Synergistic high efficiency and low energy loss of all-small-molecule organic solar cells based on benzotriazole-based π-bridge unit

Reducing energy loss (Vloss) is one of the most crucial challenges in organic photovoltaic cells. The Vloss, determined by the differences between the optical band gap (Eg) of the active layer material and the open-circuit voltage (Voc) of the device, is generally alleviated by lowering the energy difference between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) level of the donor (D) and acceptor (A). In this work, we synthesized two A-π-D-π-A-type small-molecule donors (SMDs) SM-benzotriazole (BTz)-1 and SM-BTz-2 by introducing a BTz π-bridge unit and terminal regulation. The BTz π-bridge unit significantly lowers the HOMO energy level of SMDs, resulting in high Voc and high mobility, achieving a balance of low energy loss (<0.5 eV) and high efficiency. Ultimately, the organic solar cells based on SM-BTz-2 as the donor and Y6 as the acceptor obtain a high Voc of 0.91 V, Jsc of 22.8 mA cm−2, fill factor of 68%, and power conversion efficiency (PCE) of 14.12%, which is one of the highest efficiencies based on the SMDs with triazole π-bridges to date. What’s more, the BTz π-bridge unit is a potential unit that can improve mobility and reduce energy loss.