D. Faiman

Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions

We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions ~25‐ 60 °C!. The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase ~coupled to a corresponding increase for the fill factor! was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47‐ 60 °C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility.

Optimal tilt angles of enclosed reactors for growing photoautotrophic microorganisms outdoors

The relationship between the tilt angle of a flat-plate photobioreactor and productivity of Spirulina platensis was evaluated along with the annual seasons under the climatic conditions of south Israel (latitude approx. 31°). The reactor tilt angle exerted a significant effect on the optimal population density and thus on the productivity of cell mass, owing to its control over the amount of solar radiation entering the reactor. A direct relationship between solar energy and productivity was observed: the higher the amount of solar energy that was admitted by varying the reactor tilt angle according to season, the higher was the productivity that could be sustained in the culture. Small tilt angles of 10° to 30° in summer and larger angles in the vicinity of 60° in winter resulted in maximal productivities for these seasons. Photosynthetic efficiency was calculated for the different tilt angles for all seasons. Efficiency was low in the winter due to temperature limitations. In summer it was highest in the 90° reactors, indicating that for optimal tilt angles in this season (in regards to productivity) a significant amount of radiation could not be effectively used by the culture. The results suggest a potential benefit in orientating and tilting reactors at various appropriate angles to the sun on a seasonal basis: up to 35% enhancement in annual output rate is estimated to be achievable.

EVA browning and the time-dependence of I−V curve parameters on PV modules with and without mirror-enhancement in a desert environment

Two independent experiments are described that were performed on samples of EVA-laminated photovoltaic modules which form part of a grid-connected PV system in the Negev desert. Both types of experiment provide quantitative assessments of the rate at which module output power decreases when mirror-enhancement is employed under such conditions. One of the experiments, performed on 60 modules which have been exposed to mirror-enhanced solar radiation for the past 8.4 years, compared the measured I−V curve parameters of these modules with corresponding measurements that had been made 3.4 years earlier. The second experiment consisted of monthly I−V curve measurements, spread over the last 3.4 years, on three specific mirror-enhanced modules and on another three from which the mirrors had been removed. These experiments indicated that the mirror-enhanced modules are continuing to degrade at a comparable rate to an indirect estimate that had previously been made, i.e. approximately 1% per year. On the other hand, the modules from which mirrors were removed 3.4 years ago, although visually as “brown” as the others are degrading at an, as yet, unmeasurably small rate.

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.

EVA laminate browning after 5 years in a grid-connected, mirror-assisted, photovoltaic system in the Negev desert: effect on module efficiency

Abstract Outdoor I–V curve measurements have been made on all 189 Solarex SX-146 modules that were operated for five years in a mirror-enhanced, grid-connected photovoltaic system in the Negev desert of Israel. The modules have taken on a distinct yellow-brown coloration colpared to their original “blue” appearance, a phenomenon that has been observed in systems in other parts of the world. Although no original performance figures are available for comparison purposes, some indirect measurements are reported which enable one to conclude that the power output of this ensemble of browned modules has probably degraded at an average rate of about 1% per year.

An optical system for the quantitative study of particulate contamination on solar collector surfaces

Abstract We describe a computerized microscope system that has been developed for studying the physics of dust particles which adhere to various kinds of surfaces such as those of solar collectors. The device enables investigators: (1) to obtain the particle size distribution of dust on a surface; (2) to calculate the fraction of surface area covered by dust; (3) to calculate the reduction of optical efficiency (of the solar collector under study) as a function of particle size; (4) to investigate the effect of various kinds of applied force field on the adhesion of dust particles to the surface. Some examples are given for the use of such a measuring system for the study of photovoltaic and solar-thermal collector surfaces.

REDUCING THE HEAT LOSS AT NIGHT FROM SOLAR WATER HEATERS OF THE INTEGRATED COLLECTOR-STORAGE VARIETY

Abstract Experimental data and an analysis are presented for a solar water heater of the integrated collector–storage variety. The novelty of the particular unit investigated lies in the fact that its design incorporates a device that automatically lowers the heat-loss coefficient of the system during night time. This overcomes one of the basic problems associated with solar water heaters of this kind. The analytical apparatus employed for the study is our previously-published MUE method but with the incorporation of some newly suggested experimental details that lead to improved accuracy.

Sinusoidal spectral correction for high precision outdoor module characterization

Abstract We present the results of experiments which demonstrate that a sinusoidal variation in the long-term, STC-corrected, outdoor performance of PV modules is caused by seasonal spectral changes in the received sunlight. This variation may be factored out to increase the precision of outdoor studies. We use this result (a) to quantify the rate of EVA degradation observed in the Negev desert under natural 1-sun conditions and (b) to identify a principal source of the “summer recovery” observed in modules of amorphous silicon cells.

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) eV in our samples are attributed to nonbonded intercalated O2 in the C60 lattice while other states at Ev+1.3 eV 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+1.3 eV 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.