Characteristics Of Organic Light-emitting Diodes Research Articles

Analysis of interfacial charges in aged co-host emitting layer of organic light-emitting diodes using impedance spectroscopy

This study investigated the charge injection and accumulation characteristics of organic light-emitting diodes (OLEDs) generated by spontaneous orientation polarization (SOP) of organic molecules. SOP enhances the electrical characteristics of OLED devices, such as charge injection and transport, by adjusting the interfacial charge density between the organic layers within the device. An OLED device with a mixed co-host emitting layer was fabricated to examine the degradation effect of SOP after electrical aging. Impedance spectroscopy (IS) revealed the presence of interfacial charges originating from the SOP of small molecules within the electron transport layer (ETL), in both pristine and aged devices. Furthermore, the IS simulation results using equivalent circuits were successfully interpreted based on the electrical properties of each organic layer in the pristine and aged devices. The electrical properties of the ETL, which contained 1,3,5-tris(N-phenyl benzimidazole-2-yl)benzene (TPBi), were degraded, reducing hole accumulation and electron injection.

Analysis of the characteristics of organic light-emitting diodes with single and mixed-host EML by impedance spectroscopy

Impedance spectroscopy was used to investigate the charge transportation and accumulation mechanisms in a mixed-host emissive layer (EML) of phosphorescent organic light-emitting diodes (OLEDs). 1, 3, 5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) and 4, 4′, 4″-tris(N-carbazolyl)-triphenylamine (TCTA) materials were used as the electron transport and hole transport layers, respectively, to fabricate the mixed-host EML device. The results showed enhanced current and power efficiency owing to the use of the same material as the mixed-host EML, eliminating energy barriers within the device, coupled with the negative interfacial charges in the polarized TPBi material. The hole- and electron-split devices of the mixed-host EML OLED were analyzed to comprehensively understand the enhanced electrical properties within the device. Subsequently, the capacitance-frequency (C–F) characteristics of the devices were simulated with an equivalent circuit to quantitatively determine the capacitance and resistance in each organic layer at specific voltages (0–4 V) representing each characteristic step on the capacitance-voltage (C–V) curve.

The optical characteristics of organic light-emitting diodes with various microlens arrays

It is important to increase the light extraction efficiency (LEE) of devices in which micrometer-sized spheroidal, conical, pyramidal, and truncated microlens arrays (MLAs) are coupled to organic light-emitting diodes (OLEDs). MLAs may be hexagonal or square. To specify the shapes of individual lenses and their arrays in MLAs, the respective variables x (newly defined in this paper) and the fill factor (FF) can be used. MLAs are characterised by their LEE enhancements (LEEEs) and angular intensity distributions (AIDs), and these are the key optical characteristics of MLAs coupled to OLEDs. Here, these characteristics were obtained via computational simulation using a ray-tracing method. The LEEEs of the spheroidal MLAs were the same regardless of whether the arrays were hexagonal or square. The spheroidal MLAs are suitable for light emission applications in forward direction and, for spherical MLAs of FF = 74.55%, the LEEE was 88.3%. For conical hexagonal and pyramidal MLAs, the LEEEs gradually decreased and the difference widened as the extent of truncation increased. For display applications, the largest LEEE for light rays with emission angles < 10° was 56.5% for spheroidal MLA/OLD devices. Considering all emission angles, the largest LEEE was about 120% for pyramidal MLA/OLED devices. For regular pyramidal MLAs, the intensities of emitted light by the azimuth emission angle evidenced four sawtooth shapes precluding applications in displays.

Effects of Energy‐Level Alignment on Operating Voltages of Blue Organic Light‐Emitting Diodes

AbstractAlthough organic light‐emitting diodes (OLEDs) have been studied extensively for various applications, the effect of the electron behavior on the characteristics of OLEDs has rarely been discussed owing to the difficulty in investigating the actual energy levels. Understanding the correlation between energy levels and the characteristics of OLEDs is essential to improve the performances of blue OLEDs, since the materials with relatively large band gaps used in blue OLEDs make it difficult to deliver electrons from the cathode to the emitting layer (EML). Here, it is shown that the operating voltages of blue OLEDs strongly depend on the energy barrier between the cathode and the EML, which is clarified by investigating the energy‐level alignment in blue OLEDs. It is found that a blue OLED fabricated using a superbase as the electron injection layer exhibits a lower operating voltage than conventional blue OLEDs fabricated using Li compounds. Moreover, there is a clear energy barrier between the cathode and the EML in conventional blue OLEDs, whereas there is no energy barrier in blue OLEDs fabricated using a superbase. Minimization of the energy barrier between the cathode and the EML is demonstrated to be essential to obtain blue emission at low operating voltages.

Numerical Analysis and Optimization of a Hybrid Layer Structure for Triplet–Triplet Fusion Mechanism in Organic Light‐Emitting Diodes

AbstractIn this study, a steady state and time‐dependent exciton diffusion model including singlet and triplet excitons coupled with a modified Poisson and drift‐diffusion solver to explain the mechanism of hyper triplet–triplet fusion (TTF) organic light‐emitting diodes (OLEDs) is developed. Using this modified simulator, various characteristics of OLEDs, including the current‐voltage curve, internal quantum efficiency, transient spectrum, and electric profile are demonstrated. This solver can also be used to explain the mechanism of hyper‐TTF‐OLEDs and analyze the loss from different exciton mechanisms. Furthermore, we perform additional optimization of hyper‐TTF‐OLEDs that increases the internal quantum efficiency by ≈33% (from 29% to 40%).

A simple method for the photometric characterization of organic light-emitting diodes

A simple method for the photometric characterization of organic light-emitting diodes (OLEDs) is reported. It is based on the indirect measurement of the total emitted optical power by using a calibrated photodiode and the optical emission spectrum and space emission diagram of the OLED. From this and by measuring the current–voltage characteristic of the OLED all the relevant radiometric and photometric quantities can be extracted, including the external quantum efficiency. The usual method to collect all photons emitted by a LED source in the half space uses an integrating sphere with the LED source placed at the entrance hole and a photodiode (PD) placed at an exit hole at some point on the sphere surface. Here we show that the same result can be also achieved in free space by a simple geometrical arrangement of the PD in respect of the OLED source, with no need of an integrating sphere. Moreover, we find that a large area photodiode placed in contact with the OLED surface measure about 82% of the total emitted power.

A Simple Method for the Photometric Characterization of Organic Light-Emitting Diodes

A Simple Method for the Photometric Characterization of Organic Light-Emitting Diodes

Organic Thin-Film Characteristics Modulated by Deposition Substrate Rotation Speed and the Effect on Organic Light-Emitting Diodes.

In this paper, we report on the effects of the substrate thermal evaporation process rotation speed on the electroluminescence (EL) characteristics of organic light-emitting diodes (OLEDs). In general OLED research, rotational and angle tilted deposition are widely used to maintain uniformity. However, there have been few reports on the effects of this deposition method on film characteristics. We analyzed these effects and found that the film density and its refractive index showed remarkable changes as a function of substrate rotational speed during tilted deposition. The EL characteristics of the transport layer of fluorescent OLEDs were also significantly affected. We derived the OLED optimal thickness and refractive index from our calculations.

Effects of MEH-PPV Molecular Ordering in the Emitting Layer on the Luminescence Efficiency of Organic Light-Emitting Diodes.

We investigated the effects of molecular ordering on the electro-optical characteristics of organic light-emitting diodes (OLEDs) with an emission layer (EML) of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The EML was fabricated by a solution process which can make molecules ordered. The performance of the OLED devices with the molecular ordering method was compared to that obtained through fabrication by a conventional spin coating method. The turn-on voltage and the luminance of the conventional OLEDs were 5 V and 34.75 cd/m2, whereas those of the proposed OLEDs were 4.5 V and 120.3 cd/m2, respectively. The underlying mechanism of the higher efficiency with ordered molecules was observed by analyzing the properties of the EML layer using AFM, SE, XRD, and an LCR meter. We confirmed that the electrical properties of the organic thin film can be improved by controlling the molecular ordering of the EML, which plays an important role in the electrical characteristics of the OLED.

55.1: Invited Paper: Photolithographic patterning of OLED and OPD for invisible sensor integration in mobile display

We apply a photolithographic patterning process to integrate a green phosphorescent organic light emitting diode (OLED) with a polymer‐based organic photodetector (OPD) side‐by‐side with resolution up to 635 ppi. The characteristics of OLED and OPD before and after patterning are investigated. The crosstalk between OLED and OPD subpixels and the linearity of the integrated OPD under different luminance are also studied. The results indicate that photolithography enabled integration is an encouraging route of organic and hybrid electronics (OHEs) manufacturing.

Optical characterization of organic light-emitting diodes with selective red emission

In this work, the optical properties of red-light emitting materials used for active layers in functional red OLED devices were researched. The investigated materials are: a synthesized lab scale complex vinyl-quinoline pyridine homopolymer with Ir P[QPy-Ir-(PPy)2], as well as two commercially supplied materials, Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The samples were spin-coated on ITO/glass substrates. The characterization of the materials was carried out via Spectroscopic Ellipsometry (SE), Photoluminescence Spectroscopy (PL) and Electroluminescence Spectroscopy (EL). Through SE analysis the film thickness, the optical properties and the dielectric function were obtained. PL and EL spectra were derived and evaluated in terms of the emission profile and characteristics of the different photoactive materials. Finally, the colour stability and selectivity of each material were determined, and a comparative study of the red-light emitting materials was conducted.

Solution and Evaporation Hybrid Approach to Enhance the Stability and Pattern Resolution Characteristics of Organic Light-Emitting Diodes.

The solution process and vacuum evaporation, both fabrication methods for conventional organic light-emitting diodes (OLEDs), are intrinsically restricted with regard to their ability to enhance pattern resolutions and film stability outcomes. Here, we introduce a novel approach of the solution process followed by intense pulsed light (IPL) evaporation for producing high-resolution line patterns of OLEDs. Through control of the wettability between the banks and microchannels via a mask-free selective surface treatment, we successfully deposited phosphorescent green and red inks only into the microchannels. Then, high-resolution patterns of an emitting layer (EML) layer were uniformly evaporated onto the device substrate using IPL evaporation. Ultimately, we fabricated green and red phosphorescent OLED devices with a high pixel density of a line-patterned EML with a width of 6 μm and a pitch of 13.6 μm. In addition, we demonstrated that the IPL-evaporated films have many advantages compared to those fabricated by the conventional solution process. We also showed that the IPL evaporation process can be less sensitive to problems related to the aggregation of organic molecules during a drying or annealing process. Hence, the device performance and lifetime of the IPL-evaporated OLEDs were enhanced compared to those of the spin-coated OLEDs.

Magnetic Field Implementing into the Electroluminescence of OLED Devices Doped with CoFe2O4 Nanoparticles

Cobalt ferrite magnetic nanoparticles (CoFe2O4 MNPs) were successfully prepared by citric acid-assisted sol-gel auto combustion method and used in emissive layer of organic light emitting diode (OLED). Dimensional, structural and magnetic properties of CoFe2O4 nanoparticles (NPs) were recearched and compared by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). CoFe2O4 MNPs were utilized at various concentrations (0.5 wt%, 1.0 wt% and 2.0 wt%) in the emissive layer of the OLEDs. The luminance, current efficiency and the electroluminescence characteristics of the devices with and without CoFe2O4 MNPs were investigated. An external magnetic field, Bext, has also been applied to the OLEDs doped with MNPs while under operation. Effects of MNPs on OLED characteristics under Bext were studied thoroughly. In the tailored device architecture, poly (3,4-ethylenedioxythiophene): poly polystyrene sulphonate (PEDOT: PSS) and poly(2-methoxy-5-(2-ethylhexyloxy))-1,4-phenylene vinylene (MEH-PPV) were used as a hole transport layer (HTL) and an emissive layer respectively with ITO/PEDOT: PSS/ MEH-PPV: CoFe2O4/Ca/Al device architecture. The obtained results of the fabricated OLEDs were enhanced in the presence of CoFe2O4 NPs under Bext due to providing density of states in the polymer matrices. The turn-on voltage was diminished slightly in the device doped with 0.5 % wt MNP compared to the devices with other concentrations of MNPs.

Characterization of organic light-emitting diode using a rubrene interlayer between electrode and hole transport layer

In this paper, we report the characteristic properties of an organic light-emitting diode (OLED) using a rubrene buffer layer over the fluorine-doped tin oxide (FTO) surface. Our study includes both electrical and optical properties of the device. Here, we study the OLED devices at different thicknesses of the buffer layer, which varies from 3 to 11 nm. For device fabrication, we use a thermal evaporation unit. Finally, we report that device performance in a bilayer anode form is always higher than that of a single-FTO-based device. Maximum device efficiency is found to be $$6.31~\hbox {cd A}^{-1}$$ around 8-nm thickness of rubrene layer over the FTO surface. We also study the stability of both the single-layer and double-layer anode OLED devices. Through this study, we found that both device efficiency and luminance intensity of the bilayer anode OLED remain more stable for more number of days compared with the single-FTO OLED device.

Comparison of Optical and Electrical Properties of Different Hole-Transporting Materials for Solution-Processable Organic Light-Emitting Diodes.

Achieving well-defined multilayer structure is a key to improve device performance of organic light-emitting diodes (OLEDs), especially in solution-processed OLEDs. Use of cross-linkable hole-transporting materials (HTMs) is therefore gaining much attention to achieve such multilayer structures. One of representative solution-processable HTM is TFB, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)], but it does not have any cross-linkable units in its chemical structures. In this work, we performed basic physical characterization of a new cross-linkable HTM, HL-X026, and we also investigated its OLED characteristics. Actual solvent resistance by varying the thermal curing temperatures was measured. The thermally cross-linkable HL-X026 showed better device performances than simple TFB with higher luminance and efficiencies.

Effects of the Thickness of N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine on the Electro-Optical Characteristics of Organic Light-Emitting Diodes.

We examined the electro-optical characteristics of organic light emitting diodes according to the N,N′-diphenyl-N,N′-di(m-tolyl)-benzidine (TPD) thicknesses. The thicknesses of TPD were varied from 5 nm to 50 nm. The current density of the device with a TPD thickness of 5 nm was 8.94 times higher than that with a thickness of 50 nm at a driving voltage of 10 V. According to the conduction–current characteristics of conductors, the current densities improved with a decreasing TPD thickness. Different from the current density–voltage characteristics, the current efficiency–current density characteristics showed an improved efficiency with a 50 nm TPD thickness. The current efficiencies of a device with a 5 nm TPD thickness at a driving voltage of 10 V was 0.148 and at a 50 nm TPD thickness 0.993 cd/A, which was 6.7 times higher than the 5 nm TPD thickness. These results indicated that hole transport in Organic Light-Emitting Diode (OLED) devices were more efficient with thin 5 nm TPD than with thick 50 nm TPD, while electron transport was more efficient with thick 50 nm TPD, which caused conflicting results in the current efficiency-current density and current density-voltage characteristics according to TPD thicknesses.

Preparation and Characterization of Organic Light-Emitting Diodes Using Polymer-Alumina Hybrid Passivation Film with High Hardness

We have studied hybrid passivation coating materials and methods that can achieve good surface hardness using organic and inorganic materials. The passivation layers formed using synthesized passivation agent based on urethane acrylate resins. High-hardness hybrid passivation films were prepared by adding Al2O3 nanoparticles as inorganic fillers to improve the hardness. This can compensate for the poor scratch resistance, which is a problem of existing passivation films, and the prepared films have excellent properties according to an analysis of its various physical properties, and they also have high surface hardness, thus securing their applicability to other organic electronic device fields. In addition, an optimized hybrid passivation film was applied to OLED devices in order to improve the electrical and optical characteristics as well as stability.

Device Characteristics of Top-Emitting Organic Light-Emitting Diodes Depending on Anode Materials for CMOS-Based OLED Microdisplays

We investigated the optical reflectance, surface roughness, and sheet resistance of aluminum (Al)/titanium nitride (TiN), titanium (Ti), and tungsten (W) layers on Si substrates, which are available in the CMOS foundry, for organic light-emitting diode (OLED) microdisplays. The devices with different metal anode layers exhibited different hole-injection properties and OLED performances, owing to the different optical and electrical properties of metal anode layers. Based on the OLED characteristics, the Al/TiN layer was selected as an anode layer for OLED microdisplays. A green monochromatic OLED microdisplay panel was designed and implemented using the 0.11-μm CMOS process. The density of pixels was ∼2 351 pixels per inch and the panel's active area was 0.7 in in diagonal. The resolution of the panel was 1 280 × 3 × 1 024, corresponding to SXGA. The panel was successfully operated, and the maximal luminance was ∼460 cd/m2.

A study on the sound generation at digital signage using OLED panel

With the development of ICT technology and convergence of media and ICT, smart media is spreading to people. The advertising industry through this smart screen is a rapidly growing future industry in the world. A new paradigm of bidirectional/customized features advertisement has become provided through IP-based new media such as smart TVs, portable smart devices, the Internet, IPTV (VOD), and digital signage. As one of the smart advertising industries, the digital signage is attracting attention as the fourth screen after TV, PC, and mobile with indoor/outdoor advertisement using information display. Advertising using the digital signage is installed in a lot of floating population place such as a subway station, a bus stop, an elevator, etc. Digital signage is developed with the display technology and the convergence of IT technology, but the development of appearance and screen configuration is mainly made. There are characteristics of high efficiency when communicate information with visual and auditory. However, up until now, digital signage has been difficult to provide vision and sound to people at the same time. The sound must be played through the screen; nevertheless, the smart screen does not pass the sound well, because of hardware. In this study, we introduce the technology that can generate direct sound using the characteristics of organic light-emitting diode (OLED) panel and introduce digital signage that can transmit video and sound simultaneously by using direct sound generation.

The effect of air annealing on the properties of MoO3 films and its application for organic light emitting diodes

In the present work, post-annealing modification in air was performed on the MoO3 films and its effect on the optoelectronic characteristics of organic light-emitting diodes (OLEDs) was investigated. When MoO3 was annealed at the proper temperature, the electroluminescent performances of OLEDs were improved. X-ray photoelectron spectroscopy (XPS) and Raman studies revealed that the air annealing process resulted in more crystalline and nearly stoichiometric in MoO3, leading to the enhancement of the photoluminescence (PL) intensity and transmittance. Moreover, the reduced oxygen vacancies caused by air annealing increase the work function of MoO3, resulting in the energy barrier reduction at the ITO/NPB interface, and consequently facilitating the hole injection and improving OLED performances.