Wavefunction engineering for regulation of recombination dynamics and exciton-phonon coupling in CuInS2 quantum dots

Abstract

The CuInS₂ quantum dots (QDs) exhibit exceptional potential in semiconductor light-emitting diodes and photovoltaic devices. Investigating their fundamental excitonic photophysical processes involving the recombination dynamics and exciton−phonon coupling is crucial for advancing their applications. Type-I CuInS₂/ZnS and type-II CuInS₂/CdS core/shell QDs were synthesized to investigate the impact of excitonic wavefunction-distribution on carrier dynamics and exciton-phonon interaction by femtosecond transient-absorption-spectroscopy (fs-TAS) and temperature-dependent photoluminescence spectroscopy (TD-PLS), respectively. The synthesized CuInS₂/CdS core/shell QDs have a notably longer PL lifetime than that of CuInS₂/ZnS core/shell QDs due to their type-II exciton confinement, in consistent with the theoretical calculations based on the model of spherical box with finite potential barrier. However, in the analysis of ultrafast fs-TAS dynamics, the CuInS₂/CdS core/shell QDs exhibit short excited-state lifetimes, primarily due to the elevated density of defect states. These defect states exhibit a significant thermal PL quenching effect by our TD-PLS analysis, where thermal activation energy of CuInS₂/CdS core/shell QDs is determined to be 40.1 meV, lower than that of CuInS₂/ZnS core/shell QDs (68.7 meV). This is ascribed to expanding of the electron wavefunction from CuInS₂ core to CdS shell, making electron transitions susceptible to the influence of interface defects and surface defects. Correspondingly, the evolution trend of PL lifetime on CuInS₂/CdS core/shell QDs with the temperature exhibits two parts which are attributed to thermal activation of interface defects and surface defects, respectively. These results suggest the wavefunction engineering can be employed to regulate the excitonic photophysical processes of QDs for their potential applications.