Tracking Dynamic Phase Segregation in Mixed‐Halide Perovskite Single Crystals under Two‐Photon Scanning Laser Illumination

Abstract

Photoinduced phase segregation in mixed halide perovskites has received considerable attention due to its critical roles in diminishing device performance in photovoltaic and light‐emitting applications. Here, dynamic photoinduced phase segregation and dark recovery in mixed halide perovskite single crystal microplatelets are investigated, combining depth‐resolved, temporal‐resolved, and detection‐wavelength selective spectroscopic imaging techniques. Under identical illumination, the edges and interior of microplatelets exhibit significantly different phase segregation. An intimate correlation of PL dynamics between I‐phase and Br‐phase indicates that the halide substitution is the dominant effect. In the dark, the phase‐segregated crystals reversibly recover to the stable equivalence. This work clarifies the critical role of the edge state of the microplatelets and reveals the physical mechanism of phase segregation in mixed halide perovskites, which is of crucial importance for their applications in photovoltaics and photonics.

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

“…Under optical illumination, the formation of iodide-rich phases is observed, which is rationalized by bandgap reduction and polaron effect 18,20 . Although the light-induced halide segregation (LHS) is commonly recognized, it is still regarded as an intrinsic property in mixed-halide perovskites 12,15,[22][23][24][25][26][27]43 . Our results elaborate on the incomplete understanding of LHS by unveiling the effects of strain on LHS.…”

“…However, a unified picture remains under debate, given that contradictory results were reported in the literature. For instance, Tang et al showed stable light emission in single crystals, and LHS is only occurring at the grain boundaries in polycrystalline films, whereas Mao et al reported strong LHS in the single crystals 23,25 . The doping region without LHS was also inconsistently reported [15][16][17][18][19][20][21][22][23][24][25] .…”

“…For instance, Tang et al showed stable light emission in single crystals, and LHS is only occurring at the grain boundaries in polycrystalline films, whereas Mao et al reported strong LHS in the single crystals 23,25 . The doping region without LHS was also inconsistently reported [15][16][17][18][19][20][21][22][23][24][25] . Furthermore, some studies showed that passivation or endotaxial matrices can effectively inhibit LHS, but this trend was not commonly observed by others 12,22,24,26,27 .…”

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

“…[ 72,74,79,80,88–90 ] All these factors result in a larger open‐circuit‐voltage ( V OC ) deficit (defined by E g /q − V OC ) for wide‐bandgap PSCs (>≈0.5 V) as compared to state‐of‐the‐art low‐bandgap PSCs (≈0.35–0.4 V), negating a linear increase of V OC with increasing bandgap. [ 3,52 ] We would like to stress that the relative contribution of the above mentioned factors to the V OC deficit is a complicated function of the exact perovskite composition, [ 69,72,91 ] film surface nature, [ 78,84,90 ] defect density as well as the charge extraction layers employed. [ 79,86,87 ] One popular approach to reduce the V OC deficit is to deposit a large organic cation on the surface of a 3D perovskite film, which acts as a 2D passivation agent and/or improves the energetic alignment with the charge transport layer.…”

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time‐Resolved Microspectroscopy

“…Two-photon illumination is a third-order nonlinear optical process in which two penetrating photons are simultaneously absorbed and focused deeper into bulk compared to the case of one-photon excitation. [102,[125][126][127][128] By mapping the phase segregated PL by two-photon illumination, edges, and interiors of MAPb(Br x I 1−x ) 3 microplatelets exhibit significantly different phase segregation ( Figure 9C). Such dichotomy between extrinsic (surface) and intrinsic (bulk) effects of halide substitution in lead bromide perovskites was also reported in another kind of mixed halide single crystal (MAPb(Br x Cl 1−x ) 3 ) observed by the contrast between one-and two-photon excitation.…”