Intrinsic and Extrinsic Stability of Formamidinium Lead Bromide Perovskite Solar Cells Yielding High Photovoltage

We report on both the intrinsic and the extrinsic stability of a formamidinium lead bromide [CH(NH₂)₂PbBr₃ = FAPbBr₃] perovskite solar cell that yields a high photovoltage. The fabrication of FAPbBr₃ devices, displaying an outstanding photovoltage of 1.53 V and a power conversion efficiency of over 8%, was realized by modifying the mesoporous TiO₂-FAPbBr₃ interface using lithium treatment. Reasons for improved photovoltaic performance were revealed by a combination of techniques, including photothermal deflection absorption spectroscopy (PDS), transient-photovoltage and charge-extraction analysis, and time-integrated and time-resolved photoluminescence. With lithium-treated TiO₂ films, PDS reveals that the TiO₂-FAPbBr₃ interface exhibits low energetic disorder, and the emission dynamics showed that electron injection from the conduction band of FAPbBr₃ into that of mesoporous TiO₂ is faster than for the untreated scaffold. Moreover, compared to the device with pristine TiO₂, the charge carrier recombination rate within a device based on lithium-treated TiO₂ film is 1 order of magnitude lower. Importantly, the operational stability of perovskites solar cells examined at a maximum power point revealed that the FAPbBr₃ material is intrinsically (under nitrogen) as well as extrinsically (in ambient conditions) stable, as the unsealed devices retained over 95% of the initial efficiency under continuous full sun illumination for 150 h in nitrogen and dry air and 80% in 60% relative humidity (T = ∼60 °C). The demonstration of high photovoltage, a record for FAPbBr₃, together with robust stability renders our work of practical significance.