Photovoltaic energy generation has been one of the fastest growing markets in the last years. The rising oil price, climate changes and moreover significant financial market aids in form of fed-in tariffs resulted in a growth of more than 40 % over almost the complete last decade. Most of these financial incentives include dropping fed-in tariffs in order to force the market to drop production cost consistently, which is therefore the FIGURE 1: Thin silicon solar cell (< 40 µm) with an efficiency of 20 %: Laser processes like Laser-Fired Contacts allow the manufacturing of these high-efficient next generation devices due to their selectivity, flexibility and contact-less nature. major goal in current research and development. These goals can be achieved in two main ways:
The surface photovoltage (SPV) technique is an attractive method for measuring the minority carrier diffusion length. It has the advantage of not requiring permanents sample contacts. It does require, however, an accurate knowledge of the absorption coefficient-wavelength (a-A) relationship. This relationship is well known for single-crystal silicon and SPV plots are linear for such material. However, for polycrystalline silicon we frequently find nonlinear SPV plots and we believe the single-crystal a-h data not to be applicable. Depending which portion of the plot is used for diffusion length determination, we obtain very different diffusion lengths. By making SPV, short circuit current, and spectral response measurements and comparing theory with experiment, we find a new U -A relationship that appears to depend on the stress in the sample. We find the low wavelength range of the SPV plots to be most suitable for diffusion length measurements.
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