Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized exciton-induced Stark and Phonon Bottleneck Effects
Résumé
Hybrid halide perovskites have emerged as rising materials for solution-processed photovoltaics, photodetectors, and light-emitting devices. For these applications, a deep understanding of the relaxation mechanism within the photoactive material is crucial since the rate at which hot carriers relax to the band edge will directly impact the performance of the optoelectronic devices. Several groups have investigated the cooling process in hybrid perovskite bulk materials as thin films using pump-probe spectroscopy [1–3].
More recently, the development of low dimensional perovskite structures has enabled the investigation of carrier cooling in confined materials. In these systems, slower cooling is expected even at low excitation density due to the larger energy level separation. An apparent intrinsic phonon bottleneck was observed in weakly confined nanocrystals (NCs) [4]. However, contradictory results were reported in strongly confined 2D perovskites: thin films and colloidal nanoplatelets (NPLs). [5,6] Thus, a clear understanding of the confinement effect in the ultrafast relaxation dynamics is lacking.
Here, using fs transient absorption spectroscopy (TA), we investigate the cooling rate in lead iodide-based perovskite 2D nanostructures. For such strongly confined systems, we propose an alternative method to characterize the cooling process by analyzing the TA spectral lineshape evolution of the first excitonic transitions. Indeed, the strong Stark signals in the TA spectra and the discrete nature of the optical transitions prevent to use of the classical analysis model of relaxation by extracting the time-dependent carrier temperatures or measuring the build-up of the band state bleaching, as applied previously in bulk and bulk-like perovskites nanocrystals. Using global data analysis, we extracted the rates of carrier relaxation after pump excitation above the band edge, at low and high excitation density. The ultrafast hot exciton relaxation in one- and three-monolayer thick NPLs confirms the absence of intrinsic phonon bottleneck effect, which was found independent of the nature of the internals cations. Remarkably, we found an enhanced delayed cooling rate at higher carrier densities known as the hot phonon bottleneck effect, in the 2D layered perovskite thin films compared to colloidal n=1 NPLs. This fact suggested a role of the ligands y/o sample superficial state in the cooling process.