Phase Transitions And Anion Exchange In All-Inorganic Halide Perovskites

A new generation of semiconducting materials based on metal halide perovskites has recently been launched into the scientific spotlight, exhibiting outstanding optoelectronic properties and providing promise for the development of efficient optical devices. As a vivid example, solar cells made from these materials have quickly reached conversion efficiencies exceeding 25%, now on par with well-established technologies, like silicon. Their widespread success is due, in part, to a unique ability to retain high-quality optoelectronic performance while being easily solution-processed into thin films. This feature is what defines them as a brand-new class of optoelectronic materials, with the ability to compete with traditional semiconductors requiring higher processing costs, like the III-Vs or II-IVs. However, the interesting photophysics of metal halide perovskites come with a catch; their soft ionic lattice promotes complex thermal-induced phase transitions and a high capacity for postsynthetic compositional changes, e.g., halide anion exchange. Such dynamic behavior has ultimately made understanding several important structure-property relationships ambiguous and obstructed a clear path toward commercialization due to inherent phase instability. Our aim in this Account is to highlight the fundamental aspects of metal halide perovskites that dictate a stable crystal structure and enable efficient anion exchange, through the lens of thermodynamic preference and phase formation energies. Taking the all-inorganic CsPbI3-xBrx system as a suitable case study, we focus on several ways in which its thermodynamically unstable perovskite structure can be maintained at room temperature and elucidate the restructuring pathways taken during destabilization. In addition, we will discuss the origin and mechanisms of postsynthetic anion exchange in CsPbX3 (X = I, Br, Cl) perovskites, with emphasis made toward direct visualization using in situ optical microspectroscopy and arriving at quantitative results. For several notable features of halide perovskites dealt with in this Account, e.g., strain stabilization, nonperovskite phase restructuring pathway, and lattice anion diffusion, we attempt to rationalize them using state-of-the-art materials modeling techniques.It is within this spirit that we not only modify a broad range of properties existing within metal halide perovskites but also regulate them for enhanced material functionality. For example, controlling partial phase changes and local replacement of halide composition in CsBX3 (B = Pb, Sn and X = I, Br, Cl) nanowires can facilitate the formation of optoelectronic heterojunctions, due to the abrupt change in local crystal structure and the correlated transition in optoelectronic properties. From this combined perspective, metal halide perovskites appear as highly dynamic systems, whereby structural and compositional modifications have a large impact on the underlying phase stability and optoelectronic properties. Thus, we highlight several scientific aspects important to the fundamental understanding of metal halide perovskites, ranging from the underlying mechanism and kinetics through which phase destabilization and anion exchange take place, to tuning the thermodynamic energy landscape using external stimuli. We anticipate that providing a clear perspective for these topics will help deepen our knowledge of the nature of ionic semiconductors and provide the stimulus required to build new research directions toward utilizing halide perovskites within versatile optoelectronic devices.