Photon upconversion is a promising strategy to boost the conversion efficiencies of commercially available photovoltaics beyond the Shockley-Queisser limit by harvesting photons with energy below the band gap of a host cell and converting pairs of such low-energy photons into single high-energy photons that can be returned to and absorbed by the host cell. Existing upconversion materials based on lanthanide-doped nanocrystals and sensitized triplet-triplet annihilation have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency.1 II-VI based semiconductor nanostructures offer a new paradigm for photon upconversion in which bandgap, relative band alignment, and carrier dynamics can be tailored through material composition, size, and morphology to achieve high quantum yield at desired wavelengths with broad spectral absorption.2 Recent advances in the synthetic capabilities of semiconductor nanoparticles has enabled the fabrication of complex heterostructures composed of various materials. One class of new semiconductor heterostructures are colloidal double quantum dots (QDs), where two spatially separated semiconductor nanoparticles are electronically coupled.3 Colloidal synthesis of such semiconductor heterostructures offers a route to solution-compatible, low-cost processing. We report a semiconductor nanostructure upconverting platform consisting of coupled quantum dots (QDs)3 that efficiently upconverts low-energy photons under continuous-wave illumination. The nanostructure consists of a narrower bandgap tellurium-doped cadmium selenide QD absorber and a wider bandgap cadmium selenide emitter spatially separated by cadmium sulfide nanorod. The double quantum dot nanostructure is designed such that electrons promoted by a first photon absorption in the Te-doped CdSe absorber are delocalized over the entire structure but the holes remain confined. A second low-energy photon excites the hole via intraband absorption, allowing it to escape into the cadmium sulfide and ultimately relax into the emitter QD. We will present synthesis conditions, structural characterization, and detailed optical spectroscopy of the upconversion photophysics that collectively demonstrate control over the photon energies absorbed and the corresponding photon energy gain through control over the composition and size of the absorbing QDs.
1. Zhou, J., Liu, Q., Feng, W., Sun, Y. & Li, F. Upconversion Luminescent Materials: Advances and Applications Jing. Chem. Rev. 115, 395–465 (2015).
2. Sellers, D. G. et al. Novel nanostructures for efficient photon upconversion and high-efficiency photovoltaics. Sol. Energy Mater. Sol. Cells 155, 446–453 (2016).
3. Deutsch, Z., Neeman, L. & Oron, D. Luminescence upconversion in colloidal double quantum dots. Nat. Nanotechnol. 8, 649–653 (2013).