Defect Tolerance in Halide Perovskites—A First-Principles Perspective

In recent years, the impressive photovoltaic performance of halide perovskites has been commonly attributed to their defect tolerance. This attribution seems intuitive and has been widely promoted in the field, but it has not been rigorously assessed. I will critically discuss the proposition of defect tolerance in halide perovskites based on state-of-the-art first-principles calculations. Rigorous assessments of defect formation energies and recombination rates demonstrate that halide perovskites actually <i>do</i> suffer from defect-assisted nonradiative recombination, i.e., they are <i>not</i> defect tolerant [1]. The nonradiative recombination rates in halide perovskites are comparable to or even greater than those 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; however, defect engineering is still key to improving the efficiency of perovskite solar cells.<br/>For the prototypical hybrid perovskite MAPbI₃ [MA=(CH₃NH₃)] our results indicate that iodine interstitials are most harmful, and hence iodine-rich synthesis conditions should be avoided [2]. 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, J. Appl. Phys. <b>131</b>, 090901 (2022).<br/>[2] X. Zhang, M. E. Turiansky, J.-X. Shen, and C. G. Van de Walle, Phys. Rev. B <b>101</b>, 140101 (2020).<br/>[3] X. Zhang, J.-X. Shen, M. E. Turiansky, and C. G. Van de Walle, 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).