453 research outputs found
Charge Transport Properties of a Metal-free Phthalocyanine Discotic Liquid Crystal
Discotic liquid crystals can self-align to form one-dimensional
semiconducting wires, many tens of microns long. In this letter, we describe
the preparation of semiconducting films where the stacking direction of the
disc-like molecules is perpendicular to the substrate surface. We present
measurements of the charge carrier mobility, applying temperature-dependent
time-of-flight transient photoconductivity, space-charge limited current
measurements, and field-effect mobility measurements. We provide experimental
verification of the highly anisotropic nature of semiconducting films of
discotic liquid crystals, with charge carrier mobilities of up to
2.8x10cm/Vs. These properties make discotics an interesting choice
for applications such as organic photovoltaics.Comment: 5 pages, 5 figure
Structural studies and influence of the structure on the electrical and optical properties of microcrystalline silicon thin films produced by RF sputtering
Microcrystalline silicon thin films were produced by reactive magnetron sputtering on glass substrates under several different conditions (RF power and gas mixture composition). The film structure was studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and Raman spectroscopy, allowing the determination of crystal sizes, crystallinity and mechanical strain. These parameters were evaluated by fitting a pseudo-Voigt function to the X-ray data, and by the application of the strong phonon confinement model to the Raman spectra. The degree of crystallinity and the presence of single crystals or crystal agglomerates, which was confirmed by TEM, depends on the preparation conditions, and strongly affects the optical spectra and the electrical transport properties.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XX
Influence of copper on the electronic properties of amorphous chalcogenides
We have studied the influence of alloying copper with amorphous arsenic
sulfide on the electronic properties of this material. In our
computer-generated models, copper is found in two-fold near-linear and
four-fold square-planar configurations, which apparently correspond to Cu(I)
and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic
and sulfur, grows with increasing concentration of copper. Overcoordinated
sulfur is found in trigonal planar configuration, and overcoordinated
(four-fold) arsenic is in tetrahedral configuration. Addition of copper
suppresses the localization of lone-pair electrons on chalcogen atoms, and
localized states at the top of the valence band are due to Cu 3d orbitals.
Evidently, these additional Cu states, which are positioned at the same
energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for
masking photodarkening in Cu chalcogenides
Optical modulation spectroscopy of hydrogenated microcrystalline silicon
The properties of microcrystalline silicon thin films prepared by RF sputtering
were investigated by optical modulation spectroscopy at room temperature and the
results were correlated with Raman and conductivity measurements. For comparative
purposes, a number of good quality PECVD microc-Si:H samples were also investigated.
For PECVD samples the OMS signal is very weak, and only measurable for
probe beam energies comparable to the gap of amorphous silicon. This indicates the
absence of gap states and therefore a very high crystalline fraction, as confirmed by
Raman and TEM measurements.
In what concerns RF-sputtered samples, different behaviors can be
distinguished: Some samples show a low OMS signal which can be attributed either to
a high crystallinity (low density of gap states) or to high recombination rates. These
two cases can be distinguished by electrical conductivity values and analysis of the
Raman spectra. Other samples exhibit a OMS signal similar to a-Si:H and have low
conductivity values, consistent with a Raman spectrum typical of a-Si:H.FCT-grant from PRAXIS XX
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