Enhanced hole injections in organic light-emitting devices by depositing nickel oxide on indium tin oxide anode

Chan, Isaac; Hsu, Tzu-Ching; Hong, Franklin Chau Nan

doi:10.1063/1.1505112

Cited by 210 publications

(113 citation statements)

References 21 publications

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“…3,4 The intrinsic p-type conductivity of NiO x is mainly related to its non-stoichiometric nature where Ni 3+ centers constitute the oxide defects through which holes are transferred. 5 Because of this interesting combination of electrical and optical properties, NiO x has been considered for energy storage applications, [6][7][8] in electrochromic devices as transparent electrode, 9-17 in optoelectronic devices as electron barrier, 18,19 for gas sensing, 20,21 and, more recently, in dye-sensitized solar cells (DSCs) as photoactive cathode. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] The utilization of NiO x in such diverse applications has been accompanied by the development of various preparation methods and deposition techniques, aimed at producing NiO x -based materials with variable chemical composition, electrical resistivity, compactness and morphology.…”

Section: Introductionmentioning

confidence: 99%

Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

Awais

Dini

MacElroy

et al. 2013

Journal of Electroanalytical Chemistry

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Publication information Journal of Electroanalytical Chemistry, Publisher Elsevier The UCD community has made this article openly available. Please share how this access benefits you. Your story matters! (@ucd_oa) Some rights reserved. For more information, please see the item record link above. In this contribution a novel powder coating processing technique (microblasting) for the fabrication of nickel oxide (NiOx) coatings is reported. ∼1.2 μm thick NiOx coatings are deposited at 20 mm2 s−1 by the bombardment of the NiOx powder onto a Ni sheet using an air jet at a speed of more than 180 m s−1. Microblast deposited NiOx coatings can be prepared at a high processing rate, do not need further thermal treatment. Therefore, this scalable method is time and energy efficient. The mechano-chemical bonding between the powder particles and substrate results in the formation of strongly adherent NiOx coatings. Microstructural analyses were carried out using SEM, the chemical composition and coatings orientation were determined by XPS and XRD, respectively. The electroactivity of the microblast deposited NiOx coatings was compared with that of NiOx coatings obtained by sintering NiOx nanoparticles previously sprayed onto Ni sheets. In the absence of a redox mediator in the electrolyte, the reduction current of microblast deposited NiOx coatings, when analyzed in anhydrous environment, was two times larger than that produced by higher porosity NiOx nanoparticles coatings of the same thickness obtained through spray coating followed by sintering. Under analogous experimental conditions thin layers of NiOx obtained by using the sol-gel method, ultrasonic spray-and electro-deposition show generally lower current density with respect to microblast samples of the same thickness. The electrochemical reduction of NiOx coatings is controlled by the bulk characteristics of the oxide and the relatively ordered structure of microblast NiOx coatings with respect to sintered NiOx nanoparticles here considered, is expected to increase the electron mobility and ionic charge diffusion lengths in the microblast samples. Finally, the increased level of adhesion of the microblast film on the metallic substrate affords a good electrical contact at the metal/metal oxide interface, and constitutes another reason in support of the choice of microblast as low-cost and scalable deposition method for oxide layers to be employed in electrochemical applications. Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

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Section: Introductionmentioning

confidence: 99%

Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

Awais

Dini

MacElroy

et al. 2013

Journal of Electroanalytical Chemistry

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“…Chan et al, suggested that passibility of the NiO x layer formation in the ITO anode could enhance the hole injection in OLEDs because NiO x is a p-type transparent conducting oxide with a high work function. 15 In comparison to an n-type ITO or IZO layer, the p-type NiO x could inject hole carriers more easily into the organic layer. Formation of the NiO x phase in the NIZO layer, which was confirmed by the existence of the Ni 2p and satellite peaks shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…[13][14][15] Because the NiO x layer is a well-known p-type conductor with high work function, it is more desirable for hole injection into the organic layer than ITO.…”

Section: 14mentioning

confidence: 99%

“…13,14 In particular, several groups have reported that Ni cosputtering with ITO or Ni doping on the surface region of an ITO can potentially enhance hole injection into OLEDs due to the effect of the p-type transparent conductive NiO x layer. [13][14][15] Because the NiO x layer is a well-known p-type conductor with high work function, it is more desirable for hole injection into the organic layer than ITO. 16 Similar to ITO anode films, the investigation of Ni doping or cosputtering with indium zinc oxide ͑IZO͒ is therefore necessary to improve the performance of IZO anodes.…”

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confidence: 99%

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Characteristics of Ni-Doped IZO Layers Grown on IZO Anode for Enhancing Hole Injection in OLEDs

Choi

Jeong

Kim

et al. 2008

J. Electrochem. Soc.

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The preparation and characteristics of a Ni-doped indium zinc oxide ͑NIZO͒ layer were investigated to enhance hole injection in organic light emitting diodes ͑OLEDs͒. A thin NIZO layer with a thickness of 5 nm was cosputtered onto an indium zinc oxide ͑IZO͒ anode using tilted Ni and IZO dual targets dc magnetron sputtering at room temperature in a pure Ar atmosphere. Using 3 W of Ni dc power, we can obtain a NIZO ͑5 nm͒/IZO ͑135 nm͒ double-layer anode with a sheet resistance of 30.04 ⍀/ᮀ and an optical transmittance of 83.8% at a wavelength of 550 nm. In addition, it was found that the work function of the NIZO layer was higher than that of a pure IZO anode due to the presence of a NiO x phase in the NIZO layer. An increase of Ni dc power above 7 W significantly degrades the electrical and optical properties in the NIZO layer. X-ray diffraction examination demonstrated that the NIZO layer consisted of an amorphous structure regardless of the Ni dc deposition power due to low substrate temperature. Furthermore, an OLED fabricated on the NIZO layer exhibited a higher current density, luminance, and efficiency due to improved hole injection by the high work function NIZO. These results indicate that the NIZO/IZO anode scheme is a promising anode material system for enhancing hole injection from the anode into the active layer of OLEDs. Indium tin oxide ͑ITO͒ films deposited on glass substrate have been academically and industrially used as transparent electrodes for organic light emitting diodes ͑OLEDs͒ and flexible OLEDs due to their visible transparency ͑ϳ90%͒ and low resistivity ͑2-4 ϫ 10 −4 ⍀ cm͒. 1-5 High-performance OLEDs and flexible OLEDs require a high-quality ITO anode layer with low resistance, high transparency, chemical stability, and high work function because hole injection and light-emitting properties are critically impacted by the ITO anode layer. 6 Unfortunately, conventional ITO anodes have several drawbacks, such as an imperfect work function alignment with organic materials, chemical instability, high process temperature, and easy deterioration of the ITO target. In particular, the high energy barrier between ITO and the highest occupied molecular orbit of organic materials prevents effective hole injection from the ITO into the organic layer. For these reasons, several approaches have been employed to increase the work function of ITO, including wet treatment, 7,8 plasma treatment, 9,10 UV ozone treatment, 11 selfassembly monolayer coating treatment, 12 and Ni doping of ITO. 13,14In particular, several groups have reported that Ni cosputtering with ITO or Ni doping on the surface region of an ITO can potentially enhance hole injection into OLEDs due to the effect of the p-type transparent conductive NiO x layer. [13][14][15] Because the NiO x layer is a well-known p-type conductor with high work function, it is more desirable for hole injection into the organic layer than ITO.16 Similar to ITO anode films, the investigation of Ni doping or cosputtering with indium zinc oxide ͑IZO͒ is therefor...

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“…And several kinds of approaches have been proposed to elevate the work function of ITO, such as using metal oxides with high work functions, inserting conducting polymers between ITO and organic material, and depositing metal-doped ITO layers on the ITO surface. [11][12][13] Iridium oxide ͑IrO x ͒ and ruthenium oxide ͑RuO x ͒ are transparent conducting oxides. The work functions of IrO x and RuO x ͑Ͼ5.0 eV͒ are higher than those of ITO ͑ϳ4.7 eV͒.…”

Section: Introductionmentioning

confidence: 99%

Highly efficient organic light-emitting diodes with hole injection layer of transition metal oxides

Kim¹,

Baik²,

Yu³

et al. 2005

Journal of Applied Physics

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We report on the advantage of interlayers using transition-metal oxides, such as iridium oxide (IrOx) and ruthenium oxide (RuOx), between indium tin oxide (ITO) anodes and 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (α-NPD) hole transport layers on the electrical and optical properties of organic light-emitting diodes (OLEDs). The operation voltage at a current density of 100mA∕cm2 decreased from 17to11V for OLEDs with 3-nm-thick IrOx interlayers and from 17to14V for OLEDs with 2-nm-thick RuOx ones. The maximum luminance value increased about 50% in OLED using IrOx and 108% in OLED using RuOx. Synchrotron radiation photoelectron spectroscopy results revealed that core levels of Ru 3d and Ir 4f shifted to high binding energies and that the valence band was splitting from metallic Fermi level as the surface of the transition metal was treated with O2 plasma. This provides evidence that the transition-metal surface transformed to a transition-metal oxide. The surface of the transition metal became smoother with the O2 plasma treatment. The thickness was calculated to be 0.4nm for IrOx and 0.6nm for RuOx using x-ray reflectivity measurements. Secondary electron emission spectra showed that the work function increased by 0.6eV for IrOx and by 0.4eV for RuOx. Thus, the transition-metal oxides lowered the potential barrier for hole injection from ITO to α-NPD, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency.

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Enhanced hole injections in organic light-emitting devices by depositing nickel oxide on indium tin oxide anode

Cited by 210 publications

References 21 publications

Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

Characteristics of Ni-Doped IZO Layers Grown on IZO Anode for Enhancing Hole Injection in OLEDs

Highly efficient organic light-emitting diodes with hole injection layer of transition metal oxides

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