Increased effectiveness by new technology for silicon solar cells

Petten - Researchers at the Energy research Centre of the Netherlands (ECN) have developed a simple method to improve the solar cell efficiency for a new generation of crystalline silicon industrial solar cells.

Crystalline silicon solar cells are at this moment predominantly made from p-type silicon wafers. Researchers at ECN have shown that significantly higher efficiencies can be obtained by making the solar cells from n-type silicon wafers. They used process steps, such as screen printing, of the same low-cost character as are presently used in mainstream industrial production.

A major breakthrough that the researchers obtained was a novel method (patented) for passivating the emitter of their n-type solar cells. This novel method increases the efficiency of these cells significantly over the values obtained for p-type ones, when using cell processes of comparable cost and simplicity. For single-crystalline silicon the advantage is of order 1% absolute (determined in an in-house comparison at ECN); for multicrystalline silicon the gain depends on the quality of the silicon wafer.

A best efficiency of 18.3% on single crystalline Czochralski wafers (17.9% average), and 16.4% on multicrystalline silicon was demonstrated, using standard industrial wafers of 125mm size in both cases.

Since the new cell process can be carried out using the same manufacturing equipment as the p-type cell process, it will allow rapid implementation, possibly even by modifying existing process lines. Together with other advantages and improvements based on this new cell process, low-cost screen printed n-type cells are potentially capable of becoming a strong new technology in the PV market. ECN intends to further develop and bring to commercialization several solar cell types based on this technology on short and medium time frames.

'We believe that n-type cell technology may offer several important advantages for commercial cell production', says Paul Wyers, manager of ECN Solar Energy. 'For example, the tolerance for some common impurities may be important for companies that want to employ new lower grades silicon feedstock. But there are also advantages inherent to the cell process, for any quality of wafer, such as the excellent blue response. We are well on our way to develop several cell processes that should be an interesting proposition to industry.'

Picture:
 
This is the principal picture of the new cell. The green layer (anti-reflection coating and passivating layer) is where the patented break-through was obtained. The types (n for the wafer, p+ for the emitter) constitute the fundamental difference with 'normal' cells (which are p for the wafer, and n+ for the emitter).

Literature:
ECN-scientist Valentin Mihailetchi et al., Appl.Phys. Lett. 11 February 2008, Volume 92, Issue 6, page 063510.  Download the publication via the ECN-website:  http://www.ecn.nl/publications/default.aspx?nr=ECN-W--08-007.

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