A Naphthodithiophene-Based Nonfullerene Acceptor for High-Performance Polymer Solar Cells with a Small Energy Loss

Introducing intramolecular noncovalent interactions is an effective method for enlarging the conjugated area of the donor core in nonfullerene acceptors (NFAs) to improve the photovoltaic performance. In this study, a novel NFA (NTO-4F) based on a para-substituted naphtho[1,2-b:5,6-b′]dithiophene core with alkoxy side chains was designed and synthesized, which has a featured oxygen atom at the side chains to facilitate an intramolecular noncovalent S–O interaction with thiophene at the core. NTO-4F exhibits strong absorption in the 600 to 800 nm region with a narrow optical band gap of 1.55 eV, suitable energy levels with a high-lying lowest unoccupied molecular orbital (LUMO) of −3.88 eV, and appropriate crystallinity. As a result, when NTO-4F is blended with the wide bandgap polymer, PM6, as the active layer, the optimized polymer solar cells (PSCs) achieve a power conversion efficiency (PCE) of 11.5% with simultaneously high open-circuit voltage (Voc) of 0.99 V and short-circuit current density (Jsc) of 19.1 mA cm−2, attributing to the low energy loss of 0.56 eV. Furthermore, PM6:NTO-4F-based devices exhibit excellent tolerance to thermal annealing (TA) treatment with over 10% PCEs for all devices under TA treatment from 140 to 200 °C and over 80% of the initial PCE after thermal treatment at 180 °C for 45 min.