Dorina Corlatan

Observation and simulation of space-charge effects and hysteresis in ZnS:MN ac thin-film electroluminescent devices

We have shown from basic considerations that the negative resistance effect measured in thin‐film electroluminescent devices must involve some variation of space charge in the phosphor layer. Time‐delayed voltage pulse experiments indicate that positive space charge decreases spontaneously. By introducing the presence of deep donor levels which can be ionized by accelerated electrons and, when positively charged, have some typical decay time for the release of a hole, we have created a numerical model which is able to simulate negative resistance, time‐delayed pulse experiments and the influence of temperature and frequency on the hysteretic behavior.

The influence of space charge and electric field on the excitation efficiency in thin film electroluminescent devices

In electroluminescent devices electrons move every half period from the cathodic to the anodic interface of the phosphor layer and excite atoms which can lose their energy by the emission of a photon. It has been found experimentally by others that the efficiency for excitation of the luminescent atoms is largest close to the cathodic interface. By comparing transient measurements with the steady state situation we show that in steady state the efficiency is reduced and the difference between cathodic and anodic efficiency is larger. Both phenomena could be ascribed to creation of positive space charge during the first current pulses.

Analysis of the luminescent decay of ZnS:Mn electroluminescent thin films

Abstract A procedure is described for fitting ZnS:Mn decay curves with an impulse response written as the product of a radiative exponential part and a non-radiative stretched exponential part. The effect of the non-ideal (electrical) impulse excitation is taken into account. Decay curves are analysed for ALE-samples with Mn-content ranging from 0.25 to 1.5%, and for all these samples a good fit can be found. However, for the higher concentrations unusually large values for the radiative decay time are found. Possible explanations are discussed.

Experimental investigation of currents and efficiencies in SrS:Ce thin‐film electroluminescent devices

In thin-film electroluminescent displays the electrical properties of the phosphor material are very important for the device performance. The electric field and the current in the phosphor layer may be quite inhomogeneous due to multiplication and space charge, even in devices without filamentary behavior. Measuring the ratio of luminance to current provides information about the mechanism of the light-generation process. The currents and the efficiencies for SrS:Ce devices with different thicknesses and different bottom layers are discussed and compared with numerical simulations.

Simulation and measurement of multiplication in thin-film electroluminescent devices with doped probe layers

When a single voltage pulse is applied to an electroluminescent (EL) device after previous illumination, the current through the phosphor layer will normally not be homogeneous, but increase from the cathodic side-where the electrons tunnel from-to the anodic side, due to multiplication. The positive charges that remain after the multiplication process cause a positive space charge that has been observed in various experiments and influences the efficiency. In this paper a simple numerical model is proposed for the calculation of charge transfer and light emission, in the case that multiplication takes place during a voltage pulse after previous illumination.

Properties of charge carriers in ZnS and their effect on the excitation of Mn

The subjects of this investigation are the transport of charge carriers and the spatial efficiency in thin-film electroluminescent devices based on ZnS:Mn. Measurements with optional UV illumination were performed on a set of probe layer samples. The transfer of electrons and holes was studied and by comparing transient measurements with the steady-state situation the creation of positive space charge during the first pulses is revealed and its location is estimated.

OPTIMISATION OF THE DESIGN OF A DISCRETE CdSe - FLC OASLM

Abstract We present an OASLM based on a CdSe photoconductor in combination with a ferroelectric liquid crystal. Instead of using homogeneous layers, we have developed a discrete design, in which the pixels are connected to busbars through photoconductive rectangular strips. This approach lowers the resolution of the device, but also has some advantages: dark switching can easily be eliminated, thin photoconductor layers can be used, and a dielectric mirror is no longer necessary.

UV-induced charge transfer and light emission in ZnS-based electroluminescent devices with probe layers

With transient measurements it is possible to investigate the electrical and optical characteristics of a thin-film-electroluminescent device for both polarities. With additional UV illumination the charge transfer and luminescence due to electrons and holes can be measured separately. The additional charges generated by the UV illumination make it possible to investigate electroluminescence at electric fields below the threshold in conventional thin-film-electroluminescent devices. The method and the results presented here are of great value for the fundamental understanding of charge-transport and excitation processes in electroluminescent devices.