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(NH)PbBr = FAPbBr] perovskite solar cell that yields a high photovoltage. The fabrication of FAPbBr devices, displaying an outstanding photovoltage of 1.53 V and a power conversion efficiency of over 8%, was realized by modifying the mesoporous TiO-FAPbBr 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 TiO films, PDS reveals that the TiO-FAPbBr interface exhibits low energetic disorder, and the emission dynamics showed that electron injection from the conduction band of FAPbBr into that of mesoporous TiO is faster than for the untreated scaffold. Moreover, compared to the device with pristine TiO, the charge carrier recombination rate within a device based on lithium-treated TiO film is 1 order of magnitude lower. Importantly, the operational stability of perovskites solar cells examined at a maximum power point revealed that the FAPbBr 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 FAPbBr, together with robust stability renders our work of practical significance.