Organic Light-emitting Diode Structure Research Articles

Templateless electrodeposition ZnO nanowires for charge transport optimization in OLED structures

Passivated zinc oxide nanowires (NW) were used to improve the charge injection in organic light-emitting diode (OLED) structures. Conducting polymers, deposited on the well-dispersed ZnO NW, were used to modify the electrical conductivity across the OLED structure because the charge transport is influenced by the interface interactions. Passivation with polymers improves the transport characteristics of the device due to the interaction between ZnO NW and PEDOT:PSS polymer. The hole current density increases with the ZnO NW concentration, which made the current injection more balanced and therefore enhanced the electroluminescence efficiency. A templateless electrochemical deposition method was used to grow zinc oxide nanowires on an ITO/glass substrate because parameters such as the densities and dimensions of the nanowires can be controlled to produce thin and well dispersed structures.

Comparison study of transparent RF-sputtered ITO/AZO and ITO/ZnO bilayers for near UV-OLED applications

Hybrid inorganic/organic light-emitting diodes have attracted much attention in the field of luminescent electronics due to the desired incorporation of high optoelectronic features of inorganic materials with the processability and variety of organic polymers. To generate and emit a near ultraviolet (N-UV) ray, wide band gap semiconductors can be applied in the organic light-emitting diodes (OLEDs). In this paper, zinc oxide (ZnO) and aluminum-doped ZnO (AZO) thin films are deposited by radio frequency (RF) sputtering above the ITO electrode and poly [2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) conjugated polymer is utilized as a complementary p-type semiconductor in OLED structure. The impact of ZnO and AZO thickness on the structural, electrical, optical and morphological properties of ITO/AZO and ITO/ZnO bilayers are scrutinized and compared. Results show that with the enlargement of both ZnO and AZO film thickness, the physical properties are gradually improved resulting in the better quality of transparent conducting thin film. The average electrical resistivity of 8.4×10−4 and 1.1×10−3Ω-cm, average sheet resistance of 32.9 and 42.3Ω/sq, average transmittance of 88.3 and 87.3% and average FOM of 1.0×104 and 7.4×103 (Ω-cm)−1 are obtained for ITO/AZO and ITO/ZnO bilayers, respectively. Moreover, comparing the results indicates that the strain and the stress within the ITO/AZO bilayer are decreased nearly 19% with respect to ITO/ZnO bilayer which yield higher quality of crystal. Consequently, the physical properties of ITO/AZO bilayer is found to be superior regarding ITO/ZnO bilayer. For fabricated UV-OLEDs, the turn-on voltages, the characteristic energy (Et) and the total concentration of traps (Nt) for the devices with the structures of ITO/ZnO/MEH-PPV/Al and ITO/AZO/MEH-PPV/Al are obtained 12 and 14V, 0.108 and 0.191eV, 9.33×1016 and 5.22×1016cm−3, respectively. Furthermore, according to the electroluminescence (EL) spectra, the near band emission (NBE) peak for device with the structure of ITO/ZnO/MEH-PPV/Al is attained nearly in the wavelengths of 408nm which is in N-UV region. For ITO/AZO/MEH-PPV/Al, a slightly blue shift in NBE peak is observed due to the Burstein–Moss (BM) effect. Ultimately, different charge carrier transport mechanisms of fabricated UV-OLEDs have been carefully investigated.

Optical effects of NiOx interlayer for OLEDs with AZO embedded anodes

Insertion of a nickel oxide (NiOx) in an aluminum zinc oxide (AZO) embedded organic light-emitting diode (OLED) has successfully obtained additional 20% light extraction and led to an enhanced current efficiency up to 42.7%. The introduced NiOx has a high reflective index of ∼2.1 which allows more emitted light to travel through AZO/ITO interfaces. The AZO/ITO interfaces provide deflection and scattering effect to redirect light away from being totally reflected in the reflective-index-mismatched OLED structures and thus are able to enhance external light out-coupling. Results also showed that the coating of NiOx layer increased the electrical resistance of AZO embedded ITO film which limited the current efficiency, and the introduced NiOx showed no benefit on carrier injection as expected.

Optically readout write once read many memory with single active organic layer

An optically readable write once read many memory (WORM) in Ag/Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH PPV)/ITO is demonstrated in this work. Utilising characteristics of the organic light emitting diode structure of Ag/MEH PPV/ITO and electrochemical metallization of Ag, a WORM with light emitting capability can be realised. The simple fabrication process and multifunction capability of the device can be useful for future wearable optoelectronics and photomemory applications, where fast and parallel readout can be achieved by photons.

Tri-Metal Layered Semitransparent Electrode for Red Phosphorescent Organic Light-Emitting Diodes.

In this paper, we fabricated tri-metal layered thin film semitransparent electrodes consisting of a thin conductive metal layer, sandwiched between two nickel layers. An equal red phosphorescent organic light-emitting diode (PHOLED) structure was deposited on the anodes of indium tin oxide (ITO) and three types of tri-metal layers (Ni/Al/Ni, Ni/Cu/Ni, and Ni/Ag/Ni, thickness of 3/7/3 nm in common) on a glass substrate. The optical and electrical performances of the device using Ni/Ag/Ni were improved more than the performances of the other devices due to the micro-cavity effect in accordance with the various electrode characteristics. Moreover, we fabricated the same red PHOLED structures on a flexible substrate, as a consequence, showed competitive emission characteristics compared to the devices fabricated on a glass substrate. Therefore, this study could succeed to additional research on flexible display panel and light-emitting devices with ITO-free electrodes.

46.2: A Novel Laminated Organic Light‐Emitting Diodes with a Multi‐Layered Graphene Top Anode

We report on a novel fabrication method for organic lightemitting diodes (OLEDs) by using a multilayered graphene (MLG) film and a lamination technique. The OLED structure with the MLG top electrode has higher transmittance and lower reflectance than the OLED structure with the conventional thin metal electrode. Moreover, higher luminous current efficiency of the device with the MLG top electrode has been achieved compared with that of the conventional bottom‐emission OLEDs with aluminum metal electrode.

An Organic Device with Thin Film Transistor Merged with Light Emitting Diode through Use of an Accumulation Layer in TFT As an Electrode

A light-emitting device structure is proposed which includes an organic thin-film structure merged with an organic light-emitting diode structure by utilizing a part of the electron accumulation layer in the organic thin-film transistor as a common electrode for each structure. The organic thin-film structure and the organic light-emitting diode structure each include an organic semiconductor material in which hole mobility is greater in bulk region of the material than electron mobility in the bulk region but less in the channel region. The advantages of such a light-emitting device include less complex processing and a simpler pixel circuit structure in comparison to separately fabricating OTFT and OLED structures and subsequently interconnecting them to form a pixel. Furthermore, relative to a light-emitting transistor, the proposed structure offers the advantage of a broader light emission area which is more suitable for use in display devices.

Solid state solvation effect of a donor–acceptor type fluorescent molecule and its application to white organic light-emitting diodes

We investigated the solid state solvation effect (SSSE) of a donor–acceptor (D–A) type green fluorescent emitter, 3,7-bis(benzimidazol-2-yl)-10-methylphenothiazine (BBMP), and fabricated white organic light-emitting diodes (OLEDs) by utilizing SSSE. After having confirmed that BBMP showed a large solvatochromic photoluminescence spectral shift not only in liquid solutions but also in solid solutions, we induced a sky-blue electroluminescence (EL) by doping the green-emitting BBMP in non-polar host materials. The emission spectrum of BBMP blue-shifted as the polarity of host material decreased, and white OLEDs were obtained by integrating an additional orange light-emitting material, Rubrene, into this sky-blue-emitting OLED structure. When 9, 10-di(naphtha-2-yl)anthracene(ADN) was employed as a non-polar host of BBMP, the device exhibited a sky-blue (λpeak,EL = 475 nm) emission by SSSE with evidently improved EL properties: the luminous efficiency, the external quantum efficiency, and the maximum luminance were 10.4 cd/A, 3.5%, and 46,600 cd/m2, respectively. Finally, the white OLED with an optimized structure of [ITO/NPB (20 nm)/ADN:Rubrene (20 nm)/NPB (2 nm)/ADN:BBMP (20 nm)/TPBi (20 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (100 nm)] demonstrated a luminous efficiency of 5.3 cd/A, an external quantum efficiency of 1.3%, and a maximum luminance of 15,200 cd/m2 with CIE coordinates of (0.32, 0.42) at 100 mA/cm2.

Increase of electroluminescent intensity in planar light-emitting diode structures based on PFO polymer – ZnO nanoparticles composite films operated by alternating-current voltages

We report on the considerable increase of electroluminescent (EL) intensity in planar organic light-emitting diode structures based on semiconducting polymer PFO and ZnO nanoparticles composite films operated by alternating-current (AC) voltages with respect to that operated by direct current. It was established that an increase of frequency from 0 to 30kHz results in increase of EL intensity of such structures by a factor of ~30. The EL emission takes place at positive AC bias and the EL spectral range is the same as that for the photoluminescent emission spectrum for the same sample. It was shown that applying AC voltage is effective tool in producing strong EL emission in planar composite light-emitting structures.

Measurement of the Charge Carrier Mobility in MEH-PPV and MEH-PPV-POSS Organic Semiconductor Films

The values of the charge carrier mobility in organic semiconductor materials (MEH-PPV, MEH-PPV-POSS) are obtained on the basis of an analysis of the relaxation curves of transient electroluminescence in organic light-emitting diodes (OLEDs). The data on the mobility of charge carriers are analyzed according to the Poole–Frenkel model using the dependences of the charge carrier mobility on the electric field. Physical interpretation of the transport phenomena in OLED structures based on MEH-PPV and MEH-PPV-POSS is given.

Improved Performance of OLEDs on Cellulose/Epoxy Substrate Using Adenine as a Hole Injection Layer

Organic light-emitting diodes (OLED) fabricated from all “natural” materials will lead to renewable, sustainable, and potentially inexpensive organic optoelectronics. Achieving this goal will require reimagining all the various aspects of the OLED with natural-based renewable materials. Here, we replace the common substrate, electrode, and hole injection layer in the OLED structure with cellulose, gold, and the DNA nucleobase adenine. Gold films are used as semitransparent electrodes on plant-based cellulose substrates, providing flexible anodes that are highly conductive without annealing. A lift-off fabrication method (template stripping) employed UV curable epoxy to transfer patterned gold from silicon to cellulose. The gold has excellent adhesion to the epoxy (is not damaged when wiped), and the two serve as a smoothing layer for the cellulose substrate, resulting in very uniform OLED emission. DNA and DNA nucleobases are demonstrating to be versatile materials in natural electronics due to their wide...

Numerical Investigation on Micro-Cavity Effect of Top-Emitting Organic Light Emitting Diode.

In this paper, we report our numerical investigation on the top-emitting OLED (Organic Light Emitting Diodes) with micro-cavity. Our numerical model includes an ensemble of radiating dipole antennas for light emission as well as Poisson Equation for carrier injection and transportation. We formulated a set of differential equations by the Finite Element Method. Our simulation revealed that the recombination rate is affected by the thickness of each layer comprising the OLED structure and the amount of emission is determined by the total thickness of the OLED structure due to micro-cavity effect which is observed in between the total reflection layer and the half reflection layer. Our numerical solver enables us to optimize the OLED structure and thereby improve the external quantum efficiency.

Realization of white organic light-emitting devices using single green emitter by coupled microcavities with two modes

We fabricated a novel white organic light-emitting diode (WOLED) structure based on a single green emitting layer through coupled microcavities with two modes. By optical simulation and experiment, we showed that the two resonant modes can be adjusted by varying the thickness of the special layer to shape white light using only a single green emitter. Utilizing tris(8-hydroxyquinoline)aluminum (Alq3) as an emitting layer, we obtained a series of WOLEDs whose Commission Internationale de L'Eclairage (CIE) coordinates are on the Planckian curve from to with current efficiencies in the range of 1.45–2.52 cd A−1. A maximum current efficiency of 10.12 cd A−1 and warm white emission at CIE have been demonstrated using bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)2(acac)] as an emitter material.

Room-temperature spin-polarized organic light-emitting diodes with a single ferromagnetic electrode

In this paper, we demonstrate the concept of a room-temperature spin-polarized organic light-emitting diode (Spin-OLED) structure based on (i) the deposition of an ultra-thin p-type organic buffer layer on the surface of the ferromagnetic electrode of the Spin-OLED and (ii) the use of oxygen plasma treatment to modify the surface of that electrode. Experimental results demonstrate that the brightness of the developed Spin-OLED can be increased by 110% and that a magneto-electroluminescence of 12% can be attained for a 150 mT in-plane magnetic field, at room temperature. This is attributed to enhanced hole and room-temperature spin-polarized injection from the ferromagnetic electrode, respectively.

Enhancement of light-extraction efficiency of organic light-emitting diodes using silica nanoparticles embedded in TiO₂ matrices.

We investigate two types of internal light-extraction layer structures for organic light-emitting diodes (OLEDs) that consist of silica nanoparticles (NPs) embedded in high-refractive-index TiO₂ matrices. The composite of silica NPs and TiO₂ matrices was coated on the glass substrate and fabricated with and without a SiO₂ planarization layer. An increase in the optical out-coupling efficiency by a factor of 2.0 was obtained at a high luminance of 3,000 cd/m² from OLEDs containing the silica NPs embedded in TiO₂ matrices between glass substrates and Zn-doped In₂O₃ (IZO) electrodes after additional planarization processes. This is consistent with the analytical result using the finite-difference time-domain (FDTD) method. Randomly distributed silica NPs acting as scattering centers could reduce the optical loss when extracting light. By using additional planarization processes with a PECVD-derived SiO₂ layer, one can assure that smoother surfaces provide higher out-coupling efficiency, which attain 100% and 97% enhancements in power (lm/W) and current (cd/A) efficiencies, respectively.

Review on Electroluminescence Behaviour of Organic Light Emitting Diode

In this paper report a review on electroluminescent efficiency using a deoxyribonucleic acid (DNA) complex as an electron blocking layer (EBL) material in emitting organic light emitting diode (OLED). The resulting called BioLED showed a high luminous efficiency. The DNA-based BioLED was as much more efficient and brighter than their OLED counterparts. It is found the effect of EBL is to increase the device turn-on voltage for OLED structure. This is attributed to the fact that the DNA complex increases the series resistance of the device which is turn reduces the current through the device.

Numerical Simulation on the Charge Transport in Tris(8-hydroxyquinolinato)aluminum-Based Organic Light-Emitting Diode Structure

In this paper, we report on our theoretical study on the charge transport of a multilayer structure for organic light-emitting diodes (OLEDs). Our simulation structure comprises a hole transport layer (HTL), an emission layer (EML), and an electron transport layer (ETL) between two electrodes wherein the HTL is N,N '-bis(3-methylphenyl)-N,N '-bis(phenyl)benzidine (TPD) and the ETL includes tris(8-hydroxyquinolinato)aluminum (Alq3). We discuss the carrier transport mechanism of the Alq3-based two-layer structure and thereby propose a high-efficiency device structure. We also report our investigation on the transient response during the turn-on/off period and the carrier transport in response to the variation of the injection barrier and applied voltage. This paper also reports the effect of the insertion of the EML layer and the efficiency dependence on the difference in the internal barrier height.

Plasma-tolerant structure for organic light-emitting diodes with aluminum cathodes fabricated by DC magnetron sputtering: Using a Li-doped electron transport layer

We fabricate aluminum cathodes that are almost free from plasma damage by DC magnetron sputtering for organic light-emitting diodes (OLEDs). While sputtering is widely known to have numerous advantages over conventional evaporation for mass production of devices, it can cause serious damage to organic layers. In this report, we fabricate devices that are free from plasma damage by introducing a 1%-Li-doped electron transport layer (ETL). The difference of external electroluminescence quantum efficiency between OLEDs with the structure ITO/α-NPD/ETL/Al (where ITO is indium tin oxide and α-NPD is N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine) with Al cathodes deposited by conventional evaporation or sputtering is 0.1%, and their driving voltage is identical. We find that the Li-doped ETL should be thicker than 40nm. Analysis of the depth profile of the ETL by time-of-flight secondary ion mass spectrometry indicates that considerable damage from sputtering extended to a depth of approximately 30nm, suggesting that high-energy particles penetrated about 30nm into the ETL.

Pressure Effects on the Lamination of Organic Light-Emitting Diodes

This paper presents the results of finite element simulations of the lamination process for the fabrication of organic light-emitting diodes (OLEDs). The simulations utilize mechanical properties of the individual layers of the OLED structures that are obtained using nanoindentation techniques. The simulations show that applied pressure can cause contact evolution and sink-in around dust particles that are interposed between the organic materials layers, or the organic/inorganic layers. The implications of the results are discussed for the fabrication of robust OLEDs.

Far-field luminous intensity distribution depending on the outcoupling structure of organic light-emitting diodes

The finite-difference time domain method was combined with the ray tracing technique to investigate the viewing angle characteristics of organic light-emitting diodes (OLEDs) with various optical structures for improving the outcoupling efficiency. The angular dependence of the far-field luminous intensity on OLEDs with a basic structure and on the one with scattering particles in the glass substrate exhibited an almost Lambertian distribution at their optimized conditions. The pillow lenses applied onto the front surface of the glass substrate made the intensity distribution complex with local-intensity maxima appear. This was attributed to the interactions between the curved lens surfaces and the rays emitted from the light source. This study suggests that both the microcavity in the organic layers and the optical structure for efficient outcoupling should be optimized to realize appropriate viewing angle characteristics for display or lighting applications.