A Binary Cathode Interlayer Coupling Dipole Regulation and Molecular Orientation Enables 20.1%-Efficient Organic Solar Cells.

The performance of organic solar cells is critically influenced by electron extraction at the cathode interface, which is often limited by suboptimal energy alignment and disordered molecular packing in the cathode interlayer. Here, we demonstrate that synergistic coupling between interfacial dipole engineering and molecular orientation ordering in a binary cathode interlayer comprising an n-type small molecule (NDI-Ph) and an electron-transporting polymer (PNDIT-F3N-Br) simultaneously optimizes energy-level alignment and enhances vertical charge transport. Accordingly, the binary interlayer lowers the cathode work function to 3.23 eV, promotes face-on π-π stacking in the overlying photoactive layer, suppresses trap-assisted recombination, accelerates electron extraction (0.27 µs), and prolongs carrier lifetime (3.38 µs). As a result, single-junction organic solar cells composed of PM6:BTP-eC9 achieve a power conversion efficiency of 20.1%. Notably, the same binary cathode interlayer boosts the performance of devices comprising PM6:L8-BO to 19.5%, outperforming those with single-component interlayers, underscoring its universality across state-of-the-art active layers. This work highlights the role of dipole-ordering coupling as a fundamental design principle for high-performance interfacial layers in organic photovoltaics.