Ladder-Type Nonacyclic Structure Consisting of Alternate Thiophene and Benzene Units for Efficient Conventional and Inverted Organic Photovoltaics

A ladder-type nonacyclic thienyl-phenylene-thienylene-phenylene-thienyl TPTPT unit, consisting of alternate interfused thiophene and benzene units, is designed and synthesized. This multifused distannyl-TPTPT monomer was polymerized with two electron-deficient acceptors, 4,7-dibromo-2,1,3-benzothiadiazole BT and 5,8-dibromo-2,3-diphenylquinoxaline QX monomers, by Stile coupling reaction to afford two alternating donor acceptor copolymers, PTPTPTBT and PTPTPTQX, respectively. Because of the covalent planarization of the conjugated framework, PTPTPTBT simultaneously possess excellent solubilities for solution-processability, low bandgaps with suitable position of HOMO/LUMO energy levels, and high hole mobilities. The devices based on the PTPTPTBT/PC71BM blend not only showed a promising PCE of 5.3% with conventional configuration but also achieved a high PCE of 5.9% with inverted configuration. This value is among the highest performance from the inverted solar cells incorporating a donor acceptor low bandgap polymer.