Radiative Recombination Efficiency Research Articles

Influence of Holes Capture Efficiency on Photoluminescence Temperature Dependence of n-AlGaAs/GaAs Quantum-Well Structures

The process of excess current carriers capture from wide-gap barrier layers into a quantum well plays a decisive role in the operation of devices based on semiconductor structures. Among these devices are lasers, light-emitting diodes, and photodetectors. The oscillating dependence of the capture rate on the quantum-well width is poorly experimentally studied. The purpose of this study is to clarify the influence of oscillations of holes capture efficiency on the temperature behavior of photoluminescence in modulation-doped n-AlGaAs/GaAs heterostructures. Temperature measurements of the integrated photoluminescence intensity of structures with different quantum-well widths corresponding to different capture rates of holes (under conditions of resonant and nonresonant capture) are carried out. It is shown that the efficiency of hole capture influences not only the integrated photoluminescence intensity, but its temperature dependence as well. In resonant structures at low temperatures (77–140 K), the photoluminescence intensity decreases with increasing temperature more sharply than that in structures with inefficient capture. Such a difference is attributed to the fact that, under resonance conditions, the capture rate of holes is so high that its change with temperature does not influence the photoluminescence intensity, and the temperature quenching of photoluminescence is defined only by a decrease in the efficiency of radiative recombination β in the quantum well. In nonresonant structures, the temperature behavior of photoluminescence depends not only on the quantum efficiency β, but also on the local hole capture rate which increases, as temperature is elevated. The results of this study are of interest for optimizing the parameters of heterostructures in designing of devices, for which the efficiency of the capture of nonequilibrium charge carriers into a quantum well plays a decisive role.

Efficient sky-blue perovskite light-emitting diodes via photoluminescence enhancement

The efficiencies of green and red perovskite light-emitting diodes (PeLEDs) have been increased close to their theoretical upper limit, while the efficiency of blue PeLEDs is lagging far behind. Here we report enhancing the efficiency of sky-blue PeLEDs by overcoming a major hurdle of low photoluminescence quantum efficiency in wide-bandgap perovskites. Blending phenylethylammonium chloride into cesium lead halide perovskites yields a mixture of two-dimensional and three-dimensional perovskites, which enhances photoluminescence quantum efficiency from 1.1% to 19.8%. Adding yttrium (III) chloride into the mixture further enhances photoluminescence quantum efficiency to 49.7%. Yttrium is found to incorporate into the three-dimensional perovskite grain, while it is still rich at grain boundaries and surfaces. The yttrium on grain surface increases the bandgap of grain shell, which confines the charge carriers inside grains for efficient radiative recombination. Record efficiencies of 11.0% and 4.8% were obtained in sky-blue and blue PeLEDs, respectively.

Advantages of AlGaN-based deep-ultraviolet light-emitting diodes with an Al-composition graded quantum barrier.

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.

Approaching the Intrinsic Limit in Transition Metal Diselenides via Point Defect Control.

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods, chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above 1013/cm2 to below 1011/cm2. Because these point defects act as centers for nonradiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

Optical light polarization and light extraction efficiency of AlGaN-based LEDs emitting between 264 and 220 nm

The influence of aluminum mole fraction of AlxGa1-xN/AlyGa1-yN multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated. The on-wafer EQE decreases from 0.6% to 0.00013% in this wavelength range. Polarization resolved photoluminescence and electroluminescence measurements show a change from dominant transverse-electric to dominant transverse-magnetic polarized light emission with increasing aluminum mole fraction in the MQW. The quantitative agreement with k·p calculations allow to ascribe this shift to a change of the characteristic of the Γ7+ valance band. Ray tracing simulations predict a reduction of the on-wafer LEE from 4% to 1.5%. Therefore the dramatic drop of the EQE in this wavelength range cannot be attributed to a drop in LEE and is most likely dominated by charge carrier injection and radiative recombination efficiency.

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification.

The poor stability and aggregation problem of CsPbBr3 quantum dots (QDs) in air are great challenges for their future practical application. Herein, a simple and effective ligand-modification strategy is proposed by introducing 2-hexyldecanoic acid (DA) with two short branched chains to replace oleic acid (OA) with long chains during the synthesis process. These two short branched chains not only maintain their colloidal stability but also contribute to efficient radiative recombination. The calculations show that CsPbBr3 QDs with DA modification (CsPbBr3 -DA QDs) have larger binding energy than CsPbBr3 QDs with OA (CsPbBr3 -OA QDs), resulting in significantly enhanced stability. Due to the strong binding energy between DA ligands and QDs, CsPbBr3 -DA QDs exhibit no aggregation phenomenon even after stored in air for more than 70 d, and CsPbBr3 -DA QDs films can maintain 94.3% of initial PL intensity after 28 d, while in CsPbBr3 -OA QDs films occurs a rapid degradation of PL intensity. Besides, the enhanced amplified spontaneous emission (ASE) performance of CsPbBr3 -DA QDs films has been demonstrated under both one- and two-photon laser excitation. The ASE threshold of CsPbBr3 -DA QDs films is reduced by more than 50% and their ASE photostability is also improved, in comparison to CsPbBr3 -OA QDs films.

Efficient Ruddlesden–Popper Perovskite Light‐Emitting Diodes with Randomly Oriented Nanocrystals

AbstractRuddlesden–Popper phase (RP‐phase) perovskites that consist of 2D perovskite slabs interleaved with bulky organic ammonium (OA) are favorable for light‐emitting diodes (LEDs). The critical limitation of LED applications is that the insulating OA arranged in a preferred orientation limits charge transport. Therefore, the ideal solution is to achieve a randomly connected structure that can improve charge transport without hampering the confinement of the electron–hole pair. Here, a structurally modulated RP‐phase metal halide perovskite (MHP), (PEA)2(CH3NH3)m−1PbmBr3m+1 is introduced to make the randomly oriented RP‐phase unit and ensure good connection between them by applying modified nanocrystal pinning, which leads to an increase in the efficiency of perovskite LEDs (PeLEDs). The randomly connected RP‐phase MHP forces contact between inorganic layers and thereby yields efficient charge transport and radiative recombination. Combined with an optimal dimensionality, (PEA)2(CH3NH3)2Pb3Br10, the structurally modulated RP‐phase MHP exhibits increased photoluminescence quantum efficiency, from 0.35% to 30.3%, and their PeLEDs show a 2,018 times higher current efficiency (20.18 cd A−1) than in the 2D PeLED (0.01 cd A−1) and 673 times than in the 3D PeLED (0.03 cd A−1) using the same film formation process. This approach provides insight on how to solve the limitation of RP‐phase MHP for efficient PeLEDs.

Fabrication and characterization of asymmetric metal/Ge/metal diodes with Ge-on-insulator substrate

We fabricated asymmetric metal/Ge/metal diodes with p-type Ge-on-insulator (GOI) and bulk Ge substrates, from which direct band gap electroluminescence (EL) was observed at room temperature. The integrated EL intensity for p-GOI diodes is approximately 16 times that for bulk p-Ge diodes, owing to the structure of GOI that confines carriers in the very thin Ge layer, and consequently enhances both the electron density in the conduction band and the efficiency of radiative recombination. Micro-photoluminescence spectra of p-GOI show defect-related peaks in the range of 0.70–0.75 eV, of which the intensity and energy depend on the measurement position, which implies fluctuation of defect type and density in p-GOI. Those defects contribute to the low-energy part of the broad EL spectra in p-GOI diodes.

III‐Nitride Nanowires as Building Blocks for Advanced Light Emitting Diodes

This article presents a discussion on the role of nanowire (NW) structures, prepared by reactive ion etching (RIE), in planar III‐nitride light emitting diodes (LEDs) for increasing the functionality, improving the crystalline quality, enhancing the quantum efficiency of radiative recombination, and improving the light extraction efficiency. The methods of NW fabrication by RIE, of suppressing the adverse effects of RIE‐related damage, and of incorporating air voids into the NWs are also discussed. Furthermore, metal nanoparticles producing localized surface plasmons and semiconductor quantum dots downconverting the wavelength of light emitted by LEDs are explored.

Highly efficient InGaN green mini-size flip-chip light-emitting diodes with AlGaN insertion layer

We demonstrate highly efficient InGaN-based mini-size green light-emitting diodes (mLEDs) with AlGaN insertion layer in InGaN/GaN multiple quantum wells (MQWs) using metal organic chemical vapor deposition (MOCVD). High resolution transmission electron microscopy (HRTEM) results reveal that ‘V’ defects within active region can be effectively reduced by AlGaN insertion layer. Photoluminescence (PL) and time resolved photoluminescence (TRPL) results indicate an increase of radiative recombination efficiency. Very high performance 523 nm InGaN green flip-chip mLEDs (0.025 mm2) with distributed Bragg reflector (DBR) show a high external quantum efficiency (EQE) of 38.0%, a high wall-plug efficiency (WPE) of 32.1% and a low forward voltage of 2.8 V at a working current density of 20 A cm−2, which are very promising for display application.

An Electronic Ratchet Is Required in Nanostructured Intermediate-Band Solar Cells

We investigate in this paper the intrinsic properties that have limited the efficiency of nanostructured intermediate-band solar cells. Those devices take advantage of intraband transitions, which occur on narrow energy width, and present low radiative recombination efficiency. We show that the combined effect of these two nonidealities strongly impacts the power conversion efficiency of intermediate-band solar cells. Minimum requirements in terms of those two characteristics to achieve efficiencies in excess of the Shockley–Queisser limit are computed, which show that compatible nanostructures are challenging to obtain. Especially, we evidence that currently experimentally considered materials cannot overcome the best single-junction cells. In order to solve those issues, we consider devices including an electronic ratchet mechanism. First, such devices are shown to be much less sensitive on the limitations of the nanostructures characteristics so that requirements for high efficiencies can be met. Second, we show that quantum well devices present advantages over their quantum dot counterparts, although they have attracted much less interest so far.

Andreev Reflection in a Superconducting Light-Emitting Diode.

We experimentally demonstrate Cooper-pair injection into a superconducting light-emitting diode by observing Andreev reflection at the superconductor-semiconductor interface, overcoming the contradicting requirements of an electrically transparent interface and radiative recombination efficiency. The device exhibits electroluminescence enhancement at the quasi-Fermi energy at temperatures below Tc. The theoretically predicted conductance and electroluminescence spectra based on Cooper-pair injection into the semiconductor correspond well to our experimental results. Our findings pave the way for practical superconductor-semiconductor quantum light sources.

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring

Organometal halide perovskites (OHP) are promising materials for low-cost, high-efficiency light-emitting diodes. In films with a distribution of two-dimensional OHP nanosheets and small three-dimensional nanocrystals, an energy funnel can be realized that concentrates the excitations in highly efficient radiative recombination centers. However, this energy funnel is likely to contain inefficient pathways as the size distribution of nanocrystals, the phase separation between the OHP and the organic phase. Here, we demonstrate that the OHP crystallite distribution and phase separation can be precisely controlled by adding a molecule that suppresses crystallization of the organic phase. We use these improved material properties to achieve OHP light-emitting diodes with an external quantum efficiency of 15.5%. Our results demonstrate that through the addition of judiciously selected molecular additives, sufficient carrier confinement with first-order recombination characteristics, and efficient suppression of non-radiative recombination can be achieved while retaining efficient charge transport characteristics.

Efficient Perovskite Light‐Emitting Diodes via Tuning Nanoplatelet Distribution and Crystallinity Orientation

AbstractQuasi‐2D Ruddlesden–Popper (RP) perovskites with quantum‐size‐tuned nanoplatelet crystallites are particularly attractive for light‐emitting diode (LED) applications because of their excellent charge transport, bandgap tunability, and narrow emission peaks. Favorable energy cascade structure in 2D RP perovskites can funnel the photoexcitation from large‐bandgap nanoplatelets to the lowest bandgap ones, which promises efficient radiative recombination. However, how to rationally control the nanoplatelet distribution as well as the crystallization characteristics to ensure efficient energy transfer and charge transport in perovskite films is still challenging. Here, a simple path to facilely tune the nanoplatelet distribution as well as the crystal orientation via carefully controlling postannealing condition is provided. With optimizing the thermal annealing time, compact and flat perovskite films with high photoluminescence quantum yield are obtained. Based on the optimized perovskite films, green LED with a current efficiency of 43.7 cd A−1, corresponding to an external quantum efficiency of 13.9%, is achieved. This work provides a simple path to target highly efficient perovskite LED devices.

Radiative and non-radiative recombination of thermally activated magneto-excitons probed via quasi-simultaneous photoluminescence and surface-photovoltage spectroscopy

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100 K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven high-temperature operation of advanced emission based-devices.

Self-Activated Low-Dimensional Metal Halide Phosphors and Scintillators

We report theoretical studies of exciton excitation and emission in low-dimensional metal halides, (C4N2H14X)4SnX6 (X = Br, I), Cs4PbBr6, (Ph4P)2SbCl5 (Ph4P = tetraphenylphosphonium), (C9NH20)2SnBr4, and (C4N2H14)PbCl4. These are bulk crystalline materials, in which isolated metal halide clusters/wires are separated from each other by large inorganic or organic countercations, forming 0D/1D structures. For example, in (C9NH20)2SnBr4, the anionic (SnBr4)2- clusters are separated from each other by (C9NH20)+ molecular countercations. The metal halides shown above all have 0D structures except (C4N2H14)PbCl4, which has a 1D wire structure. High photoluminescence quantum efficiencies (PLQE) (close to 100% in some cases) have been reported for the 0D metal halides. We calculated exciton excitation and emission energies in these low-dimensional metal halides; the results are in excellent agreement with the experimental values. This enables us to study the luminescence mechanisms and suggest new compounds for synthesis. We show that exciton self-trapping in the metal-halide cluster/wire leads to localized excitons, which is important for efficient radiative recombination. The relatively low PLQE (6% - 18%) in 1D (C4N2H14)PbCl4 is likely due to the exciton diffusion along the wire and the subsequent nonradiative recombination at defects. The large organic countercations in hybrid organic-inorganic 0D halides plays the important role of decoupling metal halide clusters (which are luminescent centers), leading to highly immobile excitons and high PLQE. On the other hand, in all inorganic 0D Cs4PbBr6, the significant electronic coupling between PbBr6 clusters and the small Stokes shift should enable resonant transfer of excitation energy and the subsequent energy loss at defects, which explains the strong thermal quenching of luminescence in Cs4PbBr6. We further suggest that Cs4EuX6 (X = Br,I), which are analogs to Cs4PbBr6, should exhibit improved luminescent properties compared to Cs4PbBr6 due to stronger exciton localization at Eu2+ than at Pb2+. Indeed, recent synthesis and characterization of these compounds show high light yields under gamma-ray irradiation at room temperature. In particular, the light yield of Cs4EuBr6 (78,000 photons/MeV) is the highest among the self-activated scintillators.

Influence of Bulky Organo‐Ammonium Halide Additive Choice on the Flexibility and Efficiency of Perovskite Light‐Emitting Devices

AbstractPerovskite light‐emitting diodes (LEDs) require small grain sizes to spatially confine charge carriers for efficient radiative recombination. As grain size decreases, passivation of surface defects becomes increasingly important. Additionally, polycrystalline perovskite films are highly brittle and mechanically fragile, limiting their practical applications in flexible electronics. In this work, the introduction of properly chosen bulky organo‐ammonium halide additives is shown to be able to improve both optoelectronic and mechanical properties of perovskites, yielding highly efficient, robust, and flexible perovskite LEDs with external quantum efficiency of up to 13% and no degradation after bending for 10 000 cycles at a radius of 2 mm. Furthermore, insight of the improvements regarding molecular structure, size, and polarity at the atomic level is obtained with first‐principles calculations, and design principles are provided to overcome trade‐offs between optoelectronic and mechanical properties, thus increasing the scope for future highly efficient, robust, and flexible perovskite electronic device development.

Lateral‐Polarity Structure of AlGaN Quantum Wells: A Promising Approach to Enhancing the Ultraviolet Luminescence

AbstractAluminum‐gallium‐nitride alloys (Al x Ga1– x N, 0 ≤ x ≤ 1) can emit light covering the ultraviolet spectrum from 210 to 360 nm. However, these emitters have not fulfilled their full promise to replace the toxic and fragile mercury UV lamps due to their low efficiencies. This study demonstrates a promising approach to enhancing the luminescence efficiency of AlGaN multiple quantum wells (MQWs) via the introduction of a lateral‐polarity structure (LPS) comprising both III and N‐polar domains. The enhanced luminescence in LPS is attributed to the surface roughening, and compositional inhomogeneities in the N‐polar domain. The space‐resolved internal quantum efficiency (IQE) mapping shows a higher relative IQE in N‐polar domains and near inversion domain boundaries, providing strong evidence of enhanced radiative recombination efficiency in the LPS. These experimental observations are in good agreement with the theoretical calculations, where both lateral and vertical band diagrams are investigated. This work suggests that the introduction of the LPS in AlGaN‐based MQWs can provide unprecedented tunability in achieving higher luminescence performance in the development of solid state light sources.

Solution processed ternary blend nano-composite charge regulation layer to enhance inverted OLED performances

Inverted bottom-emission organic light emitting diodes (IBOLEDs) have attracted increasing attention due to their exceptional air stability and applications in active-matrix displays. For gaining high IBOLED device efficiencies, it is crucial to develop an effective strategy to make the bottom electrode easy for charge injection and transport. Charge selectivity, blocking the carrier flow towards the unfavourable side, plays an important role in determining charge carrier balance and accordingly radiative recombination efficiency. It is therefore highly desirable to functionalize an interfacial layer which will perform many different tasks simultaneously. Here, we contribute to the hole-blocking ability of the zinc oxide/polyethyleneimine (ZnO:PEI) nano-composite (NC) interlayer with the intention of increasing the OLED device efficiency. With this purpose in mind, a small amount of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) was added as a hole-blocking molecule into the binary blend of ZnO and PEI solution. The device with a ternary ZnO:PEI:TPBi NC interlayer achieved a maximum current efficiency of 38.20 cd A−1 and a power efficiency of 34.29 lm W−1 with a luminance of 123 200 cd m−2, which are high performance parameters for inverted device architecture. The direct comparisons of device performances incorporating ZnO only, ZnO/PEI bilayers, and ZnO:PEI binary NC counterparts were also performed, which shed light on the origin of device performance enhancement.

Color Pure Green and Blue Electroluminescence Using Colloidal Quantum Confined Perovskites

Hybrid lead halide perovskites, described by a formula ABX3, recently emerged as a new class of semiconducting materials for various optoelectronic devices. Especially, colloidal perovskites exhibit numerous favorable properties, such as high photoluminescence quantum yield (PLQY), tunable bandgap, defect tolerance, high charge carrier mobility, large exciton binding energy (EB ), outstanding color purity (FWHM < 25 nm) and low cost of production, for high efficiency solution-processed light-emitting diodes (LEDs).[1] Analogously to conventional, inorganic quantum dots (QDs), quantum confinement effects could be observed upon forming colloidal perovskite nanoplatelets (NPLs) with thicknesses below ~10 unit cells (n < 10). Taking MAPbBr3 for example, we demonstrate a facile synthetic route to obtain monodisperse colloidal solutions of layered perovskites with precise control from n = 1 to n = 7-10 layers.[2] These layer controlled perovskite NPLs not only offer a color-tunable emission, but also enhance the EB . The resultant high EB is crucial to enable efficient radiative recombination of excitons. Most interestingly, our layer controlled NPLs also showed record high solid-state PLQYs (50-85%) for blue and green emission. With this approach we were able to demonstrate efficient pure-blue electroluminescence at room temperature using colloidal 2D perovskites. On the other hand, in order to meet the requirement for ultrapure green emission which targets the Rec.2020 recommendation, FAPbBr3 perovskite NPLs were utilized. Through dielectric quantum well (DQW) engineering high exciton binding energy of 162 mV was achieved which further resulted in superior PLQY value of 92%. The optimized active layer LEDs exhibited a maximum current efficiency of 13 cd/A but most importantly with its CIE 1931 color coordinates (0.168, 0.773) it represents the “greenest” LED ever reported.[3] Motivated by harsh market requirements, we were able to further improve the performance of ultrapure green device over 2-fold through compositional engineering. Using colloidal, ultrapure green emitting perovskite nanocrystals such result has never been reached so far. This opens an avenue towards low-cost and high-throughput production of solution-processed LEDs. [1] J. Jagielski, S. Kumar, W.-Y. Yu, C.-J. Shih, Journal of Materials Chemistry C 2017, 5, 5610. [2] S. Kumar, J. Jagielski, S. Yakunin, P. Rice, Y.-C. Chiu, M. Wang, G. Nedelcu, Y. Kim, S. Lin, E. J. G. Santos, M. V. Kovalenko, C.-J. Shih, ACS Nano 2016, 10, 9720. [3] S. Kumar, J. Jagielski, N. Kallikounis, Y.-H. Kim, C. Wolf, F. Jenny, T. Tian, C. J. Hofer, Y.-C. Chiu, W. J. Stark, T.-W. Lee, C.-J. Shih, Nano Letters 2017, 17, 5277.