Carrier Injection Characteristics Research Articles

Charge Carrier Injection Characteristics of Semiconductive and Metal Electrodes in XLPE

In this article, a method using a barrier layer to realize electrode unipolar charge carrier injection is proposed to study the injection characteristics of positive and negative charge carriers by different electrode materials. The space charge distribution in XLPE/polytetrafluoroethylene (PTFE) was measured by the pulsed electroacoustic (PEA) method, and the relationship between the accumulation rate of injected charge carriers and electric field strength at the electrode interface was further established. It was found that the aluminum (Al) electrode had a greater charge injection capability than the semiconductive layer, while both were stronger than that of the gold (Au) electrode. The combined analysis of space charge measurement results and theoretical model verified the accuracy of the relationship between the accumulation rate of injected charge carriers and electrode field to characterize charge injection behavior. Space charge measurement in single-layer XLPE samples verified its validation. This study provides techniques and theoretical guidance for the testing and development of semiconductive materials in XLPE direct-current (dc) cables.

2,7(3,6)-Diaryl(arylamino)-substituted Carbazoles as Components of OLEDs: A Review of the Last Decade.

Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.

Replacement Gate High-k/Metal Gate nMOSFETs Using a Self-Aligned Halo-Compensated Channel Implant

A device design technique for boosting output resistance (Rout) characteristics of long-channel halo-doped nMOSFETs for replacement gate (RMG) high-k/metal gate (HK/MG) devices is proposed based on numerical simulations. We show that the self-aligned halo-compensated channel implant (HCCI) that is carried out after dummy poly gate removal provides compensation for the conventional halo doping. This can be utilized to mitigate the magnitude of potential drop beyond saturation at the drain end so that the long-channel Rout can be boosted relative to the conventional halo-doped device. In addition, we achieve an improved Vt roll-off behavior, an advantage to hot carrier injection (HCI) characteristics at high voltage operation, and the lowered short-channel drain-induced barrier lowering (DIBL) and intrinsic delay via the optimization of the source/drain (S/D) doping profile. HCCI technique can be implemented using a process flow that is compatible with the RMG HK/MG system-on-chip (SoC) technology in the mass production.

Characterization studies of NiO:K anode buffer layer in Alq3 and TADF-based organic light-emitting diodes by secondary ion mass spectrometry and admittance spectroscopy

In this paper, we used thermal evaporation method to deposit metal oxide films as inorganic buffer layer in Organic light-emitting diodes (OLEDs), and investigated how the K2CO3-doped NiO (KNO) buffer layers affect OLEDs’ performances. The mechanisms of enhanced hole injection into the hole transport layer NPB (α-naphthylphenylbiphenyl diamine) via NiO:K2CO3 anode buffer layers in Alq3-based and TADF-based OLEDs were studied by means of J-V-L characteristics, secondary ion mass spectroscopy (SIMS), and admittance spectroscopy measurements. For Alq3-based OLEDs, devices’ opto-electrical performances showed increased of luminance from 8780 cd/m2 to 22940 cd/m2, the decreased of turn-on voltage from 3.4 V to 3.1 V at 1 mA/cm2, and increased of current efficiency from 4.3 cd/A to 3.48 cd/A when 1-nm-thick K2CO3-doped NiO film with doping concentration of 1 mol% was inserted into OLEDs. TADF-based OLEDs’ current efficiency increased from 21.7 cd/A to 29.7 cd/A with similar anode buffer layer parameters. SIMS depth profiles on OLEDs suggested higher indium (In) diffusion from ITO anode onto TADF-based OLEDs upon device operation leading to higher roll-off phenomenon. Deposition of NiO:K2CO3 as anode buffer layer onto OLEDs device reduced the diffusion of In in organic layers during device operation. Investigation of the capacitance as function of voltage and frequency yield information about carrier injection characteristics for both Alq3 and TADF-based OLEDs. Due to better carrier recombination capability, TADF-based OLEDs’ slope in V > Vbi area has fewer shifts and more stable response, leading to higher amplitude in current efficiency enhancement in comparison with fluorescents (Alq3) material.

Effects of Cl2 plasma treatment on stability, wettability, and electrical properties of ITO for OLEDs

We investigated various characteristics of chlorinated indium tin oxide (Cl-ITO) for highly efficient organic light-emitting diodes (OLEDs): the work function, surface morphology, wetting characteristic, and hole-injection property. Via the systematic analysis of Cl-ITO, we showed that the modification of ITO using the plasma treatment method has a simple process and offers higher stability than that achieved with a solution-based modification method. We fabricated Cl-ITO with a high work function of 6.04 eV, which is 1.04 eV higher than that of pristine ITO. We verified that Cl2 plasma treatment does not affect the ITO surface morphology. The Cl halogenated surface showed a low surface polarity and influenced the growth of the organic material, especially for films as thin as ∼4 nm. Therefore, we found when using ultra-thin film on Cl-ITO for design devices, the wetting characteristic of Cl-ITO should be considered. In addition, using tris-(4-carbazoyl-9-ylphenyl)-amine resulted in increased current and power efficiencies compared with those obtained using 4,4′-N,N′-dicarbazolebiphenyl. When a carrier-only device was used, the carrier-injection characteristics of Cl-ITO were higher than those of other well-known hole-injection materials. Furthermore, the Cl-ITO OLEDs showed higher stability than OLEDs with other hole-injection materials.

Research on a Micro-Nano Si/SiGe/Si Double Heterojunction Electro-Optic Modulation Structure

The electro-optic modulator is a very important device in silicon photonics, which is responsible for the conversion of optical signals and electrical signals. For the electro-optic modulator, the carrier density of waveguide region is one of the key parameters. The traditional method of increasing carrier density is to increase the external modulation voltage, but this way will increase the modulation loss and also is not conducive to photonics integration. This paper presents a micro-nano Si/SiGe/Si double heterojunction electro-optic modulation structure. Based on the band theory of single heterojunction, the barrier heights are quantitatively calculated, and the carrier concentrations of heterojunction barrier are analyzed. The band and carrier injection characteristics of the double heterostructure structure are simulated, respectively, and the correctness of the theoretical analysis is demonstrated. The micro-nano Si/SiGe/Si double heterojunction electro-optic modulation is designed and tested, and comparison of testing results between the micro-nano Si/SiGe/Si double heterojunction micro-ring electro-optic modulation and the micro-nano Silicon-On-Insulator (SOI) micro-ring electro-optic modulation, Free Spectrum Range, 3 dB Bandwidth, Q value, extinction ratio, and other parameters of the micro-nano Si/SiGe/Si double heterojunction micro-ring electro-optic modulation are better than others, and the modulation voltage and the modulation loss are lower.

Systematic color tuning of a family of firefly oxyluciferin analogues suitable for OLED applications

The firefly oxyluciferin analogues have great potential application in organic light-emitting devices because of their high fluorescent efficiency. The emission color of these analogues can be tuned by chemical functionalization. To reveal the molecular structures, optoelectronic properties, and structure–property relationships of these analogues, an in-depth theoretical investigation was elaborated via quantum chemical calculation. The calculated ionization potentials, electron affinities, and reorganization energies show that introduction of dimethylamino and double bond can significantly affect the carrier injection and transport characteristics. We also calculated the singlet-to-triplet exciton-formation cross-section ratio (σS/σT), the exciton formation fractions (χs), the absorption and emission spectra. The calculated σS/σT values range from 1.00 to 10.17, and the corresponding χs are ca. 0.25–0.77. The calculated emission spectra range from 403.93 to 631.09 nm. It can be deduced that these studied firefly oxyluciferin analogues can serve as efficient blue, green, orange and red light-emitting electroluminescent materials.

Effect of heterostructure design on carrier injection and emission characteristics of 295 nm light emitting diodes

The effects of the heterostructure design on the injection efficiency and external quantum efficiency of ultraviolet (UV)-B light emitting diodes (LEDs) have been investigated. It was found that the functionality of the AlxGa1−xN:Mg electron blocking layer is strongly influenced by its aluminum mole fraction x and its magnesium doping profile. By comparing LED electroluminescence, quantum well photoluminescence, and simulations of LED heterostructure, we were able to differentiate the contributions of injection efficiency and internal quantum efficiency to the external quantum efficiency of UV LEDs. For the optimized heterostructure using an Al0.7Ga0.3N:Mg electron blocking layer with a Mg to group III ratio of 4% in the gas phase the electron leakage currents are suppressed without blocking the injection of holes into the multiple quantum well active region. Flip chip mounted LED chips have been processed achieving a maximum output power of 3.5 mW at 290 mA and a peak external quantum efficiency of 0.54% at 30 mA.

Hybrid functional calculated optical and electronic structures of thin anatase TiO2 nanowires with organic dye adsorbates

The electronic and optical properties of thin anatase TiO2 (101) and (001) nanowires have been investigated using the screened Coulomb hybrid density functional calculations. For the bare nanowires with sub-nanometer diameters, the calculated band gaps are larger relative to the bulk values due to size effects. The role of organic light harvesting sensitizers on the absorption characteristics of the anatase nanowires has been examined using the hybrid density functional method incorporating partial exact exchange with range separation. For the lowest lying excitations, directional charge redistribution of tetrahydroquinoline (C2-1) dye shows a remarkably different profile in comparison to a simple molecule which is chosen as the coumarin skeleton. The binding modes and the adsorption energies of C2-1 dye and coumarin core on the anatase nanowires have been studied including non-linear solvation effetcs. The calculated optical and electronic properties of the nanowires with these two different types of sensitizers have been interpreted in terms of their electron–hole generation, charge carrier injection and recombination characteristics.

Charge storage characteristics of nonvolatile memories with chemically-synthesized and vacuum-deposited gold nanoparticles

Carrier injection and charge loss characteristics of nonvolatile memories with chemically-synthesized (CS) and vacuum-deposited (VD) gold nanoparticles (Au-NPs) have been investigated. Compared to CS counterparts, the memories with VD Au-NPs exhibit a higher dot density of 3.77 × 1011 cm−2, leading to a larger memory window. Further, the energy from valence-band edge to vacuum level (EVB_vac) of tunneling oxide for the samples with CS and VD Au-NPs is found to be 9.04 and 9.85 eV respectively. The small EVB_vac value of the memories with CS Au-NPs is resulted from the formation of a thin chemical oxide (SiOx) on thermally-grown SiO2 tunneling layer during the chemically synthesized process, contributing to a slow erasing behavior. Besides, the programming of the memories with VD Au-NPs is saturated at high gate bias, which has been well-explained by the electrons induced potential coupling between Au-NPs. Superior data retention property and high temperature dependence of charge loss are observed for the memories with CS Au-NPs, which can be ascribed to the thick tunneling oxide layer by the additional SiOx film.

Interface Electronic Structures of the L-Cysteine on Noble Metal Surfaces Studied by Ultraviolet Photoelectron Spectroscopy

L-cysteine, one of amino acids, is well known in the bioelectronics field as a linker between proteins and metal electrodes. The interface electronic structures between L-cysteine and metals are therefore very crucial because they dominate the charge carrier injection characteristics into such biological systems. However the interface electronic structures of L-cysteine and metals have been not well understood. In this study, the electronic structures at the interfaces of sequentially deposited L-cysteine layers on the metal surfaces of Au(111), Ag(111), and Cu(111) were investigated by thickness-dependent ultraviolet photoelectron spectroscopy (UPS). At the contact regions, the electronic structures of L-cysteine revealed modification with respect to its bulk phase and significant variation depending on the substrate. The electronic structure at the interfaces including work function, secondary electron cutoff (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection barrier, and ionization energy were estimated for each case. [DOI: 10.1380/ejssnt.2015.373]

High-color-quality white emission in AC-driven field-induced polymer electroluminescent devices

The high-color-quality white emission in an AC-driven field-induced electroluminescence (FIPEL) device consisting of a white emitting ter-polymer: poly(fluorene–benzothiadiazole–quinoline) PF–BT–QL combined with a red emitting dye: Bis(2-methyl-dibenzof,hquinoxaline)(acetylacetonate)iridium (III) Ir(MDQ)2(acac) was achieved. The wide EL emission effectively covered the visible spectral region at the concentration of 5% Ir(MDQ)2(acac) in PF–BT–QL and largely enriched the color rendering capability with a CIE (0.36, 0.38) close to the ideal equal-energy white (0.33, 0.33) and a CRI as high as 97.4, close to the blackbody curve characteristic and CCT between 3034K and 5334K which are required for high-quality white-light illumination. When further increasing the concentration of Ir(MDQ)2(acac) to 10%, leading to a more pure white with CIE (0.36, 0.37) and a CRI as high as 97.1. Surprisingly, the FIPEL devices containing 20% and 30% Ir(MDQ)2(acac) in PF–BT–QL still exhibit high-quality white emission with CIE (0.42, 0.37) and (0.32, 0.38) and CRI 93.9 and 88.9 at high electric field, respectively. To the best of our knowledge, there are no reports of two-component FIPELs with a CRI>90, especially with such a high concentration of the phosphor dopant. We attribute this to the unique carrier injection characteristics of the AC-driven field induced device. This further suggests its great potential application in display and solid state lighting.

Emission characteristics in solution-processed asymmetric white alternating current field-induced polymer electroluminescent devices

In this work, the emission characteristics of a blue fluorophor poly(9, 9-dioctylfluorene) (PFO) combined with a red emitting dye: Bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) [Ir(MDQ)2(acac)], are examined in two different asymmetric white alternating current field-induced polymer electroluminescent (FIPEL) device structures. The first is a top-contact device in which the triplet transfer is observed resulting in the concentration-dependence of the emission similar to the standard organic light-emitting diode (OLED) structure. The second is a bottom-contact device which, however, exhibits concentration-independence of emission. Specifically, both dye emission and polymer emission are found for the concentrations as high as 10% by weight of the dye in the emitter. We attribute this to the significant different carrier injection characteristics of the two FIPEL devices. Our results suggest a simple and easy way to realize high-quality white emission.

Elucidation of carrier injection and recombination characteristics with impedance and capacitance in organic light-emitting diodes and the frequency effects

Impedance (Z), phase (φ) and capacitance (C) versus bias voltage (V) characteristics are studied to clarify carrier injection and recombination characteristics in organic light-emitting diodes (OLEDs). The Z–V transition starts at a characteristic voltage (Vc), which is strongly frequency dependent, i.e. Vc shifts to a high voltage with increasing measuring frequency. The electron–hole recombination starts at a voltage above Vc revealed by the φ–V and C–V transitions, which correspond to a phase approaching 0, a sharp rise in current density and a decrease in capacitance. Hole injection starts at a low Vc and corresponds to charge carrier accumulation and a slight rise in capacitance. Cole–Cole impedance plots illustrate that the interfacial resistance corresponds to the impedance at ultrahigh frequencies and shows bias independence, while the impedance at low frequencies represents the sum of interfacial resistance and organic stacks, and exhibits considerable bias dependence.

Liquid Carbazole Substituted with a Poly(ethylene oxide) Group and Its Application for Liquid Organic Light-emitting Diodes

Abstract We report efficient carrier injection characteristics in liquid organic light-emitting diodes (OLEDs). The use of a liquid carbazole substituted with a poly(ethylene oxide) (PEO) chain provided a substantial improvement in carrier injection compared to devices constructed with carbazoles without PEO units. The external electroluminescence quantum efficiency (ΦEL) of an OLED with the liquid carbazole as an emitting layer resulted in ΦEL = 0.75%, 25 times higher ΦEL than that of our previous liquid OLED without electrolyte.

Carrier Injection and Transport in Blue Phosphorescent Organic Light-Emitting Device with Oxadiazole Host

In this paper, we investigate the carrier injection and transport characteristics in iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) doped phosphorescent organic light-emitting devices (OLEDs) with oxadiazole (OXD) as the bipolar host material of the emitting layer (EML). When doping Firpic inside the OXD, the driving voltage of OLEDs greatly decreases because FIrpic dopants facilitate electron injection and electron transport from the electron-transporting layer (ETL) into the EML. With increasing dopant concentration, the recombination zone shifts toward the anode side, analyzed with electroluminescence (EL) spectra. Besides, EL redshifts were also observed with increasing driving voltage, which means the electron mobility is more sensitive to the electric field than the hole mobility. To further investigate carrier injection and transport characteristics, FIrpic was intentionally undoped at different positions inside the EML. When FIrpic was undoped close to the ETL, driving voltage increased significantly which proves the dopant-assisted-electron-injection characteristic in this OLED. When the undoped layer is near the electron blocking layer, the driving voltage is only slightly increased, but the current efficiency is greatly reduced because the main recombination zone was undoped. However, non-negligible FIrpic emission is still observed which means the recombination zone penetrates inside the EML due to certain hole-transporting characteristics of the OXD.

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.

Theoretical study on photophysical properties of 2,1,3-benzothiadiazole-based star-shaped molecules

Star-shaped molecules with tailoring functional groups in the core and the arms have great potential application in organic light-emitting devices, because it can be designed to realize low band gap, broad absorption, and excellent solubility for low-cost solution process. To gain an insight into the structure–property relationships, a set of four-arm star-shaped molecules with 2,1,3-benzothiadiazole as the core, different π-conjugated groups as the arm, and triphenylamine or 2-(pyridin-2-yl) pyridine as the end-group were designed. In this study, a systematic investigation into them was carried out using the density functional theory and time-dependent density functional theory methods. The calculated ionization potentials, electron affinities, and reorganization energies (λ) show that the properties of the π-conjugated bridge and the end-group significantly affect the carrier injection and transport characteristics of these molecules, especially for S-BTDP and S-EBTD. Among these molecules, S-BTDP exhibits better electron injection ability due to the introduction of 2-(pyridin-2-yl) pyridine as the end-group. However, S-EBTD, with ethylene as π-conjugated bridge, has excellent hole injection and carrier transport behaviors. We also calculated the singlet-to-triplet exciton-formation cross-section ratio (σS/σT), the exciton-formation fractions (χS), and the absorption and emission spectra of these molecules. We calculated that σS/σT ranges from 1.78 to 2.76 and that χS is ca. 0.37–0.48. These molecules have two absorption bands in the range of 340–410 nm and 500–613 nm, respectively. The calculated emission spectra range from 619 to 706 nm. It can be deduced that the studied 2,1,3-benzothiadiazole-based star-shaped molecules can serve as efficient red light-emitting electroluminescent materials.

Carrier injection from polypyrrole coated gold electrode in pentacene field effect transistors

Carrier injection characteristics of conjugated polymer coated gold (Au) electrode were investigated. Using pentacene field effect transistors (FETs) with Au electrodes covered with polypyrrole (PPy) film deposited electrochemically from an aqueous electrolyte solution of HCl or PTSA (1 M) containing pyrrole monomer, transient microscopic optical second harmonic generation (TRM-SHG) measurements were carried out. Transfer characteristics of the transistors suggested that the injection of holes was assisted by the deposited polymer film improving the hole injection efficiency. The TRM-SHG results well supported the results of the electrical measurements, and we anticipate that polypyrrole could be a potential candidate as an electrode material in polymer electronics.

Modeling of injection characteristics of polar and nonpolar III-nitride multiple quantum well structures

Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well (QW) light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW (MQW) active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.