Inverted planar heterojunction perovskite solar cells with high ultraviolet stability

Long-term stability remains a challenge for commercializing metal halide perovskites in photovoltaic (PV) applications. Ultraviolet-induced degradation (UV-ID) has been considered as one of the key stability issues for crystalline silicon PVs. However, limited studies have been performed on the long-term UV-ID of perovskite PVs, and none has been found related to operational stability under UV stressing. Here, we focus on UV-ID of inverted perovskite solar cells that comprise nickel oxide (NiO) as the hole transporting layer. Under continuous UV irradiation, we observe vacancies/voids generated in the vicinity of NiO/perovskite heterojunction. Time-resolved femtosecond transient absorption and double-ion injection current measurements indicate UV irradiation would induce interface shallow traps and severe ion migration. These UV-induced degradations can be greatly suppressed by modifying the heterojunction with a self-assembled monolayer [2-(9 H-Carbazol-9-yl)ethyl] phosphonic acid (2PACz). Unencapsulated NiO/2PACz devices, operated at maximum power point (MPP), retain over 90% of their initial efficiency after exposing to a total UV dose of 35 kWh/m2. The best-performing device reaches a stabilized efficiency of 22.2%, and retains 82% of its original efficiency after 2000 h of MPP tracking under one sun illumination (100 mW/cm2) at 45 ℃ in ambient air when encapsulated.