Defect chemistry of mixed ionic–electronic conductors under light: halide perovskites as a 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 photoactive mixed conductors, such as hybrid halide perovskites, is largely unexplored. Here, we propose a model addressing this problem, combining optoelectronics and optoionics. Using methylammonium lead iodide (MAPI) as a model material, we discuss the expected influence of optical bias on the charge carrier chemistry of mixed conductors 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 (iodine in the case of MAPI) and the electroneutrality condition. Based on the trend in the quasi-Fermi level splitting in the mixed conductor, we emphasize implications of controlling point defect chemistry for the function and performance optimization of solar energy conversion devices, including those based on halide perovskites. Lastly, we show that, in the presence of multiple redox reactions mediating the ionic and electronic 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 reduce ionic defect concentrations in mixed conductors through exposure to light.