All-inorganic halide perovskites have rapidly emerged as promising next-generation low-cost solar cells due to their high photovoltaic performance and enhanced thermal stability compared to the volatile organic-inorganic hybrid perovskites. Among various all-inorganic perovskites, black-phase CsPbI₃ has been mainly studied with a band gap of ~1.73 eV, which is very suitable for photovoltaic applications. However, owing to its poor phase stability, black-phase CsPbI₃ spontaneously transforms to photo-inactive yellow phase (non-perovskite) at room temperature, limiting further development of CsPbI₃ perovskite solar cells. Recently, some progresses have been made through doping, alloying, and strain engineering, suggesting possible directions to improve the structural stability of the black phase. However, the kinetics underlying the black-yellow phase transformation and the effects of external conditions on the kinetic pathways during the phase transformation are not yet well understood.
We perform density functional theory calculation to examine the formation enthalpies of two CsPbI₃ phases (black and yellow phase) as well as phase transformation kinetics between these two with applied external pressures and atmospheric gas molecules. A solid-state nudged elastic band (SS-NEB) method was used to compute the reaction pathways of black-to-yellow phase transformation. Our calculation reveals that formation enthalpies and kinetic energy barriers associated with the black-to-yellow phase transformation are largely dependent on the external pressure. Next, to elucidate the moisture induced phase instability, the effect of H₂O on the phase transformation was examined. The SS-NEB calculations in the presence of a H₂O, H⁺, and OH^– in the system suggest the catalytic role of H₂O and OH^– on the black-to-yellow phase transition with considerably reduced kinetic energy barriers with a H₂O or OH^–. Finally, to understand the role of doping in replacing Pb²⁺ for suppressing the black-to-yellow phase conversion, we computed formation energies and kinetics during the phase transformation with dopants, such as Sn²⁺, Mn²⁺, Sr²⁺, and Bi³⁺. Our calculated results can provide theoretical guidance to improve the phase stability of black CsPbI₃ for the efficient and stable next-generation perovskite solar cells.

Jonathan Lee

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