Sep 19 2006
The Institute for Crystal Growth (IKZ) in Berlin, a member of the Leibniz Association, and BP Solar have recently signed a cooperation agreement to develop a process for the deposition of silicon on glass. It is intended that the thin crystalline silicon layer will form the semiconductor layer for the fabrication of cost-efficient solar cells.
The current supply shortage of high purity silicon feedstock for photovoltaic application has stimulated greater interest towards innovative approaches that could significantly reduce the consumption of silicon per Watt of solar module power output. The majority of today’s current solar cells are made from silicon wafers sawn from blocks of crystal produced either by ingot casting or crystal pulling (Czochralski method). Silicon wafer thickness is typically 0.20-0.25 mm but requires 0.40-0.45 mm of silicon per wafer to accommodate the kerf loss (sawdust). When sunlight is incident on a silicon solar cell, most of the photon conversion to electrical energy takes place in the top 0.02 mm of the wafer. Therefore, provided the silicon can be mechanically supported on a low-cost substrate (e.g. glass), significantly thinner films of silicon can be used reducing the feedstock requirement.
The problem that arises when depositing silicon on glass substrates is that the crystal grains are typically very small, even after high-temperature or laser re-crystallisation. The main effect is a high density of grain boundaries that result in a low efficiency of the solar cells.
The two-step process which will be developed at IKZ includes the nucleation of silicon crystallites at regularly-distributed sites on the glass surface, at a distance of at least 0.05 mm from each other, and the subsequent enlargement of these “seeds” by crystal growth from a metallic solution. The final target is a continuous multicrystalline layer with thickness under 0.05 mm. While bringing a considerable saving of silicon, the two-step deposition process should produce silicon films on glass suitable for the fabrication of efficient, low-cost solar cells.
This 3 year project will be undertaken at IKZ with the financial support of BP Solar. “We look forward to this initiative with the IKZ and it is a natural complement to our own crystalline silicon technology advances, said Eric Daniels, BP Solar technology VP.”