2, Faculty of Science and Engineering, Iwate University, Morioka, , Japan
3, Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, , Japan
Triplet-triplet annihilation up-conversion (TTA-UC) has attracted considerable attention recent years because of its high up-conversion quantum yield (UC-QY) and low excitation intensity comparable with the solar radiation. These features allow a variety of potential applications such as spectral management of sunlight for solar cell and biological imaging.
TTA-UC have been extensively studied for the systems converting from the visible (Vis) region of wavelength mostly in solution or polymeric media. We have realized efficient solid-state UC from green to blue wavelength in the binary solid fabricated from the rapid-drying casting method. In this method, we used the saturation concentration of the emitter, allowing faster solidification of emitter (matrix) .
To step forward for efficient use of solar energy, it is needed to extend the excitation wavelength to the near-infrared (NIR), which accounts for more than 50% of the entire energy of solar radiation. In this work, we develop solid-state NIR (785 nm)-to-yellow (570 nm) TTA-UC system with relatively lower excitation intensity and higher UC-QY, by using the rapid-drying casting method. For NIR-to-Vis conversion, we applied PdTPTAP (Pd-tetrakis(3,5-di-t-buthylphenyl)tetraanthrophophyrin) as a sensitizer and rubrene as a emitter. These molecules have the close triplet energy levels to each other. The binary solid of the sensitizer (guest) and emitter (host) was fabricated on a slide glass from the mixed solution of them in tetrahydrofuran at saturated concentration of the emitter. Many particles of the binary solid were obtained with the size of 10-100 µm and the thickness of 1-3 µm. Under excitation with a NIR laser diode (785 nm) though a 20x objective lens (excitation intensity 0.5-1 W cm-2 at the sample position), the particles were found to show yellow UC emission, originating from delayed fluorescence emission of rubrene. In TTA-UC system, the threshold intensity (Ith) defined as the excitation intensity of the crossing point of quadratic dependence of the UC emission to the excitation intensity at lower excitation intensities and linear dependence at higher excitation intensities. Ith of the binary particles was found to be 0.2-0.8 W cm-2 depending on particles. The Ith were in the similar order or less than that of the reported solution system with similar sensitizer . The UC-QY of 30 individual single particles was examined under an optical microscope. The values were as high as ∼1% under air condition. The UC-QY was unchanged after storing the binary solid for 80 days or more under Ar environment.
We successfully developed the NIR-to-Vis TTA-UC in binary solid by using of the rapid-drying casting method with relatively low threshold intensity and high UC-QY. This result will open the way to solid state TTA-UC by NIR excitation by using simple fabrication method of solution casting.
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