UV-ozone-treated ultra-thin NaF film as anode buffer layer on organic light emitting devices (original) (raw)
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Chinese Physics B, 2011
The use of a thin mixed layer consisting of an inert diluent material and a light emitting material between the hole-transport layer and electron-transport layer of organic lightemitting diodes leads to an increase in the external quantum efficiency. The efficiency improvement is highly dependent on the thickness of the diluted light-emitting layer and driving current. Significant improvement seen at low current densities is explained in terms of effective hole confinement by the mixed layer while a modest decreases in efficiency at higher current densities may be attributed to luminescence quenching at the hole-transport layer/inert diluents material interface. The phenomena are demonstrated with three different inert diluents materials. A maximum external quantum efficiency improvement of about 40% is found for a diluted light-emitting layer thickness between 40 Å and 60 Å.
Journal of Alloys and Compounds, 2009
The advantages of using an anode buffer layer of ZnO on the electro-optical properties of organic light emitting devices (OLEDs) are reported. ZnO powders were thermal-evaporated and then treated with ultra-violet (UV) ozone exposure to make the ZnO layers. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m 2 ) to 3 V (3.4 cd/m 2 ) and the power efficiency increased from 2.7 lm/W to 4.7 lm/W when a 1nm-thick ZnO layer was inserted between indium tin oxide (ITO) anodes and ␣-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS) results revealed the formation of the ZnO layer and showed that the work function increased by 0.59 eV when the ZnO/ITO layer was treated by UV-ozone for 20 min. The surface of the ZnO/ITO film became smoother than that of bare ITO film after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an ZnO buffer layer, resulting in a decrease of the turn-on voltage and an increase of the power efficiency of OLEDs.
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.
Applied Physics Letters, 2009
Hole injection improvement in organic light-emitting devices with Fe 3 O 4 as a buffer layer on indium tin oxide ͑ITO͒ has been demonstrated. The luminance and the current density are significantly enhanced by using the Fe 3 O 4 / ITO anode, as well as the turn-on voltage is reduced by 1.5 V compared to the devices without the buffer. Results of atom force microscopy, x ray, and UV photoelectron spectroscopy studies reveal that the enhanced hole injection is attributed to the modification of the ITO surface and the reduced hole-injection barrier by the insertion of the Fe 3 O 4 thin film between the ITO and hole-transporting layer.
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.
Japanese Journal of Applied Physics, 2007
The surface of indium tin oxide (ITO) in organic light emitting diodes (OLEDs) was treated using an atomic layer deposition with tetrakis(ethylmethylamino) hafnium (TEMAH) and O 2 precursors. The treatment for 5 cycles at room temperature resulted in a significant improvement in the electroluminescent characteristics resulting from the increased hole injection. According to the various characterizations, an ultra-thin HfO x layer without any insulating properties was formed, which modified the electronic band structure of the ITO anode. When the number of cycles or temperature was increased, an electrically insulating HfO x was formed, and the resulting OLED properties deteriorated.
Pramana, 2011
This study examined the electrical and optical properties of red OLEDs (organic lightemitting diodes) with a four-layer structure, ITO/amorphous fluoropolymer (AF)/N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine (TPD)/R-H:R-D/lithium fluoride (LiF)/Al, containing a hole injection material, AF (amorphous fluoropolymer) and an electron injection material, LiF. Compared to the basic structure (two-layer structure), the brightness and luminous efficiency of the four-layer structure, ITO/TPD/R-H:R-D/Al, increased approximately 100 times (30,000 lm/m 2) and 150 times (51 lm/W), respectively, with an applied voltage. The excellent efficiency of the external proton was also increased 150 times (0.51%). That is, the hole and electron injection layers improved the surface roughness of ITO and Al, and the interfacial physical properties. In addition, these layers allowed the smooth injection of holes and electrons. The luminance, luminous efficiency and external quantum efficiency were attributed to an increase in the recombination rates.
Journal of Applied Physics, 2014
Electrical transport in thermally stable 2, 7-bis [N, N-bis (4-methoxy-phenyl) amino]-9, 9-spirobifluorene (MeO-Spiro-TPD) thin films has been investigated as a function of temperature and organic layer thickness. ITO/MeO-Spiro-TPD interface was found to be injection limited and has been studied in detail to find barrier height for hole injection. The thickness of tetrafluoro-tetracyano-quinodimethane thin films were optimized to be used as hole injection buffer layer which resulted in switching of charge transport mechanism from injection limited to space charge limited conduction above a critical thickness of 3 nm. Hole mobility has been measured using transient space charge limited conduction (SCLC), field dependent SCLC, and top contact transistor characteristics. The charge carrier transport in interface modified hole only devices was analysed using Gaussian disorder model. The thermal stability of MeO-Spiro-TPD has been investigated by atomic force microscopy and X-ray diffraction studies. The study indicates a thermally stable and highly efficient hole transport material for application in organic semiconductor based devices. V
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.
Novel approach for deposition of thin films from low molecular weight compounds by pulverization is presented. The method was supplied for preparation of flexible organic light emitting device with tris(8-quinolinolato)-aluminum (Alq3) emissive layer. Additional film of N-N′-diphenyl-N-N′-bis (1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) was also spray deposited as a hole transporting layer (HTL) to increase the injection efficiency of the organic electroluminescent structure. Suitable substrate temperature was set to avoid dissolving and damage of both layers, caused by solvent penetration from NPB in Alq3. After optimization of the deposition conditions and because of the energy level alignment with introduction of NPB, it was measured reduction of the turn-on voltage with approximately 2 V. Current-voltage characteristics show 6 mA higher current at given voltage for the structure with HTL and the brightness-voltage characteristics show that the emission intensity is 300 cd/m2 hi...