Inverted Polymer Solar Cells with Annealing‐Free Solution‐Processable NiO

Nickel oxide (NiO) offers intrinsic p‐type behavior and high thermal and chemical stability, making it promising as a hole transport layer (HTL) material in inverted organic solar cells. However, its use in this application has been rare because of a wettability problem caused by use of water as base solvent and high‐temperature annealing requirements. In the present work, an annealing‐free solution‐processable method for NiO deposition is developed and applied in both conventional and inverted non‐fullerene polymer solar cells. To overcome the wettability problem, the typical DI water solvent is replaced with a mixed solvent of DI water and isopropyl alcohol with a small amount of 2‐butanol additive. This allows a NiO nanoparticle suspension (s‐NiO) to be deposited on a hydrophobic active layer surface. An inverted non‐fullerene solar cell based on a blend of p‐type polymer PTB7‐Th and non‐fullerene acceptor IEICO‐4F exhibits the high efficiency of 11.23% with an s‐NiO HTL, comparable to the efficiency of an inverted solar cell with a MoOx HTL deposited by thermal evaporation. Conventionally structured devices including this s‐NiO layer show efficiency comparable to that of a conventional device with a PEDOT:PSS HTL. Annealing‐free solution‐processable NiO is developed and applied in inverted polymer solar cells based on non‐fullerene system PTB7‐Th:IEICO‐4F. The inverted solar cells with annealing‐free NiO exhibit equivalence efficiency and better stability without high‐temperature annealing compared to the solar cells with the MoOx hole transport layer fabricated by thermal evaporation.