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Internal Quantum Efficiency (internal + quantum_efficiency)

Distribution by Scientific Domains

Physics and Astronomy	54%
Polymers and Materials Science	41%

Selected Abstracts

Three-Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

ADVANCED FUNCTIONAL MATERIALS, Issue 15 2009
Jang Jo
Abstract Here, an investigation of three-dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post-treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (,IQE) of the devices. To investigate the 3D BHJ morphology, novel time-of-flight secondary-ion mass spectroscopy studies are employed along with conventional methods, such as UV-vis absorption, X-ray diffraction, and high-resolution transmission electron microscopy studies. The ,IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high-performance P3HT:PCBM solar cells using the optimized solvent-annealing method is reported, and these cells show a mean power-conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100,mW cm,2. [source]

Harvesting Excitons Via Two Parallel Channels for Efficient White Organic LEDs with Nearly 100% Internal Quantum Efficiency: Fabrication and Emission-Mechanism Analysis

ADVANCED FUNCTIONAL MATERIALS, Issue 1 2009
Qi Wang
Abstract By incorporating two phosphorescent dyes, namely, iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2,]picolinate (FIrpic) for blue emission and bis(2-(9,9-diethyl-9H -fluoren-2-yl)-1-phenyl-1H -benzoimidazol-N,C3)iridium(acetylacetonate) ((fbi)2Ir(acac)) for orange emission, into a single-energy well-like emissive layer, an extremely high-efficiency white organic light-emitting diode (WOLED) with excellent color stability is demonstrated. This device can achieve a peak forward-viewing power efficiency of 42.5,lm,W,1, corresponding to an external quantum efficiency (EQE) of 19.3% and a current efficiency of 52.8,cd,A,1. Systematic studies of the dopants, host and dopant-doped host films in terms of photophysical properties (including absorption, photoluminescence, and excitation spectra), transient photoluminescence, current density,voltage characteristics, and temperature-dependent electroluminescence spectra are subsequently performed, from which it is concluded that the emission natures of FIrpic and (fbi)2Ir(acac) are, respectively, host,guest energy transfer and a direct exciton formation process. These two parallel pathways serve to channel the overall excitons to both dopants, greatly reducing unfavorable energy losses. It is noteworthy that the introduction of the multifunctional orange dopant (fbi)2Ir(acac) (serving as either hole-trapping site or electron-transporting channel) is essential to this concept as it can make an improved charge balance and broaden the recombination zone. Based on this unique working model, detailed studies of the slight color-shift in this WOLED are performed. It is quantitatively proven that the competition between hole trapping on orange-dopant sites and undisturbed hole transport across the emissive layer is the actual reason. Furthermore, a calculation of the fraction of trapped holes on (fbi)2Ir(acac) sites with voltage shows that the hole-trapping effect of the orange dopant is decreased with increasing drive voltage, leading to a reduction of orange emission. [source]

Nearly 100% Internal Quantum Efficiency in an Organic Blue-Light Electrophosphorescent Device Using a Weak Electron Transporting Material with a Wide Energy Gap

ADVANCED MATERIALS, Issue 12 2009
Lixin Xiao
A blue-light electrophosphorescence with an internal quantum efficiency of nearly 100%, much higher than that of a stronger electron transporting (ET) material, was achieved with a weak ET silane. This result contradicts the conventional notion that weak ET materials result in low efficiency. [source]

Excimer-Based White Phosphorescent Organic Light-Emitting Diodes with Nearly 100,% Internal Quantum Efficiency,

ADVANCED MATERIALS, Issue 2 2007
L. Williams
Efficient excimer emission is demonstrated in white organic light-emitting diodes (see figure) based on platinum(II)[2-(4,,6,-difluorophenyl)pyridinato-N, C2,)](2,4-pentanedionato) utilized in devices incorporating the novel host material 2,6-Bis(N -carbazolyl)pyridine (26mCPy). External quantum (power) efficiencies of 15.9,% (12.6,lm,W,1) are realized at 500,cd,m,2. [source]

Bias dependence of internal quantum efficiency and carrier capture in a green (In,Ga)N single-quantum-well diode

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
Akihiro Satake
Abstract Internal quantum efficiency (IQE) and vertical capture processes of photogenerated carriers in a c -plane green (In,Ga)N single-quantum-well light-emitting-diode have been investigated by comparing variation in photoluminescence (PL) intensity as a function of applied voltage over a wide temperature range (T = 20-300 K) under direct (,ex =380 nm) and indirect (,ex = 325 nm) excitation. Under the direct excitation the PL intensity reflecting IQE shows a maximum value at +2.4 V irrespective of temperatures. However, it decreases to 23% from the maximum as temperature increases to 300 K. The PL intensity reduction observed at the optimized forward bias is much less as temperature increases under the indirect excitation. This difference observed between the two excitation conditions indicates an important role of efficient capture of photogenerated carriers into the active radiative recombination centers from the barriers under the presence of internal polarization fields. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Surface plasmon enhanced light emission from semiconductor materials

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008
Koichi Okamoto
Abstract Surface plasmon (SP) coupling technique was used to enhance light emissions from semiconductor nanocrystals with evaporated metal layers. We found that the SP coupling can increase the internal quantum efficiencies (IQE) of emission from CdSe-based nanocrystals regardless of the initial efficiencies. This suggests that this technique should be much effective for various materials that suffer from low quantum efficiencies. We also obtained 70-fold enhancement of emission from silicon nanocrystals in silicon dioxide. Obtained IQE value is 38%, which is as large as that of a compound semiconductor with direct transition. The SP coupling technique would bring a great improvement to silicon photonics. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

High quality, high efficiency and ultrahigh In-content InGaN QWs , the problem of thermal stability

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
D. Fuhrmann
Abstract InxGa1-xN/GaN quantum well (QW) structures with Indium concentrations above 30% suited for light emitters in the green and beyond have been investigated. The structures were optimized for homogeneous Indium distributions and abrupt interfaces. We obtained very high internal quantum efficiencies (IQE) of 80% and 70% for 460 nm and 510 nm emission wavelength, respectively. However, for high In concentrations the heterostructures are thermally less stable. This is evident from systematic studies including varied GaN cap temperatures and different post annealing procedures. For elevated temperatures we observe a reduction of the PL intensity, a broadening and a shift to higher energies of the PL lines without indication of phase separation. The reason is the soft indium-nitrogen bond, the degradation likely occurs by In interdiffusion or outdiffusion via defects in the structures. The critical temperatures are well below those typical for p-GaN contact layer growth and thus need to be considered in device applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Analysis of Ge junctions for GaInP/GaAs/Ge three-junction solar cells,

PROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 3 2001
D. J. Friedman
We study Ge solar cells with epitaxial GaInP windows for application as the third junction of GaInP/GaAs/Ge three-junction solar cells. We demonstrate Ge junctions with open-circuit voltages above 230,mV, fill factors above 65%, and internal quantum efficiencies of ,90%. By varying separately the base and emitter contributions to the junction dark current, we deduce the factors limiting the performance of this device, and we project the improvement to the device performance that may be obtainable if key limiting factors such as the emitter surface-recombination velocity can be mitigated. Published in 2001 by John Wiley & Sons, Ltd. [source]

Three-Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Analysis of Improved Efficiency of InGaN Light-Emitting Diode With Bottom Photonic Crystal Fabricated by Anodized Aluminum Oxidxe

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2009
Sang-Wan Ryu
Abstract The improved performance of a bottom photonic crystal (PC) light-emitting diode (LED) is analyzed based on internal quantum efficiency (,int) and light-extraction efficiency (,ex). The bottom PC is fabricated by anodized aluminum oxide nanopatterns and InGaN quantum wells (QWs) are grown over it. Transmission electron microscopy images reveal that threading dislocations are blocked at the nanometer-sized air holes, resulting in improved optical emission efficiency of the QWs. From temperature-dependent photoluminescence measurements, the enhancement of ,int is estimated to be 12%. Moreover, the enhancement of ,ex is simulated to be 7% by the finite-difference time-domain method. The fabricated bottom PC LED shows a 23% higher optical power than a reference, which is close to the summation of enhancements in ,int and ,ex. Therefore, the bottom PC improves LED performance through higher optical quality of QWs as well as increased light extraction. [source]

Triplet Harvesting in Hybrid White Organic Light-Emitting Diodes

ADVANCED FUNCTIONAL MATERIALS, Issue 9 2009
Gregor Schwartz
Abstract White organic light-emitting diodes (OLEDs) are highly efficient large-area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high-lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer. [source]

Design of Multilayered Nanostructures and Donor,Acceptor Interfaces in Solution-Processed Thin-Film Organic Solar Cells,

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2008
Hiroaki Benten
Abstract Multilayered polymer thin-film solar cells have been fabricated by wet processes such as spin-coating and layer-by-layer deposition. Hole- and electron-transporting layers were prepared by spin-coating with poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate) (PEDOT:PSS) and fullerene (C60), respectively. The light-harvesting layer of poly-(p -phenylenevinylene) (PPV) was fabricated by layer-by-layer deposition of the PPV precursor cation and poly(sodium 4-styrenesulfonate) (PSS). The layer-by-layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor,acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best-performance device with a triple-layered structure of PEDOT:PSS|PPV|C60, where the thickness of the PPV layer was 11,nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of ,3,V. The power conversion efficiency of the triple-layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100,mW,cm,2 in air. [source]

Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells

ADVANCED MATERIALS, Issue 30 2010
George F. Burkhard
Accurately measuring internal quantum efficiency requires knowledge of absorption in the active layer of a solar cell. The experimentally accessible total absorption includes significant contributions from the electrodes and other non-active layers. We suggest a straightforward method for calculating the active layer contribution that minimizes error by subtracting optically-modeled electrode absorption from experimentally measured total absorption. [source]

Nearly 100% Internal Quantum Efficiency in an Organic Blue-Light Electrophosphorescent Device Using a Weak Electron Transporting Material with a Wide Energy Gap

Efficiency droop in nitride-based light-emitting diodes

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2010
Joachim Piprek
Abstract Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. This feature article provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop. Illustration of LED efficiency droop (details in Fig. 13). [source]

Degradation of InGaN-based laser diodes due to increased non-radiative recombination rate

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2010
N. Trivellin
Abstract With this paper we analyze the correlation between the degradation of InGaN-based laser diodes (LDs) and the increase in the non-radiative recombination rate in the active region. Several 405,nm MOCVD LDs have been submitted to CW stress, for 2000,h (stress current in the range 40,100,mA, case temperature,=,75,°C). During stress, we extensively evaluated the optical characteristics of the LDs: a technique for the evaluation of the non-radiative recombination lifetime (,nr) in the active material was developed and used for the analysis of the stress effects. We demonstrate the following: (1) degradation determines the increase in LDs threshold current (Ith) and the decrease in the ,nr; (2) degradation of Ith and ,nr have similar kinetics; and (3) the degradation rate of the LDs is almost linearly related to the stress current level. The degradation process is therefore ascribed to the decrease of internal quantum efficiency caused by the increase of the non-radiative recombination rate in the active region. [source]

Performance of high-power III-nitride light emitting diodes

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2008
G. Chen
Abstract The performance of III-nitride based high-power light emitting diodes (LEDs) is reviewed. Direct color high-power LEDs with 1 × 1 mm2 chip size in commercial LUXEON® Rebel packages are shown to exhibit external quantum efficiencies at a drive current of 350 mA ranging from ,60% at a peak wavelength of ,420 nm to ,27% at ,525 nm. The short wavelength blue LED emits ,615 mW at 350 mA and >2 W at 1.5 A. The green LED emits ,110 lm at 350 mA and ,270 lm at 1.5 A. Phosphor-conversion white LEDs (1 × 1 mm2 chip size) are demonstrated that emit ,126 lm of white light when driven at 350 mA and 381 lm when driven at 1.5 A (Correlated Color Temperature, CCT , 4700 K). In a similar LED that employs a double heterostructure (DH) insign instead of a multi-quantum well (MQW) active region, the luminous flux increases to 435 lm (CCT , 5000 K) at 1.5 A drive current. Also discussed are experimental techniques that enable the separation of internal quantum efficiency and extraction efficiency. One technique derives the internal quantum efficiency from temperature and excitation-dependent photoluminescence measurements. A second technique relies on variable-temperature electroluminescence measurements and enables the estimation of the extraction efficiency. Both techniques are shown to yield consistent results and indicate that the internal quantum efficiency of short wavelength blue (, , 420 nm) high-power LEDs is as high as 71% even at a drive current of 350 mA. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Formation of InGaN quantum dots in regularly arranged GaN nanocolumns grown by rf-plasma-assisted molecular-beam epitaxy

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2010
Hiroto Sekiguchi
Abstract InGaN quantum dots (QDs) were successfully integrated at the apex of each pyramid-topped GaN nanocolumn. Various nanocolumn arrays with different nanocolumn diameters arranged in a triangular lattice were prepared on GaN templates by Ti-mask selective-area growth (SAG) with rf-plasma-assisted molecular-beam epitaxy (rf-MBE). The photoluminescence (PL) emission wavelength from the InGaN QDs shifted from 477 to 516 nm with increasing nanocolumn diameter from 206 to 326 nm. From the Arrhenius plot of PL integrated intensity, the PL internal quantum efficiency (IQE) was evaluated to be 48.4% for the 516-nm-wavelength sample. Threading dislocations at the bottom region of the nanocolumns were bent toward the sidewall and did not propagate to the upper active layer, which contributed to the higher PL efficiency. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of temperature distribution and current crowding on the performance of lateral GaN-based light-emitting diodes

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010
Dongpyo Han
Abstract Current crowding effect is detrimental for the performance of light-emitting diodes (LEDs), causing non-uniform light emission and local heat generation. In particular, heat generated by non-uniform current distribution can badly influence the performance of LED devices. In this paper, we examine the temperature distributions of lateral InGaN/GaN multiple-quantum-well LEDs in relation to current crowding, using both simulation and experimental results. Simulation results are obtained from a 3-dimensional electrical circuit model consisting of resistances and intrinsic diodes. Temperature and luminance distributions are investigated by images taken by an infrared camera and a charge-coupled-device camera, respectively. Finally, the internal quantum efficiency is taken for each device and compared. We show that the thermal property in the lateral LED is affected by the current crowding due to the local Joule heating nearby electrodes. Therefore, uniform current spreading is very important not only for uniform luminance distribution but also for good thermal property in the LED device. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Internal parameters and optical properties of green II-VI heterostructure lasers with active region composed of multi-sheet electronically-coupled CdSe quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2010
Aliaksei G. Vainilovich
Abstract Optically pumped green-emitting ZnSe-based laser heterostructures of optimized design have been grown by molecular-beam epitaxy and studied. The structures containing five electronically-coupled stacked CdSe quantum dot (QD) sheets demonstrate the minimum threshold power density of 2.2 kW/cm2 and the maximum external quantum efficiency of , 50%. Internal laser characteristics were determined by measuring the differential laser efficiency and laser threshold of a series of samples with different cavity lengths. The characteristic gain and internal quantum efficiency of the structures have been shown to reach the values as high as ,G0 = 114 cm -1 and ,i = 65.4%, while the transparency threshold and internal losses are evaluated as low as IT = 1.22 kW/cm2 and ,i = 2.55 cm -1, respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Bias dependence of internal quantum efficiency and carrier capture in a green (In,Ga)N single-quantum-well diode

Optimizing the internal quantum efficiency of GaInN SQW structures for green light emitters

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006
D. Fuhrmann
Abstract GaxIn1,xN/GaN single quantum well (QW) structures emitting in the range of 450 nm to 620 nm have been grown by MOVPE. Temperature and excitation power dependent photoluminescence (PL) was used to determine the internal quantum efficiency (IQE) for these structures. For the blue emitting QWs high IQE values on the order of 60% were achieved. Due to a reduced growth temperature, reduced growth rate and increased V/III ratio we obtained QWs with good morphology and high In content above 25%. Thinner QWs with high In content showed a clear improvement of IQE compared to QW-structures with larger thickness but smaller In-content emitting at the same wavelength. Between ,peak = 460 nm and 530 nm we observed a slight reduction in IQE with values of 58% at 490 nm and 40% at 525 nm. But towards ,peak = 620 nm IQE decreased due to the electric field induced separation of the electron and hole wavefunction down to 1%. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Defect density dependence of luminescence efficiency and lifetimes in AlGaN active regions exhibiting enhanced emission from nanoscale compositional inhomogeneities

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006
G. A. Garrett
Abstract AlGaN epilayers grown by plasma-assisted molecular beam epitaxy and exhibiting high internal quantum efficiency (up to 30%) are incorporated into double-heterostructure devices grown on base layers of varying defect density. Growth of these AlGaN active layers, having increased emission from localization of carriers in regions of nanoscale compositional inhomogeneities, is found to benefit from base layers of reduced defect density, including thick AlGaN templates grown by hydride vapor phase epitaxy. Nonlinear radiative processes are observed at high optical excitation for layers grown on lower defect base layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]