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.
The structural stability of mixed A-site perovskite solar cells during operation is observed by in situ XRD and the de-mixing behavior is described by calculating the Gibbs free energy of mixing.
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