Patrick De Visschere

Atomic layer deposition of ZnS thin films based on diethyl zinc and hydrogen sulfide.

Abstract The atomic layer deposition (ALD) of ZnS based on diethyl zinc (DEZn) and hydrogen sulfide (H 2 S) was investigated. ZnS thin films were grown between 200°C and 350°C and the effect of other processing conditions was examined. The growth temperature, the DEZn dosing and the purge times were found to be decisive parameters, which indicate that the film growth is strongly affected by the limited stability of the film surface after the DEZn pulse. ZnS:Mn TFEL samples with a phosphor layer based on the DEZn process were shown to exhibit efficient light emission and improved stability and symmetry with aging compared with traditional chloride-based TFEL samples.

Comment on G.J. Rees “surface recombination velocity—a useful concept?”

Abstract It is shown that for calculating the effective surface recombination velocity, the drop of the minority carrier quasi-Fermi level in the space charge region should be taken into account. It is then found that a constant surface recombination velocity, independent of the excitation level, is valid up to a relatively high excitation level.

Dipole radiation within one-dimensional anisotropic microcavities: a simulation method

We present a simulation method for light emitted in uniaxially anisotropic light-emitting thin film devices. The simulation is based on the radiation of dipole antennas inside a one-dimensional microcavity. Any layer in the microcaviy can be uniaxially anisotropic with an arbitrary orientation of the optical axis. A plane wave expansion for the field of an elementary dipole inside an anisotropic medium is derived from Maxwells equations. We employ the scattering matrix method to calculate the emission by dipoles inside an anisotropic microcavity. The simulation method is applied to calculate the emission of dipole antennas in a number of cases: a dipole antenna in an infinite medium, emission into anisotropic slab waveguides and waveguides in liquid crystals. The dependency of the intensity and the polarization on the direction of emission is illustrated for a number of anisotropic microcavities.

Characterization of the electro-optical behavior of Zn2Si0.5Ge0.5O4:Mn thin-film electroluminescent devices

Zn2Si0.5Ge0.5O4:Mn was investigated as a green-emitting oxide phosphor for thin-film electroluminescent (EL) display applications. Detailed electro-optical characteristics of thin-film EL devices with SrTiO3 dielectrics on a glass substrate (TFEL) and EL devices with a single ceramic insulator (CIEL) have been analyzed. The steeper brightness–voltage dependency of the TFEL device is ascribed to the capability to provide high current densities and efficient light generation for both voltage polarities. The CIEL device on the other hand suffered from asymmetrical behavior, related with the lack of a top dielectric. It was found that the use of high-permittivity dielectrics in the TFEL device results in a small voltage drop over the insulating layers, enhancing the amount of charge transferred through the phosphor. Transient charge–voltage (αQp–V) measurements on the TFEL device evidence that a high positive space charge density of ∼0.68 μC/cm2 is present in the oxide phosphor during steady-state operation. ...

Contrast-improving methods for Digital Micromirror Device projectors

This paper discusses contrast degradation in projection displays that use the Digital Micromirror DeviceTM (DMDTM) of Texas Instruments. Various possible mechanisms for the degradation of the contrast ratio are investigated by using optical ray-trace simulations and performing some experiments. It is found that the scattering of light at the DMD is the principal source of contrast degradation. This scattering is implemented in our simulations of the DMD projector in the optical ray-trace program ASAP. This enables us to simulate the contrast ratio in a realistic way. Two methods to improve the contrast ratio are proposed. Both methods, however, reduce the total light transmission of the projector. These methods are illustrated with our simulation model, and quantitative results are given.

An exact two-dimensional model for a periodic circular array of head-to-head permanent magnets

An analytical expression is derived for the magnetic field of a two-dimensional periodic circular array of head-to-head permanent magnets, assuming there is either no gap between the magnets or that they are separated by a non-magnetic pole-piece. Some more general symmetry properties are described. The force experienced by the magnets is calculated and an analytical approximation is derived. We comment on the published dipole-approximation used for dealing with this kind of problem and also correct some mistakes.

Power Consumption and Temperature Distribution in WRGB Active-Matrix OLED Displays

In this paper, the power consumption of a white-red-green-blue (WRGB) active-matrix organic light-emitting device (OLED) display and the resulting temperature distribution across the display are analyzed as a function of the applied image and the luminance of the emitted light. It has been shown previously that temperature directly impacts the picture quality of an OLED display. Luminance, spectral radiance, power and temperature measurements are performed on a 55-in WRGB OLED display with a resolution of 1920 ×1080. A power model is presented that allows calculating the displays power consumption for a given applied image. This involves the dependency of the efficiency of the white OLED on the current density, the wavelength dependent transmission of the color filters and the contribution of each of the subpixels in producing the displays nominal white. The output of the power model is used as input for a basic thermal model that simulates the temperature distribution across the display. The thermal model is based on 3D computational fluid dynamics analysis framework (FloEFD). A good agreement between the simulations and measurements on the sample WRGB OLED display is obtained.

On the flat fermi level approximation in the space charge layer of induced-junction solar cells

Abstract Conditions are derived on which the assumption of flat quasi Fermi levels, in the space charge layer of induced-junction solar cells, is valid. It turns out that, as far as the output current of the cell is concerned, these conditions are the same as for a non-illuminated cell: the transport velocity of the carriers should be much larger in the space charge region than outside of it.

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.

Transient and local illumination of an organic photoconductive sensor

In this paper we investigate the performance of a transparent photoconductive sensor based on a double layer of organic materials (m-MTDAB / PTCBI) which are deposited on two interdigitated transparent ITO electrodes. The performance of the sensor is demonstrated with electro-optical measurements: the I(V) curves consist of two linear sections meeting at a knee voltage Vt. Linear regression performed on the I(V) curves below Vt show that the conductance is a power law of the luminance incident on the device. We present a model to describe the behaviour of the sensor below Vt. We present measurements of I(t) for a transient illumination of the sensor. Plotting the inverse of the current as a function of time we find that the transient is consistent with the model for voltages below Vt. For voltages above Vt we find that the sensor behaves like a resistor in series with a space charge (SC) region. We present a local illumination experiment that confirms the existence of a SC region between the electrodes of the photoconductive sensor for V