Currently, perovskite solar cells (PSCs) with high performances greater than 20% contain bromine (Br), causing a suboptimal bandgap, and the thermally unstable methylammonium (MA) molecule. Avoiding Br and especially MA can therefore result in more optimal bandgaps and stable perovskites. We show that inorganic cation tuning, using rubidium and cesium, enables highly crystalline formamidinium-based perovskites without Br or MA. On a conventional, planar device architecture, using polymeric interlayers at the electron- and hole-transporting interface, we demonstrate an efficiency of 20.35% (stabilized), one of the highest for MA-free perovskites, with a drastically improved stability reached without the stabilizing influence of mesoporous interlayers. The perovskite is not heated beyond 100°C. Going MA-free is a new direction for perovskites that are inherently stable and compatible with tandems or flexible substrates, which are the main routes commercializing PSCs.

Perovskites have been demonstrated in solar cells with power conversion efficiency well above 20%, which makes them one of the strongest contenders for the next generation photovoltaics. While there are no concerns about their efficiency, very little is known about their stability under illumination and load. Ionic defects and their migration in the perovskite crystal lattice are one of the most alarming sources of degradation, which can potentially prevent the commercialization of perovskite solar cells (PSCs). In this work, we provide direct evidence of electric field-induced ionic defect migration and we isolate their effect on the long-term performance of state-of-the-art devices. Supported by modelling, we demonstrate that ionic defects, migrating on timescales significantly longer (above 10 3 s) than what has so far been explored (from 10 -1 to 10 2 s), abate the initial efficiency by 10-15% after several hours of operation at the maximum power point. Though these losses are not negligible, we prove that the initial efficiency is fully recovered when leaving the device in the dark for a comparable amount of time. We verified this behaviour over several cycles resembling day/night phases, thus probing the stability of PSCs under native working conditions. This unusual behaviour reveals, that research and industrial standards currently in use to assess the performance and the stability of solar cells need to be adjusted for PSCs.Our work paves the way towards much needed new testing protocols and figures of merit specifically designed for PSCs.4

In this study, we use time-resolved Kelvin probe force microscopy to investigate current–voltage hysteresis in a hybrid lead-halide perovskite solar cell.