Improving the performance of polymer solar cells by altering polymer side chains and optimizing film morphologies

► The new combination of BDT-RT and TPD units lead to deep-lying HOMO energy levels. ► The polymer solar cells exhibited high Voc values. ► The devices based on branched side chain substituted polymer exhibited much higher photovoltaic performance than linear one. ► The film of branched side chain substituted polymer showed well-defined nanoscale morphology.