Luminance-voltage Characteristics Research Articles

Effect of thermal annealing on an emissive layer containing a blend of a small molecule and polymer as host for application in OLEDs.

In order to improve the performance of OLEDs, a host-guest mixture was used as an emissive layer. To have better host properties, a mixture of different materials with suitable properties can also be used as a host. In this study, we used a mixture of a polymer and a small molecule as the host and studied the effect of thermal annealing on the emissive layer properties by using Ir(ppy)3 as the emitter. UV-visible absorption, steady-state and time-resolved photoluminescence, scanning electron microscopy, atomic force microscopy, and optical microscopic studies were performed to study the film properties. Devices were fabricated and their current-voltage and luminance-voltage characteristics were studied. Charge-carrier mobility in the devices was studied by dark CELIV and transient electroluminescence methods. We show that, below the glass transition temperature of the polymer, the small molecules formed aggregates due to thermal annealing, which was beneficial for the device performance in the lower-temperature range, mainly due to the improved electron mobility. However, this aggregate formation was detrimental in the higher-temperature range, as it led to inefficient energy transfer due to the increased pure phase formation. At temperatures above the glass transition temperature of the polymer, the small molecules were seen to be distributed more uniformly into the polymer matrix. However, as a result of the degradation of the polymer property due to degradation of the primary chain of the phenyl ring of the polymer, this uniform distribution was not of any use and the device performance deteriorated.

Яркость и эффективность светодиода с транспортно-блокирующими слоями полиметилметакрилата и квантовыми точками: теоретическая модель, эксперимент, оптимизация

The results of studies of the influence of the thicknesses of the electron-blocking, hole-conducting and recombination layers on the luminance-voltage and other electrophysical characteristics of a multilayer light-emitting diode with an active layer based on semiconductor quantum dots are presented. Optimized thicknesses and voltages are determined to achieve the maximum efficiency of electroluminescence of the light-emitting diode.

Luminance and efficiency of a light-emitting diode with transport-blocking poly(methyl methacrylate) layers and quantum dots: theoretical model, experiment, optimization

The results of studies of the influence of the thicknesses of the electron-blocking, hole-conducting and recombination layers on the luminance-voltage and other electrophysical characteristics of a multilayer light-emitting diode with an active layer based on semiconductor quantum dots are presented. Optimized thicknesses and voltages are determined to achieve the maximum efficiency of electroluminescence of the light-emitting diode. Keywords: quantum dots, light-emitting diode, blocking layer.

Impact of 1,2-ethanedithiol treatment on luminescence and charge-transport characteristics in colloidal quantum-dot LEDs

We report on a substantial increase in luminance and luminous efficiency of green-light emitting devices (LEDs) that use colloidal CdSe@ZnS quantum dots (QDs) as a light-emitting material in response to treatment with 1,2-ethanedithiol (EDT). The maximum luminance increased from 1146 to 8075 cd m−2, and luminous yield from 0.15 to 1.41 cd A−1 as a result of treating an incomplete device with drops of EDT right after spin-coating QDs onto a ZnO-nanoparticle layer. Based on systematic studies on substrate-dependent change in photoluminescence, and current-voltage and luminance–voltage characteristics, we propose that passivation of intra-gap defect states and relative shifts of energy levels relevant to the operation of QD LEDs are two main results of EDT treatment. In particular, we argue that energy-level shift without emission-color change can be attributed to surface-dipole effects.

The utilization of low‐temperature evaporable CsN3‐doped NBphen as an alternative and efficient electron‐injection layer in OLED

Cesium azide (CsN3) doped 2,9‐bis(naphthalen‐2‐yl)‐4,7‐diphenyl‐1,10‐phenanthroline (NBphen) has been demonstrated as an efficient electron‐injection layer (EIL) in this work. Advantageous for its low evaporation temperature and air stability, CsN3 is employed as an n‐dopant to replace the reactive alkali metals or alkali metal compounds. Organic light‐emitting diodes utilizing this composite EIL exhibit highly improved current density–luminance–voltage characteristics compared to the control device with LiF. When C545T‐doped Alq3 is used as an emitting layer, a maximum current efficiency of 10.4 cd A−1 can be reached. Our results indicate that this CsN3‐doped NBphen composite layer has great potential as an alternative EIL in organic electronic devices.

Green electroluminescence from azafluoranthene derivatives in polymer based electroluminescent devices

Paper demonstrates a series of novel electroluminescent devices (OLEDs) with poly (N-vinylcarbazole) active emitting layer doped by the azafluoranthene dyes. Measured steady state electroluminescence spectra, luminance-voltage and luminance-current characteristics provide basic characterization of the designed OLEDs. All the devices exhibit sharp green electroluminescence with the emission maximum being rather weakly dependent on the type of the fluorescent dopant. The obtained results show that a series of recently synthesized azafluoranthene dyes may be considered as efficient green fluorescent emitters for electroluminescent applications.

Electrical and optical simulations of a polymer-based phosphorescent organic light-emitting diode with high efficiency

A comprehensive numerical device simulation of the electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer-based light-emitting diodes doped with phosphorescent dyes is presented. The system under investigation comprises an electron transporter attached to a polymer backbone blended with an electronically inert small molecule and an iridium-based green phosphorescent dye which serves as both emitter and hole transporter. The device simulation combines an electrical and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye's HOMO as hole transport level and the polymer LUMO as electron transport level. By fine-tuning the injection barriers at the electron and hole-injecting contact, respectively, in simulated devices, unipolar device characteristics were fitted to the experimental data. Simulations using the so-obtained set of parameters yielded very good agreement to the measured current–voltage, luminance–voltage characteristics, and the emission profile of entire bipolar light-emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of the organic light-emitting diode, including the position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the device. The simulations show that the device is severely limited by hole injection, and that a reduction of the hole-injection barrier would improve the device efficiency by almost 50%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Effects of carrier trapping and scattering on hole transport properties of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine thin films

The effects of trapping and scattering by dopant on the hole transport properties of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) have been investigated by temperature dependent current–voltage characteristics, admittance spectroscopy, and luminance–voltage characteristics. Three dopants, including 4,4′,4″-tris ( N-3-methylphenyl- N-phenylamine) triphenylamine ( m-MTDATA) behaving as hole traps, tris-(8-hydroxyquinolinato) aluminum (Alq 3) and 4,4′,4″-tri( N-carbazolyl)-triphenylamine (TCTA) acting as hole scatterers, are used in this research. It has been found that both trapping and scattering lower hole mobility in NPB, but the magnitude differs strongly. The traps are revealed to induce significantly increase in total density of traps whereas there is a slight variation for the scattering. These differences are attributed to two different charge transport mechanisms. The effect of doping on the performances of electroluminescent devices has also been studied.

Raman Spectra of Molecularly-Ordered 1-TNATA Thin Films and Organic Electroluminescence Device Properties

A molecularly-ordered thin film of vacuum deposited 1-TNATA(4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino) triphenylamine) as a hole injection layer (HIL) is placed between indium tin oxide (ITO) electrode and a hole transporting layer (HTL) in multi-layered organic electroluminescence (EL) diodes. The molecular ordering of 1-TNATA thin film was obtained by thermal annealing and electromagnetic field after vacuum deposition and the film was investigated by the Raman spectra, XRD, and AFM. It was found that the current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the multi-layered diodes were improved due to the ordering of 1-TNATA molecules, which could enhance the charge carrier mobility and luminance efficiency among the organic molecules in the molecularly-ordered film.

Raman Spectra and Current-Voltage Characteristics of 4,4′,4″-Tris(2-naphthylphenylamino)triphenylamine Thin Films

A thin film of vacuum-deposited 2-TNATA (4,4′,4″-Tris(2-naphthylphenylamino) triphenylamine), usually used as a hole injection layer (HIL) in OLEDs, was annealed after deposition and treated with electromagnetic field in order to get the film molecularly-ordered and analyzed by AFM, SEM and Raman spectroscopy. The treated film was placed between indium tin oxide (ITO) electrode and hole injection layer to investigate the current density-Voltage (J-V) and luminance-voltage (L-V) characteristics of the organic light-emitting diodes. According to the Raman spectra results, as-deposited 2-TNATA thin film was not molecularly ordered, and the films treated by the thermal annealing at near the glass transition temperature of 2-TNATA and electromagnetic field, were molecularly ordered and showed the significant increases of current density and luminance efficiency for devices at a given voltage compared to an as-deposited 2-TNATA film.

Organic Light-Emitting Diodes Based on New Silole Derivatives

Two light emissive organic light-emitting diodes (OLEDs) of high performance were prepared by vacuum thermal evaporation using two kinds of silole derivatives 1,2-bis(1-methyl-2,3,4,5-tetraphenyl-1H-silole-1-yl)ethane (BMPSiE) and 1,1′-dimethyl-3,3′,4,4′-tetraphenyl-2,2′,5,5′-tetra(thiophen-2-yl)-1,1′-bi(1H -silole) (BMPThSi) as emitting materials, N, N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(NPB) and tris(8- hydroxyquinolinolato) aluminum (Alq3) as hole transporting layer (HTL) and electron transporting layer (ETL), respectively. The luminance-voltage and current density-voltage characteristics of devices were investigated, and the difference between their performances was discussed. The results demonstrate that at a bias voltage of 20 V, the device consisting of novel emissive material BMPSiE had a maximum luminance of 9991.9 cd·m-2 and the device of BMPThSi had a maximum luminance of 15261.5 cd·m-2. The peaks of electroluminescence (EL) spectra were located at 483 and 495 nm, respectively, which were in good accordance with the photoluminescent characteristics. The Commissions International d′Eclairage (CIE) coordinates were located at (0.202, 0.337) and (0.246, 0.419), which were independent on the variation of forward bias.

Highly efficient white organic light-emitting diodes using two emitting materials for three primary colors (red, green, and blue)

The authors have demonstrated highly efficient white organic light-emitting diodes (WOLEDs) by using two emissive materials as a dopant, 1,4-bis[2-(7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)) vinyl] benzene (DAF-ph) and iridium(III) bis(5-acetyl-2-phenylpyridinato-N,C2′) acetylacetonate ((acppy)2Ir(acac)). It was found that the OLED fabricated in this study emitted a white color consisting of three primary colors (red, green, and blue). The luminance-voltage (L-V) characteristics of the WOLEDs showed the maximum luminance of 30500cd∕m2 at 14V and the maximum luminous efficiency of 38.0cd∕A, respectively. The CIEx,y coordinates of the WOLED also showed (x=0.33, y=0.40) at 10V.

Injection and transport processes in organic light emitting diodes based on a silole derivative

This paper reports on charge injection and transport in electroluminescent devices based on a silole derivative 1,1-dimethyl-2,5-bis(p-2,2′-dipyridylaminophenyl)-3,4-diphenylsilole (DMPPS). The devices are composed of tin doped indium oxide (In2O3:Sn or I TO)/poly(3,4-ethylene dioxythiophene doped with poly(styrene sulfonate)/DMPPS/metal. Current-voltage and luminance-voltage characteristics are first performed as a function of the electron injection barrier height and of the organic layer thickness. The voltage dependence of current and luminance varies with the metal cathode, i.e., Ca, Al, Cu, and Au. The effect of the DMPPS thickness in a double carrier device shows that electrons predominate and are bulk limited. An accurate investigation is carried out as a function of temperature for hole-only and bipolar devices, i.e., with gold and calcium cathodes. Hole-only devices (with Au cathode) exhibit an Ohmic behavior for low voltages. A hopping mechanism (thermally assisted tunnel transfer between localized states) agrees with experimental data, since activation energy is found close to 50meV. The electron transfer limitation is located at the DMPPS/cathode interface and the Fowler-Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With a Ca cathode, electron conduction is preponderant and is bulk limited. A power dependence J∝Vm with m>2 is consistent with the model of trap charge limited conduction. The total electron trap density was estimated to be 2×1018cm−3.

ITO 표면 처리와 음전극 변화에 따른 OLEDs의 특성 연구

In this study, we report an improved efficiency of Organic light emitting diodes(OLEDs), using <TEX>$UV/O_3$</TEX> treated anode and different cathode. We investigated the efficiency of OLEDs by <TEX>$UV/O_3$</TEX> treatment of ITO surface. We Performed <TEX>$UV/O_3$</TEX> treatment and found that <TEX>$UV/O_3$</TEX> treatment enhanced the performance of OLEDs. The fundamental structure of the OLEDs was ITO <TEX>$anode/{\alpha}-NPD/Alq_3/Al$</TEX> or Li:Al cathode. The Li:Al can improve the OLEDs efficiency dramatically in cathode because it has lower work function than Al. Current-voltage, Luminance-voltage characteristics and luminance efficiency were measured at room temperature.

Zn(HPB)2를 Hole Blocking Layer로 이용한 OLEDS의 특성 연구

Recently, organic light emitting diodes(OLEDs) is widely used as one of the information display techniques. We synthesized 2-(2-hydroxyphenyl)benzoxazole(<TEX>$Zn(HPB)_2$</TEX>). We studied the luminescent properties of OLEDs using <TEX>$Zn(HPB)_2$</TEX>. The ionization potential(IP) and the electron affinity(EA) of <TEX>$Zn(HPB)_2$</TEX> investigated using cyclic-voltammetry(C-V). The IP and EA were 6.5 eV and 3.0 eV, respectively. The PL and EL spectra of <TEX>$Zn(HPB)_2$</TEX> were observed at the wavelength of 450 nm. We used <TEX>$Zn(HPB)_2$</TEX> as an emitting layer and hole blocking layer. At the experiment about hole blocking effect, we inserted <TEX>$Zn(HPB)_2$</TEX> between emiting material layer(EML) and cathode, and between hole transport layer(HTL) and emitting material layer(EML). We measured current density-voltage and luminance-voltage characteristics at room temperature.

Synthesis and Luminescent Properties of Alq 3 Complex

New luminescent material, 6,11-dihydoxy-5,12-naphtacene-dione ·Alq 3 complex(Alq 2-Ncd), 1,4-dihydoxy-5,8-naphtaquinone ·Alq 3 complex(Al 2Nq 4) was synthesized. The Alq 2-Ncd and Al 2Nq 4 has big molecular weight and many π-electrons more than widely known Alq 3. And extended efforts have been made to obtain high-performance electroluminescent(EL) devices. We used hole transfer layer of powdered TPD to improve hole transfer and characteristics of interface in OLED. Current-voltage characteristics, luminance-voltage characteristics and luminous efficiency were measured by Flat Panel Display Analysis System(Model 200-AT) at room temperature. Photoluminescence spectra were measured by LS45(PerkinElmer instruments) at room temperature. This study indicates not only the sterical effect but also some other effects that would be responsible for the change of the emission wavelength, improvement of luminance and etc.

Origin of the improved luminance-voltage characteristics and stability in organic light-emitting device using CsCl electron injection layer

The luminance-voltage characteristics and stability were highly improved by replacing LiF with CsCl in organic light-emitting devices. To investigate the origin of these improvements, ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy were used. The additional shifts of the vacuum, highest occupied molecular orbital, and lowest unoccupied molecular orbital levels due to the CsCl layer reduce the width and height of the electron injection barrier, resulting in the improved luminance-voltage characteristics of the devices. Additionally, the intervening CsCl layer between Al and Alq3 prevents N–Al reactions among Alq3 molecules and Al, which reduces distortion or breakdown of Alq3 molecules and slows the degradation of the device.

Correlation between the Indium Tin Oxide morphology and the performances of polymer light-emitting diodes

This paper reports on performance enhancement of polymer light-emitting diodes (PLEDs) based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV) after Indium Tin Oxide (ITO) pre-treatment. Diluted aqua regia surface treatments of ITO substrates were investigated before PLEDs manufacturing. The correlation between ITO surface morphology and the properties of devices were discussed. The residual roughness, the global ITO thickness and developed anodic active area were found extremely dependent on treatment time. The performances of the ITO/BDMO-PPV/Ca/Al diodes were improved. With long treatments, the roughness was dramatically increased and poor PLEDs performances were achieved. The incorporation of a conducting polymer layer, poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonate, between the ITO layer and the emissive polymer, was also studied. With this buffer layer, no effect of the acidic treatment was observed on luminance–voltage characteristics and low turn-on voltage devices were made.

Effects of PEDOT:PSS Buffer Layer in a Device Structure of ITO/PEDOT:PSS/TPD/Alq3/Cathode

We have investigated the effects of hole-injection buffer layer in organic light-emitting diodes using poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)(PEDOT:PSS) in a device structure of <TEX>$ITO/PEDOT:PSS/TPD/Alq_{3}/cathode$</TEX>. Polymer PEDOT:PSS buffer layer was made by spin casting method. Current-voltage, luminance-voltage characteristics and efficiency of device were measured at room temperature with a variation of cathode materials; Al, LiF/Al, LiAl, and Ca/Al. The device with LiF/Al cathode shows an improvement of external quantum efficiency approximately by a factor of ten compared to that of Al cathode only device. Our observation shows that cathode is important in improving the efficiency of the organic light-emitting diodes.

Photoluminescent and electroluminescent investigations of OLEDs using Alq 2–Nq

Light-emitting display is one of the information display techniques that utilize conversion of electrical energy into light energy. In this study, new luminescent material, 1,4-dihdoxy-5,8-naphtquinone·Alq 3 complex (Alq 2–Nq) was synthesized. And extended efforts had been made to obtain high-performance electroluminescenct (EL) devices, since the first report of organic light-emitting diodes (OLEDs) based on Tris(8-hydroxyquinoline) aluminium (Alq 3). We used hole injection layer of powdered TPD to improve hole injection and characteristics of interface in OLEDs. Current–voltage characteristics, luminance–voltage characteristics and luminous efficiency were measured by Flat Panel Display Analysis System (Model 200-AT) at room temperature. PL spectra is measured by LS45 (Perkin-Elmer instruments) at room temperature.