Enhanced and controllable open-circuit voltage using 2D-conjugated benzodithiophene (BDT) homopolymers by alkylthio substitution

In this study, we explore the effects of alkylthiophene (T) and alkylthiothiophene (T-S) substituents on the benzo[1,2-b; 4,5-b']dithiophene (BDT) unit by comparing the BDTT homopolymer (PBDTT), the BDTT-alt-BDTT-S copolymer (PBDTT-BDTT-S), and the BDTT-S homopolymer (PBDTT-S) in terms of UV-visible absorption spectra, cyclic voltammetry (CV) results, computational calculations, and experimental results. The T-S substituent increased the hole mobility of the polymer and down-shifted the highest occupied molecular orbital (HOMO) energy level of the polymer, leading to slight red-shifting of the absorption spectrum. The organic photovoltaic (OPV) cells based on PBDTT-S as a donor and [6,6]-phenyl-C-71-butylic acid methyl ester (PC71BM) as an acceptor demonstrated a high power conversion efficiency (PCE) of 7.05% under AM 1.5G illumination (100 mW cm(-2)). To the best of our knowledge, this PCE value is one of the highest values reported for homopolymer donor-based OPVs. Compared to the well-known P3HT homopolymer, which shows a similar absorption profile, PBDTT-S is a promising candidate for organic photodiodes.