Synthesis and photovoltaic properties of dithienosilole-based copolymers

Two novel narrow bandgap random dithienosilole copolymers based on 3,3′-di-n-octylsilylene-2,2′-bithiophene (DTS), ethylene (E), 4,7-bis(-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DTBT), or 6,7-diphenyl-4,9-bis(4,4′-dihexyl-2,2′-bithiophen-5-yl)-[1,2,5]-thiadiazolo[3,4-g]quinoxaline (PhDTTQ) have been synthesized by means of Pd-catalyzed Stille coupling reactions. The electrochemical, optical and photovoltaic properties of the copolymers were studied. The absorption spectrum of the copolymer PDTSE-DTBTE in thin film has one absorption peak at 668 nm, and the absorption spectrum of the copolymer PDTSE-PhDTTQ displays two absorption peaks at about 594 and 841 nm. The bulk heterojunction solar cells were fabricated with the as-synthesized copolymers as the donors and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor in a 1:1 weight ratio in the device configuration ITO/PEODT/copolymer:PC61BM (1:1)/Al. The power conversion efficiency of the device based on copolymer PDTSE-DTBTE is 0.72 % with open-circuit voltage (V _oc) 0.5 V under simulated AM 1.5 G solar irradiation (100 mW/cm²).