The role of copper phthalocyanine for charge injection into organic light emitting devices (original) (raw)
Related papers
2006
In this paper, we systematically investigated the physical characteristics of the various metal phthalocyanines MPcs and the influence of the MPcs hole-injection layer on the electroluminescence performance of indium tin oxide/MPc/ naphthylphenylbiphenyl diamine NPB/Alq3/LiF/Al devices. The characteristics were measured at room temperature with a thickness variation of the MPc layer. The individual highest occupied and lowest unoccupied molecular orbital HOMO and LUMO energies of MPcs were derived from the photoelectron emission and the optical absorption measurements. The results showed that the HOMO and LUMO level energies of MPcs are dependent on their central metal atoms. The turn-on voltage for the devices is lowered by inserting MPc layers and remains virtually the same as the MPc layer thickness is adjusted in the range of 5‐15 nm. In addition, the turn-on voltage decreases significantly with the increase of the HOMO levels of the MPc films, demonstrating that the MPc/NPB int...
Structural and electronic implications for carrier injection into organic semiconductors
Applied Physics A, 2009
We report on the structural and electronic interface formation between ITO (indium-tin-oxide) and prototypical organic small molecular semiconductors, i.e., CuPc (copper phthalocyanine) and α-NPD (N,N -di(naphtalen-1yl)-N,N -diphenyl-benzidine). In particular, the effects of in situ oxygen plasma pretreatment of the ITO surface on interface properties are examined in detail: Organic layerthickness dependent Kelvin probe measurements revealed a good alignment of the ITO work function and the highest occupied electronic level of the organic material in all samples. In contrast, the electrical properties of hole-only and bipolar organic diodes depend strongly on the treatment of ITO prior to organic deposition. This dependence is more pronounced for diodes made of polycrystalline CuPc than for those of amorphous α-NPD layers. X-ray diffraction and atomic force microscopic (AFM) investigations of CuPc nucleation and growth evidenced a more pronounced texture of the polycrystalline film structure on the ITO substrate that was oxygen plasma treated prior to organic layer deposition.
Surface treatment of indium tin oxide by oxygen-plasma for organic light-emitting diodes
Materials Science and Engineering B-advanced Functional Solid-state Materials, 2003
We have carried out a systematic study to optimize the processing conditions in oxygen-plasma treatments on indium tin oxide (ITO) substrates used in organic light-emitting diodes (OLEDs). The treated ITO substrates were investigated by both contact angle measurements and X-ray photoelectron spectroscopy (XPS). It was found that oxygen-plasma treatment was quite effective in removing organic contaminants on the ITO surface, causing a reduction in contact angle. XPS revealed that the treatment led to a decrease in the surface content of carbon and an increase in the surface content of oxygen. Consequently, enhanced hole-injection, increased luminance efficiency and improved operational stability were observed in OLEDs having an ITO anode treated at the optimized conditions.
Journal of Applied Physics, 2009
We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide ͑ITO͒ in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40-5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers ͑HTL͒ that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/ m 2 ͑17.5% and 60 cd/A at 1000 cd/ m 2 ͒. The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The performance of these devices is also compared to that of devices in which ITO is modified with other well-known techniques such as air plasma treatment or the use of a layer of poly͑3,4-ethylenedioxythiophene͒/poly͑styrenesulfonate͒.
Thin Solid Films, 2008
Admittance spectroscopic analysis was used to examine the effect of a CF X plasma surface treatment on indium tin oxide (ITO) anodes using CF 4 gas and model the equivalent circuit for organic light emitting diodes (OLEDs) with the of ITO anode surface treated with CF X plasma. This device with the ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum/lithium fluoride/Al structure was modeled as a simple combination of two resistors and a capacitor. The ITO anode surface treated with the CF X plasma showed a shift in the vacuum level of the ITO, which resulted in a decrease in the barrier height for hole injection at the ITO/organic interface. Admittance spectroscopy measurements of the devices with the CF X plasma treatment on the surface of the ITO anodes showed a change in the contact resistance, bulk resistance and bulk capacitance.
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.
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.
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.
Impedance spectroscopy on copper phthalocyanine diodes with surface-induced molecular orientation
Organic Electronics, 2014
Molecular orientation and packing motif governs charge-transport property of organic semiconductor films, especially for planar small molecules. We analyze the surfaceinduced orientation of copper phthalocyannine (CuPc) molecules deposited on graphene or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) covered indium-tin-oxide (PEDOT:PSS/ITO). The CuPc films deposited on graphene are templated with preferential face-on stacking, whereas the molecules on PEDOT:PSS/ITO crystallize with edge-on ordering. Static current-voltage measurement and small-signal impedance spectroscopy are combined to elucidate the structural impact on the electrical response when those films are part of a rectifying diode. The graphene-templated diode shows enhanced out-of-plane hole conduction as compared to the diode with a PEDOT:PSS/ITO contact. Equivalent circuits describing charge injection and transport properties are proposed.
Thin Solid Films, 2001
Ž. The electronic properties of space charge layer of the copper phthalocyanine CuPc thin films in situ prepared by an UHV Ž. Ž. evaporation method and, subsequently, exposed to molecular oxygen O and molecular hydrogen H have been studied using 2 2 Ž. the high resolution photoemission yield spectroscopy PYS technique. The influence of interaction of these active gases on the work function and ionisation energy ⌽ was determined, together with the changes of effective density of filled electronic states localised in the band gap below the Fermi level and in the upper part of the valence band. In order to direct comparison to the commonly used procedure for UPS and XPS techniques, the changes of macroscopic parameters of the space charge layer were expressed by the surface Fermi level position in the band gap E ᎐E. The observed changes of surface Fermi level position upon F v adsorptionrdesorption of molecular oxygen O and hydrogen H were attributed to decreaserincrease of the induced charge in 2 2 near surface region.