The rapid increase in photoconversion efficiency (PCE) of halide perovskites cannot be separated from the degradations that alter its scaling-up to industrialization. Many instability sources need to be addressed, such as the presence of a complex sequence of phase transitions, the instability to high temperatures, the reactivity with ambient molecules. Other degradations, leading to the PCE decrease in the cell, such as the halide segregation when exposed to light sources, should be also looked at. Although the encapsulation or use of additives could help in the reduction of instability, the most pertinent pathway remains the stabilization of the intrinsic properties of perovskites. Inorganic perovskites, that reduce instabilities with ambient molecules with respect to the organic ones, should be prioritized. The stabilization strategy is focused around three main criteria: i) the stabilization of the rich sequence of phase transitions through the annihilation of soft modes, that imply structural distortions and a symmetry reduction; ii) the passivation of local (vacancies, interstitials); and iii) of more extended defects (surfaces, interfaces, etc…).

This work focuses on the optimization of the chemical composition of the complex inorganic halide perovskites AA’BB’(XX’)₃, using Density Functional Theory, in order to investigate the required conditions for a stable and high-performance inorganic perovskite. A unique home-made hybrid exchange-correlation functional is used to reproduce efficiently their structural, electronic, optic and dynamic properties, with accuracy with respect to existing experimental data. We illustrate how this approach is used to define chemical compositions, allowing to decrease the different lattice distortions by annihilating the unstable phonon modes.

Point defects are at the origin of recombination centers (causing efficiency losses), and their rate and/or their effect should thus be diminished. Although low-energy defects do not form deep traps in lead halide perovskites, we show that tuning the composition can affect both the defect formation energies and the depth of charge state energy levels. Alloying at B and X-sites can lead to significant changes in structural and electronic parameters, which can strongly increase the concentration of defects and favor the migration of halogen vacancies.

Finally, the data previously obtained, completed with other parameters (including band alignment, refractive indices…) is used as input for drift-diffusion models to predict the performance of the perovskite solar cell, with optimal material choices and layer thicknesses. We show that considering the most stable structural phase is essential to avoid inaccurate device‑performance estimations.