Near-infrared Quantum Cutting Of Tb³⁺-Yb³⁺ Co-doped Nano-crystals And Glasses

Traditional fossil fuels are all non-renewable energy, they cannot be used forever. Solar Energy is the real renewable energy and has gained more and more attantion. Si solar cells dominate the photovoltaic market. But from now on the conversion efficiency of the Si solar cells is still low due to the thermalization losses. Near-infrared quantum cutting can create two 1μm photons through'cutting'a single visible photon to decrease the thermalization losses and then enhance the efficiency of Si solar cells. Therefore the near-infrared quantum cutting of the Tb³⁺-Yb³⁺ couple was investigated.The investigation for near-infrared measurement has been carried out. The absorption spectrum measurement system, fluorescence measurement system and time-resolved spectrum measurement system were designed and built. Using electro-optic modulator to adjust the polarization direction of the laser beam. Then the light intensity would be modulated. The response time of the system is 0.02 ms.Near-infrared quantum cutting of Tb³⁺-Yb³⁺ couple in Y₂O₃ and NaYF₄ nano- crystals were studied. The emission spectra of Yb³⁺ ion was obtained excited by 473 nm laser. With the increasing of Yb³⁺ concentration, the fluorescence intensity and the lifetime of the 5D4 level of Tb³⁺ ion all decreased. This phenomenon proved that the energy transferred from Tb³⁺ ion to Yb³⁺ ion. The emission of Yb³⁺ ion was increased at the beginning and then decreased. The maximum emission intensity of Yb³⁺ ion was obtained in the sample with the concentration of 1 mol% Tb³⁺ and 3 mol% Yb³⁺, and the quantum efficiency is 110% at this concentration. The linear relationship of the pump power dependence of Yb³⁺ ion demonstrated that the quantum cutting mechanism in Y₂O₃ nano-crystal is cooperative quantum cutting. However, in NaYF₄ nano-crystal the emission intensity of Yb³⁺ ion and the pump power were nonlinear. For this reason a new quantum cutting model was established.The investigation of the quantum cutting mechanism of Tb³⁺-Yb³⁺ couple in oxyfluoride glasses was carried out. The glasses were prepared by solid-state reaction method. The enhanced emission intensity of Tb³⁺ ion and Yb³⁺ ion were obtained in oxyfluoride glasses under 473 nm laser excitation. At the concentration of 2 mol% Tb³⁺ ion and 10 mol% Yb³⁺, the maximum fluorescence emission intensity of Yb³⁺ ion was obtained and the quantum efficiency approached 140% was gained. The quantum cutting mechanism was studied by the analysis of the pump power dependence and the luminescence of Yb³⁺ ion, the steady-state rate equations demonstrated that the quantum cutting mechanism in oxyfluoride glasses is second order downconversion or one order two-step downconversion through cooperative energy transfer.