A novel, highly efficient hole injection material based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethylene‐perfluoroethersulfonic acid) (PFFSA) in organic light‐emitting diodes (OLEDs) is demonstrated. Both current–voltage and dark‐injection‐current transient data of hole‐only devices demonstrate high hole‐injection efficiency employing PTT:PFFSA polymers with different organic charge‐transporting materials used in fluorescent and phosphorescent organic light‐emitting diodes. It is further demonstrated that PTT:PFFSA polymer formulations applied as the hole injection layer (HIL) in OLEDs reduce operating voltages and increase brightness significantly. Hole injection from PTT:PFFSA is found to be much more efficient than from typical small molecule HILs such as copper phthalocyanine (CuPc) or polymer HILs such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT‐PSS). OLED devices employing PTT:PFFSA polymer also demonstrate significantly longer lifetime and more stable operating voltages compared to devices using CuPc.

By combining experimental electron-transport results through an alkane monolayer sandwiched between Si and a metal, photoemission data from the monolayer-on-Si, and theoretical calculations, we show that transport is dominated by a distribution of mixed Si molecular levels, rather than a single molecular level, as shown schematically in the figure.

Several hexaazatrinaphthylene derivatives and a tris(thieno)hexaazatriphenylene derivative have been synthesised by reaction of the appropriate diamines with hexaketocyclohexane. The crystal structure of 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene has been determined by X-ray diffraction; this reveals a molecular structure in good agreement with that predicted by density functional theory (DFT) calculations and pi-stacking with an average spacing between adjacent molecular planes of 3.18 A. Solid-state ionisation potentials have been measured by using UV photoelectron spectroscopy and fall in the range of 5.99 to 7.76 eV, whereas solid-state electron affinities, measured using inverse photoelectron spectroscopy, vary in the range -2.65 to -4.59 eV. The most easily reduced example is a tris(thieno)hexaazatriphenylene substituted with bis(trifluoromethyl)phenyl groups; DFT calculations suggest that the highly exothermic electron affinity is due both to the replacement of the outermost phenylene rings of hexaazatrinaphthylene with thieno groups and to the presence of electron-withdrawing bis(trifluoromethyl)phenyl groups. The rather exothermic electron affinities, the potential for adopting pi-stacked structures and the low intramolecular reorganisation energies obtained by DFT calculations suggest that some of these molecules may be useful electron-transport materials.

It has been demonstrated that Air Products(R) HIL (hole injection layer) material based on the conducting polymer polythienothiophene (PTT) and poly(perfluoroethylene-perfluoroethersulfonic acid) (PFFSA) dramatically improves the lifetime of polymer light emitting diodes. Compared with other conductive polymer HILs, PTT based HILs have some unique properties. The resistivity of PTT:PFFSA films is sensitive to the annealing temperature. The resistivity dependence on annealing temperature is not favorable for certain applications (e.g., in passive matrix display applications, where too low a resistivity after annealing can lead to cross-talking), or from the point view of process control. We have found that raising the pH of PTT:PFFSA dispersions can suppress the resistivity sensitivity to annealing conditions. At the same time, raising the pH of PTT:PFFSA dispersions also lowers the work function of PTT:PFFSA films. When LumationTM Green 1304 light emitting polymer is used as the emitting layer, all PTT:PFFSA based devices showed lifetime that is several times longer than that of PTT:PSSA based devices. Among the PTT:PFFSA dispersions, pH adjusted ones show a lower leakage current, lower efficiency and shorter device lifetime compared with the original dispersion. We have also explored the application of PTT:PFFSA in small molecule devices. Longer device lifetime has been obtained in devices using PTT:PFFSA as HIL and aluminum tris(8-hydroxyquinoline) (Alq3) as emitter compared with devices using copper phthalocyanine (CuPc) as HIL. We have also found that hole injection from PTT:PFFSA into hole transport materials commonly used in small molecule devices is very efficient.

Understanding the achievable degree of homogeneity and the effect of surface structure on semiconductor surface chemistry is both academically challenging and of great practical interest to enable fabrication of future generations of devices. In that respect, silicon terminated SiC surfaces such as the cubic 3C - SiC(100) 3 x 2 and the hexagonal 6H - SiC(0001) 3 x 3 are of special interest since they give a unique opportunity to investigate the role of surface morphology on oxygen or hydrogen incorporation into the surface. In contrast to silicon, the subsurface structure plays a major role in the reactivity, leading to unexpected consequences such as the initial oxidation starting several atomic planes below the top surface or the surface metallization by atomic hydrogen.

One of the major difficulties preventing the wide use of germanium (epi or bulk) as a gate material is the poor stability of its oxide, leading to reproducibility and reliability issues. In contrast to silicon, the nature and thickness of Ge “native” oxides are history dependent, and most phases of germanium oxide are water-soluble. As a result, the procedures for passivating Ge surfaces with hydrogen (HF last) are more complex and less forgiving. We have used infrared absorption spectroscopy and x-ray photoelectron spectroscopy to investigate the nature of oxidized and H-terminated Ge surfaces. The GeO2, GeO and GeC phases have been identified and quantified as a function of processing conditions. The stability of the H-terminated surfaces has been examined in air and in controlled environments. The H-passivated Ge surfaces are found to be much less stable in air than H-terminated Si surfaces.

A series of p- and n-GaAs-S-C(n)H(2n+1) || Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5-1.6m(e) for the electronic carriers that cross the junctions. We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3-1.4 eV above and 2.0-2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p-GaAs system.

N-doping of an electron-transport material, a tris(thieno)hexaazatriphenylene derivative (1), with the strongly reducing molecule bis(cyclopentadienyl)-cobalt(II) (cobaltocene, CoCp2), is investigated using ultra-violet, X-ray, and inverse photoemission spectroscopies, and current–voltage measurements. Condensed CoCp2 films show a 4 eV ionization energy, which is unusually low for vacuum-deposited molecular material and suggests that cobaltocene is promising as a molecular n-dopant. Efficient n-doping of 1 by CoCp2 is confirmed by a 0.56 eV shift of the Fermi level toward the unoccupied states of the host, and by a three orders of magnitude current increase in devices where compound 1 is interfacially doped with cobaltocene.

We elucidate the electronic structure of both filled and empty states of ordered alkyl chains bound to the Si(111) surface by combining direct and inverse photoemission spectroscopy with first principles calculations based on density functional theory. We identify both filled and empty interface-induced gap states, distinguish between those and states extending throughout the monolayer, and discuss the importance of these findings for interpreting transport experiments through such monolayers.

The structure of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT · PSS) consists of an insulating PSS layer surrounding doped PEDOT grains. In this study, X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are used to investigate the composition and electronic structure of as-loaded and lightly sputtered PEDOT · PSS films. The sputtered film shows a significant increase in the PEDOT/PSS ratio (from 0.12 to 0.7) as well as a build-up of the density of filled states close to the Fermi level, consistent with the removal of the insulating PSS layer and the uncovering of the highly doped conducting PEDOT · PSS. The thickness of PSS layer is estimated at 35 ± 5 Å. The elimination of the PSS shell reduces PEDOT · PSS work function to a value equal to that of highly doped PEDOT. The effect of the PSS removal on the charge injection barrier at the interface with the hole-transport material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) is also investigated. The increase in interface dipole and hole-injection barrier suggests a stronger interaction between the two materials, and points out the key role of the PSS layer in making PEDOT · PSS an effective hole-injection material.

Monolayers of alkyl chains, attached through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO2, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation-or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) pi* resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.

We present a comprehensive experimental and theoretical characterization of the electronic structure of four 1,1-diaryl-2,3,4,5-tetraphenylsiloles (aryl = phenyl, 2-(9,9-dimethylfluorenyl), 2-thienyl, pentafluorophenyl). Solid-state electron affinities and ionization potentials of these siloles were measured using inverse-photoelectron spectroscopy (IPES) and photoelectron spectroscopy (PES), respectively; the density of electronic states obtained from calculations performed at the density functional theory (DFT) level corresponds very well to the PES and IPES data. The direct IPES measurements of electron affinity were then used to assess alternative estimates based on electrochemical and/or optical data. We also used DFT to calculate the reorganization energies for the electron-transfer reactions between these siloles and their radical anions. Additionally, optical data and ionization potential and electron affinity data were utilized to estimate the binding energies of excitons in these siloles.

We investigate the Si-rich 3 x 3 to Si-terminated root3xroot3 phase transition of the 6H-SiC(0001) surface by atom-resolved scanning tunneling microscopy. We find a 2root3x2root3-R30degrees reconstruction, coexisting with few 3x3 domains. While a high-quality 3x3 surface preparation is achieved with a very low defect density (<2% of atomic defects), the (2 root 3x2x3 root 3)-R30 degrees reconstruction instead exhibits long atomic cracks suggesting important stress relief during the phase transition. A structural model containing 13 Si atoms per unit cell distributed in three layers above the Si terminated bulk SiC substrate is proposed. (C) 2004 American Institute of Physics.

Passivation of semiconductor surfaces against chemical attack can be achieved by terminating the surface-dangling bonds with a monovalent atom such as hydrogen. Such passivation invariably leads to the removal of all surface states in the bandgap, and thus to the termination of non-metallic surfaces. Here we report the first observation of semiconductor surface metallization induced by atomic hydrogen. This result, established by using photo-electron and photo-absorption spectroscopies and scanning tunnelling techniques, is achieved on a Si-terminated cubic silicon carbide (SiC) surface. It results from competition between hydrogen termination of surface-dangling bonds and hydrogen-generated steric hindrance below the surface. Understanding the ingredient for hydrogen-stabilized metallization directly impacts the ability to eliminate electronic defects at semiconductor interfaces critical for microelectronics, provides a means to develop electrical contacts on high-bandgap chemically passive materials, particularly for interfacing with biological systems, and gives control of surfaces for lubrication, for example of nanomechanical devices.