Rational Design of Halide Double Perovskites for Optoelectronic Applications

Abstract

Practical application of hybrid Pb-based halide perovskites needs not only to fabricate high-quality film samples on a large scale but also to properly overcome the issues of Pb toxicity and materials instability. Finding new, stable, Pb-free perovskites currently attracts significant research interest. Among various strategies, hetero-substitution of Pb to form quaternary halide double perovskites represents a promising direction to keep the high structural (and likely electronic) dimensionality nature of perovskite lattice and meanwhile offers rich chemical compositions a degree of freedom for discovering new perovskite materials. We herein present a perspective that concisely reviews the progress of rational design of Pb-free halide double perovskites by both theoretical and experimental efforts as well as their current and potential applications in various optoelectronic categories. We also envision the future research directions to realize new materials by exploring broader chemical composition space and to better utilize existing materials by considering optoelectronic properties modulation.

Progress of Lead‐Free Halide Double Perovskites

“…[179] For instance, some double perovskites such as Ag-and Bi-containing analogs are structurally 3D, whereas electronically 0D. In contrast to the report by Zhao et al, [170] it is evident that high structural dimensionality does not directly translate to high electronic dimensionality. [180] Also, calculations show that the double perovskite, Cs 2 SrPbI 6 , exhibits an indirect bandgap of 3.05 eV, which is much larger than that of 3D CsPbI 3 analog (1.48 eV), but is comparable to the calculated bandgap of the 0D Cs 4 PbI 6 analog (3.44 eV).…”

“…These features are favorable for PV application. Also, since the compound may exhibit poor carrier transport due to the presence of deep mid-gap defects, [170] c) The temperature-dependent photoconductivity of Cs 2 AgSbCl 6 showing a continuous increase in bandgap as the temperature decreases. [88] Processes such as the addition or nonaddition of HI, aging, and deposition conditions can create deficiencies and impurities, which can in turn influence the bandgap, carrier concentration, and mobility of the material.…”

“…The gap decreases with increasing ionic radius of the anion, since the halogens at the X site (F, Cl, Br, I) are known to modulate the bandgap. Beyond bandgap character and size, it is important to address other issues such as poor carrier transport [170] for this type of materials to exhibit reasonable PV properties. The measurement shows that Cs 2 TiI 2 Br 4 possesses a direct bandgap of 1.38 eV (Figure 10d,e), which is close to the ideal bandgap (1.34 eV), while Cs 2 TiBr 6 exhibits a larger bandgap (1.8 eV).…”

“…[170] Tang et al proposed a general strategy for designing layered halide double perovskites Cs 3+n M (II) n Sb 2 X 9+3n (M = Sn, Ge) with increased electronic dimensionality for PV applications. A promising solar absorber candidate should exhibit high electronic dimensionality.…”

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…First, among the 8 B(I) cations, the 5 alkali metal cations (including NH 4 + ) can be directly ruled out because they are too ionic to contribute to the band edges. [166] Since 2016, many new halide double perovskites have been synthesized, including Cs 2 AgBiX 6 (X = Br, Cl), [155][156][157] Cs 2 AgSbCl 6 , [167][168][169] Cs 2 AgInCl 6 , (MA) 2 AgBiBr 6 , [170] (MA) 2 KBiCl 6 , [161] and (MA) 2 TlBiBr 6 . [55] Second, for B(III) cations, the transition metal cations with multiple oxidation states and/or partially occupied d or f orbitals are not desirable for designing photovoltaic absorbers, because they may introduce deep defect states and too localized band edges.…”