Multiple Roles of a Non-fullerene Acceptor Contribute Synergistically for High-Efficiency Ternary Organic Photovoltaics

Ternary structure is an important design strategy to obtain high-efficiency non-fullerene organic photovoltaics (OPVs). However, the role of the third component to the standard binary system is still unclear. Here, a wide-bandgap small-molecule acceptor, denoted IDT-T, is synthesized and used together with a wide-bandgap donor polymer, PBDB-T, and a low-bandgap acceptor, ITIC, for fullerene-free ternary solar cells. The ternary cell features an enhanced power conversion efficiency (PCE) up to 12.2%, together with improved photocurrent density, open-circuit voltage (VOC), and fill factor. Studies of the thin films indicate that IDT-T functions as an energy-level mediator, a fluorescence resonance energy-transfer donor, an electron acceptor, and a crystallization modulator in the blend, which contribute synergistically in the ternary blend to deliver a higher VOC, more efficient exciton generation, suppressed bimolecular charge recombination and enhanced charge transport, and an overall high photovoltaic performance. [Display omitted] •A wide-bandgap non-fullerene acceptor, IDT-T, is employed in a ternary solar cell•The ternary solar cell delivers a PCE of 12.2% with enhanced device characteristics•IDT-T transfers energy to and mediates the crystallization of another acceptor ITIC•Multiple roles of IDT-T contribute synergistically to yield high device performance Very recently, non-fullerene acceptors (NFAs) based on low-bandgap small molecules have emerged as a new class of acceptors that rival the dominance of fullerene-based acceptors. Such discovery also stimulates promising device architectures such as ternary solar cells, with a handful that have achieved high power conversion efficiencies above 12%. The primary effort, however, has been focusing on low-bandgap NFAs that exploit complementary absorption and energy-level cascade. Herein we report a rare example of a wide-bandgap NFA that leads to high-performance ternary solar cells without relying on full absorption complementarity of all three components. Detailed studies revealed the multiple roles of this acceptor in blend films, which contribute synergistically to improved device characteristics. This work may inspire new design principles of potent wide-bandgap NFAs, which will open the door to high-efficiency organic photovoltaic devices through new opportunities such as multi-component solar cells. The marriage of non-fullerene acceptors (NFAs) and ternary solar cell architecture has brought abo