A crystalline low-bandgap polymer comprising dithienosilole and thieno[3,4-c]pyrrole-4,6-dione units for bulk heterojunction solar cells

We have used Stille coupling polymerization to synthesize a new low-bandgap conjugated polymer, PDTSTPD, that consists of an electron-rich dithieno[3,2-b:2′,3′-d]-silole (DTS) unit and an electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety. The polymer exhibited an excellent thermal stability, crystalline characteristics, a broad spectral absorption, and a deep highest occupied molecular orbital (HOMO) energy level, resulting from combination of the rigid TPD and the coplanar DTS units in the polymer backbone. Moreover, the presence of the silicon atoms along the polymer chain ensured PDTSTPD having strong interchain stacking and good hole mobility. An optimal device incorporating the PDTSTPD:PC71BM blend at a weight ratio of 1:1 provided a power conversion efficiency of 3.42%.