A Novel Thiazole-Fused Building Block for π-Conjugated Polymers Enabling Low Voltage Loss in Organic Photovoltaics

A new thiazole-fused π-extended building block, thieno[2'',3'':4',5']thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TTBTz), was designed and synthesized for wide-bandgap polymer donors in nonfullerene acceptors (NFAs)-based organic photovoltaics (OPVs). Incorporation of the TTBTz unit into polymer backbones afforded two donor polymers, PTTBTz and PTTBTzE, bearing alkyl and ester substituents, respectively. Despite the π-extended fused structure, both polymers exhibited excellent solubility and a deep HOMO level. When blended with L8-BO, a benchmark NFA, in binary OPV devices, both polymers achieved high open-circuit voltages (VOC) of 0.95 V and power conversion efficiencies exceeding 12%. Notably, the devices exhibited remarkably low nonradiative voltage losses (ΔVnr = 0.17–0.18 V), which are among the lowest values reported for halogen-free polymer donor systems. These results demonstrate that the TTBTz moiety is an effective molecular design platform for achieving low voltage loss and efficient charge generation in NFA-based OPVs.