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 (Voc) 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 45, while simultaneously exhibiting a considerably high V-oc over 0.9 V and an excellent PCE above 7.0%.