Properties Of Organic Light-emitting Diodes Research Articles

Electro-Optic Properties of High-Efficiency Organic Electronics with the Addition of An Interlayer

This study investigated the electro-optic properties of organic light-emitting diode (OLED) devices with the addition of an interlayer. A device with the perfluorododecyl-1H,1H,2H,2H-triethoxysilane–perfluorotetradecyl 1H,1H,2H,2H-triethoxysilane mixture (hybrid solution) membrane applied to the hole injection layer (HIL) using a self-assembled monolayer (SAM), along with N,N’-Bis (naphtha-len-1-yl)-N,N’-bis(phenyl)-2,2’-dimethylbenzidine (α-NPD) applied to the hole transport layer (HTL), was fabricated and subsequently assessed. Compared to the device without an HIL, the device with HIL using SAM showed a luminance of 1630.16 (Cd/m2) at the maximum cut-off voltage of 11 V, and the efficiency was increased to ≥200% from the initial turn-on voltage of 3 V to the maximum cut-off voltage. These results suggest that adding an interlayer and the consequent carrier ladder effect led to more efficient injection and transport, possibly lowering the hole injection barrier and allowing excellent power efficiency.

Organic light-emitting diodes with new dyes based on coumarin

The results of studying the luminescent properties of organic light-emitting diodes based on new luminescent compounds containing a coumarin fragment are presented. The light-emitting diodes were fabricated by spin-coating and thermal evaporation in an argon atmosphere in a clean room. Measurements of the LED characteristics were carried out by optical spectroscopy, as well as by electrical methods. It has been experimentally shown that deposition of an OLED active layer based on luminescent compounds containing a coumarin core can lead to the formation of dimers, the luminescence spectra of which differ significantly from the corresponding spectra of the original materials in toluene. A variation in the structure of the compound leads to a change in both the current-voltage characteristics of the resulting device and the luminescence spectra. These changes appeared due to the difference in the electronic structure of these materials as well as due to different values of charge carrier mobilities and the potential barriers at the heterointerface with other OLED layers. The results obtained may serve as the basis for systematizing knowledge about the dependence of the properties of new luminescent materials, which include a coumarin core, on their structure. The developed structures can become prototypes for industrially produced light-emitting devices that specifically emit white light.

Solvent Effect on the Electrical and Structural Properties for MEH-PPV Organic Light Emitting Diodes (OLED)

In this paper, the performance of the electrical properties (J-V) and surface roughness of MEH-PPV based organic light-emitting diodes (OLED) towards solvent effect was investigated. The MEH-PPV layer was deposited using spin coating technique at fixed spun speed of 3000 rpm. Two different solvents, toluene and chloroform (CHCl3) and mixture toluene:CHCl3 with the ratio of 4:1 and 1:4 were used to dilute MEH-PPV at fixed concentration of 5 mgml-1. Apparently, the mixture of solvent makes the surface roughness of the MEH-PPV films reduced to 0.15 nm and 3.59 nm under the ratio 4:1 and 1:4 respectively. Besides, the mixture solvents makes the value of turn on voltage was dropped to ∼7.2 V and ∼9 V respectively compared to non-mixture solvent. The combination of different solvent apparently gives an effect on the electrical and structural properties of organic light-emitting diode.

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes.

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.

Change in Interface Characteristics of ITO Modified with n-decyltrimethoxysilane

Herein, we examined changes in the interfacial properties of organic light-emitting diodes when n-decyltrimethoxysilane (CH3SAM) was deposited on the surface of an indium tin oxide (ITO) electrode for various deposition times. It was revealed that the interfacial properties varied with deposition time. As the latter increased, so did the measured value of the contact angle, and ITO substrate exhibited a lower wettability. The contact angle measurements for bare ITO at 1, 10, 30, and 90 min were 57.41°, 63.43°, 73.76°, 81.47°, respectively, and the highest value obtained was 93.34°. In addition, the average roughness and work function of the ITO were measured using atomic force microscopy and X-ray photoelectron spectroscopy. As the deposition time of CH3SAM on the ITO substrates increased, it was evident that the former was well aligned with the latter, improving surface modification. The work function of CH3SAM, modified on the ITO substrates, improved by approximately 0.11 eV from 5.05–5.16 eV. The introduction of CH3SAM to the ITO revealed the ease of adjustment of the characteristics of ITO substrates.

Control of the emission properties of organic light emitting diodes using plasmonic gratings of Ag nanoparticles

We report the influence of Ag nanoparticle arrays on the emission properties of organic light emitting diodes (OLED). First, we studied the features of Ag arrays related to the periodicity. The obtained results emphasize the existence of two coupling regimes dominated by near or far field effects, respectively. Thereafter, we studied the influence of these gratings on the performances of OLED devices. Our results reveal the benefit of the plasmonic structures which allow a precise tuning of the OLED properties.

Bright Deep Blue TADF OLEDs: The Role of Triphenylphosphine Oxide in NPB/TPBi:PPh3O Exciplex Emission

AbstractVery bright (≈14 000 cd m−2) deep blue exciplex organic light emitting diodes (OLEDs) peaking at ≈435 nm, where the photopic response is ≈0.033, and with CIE color coordinates (0.1525, 0.0820), are described. The OLED properties are interestingly linked to PPh3O (triphenylphosphine oxide) and attributes of the emitting layer (EML) comprising NPB interfacing host:guest TPBi:PPh3O 5:1 weight ratio. A neat PPh3O layer that is central for device performance follows the EML (NPB/TPBi:PPh3O 5:1/PPh3O). The bright electroluminescence originates from NPB/TPBi:PPh3O exciplexes involving triplets via thermally activated delayed fluorescence, as evident from the strong quenching of the photoluminescence (PL) by oxygen and interestingly, the monomolecular emission process. The transient PL decay times of a NPB/TPBi:PPh3O 5:1/PPh3O film are 43 ns in air versus 136, 610, and weak ≈2000 ns in N2. For comparison, the respective PL decay times of films of NPB:TPBi are 16 ns in air versus 131 and 600 ns in N2, and of NPB:PPh3O they are 29 ns in air versus 56, 483, and weak ≈2000 ns in N2. It is suspected that slow emitting states are associated with a PPh3O aggregate interacting with NPB.

Electrical and optical properties of organic light-emitting diodes with rhenium(I) complexes using DC and AC methods

Four rhenium(I) tricarbonyl complexes with 1,10-phenanthroline and derivative bearing electron-donating CH3 and OCH3 substituents were doped into host material poly (9-vinylcarbazole) (PVK) with a mass ratio of 8% as the emissive layer in organic light-emitting diodes (OLEDs). All complexes showed photoluminescence in the solution and embedded into a PVK matrix (λmax∼520–550 nm). The comparison with the bare PVK emission and the compound/PVK film shows that the emission of the polymer was quenched through an energy transfer process from PVK to the dopant. The electrical properties of the devices with FTO/PEDOT:PSS/Complex:PVK/Al architecture were investigated using the DC method by curves of current density-voltage and the AC method as admittance spectroscopy, which showed that the behavior of the devices is controlled by charge carrier injection rather than bulk transport.

The effect of thickness of light emitting layer on physical properties of OLED devices

In this work, we performed to study the effect of thickness (35, 55, 75 and 95 nm) of the light emitted layer (EML) on physical properties of organic light-emitting diode (OLED) devices. We used indium tin oxide (ITO) for the anode in OLED. The ITO substrates were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and optical absorption (UV–vis). X-ray diffraction studies showed that the ITO films were polycrystalline with a hexagonal or wurtzite structure of indium oxide In2O3. Atomic force microscopy analysis demonstrated that the value of surface roughness ITO is an acceptable value for the proper transfer of the carriers from the boundary of the layers. Optical transmittance spectra of the ITO film showed high transparency of about ∼94% in the visible region.After the characterization of ITO, the OLEDs with the structure of Glass/ITO/ PEDOT:PSS/Alq3/Al were fabricated. Then the current density–voltage, resistance-voltage, power-voltage, and irradiance-voltage of the OLEDs were characterized. The results showed that decreasing EML thickness leads to a reduction of turn-on voltage, resistance, and power. The peak emission intensity of 55 nm thickness EML was stronger and sharper of others.

Integration of Optical and Thermal Models for Organic Light-Emitting Diodes

This paper proposes a three-dimensional model for combinative analysis of the illuminative and thermal properties of organic light-emitting diodes (OLED). By means of the energy conversion ratio and energy conservation theory, two individual optical and thermal sub-models are integrated to form a single model constructed in a multi-physics platform. According to the measured luminous performance and temperature distribution of the fabricated OLED samples, the proposed model demonstrates sufficient accuracy. Moreover, the temperature distribution on the cross-section of the OLED can be derived from the proposed model and used as a valuable reference for manufacturers to select appropriate organic materials.

Larger VH (Hole Distribution Volume)/VM (Molecular Volume) Induced Higher Charge Mobility of Group IVA Element-Based Host Materials for Potentially Highly Efficient Blue OLEDs

Host materials have a decisive effect on the optoelectronic properties of organic light-emitting diodes (OLEDs), whether for fluorescent, phosphorescent, or thermally activated delayed fluorescence...

Photoelectric Properties of OLEDs With and Without Fluorinated Self-Assembled Monolayer and by Varying Thickness of α-NPD

In this study, we investigated the photoelectric effect and optimization of an organic light-emitting diode (OLED) depending on the presence or absence of a fluorinated self-assembled monolayer (FLSAM) and by varying the thickness of N,N'-Di (1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) from 0 nm to 50 nm. The large distinction in electronegativity between the carbon and the fluorine replacing hydrogen in the alkyl chain of FLSAM generates a strong dipole moment to elevate the vacuum level, resulting in a change of the work function. This eliminates the injection barrier between the work function of the ITO modified by FLSAM and the highest occupied molecular orbital (HOMO) level of the hole-transport layer, thus leading to excellent driving voltage characteristics. Devices without FLSAM had a driving voltage more than twice that of devices using with FLSAM. The introduction of α-NPD as the hole-transport layer enhanced the electrical conductivity by facilitating the transport of holes. However, due to the inherent insulating film properties of α-NPD, the increase in its thickness resulted in a decrease in current density.

Improving organic light-emitting diode performance with ZnO nanoparticles

The effect of ZnO nanoparticles as an electron transport layer (ETL) on the electronic properties of organic light-emitting diodes (OLEDs) was experimentally investigated in this study. OLED was fabricated with the structure of ITO/PEDOT:PSS/PVK/Eu/ZnO nanoparticle/PBD/Al, and then loaded with different amounts on ZnO nanoparticles. The scanning electron microscope field effect, optical absorption, luminescence, and current density–voltage of the OLEDs were characterized. Results showed that using ZnO nanoparticles as an ETL enhanced the performance and efficiency of the OLEDs. Moreover, increasing the loading amount of the ZnO nanoparticles increased luminescence and transparency and decreased work voltage. The characterization results showed that adding layers of ZnO nanoparticles changed the work voltage from 12.2 to 10 V, maximum luminescence from 0.19 to 0.23, and transparency from 60 to 86%.

Electroluminescence from the Microphase-separated Structure of Blended Films with a Light-emitting Polymer and an Ionic Liquid

The structures of blended films composed of a light-emitting polymer and an ionic liquid were investigated, and the relationship between the emission properties of organic light-emitting diodes (OL...

Synthesis and OLED Properties of Zinc Complexes Based on Quinaldic Acid

Metal-chelate materials based on polycyclic aromatic ligands are attractive for tuning emission colors of organic light-emitting diodes (OLED). In this work, quinaldic acid (QA) and 8-hydroxyquinoline (HQ) were employed as organic ligands and the corresponding zinc complexes (Zn(HQA), Zn(QA), Zn(HQ)) were synthesized. The structures of Zn(HQA), Zn(QA), and Zn(HQ) were determined by 1H-NMR, 13C-NMR, FT-IR, UV-Vis and XPS. Zinc complexes showed thermal stability up to 350°C under nitrogen flow by TGA. The photoluminescence (PL) was measured from DMSO solution. The Zn(HQA) emitted a yellow light around 550 nm, while Zn(HQ) emitted a green light around 500 nm. Electroluminescence (EL) device having the structure of ITO/NPB/Zn(HQA)/Liq/Al was fabricated. The device emitted a yellow light around 550 nm. The maximum luminance of 190 cd/m2 and current of 50 mA/cm2 were observed at 15 V in the Zn(HQA)-containing device.

Light-emitting transistor structures based on semiconducting polymers and inorganic nanoparticles

One of the new directions in organic electronics is the development of light-emitting organic field-effect transistors, which combine the light-emitting properties of organic light-emitting diodes and the switching properties of organic field-effect transistors. Optical and electronic properties of novel nanocomposite materials based on semiconducting polymers and inorganic nanoparticles and designed for applications in organic electronics devices were investigated. Light-emitting organic field-effect transistors with composite active layers based on the soluble semiconducting polymers PFO and MEH-PPV and ZnO nanoparticles and having asymmetric electrodes (Al and Au) that inject electrons into ZnO and holes into PFO and MEH-PPV were prepared and investigated. The data are interpreted in the context of the possibility of organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO composite films to work in both the unipolar regime and the ambipolar regime. It is shown that the mobility of charge carriers in light-emitting organic field-effect transistors based on PFO: ZnO at 300 K reaches ∼0.02 for electrons and ∼0.03 cm2/(V s) for holes, increasing with an increase in the concentration of nanoparticles up to ∼2 cm2/(V s), a value that is comparable to the maximum mobility values for conducting polymers. Light-emitting organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO emit light in the green and orange ranges of the optical spectrum, respectively, their electroluminescence intensities rising with an increase in either the negative bias or the positive bias at the source-drain and the gate as well as with an increase in the concentration of ZnO nanoparticles. The results indicate that light-emitting organic field-effect transistors based on soluble conjugated polymers and semiconducting ZnO nanoparticles are examples of multifunctional devices whose production technology is compatible with the modern ink-jet printing technology of organic electronics.

Influence of host matrix on thermally-activated delayed fluorescence: Effects on emission lifetime, photoluminescence quantum yield, and device performance

The influence of the host molecules on the photoluminescent (PL) and electroluminescent (EL) properties of organic light-emitting diode (OLED) emitters showing efficient thermally-activated delayed fluorescence (TADF) has yet to be investigated in detail. Here we demonstrate that the choice of host can cause large variations in the PL quantum yield (ΦPL∼15–70%) and delayed PL transient decay (τdel∼2–70ms) of a spiro-acridine-based TADF guest. We show that the effect of exciplex formation on ΦPL must be considered even at low concentrations of the TADF guest. Using the same TADF guest but changing the host layer, we are able to greatly vary the PL transient decay time from ∼4 to ∼70ms while maintaining a high ΦPL ∼70%, which can lead to new applications. Detailed spectral characterization during PL decay reveals a gradually increasing singlet–triplet energy gap (ΔEST) as the origin of these observations. The time-varying ΔEST is explained based on dipole interactions between the host and guest molecules. Finally, PL and electrical considerations for host selection are discussed based on the performance of OLED devices.

Electroluminescent Properties of Organic Light-emitting Diodes with Hole-injection Layer of CuPc

Copyright 2014 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. pISSN: 1229-7607 eISSN: 2092-7592 DOI: http://dx.doi.org/10.4313/TEEM.2014.15.1.41 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS Vol. 15, No. 1, pp. 41-44, February 25, 2014

Enhancement of outcoupling efficiency of organic light-emitting diodes using a planarized moth-eye structure on glass substrate

To improve optical properties of organic light-emitting diodes (OLEDs), we developed a planarized moth-eye structure by ultraviolet nanoimprint lithography (UV-NIL). The moth-eye pattern was fabricated on a glass substrate by UV-NIL using AMONIL polymer resin. It contains nanoscaled, cone-shaped, periodic pattern arrays with a 250nm diameter and a 550nm pitch. The glass substrate with the moth-eye structure exhibited a 1%-2% improvement in transmittance of visible wavelengths, but it requires a planarized layer with high-refractive-index materials. Photosensitive titanium precursor resin annealed by UV was used to improve the current efficiency by up to 26% versus conventional OLEDs.

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices.