Achieving High-Efficiency in Binary Organic Solar Cells by the Structural Fine-Tuning of Coumarin-Based Donor

The choice of molecular material plays a crucial role in improving the performance of organic solar cells. Herein, two coumarin-based small molecules are designed and synthesized in which the pi-system is extended by the introduction of double bonds. The molecules are differed by the attachment of central acceptor units. The replacement of the carbonyl group in C3 by the dicyanomethylene unit in C3-CN red-shift the absorption, reduced the energy gap, and stabilized the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. When blended with a complementary absorbing all-fused acceptor F13, the binary devices using C3 donor delivered a power conversion efficiency (PCE) of 11.09%. In contrast, C3-CN:F13-based binary device displayed an impressive PCE of 14.84% due to the obvious increase in all open circuit voltage, short circuit current density, and fill factor values. The inclusion of CN group to the molecular backbone induces a tighter intermolecular packing, which is beneficial for the charge transport properties. The study demonstrates that the subtle modification in conjugation length and tuning of the energy level by the proper choice of acceptor could be a promising way to improve the device's performance.