Maximum Internal Quantum Efficiency Research Articles

Enhanced carrier extraction of a-Si/c-Si solar cells by nanopillar-induced optical modulation

We demonstrate improved short-wavelength internal quantum efficiency (IQE) of a-Si/c-Si heterojunction (HJ) solar cells with a surface nanopillar (NP) array via simulation. The gain in IQE is attributed to the light-field modulation caused by the cavity resonance inside the NPs, in which the light energy is effectively localized within the c-Si bulk rather than the a-Si layer. The average IQE in the short-wavelength range (330–450 nm) is enhanced from 43.94% to 62.88% by the optimal NP array, with a maximum IQE of 80.98% at λ = 400 nm. The resulting current gain is over 38.25% compared to a planar HJ cell in this wavelength range, showing a well suppressed recombination-induced current loss. This light-management scheme may also find applications in other types of cells.

White emission of Yb^2+:fluoride glasses efficiently excited with near-UV light

Various Yb²⁺-containing fluoride glasses melting under a reductive atmosphere were prepared. The brightest white light emission was observed for an AlF₃-based fluoride glass not containing Hf or Zr. The largest full width at half maximum of the white emission spectra was 202 nm. In addition, incorporation of chloride into the AlF₃-based glass enabled efficient excitation with near-ultraviolet light corresponding to a GaN bandgap of 3.4 eV and the maximum internal quantum efficiency of Yb²⁺: AlF₃-based fluoride glass was 42%.

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

We present a convenient and reliable method for determining the internal quantum efficiency (IQE) in GaN-based blue light-emitting diodes (LEDs) grown on Si(111) substrates based on the carrier rate equation model. By using the peak point of the efficiency curve in photoluminescence (PL) measurements as the parameter of the rate equation analysis, the IQE can be unambiguously determined without any pre-assumed parameters. The theoretical IQE model is used to fit the measured PL efficiency curves and the IQE of LED samples are determined. The maximum IQE of the LED sample grown on the Si substrate was obtained to be 0.74, which is found to agree well with the results obtained by conventional temperature-dependent PL measurements.

Numerical Simulation of GaN-Based LEDs With Chirped Multiquantum Barrier Structure

The authors report the numerical simulation of GaN-based light-emitting diodes (LEDs) with either a conventional AlGaN electron blocking layer (EBL), uniform multiquantum barrier (UMQB) structure, or chirped multiquantum barrier (CMQB) structure. It is found that the 102-meV effective barrier height simulated from the LED with CMQB structure is larger than those simulated from the LEDs with a UMQB structure (90 meV) and with conventional AlGaN EBL (60 meV). With the large effective barrier height, it is found that LEDs with a CMQB structure exhibit smaller leakage current. It is also found that the maximum internal quantum efficiencies are 0.703, 0.842, and 0.887, for the LEDs with conventional EBL, UMQB structure, and CMQB structure, respectively. In addition, it is found that forward voltages simulated from the LEDs with CMQB structure and with UMQB structure are both smaller than that simulated from the LED with conventional AlGaN EBL. These results also agree well with the experimental data.

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

AbstractAuger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct synthesis of spherical YAG:Ce phosphor from precursor solution containing polymer and urea

In this study, spherical cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor particles were directly synthesized by a modified sol–gel method using poly(ethylene glycol) (PEG) and urea. In the absence of PEG and urea, only irregularly shaped particles were obtained through aggregation and sintering after solvent evaporation. In contrast, adding both PEG and urea to the precursor solution resulted in the formation of spherical particles. The spherical morphology was attributed to micellization by PEG and micelle agglomeration by urea in a liquid phase. Scanning electron microscopy and X-ray diffraction analyses revealed that the spherical particles were of size around 5μm, and the obtained crystal was pure a YAG phase. The emission band of the YAG:Ce phosphor prepared at 1600°C for 2h was observed at 530nm under excitation at 470nm, and the maximum internal quantum efficiency was found to be 57.6%.

A Twisting Donor‐Acceptor Molecule with an Intercrossed Excited State for Highly Efficient, Deep‐Blue Electroluminescence

AbstractIn an organic electroluminescent (EL) device, the recombination of injected holes and electrons produces what appears to be an ion‐pair or charge‐transfer (CT) exciton, and this CT exciton decays to produce one photon directly, or relaxes to a low‐lying local exciton (LE). Thus the full utilization of both the energy of the CT exciton and the LE should be a pathway for obtaining high‐efficiency EL. Here, a twisting donor‐acceptor (D‐A) triphenylamine‐imidazol molecule, TPA‐PPI, is reported: its synthesis, photophysics, and EL performance. Prepared by a manageable, one‐pot cyclizing reaction, TPA‐PPI exhibits deep‐blue emission with high quantum yields (90%) both in solution and in the solid state. Fluorescent solvatochromic experiments for TPA‐PPI solutions show a red‐shift of 57 nm (3032 cm−1) from low‐polarity hexane (406 nm) to high‐polarity acetonitrile (463 nm), accompanied by the gradual disappearance of the vibrational band in the spectra with increased solvent polarity. The photophysical investigation and DFT analysis suggest an intercrossed CT and LE excited state of the TPA‐PPI, originating from its twisting D‐A configuration. This is a rare instance that a CT‐state material shows highly efficient deep‐blue emission. EL characterization demonstrates that, as a deep‐blue emitter with CIE coordinates of (0.15, 0.11), the performance of a TPA‐PPI‐based device is rather excellent, displaying a maximum current efficiency of >5.0 cd A−1, and a maximum external quantum efficiency of >5.0%, corresponding to a maximum internal quantum efficiency of >25%. The effective utilization of the excitation energy arising from materials with intercrossed‐excited‐state (LE and CT) characters is thought to be beneficial for the improved efficiency of EL devices.

Emission Efficiency Dependence on the p-GaN Thickness in a High-Indium InGaN/GaN Quantum-Well Light-Emitting Diode

The dependencies of quantum-well (QW) internal quantum efficiency (IQE) and device behaviors on the p-layer thickness in a high-indium InGaN/GaN QW light-emitting diode (LED) are demonstrated. During the high-temperature growths of the p-AlGaN and p-GaN layers, the QWs are thermally annealed to increase their IQEs and blue-shift the emission with increasing p-layer thickness. Meanwhile, the quantum-confined Stark effect is enhanced with increasing p-layer thickness to decrease the IQEs and red-shift the emission. Based on the counteraction between the two effects, the maximum IQE and the shortest emission wavelength are observed in a sample with an optimized p-layer thickness, which includes a p-AlGaN layer of 20 nm and a p-GaN layer of 60 nm in thickness under our growth conditions. The fabricated LEDs of different p-GaN thicknesses show the similar variation trends in emission efficiency and wavelength.

GaInN‐based solar cells using GaInN/GaInN superlattices

AbstractWe report on the fabrication of GaInN‐based solar cells using GaInN/GaInN superlattices as active layers and also as underlying layers beneath the active layers. We obtained pit‐free surfaces, even with a high InN molar fraction, using the superlattices. As a result, the maximum external and internal quantum efficiencies reached 60%, and 88%, respectively. The open‐circuit voltage of the soalr cells was 1.77 V, the short‐circuit current density was 3.08 mA/cm2, and the fill factor was 70.3%. A conversion efficiency of 2.46% was achieved at room temperature under simulared 1.5 sun × AM1.5G illumination using a solar simulator. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on internal quantum efficiency of blue InGaN multiple-quantum-well with an InGaN underneath layer

Blue InGaN multiple-quantum-well (MQW) samples with different InxGa1−xN (x=0.01−0.04) underneath layers (ULs) were grown by metal organic vapor phase epitaxy (MOVPE). Temperature dependent photoluminescence showed that the InGaN UL can improve the internal quantum efficiency (IQE) of MQW effectively due to strain release. And a maximum IQE of 50% was obtained when the thickness and In content of the InGaN UL were 60 nm and 0.01, respectively. Furthermore, the larger In content or thickness of the InGaN UL makes the IQE lower. Arrhenius fit to the experiment data showed that the IQE fall was mainly caused by the quantity increase of the nonradiative recombination centers, which was believed related to the accumulated stress in InGaN ULs.

Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes

We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene—emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5×75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.

Role of triplet‐triplet annihilation in highly efficient fluorescent devices

Abstract— A study of delayed electroluminescence in model highly efficient OLEDs based on anthracene derivatives indicate that triplet‐triplet annihilation (TTA) contributes significantly to overall efficiency. Highly efficient devices (6–9% external quantum efficiencies) based on 9,10‐bis(2‐naphthyl)‐2‐phenylanthracene show that the TTA contribution depends primarily on operating current density, reaching as much as 20–30% of the overall emission intensity at moderate current densities (>5 mA/cm2). Revision of the classical estimates of maximum external quantum efficiency of fluorescent OLEDs to 8% and maximum internal quantum efficiency to 40% is recommended to account for TTA contribution (even further revision may be necessary to account for a better‐than‐20% optical outcoupling).

Quantum Efficiency and Surface Passivation Effect of Nanocrystalline Y2O3:Eu3+

Cubic nano-crystalline Y2O3:Eu3+ powders with different grain sizes were produced by chemical auto-combustion, and their structure, morphology, and fluorescent spectra were characterized. The quantum efficiency of the 5D0 level of Eu3+ was estimated, taking into account the energy transfer between Eu3+ ions located at C2 and S6 sites. Ag+ ions were introduced into the synthesis of the nanosized particles to modify the surface defects, resulting in increased emission intensity. These results indicated that the nanosized Y2O3:Eu3+ exhibits maximum internal quantum efficiency close to 90% after Ag+ ions are introduced into the synthesis of Y2O3:Eu3+. From the experimental results, it was concluded that the Ag+ ions are probably absorbed by the nanoparticle surface and do not enter the nanoparticle lattice. It was also found that the Ag+ ions can repair the surface defects and make the absorption of excitation light more efficient.

High-efficiency LEDs based on n-GaSb/p-GaSb/n-GaInAsSb/P-AlGaAsSb type-II thyristor heterostructures

A new type of light-emitting diodes (LEDs), a high-efficiency device based on an n-GaSb/p-GaSb/n-GaInAsSb/P-AlGaAsSb thyristor heterostructure, with the maximum emission intensity at wavelength λ = 1.95 μm, has been suggested and its electrical and luminescent characteristics have been studied. It is shown that the effective radiative recombination in the thyristor structure in the n-type GaInAsSb active region is provided by double-sided injection of holes from the neighboring p-type regions. The maximum internal quantum efficiency of 77% was achieved in the structure under study in the pulsed mode. The average optical power was as high as 2.5 mW, and the peak power in the pulsed mode was 71 mW, which exceeded by a factor of 2.9 the power obtained with a standard n-GaSb/n-GaInAsSb/P-AlGaAsSb LED operating in the same spectral range. The approach suggested will make it possible to improve LED parameters in the entire mid-IR spectral range (2–5 μm).

In Sb ∕ Al x In 1 − x Sb quantum-well light-emitting diodes with high internal quantum efficiencies

The properties of InSb∕AlxIn1−xSb quantum-well light-emitting diodes have been investigated as a function of temperature from 300to15K. Over the whole range, the peak emission occurred at significantly higher energies than the band gap of InSb but below the band gap of the AlxIn1−xSb barriers, confirming that emission is from the quantum wells. Maximum internal quantum efficiencies of 65% and 85% were measured at 15K for diodes containing 40 and 20nm quantum wells, respectively.

Infrared detection from GaInAs/InP nanopillar arrays

We report on the photoresponse from large arrays of 40 nm radius nanopillarswith sensitivity in the long-wavelength infrared regime. Using photoluminescencetechniques, a peak wavelength blue shift of approximately 5 meV was observed at 30 Kfrom GaInAs/InP nanopillar structures, indicating carrier confinement effects.Responsivity measurements at 30 K indicated peak wavelength response at about8 µm withresponsivity of 420 mA W−1 at −2 V bias. We have also measured the noise and estimated the peak detectivity to be3 × 108 cm Hz1/2 W−1 at 1 V reverse bias and 30 K. A maximum internal quantum efficiency of 4.5% was derivedfrom experiment. Both the photo and the dark transport have been successfully modelledas processes that involve direct and indirect field-assisted tunnelling as well as thermionicemission. The best agreement with experiment was obtained when allowances were madefor the non-uniformity of barrier widths and electric field heating of carriers above thelattice temperature.

Silicon-based light emission after ion implantation

Abstract Electroluminescence of boron and phosphorus implanted samples has been studied for various implantation and annealing conditions. Phosphorus implantation is found to have a similar effect on light emission as boron implantation. The band-to-band luminescence of phosphorus implanted diodes is observed to increase by more than one order of magnitude upon rising the sample temperature from 80 K to 300 K and a maximum internal quantum efficiency of 2% has been reached at 300 K. The remarkably high band-to-band luminescence is attributed to a high bulk Shockley–Read–Hall lifetime, likely promoted by the gettering action of the implanted phosphorus. The anomalous temperature behavior of the efficiency can be explained by a temperature dependence of the lifetime characteristic of shallow traps.

Effect of the postimplantation-annealing temperature on the properties of silicon light-emitting diodes fabricated through boron ion implantation into n-Si

The effect of the temperature of postimplantation annealing on the electroluminescence and the electrophysical and structural properties of light-emitting diodes fabricated by the implantation of boron ions into n-Si with a resistivity of 0.5 and 500 Ω cm is studied. All spectra contain strong electroluminescence (EL) peaks associated with band-to-band radiative transitions. An increase in the annealing temperature from 700 to 1100°C is accompanied by a monotonic increase in the quantum efficiency for the dominating EL peak and in the effective minority-carrier lifetime in the base of the light-emitting diodes and by the transformation of extended structural defects. Analysis of the experimental data shows that the extended structural defects formed are most likely to affect the EL properties via the formation or gettering of the radiative or nonradiative recombination centers rather than via preventing the removal of charge carriers to nonradiative recombination centers. The maximum internal quantum efficiency is reached after annealing at 1100°C (where extended structural defects are absent) and is estimated to be 0.4% at 300 K.

Silicon LEDs emitting in the band-to-band transition region: Effect of temperature and current strength

The parameters of silicon light-emitting diodes (LEDs) prepared through boron implantation into n-Si, followed by annealing at 700–1200°C, were studied. The maximum room-temperature internal quantum efficiency of electroluminescence (EL) in the region of band-to-band transitions was estimated as 0.4% and reached at an annealing temperature of 1100°C. This value did not vary more than twofold within the operating temperature range 80–500 K. The EL growth and decay kinetics was studied at various currents. Following an initial current range of nonlinear dependence, the EL intensity scaled linearly with the current. It is shown that interpretation of this result will apparently require a revision of some present-day physical concepts concerning carrier recombination in silicon diodes.

Internal quantum efficiency of stimulated emission of (λ=1.55 µm) InGaAsP/InP laser diodes

The stimulated emission (ηist) of InGaAsP/InP separate-confinement double heterostructure lasers operating at λ=1.5–1.6 µm has been studied experimentally and theoretically. Laser heterostructures with a varied design of the waveguide layer were grown by MOCVD. The maximum internal quantum efficiency ηist≈97% was obtained in a structure with a double-step waveguide characterized by minimum leakage into the p-emitter above the generation threshold. The high value of ηist is provided by low threshold and nonequilibrium carrier concentrations at the interface between the waveguide and p-emitter. The calculation yields ηist values correlating well with the experimental data.