How to Improve Performance of Porphyrin-Sensitized Solar Cells
Authors: Imahori Hiroshi 
Published: February 2020

Recently, a great deal of attention has been devoted to dye-sensitized solar cells (DSSCs) made from mesoporous TiO2 electrodes owing to the possibility of low-cost production and high power conversion efficiency (PCE). At present ruthenium polypyridyl complexes have demonstrated to be excellent TiO2 sensitizers that have achieved the highest PCE up to 11.5%. However, their practical application would be impeded substantially considering the limited resources and high cost of ruthenium metal. In this regard, organic dyes without metal or with inexpensive metal have been focused as fascinating sensitizers for DSSCs. To attain highly efficient solar energy conversion, such organic dyes should fulfill the following requirements: i) broad light absorption capability that allows us to collect solar light efficiently in visible and near infrared regions, ii) fast electron injection from the excited dyes to a conduction band (CB) of the TiO2 electrode, iii) slow charge recombination between the injected electrons and resulting dye cations and/or I3
– in the electrolyte. So far organic dyes composed of various pi-conjugative molecules have been explored as potential sensitizers for DSSCs.
Porphyrins are one of the most widely studied sensitizers for DSSCs because of their strong Soret (400–450 nm) and moderate Q bands (550–600 nm). More importantly, the optical, electrochemical, and photophysical properties can be modulated by the peripheral substitutions and/or inner metal complexations. Nevertheless, until recently porphyrins as sensitizers typically disclosed poor cell performances compared with ruthenium polypyridyl complexes as a result of the insufficient light-harvesting ability at around 500 nm as well as >600 nm. Taking into account the fact that the integrated value of molar absorption coefficient of porphyrins as a function of wavenumber over the whole spectrum is much larger than the corresponding value of ruthenium polypyridyl complexes, broadening and red-shift of Soret and Q bands in porphyrins are highly promising to surmount the problem.
Recently we have developed aromatic ring-fused, unsymmetrically pi-elongated porphyrins (i.e., naphthalene-fused and quinoxaline-fused porphyrins) that showed broadened, and red-shifted light absorption properties compared to the corresponding porphyrin references, resulting in the PCEs of 4.1% and 6.3%, respectively. As demonstrated for other sensitizers in DSSCs, introduction of both electron-donating and electron-withdrawing substituents to a core of pi system would be appealing for the modulation of the light-harvesting properties of sensitizers.

In this talk I will present our recent advance in porphyrin-sensitized solar cells.1-9 For instance, we have focused on 5,15-diazaporphyrins (DAPs).8 DAPs are a structural hybrid of porphyrins and phthalocyanines. They exhibit relatively intense Soret and Q bands at 350-450 and 500-650 nm, which are suitable for DSSCs. Nevertheless, due to the synthetic difficulty no attempt has been made to synthesize DAP for DSSCs. Carboxyphenylethynyl-substituted diazaporphyrin has been synthesized to assess the utility of diazaporphyrins in dye-sensitized solar cell for the first time. The diazaporphyrin-sensitized TiO2 cell exhibited the photocurrent generation in visible region of 400-700 nm together with a power conversion efficiency of 0.08%. Improvement of the cell performances in DAP may be possible by modulating the HOMO and LUMO levels, binding geometry of the DAP on TiO2 as well as inhibiting the aggregation behavior. I will also talk about other examples of our novel push-pull porphyrins for porphyrin-sensitized solar cells.1-9

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