• Energy Research

Halide perovskites are versatile semiconductors with applications including photovoltaics and light-emitting devices, having modular optoelectronic properties realizable through composition and dimensionality tuning. Layered Ruddlesden-Popper perovskites are particularly interesting due to their unique 2D character and charge carrier dynamics. However, long-range energy transport through exciton diffusion in these materials is not understood or realized. Here, local time-resolved luminescence mapping techniques are employed to visualize exciton transport in exfoliated flakes of the BA2MAn-1PbnI3n+1 perovskite family. Two distinct transport regimes are uncovered, depending on the temperature range. Above 100 K, diffusion is mediated by thermally activated hopping processes between localized states. At lower temperatures, a nonuniform energy landscape emerges in which transport is dominated by downhill energy transfer to lower-energy states, leading to long-range transport over hundreds of nanometers. Efficient, long-range, and switchable downhill transfer offers exciting possibilities for controlled directional long-range transport in these 2D materials for new applications.

The reduced effective mass (μ) and excitonic Rydberg (R*) are measured by magneto-optics for new perovskite semiconductors.

The authors directly show that grain size and quality have a negligible impact on the excitonic characteristics of perovskite semiconductors.

The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI$_{3}$, MAPbI$_{3-x}$Cl$_{x}$, FA (FormAmidinium)PbI$_{3}$ and FAPbBr$_{3}$ are having a tremendous impact on the field of photovoltaic cells due to their ease of deposition and efficiencies, but device performance can be significanly affected by inhomogeneities. Here we report a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both light and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers. In FAPbBr$_{3}$ a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions. Main text + supplementary information

Mixed‐halide mixed‐cation hybrid perovskites are among the most promising perovskite compositions for application in a variety of optoelectronic devices due to their high performance, low cost, and bandgap‐tuning capabilities. Instability pathways such as those driven by ionic migration, however, continue to hinder their further progress. Here, an operando variable‐pitch synchrotron grazing‐incidence wide‐angle X‐ray scattering technique is used to track the surface and bulk structural changes in mixed‐halide mixed‐cation perovskite solar cells under continuous load and illumination. By monitoring the evolution of the material structure, it is demonstrated that halide remixing along the electric field and illumination direction during operation hinders phase segregation and limits device instability. Correlating the evolution with directionality‐ and depth‐dependent analyses, it is proposed that this halide remixing is induced by an electrostrictive effect acting along the substrate out‐of‐plane direction. However, this stabilizing effect is overwhelmed by competing halide demixing processes in devices exposed to humid air or with poorer starting performance. The findings shed new light on understanding halide de‐ and re‐mixing competitions and their impact on device longevity. These operando techniques allow real‐time tracking of the structural evolution in full optoelectronic devices and unveil otherwise inaccessible insights into rapid structural evolution under external stress conditions.