Developing an Understanding-Based Selection of Hybrid-Perovskite Compounds and the Cu-in Hybrid-Perovskite (CIHP) Family

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 halides 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 [BIB + CIII] such as [Cu + Ga] or [Ag + In] and combinations thereof. The resulting group of double perovskites with formula A2BCX6 (A = K, 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 and rapidly rising 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 rather strong direct-gap optical transitions, 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 CHIPs then identifies the best-of-class Cs2[AgIn]Br6 and K2[AgIn]Br6, having direct band gaps of 1.50 and 1.44 eV, and a theoretical spectroscopic limited maximal efficiency comparable to chalcopyrites and CH3NH3PbI3• Our finding offers new routine for designing new-type stable, Pb-free halide perovskite solar absorbers.