Technology and Process

Polysilicon

The process of polysilicon production starts with silica, which occurs widely as sand, quartz, and clay. Carbothermical reduction of silica in an electric furnace, at a temperature of over 1900C produces metallurgical-grade silicon, which is then purified either via silane route or directly.

Siemens reactor process is the most widely used for producing polysilicon. In this process high temperature polysilicon rods are put in a thermal decomposition furnace. The rods are contained in a cooled bell jar. Silane or trichlorosilane (TCS) gas is passed over these rods. The silicon in the gas gets deposited on the rods. When the rods reach the required size, they are extracted. The end product is in the form of chunks or rods of polysilicon. Siemens process was used in nearly 80% of the polysilicon produced in 2008. 70% of the new startups are expected to use the Siemens process.

At the same time, a number of companies are now developing various alternative technologies, which major advantage is being less time-, energy- and, thus, cost-intensive than Siemens. For instance, the Fluidized Bed Reactor (FBR) process is gaining market share because it is expected to lower production costs. 

Anotherdeveloping technology is the direct purification of metallurgical silicon into upgraded metallurgical, or solar-grade, silicon (UMG), with over 20 companies working on proprietary technological routes to UMG. While process details vary per company, it typically requires the removal of a number of metallic impurities and reduction in the boron and phosphorous content in the metal. The resulting product – UMG - is more than 99.99% pure.

For the nearest future, however, Siemens process is expected to remain its dominant share, while yielding somewhat to FBR and other (including the UMG route) technologies:

Source: Photon Consulting, Lehman Brothers

Wafers/Cells

At present about 86% of all solar cells produced in the world are made on the basis of crystalline silicon. In 2008, 38,3 % of solar cells were made from monocrystalline silicon, 47,7% ─ from poly- or multicrystalline silicon, and 1.5% ─ in form of ribbon-sheet crystalline silicon. About 12,5 % of solar cells are produced in the form of thin films of such materials as amorphous silicon, cadmium telluride (CdTe), copper and indium diselenide (CIS) and others, applied onto various substrates.

When comparing various PV technologies, the major factors looked at are cost and performance. The performance of solar cells is assessed in terms of energy efficiency conversion rate ─ the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit.

Crystalline silicon technologies are dominant and expected to remain the major technology within the next decade. They provide for the highest efficiency at industrial levels (average at 16%, with the best rates reaching 25%, and expected to reach 17.5% on average by 2010:

Source: EPIA, Lehman Brothers

 

There are two primary methods for the production of single crystalline silicon and PV-wafers

1 - Czochralski method, CZ – silicon single crystal growth from melting of polycrystalline silicon, which is then cut into wafers and polished.

2 - Float-Zone method, FZ – silicon single crystal growth through shifting its narrow zone whilst melting inductive heating, then cut into wafers and polished. 

The production of multicrystalline silicon and PV-wafers is based on the technique of directional crystallization with the multicrystalline silicon then into square blocks and subsequently into wafers.

Most PV wafers that are currently produced are 210-240 microns thick (with the best levels of 180 microns). The wafers’ average size is 100Х100 mm (4 inches), 125Х125mm (5 inches), 150Х150mm (6 inches), 210Х210mm (8 inches). Wafer thickness is expected to fall to 150 microns in 2010. This reduction in thickness is also expected to reduce consumption to 7.5 gm/Wp by 2010 from 9 gm/Wp in 2007:

Source: EPIA

For production of photocells in the form of thin films, various modifications of chemical vapor deposition (CVD) are used. The major thin film technologies are Cadmium Telluride (CdTe), amorphous silicon (a-Si) and Copper-indium-selenium combination (CIS).

Thin-film technologies have cost advantage as they use little or no silicon. However, although best non-silicon thin-film modules achieve efficiency of 26%, on average, they have inferior conversion rates (6-10%), and, thus, can not yet compete with the mainstream crystalline silicon technologies.
With the ongoing R&D efforts and recent advances in thin-film conversion efficiencies (up to 12%), the thin-film PV sector is projected to grow at a higher rate and to increase its share in the global PV market from about 10% in 2007 up to 20% (4GW) in 2010 and 25% (about 9GW) in 2013 . 

Source: EPIA

As for particular thin-film technology, according to some industry analysts, amorphous silicon (a-Si) will continue to lead the thin-film PV space. While growing in revenues from $1.3 billion in 2008 to $4.1 billion in 2014, a-Si’s share in thin-film technologies is forecast to decline from 54% to 47% by 2011. Still, a-Si remains the most likely technological route for new entrants, as manufacturing equipment and materials are readily available. Improving efficiency of a-Si PV cells will be the key to a-Si maintaining its dominant position in the marketplace.

Other approaches in the thin-film landscape will also make their impact felt, especially CIGS and possibly organic PV and dye-sensitized cells.

In megawatts, a-Si-based thin-film technologies will possess about 70% of global manufacturing capacities in 2012, declining only slightly from 73% in 2009. The closest rival, CIGS technologies, while increasing its capacity share from 13% to 16%, still will remain far behind: