Non-Fused Ring Acceptors Achieving over 15.6% Efficiency Organic Solar Cell by Long Exciton Diffusion Length of Alloy-Like Phase and Vertical Phase Separation Induced by Hole Transport Layer

The construction of high-performance organic solar cells (OSCs) based on non-fused ring electron acceptors (NFREAs) is challenging despite their low synthesis complexity and low cost. Herein, highly efficient NFREA-based OSCs using a ternary approach by blending two in-house designed NFREAs called "C6C4-4Cl" and "BTIC-4F" with the donor PM6 are demonstrated. The two NFREAs with similar molecular skeletons tend to form an alloy-like phase that exhibits long exciton diffusion lengths and improved crystalline properties for efficient charge transfer. The complementary absorption spectra of the ternary system and energy transfer between the two NFREAs increases the current while the high lowest unoccupied molecular orbital (LUMO) energy level of BTIC-4F enhances the voltage of the OSCs. As a result, the ternary OSC with two NFREAs yields an impressive power conversion efficiency (PCE) of 15.62% using 2PACz as a hole transporting layer (HTL). Investigation of the buried interface reveals that 2PACz has a strong interaction with PM6 and induces vertical phase separation in the ternary blend. This work provides an effective strategy to improve the performance of NFREA-based OSCs by increasing the exciton diffusion length in an alloy-like acceptor phase and inducing vertical phase separation with the judicious selection of HTL.