Unsymmetrical donor–acceptor–donor–acceptor type indoline based organic semiconductors with benzothiadiazole cores for solution-processed bulk heterojunction solar cells

Bulk heterojunction (BHJ) solar cells based on small molecules have attracted potential attention due to their promise of conveniently defined structures, high absorption coefficients, solution process-ability and easy fabrication. Three D–A–D–A type organic semiconductors (WS-31, WS-32 and WS-52) are synthesized, based on the indoline donor and benzotriazole auxiliary acceptor core, along with either bare thiophene or rigid cyclopentadithiophene as π bridge, rhodanine or carbonocyanidate as end-group. Their HOMO orbitals are delocalized throughout the whole molecules. Whereas the LUMOs are mainly localized on the acceptor part of structure, which reach up to benzothiadiazole, but no distribution on indoline donor. The first excitations for WS-31 and WS-32 are mainly originated by electron transition from HOMO to LUMO level, while for WS-52, partly related to transition between HOMO and LUMO+1 level. The small organic molecules are applied as donor components in bulk heterojunction (BHJ) organic solar cells, using PC61BM as acceptor material to check their photovoltaic performances. The BHJ solar cells based on blended layer of WS-31:PC61BM and WS-32:PC61BM processed with chloroform show overall photoelectric conversion efficiency (PCE) of 0.56% and 1.02%, respectively. WS-32 based BHJ solar cells show a higher current density originated by its relatively larger driving force of photo-induced carrier in photo-active layer to LUMO of PC61BM.