Near-Infrared–to–Visible Upconversion Sensitized by Conductive Films of PbS Quantum Dots

Photon upconversion via nanocrystal-sensitized triplet fusion is a photophysical process that converts two low-energy photons into one high-energy photon. Solid-state, near-infrared-to-visible upconversion has been previously demonstrated using bilayer devices incorporating lead sulfide (PbS) quantum dots (QDs) as the sensitizer. However, the potential light-harvesting ability of previous architectures is limited by the short exciton diffusion lengths in QD films with long, insulating, native ligands. Here we demonstrate near-infrared-to-visible upconversion using conductive films of PbS QDs capped with short iodide ligands, inspired by materials used in optically thick QD solar cells. Bilayer devices with conductive PbS QDs demonstrate upconversion (λ: 808 nm → 612 nm), and the best carrier-based devices exhibit 30 ± 10% improved brightness in the linear regime relative to the best devices sensitized by the same PbS QDs with their native, oleic acid ligands. We show that conductive devices can retain 35 ± 7% of their best absolute upconversion brightness in devices thick enough to absorb 9.6 ± 0.1% of the incident NIR light (a >30× improvement over comparable devices with insulating ligands). However, we observe a performance roll-off beyond 1.5 monolayers that implies unaddressed internal-efficiency challenges. Using steady-state and transient optical spectroscopies, we attribute this roll-off to two unwanted processes caused by both types of QD films used here: (1) FRET-dependent back-transfer of excitations (from the organic to the PbS QDs); and (2) QD-film-induced changes in the morphology of the organic layer that impair rubrene-to-DBP energy transfer. These results highlight future enhancements to the architecture that could mitigate these losses and unlock the long-range sensitization capabilities of conductive PbS QD films.