Materials engineering for high efficiency thin film electroluminescent devices

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Mastio, E.A., 1999. Materials engineering for high efficiency thin film electroluminescent devices. PhD, Nottingham Trent University.

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Abstract

The effect of annealing on electroluminescent devices based on inorganic ZnS:Mn (phosphor) thin films has been investigated. Conventional thermal annealing is compared with a new technique utilising pulsed optical irradiation of the thin film structure. Typically, alternating current thin film electroluminescent (TFEL) structures consist of a phosphor thin film sandwiched between two insulating layers and the light is produced via high field electroluminescence mechanisms. In the past, conventional thermal annealing at temperatures >500 °C has been demonstrated to increase the number of active luminescent centres, but also to modify the density of electrons trapped at the phosphor / insulator interface which was suggested to limit device performance. Thus, to further improve the luminescence efficiency of TFEL devices, laser annealing was proposed as a means to heat the phosphor thin film at high temperatures but without altering the interface states density.

In the present work, laser irradiations were performed directly on the uncoated phosphor using 20 ns pulses of KrF lasers. During laser annealing the samples were placed in a pressurised environment to limit the phosphor sublimation, which is described to be the mechanism responsible for laser ablation processes. Crystalline analyses using x-ray diffraction were performed and a thermal model was developed. The combination of experimental results and theoretical analysis demonstrate that pulsed KrF laser annealing enables in-depth phase transitions within the phosphor layer with minimal energy absorption at the interfaces and cladding dielectrics. Photoluminescence investigations suggest that the phase transition is concomitant to an increase of the number of active luminescent centres. Hence, it is demonstrated that this novel annealing technique is suitable for electroluminescence applications and experimental evidence is shown by a fourfold increase in device brightness compared to thermally treated devices at 450 °C. According to the simulation, the structural, and the luminescence analyses, this improved electroluminescence efficiency is attributed to localised annealing of the phosphor thin film with minimal modifications at the phosphor / dielectric boundary, which was the premise behind this study.

Item Type:

Thesis

Creators:

Mastio, E.A.

Date:

1999

ISBN:

9781369325423

Identifiers: