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Halide perovskite solar cells have achieved unprecedented progress in efficiency and high-throughput module manufacturing; however, the stability of perovskite devices remains to be a major obstacle for their commercialization. Among various instability sources in perovskite solar cells, methylammonium (MA) has been identified to limit the thermal stability of perovskites with different band gaps. Formamidinium (FA) ions are frequently used to partially or fully replace MA ions for improved stability. Here, we show that FA ions can be less stable in precursor solution than MA ions. The −CH═NH double bonds of FA ions gradually converted into −CH═O under light in solution despite the fact that this reaction is not obvious in the solid state. The generation of −CH═O is confirmed by the FTIR measurements. FA ions containing perovskite solution stored in the dark shows comparable device efficiency with a fresh …