Enhanced Upconversion Performance in Alloyed CdSe(Te)/CdS1–xSex/CdSe Core/Rod/Emitter Nanostructures

Abstract

Colloidal quantum dot (QD) nanostructures have been shown to convert multiple low-energy photons into a single, high-energy photon through a process called photon upconversion. QD nanostructures could replace conventional upconverter materials in solar cell applications and can be tuned to absorb photons transparent to the host cell and emit usable photons, increasing the short circuit current of the host cell while maintaining open circuit voltage. Double QDs (coupled via a nanorod) have previously demonstrated 0.1% upconversion efficiency and a peak-to-peak energy gain of 380meV under pulsed excitation equivalent to 105 times solar concentration. However, for upconverters to be viable for solar energy harvesting, they must have high efficiency in low-light (1-sun) conditions. Engineering improved performance under device-relevant conditions requires a better understanding of the upconversion mechanisms. We synthesize core/rod/emitter complexes and demonstrate near-infrared (NIR)-to-visible upconversion photoluminescence (UCPL) in these structures with continuous wave (CW) illumination near low-light conditions. We further observe photon energy gains of 700meV (from 850nm to 575nm). To improve upconversion efficiency, we grow homogeneous absorber QDs and alloyed rods and systematically study the quantum yield using an integrating sphere to measure upconversion photoluminescence relative to a rhodamine 101 standard. We observe an order of magnitude increase in upconversion quantum efficiency. Further understanding the effect of morphology and composition on upconversion and carrier transfer mechanisms is critical to realizing improved optical performance and upconversion quantum efficiencies in these nanostructures.