Suppressing Halide Segregation in Wide-Bandgap Perovskite Absorbers by Transamination of Formamidinium.

All-perovskite tandem solar cells are emerging at a fast rate because of their potential to exceed efficiencies of Si-perovskite tandems, in combination with faster manufacturing, lower cost, and the ability to be processed on flexible substrates. Mixing halides is a key to achieve wide-bandgap absorbers, which however suffer from halide segregation under illumination, resulting in lowering of the bandgap. To tackle this problem, butylamine (BA) has been added to the perovskite precursor solution and is found to react with the formamidinium (FA) cation, producing N-butylformamidinium (BuFA), which accumulates at the perovskite surface and grain boundaries. The creation of the BuFA cation results in suppressed halide segregation and improved crystallization. Density functional theory calculations propose the reduction of halide defect formation upon the addition of BA, being a key to stabilize mixed-halide perovskites. Lastly, we observe a more stable performance of single junction p-i-n perovskite solar cells with the addition of BA under constant illumination at 65 °C.