Multilayer Polymer Light-emitting Diodes Research Articles

Improvement of radiative efficiency and wavelength-tuning by TPD-doping in organic light emitting diode using Poly [methylmethacrylate-co-(7-(4-trifluoromethyl)coumarin acrylamide)] (PCA)

We fabricated polymer light-emitting diodes (PLEDs) using a low-weight-molecular-doped luminescent material with a ?max in the ultraviolet region. A single layer PLED without low-weight-molecular-doping does not reveal any electroluminescence (EL) signal at all. However, a PLED with doping clearly shows EL signals. This indicates that carrier injection as well as radiative efficiency can be drastically improved by low-weight-molecular-doping. In addition, multilayered PLEDs are found to be very effective to further improve carrier injection.

High-efficiency and solution processible multilayer white polymer light-emitting diodes using neutral conjugated surfactant as an electron injection layer

High-efficiency white polymer light-emitting diodes were fabricated by using an yellow-emitting osmium complex Os(fptz)2(dppe) [fptz=3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, dppe=cis-1,2-bis-(dipheneyl-phosphino) ethylene] doped into blue fluorescent copolymer based on an ultraviolet-blue light emitting host poly[2,7-(9,9-dioctylfluorene)-co-1,3-(5-carbazolphenylene)] and a blue light emitting component 4-N,N-diphenylaminostilbene (PFCz-DPS1-OXD5) as the emissive layer and a neutral conjugated surfactant, poly[9,9-bis(6′(diethanolamino)hexyl)-fluorene] (PFN-OH), as the electron injection layer sandwiched between the emissive layer and Al cathode. The device with the configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid)/poly(N-vinylcarbazole)/Os(fptz)2(dppe)(1wt%):PFCz-DPS1-OXD5∕PFN-OH∕Al exhibited efficient white light emission at the Commission Internationale de I’Eclairage coordinates of (0.33, 0.34) and a maximum luminance efficiency of 16.9cd∕A and brightness of 22100cd∕m2.

Multi-Layer Polymer Light-Emitting Diodes Prepared by Vapor Deposition Polymerization of Polyazomethine Thin Film

A novel poly(azomethine) (PAM) thin film deposited by vapor deposition polymerization (VDP) process was used in the emissive layer of polymer light-emitting diodes (PLEDs). 1,4-Bis(4-formylstyryl)benzene (BFSB) and 4,4''-diamino-(1,1',4',1'')-terphenyl (DAT) were co-deposited to form PAM thin films at various substrate temperatures. Fluorescent quantum yield of novel PAM synthesized by using BFSB monomer increased one order of magnitude compared with the previous report. PLEDs with the device structure of indium–tin oxide (ITO)/poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS) (100 nm)/PAM (140 nm)/LiF (0.5 nm)/Al (80 nm) exhibited of electroluminescence from the PAM layer for the first time. The characteristics of PLEDs suggest the electron mobility of PAM is higher than hole mobility.

High-efficiency blue multilayer polymer light-emitting diode fabricated by a general liquid buffer method

A bright, highly efficient blue polymer light-emitting diode is fabricated by spin-coating with multilayer structure. By introducing a liquid buffer layer, the dissolution problem is overcome successfully. This general method can be applied to fabricate arbitrary polymer multilayer structure, in particular the addition of the carrier blocking layers to balance the electron and hole currents and improve the device efficiency. By adding an electron-blocking layer, the external quantum efficiency for blue emission can be increased to 5.5% (9.1 cd/A) which is even higher than the theoretical limit of 5%. The luminance reaches 26,530 cd/m 2and the Commission Internationale de L’Eclairage (CIE) coordinates is (0.15, 0.23). Further more the lifetime is increased by three times relative to the single layer device.

Crosslinkable hole-transporting materials for solution processed polymer light-emitting diodes

One of the most challenging tasks in fabricating multilayer polymer light-emitting diodes (PLEDs) by solution processes is to avoid the interfacial mixing between different layers because most of the commercially available emissive and charge-transporting materials are soluble in common organic solvents. To overcome this difficulty, extensive efforts have been invested in developing novel crosslinkable hole-transporting materials (HTMs). After thermo- or photo-crosslinking, all these crosslinked HTMs possess very good solvent resistance which greatly facilitates the subsequent processing of the emitting layer. By taking advantage of these HTMs, high efficiency red–green–blue (RGB)-emitting PLEDs, as well as white- and quantum dot based PLEDs, have been realized. This article provides a brief overview of the recent development of crosslinkable HTMs and their unique advantages in enhancing the performance of LEDs.

Multilayer fluorescent polymer light emitting diode with low voltage and high efficiency

This study reports highly efficient green multilayer polymer light-emitting diodes (PLEDs). The fluorescent polymer is poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene}] end capped with dimethylphenyl (ADS108GE). This emissive polymer has been found to behave as an electron transport emitter. A poly(N-vinylcarbazole) (PVK) layer has been introduced as both hole transporting layer/electron blocking layer. Thicknesses of both ADS108GE and PVK layers have been optimized. 3.1 lm/W have been achieved which corresponds to an improvement by a factor 2 comparing to devices without PVK. Luminances as high as 25 000 Cd/m2 at low operating voltage are reported.

Balanced charge injection in multilayer polymer light-emitting diode with water soluble nonconjugated polymer dispersed by ionic compounds

The authors have fabricated highly efficient polymeric light-emitting diode (PLED) from ionic compound dispersed water soluble nonconjugated polymer, polyurethane (PU), which was used as an ultrathin hole blocking and electron injection layer (HB-EIL) on the top of commercially available blue-emitting polymer, polyfluorene. The device with HB-EIL showed a maximum quantum efficiency of 1.7%, while the one without HB-EIL showed an efficiency of 0.6%. They propose that the better performance in PLED with PU layer was due to a well balanced charge injection in emitting layer after the enhanced electron injection due to ionic compound in the insulating PU layer.

Efficient multilayer white polymer light-emitting diodes with aluminum cathodes

Efficient multilayer white polymer light-emitting diodes (WPLEDs) with aluminum cathodes are fabricated. The multilayer structure is composed of a water soluble hole-injection layer, a toluene-soluble emissive layer, and an alcohol-soluble emissive layer. The polarity difference of the solvents used for spin coating these polymers allows for realization of the multilayer polymer structure. The recombination zone confined at the interface of the two emissive polymers avoids exciton quenching by electrodes, and white emission is realized by harvesting photons emitted from the two emissive polymers. A maximum luminous efficiency of 16.9cd∕A and a power efficiency of 11.1lm∕W are achieved for this WPLED.

High-efficiency blue multilayer polymer light-emitting diode based on poly(9,9-dioctylfluorene)

A highly efficient blue polymer light-emitting diode based exclusively on commercial poly(9,9-dioctylfluorene) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl)) diphenylamine)] is demonstrated. High electroluminescent efficiency is achieved by enhancing electron currents and making devices in multilayered structures. CsF∕Al is used as the efficient electron injection cathode, and the fabrication process is in the glove box to enhance electron mobility by reducing oxygen adsorption. The multilayer structure is prepared by the liquid buffer layer technique. The maximum efficiency is 2.5 cd∕A at deep blue with the corresponding external quantum efficiency of 2%.

Utilization of water/alcohol-soluble polyelectrolyte as an electron injection layer for fabrication of high-efficiency multilayer saturated red-phosphorescence polymer light-emitting diodes by solution processing

Highly efficient multilayer red polymer light-emitting diodes were fabricated by solution process-ing from iridium complex, bis(1-(3-(9,9-dimethyl-fluorene-2-yl)phenyl)isoquinoline-C2,N′) iridium(III)acetylacetonate, doped into polyfluorene as a host and with a water/alcohol-soluble polymer, poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-2,7-(9,9-dioctylfluorene)-4,7-(2,1,3-benzoselenadiazole)]dibromide (PFN) as electron injection layer. The device with the structure ITO∕PEDOT-PSS(50nm)∕PVK(40nm)∕PFO:PBD:Ir(DMFPQ)2acac(2%,75nm)∕PFN(20nm)∕Ba(4.5nm)∕Al(150nm) showed an external quantum efficiency of 18.0% and luminance efficiency of 9.8Cd∕A at a current density of 1.1mA∕cm2, a peak emission at λmax=636nm, and Commission International de I’Eclairage coordinates of (0.665, 0.319). The efficiency remained as high as QE=11.1%, and LE=6.0cd∕A, at a current density of 100mA∕cm2, and a luminance of 6140cd∕m2.

General method to solution-process multilayer polymer light-emitting diodes

An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5cd∕A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material.

Cover Picture: Multilayer Polymer Light‐Emitting Diodes: White‐Light Emission with High Efficiency (Adv. Mater. 17/2005)

AbstractWhite‐light‐emitting polymer diodes can be fabricated by solution processing using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer, and water‐soluble (or ethanol‐soluble) polymers and/or small molecules as the hole‐injection/transport layer (HIL/HTL) and the electron injection/transport layer (EIL/ETL), as reported on p. 2053 by Gong, Bazan, Heeger and co‐workers. Illumination‐quality light is obtained from these multilayer, high‐performance devices, with stable CIE coordinates, color temperatures, and high color‐rendering indices all close to those of “pure” white light. The cover illustration envisages the incorporation of the fabrication technique with low‐cost manufacturing technology in order to produce large areas of high‐quality white light.

Multilayer Polymer Light‐Emitting Diodes: White‐Light Emission with High Efficiency

White-light-emitting polymer diodes containing hole-injection/transport and electron-injection/transport layers have been fabricated by solution processing. Illumination-quality light is obtained from these multilayer, high-performance devices, with stable Commission Internationale d'Eclairage coordinates, stable color temperatures, and stable, high color-rendering indices, which are all close to those of “pure” white light indicated by the dotted oval in the Figure.

Covalently Bound Hole-Injecting Nanostructures. Systematics of Molecular Architecture, Thickness, Saturation, and Electron-Blocking Characteristics on Organic Light-Emitting Diode Luminance, Turn-on Voltage, and Quantum Efficiency

Hole transporting materials are widely used in multilayer organic and polymer light-emitting diodes (OLEDs, PLEDs, respectively) and are indispensable if device electroluminescent response and durability are to be truly optimized. This contribution analyzes the relative effects of tin-doped indium oxide (ITO) anode-hole transporting layer (HTL) contact versus the intrinsic HTL materials properties on OLED performance. Two siloxane-based HTL materials, N,N'-bis(p-trichlorosilylpropyl)-naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine (NPB-Si(2)) and 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si(2)), are designed and synthesized. They have the same hole transporting triarylamine cores as conventional HTL materials such as 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl)-4,4-diamine (TPD), respectively. However, they covalently bind to the ITO anode, forming anode-HTL contacts that are intrinsically different from those of the anode to TPD and NPB. Applied to archetypical tris(8-hydroxyquinolato)aluminum(III) (Alq)-based OLEDs as (1) the sole HTLs or (2) anode-NPB HTL interlayers, NPB-Si(2) and TPD-Si(2) enhance device electroluminescent response significantly versus comparable devices based on NPB alone. In the first case, OLEDs with 36 000 cd/m(2) luminance, 1.6% forward external quantum efficiency (eta(ext)), and 5 V turn-on voltages are achieved, affording a 250% increase in luminance and approximately 50% reduction in turn-on voltage, as compared to NPB-based devices. In the second case, even more dramatic enhancement is observed (64 000 cd/m(2) luminance; 2.3% eta(ext); turn-on voltages as low as 3.5 V). The importance of the anode-HTL material contact is further explored by replacing NPB with saturated hydrocarbon siloxane monolayers that covalently bind to the anode, without sacrificing device performance (30 000 cd/m(2) luminance; 2.0% eta(ext); 4.0 V turn-on voltage). These results suggest new strategies for developing OLED hole transporting structures.

Fabrication process and device characteristics of multi-layered polymer light-emitting diodes

Multi-layered polymer light-emitting diodes (ML-PLEDs) with the structure of [transparent anode / buffer-layer / emitting-layer / hole-blocking layer / cathode] have been fabricated by organic-all-wet coating process, and the device characteristics have been studied. The multi-layered device, [ITO / PEDOT:PSS / poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) / poly[(9,9-dioctylfluorene)-alt-{1,4-distyryl-2-methoxy-5-(2-ethylhexyloxy)benzene}] (PF8-DSB) / Ca / Al], has shown enhanced electroluminescent (EL) efficiency of 3.6 cd/A while the conventional device [ITO / PEDOT:PSS / MEH-PPV / Ca / Al] has shown only 1.0 cd/A in the EL efficiency. It is considered that this improvement is achieved by the adequate confinement of the recombination site of holes and electrons in the emitting layer and also by the effects of the interlayer between two polymers.

Multi-layer Polymer Light-emitting Diodes with 2,3-Dialkoxy-p-phenylene Vinylene and its Blends.

A green-fluorescent polymer, poly(2, 3-dibutoxy-1, 4-phenylene vinylene) (DB-PPV), was synthesized and used in electroluminescent (EL) devices. By introducing a small molecular electron-transport layer in a device, the EL efficiencies were improved up to 1.26cd/A. A polymer-blend with DB-PPV and poly(9, 9-dioctylfluorene) (PF8) was also examined. The EL peak wavelength was blue-shifted to 503nm and the EL efficiencies were improved in the polymer-blend device. A layered structure and a polymer-blend system both have shown contribution to the improvements of the polymer light-emitting diodes using DB-PPV.

High-performance blue light-emitting diode based on a binaphthyl-containing polyfluorene

We report efficient blue electroluminescence (EL) from multilayer polymer light-emitting diodes using a binaphthyl-containing polyfluorene as the emitting layer and a series of thermally polymerized triphenylamine/tetraphenyldiaminobiphenyl-containing polymers as the hole transport layer. The polymer possesses a high photoluminescence quantum efficiency of 44%. The polymer light-emitting diodes exhibit EL emission peak at 420 or 446 nm, depending on the hole transporting materials. The EL reaches maximum brightness and external quantum efficiency of 3070 cd/m2 and 0.82%, respectively.

A novel fabrication technique and new conjugated polymers for multilayer polymer light-emitting diodes

We report a novel fabrication technique for multilayer light-emitting diodes composed of new polyoxadiazole, POD, conjugated polymers for the first time. The fabrication technique called vapor deposition polymerization is described. Chemical modification of monomers brought about the enhancement of reactivity and the production of high molecular weight of POD. Emission color with photoexcitation was controllable from violet-blue to green by varying the chemical structures of PODs. It was found that PODs could be employed as either electroluminescent or carrier-injecting layers by the optimization of the device structure. Two types of bilayer devices, which are constructed with POD/tris (8-quinolinoato) aluminum, Alq3, and with two POD layers with different chemical structures, were investigated. Carrier injection begins in the POD/Alq3 bilayer device near 7 V, and the device emitted green light from Alq3. The maximum luminance of the POD/Alq3 device reached 3500 cd/m2. The POD/POD bilayer device emitted blue light with maximum luminance of 21 cd/m2. Electroluminescence spectra of the devices coincided with photoluminescence spectra of each emitting material used. © 1997 John Wiley & Sons, Ltd.

Hole-transporting compounds for multi-layer polymer light-emitting diodes

Polymerisation of two mono-substituted triphenylamines gave 4a and 4b. Their use as a hole-transporting layer in poly( p-phenylenevinylene) electroluminescent devices resulted in a decrease of device efficiency compared to devices without the additional layer.