Versatile Device Architectures for High-Performing Light-Soaking-Free Inverted Polymer Solar Cells.

Metal oxide charge transport layers have been widely employed to prepare inverted polymer solar cells with high efficiency and long lifetime. However, the intrinsic defects in the metal oxide layers, especially those prepared from low-temperature routes, overshadow the high efficiency that can be achieved and also introduce "light-soaking" issues to these devices. In this work, we have employed polyethyleneimine (PEI) and poly(9,9-bis(6'-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) to modify our low-temperature-processed TiO2 electron transport layer (ETL) and demonstrated that the light-soaking issue can be effectively eliminated by PEI modifications because of the formation of abundant dipole moments, whereas PFN-OX was ineffective as a result of deficient dipole moments at the interface. Excitingly, PEI modifications enable versatile device architectures to obtain light-soaking-free, inverted PTB7-Th:PC71BM solar cells with efficiencies of over 10%, by adding PEI either in the bulk or as an adjacent layer below or above the TiO2 ETL.