Controlling Defect-Assisted Nonradiative Recombination in Halide Perovskites

Halide perovskites offer impressively high solar conversion efficiencies and are being considered for applications as light emitters. These materials are often called “defect tolerant”, but we show that the impact of point defects on device efficiency has not been properly assessed to date. We have performed comprehensive studies for the prototypical hybrid perovskite MAPbI₃ [MA=(CH₃NH₃)], as well as for other halide perovskites. To achieve accurate and reliable results, our first-principles calculations are based on hybrid density functional theory with spin-orbit coupling included [1]. Rigorous calculations of nonradiative recombination coefficients show the limitations of the widely adopted rule that only defects with charge-state transition levels deep in the band gap can be efficient nonradiative recombination centers. We demonstrate that the position of the level does not directly determine the capture rates, due to strong lattice coupling and anharmonicity in the halide perovskites [2]. Our results clearly show that (1) point defects can indeed be present in relevant concentrations in the halide perovskites and (2) some of these point defects lead to nonradiative recombination rates that are just as high as in conventional semiconductors. We therefore conclude it is incorrect to call the halide perovskites “defect tolerant”. A more relevant distinction, compared to conventional semiconductors, is that halide perovskites with modest defect densities can be grown using low-cost deposition techniques.<br/>For MAPbI₃ 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 [3]. 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 [4]. 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.<br/><br/>Work performed in collaboration with X. Zhang, M. Turiansky, and J.-X. Shen, and supported by DOE.<br/><br/>[1] X. Zhang, M. E. Turiansky, J.-X. Shen, and C. G. Van de Walle, Phys. Rev. B <b>101</b>, 140101 (2020).<br/>[2] X. Zhang, M. E. Turiansky, and C. G. Van de Walle, J. Phys. Chem. C <b>124</b>, 6022 (2020).<br/>[3] X. Zhang, J.-X. Shen, M. E. Turiansky, and C. G. Van de Walle,<br/>Nat. Mater. <b>20</b>, 971 (2021).<br/>[4] X. Zhang, M. E. Turiansky, and C. G. Van de Walle, Cell Rep. Phys. Sci. <b>2</b>, 100604 (2021).