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 1800C 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 Siemens bell jar reactor with a cold water-chilled walls. 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 2009.
At the same time, a number of companies are developing alternative technologies, which potential major advantage is being less time-, energy- and, thus, cost-intensive than Siemens. For instance, technologies that use alternative deposition reactors (e.g. the Fluidized Bed Reactor, FBR) are gaining market share because they are expected to lower production costs by 15-25% compared to the traditional Siemens method.
Another developing technology is the direct purification of metallurgical silicon into upgraded metallurgical, or solar-grade, silicon (UMG). The approach gained popularity when polysilicon prices skyrocketed driven by the material shortage, and over 20 companies had been 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. However, the market potential of UMG technologies has seriously weakened as many UMG-Si producers face challenges in consistently achieving desired product purity and polysilicon prices have fallen.
On the other hand, existing Siemens-based polysilicon manufacturers have been exploring ways to cut production costs, mainly by lowering energy consumption and scaling up the production. Experts believe that leading Siemens-based producers can cut costs by 20-30% in the next 3 to 5 years, and thus alternative technologies are unlikely to gain a substantial cost advantage over large-scale plants using the advanced Siemens process. With some 70% of the new projects going to use the Siemens technology, it is expected to retain its dominant share through the next 5 years.
Wafers/Cells
At present, over 80% of all solar cells produced in the world are made on the basis of crystalline silicon. In 2009, 34% of solar cells were made from monocrystalline silicon, 47% ─ from poly- or multicrystalline silicon, and 1.5% ─ in form of ribbon-sheet crystalline silicon. About 17 % 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.
Technology details
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 thin film solar cells, 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).
Market potential for various technologies
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, as the efficiency becomes of the higher priority to customers with silicon prices falling. These technologies 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 in 2010). Although the share of monocrystalline silicon cells in the global output continues to decrease reaching 34% in 2009, with many companies planning to produce high-efficiency selective emitter cells, demand for monocrystalline wafers may rise again.
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 (average at 9%). With silicon prices falling, the market potential of thin-film technologies has been seriously eroded and they are not expected to compete seriously with crystalline silicon technologies in the nearest future.
While the thin-film sector currently encompasses some 170 active companies largely backed by venture capital, only a couple of them have by now produced in excess of 100 MW annually. Besides, the cost structure of most a-Si producers, considering its low efficiency, is not competitive with c-Si ones, and CIGS manufacturers have been facing technical issues that have forced most of them to delay commercial production. Global financial constraints also made banks and developers prefer more mature and abundant crystalline silicon modules for projects.
According to the European PV Industry Association (EPIA), the world’s PV cell manufacturing capacity in 2009 was around 24 GW. EPIA expects these capacities to grow by about 30% in 2010 after which the annual growth rate should stabilize at about 20% to reach some 65 GW in 2014. The CAGR for c-Si modules will be about 22% whereas for thin film modules it will be around 25%. The thin-film’s share will not exceed 25% by 2014:
Production capacity outlook: Crystalline vs. Thin-film technologies (technologies with market share below 0.5% are not represented)
As for particular thin-film technologies. according to industry analysts, amorphous silicon (a-Si) will continue to lead the thin-film PV space and will remain the most likely technological route for new entrants, as manufacturing equipment and materials are readily available. Other approaches in the thin-film landscape will also make their impact felt, especially CIGS:
Source: Greentech Media Research.
By region, Asia will maintain its leading positions as a cell & module manufacturer. Initially driven by the solar industry development in Japan, its leadership now is mainly supported by the dramatic growth of PV manufacturing capacities in China, Taiwan, Korea, Malaysia, Philippines, and India. In 2009, 75% of all PV cells were made in Asia (with about a third – in China alone), with the rest being divided between Europe (17%) and USA. This regional structure is projected to remain unchanged for the nearest future.
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[5] PHOTON’s PV Market Outlook — 6th PV Conference, Munich, April, 2008
[6] Lehman Brothers’ Solar Daily 24.06.2008
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[8] 2009 cell & module survey. PHOTON International 2010 March
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[13] 2009 cell & module survey. PHOTON International 2010 March
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[18] EPIA. Global market outlook for photovoltaics till 2014
[19] Greentech Media. Thin-film Market Outlook to 2015. March 2010. http://www.gtmresearch.com/report/thin-film-2010-market-outlook-to-2015 ; http://www.greentechmedia.com/articles/read/the-future-of-thin-film-beyond-the-hype
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