Improved hole-injection and external quantum efficiency of organic light-emitting diodes using an ultra-thin K-doped NiO buffer layer (original) (raw)
Related papers
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.
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.
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.
Enhanced device efficiency in organic light-emitting diodes by dual oxide buffer layer
We have demonstrated an improvement in device performance of fluorescent organic light-emitting diodes (OLEDs) by inserting a dual anode buffer layer composed of tungsten oxide (WO 3) and molybdenum oxide (MoO 3). The advantage of adding dual anode buffer layers with different deposition sequences over individual and composite oxide buffer layers has been systematically analyzed based on their electronic and optical properties. The incorporation of single and composite buffer layers has been revealed to induce a very low injection barrier for holes in tri-layer fluorescent OLEDs which results in a charge imbalance in the emission layer. In contrast, a proper sequence of buffer layers (WO 3 /MoO 3) exhibiting higher contact angle (lower surface energy) and higher surface roughness, together with a step-wise increment of potential barrier leads to a better overall charge balance in the active emission layer. Therefore, an enhanced current efficiency and power efficiency of ∼5.8 cd/A and ∼5.2 lm/W respectively were recorded for the WO 3 /MoO 3 buffer unit, which was better than the insertion of individual and composite layers.
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.
Journal of Applied Physics, 2008
We show that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing the thickness of a hole-injection layer (HIL) of molybdenum oxide (MoO3) inserted between indium tin oxide and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). From results of the electroluminescence (EL) characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO/MoO3/α-NPD.
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...
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.
IEICE Trans. Electron, 2015
This paper presents 2-(hydroxyl) quinoline lithium (Liq) used as an n-type dopant to improve white hybrid organic light-emitting diode (WHOLEDs) performance. The Liq doped tris(8-hydroxyquinolinato) aluminum (Alq 3) layer possessed enhanced electron injection, efficient hole and electron balance in the emitting layer, as one of the most essential issues for device applications. This work investigates the optimum recipe (Liq concentration and thickness) of Alq 3 :Liq n-type doped electron injection layer (EIL) for WHOLED devices by comparing the current density and efficiency results with conventional Alq 3 /LiF technique. A blocking layer or interlayer is inserted between emitting layer and EIL to avoid excitons quenched. In this work suitable material and optimum thickness for blocking layer are studied, a white small-molecular organic light-emitting diode (SM-OLEDs) based on a 1,3,5-tris (N-phenylbenzimidazol-2-yl) benzene (TPBi) stamping transfer process is investigated. The proposed stamping transfer process can avoid the complexity of the vacuum deposition process.