Increasing the Phase Stability of CsPbI3 Nanocrystals by Zn2+ and Cd2+ Addition: Synergy of Transmission Electron Microscopy and Molecular Dynamics.

Metal halide perovskites (MHPs) are emerging as promising materials for optoelectronic and photovoltaic applications due to their favorable electronic properties, including a tunable bandgap. However, achieving high stability for these materials remains a critical challenge, particularly for CsPbI3, whose photoactive phases spontaneously convert into a nonphotoactive yellow orthorhombic δ-phase under ambient conditions. This transformation results in a significant increase in bandgap and a loss of photoactive functionality. In this study, we investigate the impact of Zn2+ and Cd2+ dopants on the phase stability of CsPbI3 nanocrystals (NCs), emphasizing the formation of Ruddlesden-Popper (RP) planar defects, which are frequently observed during compositional tuning. Using transmission electron microscopy (TEM), we follow the temporal evolution of the phase transformation, where black-phase NCs agglomerate and form elongated microtubes with a yellow-phase crystal structure. Our observations demonstrate that doped samples are significantly more stable, while the dopants are key factors in the formation of the RP-like defects with specific atomic arrangements. Using a combination of quantitative TEM and molecular dynamics (MD) simulations we characterize the structure and composition of as-found RP-like defects and elucidate their role in stabilizing the photoactive phases of CsPbI3 through decreased phase transition kinetics.