Design and fabrication of temperature-insensitive InGaP-InGaAlP resonant-cavity light-emitting diodes

Visible InGaP-InGaAlP resonant-cavity light-emitting diodes with low-temperature sensitivity output characteristics were demonstrated. By means of widening the resonant cavity to a thickness of three wavelength (3/spl lambda/), the degree of power variation between 25 /spl deg/C and 95 /spl deg/C for the devices biased at 20 mA was apparently reduced from -2.1 dB for the standard structure design (1-/spl lambda/ cavity) to -0.6 dB. An output power of 2.4 mW was achieved at 70 mA. The external quantum efficiency achieved a maximum of 3% at 13 mA and dropped slowly with increased current for the device. The external quantum efficiency at 20mA dropped only 14% with elevated temperature from 25 /spl deg/C to 95/spl deg/C. The current dependent far-field patterns also showed that the emission always took place perfectly in the normal direction, which was suitable for plastic fiber data transmission.     

Impedance spectroscopy for quantum dot light-emitting diodes

Impedance spectroscopy has been increasingly employed in quantum dot light-emitting diodes(QLEDs) to investigate the charge dynamics and device physics. In this review, we introduce the mathematical basics of impedance spectroscopy that applied to QLEDs. In particular, we focus on the Nyquist plot, Mott-Schottky analysis, capacitance-frequency and capacitance-voltage characteristics, and the d C/d V measurement of the QLEDs. These impedance measurements can provide critical information on electr...     

Thermal properties of organic light-emitting diodes

Thermal management is important for the efficient operation of organic light-emitting diodes (OLED, or PHOLED) at high brightness, with the device operating temperature influencing both lifetime and performance. We apply a transmission-matrix approach to analytically model the effects of thermal conduction, convection and radiation on OLED temperature. The model predictions match experiment without requiring the use of fitting parameters. This allows for the simulation of the thermal response of various device architectures, materials combinations and environmental factors under a variety of operating conditions. Using these simulations, we find that 87% of the heat is dissipated through the air space adjacent to the glass package cap. Furthermore, an air gap between the device cathode and cap provides a significant thermal impedance. Minimizing the thickness of the internal air gap can lead to nearly room temperature operation, even at very high brightness.     

Probability characteristics of electrical noise in heterojunction light-emitting diodes

A hardware-software complex for measurements of the characteristics of electrical and optical noise in light-emitting diodes (LEDs) in the frequency range from 1 to 40 kHz is described. The electrical noise of several types of heterojunction-based LEDs are studied; these types include red-emission LEDs with AlInGaP/GaAs quantum wells and the green- and blue-emission LEDs with AlInGaN/SiC quantum wells are studied by the method of discrete samples. The spectra of all studied LEDs in the frequency range from 1 to 10 kHz have the form 1/f ^γ. It is noteworthy that, for red-emission LEDs, the exponent γ is significantly smaller than unity; this index is close to unity for the green- and blue-emission LEDs. The characteristic time of correlation of the noise of red-emission LEDs by several times exceeds the correlation times for the blue- and green-emission LEDs. It is shown that reduced functions of the amplitude distribution of the noise voltage are close to Gaussian functions with almost the same dispersion for all LED types.     

An alternative method to fabricate the planar-type resonant-cavity light-emitting diodes by using silicon oxide for short-reach communications

In this article, the silicon oxide (SiO_x) planarization technique is presented to fabricate the 650-nm resonant-cavity light-emitting diodes (RCLEDs). The performances of RCLEDs are characterized by forward voltage, light output power, external quantum efficiency, emission spectrum, and dynamic response. As a result, the device with the SiO_x-planarized layer exhibits a low operating voltage of 2.3 V at 20 mA, a maximum light output power of 304 μW at 15 mA, and the best external quantum efficiency of 3% at 1.2 mA. In addition, the SiO_x-planarized device exhibits temperature insensitivity as compared to the device without it. The RCLED with a 30-μm diameter shows the maximum 3 dB frequency bandwidth of 275 MHz at a driving current of 40 mA. Finally, the RCLED with a SiO_x-planarized layer shows a clear eye-opening feature as operating at 100 Mbit/s at 20 mA. These results indicate that such LEDs are excellent candidates for use in high-speed short-reach plastic optical fiber communications.     

Low-frequency noise measurement and analysis in organic light-emitting diodes

Low-frequency noise characteristics of organic light-emitting diodes are investigated. Two noise components were found in experimental low-frequency noise records, namely: 1) 1/f Gaussian noise from device bulk materials and 2) an excessive frequency-related part of noise related to device interfaces or defects and traps. 1/f noise is said to be related to carrier mobility. Degradation, especially photo-oxidation of the electroluminescence polymer, is a possible reason that affects carrier mobility. The excessive part of noise is believed to be related to the carrier numbers and could come from the interface deterioration, defects and traps generation and furnish. The excessive part of noise increases much faster during device stress. This shows that the degradation related interface defects and traps is much faster.     

Noise correlation and noise reduction in GaAs light-emitting diodes

The crosscorrelation between the random fluctuations of the light intensity and the current of GaAs LED's has been measured in the 1/f frequency range at room-temperature. A strong dependence of the crosscorrelation on the direction of radiation is found. The diodes with a plastic lens which increases the light intensity along the axis vertical to the emitting surface show a higher correlation than the diodes with the lens removed. From a simple LED equivalent circuit a relation linking the excess noise factor and the coherence function is calculated which fits qualitatively well with the measured results. Finally, it is shown how an optoelectronic feedback method eliminates the excess noise of the light intensity in the low frequency domain.     

Parasitic emission in inkjet-printed InP-based quantum dot light-emitting diodes

Indium phosphide-based colloidal quantum dot (QD) light-emitting diodes represent a promising technology for various lighting applications. To promote this innovative technology closer to an industrialized production environment, the fabrication methods should be adapted. Hence it is necessary to replace the common spin-coating process under an inert atmosphere, by a more cost-efficient inkjet-printing process at ambient conditions. However, in our case, this transfer results in devices with limited performance and parasitic emission channels besides the desired QD emission. In this paper, we identify the physical origin of these parasitic emission channels for three different device layouts depending on the QD material as well as the number of inkjet-printed layers. For the first type of devices, a recombination process on the dopant of the electron transporting layer (ETL) as well as an exciplex formation at the interface between QDs and ETL was identified. For the next device layout, the introduction of a hole-conducting matrix embedding the QDs leads to a shift of the parasitic emission with contributions from the matrix material. Finally, the integration of a hole injection layer leads to a reduction of the undesired emission processes. For all three kinds of devices, the spacial separation of the dopant in the ETL from the QDs is a critical factor, since it directly influences the parasitic emission channels.     

Reliability investigation of light-emitting diodes via low frequency noise characteristics

Investigation of changes of operation and noise characteristics during aging process of light-emitting diodes (LEDs) has been carried out. Several groups of different design (different optics) LEDs based on different materials (nitride-based blue and white LEDs, phosphide-based red LEDs) have been investigated. It is found that leakage current components appear due to LED’s defects and their affect is observed as increase of both the low frequency electrical noise intensity and non-ideality factor of current-leakage characteristic in small current region. No considerable changes of light intensity characteristics during LEDs aging have been observed. Noise modeling, spectral and correlation analysis of optical and electrical fluctuations show on partly correlated optical and electrical fluctuations caused by defects in the active region of the LED. Degradation processes of investigated LEDs foremost occur in the diode chip and lead to the leakage current that has important affect to the electrical fluctuation level, but practically has a weak influence to the light emission properties of LED. Phosphorous layer of white LEDs and additional optical elements have no significant influence to the reliability of investigated LEDs under given aging conditions.     

Broad-band superluminescent light-emitting diodes incorporating quantum dots in compositionally modulated quantum wells

We propose and demonstrate a technique for tailoring the emission bandwidth of /spl sim/1.3 /spl mu/m quantum dot superluminescent light-emitting diodes. A broadening of the emission is achieved by incorporating the InAs quantum dot layers in InGaAs quantum wells of different indium compositions. These structures exhibit a broader and flatter emission compared to a simple dot-in well structure comprised of wells of identical indium composition.     

AlGaInP发光二极管内量子效率测量分析

采用两种方法对650 nm AlGaInP LED内量子效率进行测量分析.一是考虑光子循环利用的影响,建立取光效率模型,使用光线追迹法模拟计算取光效率,进而反推出内量子效率.另一方法是变温L-I-V测量分析,在降温过程测量外量子效率随温度的变化,测量出一定温度范围内外量子效率出现最大值且稳定,并根据LED内部的复合机制,从而确定内量予效率值.最后分析两种测量方法,并给出了影响测量的因素.     

Visible (660 nm) resonant cavity light-emitting diodes

The demonstration of the first visible resonant cavity light-emitting diodes (RCLEDs) is reported. The devices consist of an InAlGaP strained quantum well active region surrounded by AlAs/AlGaAs distributed Bragg reflectors. Linewidths from 0.9 mm (2.6 meV) to 45 nm (12.8 meV) were obtained by varying the cavity factor (Q).<>     

Improved quantum dot light-emitting diodes with a cathode interfacial layer

Colloidal quantum dot light-emitting diodes (QLEDs) are reported with improved external quantum efficiencies (EQE) and efficiency roll-off under high current densities by introducing a thermally-evaporated organic cathode interfacial material (CIM) Phen-NaDPO. QLEDs with this new CIM modified Al cathode were fabricated, giving an upwards of 25% enhancement in the EQE relative to the bare Al device. Ultraviolet photoemission spectroscopy (UPS) suggests that this material can effectively lower the work function of Al, therefore facilitating the electron injection in QLEDs. Furthermore, Phen-NaDPO was introduced into the LiF/Al device to afford better balanced hole/electron injection in the emitting layer. Consequently, the QLEDs with the organic CIM/LiF/Al cathode further increased EQE and current efficiency by 44% and 52%, respectively, with higher luminance and lower efficiency roll-off under high current densities.     

A review on the electroluminescence properties of quantum-dot light-emitting diodes

Quantum-dot light-emitting diodes (QLEDs) are unarguably the most successful member of rapidly developing family of devices based on quantum dots (type II−VI group compounds). Herein, the electroluminescence properties and fabrication/characterization technologies of QLEDs are reviewed. Particular emphasis is devoted to the dynamic processes of charge carriers and the related characterization technology because QLEDs are electro-optic conversion devices whose performance is to a great extent determined by the carrier transport/distribution and exciton formation. The utility of spectroscopic technologies, including steady/transient electroluminescence and photoluminescence, electro-absorption spectrum, and differential absorption spectrum are explained. Additionally, displacement current measurement technology is also discussed due to its potential to characterize the trapped charges within the devices. The strategies to improve the device performance by interface modification and QD design are summarized and the corresponding physics and chemistry mechanisms are discussed. Finally, a summary and outlook are shown about the challenge faced by QLED, as well as possible pathway to enhancing the device performance.     

基于石墨烯的柔性黄光有机发光二极管制备与性能

采用喷涂技术制备了石墨烯/PEDOT:PSS复合透明导电薄膜,并在此基础上制备了柔性黄光OLED器件。通过设计DPVBi/Rubrene/DPVBi势阱结构,可以实现电荷的有效陷获,获得稳定明亮的黄光发射。实验结果表明,适当增加发光区的厚度,可以提升器件的发光效率和稳定性,在12 V时器件的效率为0.9 cd/A。该柔性黄光OLED器件在反复弯曲测试中表现出了良好的发光稳定性,发光效率没有产生明显变化。     

InGaN multiple quantum well based light-emitting diodes with indium composition gradient InGaN quantum barriers

To improve the internal quantum efficiency(IQE) and light output power of In Ga N light-emitting diodes(LEDs), we proposed an In-composition gradient increase and decrease In Ga N quantum barrier structure. Through analysis of its P-I graph, carrier concentration, and energy band diagram, the results showed that when the current was 100 m A, the In-composition gradient decrease quantum barrier(QB) structure could effectively suppress electron leakage while improving hole injection efficiency, re...     

Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.     

Degradation in short wavelength (AlGa)As light-emitting diodes

Degradation in short-wavelength (AlGa)As lasers is investigated through lifetests of such devices operated in the incoherent mode. It is shown that degradation increases with emission energy for diodes containing zinc in the p-type (AlGa)As bounding region, whereas diodes containing Ge in this region, although not satisfactory as c.w. lasers because of high resistivity, show no degradation. A way out of this difficulty is proposed through double doping with Ge and Zn, in which case degradation appears to be brought down to the Ge level.     

Influence of molecular structure on the properties of dendrimer light-emitting diodes

Iridium-based phosphorescent dendrimers have shown much promise as highly efficient light emitting materials for organic light emitting diodes (OLEDs). Here we report the effects of modifying the chemical structure on the emissive and charge transport properties of Ir(ppy)₃ based electrophosphorescent dendrimers. We investigate a novel para linked first generation (G1) iridium dendrimer. This material is compared to G1 and G2 meta linked dendrimers. We show that by blending these dendrimers into a CBP host, high external quantum efficiencies of over 10% and luminous efficiencies of 27 lm/W can be achieved.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².