Numerical simulation of charge transfer and light emission inSrS:Ce thin-film electroluminescent devices

When an AC voltage is applied to a thin-film electroluminescent device with an SrS:Ce phosphor layer, the current flowing through the phosphor layer and the emitted light can be complex functions of time. In order to explain the observed current and light waveforms, a numerical model has been constructed for the electrical and optical behavior of such a device. This model is based on the tunneling of electrons from the phosphor-insulator interface, multiplication of the electron current and creation of positive space charge in the high field region, recombination of electrons with the positive charges in the low-field region, and a back-flow of electrons from the anodic interface when the field there changes sign. The rather simple model is able to simulate a number of features observed in current and light characteristics quite well. It could serve as a reference for the interpretation of new experimental results     

Modeling of electron transport and luminance in SrS:Cu,Ag ACTFELdisplay devices

Transient and steady state responses of luminance and current in SrS:Cu,Ag ac thin film electroluminescent (ACTPEL) display devices were measured and analyzed. The results indicated the importance of bulk space charge, which was attributed to ionized activators in the bulk phosphor layer. For dented voltage pulse excitation, four luminance peaks were observed. These were labeled as the leading edge (LE) peak, dent peak, beginning of trailing edge (BTE) peak and the middle of trailing edge (MTE) peak. Ionized activators play an important role not only in affecting the net phosphor field but also in the device luminance during the trailing as well as the leading edges of the voltage pulse. It is shown that the dent peak and the BTE are due to a bulk recombination process instead of the backflow from the anodic interface. The luminance behavior of the device can be understood in terms of the bulk dipoles in the phosphor layer. A bulk dipole consists of a positively charged ionized Cu activator and a “daughter” trap (negatively charged) in its vicinity. A physical model for various optoelectronic processes is presented. The model can explain experimental data in a qualitative fashion     

Electrical modeling of interface roughness in thin filmelectroluminescent devices

If two dielectric materials with different permittivities are in contact with each other and the interface between them is rough, then the electric field near this interface will be very inhomogeneous. In thin film electroluminescent devices, light is generated when electrons move back and forth in the phosphor layer under the influence of a strong ac electric field. At high electric fields, the electrons trapped in deep states at the interface between phosphor and insulator layer tunnel into the conduction band of the phosphor. This tunnel process is very sensitive to the electric field at the interface, so for a rough interface the electron flow will be very inhomogeneous. The relation between the interface roughness and the inhomogeneous charge transfer in thin film electroluminescent devices is investigated, based on an analytical flux tube model. The importance of the inhomogeneous current for the use of gray levels and aging is discussed     

An analytical model for electron transport and luminance in SrS:Cu,Ag ACTFEL display devices

An analytical model is developed to explain electron transport and luminance mechanisms in SrS:Cu,Ag ac thin film electroluminescent (ACTFEL) display devices. The model includes shallow and deep interface states, bulk traps, and impact excitation and ionization of activators. Mathematical expressions describing optoelectronic processes in the phosphor layer and interface states are written and numerical solutions for field, current and luminance are obtained. Results of calculations are compared with experimental data. The model is able to simulate the dominant features of the experimental luminance and current waveforms.     

Simulation and measurement of multiplication in thin-filmelectroluminescent 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     

Analytical circuit model for thin film electroluminescent devices

A simple equivalent circuit is developed for alternating current thin film electroluminescent devices. With sinusoidal excitation voltage an exact analytical solution is presented for calculating the device current, transferred charge and the internal voltage across the phosphor layer. The circuit is used for modeling EL devices with widely varying phosphor layer materials that include ZnS, CaS, SrS, CaF₂, SrF₂, ZnF₂ and multilayer structures prepared with Atomic Layer Deposition. The equivalent circuit gives a straightforward way to characterize the electrical properties of these materials. The fluorides have inferior breakdown properties compared to sulfides although they are much more symmetric. The strong asymmetry of SrS is attributed to its nonuniform defect distribution     

A model for electroluminescence in SrS:Ce ACTFEL display devices

An analytical model for electroluminescence in SrS:Ce AC thin-film electroluminescent display devices is presented. The model incorporates an exact calculation of the electric field and the effect of activator ionization and bulk traps. Activator ionization is needed to explain several features of luminescence behavior in SrS:Ce devices. These features include the second luminescence peak at the trailing edge of the voltage pulse and the time lag between the luminescence and the applied voltage when the applied voltage consists of bipolar trapezoidal pulses and rectangular pulses of low voltage amplitudes. As a mechanism for the ionization of activators, field-assisted tunneling is shown to be more likely than impact ionization by hot electrons. Physical processes are described in terms of rate equations, and field, current, and luminescence waveforms are calculated for one set of device parameters. The calculated and experimental luminescence waveforms agree     

Optical Properties of Rare Earth Doped SrS Phosphor: A Review

Rare earth (RE) doped SrS phosphor has attracted a lot of attention on a wide range of photo-, cathodo-, thermo-, and electroluminescent applications. Upon doping with different RE elements (e.g., Ce, Pr, Eu, Yb), the luminescence from SrS can be varied over the entire visible region by appropriately choosing the composition of the strontium sulfide host. The main applications include flat panel displays and SrS-based powder electroluminescence (EL) for back lights. Sulfide materials known for providing Eu²⁺ based red emission band and preferred as a color conversion material in white light emitting diodes are discussed. Especially, the applications of RE doped SrS are described in light of their utility as conversion and storage phosphors. The effect of energy level splitting, EL efficiency, post-annealing, milling time, and impurity on luminescence properties for SrS are also discussed.     

基于蓝光LED的白光器件:用于固体发光的小颗粒YAG:Ce型荧光粉的合成

文章介绍了小颗粒 YAG:Ce 荧光粉的三种合成方法.这种荧光粉受到蓝光 LED 发出的光线的激发会发出黄光.在合成的 pcLEDs 里,蓝光和黄光混合形成了白光.本文解释了这种涂有 YAG:Ce 荧光粉的 pcLED 的色点的依赖性以及通过添加修复晶格点阵的催化剂以改变这种依赖型的方法.另外,文章还讨论了 Ce3 聚集中心的YAG:Ce 荧光粉的效率的测量、烧成温度、激发波长.     

Roughness of ZnS:Pr,Ce/Ta2O5 interface andits effects on electrical performance of alternating current thin-filmelectroluminescent devices

Roughness effects of neighboring dielectrics on electrical characteristics of thin-film electroluminescent devices were investigated in order to improve the understanding of physics for the devices. Atomic force microscopy analysis reveal that thicker bottom layer of Ta₂O₅ shows rougher surface resulting in the rougher surface of ZnS:Pr,Ce layer. It can be easily seen that the dc leakage current increases rapidly with increase of surface roughness. Furthermore, it is notable that the initiation field of Poole-Frenkel current conduction is lowered by increasing surface roughness of Ta₂O₅ thin film. Internal charge-phosphor field (Q _int-F_p) analysis and capacitance-ac voltage (C-V) analysis for ITO-Ta₂O₅-ZnS:Pr,Ce-Al and ITO-Ta₂O₅-ZnS:Pr,Ce-Ta₂O₅-Al show that the steady state phosphor field is smaller and C-V curve in transition region is less steep with increase of root-mean-square roughness between lower dielectric and phosphor layer in the alternating current thin-film electroluminescent (ACTFEL) devices. Therefore, we conclude that interface roughness is one of the physical factors to change the electrical performance of ACTFEL device     

Characterization of charge accumulation on multiple interfaces in phosphorescent organic light-emitting diodes

A series of green phosphorescent organic light emitting diodes (OLED) were studied using I(V), admittance spectroscopy, and capacitance-versus-voltage, C(V), measurements. We found that both the logarithmic derivative of I(V) and C(V) spectra revealed two distinct peaks related to the build-up and consequent dissipation of charge on different interfaces of the device. The first peak is common for many types of OLEDs and is caused by the external built-in potential. The second peak is a feature attributed to the specific stacking sequence of the devices studied. We argue that the secondary charge build-up occurs at the interface between hole transport layer and emission layer due to (i) the strong mismatch of HOMO level of these layers and (ii) restricted direct injection of holes onto the phosphorescent dopant molecules. Consequent dissipation of the charge is caused by greatly-enhanced supply of electrons, which in turn is caused by exponential growth of electron mobility due to the Frankel-Poole effect. Our study shows that C(V), in conjunction with I(V), measurements are not only useful for model devices with two metal-organic interfaces, but can also characterize charge accumulation in complex, multi-interface OLEDs. We also observed a strong negative contribution to the capacitance at low frequency and high biases; the emergence of the negative capacitance correlates with the onset of light emission.     

蓝光LEDs的转换荧光粉发光效率的测量

本文研究了YAG:Ce荧光粉的色点红移的现象.当YAG晶格点阵中的Y被 Gd 激发或添加了Pr作为 Ce 的共同催化剂时就会发生这种现象.测量了在(Y,Gd)AG:Ce和YAG:Ce,Pr荧光粉样品内部的效率,其激发光线的波长为 470 nm.其结果表明这两种荧光粉的发光效率分别随着Gd和Pr聚集而降低.SrGa2S4:Eu.是另外一种常用的转换蓝光 LED 的颜色的荧光粉.室温下,这种荧光粉效率很高,理论上其发光效率比 YAG:Ce 的发光效率高出 33%.对这种荧光粉效率的测量结果表明其制作过程是影响性能的主要因素.将这种荧光粉印刷在放在-个或多个蓝光 LED 前面的屏幕的表面,测得其发光效率高达 364 lm/W.     

Effect of the levels of intrinsic defects in the CdP2 band gap on electrical characteristics of corresponding structures with the Schottky barrier

The electrical characteristics of Schottky barriers formed on n-type cadmium diphosphide are studied. It is established that the space-charge region at the metal-semiconductor interface represents in fact a Schottky layer formed owing to a high concentration of deep-level centers. The charge transport in the conducting direction for these structures is related to the above-barrier emission of electrons and is consistent with the diffusion theory for one or two types of charge carriers. The high concentration of ionized centers in the space-charge region gives rise to the tunneling mechanism of breakdown in the blocking direction. The frequency dependences of the complex conductance are governed by the exchange of charge carriers between the conduction band and donors that specify the conductivity type of the material and also by the recharing of the centers with a large depth of levels. Good agreement between the reported results and the theory is obtained.     

GaN and other materials for semiconductor spintronics

Existing semiconductor electronic and photonic devices use the charge on electrons and holes to perform their specific functionality, such as signal processing or light emission. The field of semiconductor spintronics seeks to exploit the spin of charge carriers in new generations of transistors, lasers, and integrated magnetic sensors. The use of such devices depends on the availability of materials with practical magnetic-ordering temperatures. Here, we summarize recent progress in the development of GaN and other wide bandgap semiconductors that retain ferromagnetic properties above room temperature.     

Emission from outside of the emission layer in state-of-the-art phosphorescent organic light-emitting diodes

The emission zone profile in an organic light-emitting diode was extracted by fitting the experimentally measured far-field angular electroluminescence spectrum of a purposely designed device. It is based on a thin 10 nm emission layer doped with the red emitting phosphor Ir(MDQ)₂acac. We find strong indications for light emission originating from outside of the emission layer, even though the device has electron and hole blocking layers. These are commonly assumed to completely confine the charge carrier recombination and hence the light emission to the emission layer. Since the calculated internal spectrum of the emission matches the emitter photoluminescence spectrum well, diffusion of the emitter molecules outside of the emission layer is hypothesized.     

Hafnium silicate as control oxide in non-volatile memories

SONOS-type MIS capacitors with hafnium silicate as a control oxide are characterized and compared to devices featuring a conventional SONOS gate stack. Write operation is comparable for both gate stack types. Erase operation for the devices with hafnium silicate is improved since the parasitic injection of electrons from the gate is suppressed due to the low electric field in the high-k material. This reduction in leakage current through the gate enhances oxide stability. However, measurements indicate that charge retention for the gate stack with hafnium silicate is degraded for high charge densities. Band bending of the control oxide under high electric fields increases the tunneling probability for trapped charges. Additionally, initial flatband voltage decay is observed due to charge trapping in the hafnium silicate layer. Reducing the thickness of the hafnium silicate layer is possible, maintaining favorable erase properties while minimizing the charge decay rate during retention.     

The Effect of Nanoparticle Shape on the Photocarrier Dynamics and Photovoltaic Device Performance of Poly(3‐hexylthiophene):CdSe Nanoparticle Bulk Heterojunction Solar Cells

The charge separation and transport dynamics in CdSe nanoparticle:poly(3‐hexylthiophene) (P3HT) blends are reported as a function of the shape of the CdSe‐nanoparticle electron acceptor (dot, rod, and tetrapod). For optimization of organic photovoltaic device performance it is crucial to understand the role of various nanostructures in the generation and transport of charge carriers. The sample processing conditions are carefully controlled to eliminate any processing‐related effects on the carrier generation and on device performance with the aim of keeping the conjugated polymer phase constant and only varying the shape of the inorganic nanoparticle acceptor phase. The electrodeless, flash photolysis time‐resolved microwave conductivity (FP‐TRMC) technique is used and the results are compared to the efficiency of photovoltaic devices that incorporate the same active layer. It is observed that in nanorods and tetrapods blended with P3HT, the high aspect ratios provide a pathway for the electrons to move away from the dissociation site even in the absence of an applied electric field, resulting in enhanced carrier lifetimes that correlate to increased efficiencies in devices. The processing conditions that yield optimum performance in high aspect ratio CdSe nanoparticles blended with P3HT result in poorly performing quantum dot CdSe:P3HT devices, indicating that the latter devices are inherently limited by the absence of the dimensionality that allows for efficient, prolonged charge separation at the polymer:CdSe interface.     

Scanning Kelvin Probe Microscopy on Bulk Heterojunction Polymer Blends

Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub‐100 nm resolution on the phase‐separated active layer of polymer‐fullerene (MDMO‐PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport.     

On the effect of non-degenerate doping of polysilicon gate in thin oxide MOS-devices—Analytical modeling

A one-dimensional model of the polysilicon-gate-oxide-bulk structure is presented in order to analyze the implanted gate MOS-devices. The influence of the ionized impurity concentration in the polysilicon-gate near the oxide and the charge at the polysilicon-oxide interface on the flat-band voltage, threshold voltage, inversion layer charge and the quasi-static C–V characteristic is quantitatively studied. The calculations show a considerable degradation of the inversion layer charge due to the voltage drop in the gate, especially in thin oxide devices. The calculated quasi-static C–V curves agree with the recently published data of implanted gate devices.     

Electrolytic etching and electron mobility of GaAs for FET's

An electrolytic etching technique for n-GaAs is presented. The procedure is applied to post-growth etching of FET wafers to achieve uniformly thin layers from excessively thick and nonuniform material.Measurements on a Hall sample, thinned by this technique, show mobilities in good agreement with theoretical bulk mobility calculations for films as thin as 2100 Å. From Hall measurements on layers covered by the anodic native oxide, it is determined that the oxide interface traps 3·9 × 10¹¹ electrons per cm² more charge than the as-grown surface.