Role of hydrogen vacancies in efficiency of halide perovskites

Defect-induced nonradiative losses are limiting the performance of hybrid perovskite devices.  We use first-principles calculations based on hybrid density functional theory combined with a rigorous formalism for nonradiative recombination to identify defects that are detrimental to efficiency. For the prototypical hybrid perovskite MAPbI₃ (MA=CH₃NH₃) the results indicate that iodine interstitials are most harmful, and hence iodine-rich synthesis conditions should be avoided. Experimental reports have indicated, however, that iodine-poor conditions are also detrimental. We explain this puzzle by demonstrating that iodine-poor conditions lead to formation of hydrogen vacancies on the MA molecule, which act as very efficient nonradiative recombination centers.  By contrast, hydrogen vacancies are not a problem in FAPbI₃ [FA=CH(NH₂)₂], rationalizing why FA is essential for realizing high efficiency in hybrid perovskites.  Our findings also indicate the advantages of avoiding the organic cation altogether.  We show that the common belief that the organic cation suppresses defect-assisted nonradiative recombination is unfounded.  Our study suggests that all-inorganic halide perovskites hold great promise for high-efficiency optoelectronic applications.