Influence of solvent and bridge structure in alkylthio-substituted triphenylamine dyes on the photovoltaic properties of dye-sensitized solar cells

Three new triphenylamine dyes that contain alkylthio-substituted thiophenes with a low bandgap as a π-conjugated bridge unit were designed and synthesized for organic dye-sensitized solar cells (DSSCs). The effects of the structural differences in terms of the position, number, and shape of the alkylthio substituents in the thiophene bridge on the photophysical properties of the dye and the photovoltaic performance of the DSSC were investigated. The introduction of an alkylthio substituent at the 3-position of thiophene led to a decrease in the degree of redshift and the value of the molar extinction coefficient of the charge-transfer band, and the substituent with a bridged structure led to a larger redshift than that of the open-chain structure. The introduction of bulky and hydrophobic side chains decreased the short-circuit photocurrent (J _sc), which was caused by the reduced amount of dye adsorbed on TiO ₂. This resulted in a decrease in the overall conversion efficiency (η), even though it could improve the open-circuit voltage (V _oc) due to the retardation of charge recombination. Furthermore, the change in solvents for TiO ₂ sensitization had a critical effect on the performance of the resulting DSSCs due to the different amounts of dye adsorbed. Based on the optimized dye bath and molecular structure, the ethylene dithio-substituted dye (ATT3) showed a prominent solar-to-electricity conversion efficiency of 5.20%.