High efficiency phosphorescent white organic light-emitting diodes using a spirofluorene based phosphine oxide host material

High efficiency phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed by using a spirofluorene based phosphine oxide (SPPO1) as a host material in blue emitting layer. A stack structure of red:green/blue with an interlayer was used and the device performances of PHWOLEDs were investigated according to host composition in red:green emitting layer and the interlayer thickness. The use of SPPO1 as a host in the blue emitting layer resulted in a high quantum efficiency of 13.5% and a current efficiency of 27.6 cd/A with a color coordinate of (0.37, 0.43).     

表面处理提高绿色OLED外量子效率研究

该文通过喷砂打磨技术使器件的表面粗糙化,有效地压制了玻璃层内的光波导,促进光子在玻璃-空气界面的散射,大幅度提高了有机绿色发光器件的输出耦合效率。有机绿色电致发光器件的流明效率、外量子效率分别提高到44.9 cd·A-1、15.4%,提高率达到54.0%。当粗糙度介于1~4μm之间时,器件的输出耦合效率提高率随着表面粗糙度的减小而提高。同时,粗糙表面对发光器件的朗伯体特性没有带来任何影响。     

Triplet to singlet transition induced low efficiency roll-off in green phosphorescent organic light-emitting diodes

Phosphorescent organic light-emitting diodes (PHOLEDs) with an emitting layer of 4,4′-N,N′-dicarbazole-biphenyl codoped with phosphor fac-tri(phenylpyridine)iridium(III) [Ir(ppy)₃] and fluorophore N,N’-dimethy-quinacridone (DMQA) are investigated. Predominant emission from DMQA due to the efficient energy transfer from Ir(ppy)₃ to DMQA is observed. Such an energy transfer results in the transition of Ir(ppy)₃ triplet to DMQA singlet, which reduces the Ir(ppy)₃ exciton lifetime and hence suppresses the triplet-triplet annihilation and triplet-polaron annihilation of Ir(ppy)₃ excitons, leading to dramatical reduction of the efficiency roll-off of the PHOLEDs. This transition of triplet to singlet strategy provides a method to improve the efficiency roll-off of the PHOLEDs.     

Pure red emission hybrid light-emitting devices based on the blend of CdSe/ZnS quantum dots and an n-type polymer

We report on hybrid light-emitting devices (HLEDs) based on CdSe/ZnS quantum dots in polymer matrices. Color purity of the HLED with a mixed matrix consisting of polyvinylcarbazole (PVK) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) improves with the increasing concentration of PBD. We attribute this improvement to the increased electron mobility of the matrix. By using an n-type cyano-containing polymer derived from 6-(3,6-dibromo-9H-carbazol-9-yl)hexanenitrile and tetraphenylsilane to replace PVK:PBD as the matrix, further pure red emission with Commission Internationale De L'Eclairage coordinates of (0.67, 0.33) and maximum external quantum efficiency of 2.51% has been achieved.     

High-efficiency white organic light-emitting devices with a non-doped yellow phosphorescent emissive layer

Highly efficient phosphorescent white organic light-emitting devices (PHWOLEDs) with a simple structure of ITO/TAPC (40 nm)/mCP:FIrpic (20 nm, x wt.%)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C²′] iridium (acetylacetonate) (tbt)₂Ir(acac) (y nm)/Bphen (30 nm)/Mg:Ag (200 nm) have been developed, by inserting a thin layer of non-doped yellow phosphorescent (tbt)₂Ir(acac) between doped blue emitting layer (EML) and electron transporting layer. By changing the doping concentration of the blue EML and the thickness of the non-doped yellow EML, a PHWOLED comprised of higher blue doping concentration and thinner yellow EML achieves a high current efficiency of 31.7 cd/A and Commission Internationale de l'Eclairage coordinates of (0.33, 0.41) at a luminance of 3000 cd/m² could be observed.     

Synthesis and electroluminescence properties of a series of hyper-branched light-emitting polymers

A series of hyper-branched polyalkylfluorenes (HBPFs) with an electron-conducting group, triazine and a hole-conducting group, triphenylamine as molecular cores was synthesized by the Suzuki coupling reaction. This series of HBPFs with different conjugated lengths was end-capped with a hole-conducting group, triphenylamine, and a rigid moiety, tetraphenylsilane. Higher brightness and efficiency were observed for the triazine-cored HBPF-based devices as compared to the triphenylamine-cored HBPF-based devices. For the HBPF containing the triazine core and tetraphenylsilane end-cap group, brightness and efficiency can be achieved as high as 1702 cd/m² and 0.72 cd/A, respectively. Moreover, for polymers having triphenylamine as the molecular core, a narrower electroluminescence (EL) emission band was observed as compared to those of the triazine-cored HBPF-based devices. As a result, deep blue emission (Commission Internationale de L'Eclairage (CIE_x,y) coordinates (0.16, 0.09)) were observed. In addition, excimer formation of the alkyfluorene chains was induced during the electrical driving process for devices based on HBPF consisting of the triphenylamine core and tetraphenylsilane end-cap group. EL quenching of the electrically induced excimer was therefore observed. Correlations between the molecular structure, thermal stability, and optical and EL properties of the HBPFs with different electron- or hole-conducting groups as molecular cores and end-cap groups are discussed in detail.     

以聚芴为主体的高效红光磷光聚合物发光二极管

以笼型多面体硅氧烷(poss)封端的聚烷基芴PFO-poss和PVK为主体,红光磷光络合物Ir(piq)为客体制作了不同结构的器件,最终在以PFO-poss为主体的双层结构器件当中获得了5.48cd/A的电致发光效率,超过了以PVK为主体的器件效率水平.研究了以PFO-poss为主体的器件中PVK的作用,发现作为空穴传...     

Color-tuning of polymer light-emitting devices through maskless dye diffusion technique

The maskless dye diffusion technique is a method to dope dye molecules into polymer films by thermal activation. Since the patterned indium tin oxide (ITO) electrodes for the future devices are used as heat source so that the dye doping area mimics the shape of the ITO pattern heated, this method can remove the precise positioning between the ITO electrode and dye doping area which is usually required in other techniques. This paper reports some results on the polymer light-emitting devices made through the maskless dye diffusion technique. When poly(9,9-dioctylfluorene) (PDOF) was used as host material, diffusion of Coumarin 6 and a phosphorescent dye BtpIr yields green and red emission, respectively. In the case of BtpIr-diffused device, the quantum efficiency of the device was found to be about 2.5 times of the device with non-treated PDOF film. It is also found that the poly(N-vinylcarbazole) can be a host material for both green and red phosphorescent dyes.     

Correlation of recombination zone and device performances of phosphorescent white organic light-emitting diodes

We demonstrate a general method for tuning the color performance of white organic light-emitting diodes (WOLEDs) by inserting 0.5 nm thick red emitting layer in different location of blue phosphorescent emitting layer. The Commission Internationale de L'Eclairage (CIE) coordinates of WOLEDs were dependent on the position of red emitting layer and they were correlated with recombination zone of the blue phosphorescent emitting layer. Red shift of white CIE was observed as the location of red emitting layer get close to recombination zone of blue emitting layer. In addition, CIE of WOLEDs was kept stable between 100 cd/m² and 10,000 cd/m².     

Synthesis and red electrophosphorescence of a novel cyclometalated iridium complex in polymer light-emitting diodes

The preparation and characterization of a new cyclometalated iridium complex with 2-(1-naphthalene) pyridine ligand were reported. An electrophosphorescent device was fabricated by using this new iridium complex as guest and poly-(cyano-paraphenylene) as host. Red electrophosphorescence was observed with an emission peak at approximately 600 nm. An external quantum efficiency of 1.3% was achieved in this electrophosphorescent polymer light-emitting devices.     

Top emission organic light-emitting devices with CsCl capping layer

We have developed top emission organic light-emitting devices using a CsCl capping layer on top of semitransparent Ca/Ag cathode. By using a CsCl capping layer, the transmittance of top electrode can be improved by 93%. While the electrical conduction characteristic of device is not influenced by the capping layer, the current efficiency increases with increasing the transmittance of Ca/Ag/CsCl cathode. For example, as the transmittance of top electrode increases from 55 to 91% by varying CsCl thickness, the current efficiency of green fluorescent top-emitting device increases from 8 to 18 cd/A.     

Sr/Ag semitransparent cathodes for top emission organic light-emitting devices

We have developed a semitransparent cathode for the top emission organic light-emitting devices using a Sr/Ag double layer prepared by the thermal evaporation technique. The Sr (8-10 nm)/Ag (10 nm) cathode shows the transmittance of 55-76% in the visible spectral region and the sheet resistance of about 12 Ω/□. The underlying Sr layer affects the growing characteristics of Ag layer, resulting in high optical transparency. The bis[2-(2′-benzothienyl)-pyridinato-N,C^3′]iridium(acetylacetonate) doped top emission electro-phosphorescent device with a Sr/Ag semitransparent cathode has been fabricated and studied.     

Investigation of emission location in top-emitting green organic light-emitting devices by optical analysis

A series of top-emitting organic light-emitting devices with different thicknesses of carrier transporting layers (N,N′-di(1-naphtyl)-N,N′-diphenylbenzidine, tris(8-hyroxyquinloine) aluminum (Alq₃)) and emitting layer (EML, 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T)-doped Alq₃) were fabricated. C545T-doped Alq₃ was found to bring about double recombination peaks in EML. As the distance between EML and reflective anode was increased, the outcoupling efficiency greatly deviated from optically-simulated values due to charge imbalance in EML and optical loss at the EML/Alq₃ interface. The device with 30 nm of EML exhibited maximized outcoupling efficiency and further increase of EML thickness brought about decrease in efficiency due to decrease in hole-electron recombination at the EML/Alq₃ interface.     

A high triplet energy phosphine oxide derivative as a host and exciton blocking material for blue phosphorescent organic light-emitting diodes

We have designed and synthesized a blue phosphorescent host material based on a phosphine oxide moiety. 2-(diphenylphosphine oxide)-9,9′-spirobifluorene (SPPO1) was compared with N,N′-dicarbazolyl-3,5-benzene (mCP) as a blue host material in blue phosphorescent organic light-emitting diodes (PHOLEDs). The SPPO1 was effective as a host for blue PHOLEDs and the SPPO1 based blue PHOLEDs showed much higher quantum efficiency than common mCP based blue PHOLEDs. A high quantum efficiency of 16.3% and a current efficiency of 31.4 cd/A were obtained in the blue PHOLED with iridium(III) bis(4,6-(di-fluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) as a blue phosphorescent dopant. In addition, SPPO1 was also effective as an exciton blocking material for the blue PHOLED.     

High quantum efficiency in simple blue phosphorescent organic light-emitting diodes without any electron injection layer

High efficiency simple blue phosphorescent organic light-emitting diodes (PHOLEDs) without any electron injection layer were developed using a spirobifluorene-based phosphine oxide (SPPO13) as a host material in the emitting layer. A high quantum efficiency of 20.3% was obtained from the SPPO13 device, with a common device structure and quantum efficiency of 19% achieved in the simple blue device without any LiF electron injection layer. Efficient electron injection from the Al cathode to the SPPO13, without any electron injection layer, was responsible for the high quantum efficiency in the blue PHOLEDs.     

Efficient flexible phosphorescent polymer light-emitting diodes based on silver nanowire-polymer composite electrode

Blue, green, and red electrophosphorescent polymer light-emitting diodes have been fabricated on silver nanowire-polymer composite electrode. The devices are 20%-50% more efficient than control devices on ITO/glass and exhibit small efficiency roll-off at high luminances. The blue PLEDs were repeatedly bent to 1.5 mm radius concave or convex with calculated strain in the emissive layer approximately 5% (tensile or compressive).     

Enhancement of the luminance efficiency for top-emitting DB-PPV polymer light-emitting diodes with Ca/Ag cathode

The enhancement of the luminance efficiency of the top-emitting 2,3-dibutoxy-1,4-polyphenylene vinylene (DB-PPV) polymer light-emitting diodes (PLEDs) with Ca/Ag cathode has been investigated in this work. The luminance efficiency of 2.9 cd A⁻¹ (at 10 mA) for the thickness of Ca/Ag cathode of 10/15 nm is achieved by our devices. In this paper, partially oxidized CaO_x film is proposed to have the capability of hole-blocking as well as the buffer layer on condition of the suitable Ag thickness of Ca/Ag cathode. The tuning ability of colors by simply adjusting the thickness of the DB-PPV emitting layer is available for the devices.     

Enhanced efficiency in mixed host red electrophosphorescence devices

Enhanced efficiency of red phosphorescent organic light-emitting devices is observed by using a bis[2-(2′-benzothienyl)pyridinato-N,C^3′] iridium(acetylacetonato) doped 4,4′-N,N′-dicarbazole-biphenyl (CBP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) mixed host emitting layer. The CBP:TPBI mixed host device shows a maximum external quantum efficiency of 9.1%, which is dramatically improved compared to that of the CBP (6.6%) and TPBI (5.4%) single host devices. Such a mixed host strategy can also be exploited in red phosphor dibenzo[f,h]quinoxaline iridium (acetylacetonate) doped devices. Investigations reveal that the position of charge carrier recombination zone of the mixed host devices predominantly locates in the electron blocking layer/emitting layer interface. The efficiency enhancement is attributed to the optimized hole and electron injection balance and hence increased charge carrier recombination rate in the emitting layer.     

Influence of oxygen deficiency in indium tin oxide on the performance of polymer light-emitting diodes

We investigated effects of oxygen deficiency in the indium tin oxide (ITO) on the performance of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes, in which the ITO anode was deposited by radio frequency magnetron sputtering at different oxygen flow rates. We found that the degree of oxygen deficiency in the ITO films can affect the device performance significantly and is a source of current leakage. At the optimal oxygen flow rate, the leakage current of devices can be reduced and the balance between hole and electron fluxes can be promoted in the MEH-PPV layer to improve device efficiency.