Electroluminescence materials ZnS:Cu,Cl and ZnS:Cu,Mn,Cl studied by EXAFS spectroscopy

When a high-frequency ac voltage is applied to ZnS:Cu,Cl and ZnS:Cu,Mn,Cl devices, optical fluorescence, known as ac electroluminescence (EL), is observed; it depends on both the Cu (Mn) and Cl dopants. The local structure of these compounds was studied using the extended x-ray-absorption fine-structure (EXAFS) technique to understand the role of Cu and Mn. Data were taken at the K edge of Zn, Cu, and Mn, for powder material and for both new and aged (degraded, low EL) devices. The EXAFS data show that Mn substitutes for Zn in the ZnS lattice, whereas Cu has a different local structure (it cannot be fit well with the ZnS structure). For all the Cu edge data, the first shell of neighbors is best fit using an experimental standard obtained from CuS, suggesting that almost all of the Cu resides in tiny CuS-like clusters. Since these clusters are dominant in both the new and degraded samples and do not change with aging, they likely do not contribute directly to the luminescence. Consequently, our results indicate that a very small fraction of the Cu atoms are EL-active; this is consistent with previous models for which the emission centers involve isolated Cu defects or Cu pairs. Thus, a possible explanation for the rapid degradation of a device is that the isolated Cu ions electrodiffuse to the CuS-like clusters, at which point they no longer produce EL. Based on these results, a low-temperature annealing experiment (200C) was carried out that shows that a degraded device can be nearly completely rejuvenated by heating.