Single-doped (Ce3+,Tb3+), co-doped (Ce3+-Tb3+, Ce3+-Yb3+, Tb3+-Yb3+) and tri-doped (Ce3+-Tb3+-Yb3+) Y3Al5O12 phosphors were synthesized by a solid-state reaction method. The XRD, excitation and emission spectra and the fluorescence lifetime of the samples were measured. The energy transfer mechanism was also investigated. The results showed that the energy transfer efficiency from Tb3+ to Ce3+ was 51% and the energy transfer efficiency from Ce3+ to Yb3+ was 63.1%. Concomitantly, both were more efficient than that from Ce3+ to Tb3+ (7%) and from Tb3+ to Yb3+ (10.2%). Also, the Yb3+ ions received energy mainly from Ce3+ ions in Ce3+-Tb3+-Yb3+ tri-doped Y3Al5O12 phosphors. Among these materials, Ce3+-Yb3+ co-doped YAG phosphors are a better choice than others as a down-conversion material due to their higher energy transfer efficiency.Typical excitation (a) and emission (b) spectra of Ce3+-Tb3+-Yb3+ tri-doped Y3Al5O12 phosphors excited at 455 nmDownload high-res image (68KB)Download full-size image

Ce3+–Yb3+ co-doped Y3Al5O12 phosphors are one of most the potential candidates of down-conversion materials for photovoltaic cells. To improve the near-infrared emission from Yb3+ ions, Bi3+ ions were introduced into the phosphors. The experimental results showed that the intensity of Yb3+ emission was greatly enhanced after doping Bi3+ ions into the phosphors. This may be attributed to an additional pathway via Bi3+ ions for the energy transfer from Ce3+ to Yb3+. Different from the co-doped phosphors, where the energy of Ce3+ ions was transferred only to Yb3+ ions, in Bi3+–Ce3+–Yb3+ tri-doped phosphors, the energy of Ce3+ ions could be transferred not only to Yb3+ ions but also to Bi3+ ions. Then, Bi3+ ions transferred part of the energy to Yb3+ ions. This enabled Yb3+ ions to obtain more energy and enhance their near-infrared emission.

The interface energies and electronic structures of (112) grain boundaries of CuInSe2 thin films were investigated by first-principle calculations. It is found that the grain boundary with a Cu vacancy has low interface energy and exists widely in the films. The Cu deficiency may cause the charge imbalance and result in an upward band bending at the grain boundary. It also weakens the repulsion between Cu-3d orbital and Se-4p orbital and leads to the downward shift of valence band maximum. The two mechanisms, namely the band bending from the charge imbalance and the depression of the valence band maximum, have effects on the (112) grain boundaries with different defects. The change of band structure forms a potential barrier to prevent electrons or holes from approaching the grain boundary and reduces their recombination. This might be used to explain the effects of the grain boundary on carrier transportation and why polycrystalline Cu(In,Ga)Se2 thin film solar cells have better performance than single-crystal cells.