Quantum Efficiency Of Light-emitting Diodes Research Articles

Simultaneously improve the Purcell factor and internal quantum efficiency of light-emitting diodes via surface plasmon by metal conic structure

Recent advances in the development of surface plasmons (SPs) enhanced LED have provided a great opportunity to enhance either the internal quantum efficiency (IQE) or the spontaneous emission rate (SE) by employing specific metal structures. However, it is still challenging to simultaneously achieve high IQE and Purcell factor (Fp), which demonstrates the SE enhancement, without sacrificing the electrical performance of LEDs. Herein, we designed and investigated a conic metal structure applied to LEDs by comprehensively considering the electrical, optical, and data transmission performance of devices. Conic structures with various heights were implemented to investigate the variation trends of IQE and Fp with the structure design, accompanied by the planar structures as references. A more than five times increase in IQE and almost five times increase in Fp were demonstrated experimentally even with a coupling distance of 100 nm, by employing this conic structure. The theoretical analysis was verified by the experimental results and revealed the mechanism of high Fp and IQE toward high SP–photon coupling efficiency and initial IQE. This study provides a universal strategy to enhance the performance of luminous efficiency and modulation speed of LEDs without sacrificing electrical properties, making them viable for the integration of lighting, display, and communication.

Assessing the Drawbacks and Benefits of Ion Migration in Lead Halide Perovskites.

Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing.

Enhancement in external quantum efficiency of light-emitting diode based on colloidal silicon nanocrystals

Herein, we report an enhanced red emission from colloidal silicon nanocrystals (c-Si NCs) solution-processed light-emitting diode. c-Si NCs were synthesized by facile femtosecond laser ablation. Based on the structural characterization and opto-electrics properties analysis, both photoluminescence and electroluminescence arise from the radiative recombination of carriers due to quantum confined effect. The optical power density and highest external quantum efficiency have been obtained to be 0.79 mW cm−2 and ∼6.6%, respectively. These results indicate that Si NCs are very attractive as a potential optical source for future integrated chips.

Влияние внутренних отражателей на эффективность инфракрасных (850 нм) светодиодов

Investigated are IR light-emitting diodes with 850 nm radiation wavelength, based on AlGaAs/GaAs heterostructures grown by the method of MOC-hydride epitaxy with a Bragg reflector and additional “reflecting” layer Al0.9Ga0.1As, which ensures the decrease of optical losses of the generated radiation. Developed is the post growth technology for forming frontal ohmic contacts and for texturing the light-emitting surface, which ensures the decrease of ohmic losses and the increase of the radiation extraction efficiency from a crystal. External quantum efficiency of light-emitting diodes with two internal reflectors and surface texturing exceeded 9% at the current range 0.1-1.4A

Estimating internal quantum efficiency of light-emitting diodes from current–voltage curves

This work presents an approach to estimate the internal quantum efficiency (IQE) of commercial visible light-emitting diodes (LEDs) with the I–V characteristics. A narrow range of I–V curve, where the ideality factor is minimum, is chosen to be fitted by a parallel model of different current components. Then the IQE in this current range is calculated as the proportion of radiative recombination current and used as a reference to deduce the IQE from the external quantum efficiency. With this method, it founds that the peak IQE of blue and red LEDs exceeds 90% while that of green ones exceeds 60%.

Insight into the physical mechanism of AlxGa1−xN electron blocking layer in GaN-based light emitting diodes

The effect of AlxGa1−xN electron blocking layer (EBL) on suppressing electron leakage from the multiple quantum wells (MQWs) active region has been systematically investigated by conducting the first-principles calculations. Our results revealed that the Al concentration in EBL plays a dominant role in modulating the band offsets of AlxGa1−xN/GaN(0001) (0 < x ≤ 1) heterointerfaces. The existence of charge accumulation and band bending at these heterointerfaces can be attributed to the strong polarization electric field with the order of MV/cm. We further demonstrated that the EBL can suppress electron leakage from the active region as well as prevent the hole injection from the p-GaN region. Lastly, to boost the quantum efficiency of light-emitting diodes (LEDs), we proposed a “synergistic effect” of Al concentration in EBL and In-content in MQWs. These results provided a fundamental insight into the physical mechanism of the AlxGa1−xN EBL to reduce the efficiency droop in GaN-based LEDs.

Measuring the internal quantum efficiency of light-emitting diodes: towards accurate and reliable room-temperature characterization

Abstract For accurate and reliable measurement of the internal quantum efficiency (IQE) of light-emitting diodes (LEDs), the method should be theoretically solid and experimentally simple to use without any prior assumption of physical parameters or complicated equipment. In this paper, we critically review the conventional characterization techniques for measuring the IQE of LEDs, including the methods based on temperature-dependent electroluminescence and constant AB(C) models. After reviewing the limitations of the existing IQE measurement techniques, we present the recently proposed method based on the improved AB model, called room-temperature reference-point method (RTRM). The RTRM is then applied to various LED devices to show how the IQE measurement techniques can be utilized to analyze their optoelectronic performances quantitatively.

Measuring the Internal Quantum Efficiency of Light-Emitting Diodes at an Arbitrary Temperature

We present a method of measuring the internal quantum efficiency (IQE) of light-emitting diodes (LEDs) as a function of driving current at an arbitrary temperature only with experimentally measurable quantities of optical power and current. The measurement procedure starts to find an exact value of the IQE at the reference point where the injection efficiency is considered as 100% and the radiative efficiency is calculated from the infinitesimal change of the relative external quantum efficiency (EQE). Once the IQE at the reference point is exactly known, the IQE at any other point is calculated by the relative ratio of the EQE values to the reference one. We show the theoretical formalism of the proposed method and demonstrate its validity using a commercial InGaN blue LED as an example. The comparison with the IQE obtained by the conventional temperature-dependent electroluminescence is also given for various temperatures.

Engineering charge transport by heterostructuring solution-processed semiconductors

Solution-processed semiconductor devices are increasingly exploiting heterostructuring — an approach in which two or more materials with different energy landscapes are integrated into a composite system. Heterostructured materials offer an additional degree of freedom to control charge transport and recombination for more efficient optoelectronic devices. By exploiting energetic asymmetry, rationally engineered heterostructured materials can overcome weaknesses, augment strengths and introduce emergent physical phenomena that are otherwise inaccessible to single-material systems. These systems see benefit and application in two distinct branches of charge-carrier manipulation. First, they influence the balance between excitons and free charges to enhance electron extraction in solar cells and photodetectors. Second, they promote radiative recombination by spatially confining electrons and holes, which increases the quantum efficiency of light-emitting diodes. In this Review, we discuss advances in the design and composition of heterostructured materials, consider their implementation in semiconductor devices and examine unexplored paths for future advancement in the field. Heterostructuring and mixing of disparate materials provide new degrees of freedom to control carrier mobilities and exciton binding in solution-processed semiconductors. This Review highlights recent examples of heterostructured materials and devices, and examines the future direction of the field.

The relationship between the defectness of emitting nanoheterostructures of green InGaN/GaN LEDs and their threshold current values

It is shown that the threshold current of green InGaN/GaN light-emitting diodes (LEDs) can be used for evaluating quality of their light-emitting nanoheterostructures.. The threshold current exhibits correlation with the value of operating current corresponding to the maximum of the external quantum efficiency of LEDs. It has been found that LEDs with high threshold currents tested in the dc regime exhibit faster degradation than devices with lower threshold currents.

V-pits as Barriers to Diffusion of Carriers in InGaN/GaN Quantum Wells

The luminescence characteristics of V-pits in InGaN/GaN quantum wells (QW) correlated directly with the microstructure of the V-pits, as studied by use of transmission electron microscopy with cathodoluminescence. {10-11}-Faceted V-pits, formed in the QW, produce more intense blue-shifted emission than {0001}-plane QW. A dead emission center seems to be present at the corner of the V-pit which connects the R-plane and C-plane QW. High-resolution transmission electron microscopy revealed formation of indium-deficient QW at the corners of the V-pits. High potential barriers occur because of the lack of indium around the hexagonal V-pit; this effectively blocks diffusion of carriers into the threading dislocations known to be non-radiative recombination centers. V-pits thus have promise for improving the internal quantum efficiency of light-emitting diodes.

Evaluation of the internal quantum efficiency in light-emitting diodes

A new evaluation method for the internal quantum efficiency (IQE) in light-emitting diode (LED) packages is presented. In order to derive this IQE model, we introduce the thermal resistance concept composed of the effective internal optical power in LED packages. This IQE evaluation method needs precise measurements of the thermal properties, as well as the electrical and the optical properties in the LED packages. Also, numerical simulations of the internal thermal gradients in the LED packages, based on computational fluidic dynamics, should be included to evaluate the IQEs in LED packages. A high-power ceramic LED package was developed for the experiment on the IQEs, and we measured the IQEs of LED packages at various operation conditions and compared the results with those obtained by the conventional electroluminescence (EL) method. Our method is a simple EL-based method with thermal insight onto LED packages. Contrary to conventional EL methods, our method makes no assumption of the carrier injection efficiency and does not require a very low current injection level in the LED packages.

Comparison of Strain in GaN-Based Blue Light-Emitting Diode Grown on Silicon(111) and Sapphire Substrates.

We compare the strain states and device performances of GaN-based blue light-emitting diodes (LEDs) grown on Si(111) and sapphire substrates. The strain characteristics are investigated using micro-Raman spectroscopy and high-resolution transmission electron microscopy. These analyses reveal that GaN layer grown on Si has a residual tensile strain in contrast to a compressive strain for GaN on sapphire, and quantum wells (QWs) on GaN/Si experience reduced lattice mismatch than those of GaN/sapphire. When external quantum efficiencies of LED on sapphire and Si substrates are compared, the LED on Si shows better efficiency droop characteristics and this is attributed to a decrease in piezo-electric field strength in InGaN/GaN layers owing to reduced lattice mismatch.

Droop Improvement in InGaN/GaN Light-Emitting Diodes by Polarization Doping of Quantum Wells and Electron Blocking Layer

-Polarization doping in quantum wells and electron blocking layer is proposed to improve the emission intensity and efficiency droop of InGaN light-emitting diodes (LEDs). Band diagrams and the internal quantum efficiencies of LEDs are theoretically studied by the ATLAS simulation program. Numerical results show that the internal quantum efficiency of polarization doped LED structures are improved by 25% as compared to un-doped structures which can be due to accumulation of high density two-dimensional electron gas in quantum wells.

Strong modification of spontaneous emission rate in nanorod light-emitting diode structures.

This study investigates the characteristics of modifications in spontaneous emission (SE) from GaN-based nanorod light-emitting diode (LED) structures using the three-dimensional finite-difference time-domain method. The simulated nanorod LED structure is assumed to be enclosed by perfect conductors and includes InGaN multiple-quantum-well active layers emitting at 500 nm. In the simulation, the modification of the SE rate is calculated as the structural parameters of nanorods are varied. The SE rate is found to depend strongly on the radius of nanorods. Large enhancement of the SE rate is observed when the resonant modes are formed inside the nanorod cavity. For both the transverse-electric and transverse-magnetic modes, the SE rate of nanorod LED structures can be enhanced by more than six times when the radius or height of the nanorod is optimally chosen. This large increase in the SE rate can lead to considerable increase in the internal quantum efficiency of LEDs, making nanorod structures prime candidates for future high-efficiency LEDs than can overcome the efficiency limit of current LED structures.

Proposal of New Measurement Method for Internal Quantum Efficiency of Light-Emitting Diodes

A new evaluation method for effective internal optical power (IOP) and internal quantum efficiency (IQE) of light-emitting diodes (LEDs) is demonstrated. This method is based on the optical and thermal properties of LEDs. By using this proposed method, the effective IOP and the IQE of LEDs could be directly extracted from the measurements of external optical power (EOP) and junction temperature of LEDs. This method needs no assumption of the injection efficiency of carriers in the LEDs and no measurement-condition limitation of low current-injection level.

A study of LED with surface photonic crystal structure fabricated by the nanoimprint lithography

Surface photonic crystal (PC) structure can improve the external quantum efficiency of light-emitting diode (LED). However, it is very difficult to fabricate large area and uniform nanometer photonic crystal structure in this field. In this paper, two-dimensional PC with hole-like structure is successfully transferred to the surface of gallium nitride LED (GaN-LED) by the mental-polymer double-layer mask dry etching technology combined with the nanoimprint lithography. The large area nanometer PC patterns with pore diameter of 240 nm and period of 450 nm are obtained. The results show that the photoluminescence peak intensity of LED with the PC structure is 7.2 times higher than that of the conventional LED.

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.

Analysis of Current Components and Estimation of Internal Quantum Efficiency in Light-Emitting Diodes

We have decomposed light-emitting diode (LED) current into component currents by identifying the ideality factor of each component and by fitting the measured and simulated current-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ), <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dI</i> )- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> and the shape of the relative luminescence efficiency-current characteristics. Nearly unity ideality factor of the radiative recombination current is found from luminescence-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) characteristics in AlGaInP red and GaInN green and blue LEDs. The current-component analysis enables an estimation of the internal quantum efficiency (IQE) as a function of current. Compared with red LEDs, blue LEDs suffer from lower radiative recombination efficiency and much larger efficiency droop to result in lower IQE.

The use of short-period InGaN/GaN superlattices in blue-region light-emitting diodes

Optical and light-emitting diode structures with an active InGaN region containing short-period InGaN/GaN superlattices are studied. It is shown that short-period superlattices are thin two-dimensional layers with a relatively low In content that contain inclusions with a high In content 1–3 nm thick. Inclusions manifest themselves from the point of view of optical properties as a nonuniform array of quantum dots involved in a residual quantum well. The use of short-period superlattices in light-emitting diode structures allows one to decrease the concentration of nonradiative centers, as well as to increase the injection of carriers in the active region due to an increase in the effective height of the AlGaN barrier, which in general leads to an increase in the quantum efficiency of light-emitting diodes.