Iodine Exchange and Spontaneous Self-Doping in Iodide Perovskite Semiconductors

The successful incorporation of lead-halide perovskites and lead-free halide double perovskites in various optoelectronic devices has spurred remarkable discoveries and innovation in recent years. Research-scale perovskite-based devices boast efficiencies competitive with more established technologies, including as thin-film solar absorbers and light-emitting diodes. However, despite these impressive efficiencies, areal scale-up and long-term stability continue to lag and present outstanding questions. Indeed, many of the recognized limitations are intrinsic to the perovskite crystal, including thermodynamic phase instability, mobile ions, and an abundance of point defects. These characteristics manifest in many ways in devices, including moisture and temperature sensitivity and current-voltage hysteresis, underscoring the necessity for investigations of the perovskite structure and defect chemistry. <br/> <br/>Here, we highlight the predominant role of halogen vacancies and present our characterization of spontaneous iodine exchange in iodide double perovskites. This unique type of external defect reaction is mostly appreciated for its role in oxide crystals at high temperature; we show that the analagous exchange between iodine vacancies and gaseous iodine is spontaneous at room temperature and directly influences the electronic charge carrier concentration. We provide evidence that the iodine vacancy is a shallow electron donor, leading to striking <i>n</i>-type self-doping effects by manipulating the external pressure of iodine. Furthermore, single-crystal measurements allow for analysis of the thermodynamics of the exchange equilibrium and modeling of the diffusion-limited kinetics associated with transport of vacancies and iodide anions in the bulk crystal. Finally, we introduce complementary <i>in situ</i> X-ray diffraction and single-crystal diffuse X-ray scattering studies to correlate defect chemistry with the structural response at various length scales. A broad and critical evaluation of the mixed ionic-electronic conductivity and defect chemistry of the halide perovskites is discussed in the context of this underappreciated external defect equilibrium.