PVI Paper

Progress in n-type monocrystalline silicon for high efficiency solar cells

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By Bo Li, General Manager of Solar Cells, SunEdison, Inc.; Joel Kearns, Vice President for Solar R&D, SunEdison, Inc.

Future high efficiency silicon solar cells are expected to be based on n-type monocrystalline wafers. Cell and module photovoltaic conversion efficiency increases are required to contribute to lower cost per watt peak and to reduce balance of systems cost. Past barriers to adoption of n-type silicon cells by a broad base of cell and module suppliers include the higher cost to manufacture a p-type emitter junction and the higher cost of the n-type mono silicon crystal. Technologies to reduce the cost of manufacturing the p-type emitter by diffusion or implantation of boron are being developed in the industry. To reduce the cost and improve further the quality of n-type mono silicon crystal, SunEdison has developed a continuous Czochralski (CCZ) crystal pulling process, based on the technology of Solaicx, acquired in 2010. This CCZ technique allows production of a crystal with much greater resistivity uniformity, with a lower incorporation rate of lifetimereducing metals impurities, and allows crystal oxygen to be selected independent of production batch size. CCZ is expected to reduce n-type crystal cost below that of current p-type mono crystal.

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PVI Issue
There have been encouraging signs in recent months of changing fortunes for PV equipment suppliers after a difficult period of consolidation. Shipment figures, actual and forecast, have in many instances seen an upswing, as booming markets in Japan, China and the US continue to drive demand, even as some European markets continue to dwindle. It’s probably too early to call the beginnings of a new PV technology buy cycle, but it seems more a case of ‘when’ rather than ‘if ’ now, and analysts have pointed to mid-2014 as the likely point when supply and demand will be in some kind of equilibrium. Clearly the implication of this is that if demand continues to rise beyond this point, supply will have to keep up, so manufacturers will have to invest in new capacity.

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