BODIPYs for Dye-Sensitized Solar Cells - American Chemical Society

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 39873-39889

BODIPYs for Dye-Sensitized Solar Cells Hafsah Klfout, Adam Stewart, Mahmoud Elkhalifa, and Hongshan He* Department of Chemistry and Biochemistry, Eastern Illinois University, Charleston, Illinois 61920, United States ABSTRACT: BODIPY, abbreviation of boron-dipyrromethene, is one class of robust organic molecules that has been used widely in bioimaging, sensing, and logic gate design. Recently, BODIPY dyes have been explored for dye-sensitized solar cells (DSCs). Studies demonstrate their potential as light absorbers for the conversion of solar energy to electricity. However, their photovoltaic performance is inferior to many other dyes, including porphyrin dyes. In this review, several synthetic strategies of BODIPY dyes for DSCs and their further functionalization are described. The photophysical properties of dye molecules and their photovoltaic performances in DSCs are summarized. We aim to provide readers a clear picture of the field and expect to shed light on the next generation of BODIPY dyes for their applications in solar energy conversion. KEYWORDS: BODIPY, solar cell, photovoltaics, synthesis, energy conversion

1. INTRODUCTION Dye-sensitized solar cells (DSCs) have been extensively explored as an alternative photovoltaic technology.1 The state-of-the-art DSCs inherit a configuration, reported early by Grätzel,2 in which a randomly packed and interconnected dye-coated titanium dioxide nanoparticle electrode, a platinumcoated counter-electrode, and a redox couple solution are “sandwiched”. In the device, the foremost steps are the light absorption by dye molecules and subsequent charge separation at the interfaces between the dye molecules and the TiO2 semiconductor. While the extensively studied ruthenium-based dyes showed promise, the limited supply and potential environmental impact of this heavy metal ion led to the search for other alternatives.3−5 In 2014, the power conversion efficiency (PCE) of 13% was achieved when a porphyrin dye was employed.1,6 In 2015, Hanaya and coworkers reported a 14.7% energy conversion efficiency when an alkoxysilyl-anchor dye ADEKA-1 and a carboxy-anchor t dye LEG-4 were used to co-sensitize the TiO2 nanoparticles.7 Recently, BODIPY dyes have emerged as attractive light absorbers for DSCs due to their easy structural modification, strong light absorption, and photostability. Dyes in DSCs bind to the surface of TiO2 nanoparticles. Under illumination, the excited dyes inject electrons to the conduction band of TiO2, resulting in free-moving electrons. The oxidized dyes relax to the ground state by accepting electrons from a redox couple.6 Figure 1 shows the pathways for the electron production, dye regeneration, and electron recombination. The details of the operation principles and mechanisms have been described in numerous excellent reviews1,8,9 and will not be covered here; however, it is worth noting that the DSC is an electrochemical device with outputs being affected by many factors, including light absorption coverage, dye aggregation, dye loading density, frontier © 2017 American Chemical Society

Figure 1. Electron production and dye regeneration pathways in dyesensitized solar cells. Several recombination pathways include (1) relaxation of excited electrons to the ground state, (2) electrons in CB of TiO2 to the ground state of dye, and (3) injected electron from CB of TiO2 to electrolyte and (4) back transfer of electrons from TCO to electrolyte.

molecular orbital energy levels as well as redox couple potential. Therefore, the power conversion efficiency is the result of a global optimization of various processes. For example, a dye with a broader light absorption can harvest more photons for electron generation, which can be achieved by lowering LUMO (lowest unoccupied molecular orbital) or increasing HOMO (highest occupied molecular orbital) levels; however, it is important to maintain certain energy difference between LUMO of the dye molecule and the conduction band of the TiO2 nanoparticle to deter electron injection. Received: May 31, 2017 Accepted: October 26, 2017 Published: October 26, 2017 39873

DOI: 10.1021/acsami.7b07688 ACS Appl. Mater. Interfaces 2017, 9, 39873−39889

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ACS Applied Materials & Interfaces

performance of BODIPY dyes. In this review, we outline the key findings of BODIPY dyes based on the two strategies. We aim to provide readers a clear picture of the field and expect to shed light on the next generation of BODIPY dyes for their application in solar energy conversion. It should be noted that most studies discussed here are focused on n-type DSCs. Several attempts for its application in p-type DSCs were also reported1 with low energy conversion efficiency, which is out of the scope of this review and will be not discussed.

In this review, we provide readers a summary of the evolution of BODIPY dyes for solar energy conversion in dyesensitized solar cells. BODIPY, an abbreviation of borondipyrromethene, is one class of robust organic molecules that has been widely used in bioimaging due to its bright emission in the visible region.10 In the past eight years, a variety of BODIPY dyes have been synthesized; unfortunately, their applications in photovoltaics are quite limited.11,12 Due to the high absorption coefficients and versatile structural modification capability, BODIPY dyes have attracted attention in recent years for DSCs and organic solar cells. Figure 2 shows the BODIPY core

2. SYNTHETIC METHODS 2.1. Synthesis of BODIPY Core. Several methods have been reported for the synthesis of BODIPY core structures.13−15 The most widely adopted synthetic procedure involves a so-called one-pot reaction between the pyrrole and an aldehyde in dichloromethane at room temperature.10,16−18 The reaction is catalyzed by trifluoroacetic acid. After dehydration of dipyrromethane to dipyrromethene using 2, 3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), excess of trimethylamine is added to strip the proton from the pyrrole for its coordination to the B atom when boron trifluoride etherate is added in the final stage. The green fluorescence appears immediately after the addition of BF3·OEt2, which can be used conveniently to monitor the reaction. The final product is usually separated from other side products via column chromatography because of the low yield of the product and enormous amount of polymerized black residues as well as other fluorescence impurities. In many cases, repetitive chromatography procedures are required to obtain the final pure product. The resulting product is usually an orange colored solid, which is quite soluble in chloroform, dichloromethane, toluene, etc. The color of the solid is different from the color of its solution if the product has strong fluorescence. It is worth mentioning that the choice of pyrrole is very limited. In most reports, 2,4-dimethylpyrrole and 3-ethyl-2,4dimethylpyrrole are used because both aldehydes are commercially available. The use of 2,4-dimethylpyrrole also gives opportunities for the introduction of halogen atoms at the C2 and C6 positions for further structural modification. Therefore, these molecules usually have four methyl groups at the C1, C3, C5, and C7 positions, primarily due to the availability of pyrrole precursors. A variety of aldehydes is commercially available for the construction of different BODIPY dyes. For applications in DSCs, 4-carboxybenzaldehyde and 5-formyl-2thiophenecarboxylic acid are frequently used. From this perspective, the development of new pyrrole derivatives is of importance for the exploration of the structure−property relationship of BODIPY dyes. 2.2. Functionalization of BODIPY Dyes. The BODIPY core can be conveniently functionalized in several ways for their applications in

Figure 2. Schematic of two design strategies of BODIPY dyes for DSCs. D stands for an electron donor group, and A stands for an electron acceptor group.

structure, its numbering scheme, and two strategies that have been used to design BODIPY dyes for DSC applications. The two strategies are based on the position of the anchoring group. The first one is the vertical design, in which an anchoring group such as benzoic acid or phenylcyanoacetic acid group is located on the meso-position of the BODIPY core structure. In this design, the phenyl or other anchoring units align almost vertically to the BODIPY core. The second strategy is the horizontal design, in which an anchoring group such as cyanoacetic acid group is linked to the BODIPY core through its C2 position. The donor group such as a triphenylamine can also link the BODIPY core through π-conjugation, forming a premium donor−π−acceptor system. In this design, the anchoring group generally falls into the BODIPY core plane. Both strategies have been explored to improve the photovoltaic

Figure 3. Summary of four (A, B, C, and D) functionalization methods of BODIPY dyes for solar cell applications. The R′ group can be an aromatic or aliphatic group. The R group is normally an aromatic group or an aromatic derivative. 39874


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Figure 4. Absorption spectra of three BODIPY dyes with different absorption onsets. The dotted, solid, and dash-dot lines represent dyes 1, 2, and 3, respectively. Reprint with permission from ref 24. Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. reactions, which can occur at either C3 or C5, the reaction to replace F often happens at both F atoms. 2.2.4. Reactions at meso-Positions. The meso position at the BODIPY core often has an aromatic group such as phenyl or thiophene group. Therefore, the reactions often occur at these substituents. The typical one is the conversion of formyl group to a cyanoacetic acid, which is a quite common reaction to convert molecules to a usable dye for DSCs. The reaction is typically carried out in a chloroform/ethanol mixture in the presence of cyanoacetic acid and pyridine with a high yield.

dye-sensitized solar cells. Four active sites are available for structural modification, i.e. C2 (C6), C3 (C5), BF2 units, and the meso-positions as shown in Figure 3. The representative reactions will be detailed in the following sections. 2.2.1. Reactions at C2 and C6 Positions. The common reaction is the halogenation using N-bromosuccinimide (NBS) or N-iodosuccinimide (NIS). The product can be a mono- or disubstituted compound depending on the ratio of NIS (or NBS) to the BODIPY.19 Even under a dilute solution the disubstituted product always exists, which can be minimized by running the reaction at 0 °C and dropwise addition of NIS to the solution.20 The halogenated derivatives can be used as precursors for further functionalization, typically through Pdcatalyzed reactions, which will result in attaching a donor group or an anchoring group to the BODIPY core. For example, Suzuki crosscoupling reactions have been used to introduce an electron donor through a reaction between boronic acid and an iodinated BODIPY moiety.21 Another widely used Pd-catalyzed reaction is the Sonogashira coupling reaction. This method has been used to attach an electron donor or acceptor to the C2 and C6 positions to elongate π-conjugation, which is quite effective in broadening the absorption to the red region. 2.2.2. Reactions at C3 and C5 Positions and C1 and C7 Positions. The typical reaction at these positions requires the Knoevenagel reaction, which conjugates aromatic groups to the BODIPY core through a double bond.22 The reaction is typically carried out in toluene between a parent BODIPY dye, aldehyde, and piperidine in the presence of a trace amount of p-TsOH as an activator.23,24 A Dean−Stark apparatus is often used to facilitate the reaction with a decent yield (∼60−70%). In most cases, only two methyl groups at the C3 and C5 are transformed to styryl derivatives.25 Occasionally, the methyl group at the C1 and C7 positions can also be converted to styryl derivatives.26 The resulting compound has a distinctive bluish color in solution and a dark color in its solid state. It is worth mentioning that the resulting compounds have a planar geometry, i.e. the styryl groups fall in the plane of the BODIPY core. From the energy conversion perspective, such a structural characteristic is disadvantageous to DSCs due to their easy aggregation on the surface of TiO2 nanoparticles. Aggregation is known to facilitate the electron recombination and is problematic in other dyes such as porphyrin dyes. 2.2.3. Reaction at BF2 Unit. The reaction involves the substitution of two F atoms by ethynyl substitutes. This method has been used extensively by the Ziessel group.23,27,28 The reaction between the BODIPY precursor and an alkynyl Grignard reagent in dry THF gives the desired product with a decent yield. Many BODIPY dyes are stable enough to react with a Grignard reagent without introducing other side reactions. It should be mentioned that any other reactive groups on the BODIPY substrate need to be protected before the reaction, such as NH2, OH, COOH etc. Compared to the Knoevenagel

3. PHOTOPHYSICAL PROPERTIES BODIPY dyes are characterized by their extremely strong absorption and fluorescence in the visible and the near-infrared regions. The smallest BODIPY, i.e. BODIPY core without any substituent, was reported in 2006.29 This dye exhibits a maximal absorption at 497 nm and emission at 504 nm. The dye is extremely bright with a fluorescent quantum yield of 97% and a lifetime of 6.89 ns. Such photophysical properties can be fully tuned by structure modification. Substitution at the mesoposition usually does not affect the absorption spectra of BODIPY dyes significantly. This is because of the orthogonal alignment between the meso-substituent and the BODIPY core. For example, the BET dye with a benzoic acid group in its meso-position absorbs and emits at 528 and 543 nm, respectively.18 However, its fluorescence may be quenched due to a photoinduced electron transfer (PET) process between the BODIPY core and the meso-substituent.16 Absorption and fluorescence spectra will change when substituents occur at the C3 (C5) positions. When the methyl groups at C3 and C5 are replaced by aromatic groups, the absorption can be red-shifted about 40−90 nm. The changes could be more dramatic if the substituents are conjugated to the BODIPY core. Most BODIPY dyes for solar application have a methyl group in the C3 and C5 position due to the availability of pyrrole on the market. Ziessel and Akkaya and their coworkers linked a variety of aromatic groups to the C3 and C5 positions through a Knoevenagel reaction, and three representative dyes with absorption spectra are shown in Figure 4.30 Dye 1 showed a typical absorption maximum at 520 nm even though the two F atoms were replaced by 4,7-dioxaoct-1-yne groups. After the C3 position was conjugated to a pmethoxydiethylene glycol, the peak moved to 570 nm, which was further red-shifted to 640 nm after the second pmethoxydiethylene glycol was attached. The color of the dye 39875


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Figure 5. Chemical structures of BODIPY dyes (4−16) from the vertical design.

solution also changed from orange to bluish. This is an effective way to shift the absorption onset toward the red region. The substitution at the C2 and C6 positions will also broaden the absorption spectra. However, the red-shift is not as significant as modification at C3 and C5. For example, attaching a 4-ethynyl-benzoic acid to C5 position will result in ∼30 nm red shift of the spectrum.31 One feature of the absorption of BODIPY dyes is their relatively narrow absorption. This characteristic is not beneficial to the solar energy conversion, in which a broader spectral coverage is desirable. As shown in Figure 4, dye 3 absorbs quite weakly between 400 and 550 nm.24 Therefore, from a solar energy harvesting perspective, it will be beneficial to utilize several BODIPY dyes with complementary absorption capabilities in a single cell to cosensitize the TiO2 nanoparticles. The BODIPY dyes for dye-sensitized solar cells often exhibit strong fluorescence. The spectrum is often characterized by one strong peak that mirrors its absorption spectrum. The peak position is often around or above 500 nm with a Stokes shift varying from several nanometers to several dozens of nanometers. The fluorescence decay lifetime is about several nanoseconds, which is much longer than the time scale of

electron injection from the dye to the conduction band of TiO2 (typically in the range of several hundreds of femtoseconds). The fluorescence is quenched after dye molecules bind to the TiO2 nanoparticles.

4. BODIPY DYES FOR DYE-SENSITIZED SOLAR CELLS 4.1. Dyes Derived from a Vertical Design. The majority of BODIPY dyes for DSCs are based on the vertical design model, in which an anchoring group is usually benzoic acid or phenylcyanoacetic acid. The efforts in the field were devoted to enlarging the π-conjugation through two “legs” at the C3 and C5 positions, as demonstrated by several examples in Figure 5. The work is pioneered by Akkaya and coworkers. In 2008, they reported the dye 4 for DSCs.32 The dye has two triphenylamine fragments conjugated to the positions C3 and C5 through double bonds. The dye shows a molar extinction coefficient of 69500 M−1 cm−1 at the peak wavelength with an absorption onset up to 800 nm, which has not been easy to achieve in many other organic dyes. The calculated HOMO energy level of 5.09 eV and a LUMO energy level of 3.52 eV meet the requirement for its normal function in DSCs. As shown in Table 1, the fabricated device with this dye gave 1.66% PCE, 39876


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ACS Applied Materials & Interfaces Table 1. Optical and Photovoltaic Properties of BODIPY Dyes from the Vertical Designa dye

abs (nm)b

JSC (mA/cm2)

VOC (mV)

FF

η (%)

HOMO

LUMO

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

700 724 746 761 668 527, 735 693 644 695 703 504, 594, 682 504, 594, 682 504, 594, 682 665 581 666 668 645 520 665 773 385, 574, 754 503 519 502 517 493 367, 501 371, 524 377,673 385, 574, 753 424, 554 457, 528, 586, 648, 420, 502, 550, 592, 650 426,524, 556, 664 582 753 753

4.03 5.95 4.52 1.05 5.45 0.69 8.01 14.10 7.24 6.98 0.74 0.60 1.56 7.66 11.40 15.78 13.04 2.61 1.12 2.61 2.12 0.68 1.1 2.3 8.3 8.9 16.3 1.24 2.33 1.85 1.08 13.06 3.4 12.52 8.7 0.88 0.83 9.1

562 470 460 350 470 400 504 526 473 466 520 560 600 469 526 544 496 410 370 400 400 319 390 455 540 553 692 408 428 320 335 0.71 627 660 625 0.33 0.37 0.41

0.735 0.67 0.63 0.61 0.71 0.72 0.64 0.59 0.66 0.64 0.71 0.69 0.72 0.628 0.710 0.670 0.625 0.62 0.48 0.57 0.58 0.51 0.67 0.69 0.71 0.70 0.68 0.67 0.67 0.67 0.53 0.71 0.73 0.75 0.71 0.66 0.68 0.62

1.66 1.88 1.32 0.23 1.81 0.20 2.60 4.42 2.27 2.11 0.27 0.23 0.67 2.26 4.26 5.75 4.05 1.32 0.19 0.59 0.49 0.11 0.29 0.72 3.18 3.45 7.67 0.34 0.65 0.67 0.19 6.65 1.55 6.20 3.9 0.20 0.21 2.3

−5.09 −5.08 −5.05 −5.17 −5.25

−3.52 −3.56 −3.62 −3.79 −3.60

−5.06 −5.30 −4.98 −4.97 −5.66 −5.66 −5.63 −5.47 −5.59 −5.43 −5.45 −4.98 −4.84 −5.10 −5.02 −5.28 −5.08 −5.45 −5.43 −5.37 −5.22 −5.53 −5.55 −5.80 −5.28

−3.41 −3.52 −3.30 −3.31 −3.88 −3.88 −3.85 −3.63c −3.42c −3.64c −3.57c −3.22 −3.52 −3.51 −3.56 −3.84c −2.66c −3.10c −2.89c −3.02c −2.97c −3.13c −3.21c −3.70c −3.85c

−5.23 −5.71

3.32c −3.68c

−5.44 −4.90 −4.92

3.44c 3.52c 3.49c

Photovoltaic measurements were performed under simulated AM 1.5 (100 mW cm−2) conditions. A typical electrolyte consisted of 0.05 M I2, 0.1 M LiI, and 0.6 M DMPII in acetonitrile. bData were measured in solution. cEstimated from the original data using the equation (vs vacuum): EHOMO = −[Eref + Eox (vs reference)] and ELUMO = −[Eref + Ered (vs reference)]. References: Eref = 4.8 V for Fc/Fc+, Eref = 4.28 V for NHE. a

efficiency was observed. Two BODIPY core fragments were also linked to positions C2 and C6 position of another BODIPY core. The resulting 9 showed strong absorption around 530 and 750 nm. Though an efficient energy transfer from the two side BODIPY fragments to the central BODIPY core occurred, this dye showed a very poor PCE. It was suggested that the energy transfer may trigger other pathways for electron deactivation, which was evidenced by quite poor short-circuit current density. Feng and coworkers33 adopted a similar strategy and synthesized dyes 10 to 13, in which two legs were functionalized by a variety of rigid triarylamino groups. When a 9H-carbazol-9-yl group was introduced into the BODIPY core through an Ullman reaction, the absorption of resulting dye 11 is around 644 nm, whereas dyes 10, 12, and 13 have peak positions ∼700 nm. Dye 11 exhibited a significant blue-shift, which was attributed to the intense π−π coupling of coplanar benzene rings and the 9H-carazol-9-yl group. The absorption coefficients of dyes 12 and 13 are much higher than

which was the best for BODIPY-sensitized solar cells at that time. The low PCE is consistent with relatively low IPCE values across the visible region with a peak value of 22% at 750 nm. In the following study, Akkaya and coworkers25 replaced the cyanoacetic acid with the benzoic acid and prepared the dye 5. The absorption maximum increased to 724 nm with 1.88% PCE. This slight increase may come from different fabrication processes and electrolytes. The efficiency was increased to 2.46% after they increased the dye-loading time from 4 to 24 h with the addition of CDCA (chenodeoxycholic acid). Dye 6 with methoxide groups on the styrene side legs or dye 7 with two I atoms on the BODIPY core gave no better photovoltaic performance than the others. The authors ascribed this observation to the dye aggregation and lower dye loading on TiO2 nanoparticles. To reduce the dye aggregation, the authors introduced long alkyl chains and prepared dye 8. A blue shift of the absorption peak position was observed. The dye 8 showed the increased fill factor, but only a marginal increase of 39877


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Figure 6. Chemical structures of BODIPY dyes (17−21) from the vertical design. Two F atoms are replaced by styryl fragments.

Figure 7. IPCE spectra of four dyes 17 (T2A), 18 (TP2A), 19 (T2P2A), and 20 (T2P2CA) reported by Ziessel and coworkers based upon a vertical design strategy. Reprint with permission from ref 23. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

being replaced by ethynyl groups using Grignard reagents. In 2009, these authors reported the synthesis and photophysical properties of dyes 1, 2, and 3. Different to parent BODIPY dyes, these dyes showed absorption maxima in two separated regions: one around 340 nm and another between 500 and 700 nm with a shoulder on the left side of the major peaks. The conversion of fluoride to an ethynyl group did not change the absorption onset significantly compared to its parent Fsubstituted counterparts; however, when the styryl legs are introduced, the absorption broadened quite significantly. The LUMO energies of all three dyes are higher than the TiO2 conduction band (3.9 eV), which warrants the sufficient energy driving force for electron injection to the TiO2. However, it was found that the IPCE of dye 3 is much less than that of dyes 1 and 2. Such a poor performance was attributed to the small difference between HOMO level of 2 (4.98 eV) and redox potential of the I2/I3−, which slows down the regeneration of the dye. In a following study, Ziessel and coworkers23 developed new BODIPY dyes 17−21 as shown in Figure 6. In these dyes, long alkyl group substituted thiophenes were conjugated to the C3 and C5 through Knoevenagel reactions. The impact on the absorption spectra was significant, as indicated by more than 100 nm shift of absorption from 18 with one leg to 19 with two legs. Similar to the dyes 1−3, the replacement of two F atoms by two ethynyl polyethylene groups exerts only a subtle change of the absorption spectra, as evidenced by the very similar absorption spectra between 20 with two F atoms and 19 with two polyethylene groups. However, DFT calculations indicated that the HOMO and LUMO exhibited an overlap in 19 larger than that in 17. Figure

the dye 10; however, their IPCE values around 650 nm are slightly lower than 10. Both dyes showed energy conversion efficiency (2.27 and 2.11%) lower than that of dye 10 (2.60%). Surprisingly, the dye 11 with a narrower absorption spectrum gave much higher energy conversion efficiency (4.42%). The electrochemical impedance spectroscopy (EIS) indicated the higher electron recombination resistance in 11 compared to others. Notably the HOMO of 11 is much lower than other three, which may facilitate the reduction of the oxidized dye to the ground state and consequently minimize the electron recombination. As an extension of this work, Cheema and coworkers34 prepared dyes 14-16, in which different alkyl chain are tethered to N-carbazole group. It was found that the intramolecular energy transfer from the carbazole donor to benzoic acid acceptor decreases with the increase of alkyl chain length. This is possibly due to the induced changes in molecular geometry caused by long alkyl chains. The compound 16 with a longer chain performs better than the other two, indicating the impact of dye aggregation on overall energy conversion efficiency, as confirmed by transient and EIS investigations. In all of those dyes, energy levels of the excited state are higher than the conduction band energy level of TiO2 nanoparticles, indicating the electron injection is energetically favorable. However, compared to the porphyrin dyes, the photovoltaic performance of BODIPY dyes is quite poor. One plausible reason for the poor PCE of BODIPYsensitized solar cells could be the existence of two F− ions in the BODIPY core structure. The strong electron withdrawing F atoms may slow the energy injection rate. Ziessel and coworkers24 reported several BODIPY dyes with two F atoms 39878


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Figure 8. Chemical structures of BODIPY dyes (22−34) from the vertical design. Thiophene spacers are used to elongate the conjugation.

7 shows the IPCE spectra and I−V curves of four dyes in the devices. The IPCE spectrum of dye 19 showed a broad response between 560 and 760 nm (>60%), which is the best IPCE response of all BODIPY dyes reported to date. The dye exhibited an high PCE (5.75%) in the device, as listed in Table 1. When dyes 18 and 19 are used as cosensitizers, both the photocurrent and voltage increased, resulting in an impressive PCE of 6.43%, which is the best reported so far for BODIPYsensitized solar cells. In another study, Gonzalez-Valls and coworkers35 further introduced polyethylene to the styryl functionalized legs and prepared dye 21 with the benzoic acid as an anchoring group and prepared for TiO2 nanotube-based solar cells. It was found that the dye 21 adsorbed 100 times less than ruthenium dye N3. The cell with 4.4 μm long TiO2 nanotubes produced 1.3% PCE. The calculated photons at different wavelengths indicated clearly that BODIPY dyes exhibit a better photon-to-electron conversion capability compared to the N3 dye. Less electron recombination was also observed, which could be due to the long side polyethylene chains. It should be noted that the cyanoacetic acid is not an

optimal anchoring group, which is consistent with Akkaya’s study.25 It is unclear what causes this discrepancy. One plausible explanation could be the different orientations of dye molecules on the TiO2 surface due to the different anchoring groups.36 BODIPY dyes with different bridging groups between the BODIPY core and an anchoring group have also been explored. Several representative dyes are shown in Figure 8. Cakmak and coworkers37 tried fluorenyl groups as electron donors and thiophene-2-carboxylic acid as anchoring groups in 23 and 24. The absorption spectra were red-shifted from 520 nm in reference dye 22 to 665 nm in 23 and 773 nm in 24 due to the extension of π-conjugation, which increases the energy conversion efficiency in the same order. The energy conversion efficiency was higher compared to that of 22 with no substituent on the BODIPY core. However, the overall energy conversion efficiency was low. The authors argued that the inefficient electron injection from dye to the TiO2 conduction band as well as significant dye aggregation were the barriers to their high performance. Ooyama and coworkers38 linked o39879


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compared to that of 32 primarily came from broader absorption of 33. Ooyama and coworkers38 linked two diphenylamine− thienylcarbazole moieties to the C3 and C5 positions and prepared the dye 34 with a pyridine as the anchoring group. The absorption was significantly broadened with peak positions at 753 nm. However, the PCE was only 0.19%. It was found that the HOMO energy level is positive enough for sufficient dye regeneration; The LUMO is very close to the conduction band of TiO2, leading to insufficient driving force for electron injection and therefore, poor photovoltaic performance. BODIPY dyes have been incorporated into other dyes to broaden the absorption spectrum. Several representative examples are shown in Figure 10, and the photovoltaic data are also presented in Table 1. He and coworkers prepared a BODIPY dye BET.18 The dye has strong absorption around 500 nm, which is very typical for BODIPY dyes without functionalization. The dye exhibited a quite low PCE (0.16%) due to its narrow absorption in the visible region. However, the complementary nature of its absorption spectrum to that of porphyrin dyes supports its potential as a cosensitizer for porphyrin-sensitized solar cells. The PCE of a porphyrin dye 35-sensitized solar cell increased from 6.65 to 7.50% when the BET was applied as a cosensitizer. Fluorescence studies indicate an efficient energy transfer from the BET to the 35. Such a phenomenon was also observed in other cosensitized systems.41 Luo and coworkers42 also investigated photovoltaic properties of several small BODIPY dyes that are similar to BET. The replacement of para-benzoic acid by ortho-benzoic acid led to a subtle change of the absorption spectra with the decreased PCE. The introduction of I atoms decreased the fluorescence efficiency dramatically, which also decreased the PCE. Hupp and coworkers43 conjugated a porphyrin dye to a BODIPY moiety through a triple bond and synthesized the dye 36. The dye showed a strong absorption band centered around 530 nm from the BODIPY fragment and strong peaks at 450 and 650 nm from the porphyrin fragment. The DSC from this dye gave 1.55% PCE, which is the double of its counterpart without the BODIPY fragment (PCE = 0.84%). Galateia and coworkers44 took another strategy and appended two BODIPY moieties to a porphyrin and obtained a dyad 37, in which an electron donor (BODIPY moieties) and an electron acceptor (benzoic acid) are not conjugated to the central porphyrin. The dye showed strong absorption around 500 nm, which corresponds to the absorption of the BODIPY units. As shown in Figure 11, the dye showed PCEs of 5.17 and 6.20% when a pristine TiO2 and rGO/TiO2 were used for photoanodes in devices, respectively. The IPCE spectra demonstrated the strong capability of this dye in converting photons to electrons between 400 and 700 nm. Odobel and coworkers45 further linked a squaraine unit to a porphyrin ring and prepared a new dye 38. Three chromophoric subunits display complementary absorption spectra (664, 524 and 426, and 556 nm) and cover a wide window of the solar spectrum. Steady-state fluorescence experiments in solution demonstrate a significant quenching of the emission from porphyrin and BODIPY moieties in 38 with 30% photocurrent improvement compared to its squaraine analogue. This is attributed to an energy transfer from antenna to the anchoring group through the Förster energy transfer mechanism. Thus, the device produced an overall PCE of 3.9%, which corresponds to a 25% increase compared to its counterpart without a porphyrin unit, and 8% without a BODIPY unit. Nakazumi and coworkers46 linked a squaranine to a BODIPY through its meso-phenyl group and prepared

diphenylamine−thienylcarbazole moieties to BODIPY core using the Knoevenagel reaction and prepared dye 25. The dye showed a very dark color on the TiO2 film indicating its strong light absorption. However, the dye only gave 0.11% PCE due to the very low LUMO energy level. Xue and coworkers39 designed a series of BODIPY dyes 26−29 with the distance from the donor to the acceptor varying from 8.8 Å in 26 to 22.5 Å in 29 by incorporating dithieno[3,2-b:2′,3′-d]pyrrole linker. The IMV and EIS study showed that the BODIPY core interactes with the electrolyte strongly if the linker length is short. Therefore, a better efficiency was observed for 29, in which the BODIPY core is far away from the donor with minimal electron recombination, as shown in Figure 9. It was

Figure 9. Electrostatic potential plots of 26 (LB1), 27 (M62), 28 (M66), 29 (M63), and 30 (M64) and distances between donors and acceptors. Reprint with permission from ref 39. Copyright 2017 American Chemical Society.

found that dyes 26−29 all showed a negative charge at B atom on their electrostatic potential map, whereas no negative charge was observed for a reference dye 30 with 7.67% PCE. It was suggested that such a negative charge promoted the charge recombination with I−. Ooyama and coworkers40 used (CH2)6 as a bridging group and prepared two dyes, 31 and 32. These dyes easily formed aggregates with weak π-stacking on the TiO2 film.The coadsorbent CDCA did not enhance their photovoltaic performance; instead lowered the energy conversion efficiency dramatically due to the significant decrease of the dye loading. When a pyridine was used as an anchoring group in a dye 33, the CDCA enhanced the energy conversion efficiency. The dye 33 also formed aggregates on the TiO2 surface, however, the better energy conversion efficiency of 33 39880


Review


Figure 10. Chemical structures of BODIPY dyes (35−41) with incorporated complementary dyes.

Figure 11. Current−voltage characteristics (a) and IPCE spectra (b) of 37 using pristine TiO2 (black line) and rGO/TiO2 (red line). Reprinted with permission from ref 44. Copyright 2015 Royal Society of Chemistry.

LiI concentration in electrolyte was 0.2 M. In combination with results from Ziessel and coworkers,23 it shows clearly that cosensitization is an effective way to increase the PCE of BODIPY-sensitized cells; however, integration of two complementary dyes into a single unit does not always achieve the highest energy conversion efficiency for DSCs.

three dyes: 39, 40, and 41. The absorption spectra are very much the sum of the individual spectra of BODIPY and squaranine fragments. The active IPCE wavelength extends to 900 nm, making these dyes, especially 41, as a broader light absorber. The overall IPCE values are quite low (

BODIPYs for Dye-Sensitized Solar Cells - American Chemical Society

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