ダウンコンバージョンによる太陽電池の変換効率向上を目指したTm,Yb共添加ZnOナノワイヤの作製と光学評価

抄録

We report the growth and optical characteristics of Tm,Yb-codoped ZnO (ZnO:Tm,Yb) films on ZnO nanowires (NWs). The growth conditions of the NWs are optimized by changing the growth duration as well as growth temperature and VI/II ratio, which are grown on sapphire (0001) substrates. Photoluminescence (PL) characterization of the ZnO:Tm,Yb films on optimum ZnO NWs is performed with excitation by different lasers to investigate the mechanism of the energy transfer between the Tm³⁺ and Yb³⁺ ions in the films. Above-bandgap excitation of the films on optimum ZnO NWs yields several sharp peaks from Tm³⁺ and Yb³⁺ ions as well as a broad luminescence band from the oxygen vacancies in the ZnO. The PL intensity of each peak can be controlled by changing the concentration ratio of Tm³⁺ and Yb³⁺ ions. This result is due to the energy transfer between Tm³⁺ and Yb³⁺ ions where the energy transfer from Tm³⁺ to Yb³⁺ ions takes place in the sample with the high Yb mixing ratio and the reversed energy transfer process occurs with the lower Yb mixing ratio. Direct excitation of Tm³⁺ ions at 488 nm results in luminescence from the Yb³⁺ ions with specific luminescence centers. Furthermore, time-resolved PL characterization of the NWs with different concentrations of Tm³⁺ and Yb³⁺ ions reveals a change in the lifetime of luminescence from Tb³⁺ ions in the films. These results suggest the possible energy transfer between Tm³⁺ to Yb³⁺ ions in the ZnO:Tm,Yb films on ZnO NWs.