Defect chemistry of mixed ionic-electronic conductors under light: halide perovskites as master example

Shining light on a mixed ionic-electronic conductor induces variations in both its electronic and ionic behaviors. While optoelectronic processes in semiconductors with negligible ionic conductivities are well understood, the role of mobile ions in photo-active mixed conductors, such as hybrid halide perovskites, is largely unexplored. Here, we propose a model addressing this problem, relating optoelectronics and optoionics. Using methylammonium lead iodide (MAPI) as model material, we discuss the expected influence of optical bias on the material's charge carrier chemistry under steady-state conditions. We show that changes in the concentration of ionic defects under light with respect to the dark case are a direct consequence of their coupling to electrons and holes through the component chemical potential (here iodine) and the electroneutrality condition. Based on the trend in the quasi-Fermi level splitting in MAPI, we emphasize implications of controlling point defect chemistry for the function and performance optimization of solar energy conversion devices based on halide perovskites. Lastly, we show that in the presence of multiple redox reactions mediating the iodine quasi-equilibrium, either positive or negative changes in the ionic defect pair chemical potential can be obtained. These findings indicate the intriguing possibility to increase or to reduce ionic defect concentrations in mixed conductors through exposure to light.