AbstractOrganic–inorganic perovskite solar cells have achieved impressive power conversion efficiency over the past years, yet operational stability remains the key concern. One strategy to improve long‐term stability is to replace the thermally unstable organic with inorganic cations comprising the perovskite lattice. Here, for the first time, pulsed infrared light is used to drive the crystallization of inorganic mixed halide CsPbIxBr(3−x) perovskite films in solar cells with a power conversion efficiency exceeding 10%. By varying the iodide–bromine ratio systematically, it is found that to keep the inorganic perovskite black phase stable at the room temperature, the iodine content needs to be limited to lower than 60% – bromine content higher than 40%. The finding revises previous reports claiming stable compositions with higher iodine contents, which is systematically exploited to reduce the perovskite bandgap with the aim to enlarge the light absorption spectra and thus to boost the device efficiency. It is demonstrated that the newly defined stable compositional range enables devices that retain 90% of the efficiency after stressing the perovskite at 200 °C for 1 h. This result demonstrates that inorganic halide perovskites are stable materials for high‐temperature applications such as concentrated photovoltaics.