Nanowire-Based Alternating Current Oxide Powder Electroluminescent Materials and Devices

Abstract

A novel type of alternating-current (AC) powder electroluminescent (EL) device that relies on nanowire-phosphor heterogeneous junction structure has been developed. It shows promise for manufacturing of highly stable powder EL devices. To pursue this goal, manganese ion (Mn2+)-doped zinc germanate (Zn2GeO4:Mn) oxide phosphor was synthesized and used as EL powder material for the fabrication of the new types of AC powder EL devices. The Zn2GeO4:Mn oxide phosphor powder could eliminate the well-known degradation problem of zinc sulfide (ZnS)-based AC powder EL devices predominant in the current marketplace. In order to realize a high brightness at a relatively low operating voltage, a conductive semiconductor nanowire architecture using zinc oxide (ZnO) and copper oxide (CuO) nanowires with sharp-tip features, was created and integrated into conventional AC powder EL structures. Particularly, vertically-aligned n-type ZnO nanowires arrays were successfully synthesized on a Zn2GeO4:Mn polycrystalline substrate for the first time, and the growth behavior using a chemical vapor deposition (CVD) process was investigated. The density of the ZnO nanowires could be effectively controlled by some experimental parameters, such as the density of gold catalyst nanoparticles, and spatial distance between substrate and CVD source powders. This novel ZnO nanowire-decorated Zn2GeO4:Mn phosphor architecture was used to fabricate top-emission AC powder EL devices. On the other hand, a vertically-aligned ZnO nanowire array was directly synthesized on a thin film indium-tin-oxide (ITO)-coated glass substrate for fabrication of bottom-emission AC powder EL device. The nanowires were adopted to form heterogeneous junction structures between the tips of the nanowires and the Zn2GeO4:Mn EL powders, so that the composites could have a similar electrical field enhancement as in the needle-like CuxS precipitates within widely-used ZnS:Cu EL powder materials. The behavior of these top- and bottom- emission AC powder EL devices was also studied. Alternatively, vertically-aligned p-type CuO nanowires were prepared by a thermal oxidation method. Reliable heterogeneous junction structures were formed by a simple drop-coating method. The CuO nanowire-based AC powder EL device has excellent brightness maintenance with a loss of luminescent intensity under 1 % at over 10 cd/m2 luminance during 360 hours of operation. The integrating of semiconductor nanowires into conventional AC powder EL device structures offers the very promising prospect of fabrication of simple, low-cost, scaled-up EL devices that could fundamentally solve the limited operational lifetime of current ZnS-based AC powder EL devices.