Broadband Near-infrared Upconversion Nanocrystals

One of the potential pathways for exceeding the Shockley-Queisser efficiency limit of photovoltaic cells is via incoherent upconversion of low-energy photons. An ‘ideal’ upconversion system would have high efficiency, a broad response and a low saturation intensity, all encapsulated in an easily deposited system. Past colloidal upconversion nanocrystals were based on rare-earth doped oxides or on organic triple-triplet annihilation polymers. Several years ago we introduced double quantum dots as an alternative upconversion mechanism. The formation of the latter system and recent advances in the design of double quantum dots will be discussed. We also introduce a new hybrid organic-inorganic particle where upconversion is mediated by ligand induced mid-gap states. The state of the art of all these designs will be discussed, as well as the potential for low cost upconversion add-ons to present day solar cells.