High Electroluminescence of ZnS:Cu Phosphor with a CNT-Incorporated Dielectric Layer

Abstract

High brightness was obtained for alternating current-driven inorganic ZnS:Cu powder electroluminescence (EL) devices by incorporating carbon nanotubes in a barium titanate (BaTiO3) based dielectric layer. The inorganic EL devices were consisted of four layers: a top electrode, a dielectric layer, an emitting layer, and an ITO layer which was pre-coated on the glass substrate. As an emitting material, ZnS:Cu power of particle size 9 µm (green emission) was mixed with the cyanoethyl pullulan polymer (CEP), as an organic binder, and the dimethyl formamide (DMF) organic solvent. The emitting solution was spin-coated on the ITO glass at the fixed speed of 1200 rpm. The BaTiO3 power were mixed with a CNT-CEP binder, which was also spin-coated on the emitting layer. In order to optimize the EL performance, the concentration of CNTs were varied by controlling the mixing ratio of CNTs—0, 0.0018, 0.0039, 0.0071, 0.011 and 0.014 wt%. Finally, a 500-nm-thick aluminum top electrode was thermally evaporated onto the dielectric layer. Up to 50% higher brightness was obtained for the CNT concentration of 0.011 wt%. The high brightness might be caused by the high dielectric constant of the CNT-incorporated BaTiO3 dielectric layer, thanks to the micro-capacitor effect. [Z. M. Dang et al., Small 12, 1688 (2016).] Roughly speaking, the higher concentration of the CNT yielded to the higher brightness, until a certain concentration, here, between 0.011 and 0.014 wt%. It implies that too high concentration of CNTs might lead to the formation of the unwanted percolation network of a leakage current within the dielectric layer. Finally, the role of the chlorine was investigated by inclusion of chlorobenzene into the CNT-containing dielectric layer. [J.-Y. Kim et al., Adv. Mater. 25, 2308 (2013).] Based on the above results, incorporation of CNTs within the dielectric layer enhanced the brightness (and the efficiency, possibly) of the ZnS:Cu-based EL device, which would help to extend the potential applications of EL devices.