Future Students - What are Photovoltaic Devices
Thin-film Solar Cells
Thin-film Solar Cell
Solar cells are a clean and reliable method of generating electricity. For many applications, however, the high cost of solar cells makes them uncompetitive with grid supplied electricity from conventional sources such as coal fired power stations. Solar cells have therefore been limited primarily to remote applications such as for telecommunications and marine navigation systems, where low maintenance and high reliability are required. Much solar cell research has concentrated on reducing costs, thus making solar cells commercially viable for other applications. One possibility for significantly reducing costs is the development of thin film solar cell technologies.
Most commercially available solar cells are made from wafers of very pure monocrystalline or polycrystalline silicon. Such solar cells can attain efficiencies of up to 18% in commercial manufacture and over 20% in the laboratory. However, the silicon wafers used to make them are relatively expensive, making up 20-40% of the final module cost. Although silicon is highly abundant (comprising one quarter of the earth's crust), making a very pure wafer suitable for solar cell manufacture requires much energy and is therefore relatively costly. Moreover, a solar cell made using a 300-400 micrometre thick wafer generates 90% of its energy from the top 15-20 micrometres. The rest of the wafer is required simply to hold the cell together.
The alternative to these "bulk silicon" technologies is to deposit a thin layer of semiconductor onto a supporting material such as glass. Various materials can be used such as cadmium telluride, copper-indium-diselenide and silicon. Thin film solar cells made from cadmium telluride or copper indium diselenide have yet to be fully commercialised, but offer some promise of achieving low costs with reasonably good performance.
Thin film silicon solar cells have been commercially available for many years and are the most common technology employed in solar powered calculators and watches. These cells employ amorphous silicon, which is of much lower quality than the material used for bulk silicon solar cells. Because of this lower material quality, the efficiencies of amorphous solar cells are significantly lower than those of bulk silicon cells-less than 8 % for the best commercial devices. High efficiency is not critical for applications such as small consumer products, but is important for large scale applications such as supply of grid-connected electricity. Furthermore, amorphous cells tend to degrade when exposed to sunlight. Despite these problems, significant improvements have recently been made in improving and stabilising the performance of amorphous solar cells. Continued improvements could make this technology more economically viable.
Staff at our School have been researching silicon thin film solar cells since 1986. Much of this research is related to crystalline silicon structures, such as methods of crystallising amorphous thin films and maintaining the efficiency of cells made using low quality material.
A large amount of interest has been generated in our School's thin-film work. In February 1995, Pacific Power, then the main electricity generator for New South Wales, and Unisearch, the commercial arm of the University, came together in a $64 million joint venture to establish Pacific Solar. From then until early 2004, Pacific Solar worked on developing our School's work on polycrystalline silicon thin-film solar cells into a commercially viable product that was cheap enough to allow solar cells to be installed across the rooftops of the world.
In mid-2004, a new company, CSG Solar, was formed to take the technology into commercial production. Commercial products are presently expected to be available in 2006.
The CSG (crystalline silicon on glass) thin-film approach involves depositing thin layer of amorphous silicon to give a large sheet of float glass all the required junction layers. The silicon is then crystallised in a furnace to convert it to polycrystalline form.
The film is then patterned into individual cells using a laser to ablate a groove between cells. An insulating resin is then applied to the rear of the cells and patterned in two stages using ink-jet printing. The first patterning step creates a pattern of small holes in the resin which allows etching of the silicon in the region of the holes down to a buried negative-type layer. The second creates a similar pattern of holes that allows less severe etching to allow good contact to the positive-type layer underlying the resin.
CSG Thin-film Solar Cell
Metal is then deposited over the rear of the module to contact both negative and positive layers. This metal layer is patterned into strips by laser so that the correct connection between negative and positive layer results. A section of the final cell structure is shown in the figure below.
Only 1 micron of silicon is required, less than 300 times that in a conventional solar cell. Even though over 300 times less silicon is involved, present module efficiencies are 8%, not far below the median efficiency of 12.5% for conventional wafer modules. Accelerated testing shows the new modules are also likely to be even more durable than conventional modules which are already renowned for their long field life (25 year warranties are common). Those involved believe this is one of the most exciting new developments in the solar field over recent decades and believe this development could pave the way to very low cost solar cells, once manufacturing is scaled to very large volumes.