Porphyrins as excellent dyes for dye-sensitized solar cells: recent developments and insights.

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Porphyrins as excellent dyes for dye-sensitized solar cells: recent developments and insights Tomohiro Higashinoa and Hiroshi Imahori*a,b Dye-sensitized solar cells (DSSCs) have attracted much attention as an alternative to silicon-based solar cells because of their low-cost production and high power conversion efficiency. Among various sensitizers, numerous research activities have been focused on porphyrins due to their strong absorption bands in the visible region, versatile modifications of their core, and facile tuning of the electronic structures. In 2005–2007, Officer and Grätzel et al. had achieved a rapid increase in the power conversion efficiency of porphyrin DSSCs from a few percent to as much as 7%. Encouraged by these pioneering works, further high-performance porphyrin dyes have been developed in the last decade. These studies have provided us profound hints for the rational design of sensitizers toward highly efficient DSSCs. Push–pull structures and/or π-extensions have made porphyrins panchromatic in visible and even near-

Received 10th September 2014, Accepted 23rd October 2014 DOI: 10.1039/c4dt02756f www.rsc.org/dalton

infrared regions. Consequently, porphyrin sensitizers have exhibited power conversion efficiencies that are comparable to or even higher than those of well-established highly efficient DSSCs based on ruthenium complexes. So far the power conversion efficiency has increased up to ca. 13% by using a push–pull porphyrin with a cobalt-based redox shuttle. In this perspective, we review the recent developments in the synthetic design of porphyrins for highly efficient DSSCs.

1. a

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: [email protected] b Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

Tomohiro Higashino

Tomohiro Higashino was born in 1986 in Osaka, Japan. He received his BSc (2009) and MSc (2011) degrees from Kyoto University. He received a PhD degree from Kyoto University in 2014 on the chemistry of expanded porphyrins with main group elements. In 2013, he joined the research group of Professor Imahori as an Assistant Professor. His current research interests include the creation of novel pyrrole-based π-conjugated materials and their application.

This journal is © The Royal Society of Chemistry 2014

Introduction

In recent years, various environmental issues caused by consumption of a large amount of fossil fuel have attracted much attention. There is also serious concern that the world’s limited energy resources will be exhausted in the near future

Hiroshi Imahori completed his doctorate in organic chemistry at Kyoto University. From 1990–1992, he was a post-doctoral fellow at the Salk Institute for Biological Studies, USA. In 1992, he became an Assistant Professor, ISIR, Osaka University and then moved to the Graduate School of Engineering, Osaka University, as an Associate Professor. Since 2002, he has been a Professor of Chemistry, Graduate Hiroshi Imahori School of Engineering, Kyoto University. He has received the JSPS Prize (2006), CSJ Award for Creative Work (2006), Osaka Science Prize (2007), and NISTEP Researcher Award (2007). His current interests involve artificial photosynthesis, organic solar cells, and organic functional materials.

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because the world is facing a continuous growth in population and economy, especially in developing countries. From these points of view, photovoltaics and artificial photosynthesis are highly desirable technologies because clean solar energy from the sun is substantially inexhaustible. Since the seminal paper was reported by Grätzel and coworkers in 1991,1 dye-sensitized solar cells (DSSCs) have drawn much attention as an alternative to silicon-based solar cells because of their low-cost production and high power conversion efficiency (η).2–18 The schematic illustration of components and representative operational principles of DSSCs is depicted in Fig. 1. The typical DSSC consists of a dye-sensitized photoanode (TiO2) and a platinum counter electrode sandwiching an electrolyte that contains a redox mediator. The photocurrent of DSSCs is generated through the following processes. First, the photoexcited dye (S*) upon illumination injects an electron into a conduction band (CB) of TiO2 (−0.5 V vs. NHE). Then, the resultant oxidized dye (S•+) is reduced by redox shuttle, an I− ion or Co2+ complex in the electrolyte (dye-regeneration). The injected electrons move through an external circuit to the counter electrode. Finally, the I− ion or Co2+ complex is regenerated by the reduction of an I3− ion or a Co3+ complex at the surface of the counter electrode, and the circuit is completed. The cell performance of DSSCs strongly depends on sensitizers. Until recently ruthenium complexes, such as N3 and N719 (Fig. 2), have been the most efficient sensitizers exhibiting more than a power conversion efficiency (η) of 11%.4–9

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However, the limited supply and high cost of ruthenium metal may hamper their widespread application. From this point of view, organic dyes with no metal or inexpensive metal complexes have been intensively explored owing to their potential low-cost production and facile modulation of their structure and basic properties including HOMO−LUMO levels and absorption profiles.10–13 A number of organic dyes exhibiting a relatively high cell performance have been reported so far. The best DSSC performance with an η-value of 10.3% was noted when metal-free organic dye C219 was used (Fig. 2).12 Nevertheless, the cell performance of organic dye-based DSSCs is still inferior to that of ruthenium dye-based DSSCs. Moreover, the realization of η-values more than 15% is suggested for the commercialization of DSSCs. Therefore, it is crucial to develop inexpensive organic dyes or metal complexes exhibiting a high cell performance. From the beginning of DSSC history, researchers have attempted to utilize porphyrins as potential sensitizers for DSSCs by focusing on their intense absorption bands in the visible region, versatile modifications of their core, and facile tuning of the electronic structures.14–19 However, until recently the η-values of DSSCs based on typical porphyrin sensitizers were much lower than those based on ruthenium sensitizers as a result of the poor light-harvesting ability at around 500 nm and beyond 600 nm. To overcome this drawback, the introduction of a push–pull structure and/or the elongation of porphyrin π-conjugated system are effective means to improve the light-harvesting property that would allow us to gain better matching with solar energy distribution on the earth. Officer and Grätzel et al. reported a series of meso-tetraphenylporphyrins with a β-oligoalkenyl anchoring group (Fig. 3). In 2005 an η-value of 5.6% was attained for a DSSC based on GD1.20 Furthermore, in 2007 they found that a Zn-1-based DSSC displayed a higher η-value of 7.0%.21 Encouraged by the rapid increase in the η-value of porphyrin DSSCs from a few percent to as much as 7%, various porphyrins characterized by a push–pull structure and/or π-conjugated elongation have been

Fig. 1 Schematic illustration of components and operational principles of DSSCs. TCO denotes a transparent conducting oxide electrode.

Fig. 2 Representative examples of highly efficient sensitizers for DSSCs.

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Fig. 3 β-functionalized porphyrin sensitizers reported by Officer and Grätzel et al.


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synthesized in the last decade. In particular, a class of push– pull porphyrins with an electron-donating diarylamino group and an electron-withdrawing carboxyphenylethynyl anchoring group have revealed their remarkable photovoltaic performance. For instance, DSSCs based on SM315 and GY50 exhibited exceptionally high η-values of ca. 13%.22,23 Recent advances of the porphyrin-based DSSCs have been reviewed in several reports.14–19 The detailed mechanistic studies, such as the photophysics, electrical and optical spectroscopy, responsible for performance enhancement have already been reviewed by Mori and Mozer et al.17 Therefore, we will mainly focus on the development of synthetic design and insights of porphyrin structures as excellent dyes for highly efficient DSSCs in this perspective.

2. Insights for the realization of highly efficient porphyrin DSSCs 2.1

Adsorption structures of porphyrin on TiO2

Since electron transfer (ET) rates are a function of electronic coupling between the adsorbed dye and the TiO2 surface, the cell performance is influenced by adsorption structures of the dye on TiO2. Imahori and co-workers addressed the relationships between sensitization conditions and cell performances.24 For ZnP-sensitized solar cells, the η-value showed a strong dependence on sensitization solvents (Fig. 4). The η-value obtained for a sensitization time of 12 h increased from 0.55% to 3.7% in the order: DMF < CH2Cl2 < t-BuOH/ MeCN < EtOH < MeOH. This is in sharp contrast with the weak solvent dependence of N719-sensitized solar cells on the

Fig. 4 (a) Current–voltage characteristics of ZnP-sensitized solar cells and (b) sensitization time profiles of η-value (●) and of the surface coverage (Γ) (■).


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photovoltaic properties. In the cases of the aprotic solvents (i.e., DMF and CH2Cl2) the lower η-value resulted from the low surface coverage (Γ) of ZnP on TiO2, whereas in the cases of the protic solvents they were involved in the adsorption structures of the porphyrin on TiO2, yielding the higher η-value. Moreover, the sensitization time also had a large impact on the cell performance. When MeOH was used as the sensitization solvent, the η-value was increased with increasing sensitization time to reach a peak of 4.6% in 1 h and then gradually decreased to reach a plateau. In accordance with the η-profile, the Γ-value was increased initially with the sensitization time and then leveled off in 1 h. Such a drop of the η-value in the longer sensitization time was rationalized by an increase in self-quenching of the porphyrin excited singlet state by porphyrin aggregation with sensitization time. More importantly, Imahori et al. found a direct correlation between the η-value and amplitude of the injected, long-lived electrons (>10 ns) in CB.25,26 The sensitization conditions have a strong influence on the rates of electron injection and charge recombination (CR) and the amplitude of the long-lived electrons. The ET processes occur through space between the tilted porphyrin and the TiO2 rather than through a bond via the bridge. Thus, the tilt angle of the porphyrin on TiO2, which is affected greatly by the sensitization solvent and time, as well as the type of the bridge, governs the ET kinetics, the photocurrent, and the resultant cell performance. Overall, there was a linear correlation between the tilt angle and η-value, the smaller the tilt angle the higher the cell performance (Fig. 5). Similarly, D’Souza et al. probed the effect of dye-orientation on TiO2 on the photovoltaic properties of meso-carboxyphenylporphyrins where the carboxy group is attached to the para, meta or ortho position.27 The cell performance was lowered by varying the substituted positions from para to ortho, which reflects the faster CR. These studies indicate that the small tilt angle of porphyrin dye on TiO2 is favorable for retarding CR as well as obtaining large a Γ-value. He et al. also evaluated the cell performance of DSSCs using a series of porphyrin sensitizers with identical anchoring groups.28 The p-carboxyphenyl group was a better anchoring one than m-carboxyphenyl because the electron density distribution of the p-carboxyphenyl group in LUMO is higher than that of the m-carboxyphenyl in LUMO. These studies imply that rational design of a suit-

Fig. 5 Schematic representation of the porphyrin adsorption geometry on TiO2.

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able anchoring group and optimization of the sensitization conditions are essential to achieve a high cell performance.


2.2

Steric effect of substituents

The cell performance depends on the peripheral substituent of porphyrins. Imahori et al. compared the photovoltaic properties of a series of porphyrin sensitizers and found that meso-phenyl-substituted porphyrin gave a lower η-value of 1.2% than that of meso-mesityl-substituted ZnP (η = 4.6%).24 The bulky mesityl groups may suppress self-quenching by porphyrin aggregation, leading to an increase of electron injection efficiency (ϕinj ). Lin and Diau et al. developed alkoxyphenylsubstituted porphyrins (LD11 and LD12, Fig. 6).29 The cell based on ortho-substituted LD12 displayed a superior η-value (7.43%) to that of LD11 (η = 4.78%). Importantly, both short circuit current ( JSC) and open circuit voltage (VOC) of the LD12based cell were remarkably higher than those of the LD11based cell. They proposed that the long alkoxy chains at the ortho-positions wrapping the porphyrin core would inhibit the aggregation on TiO2 and retard CR between the injected electrons and I3− by a blocking effect. Recently, ortho-alkoxyphenyl groups have been regarded as an indispensable substituent for achieving highly efficient DSSCs, especially when a cobaltbased redox shuttle is used in the electrolyte (see section 5.2). 2.3

Anchoring groups

In DSSCs, carboxylic acids such as benzoic acid and cyanoacrylic acid have been the most widely used functional groups for attaching sensitizers to metal oxides. However, carboxylic acids are prone to dissociate from the metal oxide surface under severe conditions, including exposure to aqueous and alkaline electrolytes.30 Such detachment of adsorbed dyes from TiO2 is undesirable considering the durability of DSSCs for practical applications. For this reason, novel anchoring groups have been exploited, and several anchoring groups exhibiting promising performance are described here. He et al. found that 8-hydroxyquinoline (HOQ) is an attractive candidate to replace the benzoic acid group.31,32 The porphyrin dye bearing the HOQ group as the anchoring one can adsorb on TiO2. DSSCs with DPZn-HOQ and DPZn-COOH gave η-values of 3.09% and 1.76%, respectively, under the same conditions (Fig. 7).32 Co-sensitization of DPZn-HOQ with BET increased the η-value by 10%, yielding an η-value of 3.41%. In addition, the HOQ-modified porphyrin dye exhibited a

Fig. 6

Molecular structures of alkoxyphenyl-substituted porphyrins.

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Fig. 7 Molecular structures of porphyrins bearing HOQ as an anchoring group.

Fig. 8 Molecular structures of porphyrin bearing salicylic acid as an anchoring group and the reference porphyrin.

stronger resistance toward both the acid and the base. Although the η-value is moderate, the better cell performance and durability obtained by the use of HOQ make further assessment worthwhile. Jiang and co-workers used salicylic acid as a tridentate anchoring group (Fig. 8).33,34 An η-value (4.55%) of a DSSC based on PESp is twice as large as that (η = 2.26%) with PE1 possessing benzoic acid as the anchoring group.34 Although the tridentate binding mode between salicylic acid and TiO2 remains ambiguous, its mode may enhance the ϕinj-value, resulting in an increase of the η-value.

3. Push–pull porphyrins 3.1

meso-Diarylamino porphyrins

In recent years, D–π–A type organic sensitizers, which consist of an electron-donating group (D), π-spacer (π), and an electron-withdrawing anchoring group (A), have been widely investigated owing to their broad and intense absorption spectral features as well as efficient electron injection due to intramolecular charge transfer (CT) character.10–13 One can envision that porphyrins are attractive as the π-spacer moiety. In this respect, various push–pull type porphyrins have also been synthesized for DSSCs. Diau, Yeh and co-workers prepared a variety of zinc porphyrin derivatives with diarylamino group as the strong electron-donating substituent and carboxyphenylethynyl group as the electron-withdrawing anchoring substituent at meso-positions (Fig. 9).35 In 2009, by using YD1 they


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Fig. 9

Molecular structures of YD0, YD1, and YD2.

achieved an η-value of 6.0%, which is higher than that using YD0 (2.4%). Importantly, the cell performance of YD1 was comparable to that of N3 (6.1%) under their optimized conditions. The same group designed YD2, which was modified from YD1, to improve its stability.36 The cell performance of YD2 increased to an η-value of 6.8% in comparison with that of YD1. Then, the device fabrication was further optimized in collaboration with the Grätzel group. In 2010 the η-value using YD2 was remarkably improved to ca. 11%.37 This report was the first example of a DSSC using a porphyrin sensitizer that achieved an η-value of more than 10%. Introduction of multiple diarylamino groups into a porphyrin core is a fascinating methodology to enhance the lightharvesting ability. Imahori and co-workers synthesized a series of zinc porphyrins with different numbers of diarylamino groups at the meso-positions (Fig. 10).38 The light-harvesting

Fig. 10 Molecular structures of porphyrins bearing multiple diarylamino moieties.


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ability was improved with an increasing number of diarylamino groups on the porphyrin. However, DSSCs using transZnP and cis-ZnP exhibited lower η-values (3.8% and 5.5%) than that of mono-ZnP (6.5%). Considering that there are weak electronic couplings between the LUMO of porphyrin and the CB of TiO2 as a result of smaller electron densities on the anchoring carboxy groups of trans-ZnP and cis-ZnP, inefficient electron injection would lower the cell performances. With this in mind, Imahori et al. replaced one carboxyphenyl group in cisZnP with the more electron-withdrawing carboxyphenylethynyl group, while another carboxyphenyl group in cis-ZnP was replaced with the bulky mesityl group to create ZnPBAT (Fig. 10).39 The absorption of ZnPBAT was broadened and redshifted compared to that of ZnPBA without the triple bond in the electron-withdrawing anchoring group and even to YD2 with only one diarylamino group. The electron densities of the anchoring group in the LUMOs of ZnPBAT and YD2 are comparable and considerably larger than that of ZnPBA. A DSSC using ZnPBAT achieved an η-value of 10.1%, which is higher than those using ZnPBA (8.3%) and YD2 (9.1%) under their optimized conditions. Multiple electron-withdrawing anchoring groups were also employed to enhance the push–pull character as well as to strengthen the binding of the porphyrin on TiO2. Kim and coworkers synthesized push–pull porphyrins tda-1b-d-Zn and tda-2b-bd-Zn, possessing one and two β-oligo-olefinic substituents as the electron-withdrawing anchoring groups, respectively (Fig. 11).40 The doubly functionalized porphyrin tda-2bbd-Zn exhibited a higher η-value of 7.47% than that of tda-1bd-Zn. The η-value of a DSSC with tda-2b-bd-Zn is comparable to that with N3 dye (7.68%) under their optimized conditions. Kim et al. switched the β-oligo-olefinic anchoring groups to carboxyphenylethynyl substituents to create the corresponding ZnEP1 and ZnEP2.41 To the contrary, a DSSC based on ZnEP1 with one anchoring group exhibited a higher cell performance (η = 5.9%) than a DSSC based on ZnEP2 with two anchoring

Fig. 11

Molecular structures of β-functionalized push–pull porphyrins.

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groups (η = 4.0%). It is noteworthy that the lowest-energy Q-bands of tda-1b-d-Zn and tda-2b-bd-Zn (628 and 650 nm) are more red-shifted than those of ZnEP1 and ZnEP2 (592 and 638 nm). These differences imply that oligo-olefinic substituents at β-positions have more effective electronic communications with porphyrin core than carboxyphenylethynyl substituents, leading to superior light-harvesting ability and ϕinj-value. 3.2

Dialkylaminophenylethynyl porphyrins

The dialkylaminophenylethynyl group has been also used as an electron-donating moiety (Fig. 12). Lin and Diau et al. developed the meso-disubstituted porphyrin sensitizer LD13 for a DSSC.42 LD13 displayed red-shifted and broadened absorption in comparison with that of YD2. A DSSC with LD13 attained an η-value of 9.34%. Lin and Diau et al. further modulated the structure of LD13 to redesign porphyrin dye LD14 bearing four alkoxy chains at the ortho-positions of the meso-phenyl rings. This steric congestion around the porphyrin core was intended to suppress the dye aggregation on TiO2 as well as reduce CR between electrons in the CB of TiO2 and I3− in the electrolyte solution.29 In fact, the introduction of alkoxy chains led to a ca. 0.04 V increase of VOC and yielded an η-value of 10.2%. Liao and Wang et al. prepared the analogous porphyrin LW4, possessing thiophene-carboxylic acid instead of benzoic acid as the anchoring group. A DSSC based on LW4 attained a slightly improved η-value of 9.53% relative to that of LD14 (9.01%) under their optimized conditions.43 This improvement

Fig. 12 Molecular structures of porphyrins bearing a dialkylaminophenylethynyl moiety.

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was ascribed to the more effective intramolecular CT character in LW4. Pizzotti and co-workers presented similar β-disubstituted porphyrins Zn-2 and Zn-3 in which dimethylaminophenylethynyl and carboxyphenylethnyl groups are introduced at the β-positions (Fig. 12).44 DSSCs with Zn-2 and Zn-3 revealed η-values of 4.1% and 4.7%, respectively, which are slightly superior to that based on LD13 (η = 3.9%) under their optimized conditions. The absorption of β-disubstituted porphyrins Zn-2 and Zn-3 showed less broadening and red-shift than those of meso-disubstituted porphyrins LD13, LD14 and LW4, suggesting a more effective electronic communication through meso-positions than β-ones. 3.3

Triarylamine-substituted porphyrins

Triarylamine derivatives have been widely used as an electrondonating moiety in various D–π–A organic dyes.10–13 Porphyrins bearing a triarylamine moiety have also been reported (Fig. 13). Kim and co-workers synthesized porphyrin HKK-Por1 with a carbazole-containing triphenylamine moiety.45 An η-value of 5.0% was noted for the HKK-Por1-sensitized solar cell. Hung et al. designed push–pull porphyrin sensitizers possessing multiple anchoring units.46,47 The 1D–π–3A type Zn1TPA3A-sensitized solar cell exhibited an η-value of 5.4%, which is lower than that of a Zn1NH3A-sensitized solar cell (6.1%) where the phenyl groups on the N atom are replaced with alkyl chains to suppress the porphyrin aggregation on TiO2.47 Importantly, these porphyrin sensitizers bearing multiple anchoring groups showed better photostability than the mono-anchoring analogues. Compared to other push–pull porphyrins, the DSSCs based on triarylamine-substituted porphyrins display lower cell performances on account of their inferior light-harvesting ability. Indeed, the wavelength of the lowest-energy Q-band of HKK-Por1 is limited to 598 nm. Since the meso-phenyl group is tilted against the porphyrin plane by ca. 60° due to its steric hindrance, the electronic communication through the phenyl group is hampered, resulting in the lower η-values.

Fig. 13 Molecular structures of porphyrins bearing a triarylamine moiety.


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4.

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π-Extended porphyrins


4.1 Incorporation of π-chromophores into porphyrins through conjugated bridges The extension of porphyrin π-conjugation is an enchanting strategy to achieve an excellent light-harvesting ability in the visible and near infrared (NIR) regions. The effect of the introduction of additional π-chromophores through acetylenic spacers has been studied. Lin, Diau and co-workers examined the effect of the systematic introduction of acenes from benzene to pentacene into a porphyrin core through the conjugated bridges.48 With increasing the size of the acenes, the absorption became broadened and red-shifted. Among these porphyrins, the anthracene-incorporated porphyrin LAC-3 attained the highest η-value of 5.4% (Fig. 14). This was rationalized by charge delocalization between the porphyrin core and the anthracene group at the HOMO level. Lin and Diau et al. further tethered a cyclic aromatic moiety at the meso-position opposite the anchoring group.49,50 Fluorene-substituted dye LD22 gave an η value of 8.10%, whereas pyrene-substituted dye LD4 achieved an η-value of 10.1%. The performance of a DSSC based on LD4 was superior to that of DSSC based on N719 (η = 9.27%) under their optimized conditions. Push–pull porphyrins with a π-chromophore incorporated into the electron-withdrawing anchoring group were also described (Fig. 15). Yeh and Diau et al. synthesized porphyrins YD11–YD13, bearing a benzene, naphthalene or anthracene ring.51 Anthracene-incorporated YD13 showed the most redshifted absorption, but its cell performance (η = 1.9%) was low as a result of its aggregation behavior on TiO2. On the other hand, DSSCs with YD11 and YD12 displayed higher η-values of 6.8% and 6.9%, respectively, which are slightly lower than N719 (η = 7.3%). Wu and Lin et al. designed porphyrin dye LWP1 consisting of a combination of LD14 with LAC-3.52 An

Fig. 15 Molecular structures of push–pull porphyrins with π-chromophore incorporated into the electron-withdrawing anchoring group.

LWP1-sensitized solar cell displayed an η-value of 9.7%, which is slightly inferior to that of a LD14-sensitized solar cell (η = 10%) under their optimized conditions. The photovoltaic properties are strongly dependent on π-chromophores. Introduction of the larger π-conjugated chromophore creates a better light-harvesting ability, but the large π-chromophores often trigger dye-aggregation. VOC-values of DSSCs based on LD22 and LD4 are smaller than 0.7 V due to their shortened excited state lifetime and/or facile CR between electrons in TiO2 and the electrolyte. In contrast, π-chromophore incorporated push– pull porphyrins YD12 and LWP1 exhibited higher VOC-values than 0.7 V. Although it is still difficult to predict the lifetime of the excited singlet state of porphyrins in advance, modulation of the energy levels and electron density distribution of HOMO and LUMO as well as inhibition of dye-aggregation by bulky substituents are prerequisite for further enhancement of cell performance. 4.2

Fig. 14 Molecular structures of porphyrins linked with π-chromophores through conjugated bridges.


Fused porphyrins

Peripherally fused aromatic structures are one of the fascinating strategies to extend π-conjugation and thereby reinforce the absorption in the NIR region for application to materials science.53–55 In 2007 Imahori and co-workers reported the meso-naphthyl fused porphyrin fused-ZnP1 as the first example of the application of fused porphyrins to DSSCs (Fig. 16).56,57 The light-harvesting ability was significantly improved by the fusion. A DSSC with fused-ZnP1 showed an η-value of 4.1%, which is higher than a DSSC with non-fused porphyrin ZnP1 (η = 2.8%) under the same conditions.

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Fig. 16

Molecular structures of naphthalene-fused porphyrins.

Imahori and co-workers also developed a series of quinoxaline-fused porphyrins for DSSCs.58–61 The unsymmetrical π-elongation resulted in a broadened and red-shifted absorption. Moreover, the electron-withdrawing nature of the quinoxaline moiety is anticipated to accelerate electron injection. A DSSC based on ZnQMA displayed an η-value of 6.3%, which is higher than ZnP (η = 4.4%) under the same conditions.58 They further designed imidazole- and quinoxaline-fused porphyrin ZnPQI to enhance the push–pull structure (Fig. 17). This push–pull porphyrin ZnPQI showed an improved lightharvesting ability relative to ZnQMA, leading to a higher η-value of 6.8%.61 Other polycyclic aromatic compound-fused porphyrins have also been described (Fig. 18). Anderson and Snaith et al. synthesized an anthracene-fused porphyrin dye, which reveals a dramatic red-shift and broadening of the absorption spec-

Fig. 17

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Molecular structures of quinoxaline-fused porphyrins.

Fig. 19

trum.62 A DSSC using P3 exhibited a photocurrent generation onset of ∼1100 nm when using SnO2 as the photoanode. However, a DSSC based on P3 and standard TiO2 did not show any photocurrent generation in the NIR region (700–1100 nm). This was explained by the inefficient electron injection from the porphyrin excited singlet state to the CB of TiO2 due to a lowering of the excited state by the π-extension. Wang and Wu et al. reported the synthesis of perylene-fused porphyrins and their application to DSSCs.63 Perylene-fused porphyrins WW-1 and WW-2 exhibited broader IPCE spectra, extending into the NIR region up to 1000 nm. Nevertheless, owing to their aggregation tendency and the low levels and short lifetimes of their excited singlet state, these dyes displayed poor η-values of ca. 1.3% together with low maximum IPCE values of ∼30%. Diau, Yeh and co-workers prepared the triply fused porphyrin dimer YDD2 and doubly fused dimer YDD3. These dyes also encountered a low cell performance for similar reasons (Fig. 19).64 Although these fused porphyrins are potential sensitizers with panchromatic and a particular NIR response, elaborated molecular design, specifically fine-tuning of the HOMO−LUMO levels, inhibition of the aggregation, and retention of the rather long lifetime of the excited state is essential to boost the cell performance. 4.3

Fig. 18 Molecular structures of polycyclic aromatic compound-fused porphyrins.

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Molecular structures of fused porphyrin dimers.

Porphyrin arrays

Various covalent and noncovalent porphyrin arrays have been studied thanks to their excellent light-harvesting abilities, inspired by natural photosynthetic systems.65–70 The first demonstration of the utilization of covalently linked porphyrin dimers as sensitizers for DSSCs was reported by Officer and coworkers in 2009 (Fig. 20).71 They designed porphyrin dimers comprising two porphyrin dyes linked in either a linear anti (P10) or a 90° syn (P18) fashion. While the enhancement of light-harvesting ability was small on account of a negligible interaction between the two porphyrins in the ground state, JSC-values of DSSCs with the dimers were higher by up to 10% than that of DSSCs with the corresponding porphyrin


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Fig. 20 et al.72

Molecular structures of porphyrin dimers reported by Officer

monomer under the cell conditions with similar Γ-value. Later, Mozer and Mori et al. optimized the device fabrication conditions. An η-value of a DSSC based on P10 reached 5.5%, which is the highest value among the porphyrin dimer-sensitized solar cells.72 Kim et al. presented the meso–meso linked porphyrin dimers as sensitizers for DSSCs (Fig. 21).73 While the HOMO− LUMO levels were retained at the levels of the porphyrin monomer, a much broader absorption was induced by exciton

Fig. 21

Molecular structures of meso–meso linked porphyrin dimers.


Perspective

coupling. A wide IPCE action spectrum in the Soret and Q-band regions was attained for a DSSC with the dimeric porphyrin. An η-value of 4.2% was noted for the PEG-2b-bd-Zn2sensitized solar cell. Diau and Yeh et al. also described the analogous meso–meso linked porphyrin dimer YDD1 for DSSCs. A DSSC with YDD1 yielded an η-value of 5.23%, which is almost comparable to that of the reference porphyrin monomer YD0 (η = 5.14%).64 On the other hand, the meso–meso ethynyl-linked porphyrin dimer YDD0 exhibited a lower photovoltaic performance (η = 4.07%) due to dye aggregation, irrespective of the broad absorption extending to the NIR region. Segawa and co-workers reported a series of meso–meso ethynyl-linked porphyrin oligomers (Fig. 22).74–77 They introduced an electron-donating amino moiety at the meso-position of the porphyrin dimer to strengthen the push–pull character. A DSSC with DTBC exhibited an η-value of 5.2%.74 This was attributed to the rather long excited-state lifetime of DTBC. A gradient of the energy levels in the order of the zinc porphyrin and the free base porphyrin in DTBC may facilitate the electron injection from the excited states. They also synthesized the ethynyl-linked porphyrin trimer Zn-ZnA-Zn for DSSC. A DSSC using Zn-ZnA-Zn showed spectral sensitivities up to 900 nm, whereas the IPCE values were less than 25% on account of the porphyrin aggregation on TiO2. The η-value of a DSSC using Zn-ZnA-Zn reached 3.2% under the optimized conditions.76 The η-values of DSSCs based on these porphyrin arrays remain moderate, since their structural and electronic modification is synthetically difficult in comparison with the corresponding porphyrin monomers. Although ethynyl-linked porphyrin oligomers reveal a broad absorption extending to the NIR region, the maximum IPCE values are limited up to ca. 40%. These low IPCE values probably originate from insufficient electron injection and an aggregation tendency. On the other hand, a DSSC using DTBC provided higher IPCE values of up to ca. 60%. Enhancement of push–pull structures and a well-designed gradient of the energy levels of hetero-

Fig. 22 Molecular structures of meso–meso ethynyl linked porphyrin oligomers reported by Segawa et al.

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porphyrin arrays would need more consideration to strengthen the intramolecular CT character, leading to efficient electron injection and higher η-value.

5. Further improvement of DSSC performance 5.1

Co-sensitization

Co-sensitization is an effective approach to enhance the cell performance through a combination of two or more different dyes exhibiting complementary absorption in the visible and NIR regions (Fig. 23). Dye aggregation on TiO2 may be inhibited by mixing the dyes with different shapes and sizes. Kim and co-workers reported a DSSC with push–pull porphyrin 2,4ZnP-CN-COOH co-sensitized with HC-A1.78–80 While an η-value of 4.9% was obtained in the absence of a co-sensitizer, a DSSC with HC-A1 as the co-sensitizer displayed a remarkably improved η value of 8.4%.79 Co-sensitization not only increases JSC, but also VOC, because of an enhancement of the light-harvesting ability as well as suppression of CR. Yeh and Diau et al. used a cocktail of YD2-o-C8 (vide infra), CD4, and YDD6 with different absorption profiles for DSSCs. An η-value of 10.4% was achieved under the optimized cell conditions.81 The IPCE values of 75–80% and 40–60% were obtained in spectral regions of 400–700 nm and 700–800 nm, respectively, giving a high JSC value of 19.28 mA cm−2. Chen and co-workers applied a combination of HD18 and PT-C6 to DSSCs.82 Since the phenothiazine molecule PT-C6 can act as both co-adsorbent and sensitizer upon co-adsorption, the DSSCs displayed an η-value of 10.1% under their optimized conditions. Xie and coworkers presented the co-sensitization of XW4 by C1 for DSSCs.83 An η-value of ca. 8% for a DSSC with only XW4 was increased to ca. 11% for a DSSC by the co-sensitization. Diau and Lin et al. demonstrated that LD31 co-sensitized with AN-4 extended the light-harvesting ability up to ca. 800 nm and an η-value of 10.3% was attained in an iodine-based electrolyte.84 However, since LD31 has already achieved an outstanding light-harvesting ability, the co-sensitization with AN-4 resulted in only a 0.3% increase in the η-value compared to that of a DSSC based on LD31 alone (10.0%). These excellent works indicate that organic dyes possessing absorption bands around 500 nm are useful for the co-sensitization with porphyrin sensitizers that lack intense absorption around 500 nm. Additionally, relatively rigid and bulky structures would be favorable for the suppression of dye aggregations. Interestingly, the η-value of DSSCs using the cosensitizer alone seems to have no relationships with the η-values of DSSCs with the corresponding porphyrin sensitizer together with the co-sensitizer. For instance, DSSCs based on PT-C6, C1, and HC-A1 attained η-values of 8.2%, 5.7%, and 0.37%,80 respectively. At least co-sensitizers should reveal a high IPCE value at a specific wavelength region that compensates the moderate IPCE by porphyrins. Meanwhile, HC-A1 possesses the hole-conducting function because of a low oxidation potential. Thus, the use of co-sensitizers with multiple

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Fig. 23

Molecular structures of porphyrins and co-sensitizers.

functions, such as efficient light-harvesting around 500 nm, suppression of dye aggregation, and excellent hole-conducting character, would be a potential option for improving the cell performance of porphyrin DSSCs. Another concept derives from the use of an energy relay with additional photoactive dye in supramolecular assemblies (Fig. 24).85–87 While the electron and energy transfer processes


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Fig. 24

Supramolecular assemblies of MP216 + MP151 and Zn-4 + A1.

of supramolecular systems have been studied in great detail, the utilization of supramolecular interactions in solar cells is scarce. Ballester, Palomares and co-workers designed the selfassembled trisporphyrin complex MP216 + MP151.86 The pyridyl group of porphyrin MP151 coordinated to the zinc atom of porphyrin MP216 adsorbed on a TiO2 surface. The DSSC performance was obtained with a 100% increase in η-value for the supramolecular trisporphyrin MP216 + MP151 assembly (2.9%), compared to a plane zinc bisporphyrin MP216 (1.5%). Not only did the improved light-harvesting ability attain a higher photocurrent generation, but also the blocking effect of the supramolecular assembly retarded the CR process. Odobel et al. utilized a borondipyrromethene (BODIPY) dye bearing an imidazole moiety A1 as a light-harvesting antenna for a supramolecular assembly with zinc porphyrin dye Zn-4.87 The action spectra clearly displayed that the IPCE value in the 450–550 nm region was strongly enhanced when the porphyrin dye Zn-4 was connected to the antenna dye A1. For a DSSC based on this supramolecular array, an η-value of 4.6% was remarkably higher than that based on Zn-4 alone (3.6%) due to an increase of JSC by 25%. On the other hand, BODIPY dye lacking the imidazole moiety displayed no enhancement, highlighting the importance of the supramolecular assembly for efficient energy transfer. These approaches can be extended to other pigments possessing substituents coordinating to zinc metal, such as a pyridine and imidazole moiety. To extend this concept and achieve higher cell performances, optimization of additional dyes and supramolecular architecture is required. 5.2

Perspective

the relatively high overpotential of the iodide/triiodide redox shuttle for dye regeneration. Among these, CoII/IIItris(bipyridyl) has been studied as a promising candidate.88–91 The cobalt redox electrolyte can achieve a higher VOC and resultant η-value, since the potential of the CoII/IIItris(bipyridyl) redox couple (+0.57 V vs. NHE) is more positive by 0.17 V than that of the iodide/triiodide redox couple (+0.40 V vs. NHE) (Fig. 1). A drawback of the CoII/III redox couple is the fast CR in comparison with the iodide/triiodide redox couple. Blocking of undesirable CR can be attained by the use of bulky substituents around dyes.90–93 In this regard, a number of porphyrin dyes bearing long alkoxy chains at the ortho-positions of mesophenyl rings have been synthesized for DSSCs with the CoII/III redox couple. Diau, Yeh, Grätzel and co-workers designed the porphyrin sensitizer YD2-o-C8, an analog of YD2. An impressive η-value of ca. 12% ( JSC = 17.3 mA cm−2, VOC = 0.965 V, fill factor (ff ) = 0.71, 99.5 mW cm−2) was attainable by increasing the conventional VOC (0.7–0.8 V) to ca. 0.96 V when the CoII/III redox couple was used.94 A cocktail of YD2-o-C8 and complementary Y123 as a co-sensitizer led to a comparable η-value of ca. 12% (Fig. 25). Wang and Wu et al. designed porphyrin dyes bearing an electron-donating N-annulated perylene (Fig. 26).95 DSSCs based on WW-5 and WW-6 with a CoII/IIItris(bipyridyl) redox couple showed high η-values of 10.3% and 10.5%, respectively. These values were comparable with that of YD2-o-C8 under their optimized conditions (η = 10.5%). The lowest-energy

Fig. 25

Molecular structures of YD2-o-C8 and Y123.

Fig. 26

Molecular structures of WW-5 and WW-6.

Cobalt-based redox shuttle

An iodide/triiodide redox couple has been utilized as the most common electrolyte for DSSCs.88 However, other redox mediators have been searched for because its VOC is limited by


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Q-band of WW-6 (672 nm) is significantly more red-shifted than those of SM315 (668 nm) and GY50 (665 nm), indicating the superior light-harvesting ability of WW-6. However, VOC of WW-6 is relatively low (0.809 V) in spite of using the cobalt redox shuttle. This behavior is analogous to that observed for π-chromophore-incorporated porphyrins (section 4.1). Namely, N-annulated perylene moiety may induce dye aggregation and/ or a fast CR, leading to a lower η-value of ca. 10%. Grätzel and co-workers further elaborated the molecular structure of YD2-o-C8 to create SM315 (Fig. 27).22 A benzothiadiazole unit was used in SM315 as a more strongly electronwithdrawing anchoring group. The absorption of SM315 became broadened and the lowest-energy Q-band was significantly red-shifted to 668 nm. A DSSC based on SM315 yielded an η-value of ca. 13% ( JSC = 18.1 mA cm−2, VOC = 0.91 V, ff = 0.78) without a co-sensitizer. At the same time, Yeh and Grätzel et al. reported GY50, which is very similar to SM315. A DSSC based on GY50 displayed an η-value of 12.7% ( JSC = 18.53 mA cm−2, VOC = 0.885 V, ff = 0.773),23 which is comparable to the photovoltaic performance of SM315 (Fig. 26). The more bulky biphenyl substituent of SM315 is unlikely to be necessary for the suppression of CR. In contrast, the porphyrin dye GY21 lacking a phenylene spacer in the anchoring group gave a remarkably low cell performance (η = 2.52%). This suggests that a subtle change in the electronic coupling between the porphyrin and TiO2 has a large impact on the ET kinetics at the interface, varying the photovoltaic properties greatly (see section 2).

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5.3 Effects of aging and light exposure on photovoltaic performance The effect of light exposure on photovoltaic properties has been reported for various DSSCs, but the change of cell performance in DSSCs upon light exposure is dye-dependent.96–98 In 2011, for the first time Officer and co-workers described the significant and simultaneous improvement in all photovoltaic parameters ( JSC, VOC and ff) of DSSCs based on porphyrins.99 The current–voltage characteristic of DSSC with GD2 was measured immediately after cell fabrication and then remeasured after light exposure (Table 1). After 1 h light exposure, the JSC-value increased by 6% to 13.1 mA cm−2, accompanied by an additional 6% increase in both VOC and ff, leading to an η-value of 6.2%. In addition, the cells stored in the dark displayed a gradual improvement with time, which was attributed to the light exposure effect induced during the measurements of the photovoltaic performance under illumination. Mori and co-workers evaluated the details of the improvement by light exposure.100 DSSCs based on a TiO2 film with a full coverage of dye exhibited a light exposure effect with 6% and 5% increases in the JSC and VOC, respectively. On the other hand, when the dye loading was reduced by ∼50%, light exposure caused enhanced increases in the JSC by 24% and the VOC by 6%. These larger increases in the DSSC with the reduced amount of dye suggested the importance of the extra space on TiO2. Photovoltage and photocurrent transients were examined for DSSCs prepared using three different electrolyte compositions: (i) standard, (ii) DMPIm+ (dimethylpropyl-imidazolium)-rich, and (iii) Li+-rich electrolytes. Under light exposure, Li+ ions were removed from the TiO2 surface and replaced with DMPIm+ ions. This process was found to enhance the electron lifetime by decreasing CR with the redox mediator. They proposed that the light exposure effect was initiated by the formation of dye cation radicals by light rather than photoinduced changes in the levels of the semiconductor surface. Finally, they concluded that the initial limited injection and fast CR processes arise from the presence of Li+ ions on the TiO2 surface, and the improved injection and retardation of the fast CR after the light exposure is triggered by the replacement of Li+ ions with DMPIm+ ions under illumination (Fig. 28). Imahori et al. assessed an “aging effect” in which the η-value is gradually increased to reach a plateau with time.39 Immediately after cell fabrication, DSSCs with YD2, ZnPBA and ZnPBAT exhibited η-values of 8.1%, 6.2%, and 8.6%, respectively. After aging for several days under dark condition, the η-values were improved to 9.1%, 8.3%, and 10.1%, respectively. Palomares and co-workers also found an improvement in

Table 1 Photovoltaic parameters measured before and after light exposure for DSSCs with GD2

Fig. 27 Molecular structures of benzothiazole-incorporated porphyrin sensitizers.

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As prepared Light-exposed

JSC/mA cm−2

VOC/V

ff

η/%

12.43 13.14

0.665 0.705

0.63 0.67

5.2 6.2


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Perspective Table 2 Summary of the light-harvesting spectral region and photovoltaic performance for the selected porphyrin sensitizers in this perspective


Sensitizer

Fig. 28

Fig. 29

Schematic representation of light-induced cation exchange.

Molecular structure of VC-70.

Zn-1 YD2 ZnPBAT LD4 LD14 LD22 LW4 LWP1 2,4-ZnP-CN-COOH + HC-A1 YD2-o-C8 + CD4 + YDD6 HD18 + PT-C6 XW4 + C1 LD31 + AN-4 YD2-o-C8 + Y123c WW-6b SM315b GY50b,c

IPCE onseta/ nm

JSC/mA cm−2

VOC/V

ff

η/%

Ref.

740 760 800 >800 780 800 790 >800 750

14.0 18.6 19.33 19.63 19.17 17.26 17.65 17.77 15.39

0.680 0.77 0.719 0.711 0.736 0.689 0.75 0.73 0.739

0.74 0.764 0.724 0.721 0.721 0.681 0.72 0.75 0.74

7.0d 11d 10.1 10.1 10.2 8.10d 9.5 9.7 8.5

21 37 39 49 29 50 43 52 79

850

19.28

0.753

0.719

10.4

81

750 820 >800 750 800 800 790

19.36 20.15 20.27 17.66 17.69 18.1 18.53

0.735 0.736 0.704 0.935 0.809 0.91 0.885

0.710 0.71 0.718 0.74 0.735 0.78 0.773

10.1 11d 10.3 12d 10.5 13d 12.7d

82 83 84 94 95 22 23

a

cell performance with a light exposure effect.101 A DSSC using YD2-o-C8 showed an η-value of 7.6%, but there was little difference in device performance upon 90 min light soaking (7.4%). On the other hand, with light soaking an η-value of a DSSC with VC-70 was increased markedly from 5.5% to 7.3% (Fig. 29). They proposed that the loose packing of VC-70 on TiO2 due to the bulky indoline group facilitated cation exchange.

6. Summary and outlook In the last decade, extensive efforts have been devoted to the design and synthesis of porphyrin sensitizers for DSSCs. Consequently, porphyrin sensitizers revealed remarkably high cell performances that are comparable to conventional ruthenium sensitizers such as N3 and N719. Specifically, SM315 and GY50-sensitized solar cells exhibited an η-value of ca. 13%, which outperforms that of ruthenium sensitizer-based DSSCs. These remarkable improvements stem from the rather facile tuning of HOMO−LUMO levels and the intrinsic intense absorption bands of porphyrins. Thus, we believe that porphyrin dyes hold the key to further development of highly efficient DSSCs. To realize practical applications of porphyrinbased DSSCs, the following challenges still remain: (i) further improvement of the light-harvesting ability in the visible and NIR regions, (ii) suppression of porphyrin aggregation on TiO2 to reduce undesirable quenching of the excited singlet state, (iii) optimization of electron injection and inhibition of CR processes at the interface by controlling the adsorption structures on TiO2, (iv) novel redox couples that can increase VOC, and (v) long-term stability under illumination. Further exploration of push–pull structures and/or π-extensions is prerequisite


Estimated from the corresponding IPCE spectra. b With cobaltbased redox electrolyte. c 99.5 mW cm−2. d Corrected on a basis of significant figures.

for enhancing the light-harvesting ability. Bulky substituents such as ortho-alkoxyphenyl group are vital to suppress the dye aggregation as well as CR. Table 2 summarizes the light-harvesting spectral region and photovoltaic performance for the promising porphyrin sensitizers as mentioned in this perspective. It is noteworthy that these porphyrin sensitizers exhibit efficient photocurrent generation up to 750–800 nm. It is estimated that by utilizing ∼80% of the incident solar light up to 900 nm in DSSCs with the I−/I3− redox electrolyte, JSC = 27 mA cm−2 should be expected.102 Assuming VOC = 0.75 V and ff = 0.75, an η-value of 15% can be calculated. Similarly, in DSSCs with a cobalt redox electrolyte, JSC = 22 mA cm−2 should be obtained by utilizing ∼80% of the incident solar light up to 800 nm.102 Given VOC = 0.9 V and ff = 0.75, an η-value of 15% can also be estimated. In this respect, the synthesis of porphyrin sensitizers that efficiently absorb sunlight from the visible to NIR region to improve the light-harvesting ability is an effective approach to achieve higher cell performances. Besides, optimizations of the dye structure and cell fabrication can also increase VOC and ff, leading to an η-value of more than 15%. Therefore, we believe this review would be useful for the rational molecular design and cell fabrications for the further development of highly efficient DSSCs based on porphyrin dyes.

Acknowledgements This work was supported by ALCA, JST. The authors thank Prof. Seigo Ito (University of Hyogo) for the DSSC fabrications and Prof. Villy Sundström (Lund University), Prof. Helge Lemmetyinen,

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and Prof. Nikolai Tkachenko (Tampere University of Technology) for the photophysical measurements.


21

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