Injection Diodes Research Articles

Evidence of environmental strains on charge injection in silole-based organic light-emitting diodes

Using density functional theory (DFT) computations, we have demonstrated a substantial skeletal relaxation when the structure of 2,5-[bis-(4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is optimized in the gas-phase comparing with the molecular structure determined from monocrystal x-ray diffraction. The origin of such a relaxation is explained by a strong environmental strains induced by the presence of anthracene entities. Moreover, the estimation of the frontier orbital levels showed that this structural relaxation affects mainly the LUMO that is lowered of $190\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in the gas phase. To check if these theoretical findings would be confirmed for thin films of BAS, we turned to ultraviolet photoemission spectroscopy and/or inverse photoemission spectroscopy and electro-optical measurements. Interestingly, the study of the current density or voltage and luminance or voltage characteristics of an $\mathrm{ITO}∕\mathrm{PEDOT}∕\mathrm{BAS}∕\mathrm{Au}$ device clearly demonstrated a very unusual temperature-dependent behavior. Using a thermally assisted tunnel transfer model, we found that this behavior likely originated from the variation of the electronic affinity of the silole derivative with the temperature. The thermal agitation relaxes the molecular strains in thin films as it is shown when passing from the crystalline to the gas phase. The relaxation of the intramolecular thus induces an increase of the electronic affinity and, as a consequence, the more efficient electron injection in organic light-emitting diodes.

Temperature-dependent current injection and lasing in T-shaped quantum-wire laser diodes with perpendicular p- and n-doping layers

The authors measured the temperature dependence of the lasing properties of current-injection T-shaped GaAs∕AlGaAs quantum-wire (T-wire) lasers with perpendicular p- and n-doping layers. The T-wire lasers with high-reflectivity coatings on both cleaved facets achieved continuous-wave single-mode operation between 5 and 110K. The lowest threshold current was 2.1mA at 100K. The temperature dependences of differential quantum efficiency and threshold current were attributed mainly to that of current-injection efficiency.

Study on Electron Injection of Phosphorescent Polymer Light-Emitting Diodes Utilizing CsF/Metal Cathode

The effect of electron injection in polymer light-emitting diodes based on the phosphorescent material of fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) devices with CsF/MgAg/Ag cathodes were investigated. The device with MgAg (1 nm)/CsF (3 nm)/Ag cathode showed almost similar current density-voltage-luminance characteristics to that with CsF (3 nm)/MgAg (1 nm)/Ag cathode. The existence of interface between the organic layer and Cs or CsF results in low turn on voltage. To achieve the efficient electron injection and the low turn on voltage, it is necessary to exist MgAg and CsF interface at the position of about 1 nm from organic layer. To achieve the efficient electron injection, it is necessary to exist Cs layer just on the organic layer. The electron injection effects between devices with CsF and Cs are different. It is suggested that the dipole of CsF affect the efficient electron injection.

Electrical spin injection in forward biased Schottky diodes based on InGaAs–GaAs quantum well heterostructures

The authors demonstrate efficient hole spin injection from a ferromagnetic metal (Ni) contact in a forward biased light emitting Schottky diode (LESD) fabricated on a GaAs based heterostructure with a quantum well (QW). The spin polarization of the injected holes was detected by measuring the circular polarization of the electroluminescence (EL) from the near surface InGaAs∕GaAs QW. An intermediate gold layer has been used in order to improve the spin injection efficiency. Over 40% degree of circular polarization of the EL has been observed at T=2K for the LESD structure with Au–Ni–Au Schottky contact.

Enhancement of electron injection in polymer light-emitting diodes with a supramolecular insulating nanolayer on the bottom cathode

With the aim of enhancing the electron injection of a polymer light-emitting diode (PLED), a supramolecular insulating nanolayer that reacts with the cathode (Ag) was inserted between the bottom cathode and the emitting material. The PLED threshold voltage (for 1cd∕m2) was found to decrease from 5.6to4.2V as a result of the introduction of the nanolayer. Synchrotron radiation photoelectron spectroscopy results show that inserting the supramolecular insulating nanolayer results in a decrease in the work function of about 0.7eV. This reduction lowers the electron injection barrier and produces an increase in the luminance.

Charge injection in polymer light-emitting diodes based on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)

In this letter, the electrical properties of polymer light-emitting diodes based on polyfluorene derivative, named poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)] (F8P), were studied. Indium tin oxide (ITO)/polymer/metal [magnesium (Mg) and calcium (Ca)] structures were used to study the effect of the barrier height on the charge injection at the polymer/metal (anode) interfaces. For ITO/F8P/Mg devices, strong temperature influence on the dc conductivity was observed in the forward direction, and afterwards, in the reverse direction, the conductivity was totally temperature independent. On the other hand, ITO/F8P/Ca devices presented temperature-dependent conductivities for both polarities.

Surface modification of indium tin oxide anodes by self-assembly monolayers: Effects on interfacial morphology and charge injection in organic light-emitting diodes

Three silane derivatives including dodecyltrichlorosilane (DDTS), phenyltriethoxysilane (PTES) and 3-aminopropyl-methyl-diethoxysilane (APMDS) were used to modify the indium tin oxide (ITO) surfaces. The effects of various terminal groups of the self-assembled monolayers (SAMs) on the growth behavior and interfacial morphologies of N, N′-di(naphthalene-1-yl)- N, N′-diphenylbenzidine (NPB) film deposited on the SAM-modified ITO were studied, as well as their effects on the performance of organic light-emitting diodes (OLED) devices. The results show that the growth behavior of NPB film over-deposited on the SAM-modified ITO is mainly determined by the wettability of the surface. The covering ability and thermal stability of NPB film on the SAM-modified ITO decrease in the order: bare ITO > ITO/PTES > ITO/APMDS > ITO/DDTS. However, the covering characteristic of NPB films on these substrates did not show direct relation to the transport of carriers across the anode/NPB interface as evaluated from the cyclic voltammogram and OLED performance. The turn-on voltages for these SMA-modified OLED devices increase in the order: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS. The enhancing effect of PTES on the hole injection is ascribed to the similar structure of PTES to NPB. On the contrary, the inhibition effect of APMDS is caused from the interaction of the lone-pair electrons of amine group to the transport carriers. Since these devices are known to be hole dominant, the luminance efficiency increase in a similar order as that for the turn-on voltage: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS.

Study of the behaviour of a DI diode under the influence of a surface magnetic field

Double injection diodes consisting of an electron injecting cathode (n+) and a hole injecting anode (p+) exhibit s-type negative differential resistance under proper conditions similar to a silicon controlled rectifier. These devices usually have high threshold voltages and high holding voltages. This work explores the modulation of threshold voltage, in particular, with a surface magnetic field. For these devices, the threshold voltage variation under the influence of magnetic field is investigated for different anode to cathode separations.

Enhanced electron injection in polymer light-emitting diodes: polyhedral oligomeric silsesquioxanes as dilute additives

By blending monochlorocyclohexyl- polyhedral oligomeric silsesquioxanes (MCC-POSS) into poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1.4-phenylenevinylene (MEH-PPV), enhanced luminous efficiency (LE, cd A−1) and brightness were observed in polymer light-emitting diodes (PLEDs) using Al as the cathode. PLEDs made from MEH-PPV with 0.5 wt.% of MCC-POSS exhibit LE of 1 cd A−1, four times higher than that observed in simple MEH-PPV devices. x-ray diffraction studies, measurements of photovoltaic response and measurements of photoconductivity from the films of pure MEH-PPV and MEH-PPV with MCC-POSS demonstrated that the improved device performance results from mixing of MCC-POSS with MEH-PPV. The enhanced electron-injection into the emissive layer results from the effect of the ionic conductivity introduced by the addition of MCC-POSS.

Improving carrier injection in organic diodes by incorporating charge trapping molecules

We demonstrate improved charge injection in organic diodes by incorporating charge trapping molecules near the injecting electrode that dynamically alter the effective Schottky energy barrier to carrier injection between a metal electrode and the organic electronic material. Hole injection from Al and Cu anodes into the electroluminescent polymer poly[2-methoxy,5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] was improved by incorporating C60 molecules into the polymer near the anode. In operation, electrons injected from the cathode are trapped by the C60 molecules, creating an induced dipole near the anode. We demonstrate these effects by measuring changes in diode current-voltage characteristics and built-in potentials.

Effects of CsF/Metal Interface on Electron Injection in Polymer Light-Emitting Diodes

The effect of electron injection in polymer light-emitting diodes has been investigated at the interface between the CsF layer and metals of the cathode. Efficient electron injection was achieved when the thickness of the CsF layer deposited on the organic layer was 1.5 nm. When the thickness of the MgAg layer deposited on the CsF layer was more than 1 nm, efficient electron injection was achieved. For the device with a 1.5-nm-thick CsF and a 1-nm-thick MgAg cathode, the deposition orders of CsF and MgAg were independent of efficient electron injection. That is, the coexistence of CsF and MgAg near the organic layer resulted in efficient electron injection. To achieve efficient electron injection, we found that was sufficient device performance was obtained when the surface of the organic layer with CsF was covered with approximately 25% MgAg.

Efficient hole injection in organic light-emitting diodes using C60 as a buffer layer for Al reflective anodes

The hole injection of the organic light-emitting diodes with Al as a reflective anode for top-emitting devices was improved by using C60 as a thin buffer layer between Al and a hole transport layer. The driving voltage of the devices with C60 buffer layer was 5.5V compared with 11V for the devices without C60 buffer layer. The decrease of interfacial energy barrier by interface dipole formation between Al and C60 contributed to the low driving voltage of the devices.

Bipolar carrier transport in a conjugated polymer by complex admittance spectroscopy

We report the bipolar transport properties of the LUMATION™ (Sumitomo Chemical) 1300 Series green-emitting polymer investigated by means of admittance spectroscopy. Analysis of the inductive response in single-carrier polymer diodes yields electron and hole mobilities which are in excellent agreement with the results of independent measurements. Admittance measurements in dual injection diodes, in combination with the analysis of current-voltage characteristics, provide evidence that the dual injection diodes operate in space-charge-limited regime, indicative of strong recombination within the material. Our results provide strong evidence that the space-charge-related admittance response of dual-carrier diodes is dominated by combined electron-hole response, which corresponds to the sum of electron and hole mobilities. This implies that electron and hole mobilities cannot be obtained separately from admittance measurements in space-charge-limited dual-carrier devices.

Influence of dehydrated nanotubed titanic acid on polymer light-emitting diodes with phosphorescent dye

In this letter, we demonstrate that hole injection and transport in polymer light-emitting diodes with phosphorescent dye Ir(ppy)3 can be significantly enhanced by doping p-type conductive dehydrated nanotubed titanic acid into poly(vinylcarbazole) (PVK) films at 2wt.%. At the same time, both energy transfer and exciton recombination efficiency are improved because of the open and straight conformation of the PVK molecule in the nanocomposite. The performance of these devices was greatly improved, showing higher luminance, enhanced efficiency, and a lower turn-on voltage.

Effect of inorganic nanolayers on electron injection in polymer light-emitting diodes

We report the effect of inorganic nanolayers on electron injection in the polymer light-emitting diodes (PLEDs) in which a hole is the major charge carrier. The inorganic nanolayers with different dielectric constants were placed between the emitting layer and the aluminum cathode, and their influence on the device performance was investigated. The device with a nanolayer of lower dielectric constant demonstrated higher efficiency, which is significantly higher than that of one without an insulating layer. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing electron-tunneling probability, which improve the balanced injection of hole and electrons.

Improved Electron Injection on Organic Light-emitting Diodes with an Organic Electron Injection Layer

To overcome of poor electron injection in organic light-emitting diodes (OLEDs) with Al cathode, a thin layer of inorganic insulating materials, like as LiF, is inserted between an Al cathode and an organic electron transport layer. Though the device, mentioned above, improves both turn on voltage and luminescent properties, it has some problems like as thickness restriction, less than 2 nm, and difficulty of deposition control. On the other hand, Li organic complex, Liq, is less thickness restrictive and easy to deposit and it also enhances the performance of devices. This paper reports the improved electron injection on OLEDs with another I A group metal complex, Potassium quinolate (Kq), as an electron injection material. OLEDs with organic complexes showed improved turn-on voltage and luminous efficiency which are remarkably improved compared to OLEDs with Al cathode. Especially, OLEDs with Kq have longer life time than OLEDs with Liq.

Designing Interfaces That Function to Facilitate Charge Injection in Organic Light-Emitting Diodes

Layer-by-layer (LbL) assembly of triarylamine (TAA)-containing polymers has been applied for anode functionalizations in organic light-emitting diodes (OLEDs). Surface work function of the ITO electrodes was significantly altered with the functionalizations, and the values changed depending on electron affinity of the substituents (X) on the TAA units. When the functionalized ITO electrodes were utilized for the conventional TPD/Alq OLED, the multilayers of P1 (X = 4-OMe) and P2 (X = none) were found to promote better energy matching at the ITO/TPD interface to reduce the hole injection barrier. Furthermore, the multilayers having heterodeposited structure of several TAA polymers provided stepped and graded electronic profiles to facilitate hole mobility from ITO to TPD, so that the resulting OLED devices can exhibit appreciably reduced turn-on voltage and higher luminous intensities.

Extremely low-voltage driving of organic light-emitting diodes with a Cs-doped phenyldipyrenylphosphine oxide layer as an electron-injection layer

We demonstrated efficient electron injection and transport in organic light-emitting diodes using an electron-transport layer (ETL) composed of a Cs and phenyldipyrenylphosphine oxide (POPy2) co-deposited layer. In particular, an ETL composed of a Cs:POPy2 layer with an atom:molar ratio of 1:2 demonstrated an extremely low driving voltage, resulting in a high current density of 100mA∕cm2 at an applied voltage of only 3.9 V. The results of Kelvin probe and absorption measurements indicated that the formation of a CsAl alloy layer at the Cs:POPy2/Al cathode interface and the charge-transfer complex between the Cs and POPy2 contributed to enhancing the efficiency of electron injection and transport, respectively.

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.

Charge injection and transport model in organic light-emitting diodes with aluminum cathodes prepared by ion beam assisted deposition

We have fabricated highly stable organic electroluminescent devices based on spin-coated poly- p-phenylene-vynylene (PPV) thin films. The electrical properties of aluminum cathode, prepared by ion beam assisted deposition, on PPV have been investigated and compared to those by thermal evaporation. Although energetic particles of Al assisted by Ar + ion may damage the organic material, I– V– L characteristics are improved by applying thin Al buffer layer. In addition, a dense Al cathode inhibits the permeation of H 2O and O 2 into PPV film through pinhole defects, and thus retards dark spot growth. It may be deduced from highly packed structure of Al cathode with smaller junction resistance between Al and PPV. In conclusion, the lifetime of organic light-emitting device (OLED) has been extended effectively by dense Al film through ion beam assisted deposition process.