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(a) Sketch map of the LED structure. (b) Schematic diagrams of structures of the lateral LED and roughened V-LED in 2D-FDTD simulations. (c) Schematic diagrams of structures of the lateral LED, planar V-LED, and V-LED with CBL in APSYS simulations.
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Light extraction efficiency (LEE) droop as an important factor contributing to the efficiency droop of InGaN-based light-emitting diodes (LEDs) has been demonstrated and investigated in detail. The LEE droop effect is induced by the spatial dependence of the extraction efficiency of photons inside of the LED devices and the aggravating crowding eff...
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Context 1
... a LED device, as shown in Fig. 1(a), the D-LEE at an injection current density of j can be expressed by an inte- gral formula as ...Context 2
... the integration is per- formed over the LED area. The xy plane is the in plane of InGaN MQWs. In this part, it is worthy to note that z axis, which is perpendicular to the xy plane, is not considered because the thickness of the InGaN MQWs is much smaller than the size at x or y direction. Moreover, as shown in Fig. 1(a y axes, leading to symmetry and equality at x/y direction in the integration process. Therefore, in order to reduce the computation, the formula above can be simplified by leaving y direction out ...Context 3
... this article, g LEE (x) is uncovered by optical simulations using two-dimensional finite difference time domain (2D- FDTD) method. Figure 1(b) shows structures of the lateral and vertical LEDs in the numerical simulation by using 2D- FDTD method. In lateral LEDs, metal p-n electrodes were both assumed to be a perfect absorber with a length of 10 lm and a thickness of 1 lm. ...Context 4
... roughened V-LEDs, GaN pyramids on the rough surface of the n-GaN layer were assumed to have a height of 1 lm. Some of the key parameters in 2D-FDTD sim- ulations have already been presented in Fig 1(b). In order to obtain the distribution of the light extraction possibility for the emitted photons at x, we chose 20, 40, and 40 dots along the x direction for lateral LEDs, planar V-LEDs, and rough- ened V-LEDs, respectively. ...Context 5
... have further made numerical simulations to analyze the distribution of the injection current density j(x) in InGaN MQWs by using APSYS Crosslight software. Figure 1(c) presents schematic diagrams of structures of the lateral LED, planar V-LED, and V-LED with current blocking layer (CBL) in APSYS simulations. In this part, the sizes of all the LEDs were assumed to be with an area of 200 Â 200 lm 2 . ...Similar publications
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This letter reports the properties of GaN-based green light-emitting diodes (LEDs) having a p-GaN photonic crystal layer with a photonic bandgap (PCWG) and without a photonic bandgap (PCOG). With decreasing the photoluminescence (PL) detection angle from 140° to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 2...
We have used time-resolved photoluminescence to examine InGaN/GaN multiple quantum wells (MQWs) and light-emitting diodes (LEDs) before the final stages of processing at room temperature. The photoluminescence kinetics are well described by a stretched exponential exp[−(t/τ)β], indicating significant disorder in the material. We attribute the disor...
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Citations
... Another technical route is to use ultra-large mesa thin-film LEDs (usually in several mm 2 scale), which aims to push the working current density to the lower end, where the droop effect could be significantly alleviated, in order to achieve higher quantum efficiency even at high current injection conditions [33,34]. However, with this method, the mesa size must be so greatly enlarged that it will induce severe current crowding effect, particularly on the p-side, leading to a degradation in both internal quantum efficiency and light extraction efficiency [35,36]. Recently, micro-LED (µ-LED) technology has been developing rapidly as it has great potentials in next-generation displays and widespread applications, yet there are still existing great challenges for µ-LEDs to achieve high efficiency due to severe Shockley-Hall-Read (SHR) recombination and surface recombination processes. ...
Versatile applications call for InGaN-based light-emitting diodes (LEDs) to operate at ultra-high current densities with high quantum efficiency. In this work, we investigated the size-dependent effects of the electrical and optical performance of LEDs as increasing the current density up to 100 A/cm2, which demonstrated that mini-strip flip-chip LEDs were superior option to achieve better performance. In detail, at a current density of 100 A/cm2, the light output power density of these mini-strip LEDs was improved by about 6.1 W/cm2, leading to an improvement in the wall-plug efficiency by 4.23%, while the operating temperature was reduced by 11.3 °C, as compared with the large-sized LEDs. This could be attributed to the increase in the sidewall light extraction, alleviated current crowding effect, and improved heat dissipation. This work suggests an array of mini-strip LEDs would provide an option in achieving higher luminescent efficiency at ultrahigh current injection conditions for various applications.
... Hence, the excessive generated heat is an unavoidable inherent characteristic of top-emitting LEDs, that causes the junction temperature in these LEDs to increase. As a result, the efficiency is reduced, which hinders the further development of high-power LEDs [8]. Thus, bottom-emitting LEDs, namely, vertical and flip-chip LEDs, have attracted considerable attention for having the ability to enhance both light extraction efficiency (LEE) and LOP as well as having better heat dissipation [9][10][11] due to being commonly bonded with a highthermal-conductivity submount (such as copper, silicon, or ceramic) that provides a thermal path for the generated heat. ...
The effect of tin-oxide (SnO) nanoparticles, which are obtained by indium-tin-oxide (ITO) treatment, on the p-GaN surface of GaN-based flip-chip blue micro-light-emitting diode (μ-LED) arrays is investigated. A thin Ag layer is deposited on the ITO-treated p-GaN surface by sputtering. SnO nanoparticles originate from inhomogeneous Schottky barrier heights (SBHs) at Ag/p-GaN contact. Therefore, effective SBH is reduced, which causes carrier transport into the μ-LED to enhance. 10 nm thick ITO-treated μ-LEDs show better optoelectronic characteristics among fabricated μ-LEDs owing to improved ohmic contact and highly reflective p-type reflectors. Basically, SnO nanoparticles help to make good ohmic contact, which results in improved carrier transport into μ-LEDs and thus results in increased optoelectronic performances.
... [7][8][9][10] The green top-emitting LEDs (TE-LEDs) grown on sapphire substrate suffer from severe heat dissipation. [11][12][13][14] It has been reported that flip-chip LEDs (FC-LEDs) can overcome the thermal issues. [15][16][17][18] Non-uniform current spreading, however, is still a challenging problem hindering further improvement in performance of green FC-LEDs at high current densities. ...
High-performance green flip-chip light-emitting diode (FC-LED) was realized by combining double-layer electrode and highly
reflective hybrid reflector. The introduction of double-layer electrode improved uniformity of current spreading over the entire active
region. The highly reflective hybrid reflector, composed of SiO2 total internal reflection (TIR) layer and Nb2O5/SiO2 distributed
Bragg reflector (DBR), offer higher reflectance and wider bandwidth compared to conventional DBR. Numerical simulation and
experimental results indicated that the green FC-LED has advantages of uniform current spreading and high light extraction efficiency
(LEE) compared to green top-emitting LED (TE-LED). Due to uniform current spreading, the forward voltage of the green FC-LED
is 0.28 V lower than that of the TE-LED at 20 mA. Compared to TE-LED, the light emission intensity of the green FC-LED at 20 mA
is enhanced by 18% due to the higher reflectance of hybrid reflector and increased utilization ratio of active region area caused by
double-layer electrode structure.
... LEE droop inherited from the CCE has been revealed by Cao, [14] Li, [15] and our group. [16] Various countermeasures have been adopted to increase the L s to boost the EQE, such as plasma treatment of the p-type GaN region underneath the bonding pad of the electrode, [17] the geometrical parameter optimization, [18][19][20] special chip design (such as interdigitated multipixel array (IMPA) chip design, [21] interdigitated mesa geometry design, [22] ) by incorporating transparent conductive layer (TCL), [23,26] tunnel junction, [27,28] or current blocking layer (CBL). [29,30] However, there is still a lack of direct and theoretical correlation linking CCE and EQE droop in these reports, and to date there are no reports to single out the relevant contribution of CCE to IQE, LEE, and ultimate EQE droop. ...
Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes (LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect (CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency (IQE), light extraction efficiency (LEE), and external quantum efficiency (EQE) droop of the lateral LEDs. However, questions still exist for the vertical LEDs (V-LEDs). Here firstly the current diffusion length L s (I) and L s (II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L s (III) was developed by combining L s (I) and L s (II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency (WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.
... A maximum current density of 20 kA/cm 2 can be reached at forward voltage 5.94 V. The specific series resistance extracted from J-V curves is about 9.7×10 −5 ·cm 2 , suggesting the excellent electrical property of chip [18], [19]. The maximum optical power of LED chip is 7.2 mW at 15 kA/cm 2 , which can be applicable for VLC in free space and improve the signal to noise ratio of the VLC system. ...
... The effect of self-heating can be alleviated by improving the device's energy efficiency, for example by modifying its epitaxial structure, by improving the current spread, or by enhancing its light extraction efficiency. [15][16][17][18][19] Besides, it is a necessary supplement for thermal management to improve heat dissipation as the limitation of nature properties of materials and processing techniques. ...
A strategy for fabricating bendable AlGaInP light emitting diode (LED) arrays is presented in this paper. Sample LED arrays with 8 × 8 pixels were fabricated and subjected to bending. Bending only weakly affected the light output power and the current–voltage characteristics of the arrays. LED arrays suffer from a thermal problem owing to the energy loss during the electrical-to-optical energy conversion. We have designed a three-dimensional heat conduction model for analyzing the effect of the polymer substrate, the configuration of pixels, and the micro-structure on heat dissipation in bendable LED arrays. Thermal conductivity of the polymer substrate critically affected the heat dissipation, suggesting that the substrate thickness should be in the 500–1000 μm range. A larger pixel distance yielded more distributed heat sources and more uniform temperature distribution. Micro-structured polymer substrates yielded lower temperature, especially for the fins array micro-structure. Based on enhancing the polymer’s thermal conductivity and distributing LED pixels, optimizing the substrate’s micro-structure is an effective method to improve heat dissipation in bendable LED arrays. Optimized heat dissipation could effectively reduce heat accumulation in LED arrays and alleviate an increase in the junction temperature, allowing to increase the output power of the device.
... In order to enhance the LED performances by solving the current crowding effect, various approaches have been attempted. The current crowding effect can be reduced by decrease of electrode grid (geometric pattern for electrodes) interval but the degradation of light output power is also inevitable as the increased area of electrodes that reflects light back into the chip [15]. Indium tin oxide (ITO) as highly transparent and conductive p-electrode can be applied to that effect but only for the relatively smaller LED chips [16]. ...
In this study, we report the enhanced light extraction efficiency of light emitting diode (LED) devices by employing multi-chip arrayed LED structures. We analyze the optoelectronic property of multi-chip arrayed LED structures with 16, 36 and 64 LED chips in the same 1 × 1 mm² device as well as 2 × 2 and 3 × 3 mm² devices. When compared with a conventional large area single-chip LED structure with the same device size, the multi-chip arrayed LED structures generally exhibit enhanced light extraction efficiency and electrical efficiency owing to the reduced current crowding effect. The effect of chip and device size on the LED performance is discussed.
... Furthermore, since the current tends to crowd under the metal electrode, whereas large percentage of the generated photons cannot be extracted due to the electrode absorption. This certainly has a negative effect on the LEE [14]. As we know, the EQE is the product of the IQE and the LEE, so the variation of current diffusion shall have a significant influence on the IQE, LEE and thus ultimately the EQE droop level. ...
... Cao et al. [18] and Ryu and Shim [19] have investigated the CCE on the LEE and efficiency droop of LEDs with the Monte Carlo ray tracing approach. By numerical simulations, we found that LEE decreases when the injection current increases due to the aggravating CCE of the carriers underneath the electrodes [14]. In addition, Huang et al. [20] have correlated the CCE with the electrical, optical and electrical-thermal properties of LEDs using a designed pattern with different current spreading distances. ...
... As the MQWs are relatively thin, we assume v keeps invariant. We have investigated the LEE distribution under and outside of the shadow both in L-LEDs and V-LEDs before [14] and found that most of the photons emitted right below the shadow are absorbed, therefore leading to an extremely low LEE. Photons neighboring to the shadow area can also be partially absorbed, and this results in a Gaussian distribution of the photon escaping possibility along the x axis. ...
By incorporating current crowding effect into the conventional ABC model, we theoretically and directly uncover the black box of the relationship between the current diffusion and the internal quantum efficiency, light extraction efficiency, and external quantum efficiency droop of the light emitting diodes (LEDs). A general formula for the current diffusion length (L s) has been derived. The design strategies on the current blocking layer and the transparent conductive layer to increase the L s have been analyzed. The impact of the vertical resistance (R s) on the LED wall-plug efficiency (WPE) has also been addressed. We have found there exists an optimized R s , and the LED WPE is not always monotonically increasing as the R s decreases. Besides, our previous experiment results have been summarized to verify the theoretical analysis. Index Terms— Efficiency droop, light emitting diodes, current crowding effect, current blocking layer, transparent conductive layer.
... Generally, the high carrier distribution under the electrode caused by the carrier-crowding effect results in highintensity light emission in the active region, which decreases the reliability of the devices. 18,19 The electroluminescence (EL) spectra of the AlGaInP LEDs with and without a hybrid film as a TCL at an injection current of 20 mA are presented in Fig. 3(b). Although there was 10% optical loss caused by the absorption of the hybrid film, the EL intensity of the AlGaInP LEDs with a hybrid film was greatly enhanced because of the efficient carrier injection to GaP and then the active region and the effective current spreading by the hybrid film. ...
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.
... Refs. [35][36][37][38][39][40][41]. The simulated lateral profile of the vertical component of the electron current density J n, y (x) in the present devices at a current I = 100 mA (J = 40 A/cm 2 ) is reported in Fig. 7, where a slightly stronger crowding may be observed for increasing temperature both in the LDD and HDD case. ...
Electroluminescence (EL) characterization of InGaN/GaN light-emitting diodes (LEDs), coupled with numerical device models of different sophistication, is routinely adopted not only to establish correlations between device efficiency and structural features, but also to make inferences about the loss mechanisms responsible for LED efficiency droop at high driving currents. The limits of this investigative approach are discussed here in a case study based on a comprehensive set of current- and temperature-dependent EL data from blue LEDs with low and high densities of threading dislocations (TDs). First, the effects limiting the applicability of simpler (closed-form and/or one-dimensional) classes of models are addressed, like lateral current crowding, vertical carrier distribution nonuniformity, and interband transition broadening. Then, the major sources of uncertainty affecting state-of-the-art numerical device simulation are reviewed and discussed, including (i) the approximations in the transport description through the multi-quantum-well active region, (ii) the alternative valence band parametrizations proposed to calculate the spontaneous emission rate, (iii) the difficulties in defining the Auger coefficients due to inadequacies in the microscopic quantum well description and the possible presence of extra, non-Auger high-current-density recombination mechanisms and/or Auger-induced leakage. In the case of the present LED structures, the application of three-dimensional numerical-simulation-based analysis to the EL data leads to an explanation of efficiency droop in terms of TD-related and Auger-like nonradiative losses, with a C coefficient in the 10(-30) cm(6)/s range at room temperature, close to the larger theoretical calculations reported so far. However, a study of the combined effects of structural and model uncertainties suggests that the C values thus determined could be overestimated by about an order of magnitude. This preliminary attempt at uncertainty quantification confirms, beyond the present case, the need for an improved description of carrier transport and microscopic radiative and nonradiative recombination mechanisms in device-level LED numerical models. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
