Phase Segregation and Sequential Expulsion of Iodide and Bromide in Photoirradiated Ruddlesden–Popper 2D Perovskite Films

Abstract

Ruddlesden−Popper mixedhalide perovskite films, BA 2 PbBr 2 I 2 , undergo phase segregation when excited with visible light to generate bromide-and iodide-rich regions, as marked by absorption and emission changes. Upon stopping illumination, the process reverses, allowing original film compositions to be restored. However, if films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to the 2D films. The sequential disappearance of I − and Br − from pristine films (BA 2 Pb 2 Br 4 and BA 2 Pb 2 I 4 ) under photoirradiation, as observed from variances in expulsion rates, reflects differences in halide ion mobilities in these films. The photoinstability of 2D films raises questions about their use in stabilizing bulk, three-dimensional (3D) perovskite solar cells through 3D/2D interfaces.

Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites

“…Two-dimensional (2D) hybrid halide perovskites like A 2 PbX 4 (A: organic ammonium cation, X: Cl, Br, I) are semiconductors showing interesting optoelectronic properties. − In addition, can these materials show ferroelectricity, anomalous photovoltaics, and nonlinear optical properties like second harmonic generation (SHG)? The answer to this question is yes, provided A 2 PbX 4 crystallizes in a non-centrosymmetric polar space group at room temperature. − This paper deals with the rational design of such non-centrosymmetric hybrid perovskite semiconductors, which in turn show anomalous photovoltaic effects for self-powered photodetectors, and visible-to-infrared tunable SHG.…”

Reversible Phase Transformation of Colloidal 2D Lead Halide Perovskite Platelets under Photoirradiation

“…It may be interesting to note that 2D perovskite films of mixed halide perovskites (BAPb 2 Br 3.5 I 3.5 ) when in contact with DCM showed phase segregation, followed by expulsion of iodide into solution. The mechanistic aspects of photoinduced transformation in 2D perovskites discussed earlier 31 pointed out that greater mobility of iodide as compared to bromide was responsible for the quick expulsion of iodide from the perovskite lattice. 10 The observed changes in both BAOAPbI 4 and BAOAPbBr 2 I 2 suspensions did not recover when they were left in the dark, thus indicating the photoinstability of these perovskite colloids (Figure S4A,B).…”

“…This photoinduced phase segregation is reversible as it recovers when the material is stored in the dark. 29,31 While there has been significant research on the photochemistry of 2D lead halide perovskite films, 32−34 stable colloidal forms of 2D lead halide perovskite platelets have only been the focus of recent studies. 15,16,35,36 The synthesis of 2D lead halide perovskite in colloidal form enables controlled preparation of perovskite nanoplatelets with specific layer numbers.…”

“…Increased mobility of iodide as compared to bromide makes its expulsion into the solution selective. 31 Reactions 3 and 4 represent a possible scenario leading to the formation of bromide-rich domains and iodide complexes. (3)…”

“…As discussed earlier, the trapping of a hole at the iodide site causes it to migrate to the surface and get expelled and form an iodide complex. Increased mobility of iodide as compared to bromide makes its expulsion into the solution selective Reactions and represent a possible scenario leading to the formation of bromide-rich domains and iodide complexes. BA OA MA Pb 2 ( Br 3.5 I 3.5 ) + h ν → BA OA MA Pb 2 Br 7 + BA OA MA Pb 2 normalI 7 BA OA MA Pb 2 normalI 7 → 2 Pb normalI 3 − + BA + + OA + + MA + + normalI − A schematic representation of changes observed during photoirradiation of BAOAMAPb 2 (Br 3.5 I 3.5 ) mixed perovskite colloids suspended in DMF–toluene is illustrated in Scheme .…”