The hybrid organic/inorganic halide perovskites, with the formula ABX3, have exhibited a spectacular rise in the field of photovoltaics, with demonstrated power conversion efficiency as high as 22%. However, the susceptibility of these materials to degradation, primarily attributed to their organic cation, has garnered efforts to develop all-inorganic alternatives. In this regard, cesium lead iodide (CsPbI3) has emerged on account of its remarkably similar optoelectronic properties and superior chemical stability. However, the structural instability of the perovskite phase of CsPbI3 is a formidable challenge to overcome. The functional perovskite phase of CsPbI3, also known as the ‘black phase’ (α), is a structurally stable polymorph only at elevated temperature (~320 °C). At room temperature, it adopts a thermodynamically favored nonperovskite ‘yellow’ (δ) allotrope. Herein, using differential scanning calorimetry (DSC), a systematic investigation of the phase transition of CsPbI3 has been performed and the existence of a metastable state has been observed in the absence of moisture, up to a temperature of 100 °C. We have measured a reversible phase change enthalpy of 14.2 ± 0.5 kJ/mol between the ‘black’ and ‘yellow’ phases of CsPbI3. By analyzing the onset temperature and the width of the transition peaks as a function of heat flow rate, a large thermal hysteresis, which plays a critical role in maintaining the metastability in CsPbI3, has been identified. The evidence suggests an explicitly catalytic role of moisture in promoting the black-to-yellow phase transition. Additional calorimetric measurements make it possible to understand the robustness of entropically stabilized doped inorganic perovskites.

Kinetically controlled thermal hysteresis forms the basis of metastability of the perovskite phase of cesium lead iodide

254th National Meeting (2017)

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