Origin of Pronounced Nonlinear Band Gap Behavior in Lead–Tin Hybrid Perovskite Alloys

Abstract

Mixed lead–tin hybrid perovskite alloy CH3NH3(Pb1–x Sn x )­I3 attracted significant attention lately because of the reduction of its band gap below both end compounds, which makes it a promising bottom cell material in all-perovskite tandem solar cells. The effect is a consequence of a strongly nonlinear dependence of the alloy band gap on chemical composition. Here, we use electronic structure calculations at different levels of theory (density functional theory (DFT), hybrid DFT, and QSGW, with and without spin–orbit interactions) to investigate the presently elusive origin of this effect. Contrary to current conflicting studies, our results show that neither spin–orbit interactions nor the composition induced changes of the crystal structure and ordering of atoms contributes to the nonlinearity of the band gap. We find that the strong nonlinearity is primarily a consequence of chemical effects, i.e., the mismatch in energy between s and p atomic orbitals of Pb and Sn, which form the band edges of the alloy. These results unravel the nature of the band gap bowing in Sn/Pb hybrid perovskite alloys and offer a relatively simple way to estimate evolution of the band gap in other hybrid perovskite alloys.

Controlling the Growth Kinetics and Optoelectronic Properties of 2D/3D Lead–Tin Perovskite Heterojunctions

“…We show the absorbance onset variation of both the higher and lower energy features in Figure b, revealing a bowing behavior for both the 2D and 3D domains, with minima for both domains at a Pb:Sn ratio of y = 0.75. The bowing behavior is a well‐established phenomenon in 3D Pb:Sn perovskites, but here we find that it also occurs in low‐dimensional 2D domains. The origin of the bowing behavior even in 3D perovskites is still debated.…”

“…perovskites, [19][20][21][22][23][26][27][28]52,53] but here we find that it also occurs in low-dimensional 2D domains. The origin of the bowing behavior even in 3D perovskites is still debated.…”

“…The origin of the bowing behavior even in 3D perovskites is still debated. The most commonly reported origins include competition between spin–orbit coupling (reducing the bandgap) and lattice distortion (out‐of‐phase octahedral tilting in Pb‐rich compositions increasing the bandgap), and energy mismatch between Pb and Sn atomic orbitals forming the band edges of the perovskite alloy . The observation of the bowing in low‐dimensional domains adds support to the proposal of short‐range effects, such as band edge mismatch or bond bending.…”

Making by Grinding: Mechanochemistry Boosts the Development of Halide Perovskites and Other Multinary Metal Halides

“…2020, 10,1902499 Adapted with permission. [76] Copyright 2019, American Chemical Society. c) Mn-doped CsPbCl 3 nanocrystals show bright PL with large Stokes shift which can be used in luminescent solar cells concentrators.…”

Lead‐Free Tin‐Based Perovskite Solar Cells: Strategies Toward High Performance

“…The VBM and CBM of Sn‐based perovskites are less strongly bound than the Pb‐based ones, which are due to the enhanced metal‐halide orbital overlap caused by a lattice contraction. However, as the CBM is more of a nonbonding nature than the antibonding VBM, it is less shifted in response to the change in the metal‐halide orbital overlap .…”