  Photon upconversion based on triplet-triplet annihilation (TTA) of excited
triplet molecules is drawing attention due to its applicability for weak
incident light, possessing a potential for improving overall efficiencies of
solar energy conversion devices. Since energy transfer between triplet levels
of different molecules or TTA requires electron exchange between molecules by
the Dexter mechanism, inter-molecular collision is necessary and hence the
majority of previous studies have been done with volatile organic solvents as
fluidic media. This paper presents the development and characterization of a
new class of TTA-based photon upconverters with ionic liquids (ILs), which are
room temperature molten salts with negligible vapor pressure and high thermal
stability. The fabricated samples are found to be stable and the solvation
mechanism of the molecules in ILs is proposed. The upconversion quantum yield
(UC-QY) is IL dependent and the maximum value observed was approximately 10 %
for red-to-blue upconversion. An analytical model was developed to explain the
experimental results based on the assumption of efficient energy transfer
between molecules. The model agrees with the observed relationship between the
excitation intensity and UC-QY, showing that viscosities of the ILs are not a
practical problem for the usage of TTA-based photon upconversion.
