Dual-Compatible Polarity-switched Small Molecules Enable Auxiliary Charge Generation and Transport Pathways in Organic Solar Cells.

The commercialization of organic solar cells (OSCs) is limited by bottlenecks including relatively high voltage loss, insufficient donor-acceptor interfacial charge generation, and morphological instability. Traditional approaches of interfacial regulation usually struggle with unmatching energetic landscape and/or molecular compatibility. In this study, two small molecules, L8-CT and L8-2CT are designed and synthesized by end-group substitution of the star nonfullerene acceptor L8-BO, gradually switching it from electron acceptor to electron donor. Moreover, L8-2CT with full end-group substitution exhibits exceptional compatibility/miscibility with both donor (PM6) and acceptor (L8-BO) in the ternary blend. The tight molecular packing between L8-2CT and L8-BO also reduces exciton diffusion time by an order of magnitude compared to PM6:L8-BO. The introduction of L8-2CT significantly enhances the donor-acceptor molecular percolation at the interface so that enabling auxiliary charge generation and transport pathways, thereby boosting the interfacial charge generation efficiency and morphology stability. Therefore, the PM6:L8-BO:L8-2CT ternary device achieves a remarkable efficiency of 20.33%, with simultaneously enhanced photostability. This achievement fundamentally challenges the traditional design paradigms for third components in ternary OSCs.