Ultrafast multiexciton Auger recombination of CdSeS

Abstract

Multiexciton generation is a process where multiple excitons are generated by absorbing single photons. Efficient multiexciton generation in quantum dots may be a revolutionary discovery, because it provides a new method to improve the solar-to-electric power conversion efficiency in quantum dots-based solar cells and to design novel quantum dots-based multielectron or hole photocatalysts. However, the mechanism of ultrafast multiexciton generation and recombination remain unclear. In this paper, alloy-structured quantum dots, CdSeS, are prepared by the hot injection method. The generation and recombination mechanism of charge carriers in quantum dots samples are discussed in detail. The bivalent band structure of alloy-structured quantum dots is determined by ultraviolet-visible absorption spectra. It is found that the 1S_3/2(h)-1S(e) (or 1S), 2S_3/2(h)-1S(e) (or 2S) and 1P_3/2(h)-1P(e) (or 1P) exciton absorption bands of these quantum dots are at 510 nm, 468 nm and 430 nm, respectively. Femtosecond transient absorption spectroscopy and nanosecond time-resolved photoluminescence spectroscopy are used to investigate the ultrafast exciton generation and recombination dynamics in the alloy-structured quantum dots. By fitting the transient kinetics of 1S exciton bleach, an average biexciton decay time is obtained to be about 80 ps, which is almost twice the decay time of traditional quantum dots (less than 50 ps). Combined with the recently developed ultrafast interface charge separation technology that can extract multiple excitons before their annihilation, it will have a promising application prospect. Moreover, there is a hole relaxation on a the time scale of 5-6 ps via a phonon coupling pathway to lower-energy hole states in addition to the above-described ultrafast exciton-exciton annihilation process in 2S and 1P excitons. Furthermore, by nanosecond time-resolved photoluminescence spectroscopy, it can be concluded that the charge separated state is long-lived (200 ns). Our findings provide a valuable insight into the understanding of ultrafast multiexciton generation and recombination in quantum dots. These results are helpful to understand the intrinsic photo-physics of multiexciton generation in quantum dots, to implement the photovoltaic and optoelectronic applications, and to ascertain the exciton relaxation dynamics of quantum dots.