Microscopic View of the CsPbI3 Phase Transformation

The continued march of photovoltaic (PV) energy has been supported by the decrease in PV module cost and the simultaneous increase in PV module efficiency. As silicon PV approaches the thermodynamic performance limit, there has been widespread interest in deploying tandem PV using halide perovskites because of the higher performance limit of this device structure, but the stability challenges associated with halide perovskites remain challenging. While halide perovskites span a wide compositional space with their general ABX3 crystal structure, many of these compositions can undergo a phase change as one of their degradation modes. In this study, we investigate the phase change in CsPbI3, a useful model composition, from the perovskite phase to the nonperovskite hexagonal phase. Scanning electron microscopy and standard optical microscopy offer a detailed view of the nucleation and growth of the nonperovskite phase. Specifically, we investigate the nucleation sites as well as the growth of the nonperovskite phase across the domain boundaries seen in spin-cast thin films of CsPbI3. This work provides useful insight for modeling this phase change so that it can be better studied and analyzed with accelerated test protocols, and also should help further work to harden the materials against this degradation mode, ultimately leading to improved halide perovskite solar cell stability.