The upconversion of light has numerous potential applications ranging from photocatalysis, photovoltaics, imaging, and advanced manufacturing. The strategies being investigated to achieve this include energy transfer upconversion in lanthanide nanoparticles, and sensitized triplet fusion, also known as photochemical upconversion or triplet-triplet annihilation upconversion (TTA-UC). The latter involves the generation of triplet excited states by a sensitizer and their subsequent transfer to an emitter. Emitter triplets annihilate (fuse) to generate an emissive singlet state from which a higher energy photon is emitted. This strategy is spectrally adaptable, and high efficiencies are routinely achieved in solution, approaching the quantum yield ceiling of 50%.

Recently, there has been progress in translating triplet fusion upconversion into the solid state, which is desirable from a device engineering standpoint.[1] There are many possible solid-state device architectures. Broadly these can be categorized into materials wherein the sensitizer and emitter are evenly distributed, and heterogeneous devices where triplet generation is spatially separated from triplet fusion. The latter strategy is attractive, as the upconverted singlet state is protected to an extent from Förster resonance energy transfer back to the sensitizer. A simple concept would be to lay down a monolayer of sensitizer overlaid with a thin film of emitter. However, such devices suffer from poor ligh absorption.

In dye-sensitized solar cells (DSSCs), the problem of poor light absorption by a monolayer of chromophores is addressed with a nanostructured metal oxide support, such that incoming light passes through a multitude of chromophore-bearing interfaces. Indeed, DSSCs incorporating TTA have been demonstrated.[2]  As there are many steps in photochemical upconversion, it is desirable to investigate certain aspects in isolation. To concentrate on the solid-state sensitization process, here we present a strategy where the sensitizer is chemisorbed on a nanoporous Al₂O₃ support, but the emitter remains in solution.[3]

Using time-resolved spectroscopies, we show that the sensitizer triplets are long-lived, do not suffer significant aggregation-induced quenching and can be efficiently transferred to the emitter chromophores. We demonstrate devices with internal upconversion quantum yields as high as 9.4%, which is 19% of the maximum achievable value and 38% of what can be achieved with the diphenylanthracene emitter. This shows that using dyes bound to nanoporous Al₂O₃ films is a viable strategy to sensitize upconversion within a solid state material, and bodes well for future development with solid state emitters. Initial work on liquid-chromophore and fully solid state systems based on the same Al₂O₃ films will also be presented.