Layer In Light-emitting Diodes Research Articles

Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White Color-Conversion Layers in LEDs.

Being considerably more efficient than traditional incandescent bulbs or fluorescent tubes, light-emitting diodes (LEDs) are becoming the main technology for general lighting applications.1,2 In the last years, a variety of white-LEDs have been demonstrated employing, for example, organic molecules (OLEDs)3−7 or inorganic quantum dots (QDs) of different composition.8 A common way of achieving a white-light emission is to couple a near-UV or blue LED with down-conversion materials such as phosphors.9 More recently, QDs have been incorporated into LEDs, replacing conventional phosphors to tune finely the emission spectrum.10 Among the different available QDs, lead halide-based perovskite nanocrystals have recently emerged as very promising candidates for many optoelectronic applications.11−15 Colloidal perovskite nanocrystals can be synthesized and/or transformed postsynthesis, so that samples emitting at different wavelengths throughout the whole visible spectrum, with high photoluminescence quantum yield (PLQY), can be prepared easily. This can be achieved either by changing the chemical composition (by anion-exchange for example)16,17 or the shape (cubes, platelets, sheets, wires).18−21 Also, electroluminescence has been observed from bulk perovskite films,22−25 blended perovskite-in-polymer films,26 as well as from nanocrystals-only films27,28 leading to the first perovskite-based LEDs.

Electrical-optical analysis of photonic crystals GaN-based high power light emitting diodes

Although photonic crystals (PhCs) have been used to enhance the light extraction efficiency (LEE) of light emitting diodes (LEDs) in a lot of researches, the effects of PhCs on the electrical and optical characteristics of GaN-based high power LEDs have been rarely analyzed simultaneously. Herein, the 3D finite-element method (FEM) was employed to investigate the current density distribution in the active layer of LEDs while the LEE was calculated based on the finite-difference time-domain method. Then the influences of several parameters, including the radius, the filling factor, and the etching depth of PhCs on the current density distribution and external quantum efficiency (EQE) of LEDs were examined. It was found that the configurations of PhCs had a significant influence on the electrical and optical performances of LEDs. With the optimized parameters, a 62 % enhancement of EQE and more uniform current density distribution have been obtained comparing to those of LEDs with planar ITO. These analysis results can facilitate the identification of the design rules for high power, high efficiency LEDs.

Microtube Light-Emitting Diode Arrays with Metal Cores.

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

Effect of Thinning Encapsulant Layer on Junction and Phosphor Temperature of White Light-Emitting Diodes

Thinning of an encapsulant layer for light-emitting diodes (LEDs) is a current major technological trend. The influence of encapsulant thickness on the junction temperature as well as the maximum phosphor temperature for blue and white LEDs is studied systematically for the first time. Using the finite-element method and forward voltage method, it is demonstrated that, in contrast to common belief, a thinned encapsulant layer for relatively low-power LED emitters with an input electrical power of 0.5 W will lead to an increase in the junction temperature for the white LEDs fabricated using different methods, including the phosphor volume distributed in-cup method and chip-coating method, as well as for monochromatic blue LEDs. It is also demonstrated that, different from the effect of encapsulant thickness on the junction temperature, a reduction in the encapsulant thickness results in a decrease in the maximum phosphor temperature for the in-cup white LEDs and an increase in the phosphor temperature for the chip phosphor-coating white LEDs. For example, the largest phosphor temperature drop of the in-cup white LED can be as high as 5 °C. The effect of thermal conductivity of leadframe and phosphor concentration under different encapsulant thicknesses is also indicated.

Leakage Current Analysis of GaN-Based Light-Emitting Diodes Using a Parasitic Diode Model

The leakage current of GaN-based light-emitting diodes (LEDs) was analyzed with a parasitic diode model. Arrhenius plots of forward leakage currents revealed that the threshold voltages of the main diode and the parasitic diode were 2.64 and 0.94 V, respectively. The parasitic diode, however, led to an insignificant change in the reverse leakage mechanism, e.g., a predominant reverse leakage mechanism was Poole-Frenkel conduction in the high-temperature range (>300 K) and the variable range hopping in the low-temperature range (<;300 K), as reported previously. The origin of the parasitic diode could be due to the incorporated hydrogen (H) atoms into the top LEDs layers, presumably enhancing the defect-assisted tunneling through the generated H-related deep-level states, particularly generated in the p-cladding or p-AlGaN electron blocking layer.

Influencing factors of GaN growth uniformity through orthogonal test analysis

Gallium nitride (GaN) is widely used in light-emitting diode (LED) devices due to its wide bandgap and excellently optoelectronic performance. The efficiency and lifetime of LEDs are critically determined by quality of GaN, for example, growth uniformity. Metal-organic chemical vapor deposition (MOCVD) is the most popular technique to grow high-quality GaN epitaxial layers. Growth uniformity is influenced by fluid flow, heat transfer and chemical reactions in the reactor. In this paper, the growth process in a close-coupled showerhead (CCS) MOCVD reactor is investigated based on 3D numerical simulation. Influences of the operating parameters on the growth uniformity are presented. To evaluate the role of the parameters systematically and efficiently on the growth uniformity, orthogonal test method is introduced. The results reveal that the growth rate and uniformity are strongly related to the total gas flow rate, the showerhead height and the inlet gas temperature, but are weakly affected by the isothermal wall temperature, the rotating speed and the susceptor temperature under the ranges of the current study. The optimized combination of the parameters is further proposed as a useful reference for obtaining the LED layers with a balance between the growth rate and the growth uniformity in industry.

Uniformity investigation of MOCVD‐grown LED layers

Doped or undoped gallium nitride compounds (GaN/InGaN), usually grown by metal‐organic chemical vapor deposition (MOCVD) method, are at the heart of blue and green light emitting diodes (LEDs). Growth uniformities, such as the excited wavelength, luminous intensity and film thickness, critically influence their application in LED devices. In this paper, growth of GaN compounds in a MOCVD reactor, capable of a one‐time production of 36 × 2” wafers of nitrides, has been investigated. To examine growth uniformity across the wafer and from wafer to wafer, the reactor is divided into Zone A, Zone B and Zone C according to distance to the center of the graphite susceptor. Comparative analysis of each zone offers a straightforward view of the mean excitation wavelength, luminous intensity, film thickness and their standard deviations. Conformity of the growth uniformity in each zone is further checked comprehensively through averaging across‐wafer and wafer‐to‐wafer variables and their standard deviations. Zone B is found to retain excellent wavelength uniformity, since it is located at the middle of the susceptor with weaker effects of the susceptor edge and of the inlet gas flow. Zone A, at the center of the reactor, has the best mean intensity and thickness uniformities due to a well control of the infrared temperature measurement during the growth. And Zone C is worst in all uniformities and should be the main focus when optimizing the reactor. The above experimental analysis reveals the principles common to the MOCVD technique, and provides a basic for further optimization of the process window to improve the cycles with considerable reduction of the costs.

GaN-based three-junction cascaded light-emitting diode with low-resistance InGaN tunnel junctions

We report on the cascading of blue light-emitting diodes (LEDs) up to three junctions using low-resistance InGaN tunnel junctions (TJs). At a forward current density of 10 A/cm2, triple- and dual-junction LEDs operated at 9.09 and 6.07 V with total differential resistances of 6.31 × 10−2 and 4.16 × 10−2 Ω cm2, respectively. A significant increase in output power was observed from the triple-junction LED compared to the dual one, showing that all LED layers contribute to the luminescence output. Enabling high brightness at low current, cascaded LEDs can circumvent efficiency droop mechanism.

Reducing forward bias voltage of InGaN/GaN-based light emitting diodes by using (In)GaN contact layer

The electrical properties of GaN-based light emitting diodes (LEDs) fabricated with indium–tin-oxide (ITO) p-contacts were investigated as functions of the thickness of the (In)GaN contact layers and the In content. The LEDs with the GaN contact layers showed lower forward voltages (in the range of 3.48–3.03 V) than the LEDs with ITO-only contacts (4.2 V); the forward voltages of the LEDs decreased with increasing contact layer thickness (from 1–4 nm) and increasing In content. However, the output power linearly decreased with increasing In content, whereas that of the GaN contact layer LEDs became saturated at a thickness of 2 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level for the samples with the contact layers was shifted toward lower binding energies by 0.11–0.22 eV compared with that of the sample without the contact layer. However, the energy shift decreased with increasing In content. Unlike the contact-layer samples, the sample without the contact layer experienced outdiffusion of N atoms. Based on the XPS and atomic force microscopy results, the contact-layer-induced electrical improvement was described and discussed.

A Simplified Method of Making Flexible Blue LEDs on a Plastic Substrate

A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plastic substrate was demonstrated. A sticky elastomeric stamp was first brought into contact with prefabricated GaN LED array on a sapphire substrate. Laser liftoff was applied by shining laser light through the sapphire substrate. The released LED array sitting on the stamp was transferred to a polyethylene terephthalate substrate that was coated with an adhesive layer to finish the fabrication process. Careful investigation of the built-in stress in the GaN LED layer using Raman spectroscopy revealed that the maximum stress that allows for intact GaN LED layer release and transfer was 0.7 GPa. The method drastically simplifies the cumbersome conventional GaN layer transferring method while preserving the original layout of the GaN LED array. Due to its simple and practical characteristics, the method is expected to greatly facilitate the development of versatile transferrable GaN LED applications on various substrates at a much-reduced cost.

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (Tsubstrate) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by Tsubstrate/Tg, where Tg is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form "stable glasses" with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.

Hybrid film of silver nanowires and carbon nanotubes as a transparent conductive layer in light-emitting diodes

A hybrid film of carbon nanotubes (CNTs) and silver nanowires (AgNWs) that could be regarded as a parallel circuit of CNTs and AgNWs was developed, which exhibited a low sheet resistance of 23 Ω/sq and transmittance at 550 nm of 93%. The relatively high, intertube contact resistance of CNTs was reduced by the metallic AgNWs, which acted as bridges to aid carrier transport between CNTs. A hybrid film of CNTs and AgNWs was used as a transparent conductive layer in an AlGaInP light-emitting diode (LED). Including the hybrid film in the LED increased the optical output power by about 1.6 times and decreased the red shift of emission wavelength from 13.11 to 9.7.

A novel MOCVD reactor for growth of high-quality GaN-related LED layers

Gallium nitride (GaN), a direct bandgap semiconductor widely used in bright light-emitting diodes (LEDs), is mostly grown by metal–organic chemical vapor deposition (MOCVD) method. A good reactor design is critical for the production of high-quality GaN thin films. In this paper, we presented a novel buffered distributed spray (BDS) MOCVD reactor with vertical gas sprayers and horizontal gas inlets. Experiments based on a 36×2″ BDS reactor were conducted to examine influence of the process parameters, such as the operating pressure and the gas flow rate, on the growth efficiency and on the layer thickness uniformity. Transmission electron microscopy (TEM) and photoluminescence (PL) are further conducted to evaluate quality of the epitaxial layers and to check performance of the reactor. Results show that the proposed novel reactor is of high performance in growing high-quality thin films, including InGaN/GaN multiquantum wells (MQWs) structures.

Ordered and ultrathin reduced graphene oxide LB films as hole injection layers for organic light-emitting diode.

In this paper, we demonstrated the utilization of reduced graphene oxide (RGO) Langmuir-Blodgett (LB) films as high performance hole injection layer in organic light-emitting diode (OLED). By using LB technique, the well-ordered and thickness-controlled RGO sheets are incorporated between the organic active layer and the transparent conducting indium tin oxide (ITO), leading to an increase of recombination between electrons and holes. Due to the dramatic increase of hole carrier injection efficiency in RGO LB layer, the device luminance performance is greatly enhanced comparable to devices fabricated with spin-coating RGO and a commercial conducting polymer PEDOT:PSS as the hole transport layer. Furthermore, our results indicate that RGO LB films could be an excellent alternative to commercial PEDOT:PSS as the effective hole transport and electron blocking layer in light-emitting diode devices.

Investigation on bandgap, diffraction, interference, and refraction effects of photonic crystal structure in GaN/InGaN LEDs for light extraction.

In this paper, we have made a clear differentiation among bandgap, diffraction, interference, and refraction effects in photonic crystal structures (PhCs). For observing bandgap, diffraction, and refraction effects, PhCs are considered on the top p-GaN surface of light emitting diodes (LEDs), whereas for interference effect, hole type PhCs are considered to be embedded within n-GaN layer of LED. From analysis, it is observed that at a particular lattice periodicity, for which bandgap lies within the wavelength of interest shows a significant light extraction due to inhibition of guided mode. Beyond a certain periodicity, diffraction effect starts dominating and light extraction improves further. The interference effect is observed in embedded photonic crystal LEDs, where depth of etching supports constructive interference of outward light waves. We have also shed light on refraction effects exhibited by the PhCs and whether negative refraction properties of PhCs may be useful in case of LED light extraction.

Fabrication of vertical light emitting diode based on thermal deformation of nanoporous GaN and removable mechanical supporter.

A GaN vertical light emitting diode (LED) based on the novel lift-off method was demonstrated by high temperature regrowth over nanoporous (NP) GaN template formed by electrochemical (EC) etching. A two-step EC etching process was employed on a SiO2 patterned GaN surface to fabricate a nanoporous template with a controlled porosity profile, which enabled better structural stability than a single NP GaN. During the regrowth of LED structures, the high porosity GaN layer produced large coalesced voids due to the thermal deformation of nanopores. LED layers were then separated from the sapphire substrate and transferred to a Mo substrate by the removal of the SiO2 mechanical supporters that held the LED structure to suppress cracks and damage during the process. The vertical LEDs fabricated using this technique showed improved optical power emission as well as low series resistance.

Research on Charge Accumulation and Transport in Hole-Conductor-Free Perovskite (CH3NH3PbI2Cl) -based Nanostructure Solar Cells

Organic-inorganic lead halide perovskite, initially applied as active layer in light-emitting diode (LED) and field-effect transistor ( FET) devices[1], has been gradually introduced into the development of solar cells and plays such critical role as either light absorber or hole conducting material (HCM). This kind of hybrid layer structure makes perovskite as a promising candidate in photovoltaic applications, mainly due to its attractive chemical versatility that can readily tune the electronic coupling between inorganic sheets, and eventually tailor its optical/electronic properties[2]. In order to understand the performance of a photovoltaic device especially involving such novel material for its further optimization consideration, exploring the photogenerated charge accumulation and transport is imperative to be highlighted. Here we report the work on perovskite (CH3NH3PbI2Cl[3])-based nanostructure solar cell, including the cell fabrication and characterization, and particularly provide the analysis on charge carrier properties on the basis of dynamic electrochemical measurements. In this research stage, we targets more than achieving relatively fair energy conversion efficiency, instead we hope to uncover internal mechanisms of charge carrier movement and subsequently the substantial function of perovskite in photovoltaic device.The stack structure of the mesoscopic solar cell we developed for this investigation is shown in Fig. 1(a). The nanoporous semiconductor oxide membrane was fabricated by screen-printing technique on fluorine doped tin oxide (FTO) glass substrate with terpineol-based paste made by commercial-available TiO2 powder (Aldrich, d=25nm). The work electrode coated with the TiO2 paste was sintered under airflow at 550oC for 3 h. Iodide-chloride mixed-halide perovskite was synthesized from a N,N-Dimethylformamide solution of CH3NH3I and PbCl2 (3:1 molar ratio) [3]. The precursor solution was deposited on porous TiO2 surface via spin-coating or dipping process, and then was dried at 100oC. After drying, obvious color variation indicated the formation of perovskite layer. The function of the perovskite layer in nanostructure solar cells strongly depends on the deposition method. In particular, the shape of nanocrystalline CH3NH3PbI2Cl infiltrating into TiO2 porous membrane, probably with quantum-dot structure, is confirmed by electron microscopy micrographs. One thing should be noted is that, combining the band diagram shown in Fig.1(b), the extremely thin CsSnI2.95F0.05 layer (referring our previous work for its synthesis[4]) deposited on counter electrode performs more like energy ‘staircase’ for hole transport rather than pure hole conducting material. Meanwhile, a heterojunction-like cell (called ‘blank’ cell) containing only CH3NH3PbI2Cl layer on the TiO2film is produced for comparison.In order to thoroughly understand the decisive mechanisms of charge accumulation and transport behaviors on photovoltaic conversion, a systematic characterizations including photocurrent transient, open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) were carried out under different operation conditions. Correspondingly the effective lifetime and diffusivity of charge carriers are extracted to investigate the transport properties of photon-generated electrons. Particularly, chemical capacitance (Cμ) is examined since it assists to monitor the Fermi level of electrons and density of states (DoS) in perovskite absorber, which significantly differentiates from conventional dye (e.g. N719) sensitized solar cells on the aspect of charge generation and accumulation in light absorber material.Consequently, our research demonstrates the promising HCM-free nanostructure solar cells involving solution-processed perovskite material. Fair light absorption and charge accumulation capability make self-assembled CH3NH3PbI2Cl behave as both sensitizer and charge transport material, which helps to constitute new kind of solid-state sensitized solar cells with a much simpler configuration. Reference 1. D. B. Mitzi, K. Chondroudis, and C.R. Kagan, Organic–inorganic electronics. IBM J. Res. & Dev. , 2001. 45(1): p. 29-45.2. Etgar, L., Semiconductor Nanocrystals as Light Harvesters in Solar Cells. Materials 2013. 6: p. 445--59.3. Lee, M.M., et al., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012. 338 ( 6107): p. 643-647.4. Sun, X., et al., Effects of calcination treatment on the morphology, crystallinity, and photoelectric properties of all-solid-state dye-sensitized solar cells assembled by TiO2 nanorod arrays. Phys. Chem. Chem. Phys., 2013. 15: p. 18716-18720.

Sapphire Substrate Geometrical Effects on III-N Blue LED Material Properties

Solid-state lighting modules based upon III-N light-emitting diodes (LEDs) are now progressing towards widespread adoption in residential and commercial applications. LED manufacturers are driving die performance and yield upwards while reducing cost. However, color binning due to the sensitivity of the human eye response, which can resolve differences in dominant wavelength within a 2 nm range, remains one of the primary yield and cost limiters. As we continue to improve and understand growth process-related reproducibility, we have observed that some of the within-wafer and wafer-to-wafer characteristics cannot be explained through known physical phenomena from the epitaxy, and are more likely related to substrate properties.In this presentation, we will analyze a set of experiments designed to understand the effect of (0001) sapphire c-plane substrate physical dimensions (thickness, diameter, bow/warp/TTV, edge beveling) and crystal properties (misorientation, surface roughness) on uniformity and reproducibility of constituent LED layers. Our initial studies have found that precisely controlled crystal misorientation (offcut) towards the m-plane is a significant factor influencing wafer-to-wafer peak wavelength variation, GaN surface roughness, and p-doping levels. Quantitative analysis of these results with respect to variation in typical substrate manufacturing capabilities will be presented; the trends are in agreement with the materials characteristics explored in our previous studies [1, 2]. Warp and bow are critical parameters for control over GaN material characteristics. Furthermore, tight control over the total thickness variation (TTV) is needed to obtain highly uniform, repeatable sub-micron dimensions used to create current patterned substrate (PSS) wafers. [1] D. Lu, D.I. Florescu, D.S. Lee, V. Merai, J.C. Ramer, A. Parekh, and E.A. Armour, “Sapphire substrate misorientation effects on GaN nucleation layer properties”, J. Crystal Growth 272:353, (2004).[2] D. Lu, D.I. Florescu, D.S. Lee, V. Merai, A. Parekh, J.C. Ramer, S.P. Guo, and E. Armour, “Advanced characterization studies of sapphire substrate misorientation effects on GaN-based LED device development”, Phys. Stat. Sol.(a) 200:71, (2003).

Advantages of the Blue InGaN/GaN Light-Emitting Diodes with an AlGaN/GaN/AlGaN Quantum Well Structured Electron Blocking Layer

InGaN/GaN light-emitting diodes (LEDs) with p-(AlGaN/GaN/AlGaN) quantum well structured electron blocking layer (QWEBL) are designed and grown by a metal–organic chemical-vapor deposition (MOCVD) system. The proposed QWEBL LED structure, in which a p-GaN QW layer is inserted in the p-AlGaN electron blocking layer, not only leads to an improved hole injection but also reduces the electron leakage, thus enhancing the radiative recombination rates across the active region. Consequently, the light output power was enhanced by 10% for the QWEBL LED at a current density of 35 A/cm2. The efficiency droop of the optimized device was reduced to 16%. This is much smaller than that of the conventional p-AlGaN electron blocking layer LED, which is 31%.

Direct Integration of Polycrystalline Graphene into Light Emitting Diodes by Plasma-Assisted Metal-Catalyst-Free Synthesis

The integration of graphene into devices is a challenging task because the preparation of a graphene-based device usually includes graphene growth on a metal surface at elevated temperatures (∼1000 °C) and a complicated postgrowth transfer process of graphene from the metal catalyst. Here we report a direct integration approach for incorporating polycrystalline graphene into light emitting diodes (LEDs) at low temperature by plasma-assisted metal-catalyst-free synthesis. Thermal degradation of the active layer in LEDs is negligible at our growth temperature, and LEDs could be fabricated without a transfer process. Moreover, in situ ohmic contact formation is observed between DG and p-GaN resulting from carbon diffusion into the p-GaN surface during the growth process. As a result, the contact resistance is reduced and the electrical properties of directly integrated LEDs outperform those of LEDs with transferred graphene electrodes. This relatively simple method of graphene integration will be easily adoptable in the industrialization of graphene-based devices.