Strain-activated light-induced halide segregation in mixed-halide perovskite solids

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).

The analysis for other single crystals (i.e. MAPbI 3 /MAPbBr 3 ) also shows a trivial strain. We also note that 35%Br presents singular peak broadening caused by its tetragonal phase-induced broadening. The orientation-dependent strain from 0.2% to 0.4% is obtained by directly using Formula (1) (Supplementary Figure 15b). With grain size D=500 nm, the size-related broadening is 0.016° for diffraction angle at 14° and strain of 0.27% is obtained by subtracting the broadening of single crystal (0.07°) and size effect (0.016°) from the total broadening (0.17°). Single crystal is referred to as strain-free reference and equivalent to instrumental broadening here. The Schottky defect is not included for XRD broadening because the derived strain value under this assumption is close to Williamson-Hall analysis.
To further acquire the information of local strain, we use confocal Raman to analyze the strain inhomogeneity due to its sensitivity to local strain variation 1,2 . Here a 488 nm laser is used to give an optical resolution of approx. 300 nm. Since perovskite is not Raman-active at room temperature, we utilize the laser-induced decomposition product PbI x to reflect the local strain in perovskite thin film, considering the interaction between PbI x and the adjacent perovskite 1,3,4 . Two peaks are appearing at local strain in mixed-halide perovskite films 5,6 . Here MAPbI 3 is selected instead of 35%Br sample to avoid the signal interference from laser-induced halide segregation during measurement. The local halide segregation at the boundaries in 35%Br sample will affect the intrinsic strain distribution in polycrystal film.

Supplementary Note 3
To discuss the mechanism of strian-activated LHS, we use a simplified model to analyze the thermodynamics of mixed-halide perovskite with and without lattice deformation, or internal strain, as follows: Without lattice deformation: Where U, S, T is formation energy of perovskite, entropy, temperature respectively; N 0 is lattice cell number; N is carrier density, ∆E is the energy reduction by the funneling of photo-carriers to I-rich clusters; ∆E is the reduction of polaronic energy due to halide segregation, ∆ is the gain of strain energy around I-rich/Br-rich interfaces due to halide segregation.
As a function of Br concentration (x Br ), the sum of the first two terms is positive, while the third and the fourth term is negative for x Br > 20% 1,2 . The last term is positive considering the lattice mismatch between I-rich and Br-rich perovskite. Without lattice deformation (e.g. free-standing single crystals), the strain energy is close to 0 at the mixed state; however, it becomes non-trivial for the strained perovskites at the mixed state.
The halide segregation will occur if ∆ < . Only the 3 rd and the 4 th term are the driving force of halide segregation because the photo-carriers can funnel into the iodide-rich clusters to minimize the total free energy. We can derive that the gain of strain energy caused by halide segregation is lower for the strained perovskite (∆ 2 = .