Organic Interfacial Layer Research Articles

Deposition from Dopamine Solutions at Ge Substrates: An in Situ ATR-FTIR Study

Deposition from dopamine (DA) solutions at germanium (Ge) model substrates was monitored under stationary conditions using surface sensitive in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra of the interfacial organic layer formed upon contact of TRIS buffered aqueous DA solutions to a Ge internal reflection element (IRE) showed conveniently strong diagnostic IR absorption bands, which were increasing with deposition time up to at least 6 h. Comparison of IR spectra of unreacted pristine DA, surface reacted, and bulk reacted material confirmed chemical reactions of DA to a polymerizate according to the literature. The found IR bands could be assigned to aromatic as well as C-O single bond moieties. The kinetic courses of the diagnostic band integrals showed an initial increase and saturation of the deposition after around 300 min, which could be empirically represented by an exponential damping function revealing a rather small kinetic constant. Highest deposition levels were found at pH = 8.5 (TRIS buffer or NaOH) in contrast to pH = 6.2, where no deposition occurred. Minor deposition was found in the presence of salt or at ZnSe instead of Ge due to the absence of reactive hydroxyl groups. The concentration dependence of DA deposition showed an initial increase and a saturation beginning at around 0.4 mg/mL (0.0022 M), where around 50 nm thick films featuring granular surface morphologies are formed. The adsorbed species are suggested to be smaller bulk reacted DA polymerizate particles with reactive end groups. Rinsing the formed films by pure TRIS buffer resulted in a time dependent release of deposited organic material by ≈23%, which could be represented by an exponential decay function. A saturation of the release after around 100 min and a larger kinetic constant compared to deposition could be determined. The released material is suggested to be larger aggregated bulk reacted DA polymerizate particles loosely bound to the surface by weak interaction forces.

Nucleation delay in atomic layer deposition on a thin organic layer and the role of reaction thermochemistry

The atomic layer deposition (ALD) of a series of inorganic thin films on an interfacial organic layer has been examined, focusing on the existence of a delay in thin film nucleation and its dependence on the chemistry of each ALD reaction. The inorganic thin films examined were Al2O3, HfO2, Ta2O5, and TaNx, while the organic layer was a thin (∼5 Å thick) film of poly(ethylene imine), or PEI, on SiO2. The early stages (<50 ALD cycles) of inorganic thin film growth in each case have been characterized using a combination of surface-sensitive techniques including ellipsometry, x-ray photoelectron spectroscopy, and atomic force microscopy. PEI has the effect of attenuating growth to different degrees depending on the subsequently grown ALD thin film, leading to no attenuation of growth for Al2O3, but significant incubation periods, in increasing order, for Ta2O5, HfO2 and TaNx. Angle-resolved x-ray photoelectron spectroscopy of an Al2O3 thin film deposited on PEI|SiO2 demonstrates that, in this case, the PEI thin film is buried between the Al2O3 thin film and the SiO2 substrate, with no significant decrease in the density of nitrogen from the as deposited PEI film. Results from the HfO2 and the Ta-containing thin films indicate that the processes that lead to the attenuation of growth are associated mostly with the second half-cycle of ALD (H2O or NH3 exposure). The length of the incubation time is found to correlate with both the total enthalpy change of the overall ALD reaction, as well as the net internal energy change of a single ligand exchange reaction representative of the second half of the ALD reaction. These results suggest that the reaction thermochemistry, and activation barriers for the reactions involved the ALD process play a key role in determining the length of the incubation period caused by PEI.

Enhanced Performance of Printed Organic Diodes Using a Thin Interfacial Barrier Layer

Printed, organic diodes with a thin organic interfacial layer forming a Schottky barrier were fabricated and characterized. Experiments indicated that the thickness of the barrier layer is <10 nm. The interfacial layer reduces the reverse current of the diode by 2 orders of magnitude without significantly affecting the forward characteristics above 1 V. As a result, printed organic diodes with a rectification ratio of 5 orders of magnitude were fabricated. The diodes enable applications where low reverse currents are needed.

Charge Transport Mechanism and the Effects of Device Temperature on Electrical Parameters of Au/ZnPc/N-Si Structures

Gold/Zinc Phthalocyanine/n-Si metal semiconductor contact with organic interfacial layer have been developed and characterized by Current–Voltage-Temperature (I-V-T) measurements, to study its junction and charge transport properties. The junction parameters, of diode ideality factor (n), barrier height (b) and series resistance (R¬S), of the device are found to shift with device temperature. The barrier height and the diode ideality factor are found to increase and the series resistance is found to decrease with increasing device temperature. The activation energy of the charge carriers is found to be 44 meV and the peak of interface state energy distribution curves is found to shift in terms of Ess-Ev value from 0.582 eV to 0.776 eV with increasing device temperature. The data analysis implies that the Fermi level of the organic interfacial layer shifts as function of device temperature by 100 meV in the device temperature range of 283K to 343K. In terms of dominant conduction mechanism, the I-V-T data analysis confirms the fit of data to the relationship log (IV4)  V1/2 for higher device temperatures and the Poole-Frenkel type is found to be the dominant conduction mechanism for the hybrid device.

Dominant conduction mechanism and the effects of device temperature on electrical characteristics of Al/ZnPc/n-Si structures

Aluminum/Zinc Phthalocyanine/n-Si metal semiconductor contact with organic interfacial layer has been developed and characterized by Current–Voltage–Temperature ( I– V– T) measurements for the study of its junction and charge transport properties. The junction parameters, such as diode ideality factor ( n), barrier height ( φ b) and series resistance ( R S), of the device were found to shift with device temperature. The diode ideality factor was found to increase with the device temperature up to 323 K. However, a decreasing trend in the value of n was observed beyond this temperature. The barrier height and series resistance were found to increase and decrease, respectively with increasing device temperature. The peak of interface state energy distribution curves was shifted, in terms of Ess-Ev value, from 0.622 eV to 0.827 eV with 52 meV activation energy of the charge carriers. The data analysis implies that the Fermi level of the organic interfacial layer shifts as function of device temperature. In terms of dominant conduction mechanism, the I– V– T data analysis confirms the relationship log ( IV 4) ∝ V 1/2 with the device temperature in the range of 313–343 K and the Poole–Frenkel type is found to be the dominant conduction mechanism for the hybrid device.

Coverage dependent adsorption dynamics in hyperthermal organic thin film growth.

We have examined the dynamics of adsorption of diindenoperylene (DIP) on SiO(2) and SiO(2) modified with an interfacial organic layer using in situ real time synchrotron x-ray scattering, focusing on the effects of coverage. On both surfaces we observe a substantial increase in the probability of adsorption with increasing coverage, which is most dramatic at the highest incident kinetic energies. On the initially uncovered surfaces, we observe a smooth decrease in the probability of adsorption with increasing incident kinetic energy, indicative of trapping-mediated adsorption. Once both surfaces are covered by DIP, the effects of incident kinetic energy are greatly reduced, and trapping is very efficient over the range of kinetic energies examined. Possible reasons for efficient trapping at high coverage and at high incident kinetic energy include more efficient momentum transfer due to mass matching, and possibly direct molecular insertion. Comparison to results on another small-molecule, pentacene, suggests that this behavior should be common to hyperthermal growth of a variety of other small-molecule thin films.

Interface modification for highly efficient organic photovoltaics

We present highly efficient inverted polymer:fullerene bulk-heterojunction solar cells by incorporation of a nanoscale organic interfacial layer between the indium tin oxide (ITO) and the metal oxide electron-conducting layer. We demonstrate that stacking of solution-processed organic and metal oxide interfacial layers gives highly charged selective low ohmic cathodes. The incorporation of a polyoxyethylene tridecyl ether interfacial layer between ITO and solution-processed titanium oxide (TiOx) raised the power conversion efficiency of inverted organic photovoltaics to 3.6%, an improvement of around 15% in their performance over comparable devices without the organic interfacial layer.

Analysis of Organic Films and Interfacial Layers by Infrared Spectroscopic Ellipsometry

Analysis of Organic Films and Interfacial Layers by Infrared Spectroscopic Ellipsometry

ChemInform Abstract: Structural Analysis of Organic Interfacial Layers by Ellipsometry

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Improved band alignment for hole injection by an interfacial layer in organic light emitting devices

We demonstrate that a thin organic interfacial layer of 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) can be utilized to improve the band alignment of N,N′-di-(3-methylphenyl)N,N′diphenyl-4,4′diaminobiphenyl (TPD) films on [indium–tin–oxide (ITO)] (InSnO) substrates in, e.g., organic electroluminescent devices. A photoemission study of the highest occupied molecular orbital (HOMO) and vacuum level position as a function of the organic overlayer thickness reveals that due to chemisorptive bonding a thin PTCDA interlayer results in a reduced barrier between the Fermi level of ITO and the HOMO of TPD. Furthermore we detect a new molecular state 0.6 eV below the Fermi level at the PTCDA/ITO interface. Both effects are expected to improve the hole injection from the ITO anode into the TPD hole transport layer, e.g., in organic light emitting devices.

Dispersion-free membrane extraction: case studies of metal ion and organic acid extraction

Dispersion-free extraction of solutes from aqueous solutions across a hydrophilic flat-sheet microporous membrane or hydrophobic hollow fiber membrane to an organic phase containing complexing carriers was modeled. The systems of copper(II)-D2EHPA (di(2-ethylhexylphosphoric acid) as well as organic acids such as citric and lactic acids and TOA (tri- n-octylamine) were exemplified. A mass transfer model was proposed taking into account aqueous layer diffusion, interfacial chemical reaction, membrane diffusion, and organic layer diffusion on the basis of a good knowledge of extraction chemistry and the transport properties of the relevant geometry. It was shown that both the calculated extraction rates of Cu(II) and the time profiles of liquid phase concentrations of organic acids were in good agreement with the measured ones (average standard error, 11%). The rate-controlling mechanisms of such dispersion-free membrane extraction processes were finally identified and discussed.

Mass transfer modeling of citric and lactic acids in a microporous membrane extractor

The extraction of organic acids such as citric and lactic acids from aqueous solutions through a hydrophobic microporous flat-sheet membrane to an organic phase containing tri- n-octylamine (TOA) was examined experimentally and theoretically. A mass transfer model was proposed, considering aqueous layer diffusion, interfacial chemical reaction, and membrane and organic layer diffusions. It was shown that the time profiles of liquid-phase concentrations of the acids were in reasonable agreement with the measured ones (average standard error, 8%), especially well at low feed acid and/or TOA concentrations. Based on a good knowledge of kinetics of liquid–liquid extraction (LLE), the present processes were found to be governed by mixed interfacial reaction and membrane diffusion.

Nanoelectronic device applications of a chemically stable GaAs structure

We report on nanoelectronic device applications of a nonalloyed contact structure which utilizes a surface layer of low-temperature grown GaAs as a chemically stable surface. In contrast to typical ex situ ohmic contacts formed on n-type semiconductors such as GaAs, this approach can provide uniform contact interfaces which are essentially planar injectors, making them suitable as contacts to shallow devices with overall dimensions below 50 nm. Characterization of the native layers and surfaces coated with self-assembled monolayers of organic molecules provides a picture of the chemical and electronic stability of the layer structures. We have recently developed controlled nanostructures which incorporate metallic nanoclusters, a conjugated organic interface layer, and the chemically stable semiconductor surface layers. These studies indicate that stable nanocontacts (4 nm×4 nm) can be realized with specific contact resistances less than 1×10−6 Ω cm2 and maximum current densities (1×106 A/cm2) comparable to those observed in high quality large area contacts. The ability to form stable, low resistance interfaces between metallic nanoclusters and semiconductor device layers using ex situ processing allows chemical self-assembly techniques to be utilized to form interesting nanoscale semiconductor devices. This article will describe the surface and nanocontact characterization results, and will discuss device applications and novel techniques for patterning close-packed arrays of nanocontacts and for imaging the resulting structures.

Instrument-invariant method of film thickness determination by means of substrate-to-film X-ray peak intensity ratioing

A simple method for the determination of thin film thickness using X-ray spectrometry is described. The ratio of the substrate X-ray peak intensity to that of the film is taken as a measure of the thickness of the film. A calibration curve, constructed using specimens of known thickness, can then be used for thockness determination. The results are independent of the incident electron beam current. The calibration curve is made instrument invariant by means of a normalization procedure and by taking into account the respective X-ray take-off angles. Normalized calibration curves are reported for 13 different elements on a silicon substrate, covering thicknesses between 2.5 and 650 nm. The method, ideally suited to thin films on solid substrates, can also be applied to films not on substrates, and in the presence of a thin organic interfacial layer between film and substrate.