Synthesis and Characterization of Dialkylthio Side Chains Modified Benzo1,2-b:4,5-b'dithiophene-Based Conjugated Polymer for Organic Solar Cells

摘要

In this paper, two conjugated polymers named PBDT-OT-BT and PBDT-DST-BT, derived from 4,8-bis(4,5-dioctylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDT-OT) or 4,8-bis(4,5-bis(octylthio)thiophene-2-yl)benzo[1,2-b: 4,5-b']dithiophene (BDT-DST) as the electron-donating unit and benzo[c][1,2,5]thiadiazole (BT) as electron-withdrawing unit were synthesized and characterized. It was found that compared to the alkyl side chain substituted polymer PBDT-OT-BT, the two alkylthio side chain modified polymer PBDT-DST-BT exhibited lower highest occupied molecular orbital (HOMO) energy level, weaker crystallinity characteristic, and higher dielectric constant (εr) at 1 kHz. However, PBDT-DST-BT also presented relatively poor miscibility with Y6 in comparison with PBDT-OT-BT, which had a negative effect on the bulk heterojunction morphology and hindered the exciton dissociation and carrier collection. Eventually, the PBDT-DST-BT:Y6 based polymer solar cells (PSCs) achieved a power conversion efficiency (PCE) of 2.27% much lower than that of the PBDT-OT-BT:Y6 based device (PCE = 9.46%). These results would provide a deep understanding of the influence of alkylthio side groups on the properties of polymer donors in PSCs.