Titanium dioxide (TiO2) films have been deposited on SnO2 coated glass substrates by screen-printing. Film morphology and structure have been characterized by scanning electron microscopy, x-ray diffraction and BET analysis. Dye-sensitized TiO2 photoelectrochemical cells have been assembled and characterized. Cells sensitized with anthocyanin and a ruthenium complex have been investigated. A 0.77 cm2 ruthenium dye sensitized cell with 6.1% power conversion efficiency under Air Mass (AM1.5) conditions was obtained. Results obtained with a pure anthocyanin dye and dye extracted from blackberries were compared. Finally, a natural gel was found to improve the stability of anthocyanin sensitized cells.
A number of devices have been constructed which directly convert light into electrical work or into a flow of chemical products with a free energy higher than the starting materials. Solar Concentrators have been developed which increase the irradiance on the converter material to levels above that used without the device even if the incident light is diffuse. In contrast to man made converter systems, natural photosynthesis seems, at first, to rely on entirely different conversion mechanisms and physics. In this paper is presented a unification of the physics and the thermodynamics of all quantum solar converters. It will be shown that the Planck equation may describe the upper limit to the performance of systems as diverse as the Fluorescent Solar Concentrator, the novel Ruthenium dye sensitized TiO2 photoelectrochemical cells of Dr. Graetzel, silicon solar cells, Chlorophyll, and, perhaps, the unique `protonic' solar conversion found in the purple membrane of Halobacterium halobium.
Recently, a paper was published by the Lausanne Group headed by Dr. M. Gratzel which claimed to obtain a low cost 7% efficient photoelectrochemical solar cell from a Ru carboxylated bipyridyl charge transfer dye adsorbed on the very rough surface of a colloidal TiO2 film. In the current paper, a verification of this result is presented as well as the requirements for efficient collection in this new type of cell. Measurements are reported in simulated and natural sunlight which confirm that the efficiency is indeed in the range previously reported. In addition, a comparison will be made with detailed balance type calculations which support that the cell operates within the limits defined by thermodynamics and the optical absorption of the adsorbed dye. A discussion is made as to the use of the sensitization concept with other materials as well as to the basic economics of such a device.
A connection is made between the luminescence or radiative recombination in an absorber material and the current voltage characteristics of a quantum converter of light. A relationship between luminescence and voltage is derived, using detailed balance and the chemical potential of the excitation, which is similar to that obtained using the techniques of Shockley and Queisser or R. T. Ross. This model relates the absorptivity and photoluminescence efficiency of the light absorber to the I V curve. In this way both thermodynamic properties, or voltage, and the kinetics, or charge transfer and current, can be combined in order to optimize materials and configurations. The model is applied to dye sensitized Ti02 solar cells, and compared with preliminary experimental data for Ru based charge transfer dyes and inorganic compounds. The luminescence model is found to be applicable to dye sensitized converters, as well as to standard silicon solar cells, light detectors, and LEDs.
Several recent patents have described Hewlett Packard Optoelectronics Division light emitting diode, or LED, lamp designs. These designs have led to several innovative, mass produced products that are now on the market. These designs can be directly traced to the known principles of non-imaging optics, and can thus serve as a guide to solar optics fabrication.
The transduction and conversion of radiant energy into work in a quantum process are dependant on the luminescent properties of the materials involved. Materials with photoluminescent efficiencies greater than 0.1% are likely candidates for solar cells and solar converters. The luminescent optical properties of a material are directly related to the output device parameters. The chemical potential of the incoming light is a function of the photon energy and incident radiance. The amount of work per particle, or voltage, that can be extracted by a solar converter is related to chemical potential of the excitation, which can be inferred from the photoluminescence efficiency at ambient temperature. A discussion is made as to the use and optical properties of materials such as Si and GaAs, FeS2, and biological and organic dyes as efficient solar quantum converter materials. Proper choice of absorber thickness as to maximize the luminescent output observed is shown to optimize solar converter performance.
Concentrators based on geometrical optics increase the irradiance by increasing the projected solid angle, but conserve the radiance of radiation. The general principle for increasing the radiance, and thereby concentrating even diffuse radiation, resembles a light trap. Light, which enters the trap through a selective filter, is shifted in photon energy, for example, by a Stokes luminescent process. It is subsequently trapped because it is reflected by the filter. Concentration is limited, in the ideal case, by the reverse (anti-Stokes) process, which reaches equilibrium when incoming and concentrated radiation reach equal chemical potential. The laser is discussed as an example for a concentration not limited by thermodynamics. The limits imposed by quantum mechanics are derived. Real systems, with various losses, are discussed.
Pyrite (Fe52) has been investigated as a promising new absorber material for thin film solar cell
applications because of its high optical absorption coefficient of 1OL cm1, and its bandgap of 0.9 to 1.0 eV. Thin
layers have been prepared by Metal Organic Chemical Vapor Deposition, MOCVD, Chemical Spray Pyrolysis, CSP,
Chemical Vapor Transport, CVT, and Sulfurization of Iron Oxide films, 510. It is postulated that for the material
FeS2, if x is not zero, a high point defect concentration results from replacing 2 dipoles by single S atoms. This
causes the observed photovoltages and solar conversion efficiencies to be lower than expected. Using the Fe-O-S
ternary phase diagram and the related activity plots, a thermodynamic understanding is formulated for the resulting
composition of each of these types of films. It is found that by operating in the oxide portion of the phase diagram,
the resulting oxidation state favors pyrite formation over FeS. By proper orientation of the grains relative to the
film surface, and by control of pinholes and stoichiometry, an efficient thin film photovolatic solar cell material
could be achieved.