Thin film electroluminescent displays produced using sol-gel methods

摘要

An inverted double insulating thin film electroluminescent (TFEL) display has been fabricated using all sol-gel methods. This involved the production of three film types, an insulating material, a conducting film and a luminescent film. The layers have been evaluated individually and the combination effects are also looked at. The optimised film choice for the display is then given.ududThis investigation focused on manganese doped zinc sulphide (ZnS:Mn), which has a strong orange emission due to the Mn2+ 4 Ti(4G) -» 6Ai(6S) transitions. It is produced from sol-gel deposited zinc oxide films. The oxide films are converted to zinc sulphide by annealing in a hydrogen sulphide-containing atmosphere. The conversion process was investigated and it was found that it takes place in a two-step process that is controlled by diffusion. The parameters of the conversion were optimised to produce the doped zinc sulphide having the changes in structure and composition as a function of sulphidation temperature and annealing time. It has been found that, after an initial “dead time”, conversion takes place in a two-step manner where, for an initial period of approximately 60 mins. little diffusion takes place followed by faster diffusion with a diffusion coefficient of 7.8xl0~18 mV1, which is independent of sulphur concentration. It is also found that the sulphide forms the hexagonal, wurtzite phase with a strong (002) orientation.ududThe emission due to the Mn2+ 4Ti(4G) 6Ai(6S) transitions has been investigated using photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL) and the correlation between the luminescence produced by the various methods has been studied. A comparison of the spectra using PL, CL and EL has shown how these excitation methods can be used to quantify the electroluminescence produced by the zinc sulphide. The luminescent properties of ZnS:Mn films have been investigated for their application as an emission layer in the thin film electroluminescent display. The luminescence of the device depends on the structure of the device and various structures were fabricated and the luminous output investigated. It was found in this case that the optimum structure is a five layer inverted structure. The luminescent properties also depend on the insulating films used in the device and their properties. In this case two insulating materials were investigated.ududThe insulating films used in the device were both tantalum pentoxide (Ta20s) and silicon dioxide (SiC>2 ). Devices using both materials have been produced. The properties and the interaction between the emission layer and the insulating layer were investigated. The effect of the crystal structure of the Ta20s on the luminescent properties of the device was also investigated. The luminescent characteristics of the fabricated devices have been measured and a comparison is made with the characteristics of sol-gel TFEL devices using SiC>2 insulators. The TFEL devices with the Ta2 0 5 insulators have shown higher stability and more reliability in theiroperation Therefore Ta2Os is the preferred choice as an insulating material The conducting material chosen for this device is aluminium doped zinc oxide (ZnO Al) as it is a transparent conductor that is compatible with the insulating materials and enhances the performance of the device