Li-Doped ZnO Electron Transport Layer for Improved Performance and Photostability of Organic Solar Cells

Organic solar cells (OSCs) based on an inverted architecture generally have better stability compared to those based on a standard architecture. However, the photoactive area of the inverted solar cells increases under ultraviolet (UV) or solar illuminatiom because of the too-high conductivity of the UV-illuminated zinc oxide (ZnO) interlayer. This limits the potential of the inverted solar cells for industrial applications. Herein, lithium-doped ZnO (Li-ZnO) films are employed as the cathode interlayer to construct inverted OSCs. The incorporation of Li ions is found to reduce the lateral conductivity of the UV-treated ZnO films because of the presence of Li ions, preventing the high-quality-growth of ZnO nanocrystals. This addresses the problem of having too-high conductivity in the UV-treated ZnO layer, causing the increased photoactive area of inverted solar cells. The overall performance of the solar cell is shown to be higher after the incorporation of Li ions in the ZnO layer, mainly due to the increased fill factor (FF), originating from the reduced trap-assisted recombination losses. Finally, the inverted solar cells based on the Li-ZnO interlayer are demonstrated to have a much better long-term stability, as compared to those based on ZnO. This allows the ZnO-based interlayers to be used for the mass production of organic solar cell modules.