Influence of capacitance effect on alternating-current organic light-emitting diodes

The capacitance effect on ITO/poly[2-methoxy-5-[(2′-ethylhexyl)oxy]-p-phenylenevinylene] (MEH-PPV)/Al is studied by impedance spectroscopy technology, and the electroluminescence (EL) mechanism of this device driven by a sinusoidal alternating-current (AC) bias is suggested. By calculating the RC time constant of the device, we find that it is in good agreement with the lag-time between the EL and applied AC bias. Also, the influence of operating frequency on the EL intensity of the device is presented and it is concluded that a low operating frequency is good for a high device performance.     

Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3

Blue organic light-emitting devices based on wide bandgap host material, 2-(t-butyl)-9, 10-di-(2-naphthyl) anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO₃) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO₃ (5 nm)/NPB (30 nm)/[TBADN: DSA-Ph (3 wt%)](35 nm)/BPhen (12 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the MoO₃∥BPhen-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA∥Alq₃ is based on, respectively. The J-V curves of ‘hole-only’ devices reveal that a small hole injection barrier between MoO₃∥NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Effect of laser-etched indium tin oxide on optoelectrical properties of organic light-emitting diodes

This study investigated how laser and wet etching methods of ITO substrates affect the optoelectrical properties of OLEDs. Experimental results indicated that the OLED with a laser-etched ITO substrate has a lower driving voltage than that with a wet-etched ITO substrate. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, the ITO etching methods yielded different surface morphologies of ITO pattern edges. The lower luminance of the OLED with a laser-etched ITO substrate is attributable to the fringe at the edge of ITO pattern, which causes a high local electric field resulting in the recrystallization of organic materials.     

High efficiency small molecule white organic light-emitting devices with a multilayer structure

In this paper, a new white organic light-emitting device (WOLED) with multilayer structure has been fabricated. The structure of devices is ITO/N, N-bis-(1-naphthyl)-N, N^′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) (40 nm)/NPB: QAD (1%): DCJTB (1%) (10 nm) /DPVBi (10 nm) /2, 9-dimethyl, 4, 7-diphenyl, 1, 10-phenanthroline (BCP) (d nm)/tris-(8-hydroxyquinoline) aluminium (Alq₃)(50-d nm)/LiF (1 nm)/Al (200 nm). In our devices, a red dye 4-(dicyanomethylene)-2-t-butyl-6 (1, 1, 7, 7-tetramethyl julolidyl-9-enyl)-4H-pyran (DCJTB) and a green dye quinacridone (QAD) were co-doped into NPB. The device with 8 nm BCP shows maximum luminance of 12 852 cd/m² at 20 V. The current efficiency and power efficiency reach 9.37 cd/A at 9 V and 3.60 lm/W at 8 V, respectively. The thickness of the blocking layer permit the tuning of the device spectrum to achieve a balanced white emission with Commission International de’Eclairage (CIE) chromaticity coordinates of (0.33,0.33). The CIE coordinates of device change from (0.3278, 0.3043) at 5 V to (0.3251, 0.2967) at 20 V that are well in the white region, which is largely insensitive to the applied bias.     

Long-term accelerated current operation of white light-emitting diodes

Long-term degradation tests regarding white light-emitting diodes based on InGaN were performed under accelerated current conditions, and the half-life of the light's output was estimated. An estimated mean half-life of 1.5×10⁴ h was obtained under the recommended 20-mA operating condition. The change in the emission spectrum was found to be slight, and the color quality was considered generally satisfactory over the long term.     

Effect of thickness on the stability of transparent conducting impurity‐doped ZnO thin films in a high humidity environment

The resistivity of transparent conducting Al‐ and Ga‐doped ZnO (AZO and GZO) thin films prepared with a thickness in the range from 20 to 200 nm on glass substrates at a temperature below 200 °C was found to increase with exposure time when tested in a high humidity environment (air at 90% relative humidity and 60 °C). The resistivity stability (resistivity increase) was considerably affected by the thin film thickness. In particular, thin films with a thickness below about 50 nm were very unstable. The increase in resistivity is interpreted as carrier transport being dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Efficient polymer white-light-emitting diodes with a single-emission layer of fluorescent polymer blend

Efficient polymer white-light-emitting diodes (WPLEDs) have been fabricated with a single layer of fluorescent polymer blend. The device structure consists of ITO/PEDOT/PVK/emissive layer/Ba/Al. The emissive layer is a blend of poly(9,9-dioctylfluorene) (PFO), phenyl-substituted PPV derivative (P-PPV) and a copolymer of 9,9-dioctylfluorene and 4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole (PFO-DHTBT), which, respectively, emits blue, green and red light. The emission of pure and efficient white light was implemented by tuning the blend weight ratio of PFO: P-PPV: PFO-DHTBT to 96:4:0.4. The maximum current efficiency and luminance are, respectively, 7.6 cd/A at 6.7 V and 11930 cd/m² at 11.2 V. The CIE coordinates of white-light emission were stable with the drive voltages.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

Solution-processed white organic light-emitting devices based on small-molecule materials

We investigated solution-processed films of 4,4′-bis(2,2-diphenylvinyl)-1,1′-bibenyl (DPVBi) and its blends with N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) by atomic force microscopy (AFM). The AFM result shows that the solution-processed films are pin-free and their morphology is smooth enough to be used in OLEDs. We have developed a solution-processed white organic light-emitting device (WOLEDs) based on small-molecules, in which the light-emitting layer (EML) was formed by spin-coating the solution of small-molecules on top of the solution-processed hole-transporting layer. This WOLEDs, in which the EML consists of co-host (DPVBi and TPD), the blue dopant (4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl) and the yellow dye (5,6,11,12-tetraphenylnaphtacene), has a current efficiency of 6.0 cd/A at a practical luminance of 1000 cd/m², a maximum luminance of 22500 cd/m², and its color coordinates are quite stable. Our research shows a possible approach to achieve efficient and low-cost small-molecule-based WOLEDs, which avoids the complexities of the co-evaporation process of multiple dopants and host materials in vacuum depositions.     

Luminescence mechanisms of green and blue organic light-emitting devices utilizing hole-blocking layers

Optical and electrical measurements on green and blue organic light-emitting devices (OLEDs) with and without hole-blocking layers (HBLs) were performed, and the luminescence mechanisms of green and blue OLEDs utilizing HBLs were investigated by using energy band diagrams and carrier density distributions. The dependence of the electroluminescence efficiencies on the existence of HBLs was described on the basis of a luminescence mechanism. The density distributions of the electrons and the holes in OLEDs under applied electric fields were estimated from the energy band diagrams, taking into account the electronic parameters and the layer thicknesses. The luminescence efficiencies and the color chromaticities were significantly affected by the existence of the HBLs. These analyses can help improve understanding of the luminescence mechanisms at play in and the electroluminescence efficiencies of green and blue OLEDs with HBLs, and the present results provide important information on the optical properties for enhancing the efficiencies of OLEDs operating in the green and the blue regions of the spectra.     

Blue and green-blue-variable light-emitting diodes based on the blend of polymer and organic molecules

Organic light-emitting diodes based on the blend of poly (p-phenylene vinylene) (PPV) derivative and naphthyl-imine–gallium complex have been fabricated by spin-coating method. Blue emission and blue-green variation depending on the ratio of the PPV derivative to the complex and the applied voltage have been observed. The investigation on PL (photoluminescence) and EL (electroluminescence) properties demonstrates that the improvement of the luminescent efficiency is related to the injection balance between holes and electrons, and the color variation is attributed to the variation of the recombination zone. Received: 7 July 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

Efficient white organic light-emitting devices using 4,7-diphenyl-1,10-phenanthroline as block layer

A white light-emitting device has been fabricated with a structure of ITO/m-MTDATA (45 nm)/NPB (10 nm)/DPVBi (8 nm)/DPVBi:DCJTB 0.5% (15 nm)/BPhen (x nm)/Alq₃ [(55−x) nm]/LiF (1 nm)/Al, with x=0, 4, and 7. BPhen was used as the hole-blocking layer. This results in a mixture of lights from DPVBi molecules (blue-light) and DCJTB (yellow-light) molecules, producing white light emission. The chromaticity can be readily adjusted by only varying the thickness of the BPhen layer. The CIE coordinates of the device are largely insensitive to the driving voltages. When the thickness of BPhen is 7 nm, the device exhibits peak efficiency of 6.87 cd/A (3.59 lm/W) at the applied voltage of 6 V, the maximum external quantum efficiency η_ext=2.07% corresponding to 6.18 cd/A, and the maximum brightness is 18494 cd/m² at 15 V.     

Temperature‐dependent electroluminescence intensity in green and blue (In,Ga)N multiple‐quantum‐well diodes

The electroluminescence (EL) intensity has been investigated of green and blue (In,Ga)N multiple‐quantum‐well diodes grown on c ‐plane sapphire over a wide temperature range and as a function of current between 0.01 mA and 10 mA. The EL intensity of the green diode with p‐(Al,Ga)N electron blocking layer does not show low‐temperature quenching, especially at low injection levels, previously observed for the blue (In,Ga)N quantum‐well diodes. This finding rules out possi‐ bilities that the freeze‐out of holes at deep Mg acceptor levels and the failure of hole injections through the p‐(Al,Ga)N layer are directly responsible for the EL quenching at temperatures below 100 K. Variations of the EL efficiency with current level suggest that capture/escape efficiencies of injected carriers by the wells play an important role for the determination of EL external quantum efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and transport properties of cubic spinel ZnCo2O4 thin films grown by reactive magnetron sputtering

We report on the growth of cubic spinel ZnCo₂O₄ thin films by reactive magnetron sputtering and bipolarity of their conduction type by tuning of oxygen partial pressure ratio in the sputtering gas mixture. Crystal structure of zinc cobalt oxide films sputtered in an oxygen partial pressure ratio of 90% was found to change from wurtzite Zn_1−xCo_xO to spinel ZnCo₂O₄ with an increase of the sputtering power ratio between the Co and Zn metal targets, D_Co/D_Zn, from 0.1 to 2.2. For a fixed D_Co/D_Zn of 2.0 yielding single-phase spinel ZnCo₂O₄ films, the conduction type was found to be dependent on the oxygen partial pressure ratio: n-type and p-type for the oxygen partial pressure ratio below ∼70% and above ∼85%, respectively. The electron and hole concentrations for the ZnCo₂O₄ films at 300 K were as high as 1.37×10²⁰ and 2.81×10²⁰ cm⁻³, respectively, with a mobility of more than 0.2 cm²/V s and a conductivity of more than 1.8 S cm⁻¹.     

Novel transparent Yb-based cathodes for top-emitting organic light emitting devices with high performance

We have studied three kinds of transparent low-work-function Yb-based cathodes for the top-emitting organic light emitting devices (TEOLEDs) with a structure of ITO/NPB/Alq₃/cathodes and compared them with each other. For the Yb/Au cathodes, a series of Yb layers with various thicknesses have been tested and it is found that the Yb layer with a thickness of 4 nm is the optimum one. The Yb:Au (19 nm) and Yb:Ag (19 nm) co-evaporation cathodes possess very high transmittance but relative poor electron injection; whilst the Yb (4 nm)/Au (15 nm) cathode possess a little lower transmittance but much improved electron injection and the TEOLED with this cathode has the highest power efficiency among the TEOLEDs with the three kinds of Yb-based cathodes mentioned above.     

Impact of the cathode metal work function on the performance of vacuum-deposited organic light emitting-devices

Received: 7 September 1998 / Accepted: 27 November 1998 / Published online: 24 February 1999     

Combinatorial fabrication and screening of organic light-emitting device arrays

The combinatorial fabrication and screening of 2-dimensional (2-d) small molecular UV-violet organic light-emitting device (OLED) arrays, 1-d blue-to-red arrays, 1-d intense white OLED libraries, 1-d arrays to study Förster energy transfer in guest-host OLEDs, and 2-d arrays to study exciplex emission from OLEDs is described. The results demonstrate the power of combinatorial approaches for screening OLED materials and configurations, and for studying their basic properties.     

Photovoltaic and Electroluminescence Characters in Hybrid ZnO and Conjugated Polymer Bulk Heterojunction Devices

We report electroluminescence in hybrid ZnO and conjugated polymer poly[2-methoxy-5-（3′, 7′-dimethyloctyloxy）- 1,4-phenylenevinylene] （MDMO-PPV） bulk heterojunction photovoltaic cells. Photoluminescence quenching experimental results indicate that the ultrafast photoinduced electron transfer occurs from MDMO-PPV to ZnO under illumination. The ultrafast photoinduced electron transfer effect is induced because ZnO has an electron affinity a bout 1.2 e V greater than that of MDMO-PP V. Electron ＇back transfer＇ can occur if the interfacial barrier between ZnO and MDMO-PPV can be overcome by applying a substantial electric field. Therefore, electrolumi- nescence action due to the fact that the back transfer effect can be observed in the ZnO：MDMO-PPV devices since a forward bias is applied. The photovoltaic and electroluminescence actions in the same ZnO：MDMO-PPV device can be induced by different injection ways： photoinjection and electrical injection. The devices are expected to provide an opportunity for dual functionality devices with photovoltaic effect and electroluminescence character.     

The relation between lattice-parameter and particle size in ZnS:Cu Phosphor

The effect of particle size on life time in electroluminescent phosphor was investigated using fractionated samples. The life time of large-particle size phosphors was longer than that of small-particle size phosphor. It was found that particle size has a close relation with the lattice-parameter magnitude; the lattice parameter was larger in a small-particle size phosphor when the activator and coactivator content were the same. It was concluded that the lattice parameter was an important brightness life-time factor.     

On the origin of intrinsic donors in ZnO

As-grown undoped zinc oxide (ZnO) films have been annealed in zinc-rich, oxygen-rich and vacuum ambient, and the electron concentration varied greatly after the annealing process. It decreased nearly two orders of magnitude after the sample was annealed in oxygen, while increased nearly three times after annealed in metallic zinc ambient, and increased slightly after annealed in vacuum. It was found that the variation trend of the electron concentration is always the same with the expected variation of oxygen vacancy (V_O) under the three investigated conditions, it is thus speculated that V_O may be the dominant donor source in ZnO. By supplying more oxygen during the growth process to suppress V_O, ZnO films with lower electron concentration were obtained, which verifies the above speculation.