Bandgap Engineering of Lead‐Free Double Perovskite Cs₂AgBiBr₆ through Trivalent Metal Alloying

Abstract

The double perovskite family, A M M X , is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH NH PbI . Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs AgBiBr as host, band-gap engineering through alloying of In /Sb has been demonstrated in the current work. Cs Ag(Bi M )Br (M=In, Sb) accommodates up to 75 % In with increased band gap, and up to 37.5 % Sb with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs Ag(Bi Sb )Br . Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed.

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

“…The significant bandgap reduction is caused by the Tl impurity-induced band edge reconstruction, which is supported by DFT results (Figure 16f). [197] They found that Sb alloying decreases the bandgap slightly, with the smallest value of 1.86 eV for Cs 2 Ag(Bi 0.625 Sb 0.375 )Br 6 ( Figure 16g). However, Tl is highly toxic and, therefore, undesirable for photovoltaic applications.…”

“…[196] Moreover, they have shown that the Tl-alloyed Cs 2 AgBiBr 6 also exhibit long-lived carriers. [197] The bandgap reduction is attributed to the higher energy levels of Sb 5s states than that of Bi 6s states, which slightly raises the VBM. Du et al have demonstrated bandgap engineering of Cs 2 AgBiBr 6 through alloying of trivalent cations such as Sb 3+ .…”

“…However, Tl is highly toxic and, therefore, undesirable for photovoltaic applications. [197] Recently, Li et al reported bandgap narrowing through a high pressure treatment. [197] They found that Sb alloying decreases the bandgap slightly, with the smallest value of 1.86 eV for Cs 2 Ag(Bi 0.625 Sb 0.375 )Br 6 ( Figure 16g).…”

“…Yang et al proposed, by DFT calculations, that by controlling the ordering parameter of the Ag/Bi arrangement, the bandgap of Cs 2 AgBiBr 6 can vary continuously form a wide indirect bandgap of 1.93 eV for the experimentally synthesized ordered structure (ground-state structure) to a pseudodirect bandgap of 0.44 eV for the fully random alloy, which however, were reported to be thermodynamic unstable in other studies. [197] Copyright 2017, Wiley-VCH. However, because of the high-energy-lying Cu 3d 10 Reproduced with permission.…”

Progress of Lead‐Free Halide Double Perovskites

“…[145] Staying within the given x range, the double perovskite phase is maintained. The complex nature and the impact of atomic substitution on the optoelectronic character within the double perovskite family are highlighted in substituted double perovskites, Cs 2 AgBi 1−x In x Br 6 (x = 0, 0.25, 0.5, and 0.75) and Cs 2 AgBi 1−x Sb x Br 6 (x = 0, 0.125, and 0.375).…”