Explore fused-ring core incorporated A-π-D-π-A type acceptors and their application in organic solar cells: Insight into molecular conformation, optical and electrochemical properties, film morphology, and energy loss

Acceptor-π-Donor-π-Acceptor (A-π-D-π-A) type fused-ring electron acceptors (FREAs) are a class of important nonfullerene acceptors for fabricating high efficiency organic solar cells (OSCs). However, limited by the rare types of the central D core, the efficiencies of the OSCs based on A-π-D-π-A FREAs are falling behind. In this work, dithienopyrrolobenzothiadiazole (DTPBT) was employed in developing A-π-D-π-A type FREAs, and was assessed as a D core. DTPBT based FREAs were synthesized and compared with the typical A-π-D-π-A type FREAs using IDT as the D core, from aspects of molecular conformation, optical and electrochemical properties, blend film morphology, and energy loss. Compared to traditional IDT based A-π-D-π-A type FREAs (IRIC-4F), A-π-D-π-A FREAs containing DTPBT (BTOT-OD-4F) showed C-type molecular conformation, redshifted absorption and downshifted energy levels. As a result, the corresponding devices achieved an efficiency of over 13% with improved J_SC of 23.44 mA cm⁻² and a fair FF of 66.9%. Meanwhile, the energy loss of the PBDB-T: BTOT-OD-4F blends reduced effectively, due to the contribution of suppressed nonradiative recombination. Above all, DTPBT has presented its potential in developing highly efficient A-π-D-π-A type FREAs. To realize the best performance of the novel acceptors, further systematic and synergetic optimization of the molecule structure and device fabrication are needed. This work has demonstrated a new design manner of A-π-D-π-A type FREAs through exploring novel fused-ring cores, which facilitates reduced energy loss of the corresponding devices.