Light Extraction Efficiency in OLED with Sub-wavelength Structures

By simulating with finite-difference time-domain(FDTD) method, it is proved that two kinds of new photonic crystal slab(PCS) structures could enhance the light extraction efficiency of OLED. By comparing the results, the most effective PCS structure with maximum light extraction efficiency(E_ r =1.99) is got. The optimized geometric parameters and optimized performance parameters of the PCS structures are also obtained.

Improving the Light Extraction Efficiency of GaN-Based Light-Emitting Diode

The light extraction efficiency caused by total internal reflection is low. Based on the analysis of the existing technology, a new design scheme is proposed in this paper to improve the light extraction efficiency. The air gap photonic crystal is embedded on the GaN-based patterned sapphire substrate, which can reduce line misalignment and improve light extraction efficiency. The internal structure of the GaN-based LED epitaxial layer is composed of an electron emission layer, a quantum well in the light-emitting recombination region, and an electron blocking layer. Experimental results show that this method significantly improves the extraction efficiency of LED light.

Realization of high-efficiency AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping of the p-AlGaN hole injection layer

We investigate the polarization-induced doping in the gradient variation of Al composition in the pAl_(0.75)Ga_(0.25)N/Al_xGa_(1-x)N hole injection layer(HIL)for deep ultraviolet light-emitting diodes(DUV-LEDs)with an ultrathin p-GaN(4 nm)ohmic contact layer capable of emitting 277 nm.The experimental results show that the external quantum efficiency(EQE)and wall plug efficiency(WPE)of the structure graded from 0.75 to 0.55 in the HIL reach 5.49%and 5.04%,which are improved significantly by 182%and 209%,respectively,compared with the structure graded from 0.75 to 0.45,exhibiting a tremendous improvement.Both theoretical speculations and simulation results support that the larger the difference between 0.75 and x in the HIL,the higher the hole concentration that should be induced;thus,the DUV-LED has a higher internal quantum efficiency(IQE).Meanwhile,as the value of x decreases,the absorption of the DUV light emitted from the active region by the HIL is enhanced,reducing the light extraction efficiency(LEE).The IQE and LEE together affect the EQE performance of DUV-LEDs.To trade off the contradiction between the enhanced IQE and decreased LEE caused by the decrease in Al composition,the Al composition in the HIL was optimized through theoretical calculations and experiments.

Tapering-induced enhancement of light extraction efficiency of nanowire deep ultraviolet LED by theoretical simulations

A nanowire （NW） structure provides an alternative scheme for deep ultraviolet light emitting diodes （DUV-LEDs） that promises high material quality and better light extraction efficiency （LEE）. In this report, we investigate the influence of the tapering angle of closely packed AIGaN NWs, which is found to exist naturally in molecular beam epitaxy （MBE） grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic （TM） and transverse elec- tric （TE） polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.

Enhancement of light extraction efficiency in OLED with two-dimensional photonic crystal slabs

Light extraction efficiency of organic light emitting diode （OLED） based on various photonic crystal slab （PCS） structures was studied. By using the finite-difference time-domain （FDTD） method, we investigated the effect of several parameters, including filling factor and lattice constant, on the enhancement of light extraction efficiency of three basic PCSs, and got the most effective one. Two novel designs of ＂interlaced＂ and ＂double-interlaced＂ PCS structures based on the most effective basic PCS structure were introduced, and the ＂interlaced＂ one was proved to be even more efficient than its prototype. Large enhancement of light extraction efficiency resulted from the coupling to leaky modes in the expended light cone of a band structure, the diffraction in the space between columns, as well as the strong scattering at indium-tinoxide/glass interfaces.

Evaluation of light extraction efficiency for the light-emitting diodes based on the transfer matrix formalism and ray-tracing method

The internal quantum efficiency（IQE） of the light-emitting diodes can be calculated by the ratio of the external quantum efficiency（EQE） and the light extraction efficiency（LEE）.The EQE can be measured experimentally,but the LEE is difficult to calculate due to the complicated LED structures.In this work,a model was established to calculate the LEE by combining the transfer matrix formalism and an in-plane ray tracing method.With the calculated LEE,the IQE was determined and made a good agreement with that obtained by the ABC model and temperature-dependent photoluminescence method.The proposed method makes the determination of the IQE more practical and conventional.

Analysis of photonic crystal on LED extraction efficiency

Because of its very low light extraction efficiency(LEE),LED is limited to be widely used under the condition of the internal quantum efficiency which up to 90%.In order to fullfill the design of a more efficient GaN-based blue light LED,the model including deeply etched surface photonic crystals(PhCs)LED is discussed using mode analysis method from light waveguide theory.The distributions of all order modes in GaN layer are obtained by the effective index approximation.The light extraction efficiencies are also calculated by finite-difference time-domain method(FDTD).The emulated results fully coincide with the former analysis.Because the manufacture of the surface photonic crystal is feasible,the work can be very meaningful to design and manufacture the high efficiency GaN-based blue light LED in factory for a large amount.

240 nm AlGaN-based deep ultraviolet micro-LEDs:size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated.Then,the external quantum efficiency(EQE)and light extraction efficiency(LEE)are systematically investigated by comparing size and edge effects.Here,it is revealed that the peak optical output power increases by 81.83%with the size shrinking from 50.0 to 25.0μm.Thereinto,the LEE increases by 26.21%and the LEE enhancement mainly comes from the sidewall light extraction.Most notably,transversemagnetic(TM)mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design.However,when it turns to 12.5μm sized micro-LEDs,the output power is lower than 25.0μm sized ones.The underlying mechanism is that even though protected by SiO2 passivation,the edge effect which leads to current leakage and Shockley-Read-Hall(SRH)recombination deteriorates rapidly with the size further shrinking.Moreover,the ratio of the p-contact area to mesa area is much lower,which deteriorates the p-type current spreading at the mesa edge.These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm,which will pave the way for wide applications of deep ultraviolet(DUV)micro-LEDs.

A 10×10 deep ultraviolet light-emitting micro-LED array

In this work,we design and fabricate a deep ultraviolet(DUV)light-emitting array consisting of 10×10 micro-LEDs(μ-LEDs)with each device having 20μm in diameter.Strikingly,the array demonstrates a significant enhancement of total light output power by nearly 52%at the injection current of 100 mA,in comparison to a conventional large LED chip whose emitting area is the same as the array.A much higher(~22%)peak external quantum efficiency,as well as a smaller efficiency droop forμ-LED array,was also achieved.The numerical calculation reveals that the performance boost can be attributed to the higher light extraction efficiency at the edge of eachμ-LED.Additionally,the far-field pattern measurement shows that theμ-LED array possesses a better forward directionality of emission.These findings shed light on the enhancement of the DUV LEDs performance and provide new insights in controlling the light behavior of theμ-LEDs.

Progress in research of GaN-based LEDs fabricated on SiC substrate

The influence of buffer layer growth conditions on the crystal quality and residual stress of GaN film grown on silicon carbide substrate is investigated. It is found that the A1GaN nucleation layer with high growth temperature can efficiently decrease the dislocation density and stress of the GaN film compared with A1N buffer layer. To increase the light extraction efficiency of GaN-based LEDs on SiC substrate, flip-chip structure and thin film flip-chip structure were designed and optimized. The fabricated blue LED had a maximum wall-plug efficiency of 72% at 80 mA. At 350 mA, the output power, the Vf, the dominant wavelength, and the wall-plug efficiency of the blue LED were 644 roW, 2.95 V, 460 nm, and 63%, respectively.

GaN-based high-voltage light-emitting diodes with backside reflector

High-voltage light-emitting diodes （HV-LED） with backside reflector, including Ti305/SiO2 distributed Bragg reflector （DBR） or hybrid reflector combining DBR and Al or Ag metal layer, are investigated using Monte Carlo ray tracing method. The hybrid reflector leads to more enhancement of light-extraction efficiency （LEE）. Moreover, the LEE can also be improved by redesigning the thicknesses of DBR. HV-LED with four redesigned DBR pairs （4-MDBR）, and those with a hybrid reflector combining 4-MDBR and Al metal layer （4-MDBR-Al）, are fabricated. Compared to 4-MDBR, the enhancement of light-output power induced by 4-MDBR-A1 is 4.6%, which is consistent with the simulated value of 4.9%.

Wafer-bonding AlGaInP light emitting diodes with pyramidally patterned metal reflector

We demonstrate and introduce here a pyramidally patterned metal reflector into wafer-bonding AlGaInP light emitting diodes （LEDs） to improve the light extraction efficiency by using a photo-assisted chemical etched GaP：Mg layer. The pyramid patterns were fabricated employing a HF and H202 mixed solution in combination with a 532 nm laser on a GaP：Mg surface firstly, and then a gold reflector layer was evaporated onto the patterned GaP：Mg surface. After the whole chip process, the patterned gold reflector structure was confirmed to be efficient for light extraction and a 18.55% enhancement of the electroluminescent flux has been obtained by an integrating sphere, compared to the surface textured LEDs with flat reflectors.

ICP dry etching ITO to improve the performance of GaN-based LEDs

In order to improve the light efficiency of the conventional GaN-based light-emitting diodes （LEDs）, the indium tin oxide （ITO） film is introduced as the current spreading layer and the light anti-reflecting layer on the p-GaN surface. There is a big problem with the ITO thin film＇s corrosion during the electrode preparation. In this paper, at least, the edge of the ITO film was lateral corroded 3.5μm width, i.e. 6.43%-1/3 of ITO film＇s area. An optimized simple process, i.e. inductively couple plasma （ICP）, was introduced to solve this problem. The ICP process not only prevented the ITO film from lateral corrosion, but also improved the LED＇s light intensity and device performance. The edge of the ITO film by ICP dry etching is steep, and the areas of ITO film are whole. Compared with the chip by wet etching, the areas of light emission increase by 6.43% at least and the chip＇s lop values increase by 45.9% at most.