A series of cross-linked and phosphine-doped cathode interlayers (CILs), namely c-NDI:P₀, c-NDI:P₁, c-NDI:P₂, and c-NDI:P₃, are developed for inverted organic solar cells. We elucidate the relationship between the depletion region width at the heterojunction interface and the electron extraction ability of CILs. By incorporating c-NDI:P₀, a low depletion width of 0.8 nm along with a high power conversion efficiency (PCE) of 17.7% can be obtained in the inverted OSC based on PBDB-TF:BTP-eC9.

Comprehensive Summary

Cathode interlayers (CILs) play an essential role in achieving efficient organic solar cells (OSCs). However, the electronic structure at the electrode/CIL/active layer interfaces and the underlying mechanisms for electron collection remain unclear, which becomes a major obstacle to develop high-performance CILs. Herein, we investigate the relationship of the electron collection abilities of four cross-linked and n-doped CILs (c-NDI:P₀, c-NDI:P₁, c-NDI:P₂, c-NDI:P₃) with their electronic structure of space charge region at heterojunction interface. By accurately calculating the depletion region width according to the barrier height, doping density and permittivity, we put forward that the optimal thickness of CIL should be consistent with the depletion region width to realize the minimum energy loss. As a result, the depletion region width is largely reduced from 13 nm to 0.8 nm at the indium tin oxide (ITO)/c-NDI:P₀ interface, resulting in a decent PCE of 17.7% for the corresponding inverted OSCs.