Photochemical Photon Upconverters with Ionic Liquids
  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. 
Yoichi Murakami
cond-mat.mtrl-sci