High Field Electroluminescence Characteristics of Pr$^{3+}$ -Doped ZnS/TiO$_{2}$

This paper reports the fabrication, operation, and applications of rare-earth (RE) ion praseodymium (Pr
$^{3+}$
)-doped ZnS/TiO
$_{2}$
core-shell nanoparticle-based thin-film electroluminescent (ACTFEL) devices. Pr
$^{3+}$
:ZnS/TiO
$_{2}$
ACTFEL devices showed white light emission due to impact excitation of the Pr
$^{3+}$
ions by hot carriers followed by radiative RE relaxation under the high field of alternating current. The preparation of Pr
$^{3+}$
:ZnS/TiO
$_{2}$
core-shell nanoparticles was done by the colloidal synthesis method. To observe the effect of the TiO
$_{2}$
shell layer on electroluminescence (EL) brightness and properties of Pr
$^{3+}$
:ZnS, three types of molar ratios 1:7:7, 9:5:5, and 1:14:14 of Ti:Zn:S were taken in the synthesis process, and to be employed for these three different thin-film EL devices for high-field EL measurements. Each EL device consists of glass substrate/ITO (indium-tin oxide)/TiO
$_{2}$
coated Pr
$^{3+}$
-doped ZnS nanocrystalline emission layer/Al (aluminum). The brightness characteristics of Pr
$^{3+}$
:ZnS/TiO
$_{2}$
TFELDs were evaluated as a function of bias and frequency. Brightness–voltage ( B–V) measurements at 600 Hz showed the maximum efficiency of the Pr
$^{3+}$
:ZnS/TiO
$_{2}$
EL device at molar ratios 1:14:14 of Ti:Zn:S. To observe the frequency-dependent characteristics, the EL device was tested at various frequencies. The measurements showed the decrease in the threshold voltage of the EL device with increased frequencies. High-field voltage–current (V–I) characteristics also showed the frequency-dependent onset voltage and current density. It showed the minimum turn-on voltage and maximum current density at a higher frequency of 1000 Hz.