Characteristics Of Organic Light-emitting Diodes Research Articles

Device Characteristics of Organic Light-Emitting Diodes Based on Electronic Structure of the Ba-Doped Alq3 Layer

Organic light-emitting diodes (OLEDs) with a Ba-doped tris(8-quinolinolato)aluminum(III) (Alq3) layer were fabricated to reduce the barrier height for electron injection and to improve the electron conductivity. In the OLED consisting of glass/ITO/4,4',4"-tris[2-naphthylphenyl-1-phenylamino]triphenylamine (2-TNATA, 30 nm)/4,4'-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPB, 18 nm)/Alq3 (42 nm)/Ba-doped Alq3 (20 nm, x%: x = 0, 10, 25, and 50)/Al (100 nm), the device with the Alq3 layer doped with 10% Ba showed the highest light out-coupling characteristic. However, as the Ba dopant concentration was increased from 25% to 50%, this device characteristic was largely reduced. The characteristics of these devices were interpreted on the basis of the chemical reaction between Ba and Alq3 and the electron injection property by analyzing the electronic structure of the Ba-doped Alq3 layer. At a low Ba doping of 10%, mainly the Alq3 radical anion species was formed. In addition, the barrier height for electron injection in this layer was decreased to 0.6 eV, when compared to the pristine Alq3 layer. At a high Ba doping of 50%, the Alq3 molecules were severely decomposed. When the Ba dopant concentration was changed, the light-emitting characteristics of the devices were well coincided with the formation mechanism of Alq3 radical anion and Alq3 decomposition species.

대향식 스퍼터링법으로 증착된 ITO 양극 위에 제작된 OLED 성능

Indium tin oxide (ITO) has been commonly used as an anode for organic light emitting diode (OLED), because of its relatively high work function, high transmittance, and low resistance. The ITO was mostly deposited by capacitive type DC or RF sputtering. In this study we introduced a new facing target sputtering method. On applying this new sputtering method, the effect of fundamental deposition parameters such as substrate heating and post etching were investigated in relation to the resultant I-V-L characteristics of OLED. Three kinds of ITOs deposited at room temperature, at and at with after surface modification by plasma etching were compared. The OLED on ITO deposited with substrate heating and followed by etching showed better I-V-L characteristics, which starts to emit light at 4 volts and has luminescence of at 9 volts. The better I-V-L characteristics were ascribed to the relevant surface roughness with uniform micro-extrusions and to the equi-axed micromorphology of ITO surface.

Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultrathin hole-injection layer of molybdenum oxide

We show that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing the thickness of a hole-injection layer (HIL) of molybdenum oxide (MoO3) inserted between indium tin oxide and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). From results of the electroluminescence (EL) characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO/MoO3/α-NPD.

Electrical characterization of organic light-emitting diodes using dipotassium phthalate as n-type dopant

An efficient n-doped electron transport layer composed of 4,7-diphenyl-1,10-phenanthroline (BPhen) and dipotassium phthalate (PAK2) has been developed. By temperature-dependent admittance spectroscopy, the incorporation of PAK2 into BPhen is found to raise the Fermi level from 1.7eV to only around 0.5eV below BPhen’s lowest unoccupied molecular orbital, which further enhances the efficiency of electron injection from an Al cathode. When this n-doped layer is adopted in an organic light-emitting diode device, the green fluorescent 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one do-ped device can achieve a current efficiency of 16cd∕A and a power efficiency of 10.9lm∕W at 1000cd∕m2.

New cathode materials for organic light-emitting diodes: Tm:Yb and Eu:Yb

Performance characteristics of organic light-emitting diodes (OLEDs) with mono-(Al, Sm, Tm, Yb) and bimetallic (Ca:Al, Eu:Yb, Tm:Yb) cathodes are compared. It is shown that OLEDs with the Tm:Yb cathodes are the best of the studied devices in terms of the turn-on voltage, operating voltage, current efficiency, and luminance efficiency.

Electrical Characteristics of Organic Light-emitting Diodes Fabricated by Varying a Hole-size in Evaporation Boat

Electrical characteristics of organic light-emitting diodes were investigated by varying a hole-size in evaporation boat in the device structure of ITO/tris(8-hydroxyquinoline) aluminum/Al. The device was manufactured using a thermal evaporation under a base pressure of Torr. The emitting organics were evaporated to be a thickness of 100 nm at a deposition rate of . A cylindrical-shaped evaporation boat was made out of stainless steel with a small size of hole on top of the boat. Several evaporation boats were made having a different hole size on top; 0.8 mm, 1.0 mm, 1.5 mm, and 3.0 mm. We found that when the hole size on top of the evaporation boat is 1.0 mm, the average roughness is rather smoother compared to the other ones. Also, luminance and external quantum efficiency are superior to the others. Compared to the ones from the devices made with the hole-size of 0.8 mm boat. The luminance and external quantum efficiency of the device made with the hole-size of 1.0 mm boat were improved by a factor of seventy and thirty three, respectively. Also operating voltage is reduced to 2 V.

Enhancement of Light Extraction from Organic Light-Emitting Diodes with Two-Dimensional Hexagonally Nanoimprinted Periodic Structures Using Sequential Surface Relief Grating

Electric characteristics of organic light-emitting diodes (OLEDs) with two-dimensional hexagonally nanoimprinted periodic structures were investigated, and enhanced light extraction from this device was confirmed. A master mold was fabricated from an azobenzene polymer thin film patterned by sequential surface relief grating formation into a three-directional structure. The device shows a higher current density and a higher current efficiency, although leakage current is also higher than that of a device without periodic structures. These results are caused by the extraction of diffracted light from waveguided and substrate-guided light even when incident light has larger angles than the critical angle for total reflection.

Effects of ITO surface modification using self-assembly molecules on the characteristics of OLEDs

Indium tin oxide (ITO) has been commonly used as an anode of organic light emitting diode (OLED), because of relatively high work function, high transmittance, and low resistance. However, interface property between ITO and adjacent organic layer limits hole injection from the anode electrode. We have synthesized 4′-nitrobiphenyl-4-carboxylic acid (NBCA) and fabricated the hole-only device consisting of ITO/NBCA self-assembled monolayer (SAM)/TPD (1500 Å)/Al (500 Å) and the organic light emitting diode (OLED) consisting of ITO/NBCA SAM/TPD (600 Å)/Alq 3 (600 Å)/Al (600 Å). The prepared hole-only device with NBCA exhibited lower driving voltage than the device using 4-nitrobenzoic acid (NBA). OLED using NBCA also showed high external quantum efficiency.

Characteristics of organic light-emitting diodes with conducting polymer anodes on plastic substrates

The fabrication of conducting polymer films by a screen-printing method and characterization of an organic light-emitting diode (OLED) implemented using these films as an anode on plastic substrates are reported. Organic transparent electrode materials containing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (PEDOT:PSS) were used after the modification by a unique nanoparticle binder design. The electro-optical properties as well as mechanical stabilities of these films were measured. The OLED performances when employing these films were comparable to that of OLEDs using indium tin oxide (ITO) despite their relatively poor conductivity. An external quantum efficiency of the OLED using this anode is ∼3.5%, which is about twice as that of OLEDs using ITO. These results show that the organic transparent electrode of a conducting polymer film patterned by the screen-printing method is a potential candidate for an electrode of the flexible OLED.

Study on temperature characteristics of organic light-emitting diodes based on tris-(8-hydroxylquinoline)-aluminum

Both single-layer and double-layer organic light-emitting devices based on tris-(8-hydroxylquinoline)-aluminum (Alq3) as emitter are fabricated by thermal vacuum deposition. The electroluminescent characteristics of these devices at various temperatures are measured, and the temperature characteristics of device performance are studied. The effect of temperature on device current conduction regime is analyzed in detail. The results show that the current-voltage (I-V) characteristics of devices are in good agreement with the theoretical prediction of trapped charge limited current (TCLC). In addition, both the charge carrier mobility and charge carrier concentration in the organic layer increase with the rise of temperature, which results in the monotonous increase of Alq3 device current. The current conduction mechanisms of two devices at different temperatures are identical, but the exponent m in current-voltage equation changes randomly with temperature. The device luminance increases slightly and the efficiency decreases monotonously due to the aging of Alq3 luminescent properties caused by high temperature. A tiny blue shift can be observed in the electroluminescent (EL) spectra as the temperature increases, and the reduction of device monochromaticity is caused by the intrinsic characteristics of organic semiconductor energy levels.

Enhanced hole injection and transport in molybdenum-dioxide-doped organic hole-transporting layers

We have found that molybdenum dioxide (MoO2) is an excellent dopant for enhancing electrical conductivities in organic hole-transporting layers. We fabricated hole-only devices with an alpha-sexithiophene (α-6T) layer doped with MoO2 at various concentrations to investigate how doping MoO2 into the α-6T layers influences the hole-injection and hole-transport characteristics of these layers. We observed a marked increase in electrical conductivity as a result of the MoO2 doping. The 30-mol % MoO2-doped α-6T layer had a high electrical conductivity of 8.9±1.3×10−6 S/cm. From the results of our visible/near-infrared absorption spectra study of these doped layers, we confirmed that this increase in electrical conductivity is caused by a charge transfer between MoO2 and α-6T, which leads to an increase in free hole concentration in the doped layers and the formation of an ohmic contact at an electrode/α-6T interface. In the latter part of this paper, we discuss current flow and electroluminescence (EL) characteristics of organic light-emitting diodes (OLEDs) with a 30-mol % MoO2-doped α-6T hole-transporting layer and a 30-mol % Cs-doped phenyldipyrenylphosphine oxide (POPy2) electron-transporting layer. We achieved an extremely low driving voltage of 3.1 V required for a current density of 100 mA/cm2 in the doped OLEDs owing to the use of the α-6T and POPy2 layers with high carrier mobilities and the excellent p-type MoO2 and n-type Cs dopants. We demonstrated the enhancement of power efficiencies by ≈2 times in the doped OLEDs compared with undoped OLEDs and observed bright EL at low driving voltages in the doped OLEDs, for example, 100 cd/m2 at 2.3 V, 1000 cd/m2 at 2.7 V, and 10 000 cd/m2 at 3.3 V.

Organic Light-Emitting Diodes with Highly Conductive Polymer Electrodes as Anode and Their Stress Tolerance

The fabrication and characteristics of organic light-emitting diodes (OLEDs) with highly conductive polymer electrodes as an anode and the stress tolerance of the devices fabricated on polymeric substrates were studied. By inserting a wet-processed organic layer between a polymer electrode and a dry-processed hole-transport layer, the surface emission pattern from an OLED was markedly improved. For the device with a wet-processed organic layer (methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene), the uniform surface emission resulted from the uniform applied electric field in the emissive layer and the improvement in interface adherence. The OLED with a wet-processed layer as a hole injection layer showed a maximum luminance and a maximum efficiency of 10,000 cd/m2 and 3.5 cd/A, respectively. For the device fabricated on a polymeric substrate, the impact testing of the OLEDs with highly conductive polymer electrodes [poly(ethylenedioxythiophene):poly(styrene sulfonic acid)] as an anode revealed that the emission lasted for more than several ten thousand steps. A highly conductive polymer electrode had a sufficient tolerance to mechanical stress, as determined by comparing devices with indium tin oxide and a highly conductive polymer as anodes.

Fabrication and characterization of OLEDs using PEDOT:PSS and MWCNT nanocomposites

Nanocomposites of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and multi-wall carbon nanotubes (MWCNTs) were investigated as hole injection layers in organic light-emitting diodes (OLEDs) consisting of a hole transport layer of N, N′-diphenyl- N, N′-di(m-tdyl)benzidine (TPD) and a light-emitting/electron transport layer of tris(8-hydroxyquinolinato)aluminium (Alq3) sandwiched between a transparent ITO anode based on polyethylene terephthalate (PET) substrate and a metal cathode. The OLEDs were fabricated by physical vapor deposition method and the current–voltage–luminescence ( I– V– L) characteristics of the devices were examined. It was found that high light intensity can be obtained by adopting nanocomposites of PEDOT:PSS and MWCNTs as hole injection layer of OLEDs. Reasons for the improved performance of the devices were discussed. The improved electric properties of the nanocomposites might be the reason for the improvement of OLED performance achieved by incorporating MWCNTs into hole injection layer of PEDPT:PSS.

Electrical and Optical Characteristics of Organic Light-Emitting Diodes for Various Thicknesses of the PTFE Layer

Organic light-emitting diodes (OLEDs) are attractive because of possible application in display with low-operating voltage, low-power consumption, self-emission, and a capability for multicolor emission by selection of emissive materials. In a device structure of ITO/polytetrafluorethylene (PTFE)/N,N’-diphenyl-N,N’-bis(3-methyphenyl)-1,1’-biphenyl-4,4’diamine(TPD)/tris(8-hydroxyquinoline)aluminum(Alq3)/Al, the effect of the PTFE layer was investigated, which is one of the important factors affecting the electrical characteristics of organic light-emitting diodes. We measured the current-voltage characteristics and the luminance of the device for various thicknesses of the PTFE layer in an ITO/PTFE/TPD/Alq3/Al device structure. Compared with the device without PTFE, we found the optimal thickness of PTFE to be 2.0 nm from the device we studied. By using PTFE as a hole injection layer, we improved the luminance of the device by about 400 %.

Simultaneous Oxygen Plasma and Thermal Treatments of an ITO Surface to Improve the Electrical Characteristics of Organic Light-Emitting Diodes

The electrical characteristics of organic light emitting diodes (OLEDs) were improved by lowpower oxygen plasma processing of an indium tin oxide (ITO) surface at an elevated temperature of 500 ◦C in this work. ITO is widely used as a transparent electrode in light-emitting devices, and OLED device performance is sensitive to the surface properties of ITO. In this research, a simultaneously oxygen-plasma and thermally treated ITO surface showed better device performance than an oxygen-plasma-treated ITO surface in terms of current density, brightness, and power efficiency. OLED devices with the structure of ITO/TPD/Alq3/LiF/Al were fabricated, and the surface properties of ITO were investigated by using various characterization techniques. The work function of the modified ITO, as determined by using ultraviolet photoelectron spectroscopy (UPS), increased from 4.70 to 5.17 eV. An X-ray photoelectron spectroscopy (XPS) analysis showed an oxygen fraction change from 59.9 % to 70.1 % after the surface treatment. Based on the measurements, we conclude that hole injection is improved due to the high electronegativity of the oxygen adsorbed on the ITO surface. The XPS analysis also showed that the surface carbon composition decreased from 15.1 % to 4.67 % after the surface treatment. A reduction in carbon contamination is also attributed to the improvement in the electrical characteristics. Thus, we demonstrated that simultaneous oxygen plasma and thermal treatment is a proper way for improving the electrical characteristics of OLED devices.

Device Characteristics of Organic Light-Emitting Diodes Comprising Terfluorene Modified with Triphenyltriazine

The characteristics of a series of organic blue light-emitting diodes comprising terfluorene were investigated. The composition of the emitter layer was either a neat terfluorene compound, F(MB)3, F(MB)3 doped with TRZ, triphenyltriazine, as an electron acceptor, or hybrid compounds, TRZ-3F(MB)3 and TRZ-1F(MB)3, viz. TRZ functionalized with F(MB)3. The emitter layer was deposited by spin-casting from a solution on top of a PEDOT:PSS hole-injecting layer. These acceptor-containing emitters were responsible for a decreasing electroluminescence efficiency with an increasing TRZ content. The loss of efficiency can be attributed to an enhanced electron transport through the emitter layer and a consequent shift of the electron−hole recombination zone toward the exciton-quenching PEDOT:PSS interface. Compared to TRZ as the electron-accepting dopant in F(MB)3, TPBI caused a more substantial decrease in emission efficiency and a more pronounced rise in drive voltage presumably because of the protonation of TPBI by PEDOT:PSS to a much greater extent.

Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by cross-linking

X-ray photoelectron spectroscopy (XPS) measurement of the ratio of poly(3,4-ethylenedioxythiophene) (PEDT) to polystyrenesulfonate (PSS) reveals accumulation of PEDT+ at the interface between the PEDT:PSSH hole-injection layer and the organic semiconductor during diode operation. This ionic drift of PEDT+ occurs even at low fields of 1Vcm−1, which will have an impact on the operational stability of the characteristics of organic light-emitting diodes. XPS and Raman spectroscopy indicate that dedoping of PEDT+ does not occur significantly in hole-only devices. Cross-linking at the 1mol% level can stabilize the conducting polymer sufficiently against electromigration.

Modification of anode surface in organic light-emitting diodes by chalcogenes

Influence of thin chalcogen X (S, Se, Te) interlayer between anode (indium–tin oxide, ITO) and a layer of N, N′-bis(3-methylphenyl)- N, N′-diphenylbenzidine (TPD) used as a hole-transport layer (HTL) on the operating characteristics of organic light-emitting diodes (OLEDs) of composition ITO/ X/TPD/Alq 3/Yb (Alq 3 – aluminum 8-quinolinolate) has been investigated. It was found that the sulphur layer decreases operating voltage and enhances operating stability of a device while the selenium or tellurium interlayers impair these characteristics.

The effect of multilayer quantum well structure on the characteristics of OLEDs

Organic light-emitting diodes (OLEDs) with multilayer quantum well (MQW) structure were fabricated, which consisted of alternate organic materials 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) and tris(8-hydroxyquinoline) aluminum (Alq 3). PBD is used as potential barrier layer, Alq 3 used as potential well layer and emitting layer. Compared with double-layer structure, the luminescent characteristics of the devices with MQW structure were investigated. MQW structures conduce to energy transfer between wells and barriers, which is attributed to good overlap and the decrease of the distance between layers. The MQW structures make electrons and holes distribute in different wells and then increase the number of the formation of excitons to further enhance their recombination efficiency. Hence, such device achieves the maximum brightness and efficiency of 3630 cd/m 2 and 3.28 cd/A, respectively.

The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N, N′-bis-(1-naphthyl)- N, N′-1-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′- N, N′-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq 3) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq 3/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq 3 increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq 3 benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).