Role of Point Defects in Spinel Mg Chalcogenide Conductors

Close-packed chalcogenide spinels, such as MgSc₂Se₄, MgIn₂S₄, and MgSc₂S₄, show potential as solid electrolytes in Mg batteries, but are affected by non-negligible electronic conductivity, which contributes to self-discharge when used in an electrochemical storage device. Using first-principles calculations, we evaluate the energy of point defects as a function of synthesis conditions and Fermi level to identify the origins of the undesired electronic conductivity. Our results suggest that Mg-vacancies and Mg-metal antisites (where Mg is exchanged with Sc or In) are the dominant point defects that can occur in the systems under consideration. While we find anion-excess conditions and slow cooling to likely create conditions for low electronic conductivity, the spinels are likely to exhibit significant n-type conductivity under anion-poor environments, which are often present during high-temperature synthesis. Finally, we explore extrinsic aliovalent doping to potentially mitigate the electronic conductivity in these chalcogenide spinels. The computational strategy is general and can be easily extended to other solid electrolytes (and electrodes) to aid the optimization of the electronic properties of the corresponding frameworks

Bistable Amphoteric Native Defect Model of Perovskite Photovoltaics

The past few years have witnessed unprecedented rapid improvement of the performance of a new class of photovoltaics based on halide perovskites. This progress has been achieved even though there is no generally accepted mechanism of the operation of these solar cells. Here we present a model based on bistable amphoteric native defects that accounts for all key characteristics of these photovoltaics and explains many idiosyncratic properties of halide perovskites. We show that a transformation between donor-like and acceptor-like configurations leads to a resonant interaction between amphoteric defects and free charge carriers. This interaction, combined with the charge transfer from the perovskite to the electron and hole transporting layers results in the formation of a dynamic <i>n-i-p</i> junction whose photovoltaic parameters are determined by the perovskite absorber. The model provides a unified explanation for the outstanding properties of the perovskite photovoltaics, including hysteresis of <i>J–V</i> characteristics and ultraviolet light-induced degradation