Temperature-Dependent Optical Properties of PbS/CdS Core/Shell Quantum Dot Thin Films: Probing the Wave Function Delocalization

Abstract

Colloidal semiconductor quantum dots (QDs) are extraordinarily appealing for the development of cheap and large area solar cells due to high absorption efficiency, tunable bandgap energies, and solution processability. Understanding and controlling electronic wave function delocalization in QD thin films is therefore pivotal for realizing efficient optoelectronic devices. Here, a detailed study of the exciton recombination dynamics in PbS/CdS core/shell films as a function of temperature and surface chemistry is reported. By decreasing the temperature from 295 to 5.4 K, electronic wave function delocalization from the PbS core to the monolayer CdS shell is observed, demonstrating a reduction of the conduction band offset at low temperature and the formation of a quasi-type II band alignment. Interestingly, films made with core–shell particles, where the shell is thicker than a monolayer, display type I alignment also at low temperature.