Mixed halide perovskites that are thermodynamically stable in the dark demix under illumination. This is problematic for their application in solar cells. We present a unified thermodynamic theory for this light-induced halide segregation that is based on a free energy lowering of photocarriers funnelling to a nucleated phase with different halide composition and lower band gap than the parent phase. We apply the theory to a sequence of mixed iodine-bromine perovskites. The spinodals separating metastable and unstable regions in the composition-temperature phase diagrams only slightly change under illumination, while light-induced binodals separating stable and metastable regions appear signalling the nucleation of a low-band gap iodine-rich phase. We find that the threshold photocarrier density for halide segregation is governed by the band gap difference of the parent and iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect.
Light-induced halide
segregation hampers obtaining stable wide-band-gap
solar cells based on mixed iodide–bromide perovskites. So far,
the effect of prolonged illumination on the performance of mixed-halide
perovskite solar cells has not been studied in detail. It is often
assumed that halide segregation leads to a loss of open-circuit voltage.
By simultaneously recording changes in photoluminescence and solar
cell performance under prolonged illumination, we demonstrate that
cells instead deteriorate by a loss of short-circuit current density
and that the open-circuit voltage is less affected. The concurrent
red shift, increased lifetime, and higher quantum yield of photoluminescence
point to the formation of relatively emissive iodide-rich domains
under illumination. Kinetic Monte Carlo simulations provide an atomistic
insight into their formation via exchange of bromide and iodide, mediated
by halide vacancies. Localization of photogenerated charge carriers
in low-energy iodide-rich domains and subsequent recombination cause
reduced photocurrent and red-shifted photoluminescence. The loss in
photovoltaic performance is diminished by partially replacing organic
cations by cesium ions. Ultrasensitive photocurrent spectroscopy shows
that cesium ions result in a lower density of sub-band-gap defects
and suppress defect growth under illumination. These defects are expected
to play a role in the development and recovery of light-induced compositional
changes.
Photoinduced halide segregation hinders widespread application of three-dimensional (3D) mixed-halide perovskites. Much less is known about this phenomenon in lower-dimensional systems.Here, we study photoinduced halide segregation in lower-dimensional mixed iodide-bromide perovskites (PEA 2 MA n−1 Pb n (Br x I 1−x ) 3n+1 , with PEA + : phenethylammonium and MA + : methylammonium) through time-dependent photoluminescence (PL) spectroscopy. We show that layered two-dimensional (2D) structures render additional stability against the demixing of halide phases under illumination. We ascribe this behavior to reduced halide mobility due to the intrinsic heterogeneity of 2D mixed-halide perovskites, which we demonstrate via 207 Pb solid-state NMR. However, the dimensionality of the 2D phase is critical in regulating photostability. By tracking the PL of multidimensional perovskite films under illumination, we find that while halide segregation is largely inhibited in 2D perovskites (n = 1), it is not suppressed in quasi-2D phases (n = 2), which display a behavior intermediate between 2D and 3D and a peculiar absence of halide redistribution in the dark that is only induced at higher temperature for the quasi-2D phase.
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