Understanding Degradation of Doped Electroluminescent ZnS Phosphors
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Understanding Degradation of Doped Electroluminescent ZnS Phosphors Yu Jiang, Jacob Stanley, Laurel Ruhlen, Chris France, John Willey, Anna Bezryadina, Frank Bridges, and Sue Carter Physics, University of California, Santa Cruz, Santa Cruz, CA, 95064 ABSTRACT We have shown recently that the degradation of AC electroluminescence (EL) in ZnS:Cu,Cl and similar phosphors can be significantly reversed by a short anneal near 200°C. To better understand the degradation/rejuvenation processes, we investigated EL degradation and rejuvenation under different conditions. To probe for changes in the local atomic structure about the Cu sites, we collected Extended X-ray Absorption Fine Structure (EXAFS) data at the Cu Kedge EXAFS data for several as-made, thermally degraded (240°C anneal), and EL degraded powders. INTRODUCTION The electroluminescent (EL) material ZnS:Cu,Cl and similar phosphors have the unusual property that under AC excitation they exhibit bright blue-green electroluminescence at electric fields 100 times smaller than for DC driven devices. Because these materials have high external quantum efficiencies (> 50%), long device lifetimes combined with low operating voltages would result in low cost, high power efficiency devices, providing a breakthrough in next generation solid state lighting. A crucial aspect of the low voltage operation is the formation of CuxS precipitates, even at low Cu concentrations (0.15%). Under AC operation, it is assumed [1] that holes and electrons are injected into the ZnS from opposite ends of the CuxS precipitates each time the voltage is reversed; the bright blue luminescence arises from electron-hole recombination each time the voltage switches. A fundamental unsolved challenge is to understand how the charge injection, transport and recombination are affected by this novel nanostructure and the implications of this structure for power efficiency and device lifetime. Changes in the local atomic structure induced by high AC electric fields, results in a decrease in EL over time; this aging effect has previously been shown to progress faster for smaller ZnS particle sizes. EXPERIMENT In a preliminary study [2], we showed that the main Cu precipitate looks like CuS at the nearest neighbor level (from EXAFS), and more importantly that the EL degradation could be significantly reversed by annealing at 200 °C. Building upon these results, we have studied how this degradation/rejuvenation is affected for a wider range of temperatures, for which we expected, but did not observe, changes in the CuS precipitates; we have also extended our studies to samples containing orange-emitting Mn dopants to understand the role of the emissive sites. Typically, a 100V square wave at 100 kHz was used to degrade the EL in 20 hours to about 20% of initial intensity, an affect that is largely independent of doping type. Using a 2 hours annealing time, the optimum annealing temperature is 180 °C as shown in Fig. 1. At this temperature the rejuvenation can approach 70%, with most of the rejuven
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