Local Energy Landscape Drives Long-Range Exciton Diffusion in Two-Dimensional Halide Perovskite Semiconductors

Abstract

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 BA 2 MA n –1 Pb n I 3 n +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.

Sub-angstrom noninvasive imaging of atomic arrangement in 2D hybrid perovskites

“…We note that the intrinsic ferroelastic-like composition of the 2D RPP crystals, uncovered by our STM/ncAFM studies, can be a key toward understanding their outstanding photovoltaic characteristics and the occurrence of a number of intriguing optical properties unveiled recently (17)(18)(19)(20)(21)(22). Specifically, recent temperature-dependent PL studies reveal an emergence of shallow electronic states in the n = 4 (BA + ) 2 (MA + ) n−1 Pb n I 3n+1 RPP with an increased emission lifetime.…”

“…Last, owing to their one-dimensional nature (hundreds of nanometers), twin boundaries can be viewed as channels for a longdistance exciton propagation, which is directly corroborated by a recently reported phenomenon of the long-range (hundreds of nanometers) exciton funneling in RPPs (18,19,22). It is therefore envisaged that controlling the orientation of the twin boundaries via external stimuli (17,60,61) can be a plausible strategy to enhance the performance of the RPP-based photovoltaic and optoelectronic devices.…”

“…Specifically, recent temperature-dependent PL studies reveal an emergence of shallow electronic states in the n = 4 (BA + ) 2 (MA + ) n −1 Pb n I 3 n +1 RPP with an increased emission lifetime. These electronic states were previously ascribed to the presence of local domains and static disorder in the crystal that emerged at low temperature ( 22 ), although their structure has not been verified. Our findings not only provide a new quantitative insight into nanoscale domain composition of the RPPs but also suggest that low-energy electronic states revealed by PL spectra are likely associated with the domain structures observed here.…”

“…First, alternating distribution of the electrostatic potential across domain boundaries may facilitate the separation of electronhole pairs and thus lead to reduced rates of electron-hole recombination (57)(58)(59). Second, spatially extended one-dimensional twin boundaries (hundreds of nanometers) can be responsible for the long-range exciton funneling in RPPs as reported recently (18,19,22).…”

“…For instance, the most recent experimental breakthroughs show that RPP crystals render remarkably long-range exciton diffusion (hundreds of nanometers) and reduced rates of exciton recombination ( 18 , 19 ). Furthermore, it has been demonstrated that charge transport is largely determined by the intrinsic crystalline structure of the RPPs, pronounced exciton-polaronic effects ( 20 , 21 ), and peculiarities of the energy landscape ( 22 ). Nevertheless, fundamental understanding of these exciton and carrier dynamics phenomena in RPP is not complete, as it requires the microscopic knowledge of the structural properties of RPPs, which remains elusive to date.…”