Synthesis of thieno[3,4-d]thiazole-based conjugated polymers and HOMO level tuning for high VOC photovoltaic cell

► Thieno[3,4-d]thiazole was synthesized by a new and effective synthetic method. ► TTz-based copolymers showed the low-lying LUMO energy levels. ► HOMO energy levels of polymers were controlled by electron-donating building unit. ► A PCE of 2.64% with a high Voc of 0.91 V was achieved from PFTTz/PC71BM blend.