(229b) Optofluidic Encapsulation of Photon Upconversion Systems Towards Solar Energy Harvesting

Photon upconversion (UC), the emission of light at shorter wavelength than the excitation, has the potential for overcoming the thermodynamic limits of sunlight-powered devices and processes. However, conventional UC systems such as those using two-photon absorption and second-harmonic generation require high excitation intensities and as a result may be undesirable. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA).

The efficiencies of TTA-UC are determined by effective diffusion of the triplet excitons to have spatial overlap between the donor and acceptor wave functions for energy transfer. To maximize energy migration, molecular diffusivity within an inert medium is of paramount importance, especially in solid-state matrices for practical operation. Although many rubbery polymers with a low glass-transition temperature have been investigated as a potential matrix, their restricted molecular mobility resulted in much lower UC efficiency than liquid solutions.

Here, we have developed an alternative UC system composed of uniform capsules fabricated via a microfluidic emulsion approach using a photocurable resin. The capsules comprise a core-shell structure consisting of a fluidic active core which allows for highly efficient molecular interactions required for TTA-UC thus preventing the large decrease in the efficiency observed in solids, and an elastomeric shell for facile device integration with sufficient mechanical strength and air stability. We have investigated photochemical properties related to diffusive energy-transfer-driven photoluminescence in a bi-molecular UC system to utilize sub-bandgap photons of the solar spectrum.