c-Si, Thin Films Vie for Solar Cell Market

Silicon takes the lead

Silicon has been the leading solar cell material since renewable technology began to attract attention. Two major categories of silicon have been used in the manufacture of solar cells—monocrystalline silicon in the form of single wafers made from ingots, and poly- or multicrystalline silicon made from large blocks of molten material carefully cooled and solidified.

Monocrystalline silicon proved to be the most efficient but its single wafers are more costly to produce because they are cut from cylindrical ingots. They have registered the highest solar to electric conversion efficiency (25%) of any material. Multicrystalline, or poly-silicon solar cells, while less costly to produce, are a little less efficient averaging 17% to 21%. They are extremely durable however, and over the past several decades, multicrystalline silicon has become the leading material used in solar cells.

Thin films drive lower costs

Thin films found a place in the structure of solar cells as a potential method of reducing cell manufacturing costs. Thin-film solar cells are made by applying very thin layers of semiconductor material to inexpensive materials such as glass or plastic or metal. Thin-film semiconductors absorb light more easily than c-Si, further reducing the amount of material required and thus cutting costs.

At the present time, there are three leading thin film materials used in the manufacture of solar cells. Cadmium telluride (CdTe) has the lowest Wp (Watt-peak) production cost because cells made from this material are relatively easy to manufacture. Efficiencies of CdTe cells range between 9 and 12%.

Copper indium gallium selenide (CIGS) thin films have achieved the highest efficiencies of any thin film material at 12 to 14%. There has been some difficulty in controlling the uniformity of the active layer and in using steel substrates, but research in this material continues at a brisk pace.

Amorphous silicon (a-Si) has also proven to be a workable thin film technology. It has the lowest efficiency, however, registering between 7% and 10% at best. This material has long been used in consumer products powered by the sun such as calculators and watches. Some manufacturers are pursuing its use in solar cells and striving to improve efficiencies.

Gallium arsenide is a fourth thin film material that has been used to manufacture solar cells used in satellites and space exploration. Currently, GaAs is being tested for use in terrestrial solar concentrator and multijunction cells, which offer several different combinations of materials that will absorb solar rays and convert them to electricity.

Manufacturing advantages

Thin films obviously achieve lower efficiencies than crystalline silicon. Over time, however, the overall efficiencies of thin film materials in solar cells have improved and several major manufacturers are focusing on thin films for their major solar cell product lines. Some advances in the manufacturing process and its equipment have added to the potential benefits of both crystalline silicon and thin-film technologies.

Improvements in capex processes have had a positive impact on both c-Si and thin films. Applied Materials, Santa Clara, California, for example, has introduced several deposition systems that are targeted at a more efficient distribution of c-Si and thin films on various substrates. Their latest system fabricates electrical circuits on both sides of a solar cell. Keys to the performance of this new unit are a higher level of precision and control to the cell manufacturing process.

Oerlikon Solar, located in Trübbach, Switzerland, has lowered production costs for solar cells with the introduction of a new ThinFab line. The company credits its ability to make increasingly thinner films with optimized material usage. Oerlikon has dramatically increased the efficiency of thin-film silicon and enabled turn key systems for fabricating thin-film modules.

Both technologies are beneficiaries of improved and highly specialized solar cell fabrication systems. Thin films were originally adopted to lower the cost of solar cells, but crystalline silicon has also become a cost effective and efficient way of producing them.

A close race

The advantages and characteristics of both materials are similar. C-Si and thin films are light in weight, easy to fabricate, amenable to advances in capex processes and have sufficiently high efficiencies to form reliable solar cells. The problem of efficiencies is a source of constant R&D and we have seen improvements over the years. Applications for both materials are increasing daily.

BCC Research analyzed the markets for both c-Si and thin films and arrived at some interesting data, as outlined in Table 1 [1]. Our research indicates that thin films will adapt to the flexible substrate market, which shows greater potential every year. Thin films were only 16.5% of the market in 2009 and will double their growth by the end of our forecast period in 2015.

In the end, it looks as though both technologies will lead in growth over monocrystalline silicon and emerging technologies. C-Si is being used for rooftop applications, as well as other uses, and thin films are proving themselves in building-integrated PV (BiPV) and other power generating applications. Looking out five years, we see a close race between these two PV material technologies and room for both of them. As in so many other fields of electronics, the application will determine the material.