Strategy for Improved Photoconversion Efficiency in Thin Photoelectrode Films by Controlling π-Spacer Dihedral Angle

Abstract

Benzo[c][1,2,5]thiadiazole (BT) has been used in dye-sensitized solar cells (DSCs) for its light-harvesting abilities. However, as a strongly electron deficient unit, BT causes rapid back electron transfer (BET), which in turn lowers the photo conversion efficiency (PCE) of devices. Herein, we report a powerful strategy for retarding BET by controlling both the photoelectrode thickness and π-spacer dihedral angle. To achieve this, we introduced planar (BT-T) or twisted π-spacers (BT-P, BT-MP, and BT-HT) between BT units and anchoring groups, and used different photoelectrode thicknesses between 1.8 and 10 μm. Computational and experimental results show that twisted π-spacers were more efficient at retarding BET than the planar π-spacer. However, BET was found to be less important than expected, and light harvesting efficiency (LHE) played a critical role as the thickness of the photoelectrode decreased because charge collection efficiency was enhanced. The planar dye BT-T obtained the highest LHE, this value remained unusually high even in 1.8 μm photoelectrodes. As a result, BT-T gave a PCE of 6.5% (Jsc = 13.56 mA/cm2, Voc = 0.67 V and FF = 0.72) in thin 1.8 μm photoelectrodes with 3.5 μm scattering layers, which represented a roughly 40% enhancement compared to the PCE in 10 μm photoelectrodes (4.76%). Overall, these results provide a novel approach to achieving ultrathin and highly efficient flexible DSCs.