Can Lead-Free Double Halide Perovskites Serve as Proper Photovoltaic Absorber?

The emerging Pb-free double perovskites (DPs) are acknowledged as the most potential nontoxic alternatives to lead halide perovskites for thin-film photovoltaics, yet their photophysical properties significantly lag behind expectations. To tackle this issue, it is imperative to conduct a systematic investigation of the structure and optoelectronic properties and to sift through vast chemical space to extract new types of Pb-free DPs with exceptional optoelectronic characteristics and thermal stability. Through high-throughput first-principal calculations, we demonstrate that apart from a select few Pb-free DPs (e.g., Cs₂InSbCl₆ and Cs₂TlBiBr₆), other categories, even with suitable direct electronic bandgaps, exhibit inferior optical absorption due to the inversion symmetry-induced parity-forbidden transitions. The mismatch between the electronic and optical bandgap, thence, casts doubt on the reliability of the electronic bandgap as a criterion for Pb-free DPs in various optoelectronics. The assessed limited thermostability under operational conditions, however, hinders any Pb-free DPs from effectively serving as photovoltaic absorbers. Alongside the compositional engineering discussed above, the prospect of manipulating local-site symmetry and disrupting the parity forbidden transitions in stabilized Pb-free DPs through materials engineering should be recognized as a pivotal and rational avenue toward achieving high performance.