Which one is more favorable in halide perovskites, A-site migration or X-site?

Halide perovskites have emerged as promising materials in the field of optoelectronics, offering significant advantages over traditional semiconductors. Their structural versatility and remarkable optoelectronic properties have generated widespread interest for applications spanning from photovoltaics to light-emitting devices. However, the issue of fast ion migration in the materials, inducing operational instability problem, has not been well understood yet. Herein, we investigated degree of ease and pathways of ion migration in CsPbBr3 using density functional theory calculations, examining both defect formation energies and migration barriers. Our findings indicate that Cs and Br are the most diffusing ions with the assistance of corresponding vacancies (VCs and VBr) under various phases, exhibiting different activation energies. The Fermi level influences both defect formation energies and migration barriers, thereby changing the migration activation energies. Nonetheless, whether it changes the predominant migrating ion species depends on the phase. Our study elucidates the ongoing debate about the dominant transporting species between A-site vs X-site ions proposed by different research groups, providing valuable insights into identifying ways of mitigating the ion migration effect and improving perovskite-based devices.