Optimal intermediate adducts regulate low-temperature CsPbI2Br crystallization for efficient inverted all-inorganic perovskite solar cells

The emerging inorganic CsPbI2Br perovskite is regarded as a promising candidate for future photovoltaics due to its superior photo-physical properties and thermal stability, but it usually requires a high fabrication temperature (>250 degrees C) and suffers from possible phase transition from a cubic alpha-phase to a non-perovskite delta-phase under ambient conditions, restricting its commercial applications. Herein, we demonstrated an additive-assisted hot solution spin coating route to simultaneously lower the fabrication temperature and stabilize the cubic phase structure. The incorporated biuret additive, consisting of functional amino and carbonyl groups, can strongly associate with the halide plumbates in the solution and generate optimal intermediate adducts of CsI-biuret-PbX2 to seed the large crystal growth, which can significantly reduce the perovskite formation energy and enable the low-temperature (120 degrees C) fabrication of alpha-phase CsPbI2Br. It was realized that the added biuret was mainly retained at the grain boundaries that effectively passivated antisite or under-coordinated defects, boosting the crystal quality with an improved photoluminescence quantum efficiency. In addition, the biuret molecules can act as anchors among the grains to stabilize the phase structure and prevent moisture permeation. As a result, a champion efficiency of 13.3% with improved ambient air stability was achieved in inverted all-inorganic perovskite solar cells, providing a cost-effective route to prepare high quality inorganic perovskites.