Down-conversion Luminescence Properties Of Rare Earths Doped Silicon Oxide Thin Films

Recently, quantum cutting solar spectrum by using rare earth (RE) ion couples hasattracted great interest, for the down-conversion (DC) between REn+and ytterbium ion(Yb3+) can be used to enhance the optical transition of crystal Si, thus to improve thephotovoltaic conversion efficiency of Si-based solar cells. In this paper, we have studied theDC luminescent properties of cerium (Ce) and Yb co-doped silicon oxide (SiOx:Ce,Yb)and Yb heavily-doped silicon oxide (SiOx:Yb) thin films. The compositions of the films arecharacterized by using Rutherford back scattering spectroscopy (RBS); the structure of thefilms is investigated by using X-ray diffraction (XRD) and transmission electronmicroscopy (TEM). The DC luminescent properties are studied by photoluminescence (PL)spectrum, PL excitation (PLE) spectrum and decay curve (PL-decay).For the SiOx:Ce,Yb with “oxygen-rich” Si oxide, after high temperature (>1000℃)annealing, crystallized Ce (or Yb)-related silicates have formed in the matrix. While for theSiOx:Ce,Yb with “silicon-rich” Si oxide, the main crystalline phase is CeO2after the hightemperatures annealing, but one cannot rule out the presence of Ce, Yb-related silicates.Under the325nm line excitation, both the Ce3+and Yb3+PL intensities have increased withelevating the anneal temperature, which indicates the crystallized structure contributes toYb3+PL. The PLE spectra and PL-decay curves show that the DC energy transfer processCe3+�'Yb3+occur in the SiOx:Ce,Yb thin films, and the highest non-radiative cooperativeenergy down-conversion quantum efficiency of our samples is about151%theoretically.For the SiOx:Yb thin films, different crystallized Yb-related silicates have formedafter high temperature annealing, possibly related to the Si concentration of the oxide; andthe Yb silicates have different luminescent efficiency. From the PL and PLE spectra, energytransfer from transient Yb2+(and/or defects) to Yb3+possibly have occurred in the SiOx:Ybthin films. The luminescent property of SiOx:Yb varies evidently, when the thin filmsundergo different two-step annealing.In conclusion，SiOx:Ce,Yb thin film can act as potential spectral converters to improvethe photovoltaic conversion efficiency of Si-based solar cells. And the SiOx:Yb thin filmmay have other potential applications such as near infrared optical fiber laser.