Efficient Polymer Light-emitting Diodes Research Articles

Broad band and white phosphorescent polymer light-emitting diodes in multilayer structure

Efficient phosphorescent polymer light-emitting diode with poly(vinylcarbazole) (PVK) doped by two or three iridium complexes in single and bilayer structures are studied. With (tris-(2-4(4-toltyl) phenylpyridine) (Ir(mppy) 3) as the green emitter and (1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq) 2) as the red emitter the efficiency is as high as 23 cd/A with broad band emission from 500 nm to 720 nm. For white emission a second layer is added with blue emitter ((III) bis[(4,-6-di-fluorophenyl-pyridinato)N,C 2] picolinate) (FIrpic) doped in PVK. White light containing three spectral peaks results with efficiency 8.1 cd/A. As the second blue layer is replaced by the fluorescent (poly(9,9-dioctylfluorene)) (PFO) white emission with high color rendering index 86 is achieved. The efficiency is 5.7 cd/A with peak luminance 8900 cd/m 2. For a given iridium complexes ratio the relative intensity of the green and red emission depends sensitively on the second blue layer.

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.

Efficient Single Active Layer Electrophosphorescent White Polymer Light‐Emitting Diodes

Efficient electrophosphorescent white emission polymer light-emitting diodes (PLEDs) with single active layer is reported. Despite their simple architectures and straightforward fabrication procedures, double-doped devices with CIE coordinate of (0.33, 0.36) at 12 mA cm–2 can show peak PE of 6.4 lm W–1 and a peak LE of 16.1 cd A–1 for forward viewing, while triple-doped device (0.34, 0.47) can show a peak PE of 10.0 lm W–1 and a peak LE of 24.3 cd A–1. The demonstrated efficient white PLEDs hold great promise for solid state lighting application.

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.

Deep blue light-emitting diode based on high molecular weight poly(9,9-dioctylfluorene) with high efficiency and color stability

A highly efficient deep blue polymer light-emitting diode based on poly(9,9-dioctylfluorene) is demonstrated. The performance is found to increase significantly with the molecular weight. Two different molecular weights are compared, one is 71,000 and the other is 365,000. The electroluminescent efficiency and color stability are improved by slightly doping hole traps into the emission layer and bilayer structure. The maximum efficiency is 3.8cd/A with the corresponding external quantum efficiency of 3.7% at deep blue with Commission Internationale de L’Eclairage (CIE) coordinate at (0.15, 0.09). Stable blue emission is maintained up to 6600cd/m2 without growth of green shoulder in emission spectrum.

Efficient Top-Emitting Polymer Light-Emitting Diodes UsingChromium as Anode

We demonstrate a high efficiency top-emitting polymer light-emittingdiode (TPLED) with chromium (Cr) taking as the anode. The TPLEDstructure is Cr/poly-3, 4-ethylenedioxythiophene (PEDOT:PSS)/poly[2-(4-3′,7′-dimethyloctyloxy)-phenyl]-p-phenylenevinylene)(P-PPV)/Ba/Ag. The Cr (100 nm) anode is prepared by sputter-depositingin a vacuum chamber. It is found that the device emissive propertiesare affected dramatically by the thickness of both PEDOT:PSS and the Agcathode. Optimized thicknesses of PEDOT:PSS and Ag layer are 60 nm and15 nm, respectively. The diode exhibits excellent electroluminescence(EL) properties, such as a turn-on voltage of 3.32 V, luminousefficiency of 4.41 cd/A and luminance of 6989 cd/m2 at drivingvoltage of about 9 V.

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.

Enhanced Electroluminescent Efficiency Based on FunctionalizedEuropium Complexes in Polymer Light-Emitting Diodes

Efficient red polymer light-emitting diodes are fabricated with thesingle active layer from the blends of poly(N-vinylcarbazole) (PVK) inthe presence of 30 wt.% electron-transporting compound2-(4-biphenylyl)-5-(p-tert-butylphenyl)-1,3,4-oxadiazole (PBD)and europium complexes. The polyphenylene functionalized europiumcomplex shows an enhanced electroluminescent efficiency due to thelarge site-isolation effect. For the polyphenylene functionalizedeuropium complex, the maximum external quantum efficiency of 1.90% andluminous efficiency of 2.01 cd A−1 are achieved with emissionpeak at 612 nm. The maximum brightness is more than 300 cd m−2.

White polymer light-emitting diodes based on poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene- alt-9,9- n-dihexylfluorene-2,7-diyl) doped with a poly( p-phenylene vinylene) derivative

Efficient white polymer light-emitting diodes based on the polymer blend of poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene- alt-9,9- n-dihexylfluorene-2,7-diyl) doped with poly{2-[3′,5′-bis(2ʺ-ethylhexyloxy) benzyloxy]-1,4-phenylenevinylene}- co-poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated. The electroluminescence (EL) spectrum is easily controlled by changing the dopant concentration. A white light emission was realized on the device with the dopant concentration of 0.194‰ and the emission light is less sensitive to the applied voltage in a wide voltage range. The maximum luminance and the maximum EL efficiency of the single-layer device were 2330 cd/m 2 and 0.29 cd/A, respectively. By introducing an Alq 3 layer as an electron transporting and hole blocking layer, the overall performance of the double layer device was dramatically improved, the maximum luminance and the maximum EL efficiency reached 3300 cd/m 2 and 2.37 cd/A, respectively.

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%.

Efficient red phosphorescence polymer light-emitting diodes with dual function polymer

The efficient red phosphorescence polymer light-emitting diodes (PLEDs) based on aluminum (Al) cathode were demonstrated. The devices composed of neutral aminoalkyl substituted polyfluorene, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as the host and bis(1-(3-(9,9-dimethyl-fluorene-2-yl)phenyl)isoquinoline-C2,N′) iridium (III) (3-(pyridin-2′-yl)-1,2,4-triazole) [Ir(DMFPQ)2pt] as the dopant. High external quantum efficiency of 5.6% (at 16.8 V) and a luminance of 1400 cd/m2 (at 19.6 V) were obtained from the device with Al cathode only. The performances (quantum efficiency and luminance) of the devices could be comparable to those of the devices with a low work-function metal cathode (such as calcium and barium). We believe that the reason for the high performances was due to the effective electron injection from Al into PFN.

Double-Layer Structured WPLEDs Based on Three Primary RGB Luminescent Polymers: Toward High Luminous Efficiency, Color Purity, and Stability

Efficient white polymer light-emitting diodes (WPLEDs) were fabricated on the basis of three kinds of luminescent polymers: poly{2-[3‘,5‘-bis(2‘ ‘-ethylhexyloxy)benzyloxy]-1,4 phenylenevinylene}-co-poly(2-methoxy-5-(2‘-ethyl-hexy loxy)-1,4-phenylene vinylene) (BE-co-MEH-PPV), red; poly(6,6‘-bi-(9,9‘-dihexylfluorene)-co-(9,9‘-dihexylfluorene-3-thiophene-5‘-yl)) (PFT), green; and poly(9,9-dioctylfluorene-2,7-diyl) (PFO), blue. In order to overcome the complicated cascade energy transfer and deep trapping process, the two narrow band gap red and green light-emitting polymers are separated into two layers. In the bottom emission layer, hole-transporting material poly(N,N‘-bis(4-butylphenyl)-N,N‘-bis(phenyl)benzidine (poly-TPD) and electron-blocking material poly(N-vinylcarbazole) (PVK) were chosen to work together as binary-host; concomitantly, the novel red light-emitting polymer BE-co-MEH-PPV was employed as guest. In addition, in order to avoid phase separation in the top emission layer, two fluorene-deri...

Improved Performance of Top-Emissive Polymer Light-Emitting Device with Semitransparent Ag Cathode with the Aid of Au Nanoparticles

An efficient top-emissive polymer light-emitting diode (T-PLED) employing Ag as a semitransparent cathode is achieved by adding Au nanoparticles to a phenyl-substituted poly(para-phenylene vinylene) copolymer (HY-PPV). The efficiency of the T-PLED with Au nanoparticles is 3.4 cd/A, which is much higher than that of the T-PLED without such particles (0.15 cd/A). The superior performance of the device is attributed to the balance between the electron and hole currents induced by the Au nanoparticles, which increases the probability of hole–electron recombination. Furthermore, the device's stability in air can be significantly improved because of the use of stable electrodes.

Synthesis of a red electrophosphorescent heteroleptic iridium complex and its application in efficient polymer light-emitting diodes

The preparation and characterization of a heteroleptic iridium complex [2-(benzo[ b]thiophen-2-yl)pyridine]Ir(III)[2-(4H-1,2,4-triazol-3-yl)pyridine] [(Btp) 2Ir(PZ)] were reported (2-(benzo[ b]thiophen-2-yl)pyridine = Btp; 2-(4H-1,2,4-triazol-3-yl)pyridine = PZ). Electrophosphorescence was investigated in the device structure [indium-tin-oxide (ITO)/poly(ethlenedioxythiophene) (PEDOT)/poly(vinylcarbazole)(PVK)/Poly(9,9-dioctylfluorenyl-2,7-diyl) end capped with dimethylphenyl (PFO): (Btp) 2Ir(PZ)/Ba/Al] by using this iridium complex as guest and PFO as host. The red electrophosphorescent devices showed a peak emission at approximately 604 nm and shoulder at 654 nm with the Commission International de'Eclairage (CIE) coordinates of (0.64, 0.35) and external quantum efficiency of 7.7% at a doping concentration of 8 wt.% without an electron-transporting material in the emitting layer.

Effects of Sulfide Treatment of Indium Tin Oxide on Efficiency of Polymer Light-Emitting Diodes

The effects of the sulfide treatment of indium tin oxide (ITO) on the efficiency of polymer light-emitting diodes (PLEDs) were investigated in this study. We found that an increase in the work function of ITO and a decrease in the number of oxygen vacancies (VO) near the ITO surface region (due to the occupation of VO by sulfur) result in an increase in device efficiency. It is worth noting that a suitable sulfide treatment time is an important issue for producing high-external-quantum-efficiency PLEDs.

Performance improvement of polymer light emitting diodes by insertion of a silane layer

The influence of a silane layer on the performances of polymer light emitting diode(PLED)s has been studied. Glycidoxypropyltrimethoxysilane(GPS) with an epoxy functional group was used as a surface modifier for ITO substrates. The GPS layer was inserted between an ITO and a poly(3,4)‐ethylenedioxythiophene/polystyrenesulfonate(PEDOT) by wet process and the performances of PLEDs were investigated. The introduction of GPS layer increased the brightness and efficiency of PLEDs by 30%. In addition, the lifetime of PLEDs was also improved considerably by using GPS as a surface modifier.

Improvement of Current Injection and Efficiency of Polymer Light-Emitting Diodes with the Octadecyltrichlorosilane-Treated PEDOT:PSS

We studied the improvement of a polymer light-emitting diode (PLED) by inserting an interlayer between a poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and an emissive layer. An octadecyltrichlorosilane (OTS) was treated on the PEDOT:PSS as the interlayer. It improved the device efficiency of the PLED from 4.4 to 5.3 cd/A, and increased the maximum luminance from 10,000 to 17,000 cd/m 2 . It increased the forward current by 20% and reduced the reverse current by 80%. Inserting the OTS layer between PEDOT:PSS and the emissive polymer reduced the injection barrier between the PEDOT:PSS and emissive layer. Therefore, the increases of the effective work function of the anode led to better hole injection and thus, higher current density.

Control of luminescence in conjugated polymers through control of chain microstructure

The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model 'glassy' poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.

Efficient blue electroluminescence from neutral alcohol-soluble polyfluorenes with aluminum cathode

Efficient blue polymer light-emitting diodes (PLEDs) have been fabricated with a neutral alcohol-soluble polyfluorene, i.e., poly(9,9-bis(6′-diethoxylphosphorylhexyl)fluorene) (PF-EP), as the emitting layer, high work-function Al as the cathode, and poly(vinyl carbazole) as the hole-transporting layer. The PLEDs display a maximum luminous efficiency of 4.0cd∕A and the luminous efficiency >2.4cd∕A in a wide range of current densities. It is found that the promising performance of the devices is attributed to the fact that the PF-EP is not only an efficient blue light-emitting polymer, but it also can facilitate efficient electron injection at the Al/PF-EP interface.

Efficient polymer light-emitting diodes with violet blue emission based on blends of PSiF6-PPP and PSiFC6C6

Efficient polymer light-emitting diodes (PLEDs) with violet blue emission were fabricated using blends of copolymers of paraphenylene-cosilafluorene (PSiF6-PPP) and polymer of poly (9,9′-alkyl-3,6-silafluorene) (PSiFC6C6). The performances of the devices are sensitive to the blend ratio. When the mass ratio of PSiF6-PPP to PSiFC6C6 is 1: 3, the highest external quantum efficiency is 1.96% at luminance of 105 cd · m−2, its electroluminescent (EL) spectrum peaks at 398 nm and full width at half maximum is 67 nm. The improvements of the device performances were due to the energy transfer from PSiFC6C6 to PSiF6-PPP and the balanced injection of electrons and holes.