Effects of the Molecular Weight and the Side‐Chain Length on the Photovoltaic Performance of Dithienosilole/Thienopyrrolodione Copolymers

Abstract

Abstract A series of low‐bandgap alternating copolymers of dithienosilole and thienopyrrolodione (PDTSTPDs) are prepared to investigate the effects of the polymer molecular weight and the alkyl chain length of the thienopyrrole‐4,6‐dione (TPD) unit on the photovoltaic performance. High‐molecular‐weight PDTSTPD leads to a higher hole mobility, lower device series resistance, a larger fill factor, and a higher photocurrent in PDTSTPD:[6,6]‐phenyl C 71 butyric acid methyl ester (PC 71 BM) bulk‐heterojunction solar cells. Different side‐chain lengths show a significant impact on the interchain packing between polymers and affect the blend film morphology due to different solubilities. A high power conversion efficiency of 7.5% is achieved for a solar cell with a 1.0 cm 2 active area, along with a maximum external quantum efficiency (EQE) of 63% in the red region.