Abstract
Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics. Here we report that light-induced halide segregation is strain-activated in MAPb(I1−xBrx)3 with Br concentration below approximately 50%, while it is intrinsic for Br concentration over approximately 50%. Free-standing single crystals of CH3NH3Pb(I0.65Br0.35)3 (35%Br) do not show halide segregation until uniaxial pressure is applied. Besides, 35%Br single crystals grown on lattice-mismatched substrates (e.g. single-crystal CaF2) show inhomogeneous segregation due to heterogenous strain distribution. Through scanning probe microscopy, the above findings are successfully translated to polycrystalline thin films. For 35%Br thin films, halide segregation selectively occurs at grain boundaries due to localized strain at the boundaries; yet for 65%Br films, halide segregation occurs in the whole layer. We close by demonstrating that only the strain-activated halide segregation (35%Br/45%Br thin films) could be suppressed if the strain is properly released via additives (e.g. KI) or ideal substrates (e.g. SiO2).
Highlights
Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics
As the ratio of bromide-to-iodide is changed in the precursor, colored perovskite films with varied bandgaps are feasibly tuned under dark[11,12,13,14]; under illumination, photo-induced halide migration leads to bandgap instability, mainly due to lightinduced halide segregation (LHS)[12,15,16,17,18,19,20,21,22,23,24]
Device-relevant thin films are studied, finding that LHS selectively occurs at grain boundaries in 35%Br polycrystalline films while it occurs across the whole layer in 65%Br films
Summary
Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics. LHS leads to inhomogeneous bandgap across the perovskite layer, which severely limits the device performance under illumination To solve this issue, it is essential to reveal the driving force and all the relevant factors that affect this behavior in mixed-halide perovskites. We report that LHS depends on both strain and composition in mixed-halide perovskites, which unifies many controversial results on LHS We systematically study both freestanding and on-substrate single crystals of MAPb(I1−xBrx)[3] with photoluminescence (PL) tracking. Device-relevant thin films are studied, finding that LHS selectively occurs at grain boundaries in 35%Br polycrystalline films while it occurs across the whole layer in 65%Br films These results prompt us to divide mixed-halide perovskites into three distinct regimes: LHS-free regime (e.g., 15%Br), strain-activated LHS regime (e.g., 35%Br), and intrinsic LHS regime (e.g., 65%Br). We demonstrate that the strain-activated LHS can be inhibited by releasing strain in the films via passivation strategies or lattice-match substrate, etc
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