Cu–In Halide Perovskite Solar Absorbers

Abstract

The long-term chemical instability and the presence of toxic Pb in otherwise stellar solar absorber APbX₃ made of organic molecules on the A site and halogens for X have hindered their large-scale commercialization. Previously explored ways to achieve Pb-free halide perovskites involved replacing Pb²⁺ with other similar M²⁺ cations in ns² electron configuration, e.g., Sn²⁺ or by Bi³⁺ (plus Ag⁺), but unfortunately this showed either poor stability (M = Sn) or weakly absorbing oversized indirect gaps (M = Bi), prompting concerns that perhaps stability and good optoelectronic properties might be contraindicated. Herein, we exploit the electronic structure underpinning of classic Cu[In,Ga]Se₂ (CIGS) chalcopyrite solar absorbers to design Pb-free halide perovskites by transmuting 2Pb to the pair [B^IB + C^III] such as [Cu + Ga] or [Ag + In] and combinations thereof. The resulting group of double perovskites with formula A₂BCX₆ (A = K, Rb, Cs; B = Cu, Ag; C = Ga, In; X = Cl, Br, I) benefits from the ionic, yet narrow-gap character of halide perovskites, and at the same time borrows the advantage of the strong Cu(d)/Se(p) → Ga/In(s/p) valence-to-conduction-band absorption spectra known from CIGS. This constitutes a new group of CuIn-based Halide Perovskite (CIHP). Our first-principles calculations guided by such design principles indicate that the CIHPs class has members with clear thermodynamic stability, showing direct band gaps, and manifesting a wide-range of tunable gap values (from zero to about 2.5 eV) and combination of light electron and heavy-light hole effective masses. Materials screening of candidate CIHPs then identifies the best-of-class Rb₂[CuIn]Cl₆, Rb₂[AgIn]Br₆, and Cs₂[AgIn]Br₆, having direct band gaps of 1.36, 1.46, and 1.50 eV, and theoretical spectroscopic limited maximal efficiency comparable to chalcopyrites and CH₃NH₃PbI₃. Our finding offers a new routine for designing new-type Pb-free halide perovskite solar absorbers.