Alkylthio-substitution on wide bandgap conjugated polymers for non-fullerene acceptor-based organic solar cells

We investigated three conjugated polymers, PSBDB-T, P1SBDB-TS, and PSBDB-TS, having different numbers of alkylthio groups. The polymers showed a wide bandgap of 1.76-1.81 eV with broad absorption range from 300-700 nm and were used as donor polymers in organic photovoltaic (OPV) devices with the non-fullerene acceptor, IT-M. By increasing the number of alkythio groups in their chemical structures, the HOMO levels of the polymers down-shifted as-5.46 eV, -5.51 eV and -5.56 eV, and consequently, the VOC of the OPV cells gradually increased as 0.91 V, 0.92 V and 0.94 V for PSBDB-T, P1SBDB-TS, and PSBDB-TS, respectively. P1SBDBTS having three alkylthio groups (two for benzo[1,2-b:4,5-b']dithiophene and one for benzo[1,2-c:4,5-c']dithiophene-4,8-dione unit) showed the best power conversion efficiency of 6.88% among three polymers due to its high crystallinity and low bimolecular recombination in bulk heterojunction system.