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.     

Polymer White-Light-Emitting Diodes with High Work Function Cathode Based on a Novel Phosphorescent Chelating Copolymer

Polymer white-light-emitting diodes are fabricated based on the blend of poly[9,9-di-（2-ethylhexyl）-fluorenyl-2, 7- diyl]-end capped with polysilsesquioxane （PFO） and a chelating copolymer of poly[（9,9-bis（3′-（N,N-dimethylamino） propyl）-2, 7-fluorene-alt-2, 7-（9,9-dioctylfluorene） ）-co- [2, 7-（9,9-dioctlyfluorene）-alt-5,5-bis（2-（4-methyl-l-naphtha- lene） pyridine-C^2,N） iridium （III） acethylacetonate]] （PFN-NaIr）. The device with the sole aluminium cathode is able to produce a comparably white electroluminescence efficiency of 1.31 cd/A to that of the device using low work function cathodes （such as Ba, Ca, etc.）. The CIE coordinates of the white light emission consisting of red, green and blue three components are nearly at （0.34, 0.35）. The mechanism of the white light emission from the device with the AI cathode is investigated, which is related to the efficient injection of electrons through the interface of PFN-Nalr/AI.     

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.     

Efficient White Light Emission Using a Single Copolymer with Red and Green Chromophores on a Conjugated Polyfluorene Backbone Hybridized with InGaN-Based Light-Emitting Diodes

We report an efficient white-light emission based on a single copolymer/InGaN hybrid light-emitting diode. The single copolymer consists of a conjugated polyfluorene backbone by incorporating 2,1,3-benzothiadiazole （BT） and 4,7-bis（2-thienyl）-2,1,3-benzothiadiazole （DBT） as green and red light-emitting units, respectively. For the single copolymer/InGaN hybrid device, the Commission Internationale de 1＇Eclairage （CIE） coordinates, color temperature Tc and color rendering index Ra at 20mA are （0.323,0.329）, 5960K and 86, respectively. In comparison with the performance of red eopolymer PFO-DBT15 （DOF：DBT=85：15 with DOF being 9＇9- dioctylfluorene） and green copolymer PFO-BT35 （DOF：BT=-65：35） blend/InGaN hybrid white devices, it is concluded that the chemically doped copolymer hybridized device shows a higher emission intensity and spectral stability at a high driving current than the polymer blend.     

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.     

Enhancement of Stability of Polymer Light-Emitting Diodes by Post Annealing

We investigate the effect of thermal annealing before and after cathode deposition on the stability of polymer light-emitting diodes （PLEDs） based on green fluorescent polyfluorene derivative. The annealed PLEDs exhibit improved charge transport and red-shift emission compared to the as-fabricated device. The stability of the PLEDs is largely enhanced by post-annealing before and after Ca deposition, which is attributed to the enhanced charge transport and the intimate contact between the cathode and the emissive layer.     

Atomic fluorescence mapping of optical field intensity profiles issuing from nanostructured slits, milled into subwavelength metallic layers

In this work, we report on the fabrication and characteristics of light-emitting diodes based on p-GaN/i-ZnO/n-ZnO heterojunction. A 30 nm i-ZnO layer was grown on p-GaN by rf reactive magnetron sputtering, then a n-ZnO was deposited by the electron beam evaporation technique. The current-voltage characteristic of the obtained p-i-n heterojunction exhibited a diode-like rectifying behavior. Because the electrons from n-ZnO and the holes from p-GaN could be injected into a i-ZnO layer with a relatively low carrier concentration and mobility, the radiative recombination was mainly confined in i-ZnO region. As a result, an ultraviolet electro-emission at 3.21 eV, related to ZnO exciton recombination, was observed in a room-temperature electroluminescence spectrum of p-i-n heterojunction under forward bias.     

Efficient Solution-Processed Blue Electrophosphorescent Devices Based on a Novel Small-Molecule Host

Efficient blue small molecular phosphorescent fight-emitting diodes with a blue phosphorescent dye bis（3,5- difluoro-2-（2-pyridyl）-phenyl-（2-carboxypride） iridium （Ⅲ） （Flrpic） doped into a novel small-molecule host 9,9- bis[4-（3,6-di-tert-butylcarbazol-9-yl）phenyl] fluorene （TBCPF） as the light-emitting layer have been fabricated by spin-coating. The host TBCPF can form homogeneous amorphous films by spin-coating and has triplet energy higher than that of the blue phosphorescent dye Flrpic. All the devices with different Flrpic concentration in the emitting layer give emission from Flrpic indicating complete energy transfer from TBCPF to Flrpic. The device shows the best performance with a peak brightness of 8050 cd/m^2 at 10.2 V and the maximum current efficiency up to 3.52 cd/A, when the Flrpic doped concentration is as high as 16%.     

Synthesis and characterization of organic/inorganic heterostructure films for hybrid light emitting diode

Thin-film light emitting devices based on organic materials have been gathering attentions for applying a flat-panel display and a solid-state lighting. Alternatively, inorganic technologies such as Si-based thin-film technology have been growing almost independently. It is then expected that combining the Si-based thin-film technology with the organic light emitting diode (OLED) technology will develop innovative devices. Here, we report syntheses of the hybrid light emitting diode (LED) with a heterostructure consisting of p-type SiC_x and tris-(8-hydroxyquinoline) aluminum films and characterization for the hybrid LEDs. We present the energy diagram of the heterostructure, and describe that the use of high dark conductivities of the p-type SiC_x as well as inserting wide-gap intrinsic a-SiC_x at the p-type SiC_x/Alq interface are effective for improving device performance.     

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.     

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.     

On the improvement of the electroluminescent signal of organic light emitting diodes by the presence of an ultra‐thin metal layer at the interface organic/ITO

The electroluminescent (EL) signal of organic light emitting diodes (OLEDs) based on simple “hole transporting layer/electron transporting layer” (HTL/ETL) structures has been studied as a function of the anode/HTL interface, the anode being an indium tin oxide (ITO) film. It is shown that the electroluminescent (EL) signal increases when a metal ultra‐thin layer is introduced between the anode and the HTL. Experimental results show that the work function value of the metal is only one of the factors which allow improving the EL signal via better hole injection efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved electrical and optical properties of MEH-PPV light emitting diodes using Ba buffer layer and porphyrin

It has been found that insertion of a thin Ba buffer layer between the Al electrode and the MEH-PPV layer results in a significantly higher current density in ITO/MEH-PPV/Al polymer light-emitting diodes due to a reduction of the potential barrier at the cathode-polymer interface. The photoluminescence is found to increase with the addition of porphyrin-containing platinum as the central atom, showing that some of the triplet excitons decay radiatively as a result of mixing porphyrin.     

A New Conducting Polymer Electrode for Organic Electroluminescence Devices

Conducting polymer polydimethylsiloxane （PDMS） is studied for the high performance electrode of organic electroluminescence devices. A method to prepare the electrode consisting of a SiC thin film and PDMS is investigated. By using ultra thin SiC films with different thicknesses, the organic electroluminescence devices are obtained in an ultra vacuum system with the model device PDMS/SiC/PPV/Alq3, where PPV is poly para-phenylene vinylene and Alq3 is tris（S-hydroxyquinoline） aluminium. The capacitance voltage （C - V）, capacitance-frequency （C - F）, current-voltage （I - V）, radiation intensity-voltage （R - V） and luminance eFficiency-voltage （E - V） measurements are systematically studied to investigate the conductivity, Fermi alignment and devices properties in organic semiconductors. Scanning Kelvin probe measurement shows that the work function of PDMS/SiC anode with a 2.5-nm SiC over layer can be increased by as much as 0.28eV, compared to the conventional ITO anode. The result is attributed to the charge transfer effect and ohmic contacts at the interface.     

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