BODIPYs for Dye-Sensitized Solar Cells - American Chemical Society

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Review

BODIPYs for Dye-sensitized Solar Cells Hafsah Klfout, Adam Stewart, Mahmoud Elkhalifa, and Hongshan He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07688 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Applied Materials & Interfaces

BODIPYs for Dye-sensitized Solar Cells Hafsah Klfout, Adam Stewart, Mahmoud Elkhalifa and Hongshan He* Department of Chemistry and Biochemistry, Eastern Illinois University, Charleston, IL 61920

Corresponding Author Prof. Hongshan He Email: [email protected] Tel: 217-581-6231

Abstract

BODIPY, abbreviation of boron-dipyrromethene, is one class of robust organic molecules that have 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, such as porphyrin dyes. In this review, several deign 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 lights on the next generation of BODIPY dyes for their application in solar energy conversion.

Keywords: BODIPY, Solar Cell, Photovoltaics, Synthesis, Energy Conversoin

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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 randomly packed interconnected dye-coated titanium dioxide nanoparticle electrodes, platinum-coated counter-electrodes, and redox couple solution are “sandwiched”. In the device, the foremost important steps are the light absorption by dye molecules and subsequent charge separation at the interface between the dye 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 13.2% was achieved when a porphyrin dye was employed.1, 6 In 2015, Hanaya and coworkers reported 14.7% energy conversion efficiency when a carbazole-based dye ADEKA-1 and a triphenylamine-based dye LEG-4 are used to co-sensitize the TiO2 nanoparticles.7 Recently, BODIPY dyes are emerged as attractive light absorbers for DSCs due to their easy structural modification, abundant supply, and appealing color. Dyes in DSCs bind to the surface of TiO2 nanoparticles. Under illumination, the excited dyes inject electrons to the conduction band, 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 operation principles and mechanism 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 and frontier molecular orbital energy levels. 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.

Figure 1. Electron production and dye regeneration pathways in dye-sensitized 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, (3) injected electron from CB of TiO2 to electrolyte and back transfer of electron from TCO to electrolyte.


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In this review, we will provide readers a summary of the evolution of BODIPY dyes for solar energy conversion in dye-sensitized solar cells. BODIPY, an abbreviation of boron-dipyrromethene, is one class of robust organic molecules that have 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 application in photovoltaics is 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 structure, its numbering scheme and two design strategies for DSC applications. The two design strategies are generally based upon the position of an anchoring group. The first one is the vertical design, in which an anchoring group such as a 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 is linked to the BODIPY core through its C2 position. In this design, the anchoring group such as a cyanoacetic acid or a phenylcyanoacetic acid could link the BODIPY core through π−conjugation, forming a premium donor-π-acceptor system. Both strategies have been explored to improve the photovoltaic performance of BODIPY dyes. In this review, we will outline the key findings of BODIPY dyes based upon the two design strategies. We aim to provide readers a clear picture of the field and expect to shed lights 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 poor energy conversion efficiency, which is out the scope of this review and will be not discussed.

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

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, 16-17,18 The reaction is catalyzed by trifluoroacetic acid. After dehydration of dipyrromethane to dipyrromethene using 2, 3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ), excess of trimethylamine is added to strip the proton from the pyrrole for its coordination to 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, enormous amount of polymerized black residues as well as other fluorescence impurities. In many cases, repetitive chromatography procedures 3 ACS Paragon Plus Environment


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are required to get final pure product. The resulting products are usually an orange colored solid, which are quite soluble in chloroform, dichloromethane, toluene, etc. The color of the solid is different from the color of the 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, 4-dimethylpyrrole are used since both 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 C1, C3, C5 and C7 positions, primarily due to the availability of pyrrole precursors. A variety of aldehydes are commercially available for the construction of different BODIPY dyes. For applications in DSCs, 4-carboxybenaldehyde and 5-fomyl-2thiophenecarboxylic acid are frequently used. From this perspective, the development of new pyrrole derivatives is of importance for the exploration of structure-property relationship in BODIPY dyes. 2.2 Functionalization of BODIPY dyes The BODIPY core can be conveniently functionalized in several ways for their applications in dyesensitized 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.

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

a. Reactions at C2 and C6 positions. The common reaction is the halogenation using Nbromosuccinimide (NBS) or N-iodosuccinimide (NIS). The product can be a mono- or di-substituted compound depending upon the ratio of NIS (NBS) to the BODIPY.19 Even in a dilute condition the disubstituted product always exists, which can be minimized by running the reaction at 0 oC and dropwise addition of NIS to the solution.20 The halogenated derivatives can be used as precursors for further functionalization, typically through Pd-catalyzed reactions, which will result in attaching a donor group or an anchoring group to the BODIPY core. For example, Suzuki cross-coupling reactions have been used to introduce an electron donor through a reaction between boronic acid and a. iodinated BODIPY moiety.21 Another widely used Pd-catalyzed reaction for the conversion of a halogenated BODIPY to other useful species 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. 4 ACS Paragon Plus Environment

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b. Reactions at C3 and C5 positions, and C1 and C7 positions. The typical reaction at these positions require the Knoevenagel reaction, which will conjugate aromatic groups to the BODIPY core through a double bond.22 The reaction was 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 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 BODIPY core. From the energy conversion perspective of DSCs, such a structural characteristic is disadvantageous due to their easy aggregation on the surface of TiO2 nanoparticles, which is known to facilitate the electron recombination, and is problematic in other dyes such as porphyrin dyes. c. Reaction at BF2 unit. The reaction involves the substitution of two F atoms by ethyny 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 reactions, which can occur at either C3 or C5, the reaction to replace F often happens at both F atoms. d. Reactions at meso positions. The position often has an aromatic group such as phenyl or thiophene groups. 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. 3. Photophysical Properties BODIPY dyes are characterized by their extremely strong absorption and fluorescence in the visible and the near-infrared region. 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 meso position 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, BET dye with a benzoic acid group in its meso position absorbs and emit 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 5 ACS Paragon Plus Environment


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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-dioxa-oct-1-yne groups. After the C3 position was conjugated to a p-methoxydiethylene glycol the peak moved to 570 nm, which was further red-shifted to 640 nm after the second p-methoxydiethylene glycol was attached. The color of the dye 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, attach 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 co-sensitize the TiO2 nanoparticles. The BODIPY dyes for dye-sensitized applications 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 usually around several nanoseconds, which is much longer than the timescale of electron injection from the dye to the conduction band of TiO2 (typically in the range of several hundreds of femtoseconds). The fluorescence is often quenched when the dye binds to the TiO2 nanoparticles.

Figure 4. Absorption spectra of three BODIPY dyes with different absorption onsets. The dotted, solid, and dashdot lines represent dyes 1, 2 and 3, respectively. Reprint from the permission from reference 24. Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

4. BODIPY dyes for dye-sensitized solar cells 4.1 Dyes derived from a vertical design Majority of BODIPY dyes for DSCs are based upon the vertical design model, in which an anchoring group is usually a benzoic acid or a phenylcyanoacetic acid. The efforts in the field were devoted to 6 ACS Paragon Plus Environment

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enlarging the π–conjugation through two “legs” at C3 and C5 position as demonstrated by several examples in Figure 5. The work is pioneered by Akkaya and coworker. 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 69 500 M-1 cm-1 at the peak wavelength with an absorption up to 700 nm, which was not easy to achieve in many other organic dyes. The calculated HOMO energy level of 5.088 eV and a LUMO energy level of 3.517 eV meet the requirement for its normal function in DSCs. As shown in Table 1, the fabricated device with this dye only gave 1.66% PCE, which was considered the best for BODIPY-sensitized solar cells at that time. The low PCE is consistent with relative 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 electrolyte. The efficiency was increased to 2.46% after they increased the dye-loading time from 4 h to 24 h with the addition of CDCA (chenodeoxycholic acid). Dyes 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 alky 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 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 nm and 750 nm. Though a clear and efficient energy transfer from the two side BODIPY fragments to the central BODIPY core occurred, this dye showed 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 position ~ 700 nm. Dye 11 exhibited a significant blue-shift, which was attributed to 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 stronger compared to the dye 10, however, their IPCE values around 650 nm are slightly lower than 10. Both dyes showed lower energy conversion efficiency (2.27 and 2.11%) than 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 linked carbazole groups with different alkyl chain lengths tethered to N-carbazole and prepared dyes 14 -16. It was found that intramolecular energy transfer from carbazole donor to benzoic acid acceptor decreases with the increase in alkyl chain length possibly due to induced changes in molecular geometry caused by long alkyl chains. However, the compound 16 with a longer chain performs better than other two indicating the impact of dye aggregation on overall energy conversion efficiency as confirmed by transient and EIS investigations. In all those dyes, the energy level of the excited state of the dye is higher than the CB energy level of TiO2, 7 ACS Paragon Plus Environment


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indicating the electron injection is energetically favorable. However, compared to the porphyrin dyes, the photovoltaic performance of BODIPY dyes is quite poor. COOH

COOH

COOH

NC

N N

N F

B F

N B

N

F

N

N

N B

F

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I

F

N

N R

R 4

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N B

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COOH

R

5

R

R= OCH3

6

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COOH COOH

F N N B F F

F

N O

B N

N F

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R

R

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N B

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COOH

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N B

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R R R R = OC18H37

COOH

N

B

R

8

F

COOH

F NR

N

N 13

14, R = C2H5, 15, R = C7H15, 16, R= C18H37

Figure 5. Chemical structures of BODIPY dyes 4 - 16 that are designed based upon a vertical design strategy.

One plausible reason for the poor PCE of BODIPY-sensitized solar cells could be the existence of two Fions in the BODIPY core structure. The strong electron withdrawing property of F atom may slow the energy injection rate. Ziessel and coworkers24 reported several BODIPY dyes with the two F atoms being replaced by ethnyl 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 F-substituted 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 8 ACS Paragon Plus Environment

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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 to 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 on TiO2 film. Similar to the dyes 1 – 3, the replacement of two F atoms by two ethnyl polyethylene groups only exerts 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 calculation indicated that HOMO and LUMO exhibited larger overlap in 19 than in 17. Figure 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 from 560 nm to 760 nm (>60%), which is the best IPCE response of all BODIPY dyes reported to date and exhibited high PCE (5.75%) in device as listed in Table 1. When dye 18 and 19 are used as co-sensitizers, both the photocurrent and voltage increased, resulting in an impressive PCE of 6.43%, which is the best one reported so far for BODIPY-sensitized solar cells. In another study, Gonzalez-Valls and coworkers35 further introduced polyethylene to styryl functionalized legs with a benzoic acid as an anchoring group into a dye 21 for TiO2 nanotubes-based solar cells. It was found that the dye 21 adsorbed 100 times less than the 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 photonto-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 groups, which is consistent with Akkaya’s study. 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 group.36

Figure 6. Chemical structures of five representative BODIPY dyes 17 -21 that are synthesized based upon a vertical design strategy with two F atoms being replaced by styryl fragments.



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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 from the permission from reference 23. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

BODIPY dyes with different bridging groups between BODIPY core and the anchoring group have also been explored. Several representative dyes are shown in Figure 8. Cakmak and coworkers37 tried fluorenyl groups as electron donor and thiophene-2-carboxylic acid as anchoring groups in 23 and 24. The absorption spectra were red-shifted from 520 nm in 22 to 665 nm in 23 and 773 nm in 24 due to extension of π-conjugation, which increases the energy conversion efficiency in the same order. However, the energy conversion efficiency was quite low compared to 22 with no substituent on the BODIPY core. The authors argued that the inefficient electron injection from dye to the TiO2 conduction band as well as significant dye aggregation are the barrier to their high performance. Ooyama and coworkers38 linked o diphenylamine–thienylcarbazole moieties to BODIPY core through Knoevenagel reaction and prepared dye 25, which showed very dark color on the TiO2 film. 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 donor to 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 suggested that strong interaction between BODIPY core and electrolyte when the linker length is short. Therefore, a better efficiency was observed for 29, in which BODIPY core is far away from donor with minima electron recombination as shown in Figure 9. It was 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, in which six BODIPY positions with and without methyl or ethyl substituents. These dyes are easy to form weak π-stacking aggregation on the TiO2 film and co-adsorbent CDCA did not enhance their photovoltaic performance; instead lower 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 did enhance the energy conversion efficiency since 33 formed strong π -stacking aggregation on the TiO2 surface. The better energy conversion efficiency of 33 compared to 32 is 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 10 ACS Paragon Plus Environment

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significantly broadened with peak positions at 754 nm. However, the PCEs were only 0.19%. It was found that HOMO energy level is more positive than iodide/triiodide redox potential for sufficient for dye regeneration, however the LUMO is very close to the conduction band of TiO2, therefore the driving force for electron injection was insufficient leading to poor photovoltaic performance. HOOC COOH

HOOC

CN

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OC6H13 C6H13O

HO OH

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OC6H13 C6H13O

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N

F

F

26

B

HO

C6H13O

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C6H13O

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S

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29

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N

N

CN

N N

N

COOH

S

COOH S S N S

N

N

N

B F

N N B F F

S

N B

F

F

S N

N

N

N

N

B F

31

CN

OC6H13

28

N

S

N

OC6H13

S S

N

O

OC6H13

S

S N

N

25

24 HO

O

N

F

F 32

N

N 33

34

Figure 8. Chemical structures of representative BODIPY dyes 22 – 34 that are based upon a vertical design strategy. Thiophene spacers are used to enlongate the conjugation.



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

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 structure 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 porphyrin dyes supports its potential as a co-sensitizer for porphyrin-sensitized solar cells. The PCE of a porphyrin dye 35-sensitized solar cell increased from 6.65% to 7.50% when BET was applied as a co-sensitizer. Fluorescence studies indicate an efficient energy transfer from BET to 35. Such a phenomenon was also observed in other co-sensitized 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 only 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 a strong peak at 450 nm and 650 nm from the porphyrin fragment. The DSC from this dye gave 1.55% PCE, which is double of PCE (0.84%) 12 ACS Paragon Plus Environment

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from its counterpart without BODIPY fragment. 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 a PCE of 5.17% and 6.20% when a pristine TiO2 and rGO/TiO2 were used for photoanodes, respectively. The IPCE spectra showed the strong capability of this dye in converting the photons to electrons between 400 and 700 nm. Odobel and coworkers45 further linked a squaraine unit to porphyrin and prepared a new dye 38. The three chromophoric subunits display complementary absorption spectra (664 nm, 524 and 426 nm, 556 nm) and cover a wide window of the solar spectrum. Steady-state fluorescence experiments in solution demonstrate a significant quenching of the emission bands of 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 overall PCE was 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 three dyes 39, 40 and 41. The absorption spectra are very much the sum of the individual spectrum 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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