Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon. However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance. Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantum-dot light-emitting diodes that operate at infrared wavelengths with radiances (6.4?W?sr(-1)?m(-2)) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3?nm by varying the lengths of the linker molecules from three to eight CH(2) groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850?nm.     

Emission characteristics of photonic crystal light-emitting diodes

Experimentally measured optical properties of photonic crystal LEDs are reported here. Photonic crystal and photonic quasi-crystal structures were fabricated on GaN epilayer LED wafer material using both direct-write electron beam lithography and nanoimprint lithography. Some of these structures were processed to make finished LEDs. Both electroluminescence and photoluminescence measurements were performed on these structures. Devices were characterized for their current-voltage characteristics, emission spectra, far-field emission pattern, and angular emission pattern. These results are useful for fabricating photonic crystal LEDs and assessing their operational properties.     

Efficiency and stability of polymer light-emitting diodes

The operation characteristics of polymer light-emitting diodes (PLEDs) are strongly dependent on materials, processing and the structure of the device. The device structure developed at Philips Research is presented together with some typical results for brightness, efficiency, response times and stability. The PLEDs typically operate at a voltage of 3–4 V for a brightness of 100 cd m^-2 and have an efficiency ranging from 2 cd A^-1 for orange emitting polymers (610 nm) up to 16 cd A^-1 for green emitting polymers (550 nm). The response time under conditions for display operation is determined by the charge carrier transport properties and amounts to 43 ns. Lifetimes of several thousand hours have been obtained for large orange emitting devices of 8 cm² for daylight visibility at room temperature.     

Brightness of blue GaN-based light-emitting diodes

We report the brightness characteristics of KPT-1608PBC (SMD) blue light emitting diodes and the key features of emission propagation through the SMD case. The luminosity pattern of the diode is presented. The external quantum yield is determined as a function of the current through the diode, and the quantum yield is shown to correlate with the mechanism responsible for the recombination current. Original Russian Text ? N.S. Grushko, A.V. Lakalin, A.P. Solonin, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 181–183.     

Protection of organic thin-film light-emitting diodes

It is demonstrated that the lifetime of organic thin-film electroluminescent light-emitting diodes can be increased many times by the deposition of protective coatings during the fabrication cycle. Pis’ma Zh. Tekh. Fiz. 23, 88–90 (March 12, 1997)     

Transient characteristics of organic light-emitting diodes with efficient energy transfer in emitting material

We have investigated the combination effect of host–guest materials in an emitting layer on the transient property of an organic light-emitting diode (OLED). We found that an efficient energy transfer owing to the large overlap between the photoluminescence spectrum of host material and the absorption spectrum of guest material was an important factor to improve the response speed of the OLED. As a result, the rise time of optical response was mainly affected by the combination of host–guest materials, and it increased using the optimal guest material, 1,4-bis[2-[4-N,N-di(p-tolyl)amino]phenyl]vinyl]benzene (DSB). A maximum −3 dB cutoff frequency of 15.8 MHz was achieved for an OLED with DSB as a guest material.     

Perspectives on organic light-emitting diodes for display applications

Herein, we describe a number of key issues that concern the commercialization of organic light-emitting diodes for display applications. We will firstly outline the historical and market contexts that show the potential for organic electronics as a viable display technology. Next, we will discuss the chemical structures for a range of both small-molecular and polymer organic semiconducting compounds, and how the electronic properties are governed thereof. Also we will briefly discuss various common film deposition and device fabrication strategies. Then, we will describe two factors that are highly relevant for commercially viable organic light-emitting diodes, namely charge balance, and device degradation. Finally, we will outline some methods for achieving the high-volume throughput of organic electronics via well-established technologies that are used in the printing industry.     

GaN-based light-emitting diodes prepared on vicinal sapphire substrates

Thin InGaN epitaxial layers and GaN-based light-emitting diodes (LEDs) on conventional and vicinal cut sapphire substrates are prepared. It is found that indium atoms are distributed much more uniformly in the samples prepared on vicinal cut sapphire substrates. It is also found that stronger electroluminescence intensity can be achieved without the band-filling effect of localised states from the LEDs with vicinal cut sapphire substrate. With 20 mA current injection, it is found that 44% electroluminescence intensity enhancement can be achieved by using the 1deg tilted sapphire substrate     

Superfocusing of mutimode semiconductor lasers and light-emitting diodes

The problem of focusing multimode radiation of high-power semiconductor lasers and light-emitting diodes (LEDs) has been studied. In these sources, low spatial quality of the output beam determines theoretical limit of the focal spot size (one to two orders of magnitude exceeding the diffraction limit), thus restricting the possibility of increasing power density and creating optical field gradients that are necessary in many practical applications. In order to overcome this limitation, we have developed a method of superfocusing of multimode radiation with the aid of interference. It is shown that, using this method, the focal spot size of high-power semiconductor lasers and LEDs can be reduced to a level unachievable by means of traditional focusing. An approach to exceed the theoretical limit of power density for focusing of radiation with high propagation parameter M ₂ is proposed.     

Effect of residual compressive stress on near-ultraviolet InGaN/GaN multi-quantum well light-emitting diodes

Thinning was investigated to reduce the residual compressive stress in GaN-based near-ultraviolet light-emitting diode (NUV-LED) substrates. This stress has a knock-on effect of reducing piezoelectric fields in the LED structure. As the sapphire substrate thickness is reduced, the compressive stress in the GaN layer is released, resulting in wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric fields, which in turn reduces the quantum-confined Stark effect in the InGaN/GaN active region of the LED. The electroluminescence spectral peak wavelength was blue-shifted, and the internal quantum efficiency was improved by about 15% at an injection current of 50 mA. The LED with a 45-μm-thick sapphire substrate exhibited the highest light output power of ∼29 mW at an injection current of 50 mA, an improvement by about 39% compared to that of a 150-μm-thick sapphire substrate without increasing the operating voltage. The simulation results confirm that the relaxation of the compressive strain in the InGaN/GaN MQW structure results in the reduction of the piezoelectric field and improves the overlap of electron and hole wave functions with a corresponding increase in IQE.     

Changes in emission characteristics of light-emitting diodes under variation of internal factors and pulse-excitation parameters

A relationship is established between the wavelength λ at the maximum of the near-edge emission band of a light-emitting diode (LED) and the external parameters of the pulse current t_i and Q and the thermal time constant τ_i, which is determined by the product of the thermal resistance and capacitance. A dependence of the shift in λ on the relationship between the LED’s internal and external parameters is discovered. It is shown that for LED operation in the pulsed mode, the shift in λ with variation of t_i. Q, and especially τ_i can reach values on the order of 10 nm (or more), which is a considerable source of errors. Ukraine. Translated from Izmeritel’naya Tekhnika, No. 12, pp. 37–38, December, 1999.     

Detection of flame radicals using light-emitting diodes

Lasers are common tools in the field of combustion diagnostics. In some respects, however, they have disadvantages. Therefore, there is a need for new light sources delivering radiation in the required wavelength regions with high stability and reliability at low cost. Light-emitting diodes (LED) in the near- and mid-infrared spectral region have proven their potential for spectroscopic applications in the past. In the present work we demonstrate the feasibility of using ultraviolet LEDs for flame diagnostics. For this purpose, OH and CH radicals are detected in premixed methane/air flames. The LED emission is found to be stable after thermal equilibrium is reached. This was the case after a warming-up period in the order of minutes. The spectral characteristics were stable during a 24-h test.     

Layer-by-layer growth of CdSe-based nanocrystal light-emitting diodes

The partial exchange of surface-passivating trioctylphosphine oxide (TOPO) on CdSe and ZnS-clad CdSe (CdSe/ZnS) nanocrystals with primary amines was utilized to grow ultra-thin films of these nanocrystals under nonaqueous conditions. This growth was achieved using 1,12-diaminododecane in a layer-by-layer assembly format, where one of the amino groups binds with the nanocrystal surface and the other regenerates the interface for further binding of nanocrystals. The nature of the growth is dependent on the relative surface affinity between the TOPO and the primary amine toward the zinc or cadmium sites on the nanocrystals. Using this technique, high-quality luminescent films of these nanocrystals can be built with well-defined thicknesses. Electroluminescent devices have been fabricated using this methodology.     

Dielectric properties depending on frequency in organic light-emitting diodes

Dielectric properties of organic light-emitting diodes were investigated using ITO/Alq₃/Al device. In spite of various advantages in organic light-emitting diodes, a fundamental study on physical properties is not yet sufficient. Dielectric properties are used for studying fundamental physical properties of materials through a frequency-dependent response. We have investigated magnitude and phase of impedance, electrical conductivity, and the dielectric loss depending on a bias-voltage variation using ITO/Alq₃(60 nm)/Al device. The device shows a frequency-dependent response such that a major contribution is resistive below time constant and capacitive above time constant. Also, the device shows a voltage-dependent electrical conductivity in low-frequency region. A bulk resistance rapidly decreases as the frequency increases above 1 MHz. The dielectric loss shows that there appears an interfacial polarization in low-frequency region, and an orientational polarization in high-frequency region.     

Modeling and numerical study of electrical characteristics of polymer light-emitting diodes containing an insulating buffer layer

In order to improve the electrical characteristic of polymer light-emitting diodes, a simple model for the device characteristic with an insulating buffer layer at cathode is proposed. This model is based on Fowleer-Nordhein tunneling mechanism and Poission's equation. An additional tunneling factor which characterises the tunneling effect of buffer layer is introduced. The simulated current-voltage characteristic indicates how an insulating buffer layer with suitable thickness decreases the barrier height at the cathode and therefore increases the electron injection. The model is validated by experimental results of devices with BaO as the buffer material and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] as the emission material. An optimum thickness of the buffer layer is also obtained from the model, which provides a guide to device design.     

Blue organic light-emitting diodes based on solution-processed fluorene derivative

We report blue fluorescent organic light-emitting devices (OLED) by solution process utilizing a blue emitting small molecule, 2,7-bis[(9-ethyl-9H-carbazol-3-yl)ethenyl]-9,9-bis(4-n-octyloxyphenyl)-9H-fluorene (CB), which has good solubility in common organic solvent. The peak positions of absorption and emission spectra of a new fluorene-based molecule in tetrahydrofuran solution were observed at 399 and 439 nm, respectively. We achieved a maximum luminous efficiency of approximately 3 cd/A with CIE color coordinates of (0.15, 0.15) in our device.