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

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 C71 butyric acid methyl ester (PC71BM) 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 cm2 active area, along with a maximum external quantum efficiency (EQE) of 63% in the red region.