Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes (original) (raw)
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Advanced Surface Modification of Indium Tin Oxide for Improved Charge Injection in Organic Devices
Journal of the American Chemical Society, 2005
A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a π-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.
Accounts of Chemical Research, 2012
T ransparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in nextgeneration organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITOÀorganic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ. This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or holeharvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties.
MRS Proceedings, 1999
ABSTRACTWe studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen- and argon-plasma) treatments. The surface modification was investigated by surface energy, surface morphology, sheet resistance, carrier concentration, carrier mobility, and workfunction measurements. We report that the studied oxygen-plasma treatment induces: the highest surface energy with the highest polarity, the smoothest surface, the highest carrier density but the lowest mobility, the lowest sheet resistance, and the highest workfunction (stable in air). Polymer light-emitting diodes fabricated with the oxygen plasma treated substrates give the best performance in terms of electroluminescence efficiency and device lifetime. This is attributed to a favorable surface modification of ITO anodes by oxygen-plasma.
Journal of Applied Physics, 1998
We report combined studies of the influence of chemical and physical treatments on the properties of indium-tin oxide ͑ITO͒ thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes ͑LEDs͒ incorporating poly͑p-phenylene vinylene͒ ͑PPV͒ as the emitter material, with, or without, doped poly͑3,4-ethylene dioxythiophene͒ ͑PEDOT͒ as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly͑4,4Ј-diphenylene diphenylvinylene͒ were also tested. We studied chemical ͑aquaregia, degreasing, RCA protocol͒ and physical ͑oxygen and argon plasmas, Teflon, and paper rubbing͒ treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. Interestingly, we find that work-function variations induced by our oxygen-plasma treatment are unchanged after long-term storage in air and in the dark. Unexpectedly, we also find that devices incorporating a PEDOT layer benefit from an appropriate treatment of the ITO surface, for both efficiency and lifetime. The results shed light on the physics of conjugated, organic semiconductors and related devices, and in particular on the presence and the role of an anodic energy barrier on the LEDs mechanism of operation. We also discuss the implications of our integrated experimental study in relation to the modification of the ITO sheet resistance, surface and bulk composition, and surface morphology.
Journal of Applied Physics, 2009
We investigate the use of several phosphonic acid surface modifiers in order to increase the indium tin oxide ͑ITO͒ work function in the range of 4.90-5.40 eV. Single-layer diodes consisting of ITO/modifier/N , NЈ-diphenyl-N , NЈ-bis͑1-naphthyl͒-1,1Ј biphenyl-4 , 4Љ diamine ͑␣-NPD͒/Al and ITO/modifier/pentacene/Al were fabricated to see the influence of the modified ITO substrates with different work functions on the charge injection. To calculate the charge injection barrier with different surface modifiers, the experimentally measured current density-voltage ͑J-V͒ characteristics at different temperatures are fitted using an equivalent circuit model that assumes thermionic emission across the barrier between the ITO work function and the highest occupied molecular orbital of the organic material. The charge injection barrier height extracted from the model for various surface modifier-based diodes is independent of the ITO work function within the range of changes achieved through modifiers for both ␣-NPD and pentacene-based single-layer diodes.
Macromolecular Symposia, 2007
studies on the influence of chemical and physical treatments on the properties of indium-tin oxide (ITO) thin films are reported. The ITO films are utilized as transparent anodes of organic light-emitting diodes (OLEDs) incorporating poly(9,9-dihexyl-9H-fluorene-2,7-diyl) (PF6) as the hole transporter material and 8-hydroxyquinoline aluminum salt (Alq3) as emitter material. Chemical (HCl, piranha solutions), thermal (vacuum annealing), physical treatments (oxygen plasma, UV ozone) and combined treatments are studied. First, ITO layers with different treatments are characterized by using four point probe method, contact angle measurement, X-Ray diffraction (XRD), surface profilometer, scanning electron microscopy (SEM), UV-Vis-IR transmittance. Later, electrical and optical properties of OLEDs with treated ITO as anode are extensively investigated.
Optical Materials
Abstract Organic semiconductor (OSC) materials as a charge carrier interface play an important role to improve the device performance of organic electroluminescent cells. In this study, 4,4″-bis(diphenyl amino)-1,1':3′,1″-terphenyl-5'-carboxylic acid (TPA) and 4,4″-di-9H-carbazol-9-yl-1,1':3′,1″-terphenyl-5'-carboxylic acid (CAR) has been designed and synthesized to modify indium tin oxide (ITO) layer as interface. Bare ITO and PEDOT:PSS coated on ITO was used as reference anode electrodes for comparison. Furthermore, PEDOT:PSS coated over CAR/ITO and TPA/ITO to observe stability of OSC molecules and to completely cover the ITO surface. Electrical, optical and surface characterizations were performed for each device. Almost all modified devices showed around 36% decrease at the turn on voltage with respect to bare ITO. The current density of bare ITO, ITO/CAR and ITO/TPA were measured as 288, 1525 and 1869 A/m2, respectively. By increasing current density, luminance of modified devices showed much better performance with respect to unmodified devices.
Lancet, 2011
a b s t r a c t 4-[(3-Methylphenyl)(phenyl)amino]benzoic acid (MPPBA) was synthesized in order to facilitate the hole-injection in Organic Light Emitting Diodes (OLED). MPPBA was applied to form self-assembled monolayer (SAM) on indium tin oxide (ITO) anode to align energy-level at the interface between organic semiconductor material (TPD) and inorganic anode (ITO) in OLED devices. The modified surface was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). KPFM was used to measure the surface potential and work function between the tip and the ITO surface modified by SAM technique using MPPBA. The OLED devices (ITO/MPPBA/TPD/Alq 3 /Al) fabricated with SAM-modified ITO substrates showed lower turn-on voltages and enhanced diode current compare to the OLED devices fabricated with bare ITO substrates.
Accounts of Chemical Research, 2009
T he recent improvements in the power conversion efficiencies of organic photovoltaic devices (OPVs) promise to make these technologies increasingly attractive alternatives to more established photovoltaic technologies. OPVs typically consist of photoactive layers 20-100 nm thick sandwiched between both transparent oxide and metallic electrical contacts. Ideal OPVs rely on ohmic top and bottom contacts to harvest photogenerated charges without compromising the power conversion efficiency of the OPV. Unfortunately, the electrical contact materials (metals and metal oxides) and the active organic layers in OPVs are often incompatible and may be poorly optimized for harvesting photogenerated charges. Therefore, further optimization of the chemical and physical stabilities of these metal oxide materials with organic materials will be an essential component of the development of OPV technologies. The energetic and kinetic barriers to charge injection/collection must be minimized to maximize OPV power conversion efficiencies. In this Account, we review recent studies of one of the most common transparent conducting oxides (TCOs), indium-tin oxide (ITO), which is the transparent bottom contact in many OPV technologies. These studies of the surface chemistry and surface modification of ITO are also applicable to other TCO materials. Clean, freshly deposited ITO is intrinsically reactive toward H 2 O, CO, CO 2 , etc. and is often chemically and electrically heterogeneous in the near-surface region. Conductive-tip atomic force microscopy (C-AFM) studies reveal significant spatial variability in electrical properties. We describe the use of acid activation, small-molecule chemisorption, and electrodeposition of conducting polymer films to tune the surface free energy, the effective work function, and electrochemical reactivity of ITO surfaces. Certain electrodeposited poly(thiophenes) show their own photovoltaic activity or can be used as electronically tunable substrates for other photoactive layers. For certain photoactive donor layers (phthalocyanines), we have used the polarity of the oxide surface to accelerate dewetting and "nanotexturing" of the donor layer to enhance OPV performance. These complex surface chemistries will make oxide/ organic interfaces one of the key focal points for research in new OPV technologies.