Upconversion by ${\mathrm{Yb}}^{3+}$ sensitization in ${\mathrm{Er}}^{3+}$-doped ${\mathrm{YVO}}_4$ crystals has been studied under excitation with a 976 nm laser diode. In addition to the previously observed upconverted 526 and 554 nm ${\mathrm{Er}}^{3+}$ emission bands, three weak upconverted ${\mathrm{Er}}^{3+}$ emission bands are observed at 490, 660, and 670 nm at room temperature. From the quadratic pump-power dependence, it is confirmed that the upconversion is induced by a two-photon excitation process involving excited-state absorption from the ${}^4{I}_{11/2}$ state of ${\mathrm{Er}}^{3+}.$ In addition to these emission bands, downconverted ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$ emission bands are simultaneously observed at 1010 and 1470–1630 nm, respectively. It is observed that the infrared ${\mathrm{Er}}^{3+}$ emission intensity increases linearly with pump power, while the ${\mathrm{Yb}}^{3+}$ emission intensity saturates at higher pump powers.

Absorption spectra, luminescence spectra and luminescence decay time are investigated for a borosilicate glass containing low 0.1 wt% Ce3+ and high 15 wt% Tb3+ concentrations, together with the time-resolved luminescence. A broad absorption band due to parity allowed f → d transition is observed for Ce3+. Sharp absorption bands are observed in 3100–1600 nm region which are due to the 7F6 → 7F J (J = 0, 1, 2, 3, 4) transition of Tb3+, while much weak absorption bands due to the spin-forbidden 7F6 → 5D J (J = 2, 3, 4), 5G J (J = 3, 5, 6), 5L J (J = 7, 8, 9, 10), 5H J (J = 4, 5, 6), 5I J (J = 7, 8) and 5F J (J = 2, 4) transitions are observed in visible-UV region. By excitation with 266 nm laser line, two broad UV emission bands due to Ce3+ are observed at 360 and 390 nm besides the intense green emission bands due to the 5D4 → 7F J (J = 6, 5, 4, 3) transition of Tb3+ and weak blue emission bands due to the 5D3 → 7F J (J = 6, 5, 4, 3) transition. The decay time of the UV, blue and green emission is estimated to be 17 ns, 3.8 µs and 2.17 ms, respectively. The rise time of the green emission is about 4 µs after the Ce3+ emission and the Tb3+ blue emission. No rise time is observed for the latter emission.

Tandem organic light emitting diodes (OLEDs) are ideal for lighting applications due to their low working current density at high brightness. In this work, we have studied an efficient electron transporting layer of KBH₄ doped 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene (DPyPA) which is located adjacent to charge generation layer of MoO₃/NPB. The excellent transporting property of the DPyPA:KBH₄ layer helps the tandem OLED to achieve a lower voltage than the tandem device with the widely used tris-(8-hydroxyquinoline)aluminum:Li. For the tandem white OLED with a fluorescent blue unit and a phosphorescent yellow unit, we’ve achieved a high current efficiency of 75 cd/A, which can be further improved to 120 cd/A by attaching a diffuser layer.