Solar cells produce electricity just by lying in the sun; they use the energy of photons, the particles of sunlight, to directly generate an electric current, hence their scientific name, photovoltaics. Although photovoltaic technology is still considered too expensive for large-scale applications, it increasingly finds its way into “niche” markets in locations with no readily available connection to the electricity grid—such as satellites, boats, camps, or remote settlements. According to PV News, between 1995 and 2000 the world market for photovoltaics more than tripled, from 78 to 288 megawatts per year; and by 2020 it is expected to increase more than tenfold, to 4 gigawatts per year. Weizmann Institute scientists are exploring new methods and materials for making photovoltaics a viable energy source in small- and large-scale applications.
Collection Techniques
Natural sunlight is a dilute energy source: to produce a large amount of electricity, solar cells must be spread out over a very wide area. If today’s expensive photovoltaic units were replaced by optical concentrators that intensified sunlight by, say, a factor of 500, then the cell needed to produce the same amount of electricity would be 500 times smaller. The savings in silicon alone would go a long way toward making solar-produced energy economically attractive.
Focusing highly concentrated sunlight onto small solar cells, Prof. Amnon Yogev found that when filters are used to split sunlight into a number of spectral ranges, heating is reduced and efficiency of the photovoltaic unit is doubled. If the unused wavelengths are exploited for other uses, such as providing the energy for the “absorption cooling” that powers some refrigeration and air conditioning systems, the savings can be even greater.
Economic factors such as these hold the key to bringing solar energy to vast areas of the world that have no existing infrastructure for power generation and transmission based on fossil fuels. The investment necessary is modest: a 10-kilowatt photovoltaic unit could support the electric lighting, refrigeration, and communications infrastructure of an entire village. Anticipating the difficulties of maintaining solar installations in isolated areas, Institute scientists envisage building photovoltaic units complete with satellite communications networks, to allow maintenance reports to be transmitted automatically.
New Materials
Today most commercial solar cells are still made of silicon, which is stable but relatively expensive; however, several other materials are being developed for future use in photovoltaics. The goal: to produce solar cells that are both cheap and durable. The front-runners are thin-film technologies, in which a thin layer of a fine-grained photoelectronic material is deposited on an inexpensive large-area surface such as window glass.
Weizmann Institute research may speed up the application of one of the most advanced thin-film materials in solar cells. Cadmium telluride-based photovoltaics are already in the pilot stage of development and may soon be manufactured commercially on a large scale in the United States and Europe; but these cells tend to deteriorate over time. Because Institute scientists had in the past managed to solve major theoretical and practical problems related to photovoltaics, the U.S. Department of Energy asked them to address the deterioration of cadmium telluride-based units. The Institute team, headed by Profs. David Cahen and Gary Hodes of the Department of Materials and Interfaces, revealed how these solar cells can be rendered more stable. Using chemical and physical investigation methods, the scientists showed, for example, that the cells must be used in a dry, preferably oxygen-free environment. The researchers are conducting further studies aimed at enhancing the stability of cadmium telluride cells and understanding the basic science underlying their performance. Such an understanding may help not only to develop better cells but also to overcome the psychological barriers—particularly, concerns over reliability—impeding the acceptance of experimental solar energy technologies.
In another project, Profs. Cahen and Hodes are investigating the mechanism of action of an innovative type of solar cell invented in the early 1990s in Switzerland. The dye-sensitized solar cell, or DSSC, is made of an organic material that is incorporated into a porous thin film consisting of microscopic semi-conductor particles held together like beads on a string. DSSC cells are radically different from other solar cells in several respects, and until recently it was unclear how exactly they generated electric power. Profs. Cahen and Hodes, in collaboration with colleagues in Israel and abroad, have suggested a mechanism that may account for the formation of photovoltage in these cells: apparently, the dye functions much like chlorophyll, the natural photosynthetic pigment in plants. The Institute researchers are currently striving to understand the mechanisms involved in the movement of electrons through DSSC cells.
Improving the Surface
The efficiency of silicon solar cells is limited by two major problems: surface recombination—the tendency of electrons to become trapped in the surface of the semiconductor; and light reflection from the surface of the solar cells, which can decrease the energy available for electricity production by 10-20 percent.
Prof. David Cahen and his colleagues are improving solar cell performance by eliminating the surface defects common to fine-grained semiconductor materials. This research involves analyzing the defects caused by minute amounts of impurities in the semi-conductor, then controlling those defects on the molecular level. Together with Prof. Avi Shanzer of the Department of Organic Chemistry, Prof. Cahen has succeeded in improving semiconductor performance by grafting desirable properties onto organic molecules.
Prof. Shanzer is adapting models of chlorophyll, the substance that controls photosynthesis in plants, to improve the semiconductors used in photovoltaic cells. The technique involves dipping the semiconductor into a specially prepared solution containing porphyrins—the “backbone” of chlorophyll molecules. The porphyrin binds to the semiconductor surface, causing the semiconductor to absorb light more efficiently. This in turn induces a greater electric charge in the semiconductor. Developing his approach further, Prof. Shanzer’s group is working on synthesizing organic “wires” that would link light-harvesting groups such as porphyrins to a metal ion, creating molecular “antennae” to guide photons to the semiconductor surface.
