Chalcogen heteroatom engineering of solid additives with optimized excited-state properties enables the development of high-performance organic solar cells.

Solid additives (SAs) attract considerable research interest due to their remarkable ability to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). In spite of this, current research on how chalcogen heteroatom engineering of SAs affects the properties of excited states and thereby the performance of OSCs remains relatively scarce. In this work, two simple, halogen-free and low-cost SAs, benzofuran (BFZ) and 2,1,3-benzothiadiazole (BTD), are innovatively selected to conduct a systematic investigation into the influences of chalcogen heteroatom engineering of SAs on the properties of excited states and the performance of OSCs. The femtosecond transient absorption (fs-TA) measurements demonstrate that the BTD-based films exhibit faster hole transfer (HT), longer lifetime and greater quantity of intra-moiety delocalized excited states (i-DEs), as well as less non-radiative recombination mediated by triplet states. Among them, the optimal performance is attained in the BTD-treated devices, yielding a PCE of up to 19.11%, significantly superior to that of the control devices (18.12%). This study not only expands the library of halogen-free SAs but also highlights the significance of chalcogen heteroatom engineering of SAs in regulating the properties of excited states, suppressing triplet-mediated energy loss as well as advancing the performance of OSCs.