S. Goren

Structure and optical properties of C60 thin films

Abstract By varying the deposition conditions of C 60 thin films and using X-ray diffraction techniques to study the results, we obtain a continuum of structure types ranging from crystalline to amorphous. By measuring the absorption spectra of these films we find that as the film crystallinity increases the fundamental absorption edge and nearby spectral features shift progressively to longer wavelengths. Using a Tauc-type approximation, we find that the optical bandgap E g = 1.3–1.6 eV for crystalline films and E g = 2.4–2.6 eV for amorphous films. Intermediate values of E g are obtained depending upon the degree of crystallinity of the film.

CHANGES IN THE PHOTOELECTRICAL PROPERTIES AND GENERATION OF PHOTOINDUCED DEFECTS UNDER LIGHT/AIR EXPOSURE OF C60 THIN FILMS

We report on the time development of surface photovoltage (SPV) and electron paramagnetic resonance (EPR) spectra for C60 films irradiated by room light in air. Such exposure is shown to result in the progressive reduction of the SPV signal at 0.9–1.1 eV and in an increase in the signal at approximately 1.3 eV, as well as in the reduction of the signals at about 1.6 and 2.3 eV. A decrease in the dark contact potential difference signal as a result of surface band-bending reduction is also observed after exposure. These results are explained assuming that gap states at Ec−(0.9–1.1)u2009eV in our samples are attributed to nonbonded intercalated O2 in the C60 lattice while other states at Ev+1.3u2009eV are related to oxygen chemically bonded to the C60 molecules. EPR measurements reveal that light/air exposure causes an increase in the number of C60+ paramagnetic defects. A hypothesis has been suggested that the recombination centers at Ev+1.3u2009eV and the C60+ paramagnetic centers have the same origin and are attribu...

Fullerene photoelectrochemical solar cells

Abstract The photoelectrochemistry of single crystal C 60 and fullerene photoelectrochemical solar cells is studied. Illuminated and immersed, C 60 is shown to drive oxidation of several solution-phase redox couples. Utilization of a photoelectrochemical solid/liquid junction, rather than solid-state photovoltaic junction, improves the observed photocurrent. Utilization of a single crystal, rather than a polycrystalline film, of C 60 decreases dark current to the extent that light-driven charge transfer dominates. The spectral response and current-voltage behaviour in several electrolytes is studied. A low-power fullerene photoelectrochemical solar cell, utilizing a regenerative polyiodide and ferri/ferrocyanide redox couple, is demonstrated.

Solar cells from carbon

Abstract A photovoltaic device, of the Schottky-barrier variety, has been fabricated from pure buckminsterfullerene and silver. The paper presents, as motivation for this work, a number of reasons why fullerenes may turn out to be excellent materials for solar cell fabrication. A summary is given of the experimental techniques employed and the results obtained thus far.

A Photovoltaic C60-Si Heterojunction

Abstract A heterojunction was prepared by depositing a thin film of C60 on a p-Si substrate. Photovoltaic properties were observed using a UV-filtered solar simulator and natural sun light. Surface Photovoltage Spectroscopy was employed to distinguish between the nature of photoconversion in the C60 and Si layers.

Persistent photoelectric phenomena in oxygenated C60 thin films

Abstract We report on a persistent internal photopolarization effect observed in C60 thin films at room temperature. This differs from the similar “photoelectret” state previously seen in some other materials by an absence of fast photodepolarization. Our oxygenated C60 thin films also exhibit persistent photoconductivity. Both phenomena are discussed within the framework of a model of photogenerated carrier trapping at deep level centers and an inhibition of their recombination due to strong distortion of the C60 molecule which is known to occur in the presence of localized charge.

AC losses in high-temperature superconductor BSCCO hollow cylinders with induced current

Abstract The issue of the correlation of AC loss data obtained in different experimental configurations for high-temperature superconductor (HTSC) samples of different topology is investigated using a practically important example of a hollow cylinder with induced current. The basic configuration under consideration is a transformer device where the current in a BSCCO cylinder is induced by the magnetic flux from a coil positioned inside the cylinder. Using a novel contactless method, the AC losses in the cylinder are measured. It is shown that Beans critical state model is unsuitable for explanation of the experimental data. Moreover, in the case of complete penetration, the AC loss values appear to be quite different for a slab, a hollow cylinder in uniform external magnetic field and a cylinder in the investigated configuration. Thus, neither the critical state model nor the experimental data obtained on samples of simple geometry can be used to determine AC losses in HTSC hollow cylinders. A method for the AC loss evaluation in HTSC cylinders that is based on using the real E–J characteristic and the mathematical model of a transformer is proposed. It provides a good agreement of experimental and calculated values of AC losses.

Do structural defects affect semiconducting properties of fullerene thin films

Abstract We report on the time development of EPR signals (g=2.0026±0.0002) from C60 films with various crystalline structure under air/light exposure. The time development consists of two clearly distinguished regions of fast and slow growth. Improvement of the film structure, and in particular the increase in grain size, leads to a deceleration of the “fast” growth. The results are explained assuming that EPR signal growth is controlled by oxygen diffusion, along grain boundaries and into grains, during the “fast ” and “slow” periods, respectively. Fast decrease of the EPR signal as a result of in situ pumping strongly supports this model and indicates a correlation between crystalline structure and oxygen diffusion in C60 films. Such correlation is considered as one of the possible mechanisms which govern the semiconducting properties of the material.

Photovoltaic Properties of Fullerene (C 60 ) Thin films

The crystalline structure, Electron Paramagnetic Resonance and Surface Photovoltage (SPV) spectra of C{sub 60} thin films and the photovoltaic properties of C{sub 60}/Ag and C{sub 60}/Si interfaces are reported. The SPV spectra of C{sub 60} films, C{sub 60}/Ag and C{sub 60}/Si interfaces are presented and analyzed on the basis of a model of C{sub 60} film electron structure including mobility gap, band tails extending into the gap and two deep level states in the gap. I--V characteristics of the C{sub 60}/Ag and the C{sub 60}/p-Si interfaces were measured. Both device structures are shown to exhibit rectifying behavior in the dark and photovoltaic properties. The solar cell parameters are presented.

An experimental study of heating due to the AC loss in a BSCCO cylinder

Abstract Due to the very low thermal conductivity of high temperature superconductors (HTSC), AC losses can result in a non-uniform temperature distribution inside a sample. We report the results of an experimental investigation of the temperature distribution in the wall of the BSCCO hollow cylinder in AC magnetic field. The BSCCO cylinder has a relatively low critical current density of 550 A/cm 2 (at 77 K) measured under DC conditions at 1 μV/cm criterion. The temperature of the interior of the cylinder relative to the outside coolant raised 2.5 K at low external magnetic field (∼0.04 T). The change in temperature is increased linearly with frequency and as the third power of the applied magnetic field. The experimental results are compared with the theoretical predictions obtained in the framework of the critical state model complemented by the heat equation.