Low-Band-Gap Small Molecule for Efficient Organic Solar Cells with a Low Energy Loss below 0.6 eV and a High Open-Circuit Voltage of over 0.9 V

Small molecule organic solar cells (SMOSCs) have received considerable attention in recent years. However, one of the key factors limiting the performance of SMOSCs is their large energy loss ( E loss ), which is typically between 0.6 and 1.0 eV, and therefore significantly higher than those of perovskite solar cells and inorganic solar cells ( E loss < 0.5 eV). Herein, we successfully report a new acceptor–donor–acceptor (A–D–A) type dimeric squaraine electron donor ( D-IDTT-SQ ) with a low optical band gap of 1.49 eV and deep HOMO energy level of −5.20 eV. Consequently, a high open-circuit voltage ( V oc ) of 0.93 V with an impressive power conversion efficiency (PCE) of 7.05% is achieved for solution-processed bulk heterojunction SMOSCs, showing an E loss of only 0.56 eV. This is the first report wherein SMOSCs result in such a low E loss, while simultaneously exhibiting a considerably high V oc over 0.9 V and an excellent PCE above 7.0%.