n-Type Self-Doped Cathode Interfacial Layers: A Key Strategy for Realizing Thickness-Insensitive, High-Efficiency Organic Solar Cells

Organic solar cells (OSCs) hold significant promise for low-cost, lightweight, and flexible photovoltaic (PV) devices. However, thicker cathode interfacial layers (CILs) increase charge-transport resistance and recombination losses, reducing power conversion efficiency (PCE). Therefore, developing thickness-insensitive CILs with tailored electronic properties is crucial, and n-type self-doped CILs have emerged as a promising approach to enhance charge extraction, reduce interface losses, and achieve thickness-insensitive OSCs. Building on the operation principles of OSCs, n-type self-doping mechanisms surpass conventional CILs due to the enhanced charge transfer (CT) efficiency, optimized energy-level alignment, and superior interfacial stability. Subsequently, n-type self-doped small-molecule and polymer CILs are systematically classified, and key optimization strategies (molecular design, doping concentration control, interfacial engineering) are highlighted to mitigate thickness-induced efficiency loss. Finally, this review outlines OSC challenges (scalable manufacturing, stability, integration with nonfullerene acceptors) and surveys n-type self-doped CILs that overcome thickness limits, underscoring their large-scale PV potential.