Highly Efficient Light-Emitting Diodes Based on an Organic-Soluble Poly(p-phenylenevinylene) Derivative Carrying the Electron-Transporting PBD Moiety (original) (raw)
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Current efficiency in organic light-emitting diodes with a hole-injection layer
2008
We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer ͑HIL͒/hole-transport layer ͑HTL͒/electron-transport layer ͑ETL͒ organic light-emitting diodes based on 4 , 4Ј ,4Љ-tris͓N-͑3-methylphenyl͒-N-phenylamino͔triphenylamine ͑MTDATA͒ as the HIL, 4 , 4Ј-bis͓N-͑1-naphthyl͒-N-phenylamino͔biphenyl ͑NPB͒ as the HTL, and tris͑8-quinolinolato͒aluminum ͑Alq͒ as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB + radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB + radical cations at the NPB/Alq interface and a strong electric field in the NPB layer.
High hole mobility hole transport material for organic light-emitting devices
Synthetic Metals, 2013
A new hole-transporting material, 5,10,15-triphenyl-5H-diindolo[3,2-a:3 ,2-c]carbazole (TPDI) is reported for organic light-emitting device (OLED) applications. It shows excellent hole mobility (6.14 × 10 −3 cm 2 /V s), one order higher than that of NPB (4,4-bis(N-phenyl-1-naphthylamino)biphenyl), and a good HOMO level of 5.3 eV. Fabricated fluorescent blue OLEDs exhibit about 1.0 V voltage reduction and 18% external quantum efficiency (EQE) improvement by replacing TPDI instead of NPB as a hole transport layer. In the green phosphorescent OLEDs, the driving voltage improves about 1.8 V and EQE increases about 65%. This TPDI will be applicable to not only in fluorescent OLEDs but also in phosphorescent OLEDs.
Polymer light-emitting diodes with doped hole-transport layers
physica status solidi (a), 2011
We demonstrate a solution processed bi-layer PLED based on poly(p-phenylene vinylene) derivatives using orthogonal solvents. To lower the voltage drop the hole transport layer (HTL) based on poly[2,5-bis(2 0-ethylhexyloxy)-co-2,5-bis(butoxy)-1,4-phenylenevinylene] (BEH/BB-PPV (1:3)) is doped with tetracyano-tetrafluoro-quinodimethane (F4TCNQ). The conductivity of BEH/BB-PPV (1:3) was observed to increase by two orders of magnitude upon doping with F4TCNQ. The doped HTL was observed to lower the operating voltage of a double layer PLED, but suffers from additional quenching by the dopant at higher voltages due to the lack of an electron blocking functionality.
Organic Electronics, 2004
This paper reports on heterostructure small molecule organic light emitting devices (OLEDs), the design of which includes doped hole and electron transport layer (HTL and ETL) and a hole blocking layer (HBL) which can be either doped or not. Doped transport layers are expected to lower the operating voltage of devices. Insertion of a hole blocking layer increases the carrier and exciton confinement which consequently improves the recombination rate and the device efficiency. Nevertheless, an HBL tends to increase the threshold voltage. The opposing influence of doped transport layers and HBL is evidenced in this study and compromise structures are presented. The doped HTL material is N,N 0-bis(3-methylphenyl)-N,N 0-diphenylbenzidine (TPD) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ). A comparison with undoped TPD and poly(N-vinylcarbazole) (PVK) HTL is given. Devices with doped HTL show a lowering of the operating voltage from 6.5 (PVK) down to 4 V (these voltages refer to those necessary to achieve a luminance L ¼ 10 Cd/m 2). A constant current efficiency higher than 2 Cd/A is obtained in the voltage range 5-9 V with doped HTL. Insertion of a 5-20 nm thick HBL made of 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (Bathocuproine BCP) between an 8-(hydroquinoline) aluminum (Alq 3) electron transport layer (ETL) and a DCM doped Alq 3 emitting layer (EML) induces both a detrimental effect (increase of the operating voltage to 6 V attributed to a low electron mobility in BCP) and beneficial effect (strong increase of the luminance and doubling of the current efficiency which reach to about 4.5 Cd/A thanks to improved carrier and exciton confinement). An optimization of the thickness of the doped EML and of BCP is also reported. The doping of the HBL and of the ETL with 2-(4biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) leads to devices with luminance as high as 1000 Cd/m 2 at 5.3 V and a maximum efficiency of 1 Cd/A at 4 V.
Journal of Nanoscience and Nanotechnology, 2014
New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of the synthesized materials were examined by UV-vis and photoluminescence spectroscopy, and by cyclic voltammetry. 10,10 0 -bis(4-tert-butylphenyl)-N7,N7 0 -di(naphthalen-1-yl)-N7,N7 0 -diphenyl-10H,10 0 H-3,3 0 -biphenoxazine-7,7 0 -diamine (1-PNA-BPBPOX) showed glass transition temperatures (T g ) of 161°C, which was higher than that (110°C) of Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA), a commercial HIL material. The HOMO levels of the synthesized materials were 4.9-4.8 eV, indicating a good match between the HOMO of indium tin oxide (ITO) (4.8 eV) and the HOMO of N,N 0 -bis(naphthalen-1-yl)-N,N 0 -bis(phenyl)benzidine (NPB) (5.4 eV), a common hole transfer layer (HTL) material. Because the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, they have good potential for use as effective HIL materials. The synthesized materials were used as the HIL in OLED devices, yielding power efficiencies of 2.8 lm/W (1-PNA-BPBPOX) and 2.1 lm/W (2-TNATA). These results indicate that 1-PNA-BPBPOX yields a higher power efficiency, by a factor of 33%, than the 2-TNATA commercial HIL material. Also, 1-PNA-BPBPOX exhibited a longer device lifetime than 2-TNATA.
Journal of Polymer Science Part A: Polymer Chemistry, 2010
A series of novel styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine with respect to their hole-transporting behavior in phosphorescent polymer light-emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1-and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium-tin-oxide/PEDOT:PSS/polymer-blend/CsF/Ca/ Ag. In addition to the hole-transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me-ppy) 3 ] and an electron-transporting molecule (2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole). We demonstrate that two poly-mers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (T g ¼ 246 C), a luminous efficiency of 35 cd A À1 and a brightness of 6700 cd m À2 at 10 V is accessible. The phenothiazine-functionalized polymer (T g ¼ 220 C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N 0 -bis(3-methylphenyl)-N,N 0diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A À1 and a brightness of 2500 cd m À2 (10 V). V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3417-3430, 2010
2013
Increasing the efficiency and lifetime of polymer light emitting diodes (PLEDs) requires a balanced injection and flow of charges through the device, driving demand for cheap and effective electron transport/ hole blocking layers. Some materials, such as conjugated polyelectrolytes, have been identified as potential candidates but the production of these materials requires complex, and hence costly, synthesis routes. We have utilized a soluble small molecule naphthalene diimide derivative (DC18) as a novel electron transport/hole blocking layer in common PLED architectures, and compared its electronic properties to those of the electron transport/hole blocking small molecule bathocuproine (BCP). PLEDs incorporating DC18 as the electron transport layer reduce turn on voltage by 25%; increase brightness over three and a half times; and provide a full five-fold enhancement in efficiencies compared to reference devices. While DC18 has similar properties to the effective conjugated polyelectrolytes used as electron transport layers, it is simpler to synthesise, reducing cost while retaining favourable electron transport properties, and improves upon their degree for efficiency enhancement. The impact on device lifetime is hypothosized to be significant as well, due to the air-stability seen in many naphthalene diimide derivatives.