Dislocation density, multicrystalline silicon wafers, dislocation density etching, dislocation density imaging, solar cells, solar cell wafers, bulk defects, edge dislocation, efficiency enhancement, crystal defects, PV efficiency
Semiconductors, photovoltaics, polycrystalline silicon ingots, wafers and cells
Use of multicrystalline silicon-based wafers in solar cells reduces production costs but decreases efficiencies due to high defect content, dislocations, grain boundaries and impurities.
These inventions describe methods to significantly reduce dislocation density in pc-Si bricks or wafers. In one invention a high temperature anneal eliminates dislocations, a diffusion barrier slows the entry of impurities and the material is cooled to room temperature. Although dislocation density minimization is commonly attempted during ingot crystallization by a high-temperature “holding step” before cool-down, there are significant advantages to performing this at the brick or wafer level: greater surface area to volume ratio facilitates better dislocation out-diffusion and more linear time-temperature profiles, hence avoiding thermal stresses that generate new dislocations. Another invention proposes a method of using high temperatures and stresses to alleviate the crystallographic defects found in ribbon and ingot multicrystalline silicon. The proposed method uses thermal stress to cause dislocation motion and annihilation throughout the material as an alternative to applying mechanical loads and high temperature to multicrystalline silicon wafers or solid blocks. Prolonged temperature cycling has been used to decrease dislocation densities in copper hardening and have been applied to heteroepitaxially grown III/V semiconductor materials. The same principles may be applied to multicrystalline silicon thus improving the quality of multicrystalline silicon to levels comparable to monocrystalline silicon, while retaining the cost advantage of multicrystalline silicon.
Photoelectric efficiency improvements of 10-40%
Case 13974: US Patent Application Number 12/959795, filed on December 3, 2010
Case 14086: US Patent Number 8,389,999, issued on March 5, 2013
Katy Hartman, Mariana Bertoni, James Serdy, and Tonio Buonassisi. Dislocation density reduction in multicrystalline silicon solar cell material by high temperature annealing Applied Physics Letters 93, 122108 (2008). DOI: 10.1063/1.2909644
Last revised: April 30, 2013
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