 may be mainly attributed to the lower CR, because the SJW-B18-dyed TiO2 anode had enhanced surface protection due to the four alkyl chains on the ancillary ligand.66 Two Ru dyes, 106 (CYC-B6S) and 107 (CYC-B6L), were developed by Wu and co-workers125 in which alkyl-substituted carbazole moieties were attached to the thiophene-substituted bpy ligand. The ε value of the MLCT band and the η value of both sensitizers improved in comparison with those of the standard N3 dye. Interestingly, the lower efficiency of the CYCB6L-sensitized cell relative to the cell based on CYC-B6S can presumably be attributed to dye loading, which is dictated by the molecule size and binding mode. One year later, the same research group126 developed another dye, CYC-B13 (108), similar to CYC-B6S. This dye contained the electron-rich 3,4-(ethylenedioxy)thiophene (EDOT) in the thiophene moiety, in addition to having an alkyl-substituted carbazole group. Due to the presence of the EDOT group, both the extension of π-conjugation and the electron-donating ability increased, and as a result, CYC-B13 showed ∼20% enhancement in ε value compared to CYC-B6S. Consequently, the DSSC based on the CYC-B13 dye exhibited significant augmentation of the Isc value, resulting in an increase in its photovoltaic performance (η > 8%). In addition, excellent durability under light soaking at 60 °C in simulated sunlight for 1000 h was attained in comparison with CYC-B6S. Following efforts to raise the Voc of the DSSC device sensitized by heteroleptic ruthenium, while exploring how the intrinsic properties of the heteroleptic ruthenium sensitizer affect the Voc of the corresponding cell, the novel heteroleptic ruthenium dye 109, coded CYC-B7, consisting of electron-rich bithiophene and bis(heptyl)-substituted carbazole was investigated by Li et al.57 If the amounts of both CYC-B7 and CYC-B1 molecules adsorbed on TiO2 electrodes are similar, a device based on CYC-B7 may provide a higher Isc compared to a CYC-B1-sensitized cell.

A ruthenium dye, 100 (CYC-B1), with a super-high lightharvesting capability and a highly conjugated ancillary ligand, 4,4′-bis(5′-octyl-[2,2′-bithiophen]-5-yl)-2,2′-bpy (abtpy), for increasing the absorption coefficient was synthesized by Chen et al.47 Abtpy was used as the ancillary ligand not only because the use of an alkyl group can prevent water-induced desorption of the dye molecules from the TiO2 surface but also because the bridging sulfur atoms can provide excellent aromatic stability to the polyacetylene chain while conserving desirable physical properties such as high charge transport. The facile functionalization of the thiophene groups also offers fairly effective synthetic solutions to polarity, solubility, and band gap tuning. Consequently, incorporation of thiophene with a more electronrich moiety onto the bpy ligands can raise the energy levels of the metal center and the LUMO of the ligands. Therefore, under identical conditions, the PCE of the cells based on CYC-B1 was measured to be 10% higher than that of N3-sensitized cells. The CYC-B3 dye (101), which contains one fewer thiophene units than CYC-B1, caused reduced light absorption along with lower solubility, making this dye less favorable for photovoltaic cells compared to CYC-B1.124 Introduction of electron-donating α-octylethylenedioxythiophene (O-EDOT) as an alternative to the thiophene moiety in SJW-E1 (102) resulted in an improved ε of the MLCT band compared with CYC-B3.124 Combining EDOT moieties on bpy can enhance the conjugation length of the ancillary ligand without increasing the size of the ruthenium dye. Furthermore, the ether group in EDOT may also be able to interact with Li+ to increase the dye regeneration rate. Briefly, a comparison of the cells leads to the order CYCB1 > SJW-E1 ≈ HRS-1 > N3 > CYC-B3. It is interesting to note that the ε value of the N3 dye was lower than that of CYC-B3, but the cell η value was still higher. EIS studies demonstrated that the lifetime of an e− (TiO2) was in the following descending order: N3 > SJW-E1 > CYC-B3. This presumably explains the higher lifetime of N3 despite its modest light-harvesting ability.124 Incorporating electron-rich hexyl thiobithiophene antennas instead of hexylbithiophene at the 4,4′ positions of bpy as the ancillary ligand in CYC-B11 (103) led to a higher ε value (2.42 × 104 M−1·cm−1) compared to all other reported Ru(II) sensitizers containing thiophene moieties for DSSCs and to an increase of ca. 14% compared to CYC-B1.123 Introducing a sulfur atom between the alkyl substituent and the thiophene ring might result in an enhanced electronic transition dipole momentum, yielding an increase in the absorption coefficient of the MLCT band. With this new sensitizer (CYC-B11), the cell η values were 11.5% and 4.7% under irradiation of full sunlight when combined with a 9513


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ruthenium dyes. In addition, the thienothiophene-linked carbazole antennas on this dye not only effectively inhibit the desorption of the self-assembled dye molecules induced by water during the accelerated aging process but also can maintain their light-harvesting capabilities. A cell device based on CYC-B12 with a Z946 electrolyte [with composition 1 M 1,3dimethylimidazolium iodide (DMII), 0.15 M I2, 0.5 M 1-butyl1H-benzimidazole (NBB), and 0.1 M GuNCS in MPN solvent] displays a good η value of 8.2% along with excellent durability under light soaking. This cell, in combination with a volatile electrolyte [Z960 = 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M TBP, and 0.1 M GuNCS in the mixed solvent of acetonitrile/ valeronitrile (v/v 85/15), which has lower viscosity and lower concentration of triiodide compared to Z946 electrolyte], demonstrates a high photovoltaic efficiency of 9.4%.127 4,4′-Bis(5-hexylthiophen-2-yl)-2,2′-bpy, which was attached to a bpy unit as the ancillary ligand, and its corresponding complex 111 (C101) demonstrated long-term stability.50 The MLCT absorption band of C101 was red-shifted by 23 nm compared to that of Z907, and it has an extraordinary cell η value of 11%. By replacing the thiophene group of the 4,4′-bis(5hexylthiophen-2-yl)-2,2′-bpy ligand in C101 with a furan in C102 (112), the cell η value decreased, possibly owing to the lower density of dye adsorbed onto the TiO2 surface compared to the density of C101. Therefore, the C102 dye-coated cells are exposed more to the electrolyte, and thus the CR rate increases, resulting in a lower cell value. Additionally, considering that poly(3-hexylselenophene) has a lower band gap and broader photocurrent response than poly(3-hexylthiophene), Gao et al.51 designed a conjugated electron-rich selenophene unit in the bpy ancillary ligand for a high ε sensitizer, 113 (C105), in DSSCs. The ε values of C105 (with selenophene), analogues C102 and C101 (with furan and thiophene conjugation), Z907, and N719 exhibit the decreasing order C105 > C101 > C102 > N719 > Z907, which is consistent with the electropositivity trend and the size of the heteroatoms (Se > S > O). As an evolved sensitizer, C101 recorded 11% efficiency. Under similar conditions, C101 exhibits 11% efficiency.51 The same results for photophysical and photovoltaic parameters were achieved by replacing the hexylterminal chain in C101 with an electron-rich hexylthio-terminal chain, 114 (C106).128 The measured ε of the MLCT band at 550 nm for C106 was higher than the corresponding values for Z907 and C101 sensitizers, and promisingly, it has a cell η value of 11.3%. Reaching a conversion efficiency over 10% by using the above-mentioned sensitizers provided a very helpful guide for designing the next panchromatic sensitizers. A 4,4′-bis(5-octylthieno[3,2-b]thiophen-2-yl)-2,2′-bpy antenna ligand was used to synthesize the C104 dye (115), which showed a much higher ε value at the MLCT band and a higher cell value relative to Z907 under the same conditions. In addition, ruthenium dyes C103 (116) and C107 (117), comprising electron-rich 3,4-ethylenedioxythiophene and 5-octyl-2,2′-bis(3,4-ethylenedioxythiophene) units, respectively, conjugated with a 2,2′-bpy ligand, were developed.129,130 The low-energy MLCT transition absorption of C107 was 9 and 38 nm redshifted in comparison with those of C103 and Z907, respectively, and ε and cell η values follow the order C107 > C103 > Z907. Furthermore, Voc follows a reverse order, as low-density packing of the large ancillary ligand containing C107 molecules results in an increase in the CR rate constant in the same order. Design and synthesis of three heteroleptic ruthenium sensitizers, 118−120, containing (E)-3-(5 hexyloligothiophen5-yl)-2-(4′-methyl-2,2′-bipyridin-4-yl)acrylonitrile ligands in

Additionally, a slight cathodic shift of 0.04 V was observed when compared to CYC-B1 due to the presence of the terminal carbazole segment, which can destabilize the Ru−NCS mixed orbital (Figure 15). Under identical conditions, apart from the

Figure 15. Square-wave voltammograms of CYC-B7 and CYC-B1 dye molecules in DMF. Reprinted with permission from ref 57. Copyright 2010 Royal Society of Chemistry.

Voc value, the Isc, FF, and η values of CYC-B7 were lower than those of the CYC-B1 dye. By intuition, the relatively low Isc and FF values of the CYCB7-sensitized cell can be attributed to lower dye loading resulting from the sensitizer molecular size and an extensive H-aggregation on the surface of the TiO2. Nevertheless, the polarizability of the CYC-B1 dye was lower than that of CYC-B7 due to the presence of a hole-transporting carbazole moiety. This causes an induced dipole moment of the dye-loaded titania film under an electric field, which results in observation of a higher Voc compared to the CYC-B7-based DSSC. Substantial improvements to photovoltaic performance can be achieved by use of a new high light-harvesting ruthenium sensitizer, 110 (CYC-B12), that incorporates an antenna ligand consisting of the sequential connections of a thienothiophene conjugated bridge and a carbazole hole-transport moiety (Figure 16).127 The electronic absorption spectrum of CYC-B12 shows the lower-energy MLCT band slightly red-shifted relative to that of the CYC-B13 dye. It indicates that thieno[3,2-b]thiophene is a superior spacer that reinforces the light-harvesting ability of

Figure 16. Preparation of ancillary ligand in CYC-B12. Reprinted with permission from ref 127. Copyright 2011 American Chemical Society. 9514


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place of dcbpy were carried out to increase the conjugation length and thereby improve light-harvesting efficiency.131 Owing to the extended conjugation length of the ligand in the thiophene moiety, Ru-T3 (120) has a higher light-harvesting efficiency compared with Ru-T1 (118) and Ru-T2 (119). Among these dyes, cells constructed with Ru-T2 have the highest efficiency at approximately 2.84%. In spite of the higher ε of Ru-T3, the poorer photovoltaic performance of Ru-T3 was due to the lower solubility and low Isc, compared to Ru-T1 and Ru-T2, related to its bulky ancillary ligand structure and the reduced absorption of Ru-T3 on the porous TiO2 electrode. ́ Garcia-Iglesias et al.133 designed heteroleptic ruthenium sensitizers, C101 analogues 121 (TT204) and 122 (TT205), comprising bulky alkyl substituents at the 4,4′-positions of the 2,2′-bpy ancillary ligand. The obtained results for these dyes and C101 showed that the nature of the group at the 2-position of the thiophene unit on the ancillary bpy ligand has little bearing on the electronic properties and recombination kinetics of the dye (Figure 17). Additionally, as can be seen in Figure 17, the

The low-energy MLCT transition absorption of LXJ-1 was redshifted by 28 nm compared to that of Z907. The cell fabricated with this dye also exhibited good stability, keeping 96% of its initial efficiency after 1000 h of aging. The obtained results indicate that LXJ-1 is feasible and practical for large-scale production for outdoor applications at various temperatures. Another interesting investigation was reported by Paek et al.,135 using the triazole group as a bridging group for extending the π-conjugated backbone in JK-91 (128) and JK-92 (129) dyes. Due to greater delocalization of the π-conjugated systems on the ancillary ligand, a slightly positive shift was observed in the reduction potential of JK-91 compared to JK-92. In addition, the results proved that incorporation of hydrophobic hexyl chains could successfully retard electron recombination originating from direct contact between the TiO2/I3− electrons. As a final modification strategy, ligands containing both conjugated fragments and alkyl chains on the outer parts of the ancillary ligands, along with their corresponding complexes (131 and 132), were synthesized.136 The corresponding HOMO of 132 is higher relative to those of others with respect to zero vacuum energy levels due to an easily oxidizable ligand. Furthermore, the HOMO levels of these dyes are much more negative than the CB level of TiO2; thus, their LUMO levels are sufficiently negative to inject electrons into the CB of TiO2. Using the hexyloxy chains in 132 as the electron donor instead of the methoxy in 131 on diphenylamine can prevent intramolecular electron−hole recombination of 132 and enhance the lifetime of the electrons on TiO2, resulting in a cell η value 14% higher than that of N3 dye under the same conditions.136 Moreover, recently, the new dyes 133 and 134 were investigated, wherein increasing the π-conjugation length in the ligands needs to be optimized to achieve good light-harvesting abilities and enhanced photovoltaic performances in DSSCs.137 On the other hand, by employing the newly synthesized disubstituted triphenylamine with hexyloxy spacer groups [HMP-9 (135)], an efficiency of 5.34% was achieved in ZnO nanocrystallinebased DSSCs.138 In comparison with thiophene-containing oligomers, which have poor stability, especially in the solid state, fluorene-based oligomers show both improved stability and a lower HOMO level. By introducing 4,4′-bis(9,9-dibutyl-9H-fluoren-2-yl)-2,2′bpy as an ancillary ligand to synthesize the BDF dye (136), the influence of increased conjugation length on photovoltaic performance and thermal stability was investigated, and it resulted in an increased spectral response relative to that of the reference Z907 sensitizer.139 The BDF sensitizer shows an increase of approximately 29% in ε compared to Z907. Furthermore, the BDF film absorbance is lower than that of Z907 and the thermal stability of the Ru(II) sensitizer is also decreased, due to the presence of bulky fluorine moieties on the bpy ancillary ligand. A representative example was demonstrated by Kim et al.,140 who synthesized amphiphilic ruthenium sensitizers containing unsymmetrical indeno[1,2-b]thiophene, JK-188 (137), and a fused dithiophene unit, JK-189 (138), at the 4,4′ positions of bpy. The low-energy MLCT absorption band of JK-189 is approximately 21 and 23 nm red-shifted compared to that of JK188 and N719, respectively. Under the same conditions, the efficiencies of the devices decrease in the order JK-188 > N719 > JK-189 due to the better light absorption of JK-188. Furthermore, the JK-188 device presented excellent stability under a light-soaking test at 60 °C for 1000 h, maintaining 97% of the initial performance as a result of incorporation of an

Figure 17. Transient absorption kinetics of 4 mm TiO2 films sensitized with (a) C101, (b) TT204, and (c) TT205, measured in the presence (red) and absence (black) of electrolyte. Reprinted with permission from ref 133. Copyright 2012 Royal Society of Chemistry.

transient decays in the absence of an electrolyte show multiexponential behavior. Despite this, C101 devices displayed higher efficiencies, longer electron lifetimes, and larger cell voltages under working conditions, probably because of the inefficient dye packing of TT204 and TT205 on the TiO2 surface arising from the bulky groups existing on the ancillary ligands of these dyes.133 Nevertheless, bulky groups can also help to reduce the recombination rate between the injected TiO2 electrons and the redox shuttle by forming a blocking layer between them. Hence, the same research group published novel Ru(II)-bpy heteroleptic complexes TT206−209 (123−126) incorporating branched and bulkier alkyl chains compared to their linear analogues: C106 and CYC-B11.65 The TT207 (124) dye that possesses extended and red-shifted absorption caused by the dithiophene moieties in the ancillary group (CYC-B11 analogue) and the 2-methylhex-2-yl-type substitution logically attained the highest η of the series. Subsequently, LXJ-1 (127) was investigated as an extremely high ε heteroleptic polypyridyl ruthenium sensitizer containing electron-rich sulfur−furan units conjugated with 2,2′-bpy.134 9515


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Z907 and also higher overall photovoltaic performance after the completion of aging studies. Nevertheless, electronically connecting the chromophores directly to bpy without ethenyl spacers in their ancillary ligand structures not only increases their solubility but also significantly improves the overall photovoltaic performance. With the aforementioned information in mind, new polypyridyl Ru(II) complexes H102 (142) and H105 (143) were synthesized to further explore these nonvinylogous structures.143 The bulky mesityl and tri(isopropyl)phenyl groups on the ancillary ligands of H102 and H105 Ru complexes, respectively, avoid dye aggregation on the surface of the semiconductor. Additionally, theoretical calculations and experimental studies indicated that the substitution of methyl groups by isopropyl groups has no overall substantial effect on light-harvesting abilities. In addition to achieving the highest observed IPCE for DSSCs, H102 and H105 are almost identical. Cells based on these dyes showed efficiencies of 8.30% and 8.76%, respectively, under the illumination of AM 1.5G full sunlight, which can be compared to N719 (9.12%).143 The heteroleptic ruthenium complexes YS-1−5 (144−147), in which the ancillary bpy ligand contains rigid aromatic segments (fluorene-, carbazole-, or dithieno[3,2-b:2′,3′-d]pyrrole-substituted bpy) tethered with a hydrophobic hexyl substituent, were used as sensitizers for DSSCs.61 Increasing the conjugation of the ancillary bpy ligand through incorporation of an aromatic fragment is useful for the light-harvesting capability of the sensitizer and causes a bathochromic shift and hyperchromic effects on absorption compared to Z907. Among these dyes, YS-5 (147) has the longest absorption wavelength and strongest absorption due to the electron-donating character of dithieno[3,2-b;2′,3′-d]pyrrole. It also shows better coplanarity with the pyridyl ring. The oxidation potential, consistent with the electron-donating power of the ancillary ligands, decreases in the order YS-1 ≈ YS-2 > YS-4 > YS-3 > YS-5. Furthermore, according to quantum-mechanical computations, a more electron-rich ancillary bpy ligand generates a prominent contribution to the HOMO, which not only helps increase the HOMO energy level but also may retard the recombination of electrons with the oxidized dyes. Although the long hydrocarbon chains aid in overpowering the aggregation of the dye molecules, they do not ensure the suppression of dark current. However, the dark currents of DSSCs exhibit the order YS-5 > YS-1 > YS-2 > YS-4 > Z907 > YS-3. The low dark current and large Voc of a YS3-based cell are attributed to two reasons. First, the hydrocarbon chains tethered at both the end and side sites of the ancillary ligand permit YS-3 to occupy more void space, which indicates a lower dye density on the TiO2 surface. Second, more compact packing of the dyes leaves less room for the electrolytes to penetrate and reach the TiO2 surface. In addition, unlike other dye molecules in this study, the presence of more planar ancillary bpy ligands probably causes less-efficient packing of the YS-5 molecules. The RCR value decreases in the order YS-4 > YS-3 > Z907 > YS-2 > YS-5 > YS-1 (Figure 19a), consequently demonstrating that the CR rate reverses and increases in the order YS-4 < YS-3 < Z907 < YS-2 < YS-5 < YS-1. By conjugating a phenothiazine unit with bpy, PTZ1 dye (148) was carefully engineered with a combination of multiple performance-improving structural features such as (1) extension of conjugation to improve ε along with a red shift of the absorption spectrum, (2) an electron-rich S atom in addition to N, which raises electron donation, and (3) the long alkyl chain, which prevents water-induced desorption of dye molecules from the TiO2 surface and suppresses molecular aggregation.42

indeno[1,2-b]thiophene unit with a hydrophobic alkyl chain. Figure 18a indicates that electron diffusion coefficient values are

Figure 18. (a) Electron diffusion coefficients and (b) lifetimes in photoelectrode adsorbing three dyes: JK-188, JK-189, and N719. Reprinted with permission from ref 140. Copyright 2010 American Chemical Society.

hardly affected by structural changes in the dye molecules. On the other hand, the lifetime values show a significant gap among the sensitizers in the order N719 > JK-188 > JK-189 (Figure 18b). The molecular size and structure of JK-189 decrease dye loading on the TiO2 surface, giving increased dark current and lowered Voc. Li+ ion tethered by a TEOME group has been proven to retard recombination by screening the electrons in TiO2 from I3−. Nevertheless, the weak coordinating power of the flexible TEOME unit prohibits the precise role of Li+ in the retardation of the interfacial CR. To strongly coordinate the lithium ions in JK121 (139) and JK-122 (140), 1,4,7,10-tetraoxa-13-azacyclopentadecane and bis[2-(2-methoxyethoxy)ethyl]amine side groups on the bpy ligand were introduced.141 Transient absorbance spectra analysis showed that decay of the oxidized dye signal due to regeneration of the JK-121 dye is much faster than those (10 μs) with a TEOME unit and similar to that of K51 and K60. Thus, the photocurrent density is increased. In addition, the results showed that the JK-121 and JK-122 sensitizers are strongly coordinated to the Li+ ion relative to K60, hence increasing the dye regeneration rate. A new extended, thermostable, high ε bpy-based Ru(II) dye, 141, with 4,4′-bis(3,5-di-tert-butylphenyl)-2,2′-bpy as the ancillary ligand, exhibited an η value of 5.89%.142 Substitution of this ancillary ligand in H101 (141) decreases the absorption density of the dye on the TiO2 surface by increasing the diagonal size of the dye molecule. Nonetheless, the IPCE of H101 exceeded 92.5%, which is higher than that of the Z907 sensitizer (78.6%). Furthermore, thermogravimetric analysis (TGA) of H101 showed relatively extended thermal stability in reference to 9516


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Figure 20. (a) Electron probe microanalysis result showing inhomogeneous ruthenium distribution along the TiO2 film and (b) schematic diagram representing the distribution of JK-142 and JK-62 on the cosensitized electrode. Reprinted with permission from ref 144. Copyright 2011 American Chemical Society.

absorption spectrum of NCSU-10 is red-shifted by approximately 16 nm. The calculated frontier orbital energy gaps are in agreement with the experimental red-shift trend, NCSU-10′ > NCSU-10 > N719. Furthermore, a combination of spectrophotometric, electrochemical, and device response parameters for NCSU-10 and NCSU-10′ clearly showed that the anchoring group position, 4,4′- versus 5,5′-, plays a key role in Isc density and total conversion efficiency. Furthermore, DFT calculations indicated that the 4,4′-isomer is a significantly stronger electron acceptor than the 5,5′-isomer, illuminating the inferior electron injection and significantly lower Isc of the 5,5′-isomer.145 More recently, the same group146 reported the synthesis of a novel heteroleptic Ru(II) bpy complex, 151, containing a monocarbazole ancillary ligand to investigate the influence of reducing the molecular size of a mono (HD-1-mono) versus a bis(carbazole) ancillary ligand (NCSU-10) on the photovoltaic parameters of DSSCs. Compared to NCSU-10, this new dye showed a slightly blue-shifted spectrum and up to a 30% decrease in ε. Nevertheless, HOMO destabilization is in the order HD-1mono > NCSU-10 > N719 (Figure 21), which is compatible with the order of the experimental Isc, HD-1-mono > NCSU-10 > N719. Despite these trends, the η values of the sensitized devices with these dyes exhibit the order NCSU-10 > N719 > HD-1mono. It was also shown that HD-1-mono shows a 63% morefavored electron injection from the triplet state and a lower RCR on the TiO2 surface compared to NCSU-10. Subsequently, the authors synthesized HD-14 (152) and HD-15 (153) complexes containing the strong electron-donor characteristics of carbazole and the hydrophobic nature of different long alkyl chains, C7 (HD-14) and C18 (HD-15), tethered to N-carbazole.147 The results showed that the photon-harvesting efficiencies and electron-donating characteristics of carbazole-based ancillary ligands remain unaffected by different alkyl chain lengths. It was found that dye regeneration for HD-15 (3.6 μs) was slower than that for HD-14 (2.6 μs), which is likely due to the spatial effect generated by the C18 chains between the dye and the electrolyte. However, a slight drop in Voc of HD-14 and HD-15 was observed compared to that of NCSU-10. The strategy of tethering long alkyl chains to N-carbazole resulted in efficiencies of 9.27% and 9.17% for HD-14 and HD-15, respectively, versus 8.92% for

Figure 19. Electrochemical impedance spectra of DSSCs for dyes measured in the dark under −0.60 V bias: (a) Nyquist plots and (b) Bode phase plots for (■) YS-1, (●) YS-2, (▲) YS-3, (▼) YS-4, (◆) YS5, and (◀) Z907. Reprinted with permission from ref 61. Copyright 2011 Wiley−VCH.

Absorption maxima of the MLCT bands in PTZ1 are larger, and the better spectral response in comparison with Z907 sensitizer is due to the supplementary electron-donating ability of S and N. However, the lower film absorption of PTZ1 on the TiO2 surface could possibly be due to the large π-moiety and planarity-like conjugated phenothiazineprone, leading to aggregation in solution as well as on the TiO2 surface. A significant improvement in photovoltaic performance can be achieved by use of a ruthenium complex, JK-142 (149), with an ancillary bpy ligand substituted by a 3-carbazole-2-thiophenyl moiety in combination with an organic dye (JK-62) as the sensitizers in cosensitized solar cells.144 The low-energy MLCT absorption band of JK-142 at 540 nm is red-shifted by approximately 20 nm compared to that of N719. The electronrich carbazole-thiophene donor rings of JK-142 result in a cathodic shift of the oxidation potential by 0.20 V compared to that of N3. However, ineffective diffusion of JK-142 dye into the inner TiO2 surface results in a low photovoltaic performance for the single DSSC owing to its bulky molecular size. To explore the penetration depth of JK-142 in TiO2 films, electron probe microanalysis (EPMA) was performed. It showed that the relative Ru content decreased gradually to zero at ca. 8 μm, suggesting that the JK-142 molecules were adsorbed only on the front half of the TiO2 film, leaving the back half uncovered (Figure 20). Excitingly, when JK-142 with JK-62 dye as a cosensitizer was employed to fabricate photoanode electrodes, the efficiency improved up to 10.2%, which is significantly superior to that of N719 (ca. 8.68%) under identical conditions. To study the influence of the carbazole antenna and anchoring group (COOH) isomerization, two novel heteroleptic Ru(II) polypyridyl isomers based on carbazole antennas, NCSU-10 (150) and NCSU-10′ (5,5′-COOH), were synthesized without an electron donor spacer.145 Compared to that of N719, the 9517


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Figure 22. (a) Typical processes of simultaneous adsorption and sequential adsorption. (b) Front and back views of a TiO2 film stained in XS49 for 12 h. Reprinted with permission from ref 148. Copyright 2013 American Chemical Society.

Figure 21. Energy-level diagram and comparison between the groundstate oxidation potential (GSOP) and excited-state oxidation potential (ESOP) of N719, HD-1-mono, and NCSU-10. Reprinted with permission from ref 146. Copyright 2014 Royal Society of Chemistry.

effect on Voc. In addition, further studies with an ionic liquidbased electrolyte [Z952 contains 1,3-dimethylimidazolium iodide/1-ethyl-3-methylimidazolium iodide/1-ethyl-3-methylimidazolium tetracyanoborate/I2/NBB/GuNCS (12:12:16:1.67:3.33:0.67)] indicated good device stabilities up to 100% (158) under full sunlight intensity at 60 °C. The MC101 (159) dye, with a new ancillary bpy ligand comprising ethynylthiophene chromophores, was synthesized by Chandrasekharam et al.150 This sensitizer showed high ε along with a bathochromic shift in the absorption spectrum by 16 nm in comparison with C101. Incorporation of -CC- in the MC101 ancillary ligand leads to higher light-harvesting ability compared to the C101 dye ancillary ligand. Incorporation of alkyl thiophenes conjugated with bpy through -CC- bonds into the Ru(II) bpy complex acts as an electron donor, and consequently, upon excitation of the sensitizer, a faster charge transfer from HOMO to LUMO could be expected. TGA displayed decreased thermal stability for MC101 relative to C101. Additionally, the MC119 sensitizer (160) with a functionalized styryl substitution on the ancillary bpy ligand was investigated by the same group.151 The objective of these investigations was to prevent aggregation, thus facilitating a uniform monolayer of sensitizer molecules on the TiO2 surface and inhibiting CR between I3− and the electrons injected in the nanocrystalline TiO2 electrodes. Because of the presence of conjugation of the styryl moiety over the ancillary bpy ligand, MC119 exhibits a higher ε relative to the N719 standard. Moreover, Voc and cell η values of MC119 are higher compared to the N719-sensitized solar cell. The higher Voc of MC119 shows that introduction of tert-butyl and long alkyl chain hexyloxy groups into the ancillary bpy ligand reduces CR between the semiconductor and the electrolyte. In the continuing efforts to improve PCE, two new ruthenium sensitizers similar to MC119, consisting of a highly alkylsubstituted phenyl moiety as a donor group and alkylthienyl functionalities in the ancillary ligand [coded as MC219 (161) and MC102 (162)], were developed.64,152 Introduction of the alkyl and alkoxy units lifts the HOMO level of the MC219 dye relative to that of the carboxylic acid-based N719 dye, which could result from contributions from the stronger alkyl and alkoxy electron-donating groups, thus resulting in a more delocalized HOMO.64 Moreover, four butadiene-equivalent thienyls, which are directly attached to the phenyl spacer,

N719, and also in enhanced long-term stability under similar conditions. To study the effect of bulky ancillary ligands on their performance and to enhance the ε values of ruthenium dyes, two ruthenium complexes, 154 (XS48) and 155 (XS49), containing butyloxy-substituted benzene ring and 9,9-dipropyl9H-fluorene on the ancillary bpy ligand, respectively, were studied.148 Owing to the influence of the electron-donor nature of the substituents on the ancillary ligand in both dyes, their oxidation potentials have a 0.03−0.05 V cathodic shift relative to that of N3. Furthermore, a dual-function truxene-based coadsorbent, MXD1 (as an insulating molecular layer to shield against back electron transfer from TiO2 to I3− ions and to increase the light-harvesting effect in the short-wavelength region) was used to examine the influence of bulky ancillary ligands and to alleviate electron recombination. For the DSSC based on the XS49 sensitizer, which contains a large bulky ancillary ligand, coadsorbent is indispensable for achieving high efficiency. However, XS48 was independent from the coadsorbent, indicating that the butyloxy-substituted benzene ring induces good dye packing on the TiO2 surface.148 Interestingly, the simultaneous adsorption-to-sequential adsorption of XS48/ 49 and MXD1 has caused remarkably improved photovoltage, which can be primarily attributed to enhanced dye adsorption and retardation of the CR (Figure 22). The advantage of the simultaneous approach is suppression of dye aggregation and reduction of the CR. Meanwhile, the key features of the sequential approach are the sequence of the dye adsorption and the staining time of a different type of dye, both of which can be controlled to achieve a fine-tuning. The successful click-functionalization method of ancillary ligands for Ru(II) dyes 156−158 was developed by the Stengel research group.149 The dyes incorporated new ancillary ligands synthesized through the sequential connection of different alkyl functionalities with triazole as a linker. The η values of devices stained with dyes 156−158 in conjunction with a volatile electrolyte, coded as Z960, reached values of 9.9%, 8.7%, and 9.6%, respectively. The better performance of dye 158 relative to 157 can be attributed to the presence of Li+-coordinated triethylene glycol chains at the periphery of the dye molecule, which exhibits a substantial increase in the Isc value, with a small 9518


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enhance the conjugation and improve ε. Furthermore, the highly aromatic character of multiple thienyls and phenyls improves the thermal stability of MC102 in comparison with N719.152 Under the same cell fabrication and measurement conditions, an efficiency of 4.6% was observed for MC219, which is 63.8% of the efficiency of the N719-sensitized cell. To improve the interfacial and optical properties, a new Ru(II) complex, 163 (MC112), based on a dissymmetric bpy ligand was developed by introducing a 4-carboxyphenyl unit as an additional anchoring group and a hexylthiophene moiety onto the same bpy ligand, which increased the ε value of the dye, yielding a photovoltaic efficiency of 7.6% under standard conditions and displaying excellent device stability (Figure 23).153 The main

3. RUTHENIUM PHOTOSENSITIZERS BASED ON 1,10-PHENANTHROLINE ANCILLARY LIGANDS 3.1. 1,10-Phenanthroline

The 1,10-phenanthroline ligand is a well-known N−N chelating ligand that can be easily functionalized in its different positions, especially in the 5,6-positions. The first report of the DSSC applications of ruthenium complexes based on 1,10-phenanthroline derivatives goes back to the synthesis and photoelectrochemical properties discovered by Onozawa-Komatsuzaki et al.155 for heteroleptic Ru complexes 166 and 167, with 1,10phenanthroline and dipyrido[3,2-a:2′,3′-c]phenazine (dppz) as the ancillary ligands. These sensitizers are attractive, as the electron π-conjugation over the aromatic moieties allows longdistance, yet strong, electronic interaction between the dppz ligand and nanocrystalline TiO2. The efficiency of the 166sensitized solar cell (AR20) is 20% higher than that of the 167sensitized cell, but the N719-sensitized solar cell was more efficient than either of these. On the other hand, the adsorption coverage of 167 on TiO2 was higher than that of N719 and AR20 dyes. This can be attributed to the existence of a large πconjugated dppz ligand, which is easily aggregated through π−π intermolecular interactions on the TiO2 surface and, therefore, might have prohibited the transmission of light, causing a decrease in the efficiency of 167 compared to AR20 (Table 4). Dye AR25 (168) was synthesized by Palomares and coworkers156,157 with one of the bpy ligands replaced by a more conjugated ligand, such as 5,6-dimethyl-1,10-phenanthroline, and with the presence of strong electron-donating (-NH2) [169 and 170 (AR24)] or electron-withdrawing (-NO2) [171 and 172 (AR27)] substituents on 1,10-phenantroline-based dyes to achieve the desired control over molecular orbitals and high photocurrents. The ε values of the MLCT bands follow the order N719 ≈ AR20 > AR24 > AR25 > AR27. However, the efficiency of electron injection from dye• + to TiO2 CB follows the order N719 (90%) ≈ AR25 (90%) > AR20 (85%) > AR24 (70%) ≈ AR27 and is consistent with the order of cell η values, N719 ≈ AR25 ≈ AR20 > AR24 ≈ AR27. Furthermore, the results showed that AR24 and AR27 ruthenium complexes increase the electron recombination reaction between the photoinjected electrons at TiO2 and the redox-active electrolyte (I−/I3−), and for cells made with phenanthroline-substituted Ru(II) complexes, the reaction is 2-fold faster compared to that of N719, AR20, or AR25. Figure 24 shows a graphical representation of the HOMO and LUMO orbitals of heteroleptic Ru(II) complexes. In all cases, electrons on the HOMO are centered on the axial isothiocyanate ligands and, within those, on the sulfur atoms; meanwhile, electrons on the LUMO are localized on the bpy ligand for the AR20, AR24, and AR25 complexes. Two new photosensitizers, CYC-P1 (173) and CYC-P2 (174), that incorporate oligoalkylthiophene-substituted 1,10phenanthroline as ancillary ligand with the aim of tuning the overall properties of the dye complexes were synthesized and characterized by spectroscopic and semiempirical computational methods.158 The alkyl substituents on ligand P1 and ligand P2 in these new metal complexes can prevent water-induced desorption of the dye molecules from the TiO2 surface. The red shift in the absorption maximum of ligand P2 compared to ligand P1 is attributed to the longer conjugation length of ligand P2. The λmax values of the three bands in CYC-P2 are longer than those of CYC-P1, presumably because the ligand in the former has a longer conjugation length. The result suggests that the conjugation length of the ancillary ligand will affect the energy of

Figure 23. Evolution of solar cell parameters of MC112-based DSSCs measured under 1 sunlight soaking at 60 °C. Reprinted with permission from ref 153. Copyright 2014 Royal Society of Chemistry.

visible transition shows the improved ε compared to N719, although with a slightly blue-shifted absorption. The presence of the long alkyl chains introduced in MC112 can partly block recombination with the oxidized species in the electrolyte. Furthermore, the mixed bpy ligand allows MC112 to bind to TiO2 via three anchoring carboxylic groups, thus demonstrating similar interfacial properties to those of the N719 dye. Recently, bpy bearing π-delocalized dithienylpyrrole moieties was used as the ancillary ligand in the bis- and tris-heteroleptic ruthenium complexes (164 and 165) that have been used as TiO2 photosensitizers in DSSCs.154 In support of the band intensity enhancement expected from increasing the number of dithienylpyrrole (DTP) bpy ligands in the complex, the ε values for bis-heteroleptic complexes (416 and 417) are higher than those for tris-heteroleptic complexes (164 and 165). Furthermore, among these dyes, the best spectral coverage was obtained with 165, and due to the lack of NCS− groups, 416 and 417 exhibited narrower absorption domains from 400 to 600 nm. According to the results explained thus far, the best strategy for obtaining maximum efficiency in DSSCs includes the incorporation of both an aliphatic chain and π-extended groups in a bpy ancillary ligand. 9519


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Table 4. Molecular Structures of Ruthenium Photosensitizers with Various Phenanthroline-Based Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCsa

a

Both H atoms of the carboxylic acids in the general formula are replaced with N(C4H9)4. bOne H atom of the carboxylic acid in the general formula is replaced with N(C4H9)4.

the MLCT between the anchoring ligand and the metal center: the longer the conjugation length of ancillary ligands, the lower the MLCT energy. These results indicate that increasing the conjugation length of the ancillary ligand does not guarantee

higher efficiency in the Ru complexes. The decrease in intensity of the MLCT band is probably due to the change of HOMO location in the metal complexes. IPCE curves of the devices are consistent with the MLCT band of the complexes. Therefore, the 9520


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this dye has more influence on the performance of DSSCs than its electron-withdrawing effects. Subsequently, 1,3-dihydro-1,1,3,3-tetramethyl-7,8diazacyclopenta[1]phenanthren-2-one (DTDP) was used as an ancillary ligand in the synthesis of heteroleptic Ru(II) complex 179 (HJ1) to increase the optical extinction coefficient.162 The efficiency of HJ1 was comparable to the corresponding value for an N719-sensitized solar cell under the same cell fabrication conditions, which indicates the potential of DSSCs based on the HJ1 complex and emphasizes the importance of designing the molecular structure of photosensitizers to produce efficient DSSCs. 3.2. 1,10-Phenanthroline-Containing Imidazole Groups

Carbazole (CAZ), as an electron-transfer functional group, has been proven to be a good hole-transport material in optoelectronic devices. Hence, a new carbazole-based phenanthrenyl ruthenium complex, 180, along with two additional new phenanthrenyl ruthenium dyes, 181 and 182 (Table 5) for comparison, were synthesized for use in DSSCs.163 Among these complexes, 180, containing N-ethylcarbazol-3-yl, showed the most favorable photoelectrochemical properties owing to a longlived charge-separated state generated from the electron-transfer process between Ru(II/III) and the CAZ group along with a high Voc value. Because of steric congestion effects, which decrease efficient adsorption by TiO2, the η value for devices made with 180 was only 5.3%, despite its high Voc and its photocurrent contribution per dye molecule. In 2009, Lu and co-workers164 started systematic studies on variations to the light-harvesting antennas with ancillary ligands including phenyl (183), thiophene (184),164 dithiophene (185), linear trithiophene (186), and dendritic trithiophene (187)165 as well as electron-donating phenothiazine-based (188) or N,Ndiphenylamino-based (189)166 ancillary ligands. Under identical conditions, complex JF-2 (184) exhibits a higher η value, 8.3%, than that of standard N3 dye, 8.1%. This is due to the synergism of the enhanced light harvesting and directionality of the substitution, which may increase the electric transition dipole moment of dye-loaded TiO2. Surface concentrations of the dyes on TiO2 films and the cell η value follow the order JF-2 > N3 > JF-1 (183). The authors have also reported that the driving forces for charge injection and regeneration tune the localization of the frontier orbitals appropriately and broaden the distribution of MLCT bands; thus, device performance could be enhanced due to modification of the ancillary ligand with a thiophene moiety.164 The ε value of the lower-energy MLCT band of JF-6 (186) is higher than that of JF-5 (185) and JF-7 (187) because it has a longer and planar oligothiophene main chain, which not only increases the light-harvesting capability but also maintains the delocalized π-framework throughout the entire molecule. Cyclic voltammograms showed that the oxidation and reduction potentials of all dyes are very close to each other (Figure 25). However, elongation and/or bifurcation of the conjugated ancillary ligand decreases the quantity of dye loading on the TiO2 surface because of the larger volume of dye molecules, a factor that lowers device performance. The overall device performance for DSSCs follows the order JF-5 > JF-6 > JF-7.165 The ε values of the MLCT increase in the order JF-4 (189) < JF-2 < JF-3 (188), which is in good agreement with the calculated results. Furthermore, the broader absorption spectrum of JF-3 absorbed on TiO2, compared to those of JF-4 and JF-2, suggests that the molecular planar configuration between the thiophene and phenothiazine groups in JF-3 is better than that of thiophene and

Figure 24. Molecular orbitals and energy gaps of AR20, AR24a, AR25, and AR27a [DFT/6-31H(d), with an isodensity value of 0.02]. Reprinted with permission from ref 157. Copyright 2008 American Chemical Society.

overall efficiencies of CYC-P1- and CYC-P2-sensitized cells are 6.01% and 3.42%, respectively, compared to an N3-sensitized device efficiency of 7.70%, for the same device-fabrication processes and measuring parameters. (NBu4)[Ru(4,7-dpp) (dcbpy) (NCS)2], coded as YS5 (175), is a phenanthroline-based sensitizer with a broad spectral bandwidth and visible light absorption properties, in which NBu4 is tetrabutylammonium and 4,7-dpp is 4,7-diphenyl-1,10phenanthroline, reported by Sun et al.159 In comparison with N719, the ε of MLCT absorption bands in YS5 is greater at all wavelengths above 430 nm. Under AM 1.5G 1 sun illumination and the same conditions, YS5 produces an η of 6.05% with a maximum IPCE of 65% at 540 nm, compared to η = 6.32% and IPCE = ∼68% at 550 nm for N719. A heteroleptic ruthenium complex was designed and synthesized with a bathophenanthroline ligand, bearing disulfonic acid disodium salt as a sensitizer, 177 (K28), for DSSCs with a ZnO nanorod electrode.160 Under standard global AM 1.5 solar irradiation, an η value of 2.5% was obtained for the K28sensitized cell, while under the same conditions, a DSSC based on K27 (176) exhibited an η of 1.69%. The expanding πconjugation and the bulky sulfonyl groups, which reduce back electron transfer, are the reason for the higher efficiency of the K28-sensitized cell. It was also shown that the K28 complex can increase ε and enhance the spectral response in visible light by expanding π-conjugation and, of course, reducing back electron transfer. More recently, a new heteroleptic phenanthroline-based complex (178) was synthesized, characterized, and compared with other phenanthroline complexes to establish a rational relationship between the donor/acceptor character of Me, Ph, and H substituents on the 4- and 7-positions of the phenanthroline ligand attached to the Ru(II) complex.161 Results obtained from the photoelectrochemical performance of the compounds as dye sensitizers show that the improvement in performance of the solar cells was due to electronic effects from the methyl groups. The use of phenyl groups revealed that the lightharvesting efficiency or reduction of electron recombination of 9521


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Table 5. Molecular Structures of Ruthenium Photosensitizers with Various Phenanthroline Imidazole-Based Ancillary Ligands Bearing Aromatic Moieties and Photovoltaic Properties of Their Corresponding DSSCsa

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Table 5. continued

a

One H atom of carboxylic acid in the general formula is replaced with N(C4H9)4.

compared to an MLCT band at 503 nm (ε = 0.90 × 104 M−1· cm−1) for Ru(Hpip) due to the stronger electron-donating ability of the TPA group on Hipdpa. The maximum absorption wavelength of the MLCT transition with an enhanced ε value was observed to be red-shifted in Ru(Hipdpa) with respect to Ru(Hpip) because of incorporation of the electron-donating TPA group. It was also found that the η value of the Ru(Hipdpa)sensitized cell (6.85%) is comparable to the corresponding value for N3 dye (6.47%) under identical conditions and is greater than that of Ru(Hpip). In addition, the enhanced MLCT absorption shows that the electron-donating carbazole group-containing Ru(Hcpip) {192; Hcpip = 2-[4-(9H-carbazol-9-yl)phenyl]-1Himidazo[4,5-f ][1,10]phenanthroline} can capture solar radiation more effectively than a phenylcarbazole-free analogous Ru(II)

the N,N-diphenylamino group in JF-4. An η value of 9.1% was obtained for JF-3, which is greater than that of JF-2 (8.3%) and JF-4 (7.9%). EIS data showed that the overall resistance at the TiO2/electrolyte interface and of the JF-3-sensitized solar cell is small and follows the trend JF-4 < JF-2 < JF-3.166 A new heteroleptic Ru(II) complex, which contains a triphenylamine ligand grafted to imidazole phenanthrolinebased Ru(Hipdpa) {190; Hipdpa = 4-(1H-imidazo[4,5-f ][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline}, was synthesized and its performance was compared with that of Ru(Hpip) (191; Hpip = 2-phenyl-1H-imidazo[4,5-f ][1,10]phenanthroline), which has a phenyl substitution replaced with triphenylamine, and the N3 dyes.167 It is worth mentioning that the MLCT band for Ru(Hipdpa) is blue-shifted by 22 nm and intensified 9523


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of N719 under identical cell fabrication conditions. In particular, 199 produced an η value of 10.1% under AM 1.5 standard sunlight. The lowest-energy MLCT transition band in the bands at 406, 490, and 560 nm in the absorption spectrum of the complex are red-shifted by 25 nm compared to N719 and have a high ε that substantially exceeds that of N719 over the visible domain (Figure 27). This is because of the stronger electrondonating property of the cyclometalated ligand compared to the two isothiocyanate groups. Interestingly, the IPCE in this family reaches a maximum of 83% at 570 nm. The computed spectra, using both DFT for ground-state geometries and TD-DFT for excited-state energies, and their properties in the visible region are in very good agreement with the experimental spectrum (Figure 27). The progress of NCS-free sensitizers has been followed by studies of the photophysical and electrochemical properties of various substituents (-NO2, -F, -phenyl, -4-pyridine, and -thiophene-2-carbaldehyde) on the phenyl ring of the cyclometalated ligand in [Ru(dcbpy)2(ppy)]+ complexes, 200−207, by Bomben et al.172 It has been shown that introduction of substituents para to the Ru−C bond imparts the greatest influence on optical properties and redox behaviors. The electron-withdrawing character of the substituents is reflected by substantial changes in the Eox values resulting from extension of the HOMO beyond the phenyl ring of the C∧N ligand. Recently, a study of spectral and electrochemical characteristics indicated an efficiency of 7.1% for complex 200 as the sensitizer in DSSCs.173 A new class of isothiocyanate-free cyclometalated rutheniumbased dyes, HIS1 (208), was developed for sensitizing nanocrystalline TiO2 solar cells.174 This complex showed an appreciably broad absorption range. With this new sensitizer, there was an η value of 4.76%, Isc of 11.21 mA·cm−2, Voc of 0.62 V, and FF of 0.68, obtained under standard AM 1.5 sunlight. To understand the effect of an electron-withdrawing group, such as CF3, on the pyridine ring of the cyclometalated ppy, complex 209 was presented with an efficiency of 3.7%.175 Complex 209, with an electron-withdrawing group, shows enhanced optical and electrochemical properties compared to complex 199. Isothiocyanate-free thiophene-substituted Ru(II) cyclometalated complexes (210 and 211) based on thiophene-derived 2(2,4-difluorophenyl)pyridine ligands were investigated for their optical and electronic properties.176 Thiophene-based substituents led to tunable optical and energetic properties by inducing bathochromic and hyperchromic effects, which resulted in enhanced light-harvesting capacities, higher external quantum efficiencies, improved device photocurrents, and top-ranked PCEs. Introduction of the thiophene-based substituent in complexes 210 and 211 resulted in a red -shift of the visible bands and enhanced the optical properties in comparison to the 199 dye with no thiophene substitution, while easing the oxidation. Upon substitution with the dithienyl-based substituent on the pyridine ring, the η value can be enhanced to 5.7%, which is more than twice that of complex 199 based on the unsubstituted 2-(2,4-difluorophenyl)pyridine. Four novel complexes, 212−215, based on thiazole and benzothiazole cyclometalating ligands were synthesized, and their photophysical, electrochemical, thermal and photovoltaic properties were investigated.177 This is the first time that cyclometalating ligands including benzothiazole and thiazole units were used for preparing Ru(II) photosensitizing dyes. The carbazole and triphenylamine groups are electron-donating in

Figure 25. (a) Cyclic and (b) square-wave voltammograms of ruthenium sensitizers JF-5, JF-6, and JF-7 in DMF. Reprinted with permission from ref 165. Copyright 2010 American Chemical Society.

complex, Ru(Hpip).168 The photovoltaic performance of the Ru(II) dyes, in the presence of 4-TBP as an additive in the electrolyte, was also studied. The results indicate that the high PCE for Ru(Hcpip), 6.98%, is significantly greater than that of Ru(Hpip), 2.48%, even under identical conditions and is 1.08 times the η value of 6.47% observed for N3 dye. Subsequently, Ma et al.169 reported a pair of isomeric ruthenium sensitizers, 193 (BM1) and 194 (BM2), having 2thiophenimidazo[4,5-f ]-[1,10]phenanthroline (TIP) with a 9phenylcarbazole tail at the α or β position of the thiophene unit. It is noted that the DSSC based on BM1 reached an η value 21% higher than that of N3 under the same measuring procedures, which is believed to originate from the π-conjugated structures of their isomeric ancillary ligands, as evidenced by their different UV−vis absorption spectra.169 Moreover, they reported Ru(II) sensitizers 195−198 (S1−S4) based on TIP ancillary ligands [TIP (S1), 5-Br-TIP (S2), 4-Br-TIP (S3), and BTIP (S4)], in which S2 and S3 are a pair of isomers with a Br atom substituted at the α or β position of the thiophene ring.170 The obtained overall efficiencies follow the sequence S3 > S4 > S1 > S2 due to the different substituents in the ancillary ligands.

4. RUTHENIUM PHOTOSENSITIZERS BASED ON CYCLOMETALATION To remove the corrosive effects of the NCS groups and the undesirable interactions between the NCS group and electrolytes and other components in DSSCs, cyclometalated complexes have been successfully developed (Tables 6 and 7, Figure 26). 4.1. Cyclometalates Containing Phenylpyridyl and Benzothiazole Ligands

Grätzel and co-workers171 reported an efficient organometallic compound (199) with a cyclometalated 2-phenylpyridine (ppy) ligand as a sensitizer for DSSCs, with a PCE comparable to that 9524


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Table 6. Molecular Structures of Ruthenium Photosensitizers with Various Cyclometalation-based Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

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Table 6. continued

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Table 6. continued

a

The counter ion of complex is PF6−.

nature and can extend the π-electron delocalization to improve ε values and absorption spectral coverage while facilitating charge transfer. Nevertheless, the ε values of benzothiazole-containing dyes D3 (214) and D4 (215) were higher than those of dyes D1 (212) and D2 (213) due to the higher π-conjugation length of the benzothiazole ligands, and they decrease in the order D3 > D4 > D1 ≈ D2. Furthermore, the computational results obtained from DFT calculations can be correlated with the experimental results. All of these dyes were also associated with particularly good thermal stability because of the strong metal−organic bonding. However, an η value of 2.98% was obtained for dye D3 with the carbazole/benzothiazole mixed ligand. In this field, Li et al.178 demonstrated a representative example by synthesizing isothiocyanate-free cyclometalated ruthenium complexes NC102 (216) and NC103 (217) by replacing the NCS ligands of N3/N719 with 2-(4-methyl-5-phenylthiophen2-yl)pyridine and 2-(4-fluorobenzo[b]thiophen-2-yl)pyridine, respectively. Dye NC103 exhibited a higher ε than that of NC102, presumably due to the strong σ-donating cyclometalating thienylpyridine ligand. It was found that replacement of NCS with a thienylpyridine ligand not only bathochromically shifted the absorption maximum but also broadened the absorption spectrum, which may be appropriate for enhancing the light-harvesting ability to yield a high photocurrent for DSSCs. The PCE of these DSSCs based on NC102 and NC103 sensitizers reached 3.64% and 4.22%, respectively, which are consistent with the IPCE values. Moreover, the effect of different substituents [bisCF3, CF3, OMe, and diphenylamine, respectively corresponding to dyes DbisCF3 (218), D-CF3 (219), D-OMe (220), and D-DPA (221)] on the ppy ancillary ligand on the photophysical properties and photovoltaic performance was investigated.179 It was found that the absorption wavelength increases in the order D-bisCF3 < DCF3 < D-OMe < D-DPA. It was also shown that the recombination time of the devices follows the order D-bisCF3 > D-OMe > D-CF3 > D-DPA. Among all the DSSC devices, a cell based on D-CF3 showed the best performance, which can be due to its broad visible light absorption range, appropriate localization of the frontier orbitals, high charge collection efficiency, and moderate dye loading on TiO2.

4.2. Other Cyclometalates

In 2010, Chang et al.180 developed a set of N-heterocyclic carbine (NHC)/pyridine ruthenium dyes, 222 and 223, as an alternative approach to tuning the frontier orbitals of dyes. The maximum absorption spectra of the CBTR (222) solutions were dependent on the solvent used. Moreover, they observed a significant decrease in the ε of CBTR, which has a strong σ donor in comparison with N719. In addition, the absorption profile of the CBTR sensitizer indicates that red-shifted absorption properties can be enriched by increasing the conjugation length of the ancillary ligand to enhance the light-harvesting ability. The obtained results also indicate that carbene sensitizers have a similar effect to that of N719 dye on the Fermi level of TiO2 when they bind to the CB of a TiO2 electrode. The presence of the NHC ligand (a strong σ donor) in a DSSC based on the CBTR sensitizer leads to an increased Isc and results in an η value approximately 8% higher than that of an N719-sensitized DSSC.180 Additionally, theoretical calculations were performed to gain insight into the factors responsible for the photovoltaic properties of CTBR.181 A substantial improvement in photovoltaic performance can be achieved by use of the high-efficiency tris-heteroleptic cyclometalated Ru sensitizer 224 containing aliphatic substituents that can generate high efficiencies in conventional DSSCs.182 A cell efficiency of 7.3% at 1 sun was obtained with a DSSC prepared with 224, which is significantly greater than the 6.3% produced by N3 recorded under the same conditions. Notably, the high performance of 224 was maintained in devices with thinner TiO2 films, achieving 6.9% at full sun. It was also revealed that a noncorrosive cobalt mediator achieved an efficiency of 5.5% under full sun illumination. A series of tris-heteroleptic cyclometalated Ru(II) complexes, 225−227, were tested in the context of development of DSSCs in which the substituents were carefully chosen for their electronwithdrawing character to verify that a thermodynamic driving force exists for dye regeneration.183 It was shown that η ranged from 2.39% to 4.02% for complexes 225−227. Moreover, the electrochemical and UV−vis data disclosed that each complex exhibits broad MLCT bands of significant intensity in the visible region and demonstrates ground- and excited-state redox potentials that are appropriate for sensitizing TiO2. 9527


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Table 7. Molecular Structures of Cyclometalation-based Ruthenium Photosensitizers and Photovoltaic Properties of Their Corresponding DSSCs

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Table 7. continued

A notable advance is research by Chen et al.,43 who designed and synthesized carbene-based ruthenium photosensitizers 228−230 that produced photovoltaic efficiencies of up to 7.24% compared to that of N719 dye (8.63%). These carbene complexes (featuring strong donors) display low molar absorption coefficients at ca. 537−541 nm, but they provide comparable photoelectric conversion efficiencies to that of N719 dye. The IPCEs of the ruthenium complexes display broad features covering a large part of the visible spectrum from 350 to 800 nm, indicating great potential to use these types of complexes as photosensitizers for DSSCs. The photoaction spectra indicate that the complexes act as efficient sensitizers with a plateau in the 500−550 nm region and maxima in the range 45−61%. Furthermore, the IPCE curve of the device fabricated

with CifPR dye (230) is slightly broad and red-shifted in the region from 675 to 770 nm (Figure 28). Interestingly, modifications on the ancillary ligand of these sensitizers by removal of an alkoxyl group and replacement of the octyl chain with a 3,5-difluorobenzyl group showed a 13% increase in the conversion efficiency for the CifPR dye. The first example of a trichromic cyclometalated Ru dye is 231, in which one of the bidentate bpy ligands bears two triphenylamine groups to further enhance the light absorption.184 The results showed that 224 is better than 231 at protecting the surface of TiO2, probably due to less space between the dyes and/or better packing between the hexyl chains closer to the surface. An IPCE between 65% and 70% in the 9529


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Figure 28. Current density−voltage characteristics of photovoltaic devices incorporating CiPor, CifPoR, CifPR, and N719 as sensitizers under illumination with AM 1.5 simulated sunlight (100 mW·cm−2). (Inset) Monochromatic IPCE spectra of photovoltaic devices. Reprinted with permission from ref 43. Copyright 2011 Royal Society of Chemistry.

Figure 26. General graphic of polypyridyl complexes bearing cyclometalated ligands as replacements for NCS.

mediator and the TiO 2 semiconductor surface without compromising dye regeneration, thereby maintaining the device η value as high as 7%. The alkyl substituents of the heteroleptic dyes improve the V oc values by impeding unfavorable recombination processes. The IPCE traces for devices based on 233 and 236 show an onset at ca. 780 nm, with the highest response over the ∼450−600 nm range (maxima of 68.5% and 71.1% were observed at 540 and 530 nm, respectively). Polander et al.186 reported two new cyclometalated ruthenium complexes, 237 and 238, to realize the compatibility between ruthenium sensitizers and cobalt electrolytes in DSSCs. The presence of alkoxy chains with 12 carbon atoms on the cyclometalated ligand results in an increase of the IPCE up to 70% in 238. In addition, DSSCs with sensitizer 238 reached efficiencies up to 8.6% in the presence of a cobalt electrolyte. Notably, this result closely matches that obtained with iodine (8.7%), which has not been reported for a Ru(II) sensitizer thus far. Subsequently, two novel isothiocyanate-free heteroleptic Ru(II) cyclometalated complexes with long-chain hydrophobic groups, ss-22 (239) and ss-14 (240), were investigated to combine the steric properties of Z907 with the ability to control the ground- and excited-state energies that are characteristic of cyclometalated dyes to allow for compatibility with outer-sphere redox shuttles; they were applied in DSSCs using the [Co(dmbpy)3]2+/3+ redox shuttle (where dmbpy = 4,4′dimethyl-2,2′-bipyridyl).187 Functionalization of the bpy ligand with dinonyl groups, combined with addition of electronwithdrawing fluorine substituents on the ppy ligand, led to longer electron lifetimes and enhanced light absorption compared to [Ru(ppy) (bpy) (dcbpy)](PF6) and a satisfactorily positive HOMO energy level for regeneration.

Figure 27. UV−vis experimental absorption spectra of complex 199 (red line) and N719 (dashed blue line) measured in DMF. Red bars represent the computed vertical electronic excitations intensities (TDDFT) of complex 199. For three selected optically active electronic transitions (black bars with labels 1, 2, and 3 at 533, 496, and 426 nm, respectively), electron−hole density plots are shown. The hole and excited electron densities are represented in pink and cyan, respectively. Reprinted with permission from ref 171. Copyright 2009 American Chemical Society.

region 450−600 nm and an η value in excess of 7% in the DSSC were recorded despite the large molecular footprint on TiO2. In a motivative scenario, a new class of cyclo ruthenated compounds, 232−236, was reported that consist of a naphthalimide fragment [a substituted 3-(2′-pyridyl)-1,8-naphthalimide cyclometalated ligand] with a high η value in the DSSC.185 It was shown that substitution of the pyridine ring of the C∧N ligand with conjugated groups can increase ε; meanwhile, the electron density imparted on the metal center is alleviated by the 1,8-naphthalimide fragment. This latter feature preserves a Ru(II/III) redox couple more positive than 0.8 V versus normal hydrogen electrode (NHE), thereby accommodating regeneration of the oxidized dye by an iodidebased electrolyte. The structure of the sensitizer can consequently be modulated at various moieties to enhance light absorption and suppress recombination between the redox

5. RUTHENIUM PHOTOSENSITIZERS BASED ON BIS(4,4′-DICARBOXYLIC ACID-2,2′-BIPYRIDYL) 5.1. Ancillary Ligands with Strongly Electron-Donating Bidentate S, O, or N

Another interesting family of ruthenium polypyridyl sensitizers can be identified with two units of dcbpy as the anchoring ligands and one cleating ancillary ligand. Photophysical and photo9530


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Table 8. Molecular Structures of Ruthenium Photosensitizers Containing Bis(dcbpy) with Various Electron-Donating Bidentate S, O, or N Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCsa

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Table 8. continued

a

This structure contains only one dcbpy, and the other ancillary ligand is dmbpy. bBoth H atoms of carboxylic acids in the general formula are replaced with N(C4H9)4.

nm compared to that of the analogous NCS complexes. In addition, the low-energy absorption maximum of complex [Ru(dcbpy)(dmbpy)(NCS)2] is extremely solvent-sensitive and red-shifts by 45 nm upon going from water to dimethyl sulfoxide (DMSO) solvents. The oxidation potential in these complexes increases with increasing π-acceptor strength of the anionic ligands, that is, Cl− < ddtc− < NCS−, so the oxidation potential of complex [Ru(dcbpy)(dmbpy)(NCS)2] is anodically shifted compared to that of 246. Additionally, hydrogen bonds between the carboxyl groups and the lone pair of electrons on the nitrogen of the ddtc ligand or stacking of the bpy rings cause the aggregation phenomenon among the dyes in DSSCs. In complex 246, aggregation is mostly due to formation of chains along the yaxis associated with intermolecular π−π interactions of the bpy ligands, coupled with hydrogen-bond formation among COOH groups.

electrochemical studies of three ruthenium sensitizers containing diethyldithiocarbamate (ddtc) (243), dibenzyldithiocarbamate (244), and pyrrolidinedithiocarbamate (245) were developed by Argazzi and Bignozzi and co-workers (Table 8).188 For dithiocarbamate sensitizers positioned in portions of the TiO2 film where the iodide concentration is exhausted, CR efficiently competes with iodide oxidation and results in a lower photocurrent. Moreover, for the anionic forms of the sensitizers in acetonitrile, the IPCE is enhanced in the same order as E1/2: 245/TiO2 < 243/TiO2 < 244/TiO2 < N3/TiO2. To reconcile the tasks of the sensitizer to provide efficient solar light harvesting and vectorial electron injection into the semiconductor, two ruthenium complexes, 246 and [Ru(dcbpy)(dmbpy)(NCS)2], were reported that incorporate different ligands, such as dmbpy and ddtc, as the ancillary ligands.189 The low-energy MLCT band of complex 246 is red-shifted by 15 9532


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Table 9. Molecular Structures of Ruthenium Photosensitizers Containing Bis(dcbpy) with Various Aromatic Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCsa

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Table 9. continued

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Table 9. continued

a

The complex has two monodentate ligands.

In an exciting study, Anandan et al.194 reported the synthesis of mixed ligands of Ru(II) complexes (258−262) with dcbpy and substituted pteridinedione and isoalloxazine as spectator ligands and their use in electrochemical photovoltaic cells. The MLCT bands of these complexes were broad and red-shifted compared to those of [Ru(dcbpy)3]4−. The lower oxidation potential of these complexes compared to [Ru(dcbpy)3]4− is attributed to increased electron-donating properties of the ancillary ligands, which push the dd states to a higher energy. In this class, dye 260 was a better redox complex because of its ability to harvest a large fraction of visible light. Bidentate 8-oxyquinolate (OQN−) as the ancillary ligand was also used to prepare a new ruthenium complex (263).195 Spectroscopic, electrochemical, and theoretical studies indicated enhancement of the molecular orbital overlap of 263 due to degenerate mixing of Ru d (π) and OQN p (π). 8Hydroxyquinoline derivatives used as ligands for the synthesis of Ru-based dyes for DSSCs have proven to be good stabilizers for complexes in their oxidized state.195 Therefore, modifying 8oxyquinolate with methyl and employing it as a ligand for synthesis of a Ru-based dye (264) for DSSCs resulted in easier oxidation of both ligands and complexed species.196 The first example of mononuclear dyes 265−267 containing 2,4,6-trimercapto-1,3,5-triazines as versatile ancillary ligands was reported. The key point in this interesting class was tuning of the electron donor−acceptor character by means of successive acid− base equilibria involving two free thiol groups.197 Owing to the presence of six free acid−base sites, the charge-transfer excited states can be tuned by simple deprotonation of these sites, thus affecting the photoinduced electron injection properties to mesoporous TiO2. Furthermore, the contribution of two MLCT and one ligand-to-metal charge-transfer bands to the energy conversion process was revealed by deconvolution of the photoaction spectra, indicating that the first ruthenium dye presented long-distance photoinjection from a ligand-to-metal charge-transfer excited state.

As an important achievement, the diketonate ancillary ligand in a dye molecule shows interesting properties owing to the strong σ-donating nature of the negatively charged oxygen donor, which destabilizes the ground-state energy level of the dye, leading to a decreased energy shift of the MLCT transitions. Synthesis and characterization of a series of monocationic polypyridyl ruthenium complexes, 247−251, along with a βdiketonate as the ancillary ligand in place of two isothiocyanates in N3, have been reported.190,191 Due to the high electrondonating capability of diketonate ligands, the MLCT absorption maxima of these complexes are red-shifted compared with ruthenium complexes containing isothiocyanate ligands. The key role of donor strength of the substituent bound to the diketonate ligands at the ground-state energy level has been investigated in detail. When the two phenyl groups in complex 249 are replaced by three electron-donating methyl groups in dye 248, the energy level of t2g decreases from 0.74 to 0.63. The extraordinarily high photocurrent and η value for acetylacetonated complex 247 are of particular interest (13.20 mA·cm−2 and 6.0%), as these values are higher than those of its isothiocyanate complex (12.66 mA· cm−2 and 5.7%). The comparatively low photocurrents of 248 and 249 are attributed to being hardly soluble in solvent and adsorbing onto the TiO2 surface without any chemical bond; therefore, they cannot inject electrons into the semiconductor.190 Among the cells sensitized with these diketonate complexes, the photoelectrochemical cell based on 250 showed the highest conversion efficiency, owing to the strongest electron-donating character.191 An interesting series of Ru(II) polypyridyl sensitizers with strongly electron-donating dithiolate ligands were prepared for sensitization of nanocrystalline TiO2 electrodes.192 In this report, all complexes (252−255) showed a broad MLCT absorption band over the whole visible range. The energy of the MLCT transition in these complexes diminishes in the order 252 > 253 > 254 > 255. By choosing appropriate dithiolate ligands, the lowenergy absorption bands of complexes are red-shifted by approximately 150 nm, and the Ru(II/III) oxidation potentials could be modified to approximately 600 mV. Furthermore, when the relative driving forces of these complexes are considered, the CR rates increase in the order 252 < 253 < 254 ≈ 255.192 Additionally, in the investigation of ruthenium bpy dyes 256 and 257, incorporating sulfur-donor bidentate ligands, revealed that the recombination kinetics of 256 were unexpectedly fast, which was attributed to the terminal CN on the ligand binding to TiO2.193

5.2. Ancillary Ligands Containing Aromatic Groups

Tris(dcbpy)Ru(II) dichloride, 268 (Table 9), was first used to sensitize colloidal anatase TiO2 nanoparticles and polycrystalline electrodes with an IPCE of 44%.198 Then a novel sensitizer (269) was designed to vectorially translate the oxidizing equivalent (the hole) away from the nanostructured semiconducting interface. Reduced electronic coupling between the surface and the hole results in an extremely long-lived charge-separated pair, which directly leads to increased Voc in a regenerative solar cell.199 9535


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Table 10. Molecular Structures of Ruthenium Photosensitizers Containing Bis(dcbpy) with Various Monodentate Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCsa

a

The structure contains only one substituent for i, and another is H2O. bThe structure contains only one substituent for i, and another is ethanol. The structure contains only one substituent for i, and another is NCS.

c

Synthesis of Ru complexes (270−273) with triazole ligands as ancillary ligands and their applications as dye molecules were published in 1999 by Lees et al.200 Complexes containing

pyrazine-based ligands showed absorption bands broadening due to protonation of the triazole ring, and similar shifts have been 9536


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simply functionalizing the pyrid-2-yltetrazolate ligand of 288 with a phenyl group, achieving 3.4% efficiency with the new bisheteroleptic Ru dye 289.208 Introduction of a Ph substituent assisted in increasing the absorptivity of dye 289 with respect to the unsubstituted 288, exploiting the higher π-delocalization offered by the phenyl group to improve the light-harvesting capability of the sensitized photoanode. Moreover, the novel bpy-acrylonitrile ligand was used in synthesis of the molecular light-harvesting antenna of sensitizer DK108 (291).209 The ε value of the new dye is significantly lower than that of N719. Moreover, the molecular structure of DK108 was compared with its free isothiocyanate complex, which provided additional interest for the investigation of possible mechanisms for dye redox couple interactions and device degradation pathways. In short, research on NCS-free sensitizers is attractive because NCS-free sensitizers exhibit fewer corrosion effects on DSSCs. This subject is continued in later sections.

observed for corresponding mixed-ligand complexes containing bpy. In a worthwhile polymerization study, preparation of the ruthenium macromolecular complex 274 has been described using a poly ligand via a novel polymerization route employing hematin as an efficient biocatalyst.201 Investigation of the photophysical and photovoltaic properties of the monomer and its macro dye showed that the monomeric complex (1.51%) had better performance compared to the polymeric complex (0.33%). Naphthyridine and acridinedione as the macromolecule ancillary ligands coordinated on Ru(II) complexes 275−278 were also synthesized and characterized.202 This study clearly indicated that the phenyl moiety of naphthyridine could act as an insulating barrier, which in turn decreases the performance of the cells by reducing the injection of electrons from the dye to the semiconductor. In 2012, ruthenium polypyridyl complex 279 with an appended acceptor group, 4-nitronaphthalene-1,8-dicarboximide (NMI), as a photosensitizer for p-type DSSCs was synthesized and characterized.203 In this case, when the two ruthenium complexes 279 and 268 were compared, oxidation of the former complex occurs easily due to the presence of the electron-withdrawing NMI group. It has been shown that addition of an electron-accepting ligand on a Ru(II)-based photosensitizer enhances the efficiency of the cell by two processes: First, the recombination time increases, providing more time for the flow of the hole to the back contact of the photoanode to occur. Second, the regeneration of the reduced sensitizer occurs faster for the appended complex. In this interesting topic, thienyl substituents often helped to improve not only ε of the MLCT band but also stability of the dye in DSSCs. The first case of Ru(II) dipyrrinates 280−283 as NCS-free sensitizers employed as dyes in DSSCs was reported by Li et al.204 Obtained results indicated that dye 281 shows meaningfully stronger absorptions throughout the visible region and a slightly bathochromically shifted MLCT band relative to those of N3 and 280. These findings specify that the smaller separation between the dipyrrin moiety and the S atom of thienyl causes stronger coupling between the two aromatic fragments, although the lack of coplanarity between them appears to reduce the extent of the coupling. Furthermore, Ru sensitizers with benzene- (284) and thiophene- (285) substituted dipyrrinates as ancillary ligands achieved efficient sensitization of nanocrystalline TiO2 throughout the visible region extending into the nearIR region (900 nm).205 Consequently, GS3 (284) exhibited decreased conversion efficiency compared to GS10 (285), probably due to an unfavorable HOMO energy level. Synthesis and spectroscopic characterization of two heteroleptic Ru(II) complexes, 286 and 287, based on 2-(2′pyridyl)quinoxaline and 4-carboxy-2-(2′-pyridyl)quinolone ancillary ligands were also developed.206 Both compounds showed high ε values similar to those of the N719 dye, while their photovoltaic performances were not equivalent to that of N719. A notable recent advance is the research by Dragonetti et al.,207 in which a new isothiocyanate-free Ru(II) sensitizer 288 was synthesized with a pyrid-2-yltetrazolate ligand; the oxidation potential of the Ru(II) dyes containing these ligands can increase due to the stronger electron-withdrawing character of the tetrazolate ligand with respect to phenylpyridine. The first DSSC device with this complex led to an η value of 3%, which was lower than that of the cell sensitized with N719.207 Moreover, a significant increase of ∼10% in the efficiency was attained by

5.3. Ancillary Ligands Containing Monodentate Groups

Ruthenium complex 292, comprising 3-(pyrrol-1-ylmethyl)pyridine (pmp) along with a conductive polypyrrole holetransport layer, was reported for fabrication of solid-state DSSCs (Table 10).210 Similarly fabricated cells of this dye enhanced the IPCE compared to N3 due to the pmp ligand working as a binding sites for the polypyrrole chain. The effect of the pmp ligand can be explained by direct molecular wiring of the polymer chain to the excited metal center of the sensitizer. Furthermore, 4-phenylpyridine as an ancillary ligand was employed along with H2O in Ru(II) polypyridyl complexes 293 and 294 for DSSCs.211 The MLCT bands for the mono-4phenylpyridine complex are red-shifted, and the IPCE values are higher than those of the bis-4-phenylpyridine species. Furthermore, the number of azine ligands coordinated to the nonattached side, in addition to tuning the overall properties of the system for the design of molecular-level devices, can play an important role in extension of the spectral sensitivity to visible light. In addition, the IPCE was up to 40% for solar cells sensitized with Ru(II) complex 295, containing isoquinoline as the ancillary ligand.212 To explore the effect of NCS groups on photovoltaic behavior of N3, Grätzel and co-workers213 described the synthesis and photoelectrochemistry of a series of ruthenium polypyridyl sensitizers, 296−300, containing two phenylcyanamide ligands as a pseudohalide instead of NCS ligands. Light absorption of the new complexes at longer wavelengths improved due to the higher donor strength of the phenylcyanamide ligands compared with isothiocyanate and caused a red shift of 14−30 nm of the 1MLCT maximum. The Isc of all phenylcyanamide complexes increased in a nonlinear manner with light intensity, demonstrating surface aggregation. These dyes, when compared with the diisothiocyanate complex, show a decrease in IPCE, probably owing to surface aggregation (which causes a mass transport problem by blocking the semiconductor pores) and catalyzing the dark current. A transparent photoanode was prepared by immobilizing 301, comprising 4-cyanopyridine in a transparent conducting oxide substrate coated with a nanocrystalline TiO2 film.214 Analysis of experiments on TiO2 photoanodes functionalized with 301 and other species has shown a linear correlation between the quenching efficiency η and the IPCE parameters. Thiophene ligands have attracted much interest due to their ability to red-shift the absorption wavelengths. Saito et al.215 9537


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Table 11. Molecular Structures of Ruthenium Photosensitizers Based on Dipyridylamine as the Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

thiophene or pyrrole ligands can be explained through the developed charge transport at the interface of sensitizing dye molecules and hole transport materials. While 4-TBP and 1-methylbenzimidazole (MBI) as electrolyte additives increase the Voc and the η value of DSSCs, they cause problems with isothiocyanate-containing sensitizers in terms of isothiocyanate substitution reactions at elevated temperatures. However, DSSCs prepared with two N719 thermal additive substitution products, Ru-4-TBP (304) and Ru-MBI (305), had

considered thiophene- or pyrrole-bearing pyridine ligands to synthesize their ruthenium complexes with pyrrole (302) or thiophene (303) groups as the sensitizing dye molecules. Depending on the number of pyridine ligands and the degree of conjugation of the pyridine ligands, the maximum absorption wavelength decreases in the order 292 > 302 > 303 = N3. As an effective technique, the combination of in situ polymerization with these ruthenium dyes caused improvements to the photovoltaic performance of the solid DSSCs. The influence of 9538


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efficiencies 3 and 2 times lower than N719 cells, respectively, owing to the reduced electron collection efficiency.216 Another interesting investigation was reported by Shoker and Ghaddar,217 using a new family of ruthenium polypyridyl complexes, T133 (306) and T134 (307), that bear bis(tetrazolate) monodentate ligands along with an electronwithdrawing trifluoromethylphenyl (TFMP) group or an electron-rich TPA moiety. The presence of TPA groups causes a red shift in the absorption maximum of the dye in the visible and near-IR region, whereas the electron-deficient TFMP group raises the dye redox potential compared to the analogous TPAbased dye. Due to the blue shift in the absorption spectra of T133 and T134 and the slightly faster electron recombination processes, the performances of T133 and T134 are slightly lower than that of N719. However, DSSCs with these complexes displayed relatively good conversion efficiencies with opencircuit voltages comparable to that of N719 without the use of any additives due to their retarded electron-recombination processes. In summary, NCS groups play an effective role in the enhancement of light harvesting in the sensitizer, although undesirable interactions between NCS and the surroundings still remain.

decrease uptake of the sensitizer onto TiO2 surface. Consequently, the cell η values decrease in the order J6 > J9 > J5. In advanced research, Ko and co-workers222 synthesized two amphiphilic heteroleptic ruthenium complexes comprising a Nalkyl-substituted 2,2′-dipyridylamine ligand in which the 4,4′positions were substituted with hexylthiophene [in JK-85 (316)] and hexylthienothiophene [in JK-86 (317)] ancillary ligands. The MLCT band of JK-86 at the lower energy is red-shifted by 4 and 26 nm compared to those of N719 and JK-85, respectively. This red shift is attributable to the increased π-conjugation system in the ancillary ligand. Additionally, the ε and η values follow the order JK-86 > N719 > JK-85, while the Voc values follow the reverse order. This contradictory behavior can be attributed to the molecular size and intermolecular π−π stacking interactions of the dyes. On the other hand, electron recombination occurred more meaningfully in the photoelectrodes adsorbing dyes with more bulky structures, due to the relatively large TiO2 surface area unoccupied by dye molecules. Moreover, in the dark, the resistance for CR at the dyed TiO2/electrolyte interface is lower for JK-86 than for JK-85 and N719. Furthermore, the JK-86 device displayed good stability under light soaking at 60 °C for 1000 h (Figure 29).

6. RUTHENIUM PHOTOSENSITIZERS BASED ON DIPYRIDYLAMINE LIGANDS As a new class of bidentate ancillary ligand, dipyridylamine (DPA), containing a nitrogen atom between the two pyridine moieties, has been employed. Heteroleptic polypyridyl ruthenium complexes containing dipyridylamine with different alkyl chains as the ancillary ligands [DPA-C12 (308) and DPA-C14 (309)] revealed that the oxidized state of these new types of dyes is more stable than those of other polypyridyl ruthenium sensitizers with an isothiocyanate ligand.218 However, compared with the bpy ligand, the DPA-R ligand has good σ-donor and poor π-acceptor properties. Consequently, the ε of the MLCT band and the efficiencies of these dyes were lower than those of the standard N719 dye. Moreover, the lower cell η values relate to the fact that dye regeneration by I− in the electrolyte efficiently competes with the CR reaction of injected electrons trapped in the nanocrystalline TiO2 film with dye molecules in the oxidized state (Table 11). Surprisingly, triarylamine molecules have gathered great interest as organic dyes, and their excellent hole-transport abilities have been noted in DSSCs. The triarylamine moieties act as electron donors, which increases the efficiency of converting solar energy into electricity. Hence, Jin et al. developed triarylamine-functionalized ruthenium compounds J5 (310), J6 (311), and J9 (312);219 J8 (313);220 and J13 (314), and J16 (315)221 for DSSCs. Introduction of triarylamine derivatives with an alkoxy-substituted phenyl group not only is able to tune the HOMO and LUMO energy levels of the dyes but also stabilizes the photovoltaic behavior of the device under longterm light soaking and thermal stress. It is found that the efficiency of the J6-sensitized solar cell is higher than that of the J5 cell, owing to the presence of methoxy groups on the phenyl ring of triarylamine, which provides directionality to the excited state and prevents triiodide recombination in the electrolyte. Furthermore, the methoxy and tert-butyl substituents on the arylamine in J8 stabilize device performance under long-term light soaking and thermal stress. The drawback of J9 may be its relatively large molecular size compared to J6 and J5, which may

Figure 29. Evolution of solar cell parameters with JK-85 (red) and JK-86 (blue) during visible light soaking (AM 1.5 G, 100 mW·cm−2) at 60 °C. A 420 nm cutoff filter was put on the cell surface during illumination. Reprinted with permission from ref 222. Copyright 2009 American Chemical Society.

Additionally, the radius of the intermediate frequency semicircle in the Nyquist plot decreased in the order JK-85 > JK-86 ≈ N719 under illumination and open-circuit conditions, demonstrating improved charge generation and transport. Another interesting investigation was reported using amphiphilic Ru(II) sensitizers JJ-95 (318) and JJ-99 (319), having a highly conjugated dipyridylamine ligand, to obtain a clear comparison with N719.223 The polypyridyl complexes of JJ-95 and JJ-99 displayed very intense and broad absorption bands throughout the whole absorption region, and the low-energy MLCT band is red-shifted by 11 nm relative to that of N719 due to increased π-conjugation in the ancillary ligand. Cyclic voltammetry results show that the Ru(II/III) oxidation potential of JJ-95 is more positive than that of JJ-99, indicating the increased electron-donating properties of the ancillary ligand in 9539


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Table 12. Molecular Structures of Ruthenium Photosensitizers Based on Multinuclear Complexes as Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

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Table 12. continued

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Table 12. continued

was used as photoanode in a regenerative solar cell, probably because the Os(III) center is a weak oxidant. The flexible bpa ligand was engaged, and the geometry around Ru(II) permits direct Os(II)* semiconductor electron transfer and probably provides a direct pathway for CR as well. Furthermore, similar research was carried out by Lees et al.227 to explain the photophysical properties of nanoporous TiO2 surfaces modified with two polypyridyl complexes, Ru(II)-(bpt)Ru(II) (327) and Ru(II)-(bpt)-Os(II) (328), based on the bridging ligand 3,5-bis(pyridin-2-yl)-1,2,4-triazole (Hbpt). The main feature of bpt-based compounds is that the bpt bridge is very rigid and will not allow rotation around the linker. Remarkably, for the dinuclear RuRu species, an IPCE of 30% is obtained, approximately half that of the mononuclear ruthenium compound (63%), while for the RuOs species, no appreciable photocurrent is obtained. Figure 30 shows two well-defined metal-based oxidations at 1.1 V versus saturated calomel electrode (SCE) and 1.4 mV versus SCE for RuRu, while for RuOs, values of 0.660 and 1.4 V were obtained. Ruthenium homo- and heterobinuclear complexes (329− 333) with nanocrystalline thin films of TiO2 cast on an optically transparent indium tin oxide glass were sensitized, and a moderate IPCE was obtained with 329/TiO2 (39% at 470 nm).228 In comparison with IPCE values of the homo- and heterobinuclear complexes, efficiencies of the 1,4-bis([1,10]phenanthroline[5,6-d]imidazol-2-yl)benzene (bfimbz) complexes are better than those of the tetrapyrido[3,2-a:2′,3′c:3″,2″-h:2‴,3‴-j]phenazine (tpphz) complexes because of the slower rate of remote electron injection and its competition with other excited-state decay processes. First- and second-generation dendrimers (334 and 335) incorporating polypyridyl Ru(II) groups at the periphery positions were introduced, and their photophysical properties were investigated in solution and when adsorbed on the nanocrystalline TiO2 surface.229 The ε value of the MLCT band for 335 is approximately 2 times greater than ε for 334 and 6 times higher than that for the reference compound Ru(dcbpy)2(dmbpy), due to the increased number of absorbing ruthenium moieties per dendrimer upon going from Ru(dcbpy)2(dmbpy) to 335. In addition, the absorption intensities of the three dyes adsorbed on TiO2 films indicate that 334 has

JJ-95 due to the 4-hexylstyryl group. In addition, owing to their bulky structures inducing aggregation among dye molecules during adsorption onto the TiO2 surface, these novel dyes exhibited Voc and lifetimes slightly lower than those of N719. Nevertheless, a cell η value of 8.76% for JJ-99 is better than that of 8.19% for the solar cell sensitized with N719, which is attributed to the high ε of the MLCT band and extended absorption in the visible region. Consequently, the DPA ancillary ligand appears to be a good candidate to replace a bpy ancillary ligand when the appropriate substitution on DPA is used.

7. MULTINUCLEAR RUTHENIUM PHOTOSENSITIZERS Multinuclear ruthenium complexes can be used to improve light harvesting and PCE through the antenna effect. To demonstrate this attractive approach, the cyano-bridged trinuclear complex 320 was synthesized and its photovoltaic properties were studied.224 On the basis of systematic comparison between the mononuclear Ru[bpy(COO)2]34− and its trinuclear complex, the electron injection efficiency of the trinuclear dye is at least as good (60% or more) as that of its mononuclear analogue. Moreover, a series of CN-bridged trinuclear Ru complexes 321− 325 were synthesized in which TiO2 films with a specific fractaltype surface texture can be sensitized with strikingly high yields.225 The role of the COOH interlocking groups is to provide strong electronic coupling between the π* orbital of 2,2′bpy and the Ti (3d) orbital manifold of the semiconductor, allowing charge injection to proceed at quantum yields close to 100%. In addition, the monochromatic IPCE is strikingly high, exceeding 80% in some cases. Surprisingly, the optimal obtained results with dye 320 gave an η value of 11.3% (Table 12). These remarkable advances in photovoltaic performance have been motivation to explore more multinuclear dyes accordingly. In another exciting approach, inorganic donors were used to fine-tune the driving force for intramolecular electron transfer and the spectral properties of materials due to the high degree of molecular flexibility from systematic changes in the ancillary ligands. Thus, the first bimetallic sensitizer designed to demonstrate diodelike behavior at the semiconductor surfaces was a bimetallic RuOs compound (326) anchored to nanocrystalline TiO2.226 No significant photocurrent was detected when TiO2|Ru-bpaOs [where bpa = 1,2-bis(4-pyridyl)ethane] 9542


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8. RUTHENIUM PHOTOSENSITIZERS BASED ON PYRIDYL-BENZIMIDAZOLE In recent years, a number of pyridyl-benzimidazole derivatives containing different substituted groups have shown attractive chemical and photovoltaic properties. Heterocyclic ligands containing oxazole, thiazole, and imidazole derivatives were used to make corresponding ruthenium complexes 338 and 339 for DSSC applications.232 Interestingly, ε values of the lowerenergy MLCT bands of all these dyes are considerably lower than that of N719, but the amounts of RD1−RD3 (338−340) absorbed on TiO2 films are 2 times higher than that of N719. The imidazole ligand with a stronger electron-donating property might lead to better charge separation, so that the chargecollection efficiency of RD1 becomes much larger than those of RD2 and RD3. However, their oxidation potentials show a trend RD1 > RD2 > RD3, resulting in Voc decreasing in the reverse order: RD3 > RD2 > RD1 (Table 13). To enhance the spectral response over a wide wavelength region, as well as maintain a sufficient thermodynamic driving force for electron injection, the ligand 1-(2,4,6-trimethylbenzyl)2-(2′-pyridyl)benzimidazole and its heteroleptic Ru(II) dye 341 (CS23) were synthesized.58 Compared to Z907, a red shift of 7 nm in the lowest-energy MLCT absorption and a decrease in ε were observed in CS23 due to more extensive πbback-donation on the ancillary ligand. However, a DSSC based on CS23 exhibited nearly the same efficiency as the reference ruthenium complex, Z907, under the same conditions. In an effort to easily prepare ruthenium sensitizers, Huang et al.233 synthesized benzimidazole (BI)-based ruthenium sensitizers 342−344 and investigated their light-to-electrical energy conversion properties. The presence of a hydrophobic chain in the imidazolyl ligand of RD10 (343) longer than that of dye RD1 hinders CR, but the presence of a benzyl substituent in the imidazolyl ligand (342) may also hinder CR. In contrast, RD11 (344), with a dimeric imidazolyl ligand, may subsequently increase CR, thus diminishing electron density in the CB of TiO2. From the results based on photocurrent and photovoltage decays, the electron lifetimes of the devices indicate the systematic trend RD5 > RD10 > RD1 > RD11, which is consistent with cell performance. Among these four dyes, RD5 presented a better absorption intensity and had a higher Isc, and owing to the alleviated recombination dynamics, it had an augmented Voc relative to the others. The observed CR kinetics indicated the order of the Voc values as N719 > RD5 > RD10 > RD1 > RD11 (Figure 31). Two years later, the same research group published research on heteroleptic ruthenium complexes RD12−RD15 (345−348), containing fluoro-substituted BI ligands for DSSCs.234 Absorption spectra of the RD12−RD15 dyes are similar to the spectrum of the RD5 dye, demonstrating that the effect of F− substituents on the light-harvesting property is minor. BI ligands in Ru dyes of this series have the effect of retarding the CR but also leading to a downward shift of the TiO2 potential relative to an N719 device. The obtained energy-level diagram (Figure 32) from cyclic voltammetry measurements for the RD dyes shows that LUMO levels become increasingly stabilized as the number of fluorine atoms increases, which is consistent with the electron-withdrawing nature of the fluoro substituents. In addition, with increasing number of fluorine atoms, the Voc values decrease in the order RD15 > RD13 ∼ RD12 > RD14 > RD5; meanwhile, the trend of Isc for dyes in this series is the reverse. Additionally, in these dyes, the position of the fluoro

Figure 30. Cyclic voltammograms of (A) TiO2-RuRu and (B) TiO2RuOs. Reprinted with permission from ref 227. Copyright 2001 American Chemical Society.

the highest absorption intensity in the MLCT region compared to 335 and Ru(dcbpy)2(dmbpy), owing to the higher density of the adsorbed ruthenium moieties. Among these multinuclear ruthenium dyes, 334 demonstrated the highest efficiency value, which may be attributed to the greater light-harvesting properties in the red region of the visible absorption spectrum, the dendritic effect, and/or an antenna effect. The porphyrin and phthalocyanine families are of great interest due to extended visible light harvesting. Two new dyads were developed that are composed of ruthenium phthalocyanine (PcRu) and bis(bpy)-Ru(II) chromophores bearing carboxyl functionality to incorporate the excellent electron injection efficiency of (bpy)Ru complexes with the strong light absorption of the PcRu complexes (336).230 Interestingly, the amount of surface-absorbed dyad molecules is approximately half that of [Ru(dcbpy)(terpyridine)(NCS)2], which does not include the phthalocyanine complex, demonstrating that the presence of PcRu constituent meaningfully increases the steric bulk, thereby reducing the number of absorbed molecules on the TiO2 surface. Furthermore, DSSCs based on N719 and [Ru(dcbpy)(terpyridine)(NCS)2], compared to those sensitized with 336, showed superior performance; thus, the inclusion of a metallophthalocyanine unit has an overall detrimental effect. Recently, a novel dinuclear type with a highly conjugated ligand based on the Ru(II) polypyridyl complex of KO-20 (337) was synthesized and employed in DSSCs and was shown to have an efficiency of 6.5%.231 In comparison with the most efficient dyes, N719 and Z907, the ε of KO-20 was approximately double the efficiency, which can be attributed to conjugation extension/ variation at the core of the Ru(II) bpy sensitizer, presenting notable differences in the photovoltaic performances. It seems that multinuclear complexes and dendrimers will be promising candidates for improving photovoltaic performance in DSSCs. 9543


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Table 13. Molecular Structures of Ruthenium Photosensitizers Based on Pyridyl-Benzimidazole as Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

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Table 13. continued

BI ligands to promote the light-harvesting ability of dyes.63 Thiophene substituents effectively enhance the ε of these dyes and effectively shift the absorption band of the MLCT band toward the longer-wavelength region with enhancement of the red shift in the order RD18 ∼ RD16 > RD17 > RD12. Meanwhile, the hexyl substituent plays hardly any role in enhancing absorption coefficients and only a minor role in the blue shift of the absorption features. However, increasing the number of thiophene groups in the BI ancillary ligands increases the light-harvesting ability and broadened spectral features, resulting in higher Isc, and the corresponding IPCE of the devices is in the order RD18 > RD17 ∼ RD16 > RD12 > N719; however, the Voc of the devices displays the opposite trend to that of Isc. Moreover, thiophene substitutions shift the TiO2 potential downward and accelerate CR, but the inclusion of a long hexyl chain on the thiophene moiety hinders CR to account for the variation in Voc in the mentioned series. From stability tests of device performance at ambient temperature, surprisingly, the RD18 and RD12 devices showed only ∼2% degradation in cell performances over 2000 h. Recently, moreover, studies of interfacial electron-transfer dynamics for BI-based ruthenium complexes (RD5, RD12, and RD15−RD18) sensitized on TiO2 films exhibited fast and slow rise components that are assigned to electron injections of the 1 MLCT and 3MLCT states, respectively.235 The transients displayed a decay feature on the time scale of 10 ns only for films of RD5, RD12, and RD15; however, this type of decay feature was not observed for RD16−RD18 films. Thus, the results showed that fluorine substitution in the BI ligands retards back electron transfer, whereas thiophene substitution in the ligands retards electron injection.

Figure 31. Time-resolved profiles of infrared transient absorption of TiO2 films sensitized with RD1, RD5, RD11, and N719. Reprinted with permission from ref 233. Copyright 2010 Royal Society of Chemistry.

Figure 32. Schematic of potential levels of RD5, RD12−RD15, and N719, showing the HOMO and LUMO levels obtained from electrochemical and spectral measurements. Reprinted with permission from ref 234. Copyright 2012 American Chemical Society.

9. RUTHENIUM PHOTOSENSITIZERS WITH TWO FUNCTIONALIZED PYRIDYL AZOLATE ANCILLARY LIGANDS To find new isothiocyanate-free ruthenium sensitizers with enhanced light-harvesting abilities, pyridyl azolate derivatives were presented and used as the ancillary ligand. For the first time, isothiocyanate-free, charge-neutral ruthenium sensitizers, TFRS1−3 (352−354) (Table 14), were synthesized with a 2pyridylpyrazolate ancillary ligand.236 The ε values increase in the order TFRS-1 < TFRS-2 < TFRS-3, and Isc follows the same trend due to extension of π-conjugation of the backbone of pyridylpyrazolate chelates in the aforementioned order. Consequently, Voc and η values of TFRS-1 are higher than those of the N719 dye, demonstrating that pyridylpyrazolate chelate is a better insulator than NCS, which blocks the dark current and improves the Voc. Nevertheless, the Voc values of TFRS-2 and TFRS-3 are lower than that of the TFRS-1 dye due

substituents has no major effect on Voc and Isc. The increase in Voc upon fluoro substitution might be a result of two factors: an upward shift in the potential and a retardation of CR. Furthermore, increasing the number of fluorine atoms produces a more negative potential shift, but more than two substituted fluorine atoms also result in a decrease in Isc because of the lower LUMO level and the smaller amount of dye loading. It is also found that substituted fluorine atoms in the ortho position of the donor−acceptor dye performed better than meta-substituted fluorine in an isothiocyanate-free ruthenium sensitizer.234 Finally, overall device performance displays a trend with the order RD12 > N719 > RD15 ∼ RD5. Subsequently, the authors investigated heteroleptic ruthenium complexes RD16−RD18 (349−351) containing fluoro-substituted and thiophene-based 9545


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Table 14. Molecular Structures of Ruthenium Photosensitizers Based on Pyridyl Azolates as Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

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Table 14. continued

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Table 14. continued

to the relatively larger molecular size, resulting in reduced dye loading on TiO2 as well as defective packing and giving rise to an increase in the dark current. In addition, EIS studies under dark current conditions emphasized that the CR kinetics are slower for TFRS sensitizers than for the N719 dye (Figure 33). Furthermore, Ru(II) metal complexes [TFRS-4 (355), TFRS21 (356), TFRS-22 (357), and TFRS-24 (358)] comprising two functionalized 2-pyridyl azolate ancillaries have been systemically modified with the goal of finding better and more stable sensitizers for DSSC applications.237 Notably, all triazolate dyes exhibit relatively blue-shifted absorptions compared to their pyrazolate counterparts. Moreover, cyclic voltammetry studies show that TFRS-21, -22, and -24 complexes have a positive shift

relative to their pyrazolate counterparts, apparently due to more electronegative triazolate chelates. Additionally, TFRS-4, containing a hexylthiothiophene group on pyrazolate chelates, shows a distinctive red shift in MLCT absorption, which increases lightcapturing capabilities in the visible and near-IR regions and results in the best efficiency among all the sensitizers studied in this family. On the contrary, triazolate sensitizers, including TFRS-21, -22, and -24, display comparatively inferior Isc and Voc values relative to those of their pyrazolate counterparts, resulting from the enlarged optical gap caused by the electron-withdrawing nature of the triazolate chelates and possible interaction between the triazolate group of the sensitizer and the iodine in the electrolyte that induced rapid CR.237 Subsequently, two 9548


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device performance data with these dyes show that greater insertion of π-conjugation without the use of heteroatom appendages leads to higher leaning toward binding the I−/I3− redox couple. However, the bulky tert-butyl group introduces two contradictory effects on conversion efficiency: from one aspect, light absorption and Isc increase, while from another aspect, Voc decreases due to increased recombination. To obtain deeper insight into this behavior, transient photovoltage (TPV) measurements (Figure 35) were performed, and the results

Figure 33. Electrochemical impedance spectra, measured in the dark, for cells employing different dyes (TFRS dyes and N719). Reprinted with permission from ref 236. Copyright 2010 Royal Society of Chemistry.

judiciously engineered azolate-containing Ru(II) sensitizers, TFRS-41 (359) and TFRS-42 (360), were investigated by Wu et al.238 to probe the effective dialkoxyphenyl-substituted thienyl substituents at the azolate ancillaries to build an adequately covered blocking layer on the TiO2 surface. It is notable that TFRS-42 displayed the highest η-value for the [Co(bpy)]2+/3+ electrolytic system due to its charge neutrality, greater spatial congestion, and absence of isothiocyanate ligands, all of which are essential for reducing recombination across the TiO2/ electrolyte interface. The obtained results revealed that sensitizer TFRS-42, featuring 2,6-dialkoxyphenyl substituents, was capable of suppressing electron recombination between TiO2 and oxidized Co(III) species and retained the fast dye regeneration characteristics (Figure 34). A new class of Ru(II) sensitizers [TFRS-51−TFRS-54 (361− 364)] with a dcbpy anchoring ligand and two trans-oriented isoquinolinyl (or quinolinyl) pyrazolate ancillaries was designed in which the absorptivity was enhanced solely by fused chromophores without any sulfur atoms.239 The fabricated

Figure 35. (a) TiO2 electron density versus voltage, deduced from charge extraction measurements, and (b) electron lifetime versus TiO2 electron density, deduced from TPV measurements, for DSSC devices containing TFRS sensitizers. Reprinted with permission from ref 239. Copyright 2013 Royal Society of Chemistry.

showed that the presence of tert-butyl groups in TFRS-52 and TFRS-54 dyes results in longer device electron lifetimes, indicating greater retardation in recombination losses of TiO2 electrons to the electrolyte relative to TFRS-51 and TFRS-53. The highest efficiency of approximately 10.1% was recorded for a device sensitized with TFRS-52. Moreover, under light soaking at 60 °C, the device displayed brilliant stability by retaining ≥95% of its initial efficiency after 1000 h.239 The same research group published a detailed study on three isomeric Ru(II) metal complexes, TFRS-80a−c (365−367), all with similar electrochemical properties and identical optical responses. However, they showed varied steric impedance upon deposition onto the TiO2 surface according to the coordination orientation of the ancillary ligands.240 All TFRS-80a−c DSSCs with a [Co(phen)3]2+/3+ electrolyte demonstrated higher efficiencies (η = 8.36−9.06%) in comparison with devices with an I−/I3− mediator. This can be attributed to the isothiocyanate-free architecture, the improved light-harvesting capability, and the possession of conjugated and bulky 5-[2,6-bis(hexyloxy)phenyl]thiophen-2-yl functional moieties. In addition, hole-

Figure 34. Transient absorption kinetics of TFRS-41 and -42 devices, recorded in the presence and absence of Co(II/III) redox couples at λ = 550 nm, following excitation at λ = 500 nm (OD = optical density). Reprinted with permission from ref 238. Copyright 2014 Wiley−VCH. 9549


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Table 15. Molecular Structures of Ruthenium Photosensitizers Based on Tridentate Ancillary Ligands and Photovoltaic Properties of Their Corresponding DSSCs

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Table 15. continued

efficiencies of complexes possessing bipyrazolate are higher than those of bitriazolate complexes and are in the order TFRS-63 > TFRS-62 > TFRS-61 > TFRS-66 > TFRS-65 > TFRS-64. Another interesting investigation in this family was reported via a click-chemistry approach to synthesize 1,2,3-triazolylpyridine ligands and their corresponding Ru sensitizers 376 and 377.243 The lowest-energy MLCT band of dye 377 is blueshifted by 30 nm relative to dye 376. Under similar conditions, the efficiency of dye 376 bearing the 1,2,3-triazol-4-yl ligand is superior to that of the 1,2,3-triazol-1-yl counterpart in dye 377 due to faster electron transport into the TiO2 film and the lower recombination rate in comparison with dye 377-sensitized devices. In summary, ruthenium sensitizers containing pyridyl azolate derivatives as the ancillary ligand shows both advantages and disadvantages for consideration as NCS-free sensitizers.

diffusion coefficients follow the trend TFRS-80a > TFRS-80b > TFRS-80c due to effective blockage of π−π interactions by 2,6dihexyloxylphenyl substituents.240 In addition, comprehensive spectroscopic measurements and DSSC performance characteristics showed that the symmetrical isomers TFRS-2 and TFRS52, with trans-substituted pyrazolate fragments, are better DSSC sensitizers than their asymmetrical counterparts with cissubstituted pyrazolate fragments, TFRS-2b (368) and TFRS52b (369).241 The results indicated that electron lifetimes and RCR descend in the order TFRS-52 > TFRS-2 > TFRS-52b > TFRS-2b. A representative example was demonstrated by Yeh et al.,242 who synthesized a new series of Ru(II) sensitizers, TFRS-61−66 (370−375), by incorporating novel dianionic bipyrazolate or bitriazolate ancillary ligands to replace the dual isothiocyanates in N3 (or N719) with the aim of improving device stability. In comparison with TFRS-61 and N3, TFRS-64 comprises two relatively strong electron-withdrawing triazolate fragments, which is expected to further reduce electron density at the central Ru(II) atom, causing a significant blue shift in the MLCT band position. Furthermore, the presence of the thiophene appendage on bpy (TFRS-62 and -65) reveals a substantial hyperchromic effect and bathochromic shift, which are effectively improved in absorptivity through extension of the absorption wavelength to red as well as having an enhanced ε (TFRS-61 and -64). Furthermore, the extra insertion of a hexylthiolate side group to the thiophene appendage (TFRS-63 and -66) is capable of providing further delocalization and, hence, higher absorptivity, resulting from extension of the π-electron delocalization. Electrochemical data analysis reveals that E0ox for complexes containing a bitriazolate moiety is more positive than that for its congener with the bipyrazolate moiety (TFRS-64 > TFRS-61, TFRS-65 > TFRS-62, and TFRS-66 > TFRS-63). The CR of this series of dyes follows the order TFRS-63 > N3 > TFRS-65 > TFRS-61 > TFRS-66 > TFRS-62 > TFRS-64, where the smaller CR value means a faster CR between the electrons in the TiO2 and electrolyte and, thus, shorter TiO2 electron lifetimes. The

10. RUTHENIUM PHOTOSENSITIZERS BASED ON TRIDENTATE ANCILLARY LIGANDS Synthesis and characterization of several complexes of Ru(II), 378−382 (Table 15), containing 2,6-bis(1-methylbenzimidazol2-yl)pyridine (bmipy) or Ph-bmipy as tridentate ancillary ligands and their monodentate ligands were reported.244 Introduction of one phenyl group in position 4 of the bmipy ligand gives a redshifted absorption maximum for the corresponding phenylated 379 complex. Moreover, an improved ε for the MLCT maximum was achieved compared to that of the unsubstituted isothiocyanato complex. Nevertheless, when phenyl substitution was introduced between the bpy and the carboxyl group, it acted as an insulating barrier (distance effect) for the excited electron and did not enhance the absorption coefficients of its complex at wavelengths above 620 nm. In addition, the authors claimed that the weaker donor strength of the NCS results in a blue shift of the spectrum compared to those of the halide complexes. However, visible spectra of the halide complexes 380 and 381 display an enhanced red response, but the complexes show significantly reduced overall IPCE values. The stronger electron-donating 9551


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Table 16. Molecular Structures of Ruthenium Photosensitizers and Photovoltaic Properties of Their Corresponding DSSCs

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Table 16. continued

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Table 16. continued

effect of the halide ligands causes an increased energetic destabilization of the Ru (t2g) level compared to that of the isothiocyanate ligand. Subsequently, preparation, characterization, and photoelectrochemical studies of 383245 and 384246 with a 2,6-bis(3,5dimethyl-N-pyrazoyl)pyridine (bdmpp) ligand were also reported. The -NCS was responsible for tuning the metal t2g

orbitals of Ru(II), leading to the likely stabilization of the hole that is generated on the metal after injecting an electron into the semiconductor. The stronger π-acceptor character of the transisothiocyanate ligand compared to the Cl− ligand is possibly the cause of the blue shift observed in the Ru−NCS complex. In subsequent studies, substituted Ru(II)−bdmpp complexes were investigated to understand the influence of NCS− and Cl− 9554


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11. OTHER RUTHENIUM PHOTOSENSITIZERS In addition to the ∼400 ruthenium complexes introduced thus far, unclassified complexes are presented as other ruthenium complexes in this section. To facilitate dye/redox couple interactions and dye regeneration in nanocrystalline TiO2 solar cells, a supramolecular ruthenium complex 396 (Table 16) containing a cyclodextrin (CD) ligand was designed and synthesized as an efficient supramolecular light-absorbing antenna.252 The incorporation of α-CD supramolecular functionality into the Ru dye via a covalent linkage leads to an increase in Isc, Voc, and η relative to the Ru complex without a CD cavity; meanwhile, FF remains unaffected. This is attributed to guest−host interactions between the dye and I−/I3− redox couple. Autonomous studies showed complexation of the iodide redox couple with the CD in 1-α-CD. These results show that the CD moiety is able to act as a moderator and fine-tune the photoelectrode/electrolyte interface. Subsequently, Martineau et al.253 reported the preparation of heteroleptic ruthenium complexes 397−400 involving pyrroleor pyrrolidine-containing bpy ligands. The oxidation−reduction potentials in 397−400 were higher than those of the corresponding homoleptic complexes, indicating that the HOMO remained at lower energy levels. However, complexes 397 and 398 encompassing a pyrrole moiety showed the best ε due to electron delocalization, which increased the electric dipole moment associated with the charge-transfer transition. Additionally, the best performances were attained in dyes 397 and 398, providing more compatibility in a cobalt-based mediator relative to the usual I−/I3− mediator. Subsequently, 4,5-diazafluoren-9-one (dafo) and 1,10-phenanthroline-5,6-dione (phendione) chromophore units were used for the synthesis of novel Ru(II) dyes 401−404, and their characterization by spectroscopic techniques was described.254 Generally the phendione ligand increases the oxidation potential by lowering the HOMO level. A notable conclusion in this work is that the CN ligand increased the Isc to more than twice that of the NCS ligand, with the same chelating ligand directly bonded to the Ru metal in the complex. Another tris-heteroleptic complex of ruthenium, K30 (405), was prepared by a novel synthetic route and characterized with respect to its absorption, luminescence, and redox behavior.255 The complex presented the characteristic features of strong absorption over a broad spectral range of visible light, which is preferable for efficient solar light energy conversion. Dai et al.256 reported four new heteroleptic ruthenium sensitizers (409−412) containing uncommon 5,5′-disubstituted 2,2′-bpy as ancillary ligands, including various substituents at the 5,5′-positions of the bpy moiety such as alkylthiophene and triphenylamine and/or the incorporation of these on the side groups. The ε values of the MLCT band of these ruthenium dyes show the order 409 > 410 > 412 > 411, which are lower than that of N719 and exactly the inverse of wavelength. These results show that the increasing conjugation length of the 5,5′disubstituted bpy not only decreases the MLCT energy but also reduces the absorption intensity of the MLCT transition. In the same experiments, these bulky molecular structures indicated lower η due to the lower ε compared with N719, which can be caused by their weaker light-harvesting capacity and bulky molecular structures. Hence, two novel Ru(II) sensitizers, DV42 (413) and DV51 (414), bearing an unsymmetrical pyridine-quinoline hybrid ligand with a long aromatic moiety, were reported to study the

ligands on the efficiency of solid-state nanocrystalline solar cells.247 It is indicated that bdmpp is a weaker acceptor than dcbpy and that halide ions, such as Cl−, are stronger σ and π donors than the NCS− ligand. Solar cells fabricated with Ru− NCS showed a significantly better performance than those made with the Ru−Cl dye, and the cell η value was approximately 3 times higher than that obtained with the Ru−Cl dye. These differences are attributed to the influence of the isothiocyanato ligand on light absorption, redox properties, increase in the charge injection efficiency, and also reduction of the dark current of the corresponding solid-state solar cells. Heteroleptic Ru(II) complexes 385−388, similar to the abovementioned dyes with methyl substitution on 2,6-bis(1pyrazolyl)pyridine, were reported by Philippopoulos and coworkers.248 The maximum absorption wavelengths of complexes 385, 387, and 388 are blue-shifted in the order Cl−, NCS−, CN−, which is consistent with the reduction in the electron density on the metal. Dye 386 (with two carboxyl anchoring groups) yields an electron density approximately 2 times higher than that of the solar cells sensitized with 385, 387 (bearing one carboxyl group) and 388 (without any carboxyl groups), because the photovoltaic performance of solar cells depends on the number of carboxyl functional groups. Novel DSSCs were also fabricated incorporating tridentate ligand Ru(II) dyes 389−391 containing 2,4,6-tris(2-pyridyl)-striazine (tptz) and NCS, Cl, or CN attached to sol−gelprocessed TiO2 electrodes.249 These dyes are composed of dcbpy functions as the anchoring ligand and tptz spectator as the ancillary ligand with sufficient visible light absorption. The MLCT bands of these complexes are broadened and red-shifted compared to those of [Ru(dcbpy)3]4− due to the greater πacceptor ability of the ligand or the greater ease of back-donation to the ligand. Compared to other tptz ruthenium complexes, the isothiocyanato-coordinated ruthenium complex coated on SnO2 conducting glass shows a maximum efficiency of 6.15%. To understand how modifications of the 2,6-bis(benzimidazol-2-yl)pyridine-based ancillary ligand with phenyl or other alkyl groups can affect solar-cell efficiency, three ruthenium photosensitizers, Y1 (392), Y2 (393), and Y3 (394), containing 2,6-bis(benzimidazol-2-yl)pyridine were synthesized.250 It is interesting to note that the benzyl and hexyl groups significantly affect ε and enhance the light-harvesting capability of dyes without regard to the size of the molecules. The ε of the low-energy MLCT bands in Y1 is higher than that in Y2 and Y3, which may stem from the enhanced π−π stacking of dye Y1 with the benzyl group. Hence, structural modification of dyes Y2 and Y3 with alkyl groups could be effective in disassociating π−π stacking or dye aggregation, therefore increasing the solar cell efficiency. Moreover, a Y3-sensitized solar cell has higher conversion efficiency compared to Y1 and Y2 devices because the hexyl chains act as an effective blocking layer between the TiO2 layer and the I−/I3− electrolyte to avoid recombination processes. Dye DV56 (395), as a novel heteroleptic ruthenium sensitizer bearing a terpyridine ligand with an appended phenylanthracene moiety, was investigated for its application in DSSCs.251 Contact-angle measurements demonstrated that DV56-sensitized TiO2 films are hydrophilic. In addition, this dye has an MLCT absorption band centered at 514 nm and appears to be an ideal candidate for resonance Raman spectroscopy investigations on DSSCs aiming at elucidating both the operation and degradation mechanisms. Of course, more photovoltaic studies were suggested to obtain more details regarding this special case. 9555


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some less conventional polypyridyl complexes that have led to promising results so far. The content of the review is a useful source of outstanding references to determine whether ruthenium complexes based on polypyridyl ancillary ligands are suitable for DSSC applications. Moreover, this review shows the progress of ruthenium polypyridyl complexes and, in particular, the exclusive position of the polypyridyl ancillary ligand in this exciting field. Furthermore, it has been established that evolution of the ancillary ligand structure and application of the resulting dyes in DSSCs are in progress, with the crucial goal of increasing PCE by controlling the light-harvesting and charge-transport processes occurring at the sensitizer dye component. As shown in this review, the growth in the study of DSSCs has been particularly encouraging to date. However, maximizing the open-circuit voltage, decreasing CR, and resolving long-term stability are demanding issues that require continuing efforts. Of course, ancillary ligand engineering compatible with cobalt-based electrolytes in DSSCs is likely to be the focus of future investigations.

influence of exceedingly long aromatic moieties on the hydrophobicity and wettability of the corresponding Ru(II) polypyridyl dyes.257 Compared to Z907 dye, the absorption spectra of the new dyes shifted to long wavelengths, which is attributed to the extended aromatic conjugation of DV42 and DV51. Moreover, the 0.1 V difference in oxidation potential of the two dyes may be attributed to the contribution of NCS− versus Cl− ligands on the HOMO levels of the respective complexes. However, the stronger electron-donating nature of the Cl− ligand than that of the NCS− ligand enhances the easier oxidation of the corresponding Ru(II) complex. Highly efficient electron transfer from the iodine ions to the NCS− ligands caused improvements in DV51 regeneration, despite the driving force for DV51 being 100 mV smaller than that of DV42. A functionalized ruthenium complex, K314 (415), with carboxyl and sulfonyl groups was reported by Erten-Ela and Ocakoglu,258 and its photovoltaic properties were compared with those of [Ru(L)2(NCS)2] (K313, where L = 4,7-diphenyl-1,10phenanthrolinedisulfonic acid disodium salt) under standard AM 1.5 sunlight. K314 exhibited greater efficiency than K313 (η = 5.09% for K314 and and 4.02% for K313), which can be attributed to higher electron injection yield and slower CR rate due to bulky sulfonyl groups avoiding aggregation on the TiO2 surface. The first report259 of synthesis and DSSC application of trisheteroleptic metal−dipyrrinate complexes (418 and 419) was published with enhanced light-harvesting ability, lower excitedstate reduction potentials, and much higher efficiency compared to the bis-heteroleptic complex 281. Electron injection analysis revealed that the highest injection yield was achieved with the dye that acts as the strongest photoreductant, following the incorporation of electron-donating substituents into the ancillary bpy ligand. The extraordinary spiro-conjugated donor−acceptor systems exhibit ultrafast electron transfer and open new possibilities for the development of novel charge transfer chromophores. Keeping these viewpoints in mind, two novel ruthenium sensitizers, 420 and 421, were studied that comprised two electron-donating carbazole units and an electron-accepting 4,5diazafluorene.67 The obtained interesting results show that introduction of carbazole via the sp3-hybridized C9 atom of the 4,5-diazafluorene unit yielded significant changes in optical absorption spectra and subsequently altered intramolecular charge-transfer processes, which was reflected in the photovoltaic performance. Finally, to study the influence of replacement of NCS− in Ru(II) metal complexes with monodentate ligands having electron-withdrawing groups on frontier orbital energies and photovoltaic performance, dye HD-11 (422) was reported by Cheema et al.260 This dye showed a red shift in the MLCT of 50 nm and a higher molar absorptivity compared to N719. The presence of a strong electron-withdrawing CF3 group tethered to pyrazole provides more free energy for dye regeneration but less driving force for electron injection into the CB of TiO2. In summary, the development of sensitizers with differing molecular structures is rapidly progressing to achieve new horizons.

AUTHOR INFORMATION Corresponding Authors

*E-mail [email protected] (H.S.). *E-mail mdkhaja.nazeeruddin@epfl.ch (M.K.N.). Notes

The authors declare no competing financial interest. Biographies Babak Pashaei is a Ph.D. student (2014−present) in inorganic chemistry at the University of Zanjan as a member of Professor Shahroosvand’s group. He obtained his M.Sc. degree in 2013 at the Institute for Advanced Studies in Basic Sciences (IASBS) with a thesis on “Aluminum or Magnesium−Manganese Oxides as Models for Water Oxidizing Complex in Photosystem II” under supervisor Professor Mohammad Mahdi Najafpour. He has started his Ph.D. thesis on “Investigation of Ancillary Ligand Effects in Ruthenium-Sensitized Solar Cells and Light Emitting Diodes”. Hashem Shahroosvand was born in 1978 in Tehran, Iran. He obtained his B.S. degree in chemistry from Shahrood University of Technology in 2003. He received a Ph.D. in inorganic chemistry under the supervision of Professor Mozhgan Khorasani Motlagh at the University of Sistan & Baluchestan in March 2008. Then he conducted postdoctoral work in the groups of Professor Ezeddin Mohajerani at the Laser Research Institute and Professor Nasser Safari in the Chemistry Department, University of Shahid Beheshti (2008−2009). In 2010, he joined the University of Zanjan, where he is currently an associate professor in inorganic chemistry. His current scientific research interests are in the design and synthesis of functionalized polypyridyl complexes and their OLED and DSSC applications. Michael Grätzel directs the Laboratory of Photonics and Interfaces at EPFL. He pioneered the use of mesoscopic materials in energy conversion systems, in particular photovoltaic cells, lithium ion batteries, and photoelectrochemical devices for the splitting of water into hydrogen and oxygen by sunlight. He discovered a new type of solar cell based on dye-sensitized nanocrystalline oxide films, acting as a reference group in the world of electrochemistry. He has published 1060 peer-reviewed scientific publications and 40 review/invited book chapters, is the inventor or coinventor of over 50 patents, and has an h index of >150.

12. CONCLUSION This review systematically introduces several approaches to designing ruthenium complex sensitizers based on different ancillary ligands, not only covering the most frequently reported complexes or those investigated in depth but also describing

Professor Md. K. Nazeeruddin received his M.Sc. and Ph.D. in inorganic chemistry from Osmania University, Hyderabad, India. He joined the 9556


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(13) Nazeeruddin, M. K. Michael Graetzel Festschrift, A Tribute for His 60th Birthday. Coord. Chem. Rev. 2004, 248, 1161−1164. (14) Hagfeldt, A.; Graetzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49−68. (15) Nazeeruddin, M. K.; Zakeeruddin, S.; Lagref, J.-J.; Liska, P.; Comte, P.; Barolo, C.; Viscardi, G.; Schenk, K.; Grätzel, M. Stepwise Assembly of Amphiphilic Ruthenium Sensitizers and Their Applications in Dye-Sensitized Solar Cell. Coord. Chem. Rev. 2004, 248, 1317−1328. (16) Meyer, G. J. Molecular Approaches to Solar Energy Conversion with Coordination Compounds Anchored to Semiconductor Surfaces. Inorg. Chem. 2005, 44, 6852−6864. (17) Robertson, N. Optimizing Dyes for Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2006, 45, 2338−2345. (18) Xie, P.; Guo, F. Molecular Engineering of Ruthenium Sensitizers in Dye-Sensitized Solar Cells. Curr. Org. Chem. 2007, 11, 1272−1286. (19) Hagfeldt, A.; Grätzel, M. Molecular Photovoltaics. Acc. Chem. Res. 2000, 33, 269−277. (20) Kalyanasundaram, K.; Grätzel, M. Applications of Functionalized Transition Metal Complexes in Photonic and Optoelectronic Devices. Coord. Chem. Rev. 1998, 177, 347−414. (21) Grätzel, M. Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2003, 4, 145−153. (22) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. Molecular Design of Coumarin Dyes for Efficient Dye-Sensitized Solar Cells. J. Phys. Chem. B 2003, 107, 597−606. (23) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant, J. R. Molecular Control of Recombination Dynamics in Dye-Sensitized Nanocrystalline TiO2 Films: Free Energy vs Distance Dependence. J. Am. Chem. Soc. 2004, 126, 5225−5233. (24) Haque, S. A.; Handa, S.; Peter, K.; Palomares, E.; Thelakkat, M.; Durrant, J. R. Supermolecular Control of Charge Transfer in DyeSensitized Nanocrystalline TiO2 Films: Towards a Quantitative Structure−Function Relationship. Angew. Chem., Int. Ed. 2005, 44, 5740−5744. (25) Zhang, L.; Cole, J. M. Anchoring Groups for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 3427−3455. (26) Li, M.; Xiao, Z.; Huan, Z.; Lu, Z. New Binding State Useful for Attachment of Dye-Molecules onto TiO2 Surface. Appl. Surf. Sci. 1998, 125, 217−220. (27) Jing, B.; Zhang, H.; Zhang, M.; Lu, Z.; Shen, T. Ruthenium(II) Thiocyanate Complexes Containing 4[Prime or Minute]-(4-Phosphonatophenyl)-2,2′[Prime or Minute]:6[Prime or Minute],2[Prime or Minute][Prime or Minute]-Terpyridine: Synthesis, Photophysics and Photosensitization Tonanocrystalline TiO2 Electrodes. J. Mater. Chem. 1998, 8, 2055−2060. (28) Trammell, S. A.; Moss, J. A.; Yang, J. C.; Nakhle, B. M.; Slate, C. A.; Odobel, F.; Sykora, M.; Erickson, B. W.; Meyer, T. J. Sensitization of TiO2 by Phosphonate-Derivatized Proline Assemblies. Inorg. Chem. 1999, 38, 3665−3669. (29) Rice, C. R.; Ward, M. D.; Nazeeruddin, M. K.; Grätzel, M. Catechol as an Efficient Anchoring Group for Attachment of Ruthenium−Polypyridine Photosensitisers to Solar Cells Based on Nanocrystalline TiO2 Films. New J. Chem. 2000, 24, 651−652. (30) Schwarz, O.; van Loyen, D.; Jockusch, S.; Turro, N. J.; Dürr, H. Preparation and Application of New Ruthenium(II) Polypyridyl Complexes as Sensitizers for Nanocrystalline TiO2. J. Photochem. Photobiol., A 2000, 132, 91−98. (31) Aranyos, V.; Grennberg, H.; Tingry, S.; Lindquist, S.-E.; Hagfeldt, A. Electrochemical and Photoelectrochemical Investigation of New Carboxylatobipyridine(bis-Bipyridine)Ruthenium(II) Complexes for Dye-Sensitized TiO2 Electrodes. Sol. Energy Mater. Sol. Cells 2000, 64, 97−114. (32) Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M. Dye-Sensitized Solar Cells: A Brief Overview. Sol. Energy 2011, 85, 1172−1178. (33) Asghar, M. I.; Miettunen, K.; Halme, J.; Vahermaa, P.; Toivola, M.; Aitola, K.; Lund, P. Review of Stability for Advanced Dye Solar Cells. Energy Environ. Sci. 2010, 3, 418−426.

Deccan College of Engineering and Technology, Osmania University, as a lecturer in 1986 and subsequently moved to Central Salt and Marine Chemicals Research Institute, Bhavnagar, as a research associate. He was awarded the Government of India’s fellowship in 1987 to study abroad. After a one-year postdoctoral stay with Professor Graetzel at the Swiss Federal Institute of Technology Lausanne (EPFL), he joined the same institute as a senior scientist. In 2014, EPFL awarded him the title of professor. His current research at EPFL focuses on DSSCs, perovskite solar cells, CO2 reduction, hydrogen production, and light-emitting diodes. He has published more than 500 peer-reviewed papers and 10 book chapters, and he is the inventor/coinventor of over 60 patents. He appeared in the ISI listing of most cited chemists and has more than 47 000 citations, with an h-index of 105.

ACKNOWLEDGMENTS H.S. thanks Mrs Parisa Abbasi and Professor Sara Tarighi for their scientific help. B.P. and H.S. acknowledge joint financial support by the University of Zanjan and Federation of Supports of Excelled Scientists of IRAN under grant agreement 11/66332, (1394/02/30). M.G. and M.K.N. thank the European Union Seventh Framework Program (FP7/2007−2013) under Grant Agreement 604032 of the MESO project and (FP7/2007−2013) ENERGY.2012.10.2.1; NANOMATCELL, Grant Agreement 308997. M.K.N. acknowledges funding by the Swiss National Science Foundation NRP 70. REFERENCES (1) Costello, A.; Abbas, M.; Allen, A.; Ball, S.; Bell, S.; Bellamy, R.; Friel, S.; Groce, N.; Johnson, A.; Kett, M.; et al. Managing the Health Effects of Climate Change. Lancet 2009, 373, 1693−1733. (2) Scott, F. The Low Carbon Energy Lift: Powering Faster Development in Sub-Saharan Africa. Green Alliance: 2014; http://www.green-alliance. org.uk/lowcarbonenergylift.php. (3) Twenty-First Session of the Conference of the Parties (COP) and Eleventh Session of the Conference of the Parties Serving as the Meeting of the Parties to the Kyoto Protocol (CMP); Paris, France, 30 November to 11 December 2015; http://unfccc.int/meetings/paris_ nov_2015/meeting/8926/php/view/reports.php. (4) The ‘billiard ball’ model is from Overseas Development Institute. Zero Poverty ... Think Again, 2014; http://www.odi.org/sites/odi.org. uk/files/odi-assets/publications-opinion-files/8863.pdf. (5) Reynal, A.; Palomares, E. Ruthenium Polypyridyl Sensitisers in Dye Solar Cells Based on Mesoporous TiO2. Eur. J. Inorg. Chem. 2011, 2011, 4509−4526. (6) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (7) O’regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737− 740. (8) Argazzi, R.; Iha, N. Y. M.; Zabri, H.; Odobel, F.; Bignozzi, C. A. Design of Molecular Dyes for Application in Photoelectrochemical and Electrochromic Devices Based on Nanocrystalline Metal Oxide Semiconductors. Coord. Chem. Rev. 2004, 248, 1299−1316. (9) Polo, A. S.; Itokazu, M. K.; Iha, N. Y. M. Metal Complex Sensitizers in Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2004, 248, 1343−1361. (10) Vougioukalakis, G. C.; Philippopoulos, A. I.; Stergiopoulos, T.; Falaras, P. Contributions to the Development of Ruthenium-Based Sensitizers for Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2011, 255, 2602−2621. (11) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (12) Bomben, P. G.; Robson, K. C.; Koivisto, B. D.; Berlinguette, C. P. Cyclometalated Ruthenium Chromophores for the Dye-Sensitized Solar Cell. Coord. Chem. Rev. 2012, 256, 1438−1450. 9557


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Influence of Ancillary Ligands in Dye-Sensitized Solar Cells

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