Improved Carrier Injection Research Articles

Interface Modification of Cathode Electrode Using Dimmethyldicyanoquinonediimine as a Charge Transfer Layer in Organic Photovoltaic Cell

Various metals for the cathode electrode of organic electronic devices have been used in order to improve carrier injection and contact resistance etc. However, metal electrodes have some disadvantages such as rough surfaces, inadequate interfacial durability and unsuitable work functions. In the present work, we have fabricated an organic photovoltaic cell consisting of ITO/PEDOT:PSS/P3HT:PCBM/DMDCNQI/Al. The dimmethyldicyanoquinonediimine (DMDCNQI) compound was used as an organic n-type charge transfer complex between the cathode electrode and an organic active layer to improve contact resistance and electron transport ability. The prepared device shows a high short-circuit current density of 10.39 mA/cm2 and a maximum power conversion efficiency of 3.10%.

Effect of Trapezoidal-Shaped Well on Efficiency Droop in InGaN-Based Double-Heterostructure Light-Emitting Diodes

We investigated the effects of different well shapes on the external quantum efficiency (EQE) and the efficiency droop in wide-well InGaN/GaN double-heterostructure light-emitting diodes. For forward current densities in the measurement range of greater than 135 A/cm2, the device featuring a trapezoidal well exhibited improved EQEs and alleviative efficiency droop, relative to those of the device featuring a rectangular well. The decreased Auger loss has been proposed as the main reason for the greater maximum efficiency that occurred at high current density (>50 A/cm2). For the devices incorporating trapezoidal and rectangular wells, the EQEs at 200 A/cm2decreased by 14 and 40%, respectively, from their maximum values, resulting in the EQE at a current density of 200 A/cm2of the device featuring a trapezoidal well being 17.5% greater than that featuring a rectangular well. These results suggest that, in addition to the decreased Auger loss, the alleviation in efficiency droop at higher current densities might be due to higher internal quantum efficiency resulted from the improved carrier injection efficiency of the trapezoidal well.

Improving organic light-emitting diode performance with patterned structures

The influence nano- and micro- structures on light extraction and carrier injection characteristics in organic light-emitting diodes (OLEDs) is examined. The results demonstrate device enhancement from two distinctive patterning scales: embedded microstructure arrays for improved light extraction and imprinted nanostructures for carrier injection enhancement. Further, based on these results, a two-level imprinting stamp is designed which simultaneously produces a micron-sized pattern for facilitating light extraction and a nanometer-sized pattern to improve carrier injection. Additionally, this scalable patterning technology circumvents imprinting misalignment while preserving its compatibility with rollto- roll technology.

A New Phosphine Oxide Host based on ortho‐Disubstituted Dibenzofuran for Efficient Electrophosphorescence: Towards High Triplet State Excited Levels and Excellent Thermal, Morphological and Efficiency Stability

An efficient host for blue and green electrophosphorescence, 4,6-bis(diphenylphosphoryl)dibenzofuran (o-DBFDPO), with the structure of a short-axis-substituted dibenzofuran was designed and synthesised. It appears that the greater density of the diphenylphosphine oxide (DPPO) moieties in the short-axis substitution configuration effectively restrains the intermolecular interactions, because only very weak π-π stacking interactions could be observed, with a centroid-to-centroid distance of 3.960 Å. The improved thermal stability of o-DBFDPO was corroborated by its very high glass transition temperature (T(g)) of 191 °C, which is the result of the symmetric disubstitution structure. Photophysical investigation showed o-DBFDPO to be superior to the monosubstituted derivative, with a longer lifetime (1.95 ns) and a higher photoluminescent quantum efficiency (61 %). The lower first singlet state excited level (3.63 eV) of o-DBFDPO demonstrates the stronger polarisation effect attributable to the greater number of DPPO moieties. Simultaneously, an extremely high first triplet state excited level (T(1)) of 3.16 eV is observed, demonstrating the tiny influence of short-axis substitution on T(1). The improved carrier injection ability, which contributed to low driving voltages of blue- and green-emitting phosphorescent organic light-emitting diodes (PHOLEDs), was further confirmed by Gaussian calculation. Furthermore, the better thermal and morphological properties of o-DBFDPO and the matched frontier molecular orbital (FMO) levels in the devices significantly reduced efficiency roll-offs. Efficient blue and green electrophosphorescence based on the o-DBFDPO host was demonstrated.

Management of the electrical injection uniformity in broad-area top-emitting VCSELs

The electrical properties of broad-area 850 nm top emitting VCSELs have been investigated in order to improve carrier injection uniformity in their active zone. First, we have demonstrated using an electrical simulation tool that a multi-point localized injection design associated with a spreading layer at the top of the device (ITO) can lead to a significant improvement of carrier injection and on its spatial distribution. Secondly, the electrical contrast achievable by applying this method with localized etchings has been experimentally measured. Finally, stripe-shaped devices with output power up to 50 mW in a continuous-wave operation at room temperature have been demonstrated.

Effect of Solvent-Assisted Thermal Treatment on the Performance of Polyfluorene-Based Polymer Light Emitting Diodes

Solvent-assisted thermal annealing plays an important role in controlling the morphology of polymers; indeed, it affects the performance of polymer light emitting diodes (PLEDs). Using a polyfluorene derivative, poly(9,9-di-2-adamantylhexyl fluorene-2,7-diyl) (PDAHF), possessing bulky rigid side groups as the emitting material and suitable thermal annealing conditions, we observed improved carrier injection and no photoluminescence quenching. The boiling point of the solvent and the glass transition temperature of the polymer also played important roles. Blue PLED devices having the configuration indium tin oxide (1300 Å)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (800 Å)/1 wt % PDAHF (650 Å)/LiF (10 Å)/Ca (100 Å)/Al (2000 Å) were prepared using -xylene or toluene as the solvent. Thermal annealing at (-xylene) or (toluene) resulted in devices exhibiting blue emissions, with maximum luminances at 12.5 V of (-xylene) and (toluene) as a result of improved carrier injection, which increased the degree of charge recombination. The current efficiencies were 4.0 cd/A (-xylene) and 4.4 cd/A (toluene) at . The Commission Internationale de l'Eclairage coordinates were good at (0.14, 0.17). These results indicate that searching for a suitable solvent combined with appropriate thermal annealing conditions is a promising approach toward achieving high-efficiency PLEDs.

Electronic and chemical properties of cathode structures using 4,7-diphenyl-1,10-phenanthroline doped with rubidium carbonate as electron injection layers

The electronic properties and chemical interactions of cathode structures using 4,7-diphenyl-1, 10-phenanthroline (Bphen) doped with rubidium carbonate (Rb2CO3) as electron injection layers were investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron injection efficiency than those with cathode structures of Bphen/LiF/Al. Ultraviolet and x-ray photoemission spectroscopy shows that n-type doping effects resulting from Rb2CO3 and the gap states created by aluminum deposition are both keys to the improved carrier injection efficiency. Moreover, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.

Improved Current Gain in GaN/SiC Heterojunction Bipolar Transistors by Insertion of Ultra-Thin AlN Layer at Emitter-Junction

GaN/SiC Heterojunction Bipolar Transistors (HBTs) with ultra-thin AlN insertion layers at the n-GaN/p-SiC emitter junction are proposed to improve carrier injection efficiency. The current-voltage characteristics of n-GaN/AlN/p-SiC heterojunctions have exhibited very small reverse leakage and good rectification. The capacitance-voltage measurement have revealed that the conduction band offset between n-GaN and p-SiC has been reduced from -0.74 eV to -0.54 eV by insertion of AlN, indicating that the GaN/AlN/SiC heterojunction may show better electron-injection efficiency. A significantly improved common-base current gain (α~0.2) is obtained for GaN/AlN/SiC HBTs with initial N* pre-irradiation, while it was very low (α~0.001) for GaN/SiC HBTs without AlN layers.

Demonstration of Schottky Barrier NMOS Transistors With Erbium Silicided Source/Drain and Silicon Nanowire Channel

We have fabricated silicon nanowire N-MOSFETs using erbium disilicide (ErSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> ) in a Schottky source/drain back-gated architecture. Although the subthreshold swing (~180 mV/dec) and drain-induced barrier lowering (~500 mV/V) are high due thick BOX as gate oxide, the fabricated Schottky transistors show acceptable drive current ~900 muA/mum and high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ratio (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> ). This is attributed to the improved carrier injection as a result of low Schottky barrier height (Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> ) of ErSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> /n - Si(~0.3 eV) and the nanometer-sized (~8 nm) Schottky junction. The carrier transport is found to be dominated by the metal-semiconductor interface instead of the channel body speculated from the channel length independent behavior of the devices. Furthermore, the transistors exhibit ambipolar characteristics, which are modeled using thermionic/thermionic-field emission for positive and thermionic-field emission for negative gate biases.

Programming Speed Enhancement by NH3 Plasma Nitridation of Tunneling Oxide for Ge Nanocrystals Memory

The characteristics of Ge nanocrystals charge-storage memory stacks with plasma surface treatment of the tunneling oxide layer have been studied in this paper. The tunneling silicon oxide (TO) surface was nitrided by plasma treatment, which appears to modify the bonding type of the substrate surface and enhance the nucleation sites for Ge nanocrystals. The chemical bonding depth profile of the /TO layers and the Ge nanocrystals structure were examined by X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. The trap charge densities were estimated to be as high as from hysteresis loop for sweep. The nitrided interface (oxynitride) between the trapping layer and the tunneling silicon oxide has a bandgap lower than the silicon oxide, which causes an asymmetric tunneling barrier under programming and erasing conditions. An effective thinner barrier under programming bias results in enhanced programming speed due to improved carrier injection efficiency. It was also observed that the nitrided interface reduces Ge diffusion into the TO layer, which is known to result in degradation of the properties of Ge nanocrystal-based memory devices.

Enhanced Current Efficiency from Bio-Organic Light-Emitting Diodes Using Decorated Amyloid Fibrils with Conjugated Polymer

We demonstrate the use of self-assembled bionanostructures in polymer light-emitting diodes. Amyloid fibrils formed by protein misfolding were decorated with a soluble luminescent conjugated polymer. This conjugated polymer complex with amyloid fibrils was used as the active layer in a light emitting diode, resulting in a 10-fold increase in external quantum efficiency compared with pristine polymer, because of improved carrier injection.

Improvement of radiative efficiency and wavelength-tuning by TPD-doping in organic light emitting diode using Poly [methylmethacrylate-co-(7-(4-trifluoromethyl)coumarin acrylamide)] (PCA)

We fabricated polymer light-emitting diodes (PLEDs) using a low-weight-molecular-doped luminescent material with a ?max in the ultraviolet region. A single layer PLED without low-weight-molecular-doping does not reveal any electroluminescence (EL) signal at all. However, a PLED with doping clearly shows EL signals. This indicates that carrier injection as well as radiative efficiency can be drastically improved by low-weight-molecular-doping. In addition, multilayered PLEDs are found to be very effective to further improve carrier injection.

Layer‐by‐Layer Spin Self‐Assembled Hole Injection Layers Containing a Perfluorinated Ionomer for Efficient Polymer Light‐Emitting Diodes

AbstractDeposition of hole injection layers including a perfluorinated ionomer has been demonstrated using layer‐by‐layer spin self‐assembly for enhanced device efficiency and lifetime in PLEDs. We show that the LBL spin self‐assembled thin films enable to control work functions of indium‐tin oxide anodes by changing the PFI concentration and that a resulting green‐emitting device has an enhanced luminescence efficiency and 18 times longer half lifetime than a device using a conventional HIL. We also fabricate a gradient of energy levels by the LBL self‐assembly of the PFI that results in a work function of 5.74 eV, which can be used to improve carrier injection even for an emitting layer whose ionization potential is over 5.7 eV.magnified image

Efficient White‐Electrophosphorescent Devices Based on a Single Polyfluorene Copolymer

AbstractAn efficient white‐light‐emitting polymer (W3) is realized by covalently attaching a green fluorophore and a red phosphor into the backbone and the side chains, respectively, of polyfluorene at a concentration of 0.04 mol %. In addition, charge‐transporting pendant units are included to improve carrier injection and transport. White‐electrophosphorescent devices with the structure ITO/PEDOT:PSS/W3/CsF/Al (ITO: indium tin oxide; PEDOT:PSS: poly(styrenesulfonate)‐doped poly(3,4‐ethylenedioxythiophene)) exhibit a low turn‐on voltage of 2.8 V and a luminance of ca. 103 cd m–2 at below 6 V. The peak luminance and power‐conversion efficiencies are 8.2 cd A–1 and 7.2 lm W–1, respectively. Furthermore, the device shows relatively stable white emission: the Commission Internationale d'Éclairage (CIE) chromaticity coordinates of the devices change only slightly from (0.35,0.38) at 10 mA cm–2 to (0.33,0.36) at 100 mA cm–2, with an almost constant color render index (CRI) value of 82 at all measured current densities.

Self-aligned carbon nanotube transistors with charge transfer doping

This letter reports a charge transfer p-doping scheme which utilizes one-electron oxidizing molecules to obtain stable, unipolar carbon nanotube transistors with a self-aligned gate structure. This doping scheme allows one to improve carrier injection, tune the threshold voltage Vth, and enhance the device performance in both the “ON-” and “OFF-” transistor states. Specifically, the nanotube transistor is converted from ambipolar to unipolar, the device drive current is increased by 2–3 orders of magnitude, the device OFF current is suppressed and an excellent Ion∕Ioff ratio of 106 is obtained. The important role played by metal–nanotube contacts modification through charge transfer is demonstrated.

Chemical Optimization of Self-Assembled Carbon Nanotube Transistors

We present the improvement of carbon nanotube field effects transistors (CNTFETs) performances by chemical tuning of the nanotube/substrate and nanotube/electrode interfaces. Our work is based on a method of selective placement of individual single walled carbon nanotubes (SWNTs) by patterned aminosilane monolayer and its use for the fabrication of self-assembled nanotube transistors. This method brings a relevant solution to the problem of systematic connection of self-organized nanotubes. The aminosilane monolayer reactivity can be used to improve carrier injection and doping level of the SWNT. We show that the Schottky barrier height at the nanotube/metal interface can be diminished in a continuous fashion down to an almost ohmic contact through these chemical treatments. Moreover, sensitivity to 20 ppb of triethylamine is demonstrated for self-assembled CNTFETs, thus opening new prospects for gas sensors taking advantages of the chemical functionality of the aminosilane used for assembling the CNTFETs.

Electrical doping: the impact on interfaces ofπ-conjugatedmolecular films

Organic–metal and organic–organic interfaces play crucial roles in charge injection in, andtransport through, organic thin film devices. Their electronic structure, chemical propertiesand electrical behaviour must be fully characterized and understood if engineering andcontrol of organic devices are to reach the levels attained for inorganic semiconductordevices. Recent fundamental, as well as device, work has demonstrated that electricaldoping provides a very interesting way to improve carrier injection into molecular filmsand, eventually, control molecular level alignment at their interfaces. This brief reviewemphasizes the current understanding of the effects of doping on organic interfaces.

Electronic structure and current injection in zinc phthalocyanine doped with tetrafluorotetracyanoquinodimethane: Interface versus bulk effects

Abstract The p-type doping of zinc phthalocyanine (ZnPc) with the highly electronegative tetrafluorotetracyanoquinodimethane (F4-TCNQ) is investigated via direct and inverse photoemission spectroscopy and in situ current–voltage (I–V) measurement. The electron affinity of F4-TCNQ and the ionization energy of ZnPc are found to be energetically compatible with an electron transfer between the highest occupied molecular orbital (HOMO) of the host and the lowest unoccupied molecular orbital of the dopant. The Fermi level is near mid-gap in undoped ZnPc, and drops to 0.42 and 0.18 eV above the HOMO in the 0.3% and 3% doped films, respectively, consistent with efficient p-doping. The dependence of the Au/ZnPc:0.3%F4-TCNQ/Au I–V characteristics on the thickness of the organic film is analyzed in terms of injection-limited versus space-charge-limited current. The analysis demonstrates that the large doping-induced increase in hole current is primarily due to improved carrier injection via tunneling through the narrow interface space charge layer.

Strained Layer Multiquantum Barriers with Improved Carrier Injection and Confinement

The effects of using strained layer superlattice in multiquantum barriers (MQBs) were analyzed with focus on InP-based materials and their application to strained layer multiquantum well lasers (SL-MQW LDs). Tensile strained barriers are shown to largely increase the effective barrier height of MQBs. A new barrier material, AlInP, which is 1–1.5% tensile strained on InP, is shown to have inherent advantages of large conduction band edge discontinuity ΔEc and low aluminum content compared to conventional AlInAs on InP. Those advantages are confirmed for strain-compensated MQB with tensile barrier/compressive well in terms of increasing the effective barrier height. Some disadvantages of the strain-compensated MQB are also pointed out; the increase in density of holes injected into the MQB region under lasing conditions and the high p-doping concentration required. To solve those problems, an improved method of carrier injection through minibands with gradually enhanced energy levels toward the MQW region is presented, which is shown to enable elimination of the carrier transport effect and to result in marked improvement in laser performances.

N -type delta doped strained quantum well lasers for improved modulation bandwidth

Improved dynamic properties of strained single quantum well (QW) lasers were obtained by the incorporation of an n-type δ doping at close proximity to the strained QW active layer. The resultant modulation bandwidth was almost doubled, from 3.5 GHz to more than 6.2 GHz. The increased modulation bandwidth is attributed to an improved carrier injection resulting from the enhancement of the carrier transit time into the QW, as well as a decrease in the significance of the adverse contribution of carrier transport effects.