Hole Barrier Research Articles

Correlation between carrier recombination and valence band offset effect of graded Cu(In,Ga)Se2 solar cells

Carrier recombination in Cu(In,Ga)Se2 (CIGS) solar cells is one of the critical factors determining cell performance loss. Additionally, heterointerface recombination is the most detrimental recombination pathway based on the Shockley–Read–Hall mechanism. Various methods have been applied to impede interface recombination, and the introduction of a valence band offset (ΔEV) produced by a homogeneous Cu-deficient layer (CDL) has been proved to be effective for improving cell efficiency. However, the relationship between ΔEV and recombination processes has not yet been fully clarified. In this work, through a Se interval irradiation process, ΔEV was incorporated into CIGS solar cells with bandgap grading so as to explore the correlation between the ΔEV effect and heterointerface recombination, which was demonstrated by both SCAPS-1D simulation and experiment. Moreover, illumination-dependent open-circuit voltage (Suns–Voc) and temperature-dependent open-circuit voltage (Voc–T) measurements were performed to quantitatively estimate the recombination rates at the interface, in the space-charge region (SCR), and in the quasi-neutral region (QNR), contributing to elaboration of the mechanism by which ΔEV serves as a hole barrier at the interface, thereby reducing heterointerface recombination and enhancing cell performance.

Quantum dot lasers with asymmetric barrier layers: Close-to-ideal threshold and power characteristics

A theory of static (threshold and power) characteristics of novel diode lasers – quantum dot (QD) lasers with asymmetric barrier layers (ABLs) – is developed. The barrier layers are asymmetric in that they have considerably different heights for the carriers of opposite signs. The ABL located on the electron- (hole-) injecting side of the structure provides a low barrier (ideally no barrier) for electrons (holes) [so that it does not prevent electrons (holes) from easily approaching the active region] and a high barrier for holes (electrons) [so that holes (electrons) injected from the opposite side of the structure do not overcome it]. The use of ABLs should thus ideally prevent the simultaneous presence of electrons and holes (and hence parasitic electron – hole recombination) outside the QDs. It is shown in this work that in such a case of total suppression of parasitic recombination, the QD lasers with ABLs offer close-to-ideal performance: the threshold current density is below 10 A cm−2 at any temperature, the absolute value of the characteristic temperature is above 1000 K (which manifests a virtually temperature-independent operation), the internal differential quantum efficiency is practically unity, and the light – current characteristic is linear at any pump current.

Driving and Application of Blue Light Organic Light Emitting Devices

The DPVBi (4,4′-bis(2,2-diphenylvinyl-1,1′-biphenyl) is a blue-light organic fluorescence doped material, which can be used as a hole barrier layer or a luminescent layer for fabricating organic light-emitting devices. A blue light device with stable color stability and high efficiency was prepared by co-doping blue light dye DPVBi and red light dye DCJTB as light-emitting layer. In order to prevent the infiltration of O2 and moisture inside the device from affecting the luminescence lifetime of the device, the device was encapsulated by atomic layer deposition. Since the driving voltage of the organic light-emitting device is generally above 5 V and the power consumption is low, in order to facilitate driving with a low voltage, a boost driving circuit based on the XL6009 chip was designed. The driver of the fabricated blue-light device was tested. The results showed that circuit had low-voltage drive characteristics and could be widely used in small toys, lighting, and portable devices. Through the test to achieve the desired goal, the requirements of low voltage and low energy consumption were realized, and the life of the light-emitting device can be tested, which has certain practicability and reference value.

Enhanced Gate Reliability in GaN MIS-FETs by Converting the GaN Channel into Crystalline Gallium Oxynitride

We demonstrated the enhanced threshold voltage (VTH) stability and gate reliability of the enhancement-mode (E-mode) GaN-based MIS-FETs under reverse-bias stress (i.e., stress at off-state with high drain voltage), which is achieved by converting the conventional GaN (Eg ∼ 3.4 eV) channel into a crystalline GaOxN1–x (Eg ∼ 4.1 eV) layer. In the MIS-FETs stressed at off-state with a large drain voltage, holes will be generated in the high-electric-field region by impact ionization, and subsequently, degradation of the gate dielectric is caused by the holes passing through the dielectric film. As the valence band offset between the GaOxN1–x and GaN is ∼0.6 eV, an energy barrier for holes will be formed surrounding the gate, which can prevent holes from flowing to the gate side and therefore reduce the hole-induced gate dielectric degradation. The crystalline gallium oxynitride layer converted from GaN could also be a promising method to improve channel reliability for many GaN-based structures and processes.

Effect of chemical impurities on charge injection barriers at the interface of copper and polyethylene

Chemical impurities in dielectric materials are one of the reasons that reduce the height of charge injection barrier. These impurities create energy states in dielectric band gap, which impact on the injection process. Chemical impurities that are studied in this paper include carbonyl, vinyl, and conjugated double bond impurities. These types of chemical impurities are the most common in technical dielectric polymers such as polyethylene. These impurities are studied using the computational quantum mechanics in the scope of density functional theory (DFT). The computed barrier in the present work is carried out using a new technique based on correcting the underestimation in polyethylene band gap due to the DFT computations. It is found that terminal carbonyl and conjugated double bond produce the lowest injection barrier for electrons and holes, respectively. The depth of these impurity states are compared with that developed in bulk PE and at the interface with other metals. The similarity in calculated barrier for electrons and holes is in agreement with the experimental observations. This paper concludes that chemical impurities cannot alone explain the activation energy of conduction of 1 eV.

Performance improvements of AlGaN-based deep-ultraviolet light-emitting diodes with specifically designed irregular sawtooth hole and electron blocking layers

A new structures for efficient AlGaN based deep-ultraviolet light-emitting diode (DUV -LED) with irregular sawtooth hole barrier layer (HBL) and electronic barrier layer (EBL) is proposed in this work. Optical output power and internal quantum efficiency(IQE), the energy band diagrams, radiative recombination rate are investigated by APSYS software. The results indicate that light output power of the DUV-LED with the new structure is enhanced by 131.8%, and internal quantum efficiency is increased by 125.0% at an injection current of 180 mA compared with the conventional AlGaN based DUV-LED structure. Further research shows that irregular sawtooth hole blocking layer(HBL) and electron blocking layer(EBL) can obviously improve the carrier radiation recombination rate, modulate carrier distribution, enhance electron injection efficiency, and confine hole leakage; thus the IQE and optical output power are enhanced.

Optimization of in-device depleted passivation layer for InGaAs photodetectors

Lattice matched InGaAs/InP photodetector structure with an InP layer for in-device passivation purposes has been analyzed numerically, an optimized design for mesa type devices has been proposed and experimental verification has been shown. Structural and electrical parameters such as thickness and doping concentration of InP layer have been studied in order to avoid any hole barrier possible in the valence band and to achieve a high photo-current with pixel isolation while reducing the surface leakage dark current component substantially. The passivation effect has been quantified for a variety of surface instability levels in comparison with the traditional InGaAs/InP hetero-junction structure and fully depleted InP layer has shown to be able to isolate pixels successfully and provide passivation that can help reduce the total dark current more than one order of magnitude. Photo-response in the proposed structure has been observed to be enhanced as well due to the increased collection efficiency.

Device simulation of Cu(In,Ga)Se2 solar cells by means of voltage dependent admittance spectroscopy

The simulation of solar cell devices is important for the understanding of defect physics and loss mechanisms in real solar cells. On the other hand, voltage dependent admittance spectroscopy delivers essential information for establishing a baseline simulation model of Cu(In,Ga)Se2 (CIGSe) solar cells. Here we give an explanation for the weak temperature dependence of the N1-signal, the latter being not compatible with a bulk defect or with a simple hole barrier at the Mo back contact. Furthermore, we find a Ed,IF – EV ≈ 0.3 eV deep recombination-active acceptor state at the absorber/buffer interface made of air-light exposed CIGSe absorbers. This gives us the ability to explain the reduction of power conversion efficiency of solar cells made from air-light exposed absorbers. From the voltage dependent capacitance step of this interface defect we can deduce the formerly unknown position of the Fermi level at the hetero junction in equilibrium which is close to mid-gap. Simulation of dark J-V curves allows a refinement of the parameter of this absorber/buffer interface defect, resulting in a defect density of Nd,IF ≈ 3.5·1011 cm−2 as well as capture cross sections of σn ≈ 4·10−16 cm2 for electrons and σp ≈ 3·10−11 cm2 for holes.

Organic Bidirectional Phototransistors Based on Diketopyrrolopyrrole and Fullerene

AbstractIt is shown that simple bilayer devices consisting of the diketopyrrolopyrrole (DPP) monomer Ph‐TDPP‐Ph as donor and C60 as acceptor feature J–V‐characteristics of a bidirectional organic phototransistor where illumination intensity plays the role of the gate voltage as compared to a conventional field‐effect transistor. The output current may therefore be controlled both electrically and optically. The underlying mechanism is based on the good charge transport in Ph‐TDPP‐Ph and C60, the intrinsic dissociation properties of C60, and the presence of an injection barrier for holes. In addition to this, it is demonstrated that the observed behavior of the DPP/C60 system allows the realization of basic logic elements like NOT‐, AND‐, and OR‐Gates, which may provide the basis for advanced analog and digital applications.

Effects of Hydrogen Treatment in Barrier on the Electroluminescence of Green InGaN/GaN Single-Quantum-Well Light-Emitting Diodes with V-Shaped Pits Grown on Si Substrates**Supported by the National Key R&D Program of China under Grant Nos 2016YFB0400600 and 2016YFB0400601, the State Key Program of the National Natural Science Foundation of China under Grant No 61334001, the National Natural Science Foundation of China under

Effect of hydrogen (H2) treatment during the GaN barrier growth on the electroluminescence performance of green InGaN/GaN single-quantum-well light-emitting diodes (LEDs) grown on Si substrates is experimentally investigated. We prepare two LED samples with different carrier gas compositions during the growth of GaN barrier. In the H2 free LED, the GaN barrier is grown in full nitrogen (N2) atmosphere. For the other H2 treated LED, a mixture of N2 and H2 was used as the carrier gas. It is observed that V-shaped pits decrease in size after H2 treatment by means of the scanning electron microscope. Due to the fact that the p–n junction interface would be closer to the p-GaN as a result of smaller V-shaped pits, the tunneling barrier for holes to inject into the InGaN quantum well would become thicker after H2 treatment. Hence, the external quantum efficiency of the H2 treated LED is lower compared to the H2 free LED. However, LEDs would exhibit a better leakage behavior after H2 treatment during the GaN barrier growth because of more effective blocking of the threading dislocations as a result of the H2 etching at V-shaped pits.

Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers

In this work, rear-contact passivated Cu(In,Ga)Se2 (CIGS) solar cells were fabricated without any intentional contact openings between the CIGS and Mo layers. The investigated samples were either Na free or one of two Na supply methods was used, i) a NaF precursor on top of the Al2O3 rear passivation layer or ii) an in situ post-deposition treatment with NaF after co-evaporation of the CIGS layer. The thickness of the ALD-Al2O3 passivation layer was also varied in order to find an optimal combination of Na supply and passivation layer thickness. Our results from electrical characterization show remarkably different solar cell behavior for different Na supplies. For up to 1 nm thick Al2O3 layers an electronically good contact could be confirmed independently of Na deposition method and content. When the Al2O3 thickness exceeded 1 nm, the current was blocked on all samples except on the samples with the NaF precursor. On these samples the current was not blocked up to an Al2O3 layer thickness of about 6 nm, the maximum thickness we could achieve without the CIGS peeling off the Al2O3 layer. Transmission electron microscopy reveals a porous passivation layer for the samples with a NaF precursor. An analysis of the dependence of the open circuit voltage on temperature (JVT) indicates that a thicker NaF precursor layer lowers the height of the hole barrier at the rear contact for the passivated cells. This energy barrier is also lower for the passivated sample, compared to an unpassivated sample, when both samples have been post-deposition treated.

Hole barrier height reduction in inverted quantum-dot light-emitting diodes with vanadium(V) oxide/poly(N-vinylcarbazole) hole transport layer

This study demonstrates superior electrical and electroluminescence performance of inverted quantum-dot light-emitting diodes (QD-LEDs) with a V2O5/poly(N-vinylcarbazole) (PVK) hole conduction layer. Hole- and electron-only device measurements reveal a more balanced charge carrier injection as well as the higher hole conduction capability in the inverted QD-LED than the standard one. Smooth stepwise hole conduction energy levels with a remarkably reduced hole barrier height (Δh) from 1.74 to 0.89 eV at QD/PVK are found to be responsible for high hole conduction and high luminous efficiency in the inverted QD-LED, which is validated by ultraviolet photoelectron spectroscopy measurements. The down-shifted electronic energy levels of PVK for reducing the Δh are discussed from the point of view of molecular orientation of PVK governed by interfacial atomic interaction with underlayers of V2O5 and QD for standard and inverted device structures, respectively.

Enhanced irradiance sensitivity of 4H-SiC based ultraviolet sensor introducing laterally gated Al/SiO2/SiC tunnel diode structure with low gate bias

A laterally gated Al/SiO2/SiC structure composed of an inner circular tunnel diode (TD) and an outer annular TD is investigated in this work to demonstrate the improved ultraviolet (UV) response in comparison with a single metal–oxide–semiconductor (MOS) TD or a conventional photodiode scheme. The current of the inner TD (ID) can interrelate with the bias of the outer TD (VG) against the voltage of inner TD (VD). This coupling effect will increase if the outer TD is negatively biased, thereby augmenting the optical current ratio and the responsivity. The lateral flow of holes is determined by the variation of the intermediate hole barrier between the inner and outer TDs. Once the hole potential at the outer TD overtops that at the inner TD, photogenerated holes from the outer TD can flow to the inner TD. Moreover, the suggested coupled TD design exhibits distinct increments in photocurrent under various illumination levels because of the coupling effect. For VD = −1.5 V and VG = −2 V, the optical current ratio reaches 12 841 at 303 K, and the obtained responsivity attains 0.0052 at 383 K with a light irradiance of 50 mW/cm2. Hence, the laterally gated MOS device can operate at elevated temperatures and heightened optical intensities, making it a promising UV sensor with enhanced irradiance sensitivity.

Band offsets at amorphous-crystalline Al2O3–SrTiO3 oxide interfaces

2D electron gases (2DEGs) formed at oxide interfaces provide a rich testbed for fundamental physics and device applications. While the discussion of the physical origins of this phenomenon continues, the recent discovery of oxide 2DEGs at non-epitaxial interfaces between amorphous and crystalline oxides provides useful insight into this debate. Furthermore, using amorphous oxides offers a low-cost route towards realizing 2DEGs for device applications. In this work, the band offsets of a simple model system of an amorphous-crystalline oxide interface are investigated. The model system consists of amorphous Al2O3 grown on single-crystalline (001) SrTiO3. X-ray photoelectron spectroscopy is employed to study the chemical states, bandgap, and band offsets at the interface. The density of ionic defects near the interface is found to be below the detection limit, and the interface is found to be insulating. Analysis of the relative band structure yields significant interfacial barriers, exceeding 1.05 eV for holes and 2.0 eV for electrons. The barrier for holes is considerably larger than what is known for related material systems, outlining the promise of using amorphous Al2O3 as an effective and simple insulator, an important building block for oxide-based field effect devices.

Impact of Cell Layout and Device Structure on On-Voltage Reduction of 6.5-kV n-Channel SiC IGBTs

A box cell layout and a hole-barrier structure were used to realize low-on-voltage n-channel 4H-SiC IGBTs with 6.5-kV blocking capability. Box cell layout can increase the channel width, leading to reduction of the channel resistance and an enhancement of electron injection from an emitter. Hole-barrier structure, which is a potential barrier for holes to prevent them from flowing out of the emitter, can enhance conductivity modulation. An on-voltage of 3.98 V at a collector current of 100 A/cm2 was achieved from a fabricated SiC IGBTin this study. Since the on-voltage of a SiC IGBT with a conventional structure was 4.81 V at the same collector current, the effect of our new structure was successfully shown to reduce the on-voltage of SiC IGBTs. An estimation of each voltage component involved in the on-voltage was also carried out by utilizing a device simulation, and the estimation shows that a SiC IGBT incorporating a box layout and hole-barrier structure will thus have quite a low drift-layer voltage and an on-voltage close to the limit determined by the bipolar built-in voltage.

Temperature dependence of electrical characteristics for diamond Schottky-pn diode in forward bias

A diamond Schottky-pn diode (SPND) was fabricated and its electrical properties were investigated by measuring the current-voltage characteristics. The diode showed high current density (>104 A/cm2 at 7 V) at room temperature with low differential on-resistance (~10−5 Ωcm2) under a wide temperature range (295–600 K). We constructed Richardson plots based on the characteristics and then estimated the potential barrier height on the hole current. From these results combined with the results of Poisson equation analysis, we viewed the change in hole barrier as a function of applied voltage and obtained the value of the hole activation energy as ~0.06 eV in the p+-type layer around the flat-band voltage. We clarified the transport mechanisms of the SPND and the method for estimating the activation energy in heavily boron-doped diamond exhibiting hopping conduction.

Performance enhancement of pentacene-based organic thin-film transistors using 6,13-pentacenequinone as a carrier injection interlayer

This work demonstrates pentacene-based organic thin-film transistors (OTFTs) fabricated by inserting a 6,13-pentacenequinone (PQ) carrier injection layer between the source/drain (S/D) metal Au electrodes and pentacene channel layer. Compared to devices without a PQ layer, the performance characteristics including field-effect mobility, threshold voltage, and On/Off current ratio were significantly improved for the device with a 5-nm-thick PQ interlayer. These improvements are attributed to significant reduction of hole barrier height at the Au/pentacene channel interfaces. Therefore, it is believed that using PQ as the carrier injection layer is a good candidate to improve the pentacene-based OTFTs electrical performance.

Inverted bulk-heterojunction organic solar cells with the transfer-printed anodes and low-temperature-processed ultrathin buffer layers

We studied the effects of a hole buffer layer [molybdenum oxide (MoO3) and natural copper oxide layer] and a low-temperature-processed electron buffer layer on the performance of inverted bulk-heterojunction organic solar cells in a device consisting of indium–tin oxide (ITO)/poly(ethylene imine) (PEI)/titanium oxide nanosheet (TiO-NS)/poly(3-hexylthiopnehe) (P3HT):phenyl-C61-butyric acid methylester (PCBM)/oxide/anode (Ag or Cu). The insertion of ultrathin TiO-NS (∼1 nm) and oxide hole buffer layers improved the open circuit voltage VOC, fill factor, and rectification properties owing to the effective hole blocking and electron transport properties of ultrathin TiO-NS, and to the enhanced work function difference between TiO-NS and the oxide hole buffer layer. The insertion of the TiO-NS contributed to the reduction in the potential barrier at the ITO/PEI/TiO-NS/active layer interface for electrons, and the insertion of the oxide hole buffer layer contributed to the reduction in the potential barrier for holes. The marked increase in the capacitance under positive biasing in the capacitance–voltage characteristics revealed that the combination of TiO-NS and MoO3 buffer layers contributes to the selective transport of electrons and holes, and blocks counter carriers at the active layer/oxide interface. The natural oxide layer of the copper electrode also acts as a hole buffer layer owing to the increase in the work function of the Cu surface in the inverted cells. The performance of the cell with evaporated MoO3 and Cu layers that were transfer-printed to the active layer was almost comparable to that of the cell with MoO3 and Ag layers directly evaporated onto the active layer. We also demonstrated comparable device performance in the cell with all-printed MoO3 and low-temperature-processed silver nanoparticles as an anode.

Enhanced device efficiency in organic light-emitting diodes by dual oxide buffer layer

Abstract We have demonstrated an improvement in device performance of fluorescent organic light-emitting diodes (OLEDs) by inserting a dual anode buffer layer composed of tungsten oxide (WO3) and molybdenum oxide (MoO3). The advantage of adding dual anode buffer layers with different deposition sequences over individual and composite oxide buffer layers has been systematically analyzed based on their electronic and optical properties. The incorporation of single and composite buffer layers has been revealed to induce a very low injection barrier for holes in tri-layer fluorescent OLEDs which results in a charge imbalance in the emission layer. In contrast, a proper sequence of buffer layers (WO3/MoO3) exhibiting higher contact angle (lower surface energy) and higher surface roughness, together with a step-wise increment of potential barrier leads to a better overall charge balance in the active emission layer. Therefore, an enhanced current efficiency and power efficiency of ∼5.8 cd/A and ∼5.2 lm/W respectively were recorded for the WO3/MoO3 buffer unit, which was better than the insertion of individual and composite layers.

Te Layer to Reduce the CdTe Back-Contact Barrier

The CdTe valence-band maximum (VBM) at 5.8 eV below the vacuum level is mismatched with potential contacting metals such as Ni (work function ∼5.2 eV) for the p-type CdTe commonly used in solar cells. Consequently, downward band bending in the CdTe forms both a hole barrier and a reverse field for electrons. Simulations reported here explain that a Te layer, which has been shown experimentally to enhance CdTe cell performance, is effective because it divides the larger CdTe barrier into two smaller ones. Comparison of simulations with experimental data implies a Te VBM of 5.40–5.45 eV, which is intermediate between the CdTe VBM and the Ni work function. An important consequence is a major reduction in the need for copper (Cu) at the back contact; with the Te layer, Cu in smaller amounts can be used for doping bulk CdTe instead of the larger concentration needed to reduce the band bending. The minimum thickness for the Te layer to enhance performance is calculated to be ∼20 nm, which is consistent with experimental variations in thickness and is also near the depletion width for the 1018 cm−3 Hall density of Te in those experiments. It also predicts that a Te layer used in combination with CdMgTe (CMT), whose conduction-band expansion can reflect electrons away from back interface, can further enhance cell performance by mitigating CMT's small valence-band expansion.