Hole Injection Characteristics Research Articles

Evolution Between Exciton and Exciplex Emission in Planar Heterojunction OLEDs with Different Hole-Injection Characteristics

Planar heterojunction-based organic light-emitting diodes (PHJ OLEDs) usually have simultaneous exciton and exciplex emissions and understanding their evolution processes is crucial for designing high-performance white OLEDs, but relevant evolution mechanisms are still unclear. Here, unreported competitions between exciton and exciplex emissions and an undiscovered energy-transfer channel from triplet exciton to triplet exciplex states in PHJ OLEDs with different hole-injection abilities are investigated by separately measuring their current-dependent electroluminescence (EL) spectra and magneto-EL (MEL) traces at different temperatures. Interestingly, the ratio of emission intensity between exciton and exciplex states ($R={I}_{\mathrm{exciton}}/{I}_{\mathrm{exciplex}}$) from the device with a good hole-injection ability rises with increasing bias current at each temperature, whereas that from the device with a poor hole-injection ability hardly changes. In addition, R from the devices with good and poor hole-injection abilities show nonmonotonic variation and monotonic reduction with decreasing operational temperature at each bias current, respectively. These various current- and temperature-dependent competitions between exciton and exciplex emissions are attributed to distinct current- and temperature-dependent electron-tunneling effects occurring at organic heterojunction interfaces of devices with different hole-injection abilities. More intriguingly, using MEL as a fingerprint detection tool, we find there is a Dexter energy-transfer (DET) channel from the triplet exciton to triplet exciplex (${T}_{1}\ensuremath{\rightarrow}{\mathrm{EX}}_{3}$) state and the DET can enhance the reverse intersystem crossing (RISC) process from triplet to singlet exciplexes (${\mathrm{EX}}_{3}\ensuremath{\rightarrow}{\mathrm{EX}}_{1}$), which cannot be found using the conventional probing tool, EL spectra. Because the numbers of ${T}_{1}$ excitons in these devices have different current and temperature dependencies, various current- and temperature-dependent DET and RISC processes happen, which cause abundant MEL behaviors. Obviously, this work deepens the physical understanding of the competition and DET processes between exciton and exciplex states in PHJ OLEDs.

Numerical Investigation on Hole-Injection Characteristics of NiO/SiC Heterojunction

Numerical investigation on hole-injection characteristics of NiO/SiC heterojunction is carried out in this paper. Theory analysis and numerical simulation both indicate the excellent hole-injection characteristic of p-NiO/n-SiC heterojunction. The pn junction diode and pnp phototransistor are constructed and simulated to evaluate hole-injection characteristics p-NiO/n-SiC heterojunction. The results indicate that the p-NiO/n-SiC heterojunction shows great potential advantage in enhancing current gain of pnp phototransistor. By using NiO/SiC heterojunction as the emitter junction, the current gain of SiC based pnp phototransistor can be increased by about 96.3 times.

Improvement of hole injection characteristics in wet-processed organic field-effect transistor based on oxidation of silver electrode surface

Source–drain electrodes for organic field-effect transistors (OFETs) were fabricated with Ag nanoink on a Si wafer with a 300 nm thick oxide layer using the repellent patterning method, and a depth-profile analysis of the composition and physical properties of the electrode surface was performed once the electrode was oxidized by UV–ozone irradiation. Additionally, OFETs with a wet-processed electrode and 9,10-diphenylanthracene layer were fabricated, and their electrical characteristics were measured. The chemical composition of the Ag electrode surface changed to silver oxide (Ag2O or AgO) due to the longer oxidation treatment time, and the work function value increased. In the OFET with the electrode oxidized for 600 s, increased drain current ∣I D∣ was observed around a gate voltage of 0 V. Furthermore, good OFET characteristics were obtained [maximum ∣I D∣ = 326.2 μA, mobility μ = 0.91 cm2V −1·s−1], which were similar to those of the OFETs manufactured using a dry process.

Enhanced Hole Injection Characteristics of a Top Emission Organic Light-Emitting Diode with Pure Aluminum Anode.

A top emitting organic light-emitting diode (OLED) device with pure aluminum (Al) anode for high-resolution microdisplays was proposed and fabricated. The low work function of the Al anode, even with a native oxide formed on the Al anode surface, increases the energy barrier of the interface between the anode and hole injection layer, and has poor hole-injection properties, which causes the low efficiency of the device. To enhance the hole-injection characteristics of the Al anode, we applied hexaazatriphenylene hexacarbonitrile (HATCN) as the hole-injection layer material. The proposed OLED device with a pure Al anode and native oxide on the anode surface improved efficiency by up to 35 cd/A at 1000 nit, which is 78% of the level of normal OLEDs with indium tin oxide (ITO) anode.

Low-temperature and solution-processable inorganic hole injection layer for flexible quantum-dot light-emitting diodes

A low-temperature solution-processable inorganic vanadium oxide (V2O5) hole injection layer (HIL) was synthesized for flexible quantum-dot light-emitting diodes (QLEDs). Efficient hole injection characteristics were observed in the hole-only devices; furthermore, the process temperature of V2O5 was as low as 30 °C. We investigated the source of the efficient hole injection behavior using ultraviolet and x-ray photoelectron spectroscopy. The density of gap states was found to increase in accordance with process temperature reduction. Therefore, QLEDs with low-temperature solution-processable V2O5 HILs were fabricated on a glass substrate, which showed excellent characteristics. The maximum luminance and luminous efficiency of the device were 56,717 Cd/m2 and 4.03 Cd/A, respectively. Due to the low-temperature process of the V2O5 HIL, it was also possible to fabricate QLEDs on a flexible plastic substrate without mechanical or thermal deformation of the substrate. Our results suggest that the low-temperature V2O5 inorganic HIL is a feasible alternative to organic HILs for flexible QLEDs.

Interfacial electronic structure between a W-doped In2O3 transparent electrode and a V2O5 hole injection layer for inorganic quantum-dot light-emitting diodes.

The interfacial electronic structure between a W-doped In2O3 (IWO) transparent electrode and a V2O5 hole injection layer (HIL) has been investigated using ultraviolet photoelectron spectroscopy for high-performance and inorganic quantum-dot light-emitting diodes (QLEDs). Based on the interfacial electronic structure measurements, we found gap states in a V2O5 HIL at 1.0 eV below the Fermi level. Holes can be efficiently injected from the IWO electrode into poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(4-sec-butylphenyl)diphenylamine)] (TFB) through the gap states of V2O5, which was confirmed by the hole injection characteristics of a hole-only device. Therefore, conventional normal-structured QLEDs were fabricated on a glass substrate with the IWO transparent electrode and V2O5 HIL. The maximum luminance of the device was measured as 9443.5 cd m−2. Our result suggests that the IWO electrode and V2O5 HIL are a good combination for developing high-performance and inorganic QLEDs.

Revealing the influence of hole injection material's molecular orientation on OLED's performance

Abstract In this study, we introduced N,N′-bis(4-trifluoromethoxybenzyl) naphthalene-1,4,5,8-tetracarboxylic acid di-imide (NDI-BOCF3) and N,N′-Bis(4- trifluoromethoxyphenyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI- POCF3) as hole injection layers (HILs) in organic light emitting diodes (OLEDs) and investigated the relationship between HIL's molecular orientation and the hole injection characteristics. NDI-BOCF3 and NDI-POCF3 have similar optical and electrochemical properties; however, it is quite surprising that both of these materials demonstrated totally different function when used as HILs in OLEDs. NDI-BOCF3 is observed to enhance the hole injection, in contrast, NDI-POCF3 hinders hole injection. X-ray Diffraction (XRD), Electrostatic Surface Potential (ESP) calculations and Ultraviolet Photoelectron Spectrometry (UPS) were applied to explore the mechanism of NDI-BOCF3 and NDI-POCF3 as HILs. By comparing molecular orientation and electrostatic potential distribution of these two materials, we identified that the change in anode's work function caused by different molecular orientation of HILs results in completely different hole injection properties.

Polarization-Engineered High-Efficiency GaInN Light-Emitting Diodes Optimized by Genetic Algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.

Anomalous hole injection deterioration of organic light-emitting diodes with a manganese phthalocyanine layer

Metal phthalocyanines (MPcs) are well known as an efficient hole injection layer (HIL) in organic devices. They possess a low ionization energy, and so the low-lying highest occupied molecular orbital (HOMO) gives a small hole injection barrier from an anode in organic light-emitting diodes. However, in this study, we show that the hole injection characteristics of MPc are not only determined by the HOMO position but also significantly affected by the wave function distribution of the HOMO. We show that even with the HOMO level of a manganese phthalocyanine (MnPc) HIL located between the Fermi level of an indium tin oxide anode and the HOMO level of a N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine hole transport layer the device performance with the MnPc HIL is rather deteriorated. This anomalous hole injection deterioration is due to the contracted HOMO wave function, which leads to small intermolecular electronic coupling. The origin of this contraction is the significant contribution of the Mn d-orbital to the MnPc HOMO.

Interface Control by Surface-Initiated Deposition Polymerization and Its Application to Organic Light-Emitting Devices

Surface-initiated deposition polymerization was applied to the preparation of a hole transport layer (HTL) on an indium–tin oxide (ITO) electrode. A silane-based self-assembled monolayer (SAM) that has an azo terminal group was prepared on an ITO surface on which a vinyl monomer of carbazole (CPA) or tetraphenyldiaminobiphenyl (vTPD) was deposited by physical vapor deposition. The polymerization of the HTL was achieved by electron-assisted deposition. UV irradiation, as well as conventional evaporation, was not capable of growing polymer films on the SAM. Organic light-emitting diodes (OLEDs) were prepared by depositing tris(8-quinolinolato) aluminum (Alq3) on the HTL. The surface-initiated deposition polymerization was effective in markedly increasing device current flow and reducing turn-on voltage. However, luminescence efficiency was not improved by this method owing to improper carrier balance under excessive hole injection. Nevertheless, surface-initiated deposition polymerization was effective in improving film morphology, stability, and hole injection characteristics.

Hole-injection from a polar mono-molecular layer derivatised ultra-thin gold electrode into a triphenylamine derivative

This Letter, reports the hole-injection characteristics of an ultra-thin, transparent gold electrode derivatized with a thiolate polar monolayer into the amorphous molecular semiconductor, N, N′-bis(3-methylphenyl)- N, N′diphenyl-1,1′biphenyl- 4,4′-diamine (TPD). Thermionic emission with image force lowering gives an excellent description of the injection mechanism over a large range of temperatures and electric field strengths, yielding a zero field Schottky barrier height of ∼0.4 eV. This study demonstrates that derivatizing the surface of this novel gold electrode with a 4-nitrophenylthiolate monomolecular layer is an effective means of circumventing the abrupt negative vacuum level shift, which forms when TPD is thermally deposited onto gold electrodes.

High-Efficiency Carrier Injection Characteristics of Dixanthene Derivatives in Organic Light-Emitting Diodes

We demonstrate that benzo[1,2,3-kl:4,5,6-k'l']dixanthene (BDX) derivatives show high-efficiency carrier injection in organic light-emitting diodes (OLEDs). Using 3,11-dibromobenzo[1,2,3-kl:4,5,6-k'l']dixanthene (BDX6) as a hole injection layer (HIL), we achieved a low driving voltage of 8.12±0.10 V in obtaining a current density of J=100 mA/cm2. The hole injection characteristics were superior to those of a device with a conventional CuPc layer as a HIL. On the other hand, inserting benzo[1,2,3-kl:4,5,6-k'l']dixanthene (BDX1) as an electron injection layer (EIL) efficiently decreased driving voltage, indicating a superior electron injection capability compared with a device without a BDX1 EIL. We show that carrier injection efficiency can be well controlled by the substituent groups of BDX, thus providing efficient hole and electron injection layers.

Interfacial charge transfer and hole injection in α-NPD organic overlayer–CVD diamond substrate system

We have investigated interfacial charge transfers and hole injection characteristics between oxygen-terminated boron-doped homoepitaxial (100) chemical-vapor-deposited (CVD) diamond films and thermally deposited N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4′′diamine (α-NPD) mainly using X-ray photoelectron spectroscopy (XPS), Hall effect measurements and current–voltage (J–V) characteristic measurements. On one hand, no chemical shift was observed for C 1s and N 1s XPS spectra taken from the α-NPD overlayer–CVD diamond substrate system. On the other hand, the low-temperature hole concentration of the diamond substrate decreased by ≈10–17% after the α-NPD depositions, verifying that small but substantial amounts of holes were transferred from the diamond substrates to the α-NPD films. However, compared with those obtained from the specimens before the α-NPD depositions, the sheet electrical conductivities of the specimens after the α-NPD depositions increased at T ≥ 230K while decreasing at T ≤ 200K. In addition, J–V data measured from layered α-NPD–diamond hetero-structures show evidence that the junction worked as a diode. These facts suggest that the α-NPD–CVD diamond system may be useful for a transparent anode electrode with a hole transport layer appropriate for organic light emitting diodes.

Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current–voltage characteristics

The current–voltage characteristics I(V) of model organic devices are studied under ultra-high-vacuum conditions. Active materials are N,N′-bis-(1-naphthyl)-N,N′-diphen-yl1-1,1-biphenyl1-4,4′-diamine (α-NPD) and pentacene, electrode materials are polycrystalline Au and the conductive polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Despite a similar work function of electrode material surfaces (∼5 eV), hole injection from PEDOT/PSS is significantly more efficient than from Au, due to a smaller hole injection barrier. Hole injection characteristics from Au electrodes for devices made from α-NPD are independent of deposition sequence and substrate used. Pentacene devices exhibit serious asymmetries in that respect. These are caused by a strong dependence of morphology and preferred molecular orientation on the substrate for the crystalline material.

Analysis of dual-gate LIGBT with gradual hole injection

The device characteristics of a gradual hole injection dual-gate lateral insulated gate bipolar transistor (GHI-LIGBT) is analyzed with the experiment and two-dimensional (2-D) numerical simulation. The hole injection mechanism in the GHI-LIGBT is investigated in detail and the device characteristics of the GHI-LIGBT are studied in terms of the various device parameters such as the p/sup +/ injector length and the injection gate length. The hole injection characteristics and latch-up characteristics in the GHI-LIGBT are also presented as a function of temperature. The experimental results demonstrate that an on-state voltage drop in GHI-LIGBT is reduced by 1 V, compared with the conventional SA-LIGBT and the negative resistance regime is eliminated since the injection of holes is initiated at the lower anode voltage. It is also found that the low on-state voltage drop in the GHI-LIGBT can be achieved without sacrificing the inherent high switching speed in shorted anode (SA) LIGBT structures.

Polymer-based light-emitting devices: investigations on the role of the indium—tin oxide (ITO) electrode

We have studied the cleaning procedure dependence of the chemical composition and work function for different indium—tin oxide (ITO) samples using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Also, the surface morphology of ITO was investigated by atomic force microscopy (AFM). Despite the pronounced differences in surface morphology and the In Sn ratio, the variation of the work function between different ITO samples was very small after each cleaning procedure. The work functions of ITO samples cleaned with organic solvents and hydrogen peroxide were 4.4–4.5 and 4.7–4.8 eV, respectively. Ne-ion sputtering preferentially removed oxygen, which resulted in a partial reduction of the surface and a lowering of the work function to 4.0–4.1 eV. Used as hole-injecting electrodes in organic and polymer light-emitting devices (LEDs), different ITOs resulted in pronounced differences in the LED performances despite their almost identical work functions after the hydrogen peroxide treatment. This reveals that the work function of the ITO is not the only factor to determine the hole-injection characteristics in polymer-based LEDs.

Space-charge-limited charge injection from indium tin oxide into a starburst amine and its implications for organic light-emitting diodes

We have investigated the hole-injection characteristics from indium tin–oxide (ITO) into 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA) and have measured the hole-carrier drift mobility of this compound in single-layer ITO/m-MTDATA/Au structures. We have found that ITO is able to provide trap-free space-charge-limited currents over a wide range of film thicknesses and have established unambiguously that the ITO/m-MTDATA is an ideal Ohmic contact at high electric fields. Our observations clarify the role of m-MTDATA as a voltage-lowering hole-injecting buffer layer in organic light-emitting diodes.

Electron and hole injection by a metal-depletion layer contact

The injection of electrons and holes by various metal probes into the depletion region of a beveled silicon p- n diode was experimentally studied. The electrical and thermal properties of metal pressure contacts are presented and used to identify the nature of the injection mechanism. The characteristics of electron injection are shown to be of the space-charge-limited type and the hole injection characteristics follow a high field process. A potential energy diagram of the metal-depletion layer contact is presented and appears to be in agreement with the observed injection phenomena.