Ultrafast dynamics of excitons and charge carriers in colloidal perovskite nanostructures studied by time-resolved optical spectroscopies
VILLAMIL FRANCO Carolina
Mercredi 30/09/2020, 14:00-17:01
Neurospin Bât 145, CEA-Saclay
Halide perovskites have emerged as very promising photoactive materials due to their outstanding optoelectronic properties combined with low-cost processability. In spite of their successful implementation in photovoltaic or light-emitting devices, a better understanding of the photo-induced relaxation and recombination dynamics is needed in order to enhance the device performances. To this end, time-resolved photoluminescence and femtosecond transient absorption spectroscopy were used to investigate the effects of confinement and composition in different lead iodide perovskite nanostructures.
For the investigation of the hot exciton/charge carrier dynamics (“cooling”), a global analysis method was used, where the temporal evolution of the spectral lineshapes was modeled with a sequential kinetic scheme. This method was successfully applied to effectively describe the continuous energy relaxation in weakly-confined nanoplates and allowed disentangling the hot phonon bottleneck from the Auger reheating effects at high excitation fluence. The global analysis was also applied to the cooling dynamics in strongly-confined 2D nanoplatelets. As in the weakly-confined samples, the NPL cooling rate decreases with the excitation fluence. However, it remains ultrafast, evidencing the absence of an intrinsic phonon bottleneck. This leads us to propose a ligand-mediated relaxation pathway that participates to the ultrafast hot exciton cooling in 2D NPLs in the strong confinement regime.
In addition, multiple exciton recombination dominated by non-radiative Auger recombination (AR) was studied in the strongly-confined 2D perovskite NPLs. At moderate fluence, the AR is diffusion-limited and thus highly depends on the initial inter-exciton distance and sample dimensionality. Typical AR occurs then on a timescale of several hundreds of picoseconds. In contrast, high excitation fluences produce “overlapping” excitons with AR times of less than 10 ps (intrinsic, reaction-limited AR).
Finally, the exciton population dynamics of 2D NPLs were studied after excitation in the ultraviolet. The strong dependence of the AR with the inter-exciton distance allows the identification of multiple exciton generation (MEG), which involves the reaction of “geminate”, spatially close biexcitons, produced by the absorption of a single high-energy photon.