Performance Improvement of Organic Light Emitting Diodes Using Poly( N -vinylcarbazole) (PVK) as a Blocking Layer (original) (raw)
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Applied Physics Letters, 2011
Polymer light-emitting diodes based on poly(N-vinylcarbazole) (PVK) with molecular weights M W of 1.1 Â 10 6 and $7.5 Â 10 4 are compared. For devices without an electron transport layer (ETL), the high M W PVK yields higher external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low M W PVK yields higher efficiency (1.13% vs 0.83%). This intriguing difference is believed to result from higher energetic disorder in the high M W polymer and different recombination zone-quenching metal electrode distances, in agreement with Konezny et al. [Appl.
Current Applied Physics, 2009
The effect of tetrabutylammonium hexafluorophosphate (TBAPF 6) doping on the electrical and electroluminescence properties of single-layer polymer light emitting diodes (PLEDs) with ITO/PVK:PBD/Al structure were investigated where indium tin oxide (ITO) was used as anode, poly(9-vinylcarbazole) (PVK) as polymeric host, 2-(4biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) as electron-transporting molecule and aluminium (Al) as cathode. The emitting layers were spin-coated onto the ITO-coated glass substrates. It was found that the doped devices underwent transition at the first voltage scan where the current increased drastically at certain voltage. After the transition, the threshold voltage for current injection as well as the light emission decreased significantly as compared to undoped device. The turn on voltage of the doped device was 5 V. The significant improvement was attributed to the reduction of both electron and hole injection energy barriers caused by accumulation of ionic species at the interface. In conclusion, doping of TBAPF 6 has been shown to be a valuable approach to reduce the turn-on voltage and increase the EL intensity of PLEDs.
2016
iv ABSTRACT This research work aims at improving the device efficiency of solution processed OLED and at the same time to do in-depth study on the device charge injection and transport. The first research project demonstrates high efficiency solution process red OLED device by doping small molecules 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TcTa) into Poly(9-vinylcarbazole) (PVK) as mixed hole-transporting hosts. The device performance increased from 2 cd/A to 4 cd/A. This is attributed to the energy barrier reduction and better charge balance in the device. The analysis of temperature-dependent hole mobility in PVK:TcTa film indicates that the energetic disorder of PVK:TcTA decreases with increasing concentration of TcTa implying that hole transport is predominately hopping among more ordered TcTa molecules even at low concentration. Second project presents the fabrication of tandem OLED device where a novel solution process charge-generating unit (CGU) using orthogonal solvents i...
Synthetic Metals, 2006
This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3 ,7 -dimethyloctyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq 3 ) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nmthick ImPE layer, at 1 cd/m 2 , the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq 3 for low luminances (<20 cd/m 2 ). However, for luminances higher than 350 cd/m 2 , it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.
PVK-based organic light emitting diodes with low turn on voltage
Materials Chemistry and Physics, 2003
Poly(N-vinylcarbazole) (PVK) thin films deposited by evaporation have been used in electroluminescent organic diodes. The turn on voltages of the current-voltage (I-V) and electroluminescence-voltage (EL-V) characteristics are not very different and they are smaller than the one obtained in such PVK-based devices, when the PVK film is obtained by spin coating. It is shown that the carrier injection process is dominated by tunneling field effect. During evaporation the chain length of the polymer is shortened and the PVK films contain a high density of dangling bonds. It is proposed that these dangling bonds introduce a broad distribution of localized states in the band gap. Therefore, the carrier injection occurs via hopping among an energetic distribution of localized states rather than by band motion, which is equivalent to a reduction of the energy barriers as experimentally measured.
High performance organic polymer light-emitting heterostructure devices
Applied Physics Letters, 1999
We report a high performance electroluminescence device based on bi-layer conjugated polymer structures consisting of a hole transporting ͑amine-fluorene͒ and an emissive ͑benzothiadiazole-fluorene͒ polymer layers prepared by the spin-coating technique on the glass substrate. Devices showed green emission with an electroluminescence peak located at around 545 nm and a full width at half maximum of about 80 nm. Our devices have also shown a high brightness (ϳ10 000 cd/m 2 at 0.84 mA/mm 2 ), good emission efficiency (ϳ14.5 cd/A͒ and luminous efficiency ͑2.26 lm/W͒, a large external quantum efficiency ͑3.8%͒, and a reasonable forward-to-reverse bias current rectification ratio (Ͼ10 3 at Ϯ25 V͒.
Improving the performance of organic light-emitting diodes
SPIE Newsroom, 2008
Organic light-emitting diodes (OLEDs) are promising candidates for large-area full-color flat panel displays due to their ease of fabrication and convenience for many applications. 1 OLEDs work through the passage of an electric current across a fluorescent or phosphorescent organic layer resulting in an excitation/emission profile of the material used. With OLEDs, the injection efficiency of electrons is a critical parameter and depends to a great extent on the work function (the minimum energy needed to move an electron out of a substance) of the electrode. A thin hole-injection layer (HIL) or an anode buffer layer (ABL) between the indium tin oxide (ITO) anode and the organic emitting layer are usually adopted to enhance the performance of the hole-injection process. 2-6 Thus, current electroluminescent devices typically have the following layered configuration: ITO anode/HIL or ABL/organic emitting layer/tris(8-hydroxyquinoline) aluminum (Alq 3 )/lithium fluoride (LiF)/aluminum cathode. Our recent work suggests that either an HIL composed of metal phthalocyanine (MPc) or an ABL of Li-doped zinc oxide (LZO) should improve the holeinjection efficiency. The organic, inorganic, and Al layers of our test device were successively deposited using vacuum vapor evaporation at room temperature. The LZO powders with a doped concentration of 5% Li were prepared by sintering a mixture of ZnO and Li 2 CO 3 powders in air. Various MPc layers were tested for their effect on injection efficiency (see ). The turn-on voltage of the devices decreases from 5.3V to 4.3V when CoPc or CuPc layers are inserted: see (b). Compared to the non-MPc device, higher emission efficiency was observed in all MPc devices. The CuPc device achieved the highest efficiency as shown in . For the same emission intensity, the higher efficiency suggests that a much lower current density is required. Figure 1.
A comparison of hole blocking/electron transport polymers in organic LEDs
1999
Three main-chain aromatic polyethers with different electroactive heterocyclic moieties, 1,4-quinoxaline, 1,3,4-oxadiazole and 1,3,5-triazine, have been synthesized. The polymers are amorphous with glass transition temperatures above 200 8C. The polymers with these high electron affinity units were used as hole blocking/electron transport layers (HBETL) in lightemitting diodes (LEDs) having the HBETL casted on top of a hole transport/emitting PPV layer. In order to compare the influence of the different polyethers on the LED characteristics, three multilayer devices (ITO/PPV/HBETL/Al) with different HBETLs were investigated. Relative to the single layer PPV device, quantum efficiencies were improved by two orders of magnitude in all multilayer devices and power efficiency was increased using poly(quinoxaline ether) as HBETL. To investigate the electrochemical behavior of the three HBETLs, cyclic voltammetry measurements were carried out and the HOMO/LUMO energy values determined from redox potentials were used to understand the hole blocking property. Lowering the onset voltage using the poly(quinoxaline ether) as HBETL in two-layer devices is compatible with the high electron affinity of this polymer.
Study and comparison of conducting polymer hole injection layers in light emitting devices
Organic Electronics, 2005
A set of polyaniline-and poly(3,4-ethylene dioxythiophene)-based materials were studied as hole injection layers in polymer light emitting devices. The choice of polymeric counterion/dopant poly(styrenesulfonic acid), and poly(acrylamido-2-methyl-1-propanesulfonic acid), and poly(acrylamide) blended with polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid) was found to influence both work function and film morphology, which in turn affects device performance. The work functions of the polymer films spanned the range of over 1 eV and the surface region of the films were found to be low in conducting polymer content compared to the bulk. This was particularly the case of the polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid) blended with poly(acrylamide) which showed device efficiency equal to that of the poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonic acid) reference. The turn on voltage, however, was significantly larger, likely due to the insulating poly(acrylamide)-rich surface region of the polyaniline/poly(acrylamido-2-methyl-1-propanesulfonic acid)/poly(acrylamide) film. The polymer blend of polyaniline/ poly(styrenesulfonic acid) yielded the highest work function (5.5 ± 0.1 eV).
Journal of The Electrochemical Society, 2007
We report on the advantages of an anode buffer layer of Li-doped ZnO ͑LZO͒ on the electro-optical properties of organic light-emitting diodes ͑OLEDs͒. LZO layers with different thicknesses were prepared by thermally evaporating the LZO powders and then treating them with ultraviolet ͑UV͒ ozone exposure. The turn-on voltage of OLEDs decreased from 4 V ͑4.2 cd/m 2 ͒ to 3 V ͑5.1 cd/m 2 ͒, the maximum luminance value increased from 16780 to 28150 cd/m 2 and the power efficiency increased from 2.74 to 5.63 lm/W when a 1 nm thick LZO layer was inserted between indium-tin oxide ͑ITO͒ anodes and -naphthylphenylbiphenyl diamine hole-transporting layers. X-ray and ultraviolet photoelectron spectroscopy were performed to show that the formation of the LZO layer and the work function increased by 0.64 eV when the LZO/ITO layer was treated by UV-ozone for 20 min. The surface of the LZO/ITO film became smoother after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an LZO buffer layer, resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.