Films For Light-emitting Diodes Research Articles

Optically optimization of thin film light-emitting diodes based on transfer matrix

The evolution of white LEDs, both organic and inorganic, has marked a transformative era in lighting technology known as solid-state lighting. This revolution is driven by electroluminescence, where electron-hole pairs recombine to emit light. While OLEDs have achieved high internal quantum efficiency, limited out-coupling efficiency remains a challenge. Improving light out-coupling efficiency is crucial for enhancing LED performance. This paper employs the Transfer Matrix Method to analyze the optical properties of OLEDs and aims to enhance light coupling, efficiency, and overall impact of LED and OLED technologies.

Optically optimization of thin film light-emitting diodes based on transfer matrix

The evolution of white LEDs, both organic and inorganic, has marked a transformative era in lighting technology known as solid-state lighting. This revolution is driven by electroluminescence, where electron-hole pairs recombine to emit light. While OLEDs have achieved high internal quantum efficiency, limited out-coupling efficiency remains a challenge. Improving light out-coupling efficiency is crucial for enhancing LED performance. This paper employs the Transfer Matrix Method to analyze the optical properties of OLEDs and aims to enhance light coupling, efficiency, and overall impact of LED and OLED technologies.

Enhancement of the optical properties of CsPbBr3 perovskite nanocrystals using three different solvents.

Green-emitting CsPbBr3 perovskite nanocrystals were synthesized by the modified hot-injection method using three different solvents. The produced nanocrystals showed a narrow green emission band centered at 515-520 nm with full width at half-maximum (FWHM) values of approximately 18-20 nm. The highest photoluminescence quantum yield (PLQY) was obtained for the nanocrystal sample synthesized using a paraffin liquid solvent, with a value of 70.1% under excitation at 450 nm. The CsPbBr3 nanocrystals film light-emitting diodes (LED) chip module showed a luminous efficacy of 40.7lm/Wrad. The white LED (WLED) with green CsPbBr3 and red CsPbI3 nanocrystal films emitted bluish-white light with a high color rendering index of 89, and the luminous efficacy of the WLED reached 16.3lm/Wrad.

Investigation of Light-Extraction Mechanisms of Multiscale Patterned Arrays With Rough Morphology for GaN-Based Thin-Film LEDs

Micro-scale patterned arrays and nano-scale rough morphology are promising for improving the light-extraction performance of GaN-based thin film light-emitting diodes (TFLEDs), while the light-extraction mechanisms of the multiscale architectures combining these two structures have not been investigated yet. In this report, we have adopted a pattern transfer and wet etching combined method to fabricate multiscale patterned arrays with rough morphology (msPARM) on n-GaN layers for TFLEDs and investigated their light-extraction mechanisms by the finite-difference time domain and ray-tracing combined method. The results show that the TFLEDs achieve the maximum radiant efficacy using the msPARM with an etching time of 8 min, which is increased by 16.3% and 1.7% compared with that achieved using only the patterned arrays or only the rough morphology, respectively. Most importantly, optical simulation reveals that the msPARM can provide a high transmittance for light with large emission angles from the active region using the inclined surface of micro-scale concave cones, while effectively suppressing the reflection loss for light with small emission angles using the scattering effect of nano-scale rough morphology, resulting in enhancing the light-extraction of the TFLEDs. Consequently, this study can provide a better understanding to design the multiscale structures for achieving high efficiency LEDs.

The underlying micro-mechanism of performance enhancement of non-polar n-ZnO/p-AlGaN ultraviolet light emitting diode with i-ZnO inserted layer

Non-polar a-plane n-ZnO/p-AlGaN and n-ZnO/i-ZnO/p-AlGaN heterojunction film light-emitting diodes (LEDs) are fabricated with good crystalline quality. The optical measurements show obvious performance enhancement with i-ZnO layer insertion. Off-axis electron holography reveals a potential drop of ∼1.5 V across the heterojunctions with typical p-n junction characteristics. It is found that the electrostatic potentials are inclined and the corresponding electrostatic fields are opposite to each other in n-ZnO and p-AlGaN regions. The electrostatic fields are mainly attributed to strain induced piezoelectric polarizations. After an insertion of an i-ZnO layer into the p-n heterojunction, comparatively flat electrostatic potential generates in the intrinsic ZnO region and contributes to faster movements of the injected electrons and holes, making the i-ZnO layer more conductive to the radiative recombination with enhanced exciton recombination possibilities and at last the LED performance enhancement.

White perovskite based lighting devices.

Hybrid organic-inorganic and all-inorganic metal halide perovskites have been one of the most intensively studied materials during the last few years. In particular, research focusing on understanding how to tune the photoluminescence features and to apply perovskites to optoelectronic applications has led to a myriad of new materials featuring high photoluminescence quantum yields covering the whole visible range, as well as devices with remarkable performances. Having already established their successful incorporation in highly efficient solar cells, the next step is to tackle the challenges in solid-state lighting (SSL) devices. Here, the most prominent is the preparation of white-emitting devices. Herein, we have provided a comprehensive view of the route towards perovskite white lighting devices, including thin film light-emitting diodes (PeLEDs) and hybrid LEDs (HLEDs), using perovskite based color down-converting coatings. While synthesis and photoluminescence features are briefly discussed, we focus on highlighting the major achievements and limitations in white devices. Overall, we expect that this review will provide the reader a general overview of the current state of perovskite white SSL, paving the way towards new breakthroughs in the near future.

A Long-Term View on Perovskite Optoelectronics.

Recently, metal halide perovskite materials have become an exciting topic of research for scientists of a wide variety of backgrounds. Perovskites have found application in many fields, starting from photovoltaics and now also making an impact in light-emitting applications. This new class of materials has proven so interesting since it can be easily solution processed while exhibiting materials properties approaching the best inorganic optoelectronic materials such as GaAs and Si. In photovoltaics, in only 3 years, efficiencies have rapidly increased from an initial value of 3.8% to over 20% in recent reports for the commonly employed methylammonium lead iodide (MAPI) perovskite. The first light emitting diodes and light-emitting electrochemical cells have been developed already exhibiting internal quantum efficiencies exceeding 15% for the former and tunable light emission spectra. Despite their processing advantages, perovskite optoelectronic materials suffer from several drawbacks that need to be overcome before the technology becomes industrially relevant and hence achieve long-term application. Chief among these are the sensitivity of the structure toward moisture and crystal phase transitions in the device operation regime, unreliable device performance dictated by the operation history of the device, that is, hysteresis, the inherent toxicity of the structure, and the high cost of the employed charge selective contacts. In this Account, we highlight recent advances toward the long-term viability of perovskite photovoltaics. We identify material decomposition routes and suggest strategies to prevent damage to the structure. In particular, we focus on the effect of moisture upon the structure and stabilization of the material to avoid phase transitions in the solar cell operating range. Furthermore, we show strategies to achieve low-cost chemistries for the development of hole transporters for perovskite solar cells, necessary to be able to compete with other established technologies. Additionally, we explore the application of perovskite materials in optoelectronic applications. We show that perovskite materials can function efficiently both as a film in light-emitting diodes and also in the form of nanoparticles in light-emitting electrochemical cells. Perovskite materials have indeed a very bright future.

Influence of etching AlN buffer layer on the surface roughening of N-polar n-GaN grown on Si substrate

Light extraction efficiency of thin-film GaN-based light-emitting-diode (LED) chip can be effectively improved by surface roughening. The film transfer is an indispensable process in the manufacture of thin-film LED chip, which means transferring the LED film from the growth substrate to a new substrate, and then removing the growth substrate. After the growth substrate is removed, the buffer layer is used to cushion the mismatch between the substrate and the n-GaN exposed, which has a significant influence on the roughening behavior of n-GaN. Unlike the GaN buffer layer grown on sapphire substrate, AlN buffer layer is usually used when n-GaN is grown on Si substrate. In this paper, the surface treatment of the AlN buffer layer by reactive ion etching (RIE) is used to improve the surface roughening effect of N-polar n-GaN grown on the silicon substrate in the hot alkali solution (85 ℃, 20% KOH mass concentration of solution), and the mechanism of the influence of the surface treatment on the roughening behavior is discussed by X-ray photoelectron spectroscopy (XPS) and other advanced methods. The degree of etching surface AlN buffer layer is detected by energy dispersive spectrometer (EDS), the sample surface state after RIE etching is analyzed by XPS, the morphology of the surface roughening is observed by scanning electron microscope (SEM) and the effect of surface roughening on the optical power of LED devices is verified by the photoelectric performance test. The EDS results show that the AlN buffer layer remains after RIE etching 10-30 min and the AlN disappears after RIE etching for 40 min. The SEM results show that surface states of AlN buffer layer have a great influence on the roughening behavior of n-GaN in KOH solution. The sample with part of AlN buffer layer has a good roughening effect and proper size hexagonal pyramid distributing uniformly. In addition, the rate of coarsening is too fast for the samples with AlN buffer layer completely removed, while the rate is too slow for the samples without any etching process. In summation, using RIE etching to remove a part of the AlN buffer layer can effectively improve the roughening effect of N-polar n-GaN in KOH solution. We believe that lots of N-vacancies are produced on the surface of the sample after RIE etching, which provides the electrons, thereby causing the surface Fermi level to be elevated. The XPS analysis shows that the RIE etching can improve the electronic binding energy of Al 2p of AlN buffer layer, resulting in a shift of the surface Fermi level near to the conduction band, and reducing the Schottky barrier between the KOH solution and the surface of the sample, which is beneficial to the surface roughening. To remove a part of the AlN buffer by using plasma etching layer can improve the roughening effect of N-polar n-GaN in KOH solution, resulting in the output power of the corresponding LED device being improved obviously.

Effect of Light Absorption in InGaN/GaN Vertical Light-Emitting Diodes.

For evaluating the effect of light absorption in vertically structured thin film light-emitting diodes (VLEDs), we investigate the dependence of the efficiencies on the several specific parameters including thickness and doping concentration (N(D)) of the n-GaN layer, a design of hetero-structures of the n-GaN layer, and a number of pairs of multi-quantum wells (MQWs). Generally, there is a complementary relation between internal quantum efficiency (IQE) and light extraction efficiency (LEE). However, we confirmed that LEE determined by light absorption is more dominant than IQE in VLED structures with a textured surface, from numerical simulation and experimental results. Effect of light absorption is more prominent in the vertical chip with a textured surface than in that with a flat surface, because a travel length of light extracted from the textured surface is longer. Minimizing light absorption in VLEDs is a key technology for improving light output, and light absorption speaks for the index of enhancement by the general technologies for improving LEE.

Changes of micro zone luminescent properties and stress of GaN-based light emitting diode film grown on patterned silicon substrate, induced by the removal of the substrate and AlN buffer layer

At present, there are mainly two kinds of methods to prevent crack and reduce tensile stress of the silicon substrate GaN based light emitting diode (LED) epitaxial films: one is to use the patterned silicon substrate and the other is to grow a thick AlGaN buffer layer. The two kinds of methods have their own advantages and disadvantages. Although the patterned silicon substrate GaN based LED has industrialized and is gradually accepted by the market, there remain many scientific and technical problems, to be resolved, and a lot of research gaps worth studying deeply. Among these problems, to clearly investigate the different micro zone photoluminescence and the stress states in a single-patterned GaN based LED film grown on patterned silicon substrate. The studies of the stress interaction between the buffer layer and the quanturn well layer and the effect on the luminescent properties have important guiding significance for improving the quality and performance of the devices. Different micro zone photoluminescence (PL) properties in single-patterned GaN-based LED films grown on patterned silicon substrates, nondestructive free-standing LED thin film after removing away the silicon substrate, and the free-standing LED films after removing away the AlN buffer layer are studied. The variations of the bending degree of the free-standing LED thin films before and after removing away AlN buffer layer are inverstigated by using fluorescence microscopy and scanning electron microscopy. The results show as follows. 1) After removing away the silicon substrate, the free-standing LED film bends to the substrate direction in a cylindrical bending state. After removing away the AlN buffer layer, the LED film bends into flat. 2) For LED thin films on silicon substrates or off silicon substrates, their PL spectra have significant differences in different micro zones for the same pattern. When the AlN buffer layer is removed from the substrate its PL spectrum tends to be consistent in the different micro zones of the same pattern. When the patterned silicon substrate GaN-based LED thin film is removed from the silicon substrate, the PL spectrum is redshifted in each micro zone. After AlN buffer layer is removed from the substrate, the PL spectra present different degrees of blueshift in each micro zone. 3) The LED films before and after removing away the AlN buffer layer show some differences in droop effect.

A fluctuational electrodynamics model for the optimization of light-extraction efficiency in thin-film light-emitting diodes

The rapid development of thin film light-emitting diodes (LEDs) has enabled the enhancement of the light extraction beyond geometrical limits but more quantitative understanding of the underlying optical processes is required to fully optimize the extraction. We present first-principle calculations of the light extraction efficiency and optical energy flow in thin-film LEDs. The presented model generalizes the methods of fluctuational electrodynamics to excited semiconductors and simultaneously accounts for wave optical effects, e.g., interference and near-field coupling as well as the internal absorption of the light-emitting material in determining the rate of light emission and internal dissipation in the optical cavity formed by a planar LED. The calculations show that in structures with a metallic mirror, the emissivity of the active region can approach unity at selected wavelengths, even when the nominal emissivity of the active region is only moderate. However, the results also show that near-field coupling of emission from the active region to the mirror can provide a substantial non-radiative loss channel reducing the maximum light extraction efficiency to 0.67 in our example setup. These losses can be partly compensated by the efficient photon recycling enabled by thick active regions that quench emission to confined modes and thereby reduce parasitic absorption.

AlGaInN-based light emitting diodes with a transparent p-contact based on thin ITO films

A method for obtaining transparent conductive ITO (indium-tin oxide) films aimed for use in light emitting diodes of the blue spectral range is developed. The peak external quantum efficiency of light-emitting diodes with a p-contact based on the obtained films reaches 25%, while for similar light-emitting diodes with a standard semitransparent metal contact, it is <10%. An observed increase in the direct voltage drop from 3.15 to 3.37 V does not significantly affect the possibility of applying these films in light-emitting diodes since the optical power of light-emitting diodes with a transparent p-contact based on ITO films exceeds that of chips with metal semitransparent p-contacts with a working current of 20 mA by a factor of almost 2.5. Light-emitting diodes with p-contacts based on ITO films successfully withstand a pumping current that exceeds their calculated working current by a factor of 5 without the appearance of any signs of degradation.

Light extraction limits in textured GaN-InGaN light-emitting diodes: Radiative transfer analysis

We present a study on the light extraction properties of thin film light-emitting diodes (LEDs) based on the radiative transfer theory. We show that the well known ergodic limit for absorptivity in textured solar cells also applies to emissivity in LEDs accordance with the Kirchhoff’s radiation law. This limit for the emission enhancement by surface texturing in LEDs is fundamental and cannot be exceeded even with index-matched optics. We further carry out numerical calculations accounting for realistic absorption in typical GaN-InGaN LEDs to compare their performance with the ergodic limit for non-absorbing structures. The results show that the optical power of InGaN-GaN LED designs can be improved by a substantial factor of 2–4 with textured surfaces and engineering of the emission pattern and provide a guideline for more efficient LED designs.

Optoelectronic Characteristics of Direct-Current and Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Silicon Nitride Film

Direct-current and alternating-current white thin- film light-emitting diodes (DCW and ACW TFLEDs) have been fabricated and demonstrated with the intrinsic hydrogenated amorphous silicon nitride (i-a-SiN:H) film as the luminescent layer. The achievable brightness of the representative DCW and ACW TFLEDs were 200 and 170 cd/m2 at an injection-current density of 600 and 100 mA/cm2, respectively. The electroluminescence (EL) threshold voltage of the DCW TFLED was 10.9 V, and its peak wavelength and full-width at half-maximum (FWHM) of the EL spectrum were about 455 and 230 nm, respectively. For the ACW TFLED, the EL threshold voltage was 8.4 V, and its peak wavelength and FWHM of the EL spectrum were about 535 and 260 nm, respectively. In addition, their current-conduction mechanism was also investigated. Within the lower applied-voltage region, they showed an ohmic current, while for the higher applied-voltage region, the Frenkel-Poole emission was the main mechanism. It was also found that the H2 -plasma treatment of luminescent i-a-SiN:H layer of an ACW TFLED played an important role in improving device performances, such as decreased EL threshold voltage, increased brightness, and broadened and blue-shifted EL spectrum. The EL spectra of an ACW TFLED under either DC forward or reverse bias or the sinusoidal AC voltage were qualitatively very similar, with a peak wavelength at about 535 nm and a broad FWHM about 260 nm. Moreover, the EL intensity of an ACW TFLED increased with an AC frequency of up to 180 kHz and, then, decreased rapidly and became very weak as the frequency was up to about 500 kHz.

Change in stress of GaN light-emitting diode films during the process of transferring the film from the Si(111) growth substrate to new substrate

InGaN multiple-quantum-well(MQW) light-emitting diode(LED) thin films were successfully transferred from the original Si(111) substrate to new Si substrate, and then the vertical structure LEDs were fabricated. After the original substrate removal, the residual tensile stress in GaN layer of the transfferred film is partially relaxed, while the compressive stress in InGaN well layer is increased. When the buffer layer of the transferred LED film was eliminated, the tensile stress in GaN layer was shown to increase. However, the compressive stress in InGaN well layer was kept unvaried. The performance of the vertical LEDs was significantly improved compared with the lateral LEDs.

Anomalous Poole–Frenkel Mode of Current-Conduction Mechanism in the P-I-N Thin-Film Light-Emitting Diodes

The current density-applied voltage (J-V) characteristics of the p-i-n thin film light-emitting diode (TFLED) were studied. It is found that when the applied voltage is less than the threshold voltage, the current densities of reverse and forward bias are essentially equal. This indicates that the current is limited by the i-layer, which has the highest resistance. In order to study the current-conduction mechanism in the i-layer, numerical approximation in a one-dimensional model is used to calculate the internal distributions of the potential, electric field, and carrier concentration, etc. The theory of anomalous Poole-Frenkel emission gives a calculated result, which agrees with the experimental result. The relationship between the implicated generation rate and radiative recombination rate with electric field is then analyzed. It is found that the electron impact ionization is the primary cause of electron-hole pairs production which upon recombination results in light emission in the i-layer. The current density was calculated by setting appropriate initial conditions for ionization. Radiative recombination rate needed to fit depends on the magnitude of electric field in the i-layer, being slower as the magnitude of electric field increases.

Solution-processed anodes from layer-structure materials for high-efficiency polymer light-emitting diodes.

The development of low-cost, large-area electronic applications requires the deposition of active materials in simple and inexpensive techniques at room temperature, properties usually associated with polymer films. In this study, we demonstrate the integration of solution-processed inorganic films in light-emitting diodes. The layered transition metal dichalcogenide (LTMDC) films are deposited through Li intercalation and exfoliation in aqueous solution and partially oxidized in an oxygen plasma generator. The chemical composition and thickness of the LTMDC and corresponding transition metal oxide (TMO) films are investigated by X-ray photoelectron spectroscopy. The morphology and topography of the films are studied by atomic force microscopy. X-ray powder diffraction is used to determine the orientation of the LTMDC film. Finally, the LTMDC and their corresponding oxides are utilized as hole-injecting and electron-blocking materials in polymer light-emitting diodes with the general structure ITO/LTMDC/TMO/polyfluorene/Ca/Al. Efficient hole injection and electron blocking by the inorganic layers result in outstanding device performance and high efficiency.

Optoelectronic characteristics of a -SiC:H-based pin thin film LEDshaving a thin Mo buffer layer in contact with p -type a -Si:H

To improve the electroluminescence (EL) properties of the a-SiC:H-based pin thin film light-emitting diodes (TFLEDs) with the promotion of hole injection efficiency, a thin Mo metal film was used as a buffer layer to prevent reactions between the ITO (indium-tin-oxide) electrode and the p+-a-Si:H layer. With the formation of semi-transparent Mo silicide after annealing, a lower EL threshold voltage and a significantly higher brightness for a finished TFLED were achieved. The brightness of the obtained device was 1300 cd/m2 at an injection current density of 600 mA/cm2, and its EL threshold voltage was 14 V.

Amorphous Visible-Light Thin Film Light-Emitting Diode Having a-SiN:H as a Luminescent Layer

Hydrogenated amorphous silicon nitride (a-SiN:H) has been applied for the first time as a luminescent active layer (i-layer) in an amorphous visible-light thin film light-emitting diode (TFLED). The TFLED has the structure of glass substrate/ITO/p a-SiC:H/i a-SiN:H/n a-SiC:H/Al. Visible red and yellow emissions can be observed at room temperature from the TFLEDs in which the optical energy gap of i-a-SiN:H is larger than 2.4 eV. The brightness of the red TFLED was 0.5 cd/m2, with a forward injection current density of 2000 mA/cm2 for the 0.033 cm2 cell area.