Effect of side chains on solubility and morphology of poly(benzodithiohene-alt-alkylbithiophene) in organic photovoltaics

•According to the type of side chains, solubility and the absorption coefficients of the polymers were enhanced.•The structure−property relationships caused by the difference in side chains of bithiophene were investigated.•PBDTBiTh(2EH) exhibited an increase in Voc, Jsc and power conversion efficiency (PCE) which was double that of PBDTBiTh(12C). It was reported that the side chains play especially an important role in enhancing physical properties and energy levels. Polythiophene based on benzodithiophene has excellent carrier mobility, but high HOMO level. We synthesized polythiophenes, PBDTBiTh(2EH) and PBDTBiTh(12C), were polymerized using the Stille coupling reaction and had thiophene with a 2-ethylhexyl or n-dodecyl side chain. Upon introducing the 2-ethylhexyl side chain, the absorption coefficients of the monomers and polymers were enhanced. Also, the edge-on orientation was fortified and the HOMO level was decreased to −5.37eV. PBDTBiTh(2EH) showed a power conversion efficiency (PCE) of 2.1%, which was double that of PBDTBiTh(12C).