Thin-film electroluminescent structures on substrates with rough surfaces

It is shown that the use of substrates with inner rough and outer diffusely scattering surfaces in film electroluminescent structures makes it possible to substantially increase the coefficient of radiation extraction (by a factor of 1.3–2.5) and the brightness of luminescence (by a factor of 1.2–3.5) as compared to the structure on a smooth surface, which can be employed to enhance the brightness of individual colors of the luminescence of electroluminescent emitters. The results of the investigations indicate a substantial decrease in the constant of the time of brightness buildup (by a factor of 6–8 for MDSDM structures and by a factor of ∼3.8 for MDSCM structures) and the appearance of two segments of brightness decay for MDSMD structures with different constant decay times in going from the structure on a smooth substrate to the structures built on substrates with an inner rough surface. Unlike this, MDSCM structures have two segments of brightness decay on both a smooth substrate and a rough substrate. Ul’yanovsk State University, 42, L. Tolstoi St., Ul’yanovsk, Russia, 432700. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 787–793, September–October, 1998.     

Infrared quenching of electroluminescence in ZnS: Mn thin-film electroluminescent structures

Electroluminescence from thin-film electroluminescent devices is found to be quenched after IR irradiation of the devices in the interval between exciting voltage pulses. The IR irradiation decreases the emission intensity in the spectral range 530–540 nm, while increasing it between 640 and 690 nm. These effects are explained by IR-induced charge exchange between the deep centers due to V _S ²⁺ and V _S ⁺ sulfur vacancies, an increase in the concentration of the latter vacancies, and the redistribution of the channels of impact excitation of Mn²⁺ and V _S ⁺ centers in favor of V _S ⁺ centers. The cross section and rate of impact excitation of V _S ⁺ centers, the photoexcitation cross section for V _S ²⁺ centers, the IR radiation absorption coefficient, the internal quantum efficiency of electroluminescence, and the probability of radiative relaxation of Mn²⁺ centers, as well as the electron multiplication factor in the phosphor layer, are evaluated.     

Effect of photoexcitation on the electrical performance of ZnS: Mn thin-film electroluminescent structures

It is shown that the kinetics of the charge and current passing through a thin-film electroluminescent emitter, as well as the I-V characteristics of the emitter, greatly diverge under blue, red, and IR pulsed illumination with photon energies of ≈2.6, ≈1.9, and ≈1.3 eV, respectively, and a photon flux density of 4×10¹⁴–3×10¹⁵ mm⁻² s⁻¹. Results obtained indicate that, during the operation of the emitter, deep centers associated presumably with V _Zn ²⁻ zinc vacancies and V _S ⁺ and V _S ²⁺ sulfur vacancies exchange charge. These centers lie above the valence band by ≈1.1, ≤1.9, and ≤1.3 eV, respectively. Their concentrations are estimated as (3–4)×10¹⁶ cm⁻³ for V _Zn ²⁻ and V _S ⁺ and ≈1.5×10¹⁶ cm⁻³ for V _S ²⁺ . It is demonstrated that positive and negative space charges forming in the near-anode and near-cathode regions of the phosphor layer specify the electric performance of the emitters.     

基于电致发光效应的光学电压传感器

本文综述了基于电致发光效应的光学电压传感器机理、分类及其主要特性,分析总结了此类传感器的研究现状及其存在的主要问题,同时提出未来研究课题的建议。电致发光型电压传感器的主要优点在于不需要载波光源,因而可以有效避免以往光学电压器中工作光源性能不稳定所引起的传感器性能变化;此外,此类电压传感器结构简单、体积小、重量轻、成本低,可以实现较高的性能价格比。今后研究的主要问题包括合理选择电压传感材料与器件、提高传感器的温度和湿度稳定性等。电致发光型电压传感器在电力工业和航空航天等领域的科学研究与实验中将具有广泛的应用前景。     

Exciplex formation and electroluminescent absorption in ultraviolet organic light-emitting diodes

We investigated the formation of exciplex and electroluminescent absorption in ultraviolet organic light-emitting diodes(UV OLEDs) using different heterojunction structures.It is found that an energy barrier of over 0.3 eV between the emissive layer(EML) and adjacent transport layer facilitates exciplex formation.The electron blocking layer effectively confines electrons in the EML,which contributes to pure UV emission and enhances efficiency.The change in EML thickness generates tunable UV emission from 376 nm to 406 nm.In addition,the UV emission excites low-energy organic function layers and produces photoluminescent emission.In UV OLED,avoiding the exciplex formation and averting light absorption can effectively improve the purity and efficiency.A maximum external quantum efficiency of 1.2%with a UV emission peak of 376 nm is realized.     

有机电致发光器件中载流子的输运和复合发光

以高场作用下载流子对三角势垒的FowlerNordheim隧穿理论为基础,建立了单层有机电致发光器件中载流子输运和复合发光模型,给出了薄膜中电子空穴对的解离和复合概率及电子和空穴的密度分布.计算并讨论了外加电压和注入势垒对器件电流和复合效率的影响. 关键词： 电致发光器件 载流子输运 载流子复合     

同时掺杂磷光和荧光染料的白色有机电致发光器件性能研究

以磷光染料Ir(piq)₂(acac)作为发光掺杂剂,掺入空穴传输性主体材料NPB中得到红色发光层,荧光材料TBP掺入到主体CBP中作为蓝色发光层,制备了结构为ITO/NPB/NPB：Ir(piq)₂(acac)/CBP/CBP：TBPe/BCP/ALq/Mg：Ag的双发光层白色有机电致发光器件.其中ALq₃、未掺杂的NPB和CBP及BCP层分别作为电子传输层、空穴传输层和激子阻挡层.实验中通过调节发光层厚度及Ir(piq)_{2关键词： 磷光 激子阻挡层 有机电致发光}     

大尺度分层介质粗糙面电磁散射的特性研究

首先建立大尺度分层介质粗糙面散射的物理模型, 基于Stratton-Chu积分方程和Kirchhoff近似导出了粗糙面散射场的计算公式. 采用高斯随机粗糙面来模拟实际的分层介质粗糙面, 通过数值计算得到了正下视单站雷达接收到的后向散射回波. 理论推导了散射场强度与表面粗糙度之间的定量关系, 并从数值仿真的角度分析了表面和次表面的粗糙度对散射回波的影响, 给出了散射场随粗糙度变化的曲线. 最后考察了分层介质的电特性参数(介电常数和电导率)对分层粗糙面散射场的影响, 并对计算结果做出了分析.     

粗糙面重构问题的混合算法研究

基于Desanto的谱表示法,采用数值和近似算法相结合的混合算法对一维粗糙面的重构问题进行了研究.对于正问题,采用数值算法——矩量法(MOM)得到一维粗糙面的相关散射数据,对于逆问题,考虑不同粗糙度的粗糙面,分别采用两种近似算法——微扰近似(SPA)、基尔霍夫近似(KA)与矩量法的混合算法,对粗糙面轮廓进行了重构;数值结果以高斯粗糙面为例,给出了混合算法对不同粗糙度粗糙面的重构算例,并进行了数据比较和分析. 关键词： 粗糙面重构 矩量法 微扰近似 基尔霍夫近似     

Investigation of the current-voltage characteristics of thin-film ZnSe-ZnTe heterojunction structures

This paper investigates the current-voltage characteristics of thin-film ZnSe-ZnTe heterojunction structures grown on zinc substrates. It is shown that the maximum rectification coefficient for such systems is observed when the condition d_1,2=d₀ is satisfied. On the basis of the general equation of the current-voltage characteristics of heterojunctions taking into account the boundary conditions, equations are derived which describe the behavior of the forward and back branches of the current-voltage characteristic of the heterojunction under consideration, and an explanation is given of the divergence between the theoretical and experimental results.     

Electroluminescence parameters of thin-film ZnS: Mn electroluminescent devices

A new technique is suggested that makes it possible to determine the probability of radiative recombination of impact-excited Mn²⁺ luminescence centers, as well as to find the time dependences of the electron multiplication factor, impact ionization length, and impact ionization coefficient from the time dependences of the instantaneous internal quantum yield. These dependences follow from the time dependences of the luminance, as well as of the current and charge passing through the phosphor layer, when the devices are excited by a low-frequency ramp voltage.     

Luminescent impurity doping trends in alternating-current thin-film electroluminescent phosphors

The doping properties of three alternating-current thin-film electroluminescent (ACTFEL) phosphor host/luminescent impurity systems, ZnS:Mn, SrS:Ce, and SrS:Cu, are elucidated, and the ACTFEL device implications of these properties are assessed. Mn is isovalent, Ce is a donor, and Cu is an acceptor. Moreover, Ce is readily ionized in SrS, so that it behaves as a double donor. The distinctly different doping nature of these three luminescent impurities leads to dramatically disparate defect and device physics trends. The donor/acceptor nature of Ce/Cu in SrS results in charge neutrality being achieved in SrS:Ce and SrS:Cu via self-compensation-induced vacancy creation; subsequent defect complexing between oppositely charged luminescent impurities and self-compensation-induced vacancies results in more complex ACTFEL device behaviors such as dynamic space charge, trailing-edge emission, charge collapse, color tuning, and electroluminescence (EL) thermal quenching. In contrast, the isovalent nature of ZnS:Mn leads to more ideal ACTFEL device operation. This suggests that the optimal ACTFEL phosphor luminescent impurity is isovalent.     

Negative differential resistance in thin-film electroluminescent emitters based on zinc sulfide

S-and N-type negative differential resistance (NDR) has been observed in thin-film electroluminescent emitters based on zinc sulfide doped with manganese, and conditions for its emergence have been identified. It has been found that when a negative half-wave of voltage is applied to the nontransparent top electrode, an S-type NDR with a region of decreasing current is observed, and when it is applied to the transparent bottom electrode, the NDR will be N-type. The emergence of NDR is due to space charges which form in the near-cathode and near-anode layers of the phosphor.     

Investigation on global positioning system signal scattering and propagation over the rough sea surface

This paper is devoted to the study of polarization properties, scattering properties and propagation properties of global positioning system (GPS) scattering signal over the rough sea surface. To investigate the polarization and the scattering properties, the scattering field and the bistatic scattering coefficient of modified Kirchhoff approximation with using the tapered incident wave is derived in detail. In modeling the propagation properties of the GPS scattering signal in the evaporation duct, the initial field of parabolic equation traditionally computed by the antenna pattern with using fast Fourier transform (FFT) is replaced by the GPS scattering field. And the propagation properties of GPS scattering signal in the evaporation duct with different evaporation duct heights and elevation angles of GPS are discussed by the improved discrete mixed Fourier transform with taking into account the sea surface roughness.     

Quantum yield and luminous efficacy of thin-film ZnS electroluminescent emitters

The instantaneous values of the internal quantum yield and luminous efficacy of thin-film electroluminescent emitters are experimentally studied as functions of time, the mean field in the phosphor layer, and the charge passing through this layer. Also, the dependences of the internal quantum yield and luminous efficacy on the exciting voltage amplitude are explored. At exciting voltage frequencies above 10 Hz, the time dependences of the instantaneous quantum yield and luminous efficacy exhibit a dip in the range where the brightness and current through the phosphor layer grow and a peak in the range where the brightness and current decline. The dip and peak are related to the different rates of rise and fall of the brightness and current.     

Effect of driving voltage pulse shape on the luminance of thin-film electroluminescent devices

The dependence of the average luminance on the parameters of the metal-insulator-semiconductor-insulator-metal structures in thin-film electroluminescent devices (TFELDs) and on the excitation conditions is found by solving kinetic equations for the variation in the concentration of excited emission centers in the phosphor layer of TFELDs for different driving voltage pulse shapes (triangular, trapezoidal, sinusoidal, and rectangular with an exponential leading edge). It is shown that for equal amplitudes and pulse repetition rates of a sign-changing symmetric voltage, the average luminance and luminous efficacy of TFELDs increase as the rate of rise of the voltage is increased for different driving voltage pulse shapes in the following sequence: triangular, sinusoidal, trapezoidal, rectangular. The calculations are confirmed by experiment. Zh. Tekh. Fiz. 69, 64–69 (February 1999)     

ZnS…Cu电致发光电压传感器及其温度漂移补偿

利用ZnS…Cu电致发光粉末与环氧树脂胶混合,设计制作了一种梯形电极结构的电压传感单元,实现了电致发光电压传感器输出信号的温度漂移补偿。电致发光电压传感信号通过2根塑料光纤传输到2个硅光电探测器,并选择其开路电压作为传感器的输出信号。在同一外加电压条件下,梯形电极区域内的电场分布是不均匀的,因而不同场点的发光亮度不同。通过测量梯形电极区域内2个不同发光点的发光强度随外加电压的变化,并对两路输出电压传感信号进行数据拟合与计算,可获知被测电压的有效值,并可实现对输出信号温度漂移的补偿。在-40~60℃范围内,采用上述温度漂移补偿方法测量了有效值在0.7~1.5 k V范围内的工频电压,传感器输出信号的非线性误差低于1.6%,验证了该温度漂移补偿方法的有效性。