Diffusion length of tri-crystalline silicon during solar cell processing

Low costs and high efficiency are driving the development of crystalline silicon solar cells towards thinner wafers. The optimum wafer thickness with respect to high efficiencies is around 60-100 μm. Tri-crystalline silicon is a promising material to enter this region on production scale. It shows an improved mechanical stability and can be sawn into thinner wafers. The electric properties are investigated by diffusion length measurements performed after each solar cell process step with the laterally resolved ELYMAT technique. Depending on the damage etching process the diffusion length L is between 100 and 300 μm. For alkaline etched wafers a correlation between L and the etch pit density is found. After emitter diffusion the diffusion length increases to values between 400 μm and 500 μm and stays at that level even for subsequent processing steps