Nature has utilized chlorophylls in plants as antennae to harvest light for the conversion of solar energy in the photosynthetic processes. In the photosynthetic cores of bacteria and plants, solar energy is collected at chromophores based on porphyrin;[85] the captured radiant energy is converted efficiently to chemical energy. Inspired by the natural photosynthesis, scientists utilized artificial chlorophylls, “the porphyrins” as efficient centers to harvest light for solar cells sensitized with a porphyrin. This efficient energy transfer in naturally occurring photosynthetic reaction centers motivated a lot of researchers to design and synthesize numerous porphyrins for DSSC applications.[30-31, 86-90] Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits inter connected at their α-carbon atoms via methane bridges as shown in Figure 1-13. Porphyrin macrocycles are highly conjugated systems and as a consequence, they typically have very intense absorption bands in the visible region; the name "porphyrin" comes from a Greek word for purple.
Figure 1-13. General structure of porphyrin.
Due to their aromatic structure porphyrins have a similar chemical behavior as simple aromatics. The inherent advantages of porphyrin-based dyes are their rigid molecular
structures with large absorption coefficients in the visible region. Also, their various reaction sites, i.e., four meso and eight β-positions, are available for functionalization through which fine tuning of the optical, physical, electrochemical and photovoltaic properties of porphyrins thus becomes feasible. In principle, the free meso-carbon atoms are more reactive than the β-carbon atoms. Though regular N4 porphyrins are the most efficient and massively studied porphyrin analog, other porphyrinoids such as chlorins, bacteriochlorins, phthalocyanines and subphthalocyanines, corroles and thiaporphyrins have been efficiently used as sensitizers for DSSCs. In this section we reviewed various porphyriniod sensitizers for DSSCs.
1.5.3.1 Chlorins and Bacteriochlorins
The use of chlorins in a DSSC was first reported in 1993 by Kay and Grätzel.[91] In that report a variety of metallo and free-base carboxychlorins were prepared from natural chlorophylls via metallation and/or saponification. The best DSSC reported was with a copper chlorophyll derivative, Cu-2-α-Oxymesoisochlorin e4 as sensitizer (Scheme 1-3), which gave an efficiency value of 2.6%.[91-92] Since this first report, chlorins have been studied in DSSCs using both free-base[93] and zinc metalated[94] forms. Ikegami et al. reported in 2008, the best performance of a DSSC using Chlorin-e6 (Scheme 1-3), η = 4.35% was observed after optimizing the co-adsorbent to avoid molecular aggregate formation between dye molecules.[95] The research by Wang et al. has had a profound impact on advances in chlorin and bacteriochlorin DSSCs,[96-104] starting in 2006 with work on chlorin PPB a in which an η
= 4.2% was achieved using β-carotene as a co-adsorbent (Scheme 1-3).[97] Later PPB a was tested without the presence of the co-adsorbent and compared to other chlorins, bacteriochlorins, and porphyrins; in these tests an η of 3.8% was achieved and PPB a was the best sensitizer tested.[100] Dyes Chlorin 1-4 (Scheme 1-3) have carboxylic acid groups linked to the chlorin macrocycle via an ethylene moiety and gave η values in the range of 6.5%-8.0% in DSSC tests, comparable to N719 which gave η = 9.3% under the same experimental conditions.[104] An improved η value of 7% has been reported for Chlorin 2 compared to Chlorin 1 (η = 6.5%).[102, 104] Chlorin 3 gave an η = 8.0% which is the best η value to date for a chlorin sensitizer.[104] Work on bacteriochlorin sensitizers by Wang et al. has yielded the most efficient bacteriochlorin DSSC to date. The dye BChlorin-1 (Scheme 1-3) uses dialkyl substitution at its second reduced pyrrole ring to increase the stability of the bacteriochlorin skeleton and avoid oxidation to the corresponding chlorin (a general problem with bacteriochlorins). The η value of BChlorin-1 sensitized DSSC was found to be 6.2% and was improved to 6.6% when chenodeoxycholic acid was used as a coadsorbent.[101]
Scheme 1-3. Chlorin and Bacteriochlorin Sensitizers.
1.5.3.2 Porphyrin Sensitizers
Regular porphyrins are the most studied sensitizers among porphyrinoids for the application in DSSCs. Through available four meso and eight β-positions, a variety of porphyrin derivatives were designed, synthesized and applied in DSSCs. Most of the design strategies are based on the mode of attachment of anchor either on β-position or on meso-position of porphyrin ring. Several other variations were also tried, like inserting different metals inside the porphyrin core, Zn/free base porphyrin heterodimers, checking the scope with different anchoring groups.[105-109] In this section we have systematically discussed the mode of attachment of the linker strategy.
a) Attachment through β-position
Modification at β-position with π-extending functional groups can result in red shifts in absorption spectrum and also increase the possibility of electron transfer from the substituent
due to the splitting of the four frontier molecular orbitals. The first example of β-substituted porphyrins was reported by Officer, Grätzel and co-workers in 2004. They reported a series of β-substituted zinc porphyrins, among them Zn-1a (Scheme 1-4) attained promising efficiency of 4.8%.[105] Further they investigated different derivatives, out of which Zn-3 (Scheme 1-4) achieved η = 5.6% in the presence of co-adsorbent chenodeoxycholic acid (CDCA).[110]
Scheme 1-4. Molecular structures of β-substituted porphyrins.
Later in 2007, the same group reported another series of porphyrin sensitizers with the best performer GD2 (Scheme 1-4) obtaining impressive efficiency of η = 7.1% with liquid electrolytes.[111] Kim and co-workers applied this strategy of attaching the π-conjugated linker at β-position of the porphyrin ring to design doubly anchored porphyrin sensitizers. They revealed that zinc porphyrin 2b-bdta-Zn (Scheme 1-4) with double malonic acid linkers effectively enhance the electron injection and retarded charge recombination.[112] The similar group in 2011, reported β-functionalized push-pull zinc poprhyrins with diarylamino donor, tda-2b-bd-Zn (Scheme 1-4) presenting the best performance of η = 7.5% which is comparable with N3 dye (η = 7.7%) under the similar conditions.[113] In 2013, the same group
also reported β-Ethynylbenzoic acid substituted push–pull porphyrins, ZnEP1 and ZnEP2 (Scheme 1-5). Surprisingly ZnEP1 with single anchoring arm (η = 5.9%) performed better than ZnEP2 with η = 4.0%. The overall conversion efficiency of ZnEP1 was comparable with YD1 (η = 6.2%).[114]
Scheme 1-5. Molecular structures of β-substituted porphyrins.
In a report, Pizzotti and co-workers synthesized five β-substituted porphyrins with ethynylphenyl linker and different anchoring acid groups and compared their DSSC performance with meso-substituted porphyrin derivatives.[115] They have shown that, zinc porphyrin 4 (Scheme 5) attained η = 4.6%, while the push-pull zinc porphyrin 5 (Scheme 1-5), achieved overall conversion efficiency of 4.7%. They also stated that the efficiency of 5 is better than the meso derivative (η = 4.2%). Although some of the above mentioned dyes show that β-substituted porphyrins performed better than meso-substituted porphyrins, their progress is limited to the overall conversion efficiency of 7.5%. Also it is noteworthy that the π-conjugation at the β-substituted porphyrins has a narrow effect to extend the absorption spectra to greater wavelengths. These results encourage researchers to change the design of extending the π-conjugation through functionalization the porphyrins at meso-position.
b) Attachment through meso-position
The concept of the meso ethynyl substituted porphyrins was first reported by Anderson[116]
and Therian.[117] The first meso-substituted free base porphyrin for DSSC application was reported in 2000 by Cherain and Wasmer[118] with η = 3.5%. After a long gap, in 2007 Galoppini et al. reported tetrachelated zinc porphyrins with meta-substituted linker on four meso positions to suppress dye aggregation. To solve the porphyrin aggregation, 3,5-di-tert-butylphenyl group were introduced at the meso-positions of the porphyrin ring. Following this concept, Yeh and co-workers designed and synthesized a library of meso- and β-substituted porphyrins with carboxyl anchoring group. Their study revealed that dyes with meso-substituents are better in terms of efficiency than their β-substituent counterparts. They designed dye YD1 (Scheme 1-6), a push-pull porphyrin with a D-π-A skeleton, having diphenylamine as donor group, porphyrin chromophore as a π spacer and 4-ethynylbenzoic acid as acceptor group, to achieve 6.0% efficiency under AM 1.5 G illumination.[119] It is noteworthy that it is comparable with N3 dye (η = 6.1%) under the similar conditions. The superlative performance of YD1 reflects its remarkable short-circuit photocurrent density (Jsc) which arises from the large IPCEs broadly extending beyond 700 nm. The electron donor in YD1 plays a role not only spectrally to extend the absorption to a greater wavelength but also spatially pushing the excited electrons towards TiO2 for an improved separation of charge. Another promising porphyrin YD2 (Scheme 1-6) based on design of YD1 is reported by the same group with the tert-butyl groups were replaced by hexyl chains in the diphenylamine donor. The device performance of YD2 was improved to η = 6.8%. The electron donating nature of the amino substituents in YD1 and YD2 appears to be accountable for their higher open-circuit voltage (Voc).
N