Silicon compatible organic light emitting diode

As an effort toward a goal of monolithic optoelectronics for silicon (Si) chip-to-chip connections, we have fabricated organic light emitting diodes (LED's) using either heavily N-doped silicon (Si) as a cathode or P-doped Si as an anode. A thin silicon dioxide (SiO₂ ) layer, thermally grown on Si before deposition of a polymer or a molecular organic layer, enhances the electron injection into the semiconducting emissive layer. Without the thin oxide layer, no light was observed from LED's made from either (2-methoxy, 5-(2'-ethyl-hexoxy)-1, 4-phenylene vinylene) (MEH-PPV) or 8-hydroxyquinoline aluminum (Alq). With the SiO₂ layer,the internal quantum efficiencies as high as 0.02% and 0.5% have been observed for MEH-PPV and Alq, respectively, and the turn-on voltages were as low as 2.5 V and 8 V, again for MEH-PPV and Alq, respectively. From the LED response time measurement, we identified RC constant and the recombination time of transport-related traps as the speed limiting factors     

Remodeling light emitting diode in low current region

A general expression is proposed for the current-voltage (I-V) characteristics of junction diode by which it is possible to determine forward voltage across diode at actual current and temperature. Expressions fur the band gap and temperature coefficient of junction voltage based on the experimental measurements are also presented. The expressions include only experimentally obtained and derived values. These expressions have been used to check the performance of logarithmic amplifier using light emitting diode (LED) as a feedback clement, at different temperatures. Further, it has been shown that it is possible to predict the behavior of a logarithmic amplifier without temperature compensation techniques within 5% accuracy for the current range 10^-12-10^-5 A and for the temperature range -5°-60°C. The alternative could be the use of temperature compensation circuitry which may make electrometer circuit more complex. The proposed scheme is useful in applications where it is difficult to incorporate temperature compensation technique and space and power are at premium     

Analysis of alternating current driven electroluminescence in organic light emitting diodes: A comparative study

The alternating current (AC) responses of double-injection and double-insulated organic light-emitting diodes (OLEDs) were investigated and compared. To reveal the electroluminescent (EL) processes in these devices, the AC voltage and frequency dependence of the EL intensity and capacitive current were studied in the time domain with a focus on phase difference analysis. It was found that the voltage-dependent transit time and frequency-dependent carrier distribution were important for the AC-driven performance of the double-injection OLEDs. In contrast, although the double-insulated OLEDs shared some similarities with the double-injection OLEDs, they had some unique characteristics, which were the absence of resistive current and phase shift of EL profiles. It was revealed that the EL in the double-insulated OLEDs was driven by the displacement current generated by the ionization of the doped layers, which, however, formed space charge regions and undermined the EL emission. The space charge redistributed the electric field across the devices after the initiation of EL, making the EL maintain for a limited time interval. This effect was significant under low frequency and high AC voltage. Comparing the phase difference between both devices, it was indicated that the space charge effect was responsible for the observed EL phase shift and the asymmetric EL profiles at low frequency and high AC voltage in the double-insulated OLEDs. The proposed model was also of help to understand the EL saturation phenomena with AC frequency and voltage in those devices.     

Guided electromagnetic waves in organic light emitting diode structures

A detailed analysis of the optical waveguide modes in organic light emitting diodes (OLEDs) is presented. The modes are compared with respect to their electric field profile, attenuation coefficient and optical confinement factor. The first transversal electric (TE) mode is best suited for an optimized energy transfer from emitting dipoles. The transversal magnetic (TM) modes are characterized by a large plasmon character with high absorption. The propagation constants of distinct modes have been measured in test devices by grating coupling.     

Magnetic field effects on electroluminescence in phosphorescence organic light emitting diodes

In this letter, we presented a method to study the MFEs on the triplets in phosphorescent OLEDs. The magnetic electroluminescence (MEL) was obtained by doping a red phosphorescent guest with low concentration into a fluorescent host, where the guest and host can simultaneously emit. Experimentally two different MEL shapes of Lorentz and linear were observed, depending on the used host materials. We presented two different mechanisms to explain their difference. The diffusion process of triplets from host to guest and prolonged lifetime of triplet by magnetic field were attributed to the formation of the Lorentz shape, and it is considered that the linear shape was caused by magnetic field increased Dexter energy transfer rate and determined by the triplet energy difference between guest and host. It can be seen that the competition of two processes lead to the formation of the two different MEL shapes in the phosphorescent OLEDs.     

Simple-structured efficient white organic light emitting diode via solution process

High-efficiency white emission is crucial to the design of energy-saving display and lighting panels, whereas solution-process feasibility is highly desirable for large area-size and cost-effective roll-to-roll manufacturing. In this study, we demonstrate highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs) with solution-processed single emissive layer. The resultant best white OLED shows excellent electroluminescence performance with forward-viewing external quantum efficiency, current efficiency and power efficiency of 22.7%, 48.8 cd A^− 1 and 27.8 lm W^− 1 at 100 cd m^− 2, respectively, with a maximum luminance of 19,590 cd m^− 2. Furthermore, we also observed an increment of 112% in the power efficiency, 86.9% in the current efficiency and a decrement of 39.2% in the external quantum efficiency at 100 cd m^− 2 as the doping concentration of blue dye was increased from 10 wt% to 25 wt% in the devices. The better efficiency performance may be attributed to the effective exciton-confining device architecture and low-energy barrier for electrons to inject from the hole-blocking electron-transport layer to the host layer.     

Room-temperature electroluminescence of p-ZnxMg1-xO:Na/n-ZnO p-n junction light emitting diode

tifying behavior and obvious electroluminescence was realized by feeding a direct current up to 40 mA. Furthermore, its structural and electric characters are discussed as well.     

Room-temperature electroluminescence of p-ZnxMg（1-x）O：Na/n-ZnO p-n junction light emitting diode

p-ZnxMg1-xO：Na/n-ZnO p-n junction light emitting diode （LED） was produced on n-ZnO （0001） single-crystal substrate using pulsed laser deposition. The realization of band gap engineering was achieved by the incor-poration of Mg in ZnO layers and was confirmed by photoluminescence spectrum. The p-type ZnxMg1-xO：Na film with low resistance was obtained at 500 ℃ and in which, Na has taken effect evidenced by Hall and X-ray photo-electron spectroscopy measurements. The current-voltage curve of LED showed a rectifying behavior and obvious electroluminescence was realized by feeding a direct current up to 40 mA. Furthermore, its structural and electric characters are discussed as well.     

Enhanced organic light emitting diode and solar cell performances using silver nano-clusters

Silver nano-clusters (NCs) were incorporated into organic light emitting diodes (OLED) and solar cells by means of thermal evaporation. Silver NCs enhance the efficiency of both OLEDs and polymer solar cells under tailored device architecture. In tris-(8-hydroxyquinoline) aluminum (Alq₃) based small molecule OLEDs, silver NCs were deposited under the Al cathode. The electron injection from the cathode to organic layer is promoted significantly owing to silver NCs induced lightning rod effect, the Alq₃ OLEDs luminous efficiency is increased up to a factor of 6. In poly(3-hexylthiophene) (P3HT) polymer solar cells, the active layer absorption is enhanced in the presence of silver NCs, which can be ascribed to NCs induced light scattering effect as well as to plasmon enhanced local electric field effect. As a result, photocurrent of the solar cells is increased and the power conversion efficiency (PCE) is improved up to 20%. The comparative study of surface plasmon effects in different organic optoelectronic devices reveals interesting features of the surface plasmon and allows optimization of optoelectronic devices from a novel point of view.     

Real-time external sensing and compensation method for organic light emitting diode displays

A real-time external sensing and compensation method for active matrix organic light emitting diode displays is proposed. The proposed method senses and compensates for the variation of electrical characteristics of poly-Si thin-film transistors (TFTs) during display time. Experimental results show that the proposed method successfully senses the electrical characteristics of TFTs in real-time and compensates for the non-uniform emission current error.     

Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η = 8.01% under standard 100 mW/cm² AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh₂)₂/PC₇₀BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V < 0.65 V) and the interface state assisted thermionic emission at high forward bias (V > 0.65 V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.     

具有电流阻挡层的垂直结构氮化镓发光二级管的光电特性研究

本文对具有电流阻挡层（CBL）的氮化镓垂直结构发光二极管（LED）进行了研究。不带有CBL、带有非欧姆接触CBL和带有二氧化硅CBL的垂直LED芯片样品被制作出来,并对它们进行了光电性能的测试。结果表明这种带有非欧姆接触CBL和带有二氧化硅CBL的氮化镓垂直结构LED的光输出效率相比不带有CBL的分别高出40.6%和60.7%。不带有CBL的、带有非欧姆接触CBL和带有二氧化硅CBL的氮化镓垂直结构LED在350毫安下的效率分别下降到各自最高效率的72%、78%和85.5%。另外,带有非欧姆接触CBL的氮化镓垂直结构LED具有更优越的抗静电性能。     

Improved efficiency in fluorescent blue organic light emitting diode with a carrier confining structure

A blue organic light emitting device (OLED) with improved efficiency and good color purity is reported. The highest occupied molecular orbital (HOMO) level of the hole transport layer (HTL) and that of the emissive layer (EML) differs by 0.3 eV. This energy level mismatch confines the carriers at the HTL/EML interface. Conventional devices have only one HTL/EML interface, with a current efficiency of 2.9 cd/A. Without adding a separate hole blocking layer, incorporating multi-layers of the same HTL and EML increases this efficiency to 5.8 cd/A, with only a small increase in operating voltage yielding increased power efficiency also. But, there are an optimum number of layers, beyond which efficiency loss results. Also, including the multilayer structure simultaneously improves the blue color co-ordinates. To gain insight into the role of multilayer structures in modifying charge transport and recombination zone a simulator was developed. The simulated results could qualitatively explain the experimental observations.     

Optical and electrical characteristics of GaN vertical light emitting diode with current block layer

A GaN vertical light emitting diode(LED)with a current block layer(CBL)was investigated.Vertical LEDs without a CBL,with a non-ohmic contact CBL and with a silicon dioxide CBL were fabricated.Optical and electrical tests were carried out.The results show that the light output power of vertical LEDs with a non-ohmic contact CBL and with a silicon dioxide CBL are 40.6%and 60.7%higher than that of vertical LEDs without a CBL at 350 mA,respectively.The efficiencies of vertical LEDs without a CBL,with a non-ohmic contact CBL and with a silicon dioxide CBL drop to 72%,78%and 85.5%of their maximum efficiency at 350 mA,respectively. Moreover,vertical LEDs with a non-ohmic contact CBL have relatively superior anti-electrostatic ability.     

蓝色发光二极管的技术与实际应用

综述了蓝色发光二极管的技术及不同材料蓝色发光二极管的制作与特性，并介绍了蓝色发光二极管的实际应用。     

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.     

Temperature dependence of electroluminescence in a tris-(8-hydroxy)quinoline aluminum (Alq3) light emitting diode

Organic electroluminescent devices using tris-(8-hydroxy) quinoline aluminum (Alq₃) as the emissive layer and N,N'-diphenyl-N,N' bis (3-methylphenyl)-[1-1'-biphenyl]-4-4'-diamine as the conventional hole transport layer have been fabricated. The temperature- and field-dependent quantum efficiency have been investigated over the temperature range from 1 to 300 K using a model developed by Shen et al. to explore the physics at the organic heterointerface in the present device structure with the formation of an accumulation layer. It has been observed that electron luminescence intensity decreases with decreasing temperature down to 160 K, then saturates in the low-temperature region. The quantum efficiency increases with decreasing temperature and finally reaches an almost constant value. From the analysis, it is seen that the model can explain the luminescence behavior of the device satisfactorily down to 120 K but fails to explain the low-temperature behavior. The efficiency has also been studied with voltage and it is seen that there is an optimum voltage required to get the maximum efficiency     

Tunneling current and electroluminescence in InGaN: Zn,Si/AlGaN/GaN blue light emitting diodes

We investigate electrical and optical characteristics of Nichia NLPB-500 double-heterostructure blue light-emitting diodes (LEDs), measured over a wide temperature range from 10 to 300K. Current-voltage characteristics have complex character and suggest involvement of at least two different tunneling mechanisms. The peak energy of the optical emission follows the applied bias for voltages between 2.3–2.6 V and can be tuned in large spectral range from 2.3 up to 2.8 eV (yellow to blue). This behavior can be understood invoking the photon-assisted tunneling model which was previously successfully applied to highly doped GaAs LEDs. Even at the lowest temperatures, light emission still continues while the increase in the series resistance does not exceed a few tens of kΩ, which indicates absence of complete carrier freeze-out. On leave from P.N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Russia On leave from High Pressure Research Center, Warsaw, Poland     

Temperature dependence of electroluminescence degradation in organic light emitting devices without and with a copper phthalocyanine buffer layer

Temperature dependence of electroluminescence degradation was investigated in two types of organic light emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ₃) emitter molecule, one without and another with copper phthalocyanine (CuPc) buffer layer at the hole-injecting contact interface. Electroluminescence degradation in time was measured for devices operated at 22 and 70 °C. Results unexpectedly showed that devices without the CuPc buffer layer demonstrated negligible change in half-life when operated at 22 or 70 °C, while devices with the CuPc layer showed the expected decrease in half-life when the temperature was increased. The results are explained within the framework of recently proposed OLED degradation mechanism, which identifies AlQ₃ cations as unstable, leading to device degradation.     

Impact of temperature on the efficiency of organic light emitting diodes

Organic light emitting devices (OLEDs) are known to heat up when driven at high brightness levels required for lighting and bright display applications. This so called Joule heating can in the extreme case lead to a catastrophic failure (breakdown) of the device. In this work, we compare the effect of Joule heated and externally heated OLEDs by their electrical and optical response. A reduction in resistance is observed at elevated temperatures, both, for Joule heating, and for externally heated samples driven at low current density. In both cases, we attribute the change in resistance to a higher mobility of charge carriers at the elevated temperatures. Additionally, we observe a quenching of the emission efficiency in heated single layers as well as in OLEDs, treated with an external heat source as well as on Joule heated samples.