Exciton localization in solution-processed organolead trihalide perovskites

Abstract

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

Investigation of localized and delocalized excitons in ZnO/ZnS core-shell heterostructured nanowires

“…As shown in Figure 5B, the curves of FWHM show the "valley-shape" features that gradually disappear after thermal treatments. The "S-shape" and "valley-shape" characteristics obtained from the samples' emission have further confirmed the existence of localized states, which is consistent with previous reports [1,2,[6][7][8].…”

“…The localized states can expand the electronic density of state (DOS) to form an exponential tail instead of the sharp conduction and valance band edges. Localized states can be found in the semiconductor quantum structures and has become one of the characteristics which have been demonstrated to be responsible for the high emission efficiency of a variety of semiconductor composites [2][3][4]. For example, in the case of dilute nitride semiconductors, the localized excitons always present a socalled "S-shape" peak position feature obtained from the temperature-dependent photoluminescence (TDPL) measurement and a "valley shape" of full width at half maximum (FWHM) [1,5].…”

“…In this paper, through the growth of ZnO/ ZnS core-shell heterostructured NWs, we have introduced localized states in the core-shell interface between the ZnO and ZnS. After subsequent thermal treatments, the optical properties of the localized excitons have been discussed based on laser spectroscopy [1][2][3][4][5][6][7][8]. The underlying physical mechanism of the change of localized excitons after thermal treatments has also been proposed and discussed in detail.…”

Lead halide perovskites: Recombining faster, emitting brighter

“…However, the exciton binding energies of bulk perovskites are lower than or comparable to thermal energy [6], too low to support dominant excitonic emission at room temperature. The enhancement of excitonic emission can be realized by localization [7] or quantum confinement of excitons. In particular, exciton confined in various dimensions, such as nanocrystals and nanoplatelets, shows greatly enhanced exciton binding energy and oscillator strength [8], which are fundamental parameters determining the luminescence efficiency.…”

Perovskite Quantum Dot Light-Emitting Diodes

“…It means that the identification of dominant recombination mechanisms in perovskites will help interpret the seemingly counterintuitive facts that perovskites can act as both extraordinary photovoltaic materials and superior optical gain mediums for LED and lasers. In general, photovoltaic materials require efficient separation of photocarriers, and emissive materials require high recombination rates [28]. If the obtained exciton-binding energy of the perovskites is comparable to the thermal energy at RT (~26 meV), excited states will tend to dissociate into free carriers rather than recombination radiatively.…”