PV Research Concentrates on Lowering Cost and Thickness

Two recent breakthroughs have lowered the cost of solar power and may revolutionize the PV industry.

WASHINGTON, DC, US, 2001-10-16 [] Scientists at the University of Arizona have been funded with grants of more than US$1 million to develop organic molecules that will ‘self assemble’ into thin-film solar panels. They are designing and synthesizing molecules that will organize themselves from a solution into coatings of one-thousandth the thickness of a human hair. “It would be efficient enough at energy conversion to economically generate power,” says Neal Armstrong, principal investigator on a three-year, $490,000 grant from the U.S. National Energy Research Laboratory. The grant is designed to develop new organic ‘liquid crystal’ PV materials that could be inexpensively wet-processed into large area panels, producing a solar panel array on a flexible plastic substrate that would be inexpensive and which could be rolled out like wallpaper on a roof. At the same time, engineers at Motorola Labs claim to have solved a 30-year-old problem of the semi-conductor industry by successfully combining the best properties of workhorse silicon technology with the speed and optical capabilities of compound semiconductors, known as III-V materials. In thin layers, the technology involves III-V semiconductor materials such as gallium arsenide/nitride, indium phosphide, and other high performance/light-emitting compounds, to be grown on a silicon substrate. Until now, this has been a virtually impossible task due to fundamental material mis-match issues, say officials at Motorola, but the company says the discovery opens the door to less expensive devices in potential markets such as photovoltaics, data storage, lasers for DVD players and other consumer products, medical equipment, radar, automotive electronics and lighting. The new semiconductors will make major cost savings in optical communications, high-frequency radio devices and high-speed microprocessor-based subsystems. In Arizona, Armstrong says new inorganic thin films such as cadmium telluride and cadmium-indium-galleium-selenide are already entering the PV market, and their efficiencies and potentially lower cost are impressive. The heavy metals, tellurium and selenium, raise environmental concerns, both during their manufacture and ultimate disposal. Potential organic solar cells would be a less-toxic and more environmentally friendly way to tap solar energy because most of the organic materials under study are environmentally benign, both in processing and in discard. His colleague, optical sciences professor Bernard Kippelen, says organic material for optical or optoelectronic applications were perceived as unstable and could have only a limited lifetime. With recent research, people know that if you synthesize the materials correctly, purify them and keep them from water and oxygen, even organic materials can have very long lifetimes, he explains. The challenge for PV is to achieve higher electrical ‘mobility’ in films that rapidly carry charge. “We don’t want to make predictions that sound overly optimistic but, theoretically, there is no reason that we cannot make organic solar cells with 20 percent efficiency,” says Kippelen. Researchers from the University of Cambridge and the Max Planck Institute reported in the September issue of Science magazine that they have developed a potentially efficient self-assembled organic thin film PV, proving the concept works. This developed from earlier work that showed how the right kind of self-assembly would increase PV efficiency. Initially, the Arizona scientists focused on self-assembling liquid crystals developed from a common deep blue-green pigment called phthalocyanine. Under conditions of heat, these disk-shaped molecules line up like a stack of coins, solidifying as long, rod-like molecular stacks in a well organized film. They are working to engineer molecules that stack themselves vertically, rather than horizontally, on the substrate for higher electrical mobility. Armstrong says it is no small feat but adds that a conversion efficiency of 10 percent is a realistic goal, based on recent work at UA and studies by several other groups in Europe and Japan. Such a molecular film could also improve a potentially important type of organic solar cell, the dye sensitized cell, first proposed in the 1990s by scientists at Ecole Polytechnique in Lausanne, Switzerland and currently being manufactured in Australia. These solar cells have reached conversion efficiencies of 10 percent but are said not to be widely practical because they contain liquid electrolytes which can evaporate and decompose and can be hard to process into large area PV arrays. Dye sensitized organic solar cells have a transparent electrode coated with a porous network of titanium dioxide nanoparticles, the semiconductor. By itself, titanium dioxide cannot absorb visible sunlight efficiently so a photosensitive dye is added to the network, the sensitizer, which absorbs photons from sunlight and releases electrons that flow as an electrical current to an electrode. But once photoactive dye molecules have given up electrons, they must be very quickly recharged. The Arizona researchers want to replace the liquid electrolyte currently used to regenerate dye in these organic solar cells with a faster charge-transporting film. Researchers say they can make the titanium dioxide network even more porous so photoactive dye covers a greater surface area, thereby increasing the cell’s light absorption potential. Sensitizing dyes currently used do not absorb the full solar spectrum, harvesting only about 45 percent of light, so they are seeking dyes that absorb more of the infrared spectrum. The U.S. Department of Energy, with the Office of Naval Research, are funding organic solar cell research through new programs, and other basic and applied research funding sources are increasingly interested in the technology.