Exciton-exciton annihilation in thin indium selenide layers.

The photocarrier recombination in van der Waals layers may determine the device performance based on these materials. Here, we investigated the photocarrier dynamics in a multilayer indium selenide nanofilm using transient absorption spectroscopy. The sub-bandgap transient absorption feature was attributed to the indirect intraband absorption of the photocarriers, which was then exploited as a probe to monitor the photocarrier dynamics. With increasing pump intensities, the photocarrier decay was accelerated because of the rising contribution from a bimolecular recombination channel that was then assigned to exciton-exciton annihilation. The rate constant of the exciton-exciton annihilation was given as (1.8 ± 0.1) × 10 cm ps from a global fitting of the photocarrier decay kinetics for different pump intensities. Our finding suggests that, in contrast with their monolayer counterpart, the exciton-exciton annihilation is rather inefficient in multilayers due to their weaker Coulomb interaction. Hence, compared with monolayers, the lifetime of photocarriers in multilayers would not be significantly reduced under high-intensity pump conditions, and the apparent photocarrier lifetime could be further improved just by suppressing the monomolecular recombination channels such as trapping.