Molecular engineering of boryl oxasmaragdyrins through peripheral modification: structure-efficiency relationship.

Abstract

Expanded porphyrins with the absorption profile down to the infrared region through increased π-conjugation are suitable candidates for a low energy sensitizer. Oxasmaragdyrin boron complexes, a class of aromatic-core-modified expanded porphyrin with 22 π-electrons, have been recently utilized as an efficient low energy sensitizer in dye-sensitized solar cells. In this paper, we have prepared a series of eight novel boryl oxasmaragdyrins through molecular engineering on the periphery and their overall photovoltaic performances in dye-sensitized solar cells are evaluated. With the help of photophysical, electrochemical, and photovoltaic studies, it is revealed that molecular structure, especially the number and position of the donor-acceptor groups play a pivotal role in their photovoltaic performance. Presence of the two well-separated split Soret bands in the 400-500 nm region of UV/Vis spectrum ensures broader coverage of absorption wavelengths. Even though the two-anchoring-group dyes (SM5-SM8) bind strongly to TiO2 compared to one-anchoring-group dyes (SM1-SM4), the latter have superior photovoltaic performance than the former. Dye SM1, with two hexyloxyphenyl donors and one carboxylic acid anchor showed the best overall conversion efficiency of 4.36% (JSC = 10.91 mA cm(-2); VOC = 0.59 V; FF = 0.68). This effective modulation of photovoltaic performance through structural engineering of the dyes will serve as a guideline for the future design of efficient low energy light-harvesting sensitizers.