Effects of Bulky Substituents of Push-Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells

To evaluate the effects of substituent bulkiness around a porphyrin core on the photovoltaic properties of porphyrin-sensitized solar cells, long alkoxy groups were introduced at the meso-phenyl group (ZnPBAT-o-C8) and the anchoring group (ZnPBAT-o-C8Cn, n = 4, 8) of an asymmetrically substituted push-pull porphyrin with double electron-donating diarylamino groups and a single electron-withdrawing carboxyphenylethynyl anchoring group. The spectroscopic and electrochemical properties of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn were found to be superior to those of a push pull porphyrin reference (YD2-o-C8), demonstrating their excellent light-harvesting and redox properties for dye-sensitized solar cells. A power conversion efficiency (eta) of the ZnPBAT-o-C8-sensitized solar cell (eta = 9.1%) is higher than that of the YD2-o-C8-sensitized solar cell (eta = 8.6%) using iodine-based electrolyte due to the enhanced light-harvesting ability of ZnPBAT-o-C8. In contrast, the solar cells based on ZnPBAT-o-C8Cn, possessing the additional alkoxy chains in the anchoring group, revealed the lower eta values of 7.3% (n = 4) and 7.0% (n = 8). Although ZnPBAT-o-C8Cn exhibited higher resistance at the TiO2 dye electrolyte interface by virtue of the extra alkoxy chains, the reduced amount of the porphyrins on TiO2 by excessive addition of coadsorbent chenodeoxycholic acid (CDCA) for mitigating the aggregation on TiO2 resulted in the low eta values: Meanwhile, the ZnPBAT-o-C8-sensitized solar cell showed the lower eta value of 8.1% than the YD2-o-C8-sensitized solar cell (eta = 9.8%) using cobalt-based electrolyte. The smaller eta value of the ZnPBAT-o-C8-sensitized solar cell may be attributed to the insufficient blocking effect of the bulky substituents of ZnPBAT-o-C8 under the cobalt-based electrolyte conditions. Overall, the alkoxy chain length and substitution position around the porphyrin core are important factors to affect the cell performance.