Electron Blocking Effect Research Articles

Demonstrating the electron blocking effect of AlGaN/GaN superlattice cladding layers in GaN-based laser diodes

Electron leakage currents seriously limit the power conversion efficiencies (PCEs) of gallium nitride (GaN)-based laser diodes (LDs). To minimize the leakage currents, electron blocking layers are generally applied in the p-type region. However, few works have discussed the electron blocking effect of a p-cladding layer, which is found to be critical in suppressing the leakage currents of an LD. In this work, we compare the blocking performance of uniform AlGaN p-cladding layers and AlGaN/GaN superlattice (SL) p-cladding layers with the same average Al component respectively. Both light-emitting diodes (LEDs) and LDs with the same epitaxy structures are characterized by light–current (L–I) and current–voltage (I–V) measurements. The latest analytical model of leakage currents is applied to fit the L–I curves of LEDs, where smaller leakage coefficients are observed in the SL structures compared with the uniform-layer structures. Eighty LDs with varying ridge widths are studied by comparing the threshold current densities, slope efficiencies, and PCEs. The SL-based p-cladding layer shows statistically significant advantages over a uniform AlGaN layer. The blocking effects of both scattering- and bound-state electrons in SLs are investigated theoretically. Repetitive reflection and thermal relaxation are responsible for the blocking effect of scattering-state electrons. Simulation results indicate that the tunneling effect of bound-state electrons through a miniband mechanism is insignificant at a large injection level due to a negative differential conductivity by the Esaki–Tsu effect. We demonstrate a better electron blocking performance of p-cladding layers based on SLs than uniform AlGaN layers in GaN-based LDs.

Thermally Conductive and Electrically Insulated Silicone Rubber Composites Incorporated with Boron Nitride-Multilayer Graphene Hybrid Nanofiller.

Thermally conductive and electrically insulating composites are important for the thermal management of new generation integrated and miniaturized electronic devices. A practical and eco−friendly electrostatic self−assembly method was developed to prepare boron nitride−multilayer graphene (BN−MG) hybrid nanosheets. Then, BN−MG was filled into silicone rubber (SR) to fabricate BN−MG/SR composites. Compared with MG/SR composites with the same filler loadings, BN−MG/SR composites exhibit dramatically enhanced electrical insulation properties while still maintaining excellent thermal conductivity. The BN−MG/SR with 10 wt.% filler loading shows a thermal conductivity of 0.69 W·m−1·K−1, which is 475% higher than that of SR (0.12 W·m−1·K−1) and only 9.2% lower than that of MG/SR (0.76 W·m−1·K−1). More importantly, owing to the electron blocking effect of BN, the electron transport among MG sheets is greatly decreased, thus contributing to the high−volume resistivity of 4 × 1011 Ω cm for BN−MG/SR (10 wt.%), which is fourorders higher than that of MG/SR (2 × 107 Ω·cm). The development of BN−MG/SR composites with synergetic properties of high thermal conductivity and satisfactory electrical insulation is supposed to be a promising candidate for practical application in the electronic packaging field.

Compositionally graded AlGaN hole source layer for deep-ultraviolet nanowire light-emitting diode without electron blocking layer

The electron blocking layer (EBL) plays a vital role in blocking the electron overflow from an active region in the AlGaN-based deep-ultraviolet light-emitting diode (DUV-LED). Besides the blocking of electron overflow, EBL reduces hole injection toward the active region. In this work, we proposed a DUV nanowire (NW) LED structure without EBL by replacing it with a compositionally continuous graded hole source layer (HSL). Our proposed graded HSL without EBL provides a better electron blocking effect and enhanced hole injection efficiency. As a result, optical power is improved by 48% and series resistance is reduced by 50% with 4.8 V threshold voltage. Moreover, graded HSL without EBL offer reduced electric field within the active region, which leads to a significant increment in radiative recombination rate and enhancement of spontaneous emission by 34% at 254 nm wavelength, as a result, 52% maximum internal quantum efficiency with 24% efficiency drop is reported.

Numerical study on the electron-blocking effect and optimized operation parameters of ceria-SOFCs with the pure Sm doping CeO2 electrolyte

Ceria-based electrolytes have been widely used for low temperature solid oxide fuel cells (SOFCs) owing to their high conductivity, however, the reduction of Ce4+ to Ce3+ increases the electronic conductivity, leading to the internal short circuit. Herein, a numerical model considering the charge transport of electron and oxygen-ion in the pure SmxCe1-xO2-δ electrolyte for ceria-SOFCs was built, to evaluate the electron-blocking effect and optimized the operation parameters. The effects of the electrolyte thickness, operating temperature and Sm-doping content in the pure SmxCe1-xO2-δ electrolyte on the electrochemical performance are carefully evaluated in terms of I–P curves, open circuit voltage, leakage current density, oxygen partial pressure distribution and cell efficiency. The simulation results are found that the leakage current density can be greatly decreased with the reducing operating temperature and increasing electrolyte thickness, thus leading to high open circuit voltage and cell efficiency. Accordingly, 20%mol Sm doping in SmxCe1-xO2-δ electrolyte has the highest open circuit voltage and cell efficiency, resulting in that the smallest leakage current and the optimal power density can be reached. Our numerical results suggest that the optimized operation parameters of ceria-SOFCs can effectively reduce the leakage current density and increase the electrochemical performance.

Electronic anti‐injection effect for carbonyl in anchor group based on diphenylacetylene D 2 ‐π‐A sensitizer in dye‐sensitized solar cells

Up to now, in developing organic molecules for dye-sensitized solar cells, the determining role of acceptors has not yet received enough attention. Herein, three novel sensitizers were prepared with D2-π-A structure, following cyanoacetic acid, acetic acid, and rhodanine-3-acetic acid as the acceptors, respectively. It was found that the thiocarbonyl and carbonyl groups, which are separated by two atoms with carboxyl group, effectively promoted molecular light harvesting, intramolecular charge transfer, and improved energy level matching, but a significant electron-blocking effect occurred, which limited the enhancement of photoelectric conversion performance with rhodanine-3-acetic acid. From PT-1 to PT-3, the power conversion efficiency decreased sharply from 5.27% to 2.70%. As a result, an electronic anti-injection effect of electron-drawing unit in the anchor group compared with cyanoacetic acid was found for the development of organic sensitizer design.

Artemisinin (ART)-Induced “perovskite/perovskite” bilayer structured photovoltaics

Halide perovskites have attracted significant attention due to their high photovoltaic (PV) performance. The transition of perovskite PVs into practical applications will require significantly higher reliability while maintaining high efficiency. Here we provide breakthroughs in achieving this goal of enhanced efficiency and reliability. A medical molecular additive, artemisinin (ART), is found to result in the spontaneous formation of a “perovskite/perovskite” bilayer structure during the film casting process. The top-perovskite-layer shows up-shifted Fermi level resulting in an excellent electron blocking effect (EBE) responsible for a high open-circuit-voltage. Further, this bilayer structure enhances hydrophobicity due to the nanoscale wrapping by ART molecules. Combined, “perovskite/perovskite” bilayer results in reliable PV cell with simultaneously improved efficiency (20.43% versus 18.28% from control cell) and stability (95% efficiency was preserved after one-month ambient exposure). These promising results demonstrate that medical biomolecule engineered PVs could pave a new perspective to reliable PV implementation.

Improving the Performance and Stability of Perovskite Light-Emitting Diodes by a Polymeric Nanothick Interlayer-Assisted Grain Control Process.

CsPbBr3 is a promising light-emitting material due to its wet solution processability, high photoluminescence quantum yield (PLQY), narrow color spectrum, and cost-effectiveness. Despite such advantages, the morphological defects, unsatisfactory carrier injection, and stability issues retard its widespread applications in light-emitting devices (LEDs). In this work, we demonstrated a facile and cost-effective method to improve the morphology, efficiency, and stability of the CsPbBr3 emissive layer using a dual polymeric encapsulation governed by an interface-assisted grain control process (IAGCP). An eco-friendly low-cost hydrophilic polymer poly(vinylpyrrolidone) (PVP) was blended into the CsPbBr3 precursor solution, which endows the prepared film with a better surface coverage with a smoothened surface. Furthermore, it is revealed that inserting a thin PVP nanothick interlayer at the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/emissive layer interface further promotes the film quality and the performance of the derived LED. It is mainly attributed to three major consequences: (i) reduced grain size of the emissive layer, which facilitates charge recombination, (ii) reduced current leakage due to the enhanced electron-blocking effect, and (iii) improved color purity and air stability owing to better defect passivation. As a result, the optimized composite emissive film can retain the luminescence properties even on exposure to ambient conditions for 80 days and ∼62% of its initial PL intensity can be preserved after 30 days of storage without any encapsulation.

GaN/InAlN chirped short period superlattice as strain free top cladding for blue laser diode

This work presents the theoretical study on strain-free chirped short-period superlattices (C-SPSLs) as the top waveguide core and cladding layers for InGaN laser diodes (LDs) emitting at 450 nm. The total 530 nm thick layers of reference LD containing 100 nm p-GaN waveguide, 30 nm Al0.15Ga0.85N electron blocking layer (EBL) and Al0.065Ga0.935N cladding layers are replaced by 300 nm thick C-SPSL which contains the 33-periods of 10 monolayers (ML) GaN/ 2 ML In0.18Al0.82N, 48-periods of 6 ML GaN/2 ML In0.18Al0.82N, and 97-periods of 2 ML GaN/ 2 ML In0.18Al0.82N. The optical confinement factor of reference LD is 3.28% whereas for C-SPSL LD it is 3.42%. The better optical confinement at reduced thickness using C-SPSL is due to the high refractive index contrast. The strain in the C-SPSL is negligible. The electron leakage has reduced from 2.54 kA cm−2 to 0.166 kA cm−2 at ∼10 kA cm−2 injected current density, whereas the hole transportation has improved by 2.31 kA cm−2. The C-SPSL configuration also favors the electron blocking effect, thus EBL is not required in C-SPSL design. The light output power at a current injection of 500 mA (10 kA cm−2) is 248 mW for the new structure and ∼143 mW for reference LD. The slope efficiency is improved from ∼0.55 to ∼0.89 W A−1. Also, the dynamic resistance from I–V characteristic is ∼1.12 Ω which is lowered to ∼0.94 Ω.

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.

Efficiency enhancement in quantum dot light-emitting devices employing trapping-type electron buffer layer

Abstract Core/shell quantum dots (QDs) always favor electron injection more than hole injection because of the deeper energy levels. Therefore, it is still challenging to achieve highly efficient quantum dot light-emitting devices (QLEDs) needing excellent carrier injection balance. Here, efficient QLEDs employing 4,4,4-tris(N-Carbazolyl)-triphenylamine (TCTA) doped with phosphorescent material tris(2-phenylpyridine) iridium [Ir(ppy)3] as trapping-type electron buffer layer are reported. Compared with conventional devices, efficiency enhancement reaching to 89% can be observed in such QLEDs. It is proved that the electron trap and blocking effect can efficiently improve carrier injection balance without sacrificing the operating voltage and luminance, mainly resulting in the considerable efficiency enhancement.

Measuring the nonlocality of different types of Majorana bound states in a topological superconducting wire

According to the degree of topological protection, Majorana bound states (MBSs) can be divided into three types: ideal zero-energy MBSs (IZMs), finite-energy MBSs (FEMs) and zero-energy MBSs at parity crossing points (PZMs). Herein, we investigate the nonlocality of these three types of MBSs by comparing the conductance spectra of a normal lead–topological superconducting wire–normal lead (NSN) junction and an NS junction. We find that for the FEM-related tunnelling process, the decrease in the nonlocal processes is trivially accompanied by an increase in the local processes, whereas for the IZM-related tunnelling process, the left and right tunnelling processes are completely independent. Remarkably, PZMs induce a nonlocal electron-blocking effect in which incoming electrons from the left lead cannot participate in local Andreev reflection unless the right lead is present, even though no nonlocal tunnelling processes occur in the right lead of an NSN junction. We show that this PZM-mediated nonlocal electron-blocking effect is due to the nonlocal coupling of the left lead to the more distant PZM and that the phase difference between the two end PZMs is . Our findings provide an experimentally accessible method for characterizing MBSs by probing their different nonlocal signatures.

Experimental and numerical studies of a bifunctional proton conducting anode of ceria-based SOFCs free from internal shorting and carbon deposition

A proton conducting anode NiO-BaZr0.1Ce0.7Y0.2O3-δ (BZCY) is investigated to play a dual role in avoiding internal shorting and carbon deposition of ceria-based SOFCs by experimental and numerical methods. The electronic barrier layer BaxCe1−ySmyO3-δ (BCS) can be in-situ formed at the interface of anode and electrolyte, which improves the open-circuit voltage (OCV) and the peak power density. To deeply analyze the electron-blocking effect and optimize the thickness ratio of the BCS layer, electronic conductivities, leakage current densities and cell efficiencies considering the internal shorting and polarization are calculated by a mathematic model. Importantly, this ceria-based cell with the proton conducing anode also shows superior electrochemical performance and stability using wet methane as fuel. A thermodynamic equilibria calculation considering the water adsorption capacity of the anode is performed to predict the equilibrium composition and carbon deposition boundaries, which quantitatively explains the excellent carbon-tolerance of the cell. The superior bifunctional proton conducting anode enables ceria-based electrolytes to be promising electrolyte candidates toward the application of low temperature SOFCs and provides an efficient path for avoiding carbon deposition.

Highly efficient solution-processed phosphorescent organic light-emitting devices with double-stacked hole injection layers

In this paper, solution-processed nickel oxide (NiOx) is used as hole-injection layers (HILs) in solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). Serious exciton quenching is verified at the NiOx/emitting layer (EML) interface, resulting in worse device performance. The device performance is significantly improved by inserting a layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) between the EML and NiOx. The solution-processed blue PhOLED with the double-stacked NiOx/PEDOT:PSS HILs shows a maximum current efficiency of 30.5 cd/A, which is 75% and 30% higher than those of the devices with a single NiOx HIL and a PEDOT:PSS HIL, respectively. Improvement of device efficiency can be attributed to reducing exciton quenching of the PEDOT:PSS layer as well as the electron blocking effect of the NiOx layer.

Enhanced performance of SubPc planar heterojunction solar cells with a novel luminescent sensitizer of 4,4′-bis[(N-carbaz ole)styryl]biphenyl

Here, the conventional luminescent material 4,4′-bis[(N-carbazole) styryl] biphenyl (BSB-Cz) is used as luminescent sensitizer in boron subphthalocyanine chloride (SubPc)/C60 planar heterojunction (PHJ) solar cells for the first time. When inserting 3-nm BSB-Cz between the MoO3 and donor layer, the power conversion efficiency (PCE) of SubPc/C60 PHJ is improved from 1.21% to 1.72%. BSB-Cz at 3–8nm all enhances the short-circuit current (JSC) and open-circuit voltage. As BSB-Cz is capable of absorbing short-wavelength solar light and re-emitting long-wavelength light which is exactly located at the absorption wavelength range of SubPc and C60. The JSC increase is attributed to the electron-blocking effect of BSB-Cz together with matched photoluminescence spectrum of BSB-Cz and the absorption spectra of SubPc as well as C60, which enhances the incident photon number for SubPc and C60. AFM images indicate that the bumpy surface of 3-nm BSB-Cz is filled in by the subsequent SubPc layer, thus decreases the surface roughness. However, thicker BSB-Cz results in significantly increased RMS roughness with decreased fill factor. The thickness of BSB-Cz maybe decides the only donor or two competing donors at the donor/acceptor interface, which affects the exciton disociattion and recombination in the overall device.

Ultrathin Al2O3 interface achieving an 11.46% efficiency in planar n-Si/PEDOT:PSS hybrid solar cells

Hybrid organic–inorganic photovoltaic devices consisting of poly(3,4-etyhlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and n-type silicon have recently been investigated for their cost-efficiency and ease of fabrication. We demonstrate that the insertion of an ultrathin Al2O3 layer between n-Si and PEDOT:PSS significantly improves photovoltaic performance in comparison to the conventional interfacial oxide employing SiO2. A power-conversion efficiency of 11.46% was recorded at the optimal Al2O3 thickness of 2.3 nm. This result was achieved based upon increased built-in potential and improved charge collection via the electron blocking effect of Al2O3. In addition, the hydrophilicity enhanced by Al2O3 improved the coating uniformity of the PEDOT:PSS layer, resulting in a further reduction in surface recombination.

Readily synthesized dopant-free hole transport materials with phenol core for stabilized mixed perovskite solar cells

Abstract With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PVSCs), device lifetime has become one of the extensive research interests and concerns. To enhance the device durability, developing high performance dopant-free hole transport materials (HTMs) is a promising strategy. Herein, two new C3 -symmetric HTMs with phenol core, TCP-OH and TCP-OC 8 are readily prepared and show ultra-wide energy band-gap and excellent film-formation property. PCEs of 16.97% and 15.28% are achieved with pristine TCP-OH and TCP-OC 8 film as HTMs, respectively, even though their hole mobilities are as low as 10 −6 cm 2 V −1 s −1 . Phenol acts as hole trap in traditional concept, however, TCP-OH shows higher hole mobility than that of TCP-OC 8 . Moreover, TCP-OH shows higher glass transition temperature and better matching band alignment than those of TCP-OC 8 . Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs. Moreover, our study demonstrates an interesting viewpoint to design HTMs with the balance of hole mobility and electron blocking effect.

Identifying the influence of GaN/InxGa1-xN type last quantum barrier on internal quantum efficiency for III-nitride based light-emitting diode

GaN/InxGa1-xN-type last quantum barrier (LQB) proves to be useful for Ⅲ-nitride based light-emitting diode (LED) in enhancing the internal quantum efficiency (IQE) and suppressing the efficiency droop level that often takes place especially when the injection current is high. In this work, GaN/InxGa1-xN-type LQB reported by the scientific community to enhance the IQE is first reviewed and summarized. Then, the influences of indium composition and thickness of the InxGa1-xN layer on the performance of LED incorporated with the GaN/InxGa1-xN-type LQB are studied. Through analyzing energy band diagrams calculated with APSYS, we find that the[0001] oriented LQB features an electron depletion due to the polarization induced negative charges at the GaN/InxGa1-xN interface. The electron depletion enhances the electron blocking effect and reduces the electron accumulation at the InxGa1-xN/AlGaN interface, leading to an improved IQE for the LED. In addition, increasing the indium composition of the InxGa1-xN layer will generate more negative interface charges, which result in further increased conduction band barrier height for the electrons and reduced electron leakage. On the other hand, for the GaN/InxGa1-xN-type LQB with a fixed indium composition, there exists an optimum thickness for the InxGa1-xN layer in maximizing the improvement of IQE for the LED, mainly because the interaction between two mechanisms co-exists when varying the thickness of the InxGa1-xN layer, i.e., the initial increase in the InxGa1-xN layer thickness will lead to an increased conduction band barrier height, which prevents electrons from leaking into the InxGa1-xN layer. However, further increasing the InxGa1-xN layer thickness to a certain value, tunneling effect will kick in as a result of the simultaneously reduced GaN thickness-the electrons will tunnel through the thin GaN layer in the LQB from the quantum wells to the InxGa1-xN layer. This will cause electrons to increase in the InxGa1-xN layer. Therefore, as a result of the interaction between the above-mentioned two mechanisms, there is an optimum thickness for the InxGa1-xN layer such that the electrons in the InxGa1-xN layer will reach a minimal value, which in turn will lead to a maximized conduction band barrier height for the AlGaN electron blocking layer and facilitate the performance of LEDs.

Interface Engineering of Organic Schottky Barrier Solar Cells and Its Application in Enhancing Performances of Planar Heterojunction Solar Cells.

In this work, we describe the performance of organic Schottky barrier solar cells with the structure of ITO/molybdenum oxide (MoOx)/boron subphthalocyanine chloride (SubPc)/bathophenanthroline (BPhen)/Al. The SubPc-based Schottky barrier solar cells exhibited a short-circuit current density (Jsc) of 2.59 mA/cm2, an open-circuit voltage (Voc) of 1.06 V, and a power conversion efficiency (PCE) of 0.82% under simulated AM1.5 G solar illumination at 100 mW/cm2. Device performance was substantially enhanced by simply inserting thin organic hole transport material into the interface of MoOx and SubPc. The optimized devices realized a 180% increase in PCE of 2.30% and a peak Voc as high as 1.45 V was observed. We found that the improvement is due to the exciton and electron blocking effect of the interlayer and its thickness plays a vital role in balancing charge separation and suppressing quenching effect. Moreover, applying such interface engineering into MoOx/SubPc/C60 based planar heterojunction cells substantially enhanced the PCE of the device by 44%, from 3.48% to 5.03%. Finally, we also investigated the requirements of the interface material for Schottky barrier modification.

Peripheral Hole Acceptor Moieties on an Organic Dye Improve Dye-Sensitized Solar Cell Performance.

Investigation of charge transfer dynamics in dye‐sensitized solar cells is of fundamental interest and the control of these dynamics is a key factor for developing more efficient solar cell devices. One possibility for attenuating losses through recombination between injected electrons and oxidized dye molecules is to move the positive charge further away from the metal oxide surface. For this purpose, a metal‐free dye named E6 is developed, in which the chromophore core is tethered to two external triphenylamine (TPA) units. After photoinduced electron injection into TiO2, the remaining hole is rapidly transferred to a peripheral TPA unit. Electron–hole recombination is slowed down by 30% compared to a reference dye without peripheral TPA units. Furthermore, it is found that the added TPA moieties improve the electron blocking effect of the dye, retarding recombination of electrons from TiO2 to the cobalt‐based electrolyte.

Effects of emissive layer architecture on recombination zone and Förster resonance energy transfer in organic light-emitting diodes

Recombination zone and Förster resonance energy transfer (FRET) in multilayer organic light-emitting diodes (OLEDs) were investigated. Basis device architecture is indium tin oxide (ITO)/N, N′-diphenyl-N, N′-bis(1-naphthyl-phenyl)-1, 1′-biphenyl-4, 4′-diamine (NPB)/4-(dicyanomethylene)-2-tert-butyl-6-(1, 1, 7, 7- tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/NPB (spacer)/tris-(8-hydroxyl quinoline) aluminum (Alq3)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP)/Al. Exciton recombination zone is located at DCJTB and Alq3 layers. When the NPB spacer is 10-nm-thick, Alq3 emission governs in electroluminescent (EL) spectra owing to absence of FRET between DCJTB and Alq3. FRET occurs while the NPB spacer is 5-nm-thick and thus DCJTB emission is dominant in EL spectra. As the emissive layout of DCJTB/Alq3/NPB substitutes for DCJTB/NPB/Alq3, both DCJTB and NPB emissions are observed due to electron-blocking effect of NPB.