PbS/CdS Core/Shell Quantum Dots Designed to Enable Efficient Photon Upconversion for Solar Energy Applications

Colloidal semiconductor quantum dot heterostructures are an attractive platform for photon upconversion in solar energy applications due to their wide absorption bandwidths and highly tunable optical properties. NIR-to-visible photon upconversion has been previously demonstrated in PbS/CdS/CdSe core/multishell heterostructures, but their reported upconversion efficiencies are low. The upconversion performance could be significantly improved by engineering the PbS/CdS core/shell intermediate structure to achieve the quasi-type II band structure and carrier separation behavior known to promote the upconversion process. Here we address two critical challenges to realizing an optimized PbS/CdS intermediate structure that could enable efficient upconversion in full PbS/CdS/CdSe structures. We first use computational simulations to predict the band alignment and carrier behavior in PbS/CdS and PbS/CdS/CdSe as a function of PbS core size and CdS shell thickness. We use the results to develop synthesis targets for PbS/CdS predicted to achieve effective carrier separation and improved upconversion performance. Next, we synthesize a library of PbS/CdS quantum dots via cation exchange across three particle sizes. We analyze the reaction products using absorbance, PL, and transmission electron microscopy to create a framework for the predictive synthesis of PbS/CdS with the target core and shell dimensions. Finally, we combine our computational and experimental findings to identify and understand a trade-off in design and synthetic factors required to realize PbS/CdS structures that provide a foundation for efficient NIR-to-visible upconversion.