Characteristics Of Organic Light-emitting Devices Research Articles

Modeling of carrier transport in organic light emitting diode with random dopant effects by two-dimensional simulation.

To model the carrier transport in organic light-emitting diodes (OLEDs) with random dopant effects in the emitting layer, two-dimensional simulation was used. By including the Gaussian shape density of states and field-dependent mobility in the Poisson and drift-diffusion solver, the carrier transport, trapping in the dopant state, and radiative recombination were accurately modeled. To examine the model, the current-voltage characteristics of organic light-emitting devices were compared. The host material in the emitting layer was 2,2-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)biphenyl (BImBP), which was doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) at various concentrations. By including the random doping model, the trend of mobility was altered and the radiative efficiency fitted experimental values well.

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices.

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.

Characteristics of organic light-emitting devices consisting of dye-doped spin crossover complex films

Two organic light-emitting devices (OLEDs) were fabricated to investigate the mechanism of electroluminescence (EL) switching accompanying the spin transition of [Fe(dpp)2](BF4)2 (dpp=2,6-di(pyrazol-1-yl)pyridine) observed in an OLED with the structure indium tin oxide (ITO)/[Fe(dpp)2](BF4)2:chlorophyll a/Al, consisting of a chlorophyll a (Chl a)-doped [Fe(dpp)2](BF4)2 film. One OLED consisted of poly(N-vinylcarbazole) (PVK) between an ITO electrode and the active layer, and the other contained the electron transporting dye Nile Red (NR) as an emitting dopant material instead of the hole transporting Chl a. In both devices ITO/PVK/[Fe(dpp)2](BF4)2:Chl a/Al and ITO/[Fe(dpp)2](BF4)2:NR/Al, EL emission from the dye compound was observed, irrespective of the spin state of [Fe(dpp)2](BF4)2. It was determined that the EL switching accompanying the spin transition was dominated by a change in the molecular orbital level concerning electron transport in [Fe(dpp)2](BF4)2.

Correlations of impedance–voltage characteristics and carrier mobility in organic light emitting diodes

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage ( Z– V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z– V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z– V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.

Optoelectrical characteristics of organic light-emitting devices fabricated with different cathodes

Organic light-emitting devices (OLEDs) with various cathode structures were prepared on indium tin oxide (ITO) substrates by vacuum sublimation technique, and the effects of the device cathodes on the electroluminescence (EL) characteristics of OLEDs were studied in terms of the luminance, efficiency, driving voltage and threshold voltage. The results demonstrate that the optical and electrical performance of OLEDs depend on the properties of the devices' cathodes and the characteristics of the cathode–organic interface and the organic–organic interface. The optoelectrical performance of a device with composite cathodes is better than that of the devices with metal alloy and pure metal cathodes. The improvement in the device performance can be attributed to a more efficient electron injection at the cathode–organic interface, a better balanced hole and electron recombination in the light-emitting layer and fewer accumulated charges near the organic–organic interface.

Influence of p-isopropenylcalixarenestyrene copolymer buffer layer over Alq3based OLEDs

The characteristics of organic light-emitting devices based on aluminum tris-(8-hydroxy-quinoline) (Alq3), N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) with novel p-isopropenyl-calix[8]arenestyrene copolymer buffer layer and Al cathode were investigated. The devices with TPD/Alq3/Al were also fabricated in the same way for comparison. The p-isopropenylcalix[8]arenestyrene copolymer used as buffer layer greatly improved the performance of the device and increased the device efficiency and stability.

Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices

In this paper, we experimentally and theoretically investigated the optical characteristics of organic light-emitting devices (OLEDs), having different pixel sizes and attached with patterned microlens array films. For a regular microlens array, though it can extract the waveguiding light and increase luminous current efficiency for a large-pixelated OLED, we observed that it decreases the luminance to an even lower level than that of the planar OLED as its pixel size is close to the microlens dimension. Although a microlens can effectively outcouple the light rays originally at incident angles larger than the critical angle, it also can impede the outcoupling for the light rays originally at incident angles smaller than the critical angle. Enhancement or reduction of the light extraction depends on the relative positions of the light emitting point and the microlens. Therefore, we proposed a center-hollowed microlens array, of which the microlenses directly upon the pixel are removed, and proved that it can increase the luminous current efficiency and luminous power efficiency of a small-pixelated OLED. By attaching this patterned microlens array, 87% of luminance enhancement in the normal direction was observed for a 0.1x0.1 mm2 OLED pixel. On the other hand, a regular microlens array resulted in 4% decrease under the same condition.

Influence of PVK: NPB hole transporting layer on the characteristics of organic light-emitting devices

A doping system consisting of NPB and PVK is employed as a composite hole transporting layer (CHTL). By adjusting the component ratio of the doping system, a series of devices with different concentration proportion of PVK:NPB are constructed. The result shows that doping concentration of NPB enhances the competence of hole transporting ability, and modifies the recombination region of charge as well as affects the surface morphology of doped film. Optimum device with a maximum brightness of 7852 cd/m2 and a power efficiency of 1.75 lm/W has been obtained by choosing a concentration proportion of PVK:NPB at 1:3.

Effect of NaCl buffer layer on the performance of organic light-emitting devices (OLEDs)

The characteristics of organic light-emitting devices based on tris-(8-hydroxyquinoline) aluminum (Alq 3 ) with NaCl thin films inserted at both anode/organic and organic/cathode interfaces are investigated. We optimize the thicknesses of NaCl at both interfaces, and we get the optimal device with the structure of ITO/NaCl(2.0 nm)/N,N'-bis(l-naphthyl)-N,N'-diphenyl-1,1' biphenyl-4,4'-dimaine (NPB)/Alq 3 /NaCl(2.0 nm)/Al. The device shows much better electroluminescent (EL) performance than that of the as-stated standard device with only LiF buffer layer. The maximal current efficiency and power efficiency is improved by 31% and 67%, the turn-on voltage is also reduced from 4.2 V for the standard device to 3.1 V for our target device. This is due to enhanced electron injection and blocked hole injection, which results in more balanced hole and electron currents.

Plasma treatment of ITO surfaces to improve luminescence characteristics of organic light-emitting devices with dopants

Indium-tin-oxide (ITO) surfaces were treated with O 2 and H 2 plasmas. The contact angle of water, X-ray photoelectron spectroscopy (XPS) spectra, and luminescence characteristics of organic light-emitting devices (OLEDs) fabricated on plasma-treated ITO surfaces using poly( N-vinylcarbazole) (PVK) films doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) or Nile Red were investigated. Using O 2 plasma treatment, the contact angle was reduced from 35° to 13°. The luminance of OLEDs with rubrene was significantly improved. The luminous color of an OLED inserted poly (3,4-ethylenedioxythiophene)-polystyrene sulphonic acid (PEDOT-PSS) layer between Nile Red-doped PVK film and O 2-plasma-treated ITO surface turned white as the applied voltage increased.

Electromagnetic modeling of organic light-emitting devices

Based on the rigorous electromagnetic wave theory, a numerical model for simulating the radiation characteristics of organic light-emitting devices (OLEDs) is developed. In particular, a novel method for overcoming the numerical difficulty in taking the thick glass substrate into account is proposed. The numerical results confirm the importance of the effects of the thick glass substrate. The algorithms based on the numerical model are then used for evaluating the dependencies of OLED radiation characteristics on various parameters, including the thickness of different device layers and the cathode metal variety. In the study of the effect of emission layer (EML) thickness, it is found that the radiation spectral peak red shifts with increasing EML thickness. This trend is consistent with the experimental result.

Optimal thickness of hole transport layer in doped OLEDs

Current-voltage (I–V) and electroluminescence (EL) characteristics of organic light-emitting devices with N,N’-Di-[(1-naphthalenyl)-N,N’-diphenyl]-(1,1’-biphenyl)-4,4’-diamine (NPB) of various thicknesses as the hole transport layer, and tris(8-hydroxyquinoline)aluminum (Alq3) selectively doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) as the electron transport layer, have been investigated. A trapped charge induced band bend model is proposed to explain the I–V characteristics. It is suggested that space charge changes the injection barrier and therefore influences the electron injection process in addition to the carrier transport process. Enhanced external quantum efficiency of the devices due to the electron blocking effect of an inserted NPB layer is observed. The optimal thickness of the NPB layer is experimentally determined to be 12±3 nm in doped devices, a value different from that for undoped devices, which is attributed to the electron trap effect of DCM molecules. This is consistent with the result that the proportion of Alq3 luminescence in the total electroluminescence (EL) spectra increases with NPB thickness up to 12 nm under a fixed bias.

Improving organic light-emitting devices by modifying indium tin oxide anode with an ultrathin tetrahedral amorphous carbon film

The characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with an ultrathin tetrahedral amorphous carbon (ta-C) film on indium tin oxide have been investigated. The device with a 1.0-nm ta-C layer has the highest current and power efficiency. The current efficiency of a device with a ta-C layer thickness of 1.0nm is 3.7cd∕A at 20mA∕cm2, however, the current efficiency of a standard device without a ta-C layer is 2.56cd∕A at the same current density. The current efficiency is improved by 46% compared to the standard device. Although there is a 1.2-V increase in driving voltage for 100-cd∕m2 luminance, the power efficiency is still improved by 22% compared to that of the standard device. The improvement of the efficiency is due to smoothing indium tin oxide surface, blocking hole injection from anode and balancing hole and electron currents. The optimal thickness of ta-C layer for hole injection mechanics can be understood by tunneling.

The role of MgF2 buffer layer in tris-(8-hydroxyquinoline)aluminium-based organic light-emitting devices with Mg:Ag cathode

The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminium with a thin layer of MgF2 inserted at the indium-tin-oxide (ITO) and organic interface or the organic and Mg:Ag cathode interface are investigated. A 1.0 nm MgF2 thin layer can enhance electron injection when it is inserted only between organic electron transporting layer and Mg:Ag alloy cathode, but can block hole injection when it is inserted only between the ITO anode and organic hole transporting layer. By inserting MgF2 at both sides on the ITO anode and under the Mg:Ag cathode, the current efficiency of the device is improved by 74%, and power efficiency is also improved by 18% at a current injection of 20 mA cm−2, compared to the standard device without MgF2 buffer layer. This is due to the increased electron and decreased hole injection, which results in more balanced electron and hole injection, and more efficient exciton formation. The increased electron injection can be well understood by the tunnelling effect model.

The role of LiF buffer layer in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices with Mg:Ag cathode

— The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with a thin layer of LiF inserted at the ITO and organic interface or organic and Mg:Ag cathode interface were investigated. A thin layer of LiF can enhance the electron injection when it was inserted only between the organic electron-transporting layer and the Mg:Ag alloy cathode, but can block hole injection when inserted between the ITO anode and the organic hole-transport layer. By inserting both a 1.0-nm LiF layer at side of the ITO anode and a 0.5-nm LiF layer under the Mg:Ag cathode, the device, at a current injection of 10 mA/cm2, exhibited the highest current efficiency of 8.2 cd/A and power efficiency of 1.93 lm/W for all the types of devices investigated in this study. Both the current efficiency and power efficiency of the device were improved by 1.2 times at a current injection of 10 mA/cm2, compared to the standard device without any LiF buffer layer. This is due to the increased electron injection and decreased hole injection that off-sets the imbalance of electron and hole injection and brings it towards the balanced injection of electrons and holes, thus reducing the non-productive hole current.

Enhanced performance of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices with LiF/Mg:Ag/Ag cathode

The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum devices with LiF/Mg:Ag/Ag cathodes were investigated. The devices with Mg:Ag/Ag, LiF/Al and Al-only cathodes were also fabricated in the same run for comparison. Similar to the LiF/Al cathodes, the LiF/Mg:Ag/Ag cathode greatly improved the performance of the device over the Mg:Ag/Ag cathode. A LiF layer of 0.5 nm significantly enhanced the electron injection and resulted in lower turn-on voltage and increased device efficiency. The turn-on voltage of the device with 0.5 nm LiF as buffered layer is as low as 2.8 V. At injection current density of 20 mA/cm2, the current efficiency and power efficiency of the device is 5.56 cd/A and 1.94 lm/W, respectively. Compared to the device with Mg:Ag/Ag only cathode, the turn-on voltage is 1.6 V lower, the current efficiency improved by 75 %, and power efficiency is almost double. The performance of the device is also much improved over the device with LiF/Al cathode.

Electron blocking and hole injection: The role of N,N′-Bis(naphthalen-1-y)-N,N′-bis(phenyl)benzidine in organic light-emitting devices

The current density-luminance-voltage characteristics of organic light-emitting devices (OLEDs) with N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium–tin–oxide/NPB/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) (30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM EL to the whole EL decreases with increasing NPB thickness. This suggests that the NPB layer blocks electron transport to the Alq3:DCM layer. The Förster energy transfer from the 60 nm Alq3 layer to the DCM molecules is ruled out by the EL behavior observed after quenching excitons in the Alq3 layer. The origin of the difference in the optimal N,N′-Bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) thicknesses reported by other two different groups is also discussed.

Buffer-layer-induced barrier reduction: Role of tunneling in organic light-emitting devices

Based on the WKB approximation of the tunneling model, we calculate the J–V characteristics of organic light-emitting devices (OLEDs) having buffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.

Influence of space charges on the current–voltage characteristic of organic light-emitting devices

The role of space charges on the device characteristics of multilayer organic light-emitting devices (OLEDs) is investigated. We studied OLEDs consisting of copper phthalocyanine (CuPc) as buffer and hole injection layer, N, N′-di(naphthalene-1-yl)- N, N′-diphenyl-benzidine (NPB) as hole transport layer, and tris(8-hydroxyquinolinato)aluminum (Alq 3) as electron transport and emitting layer sandwiched between high and low work function metal electrodes. Detailed current–voltage measurements show that the device characteristics at low bias depend strongly on sweep direction as well as on sweep speed, indicating that space charges accumulate within the organic layers. On the one hand these space charges increase the electric field for electron injection at the cathode, on the other hand they screen the applied electrical field and thus determine the steepness of the current–voltage characteristics. Reducing these space charges by fabricating optimized structures where the limiting interfaces between the different organic layers are graded results in a significantly enhanced current flow and higher brightness at a given voltage.

Temperature dependent device characteristics of organic light-emitting devices

Current–voltage characteristics of single and hetero-layer light-emitting devices with an aromatic diamine (TPD) as hole transport material and tris-8-(hydroxyquinoline) aluminum (Alq 3) as electron transport material and emitter have been investigated over a wide temperature range and for various film thickness in order to identify the limiting mechanism: charge carrier transport or injection. From the observed thickness and temperature dependence, pure injection limitation can be ruled out as dominant mechanism. Instead, the voltage dependence of the current density can be well described by power laws j∝ V m+1 (with V corrected by the built-in potential) with temperature dependent exponents m ranging from 4 to 25. This can be interpreted in terms of space charge limited currents (SCLC) in Alq 3 with an exponential energetic distribution of traps where m is given by m= E t/ kT. A reasonable trap energy of 0.15–0.2 eV is obtained by a temperature dependent analysis of the I–V characteristics. However, the thickness dependence cannot be satisfactorily explained by the simple SCLC-model. This indicates that more sophisticated models are required.