Publication date: 15th May 2025
Solid-state NIR/SWIR-to-VIS triplet-fusion upconversion has many possible applications that require high efficiency at low light intensities. The performance of present-day upconversion devices that are sensitized by oleate-capped lead sulfide (PbS) quantum dots (QDs) is limited by short exciton diffusion lengths—the best reported devices use very thin QD layers (~monolayer), where only 0.1-6% of incident light is absorbed. Inspired by the success of QD photovoltaics, we are exploring carrier-based device architectures where longer-range transport could increase upconversion performance at higher optical densities. Here we report solid-state, nanocrystal-sensitized upconversion in devices made with PbS QDs capped with TBAI ligands. Compared to devices using PbS QDs with long-chained ligands, our devices demonstrate strongly improved absorbance and improved upconversion performance for QD film thicknesses up to ~13 monolayers. However, the external upconversion efficiency remains modest, and progressively decreases in devices with more than two monolayers of PbS-TBAI QDs. Our spectroscopic studies show that FRET-like back-transfer of excitations from the emitter to the QDs is a contributing factor, but does not appear to be the primary challenge. This focusses attention on mitigating possible losses in charge transport and at the QD-emitter interface.
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