We have performed photovoltage mapping on polycrystalline silicon solar cells through the potential measurements by KFM under light illumination, in order to investigate an influence of the grains and their boundaries on the photovoltaic properties of the solar cells.
Our KFM system is based on a commercial AFM system operating in a high vacuum at room temperature and in tapping mode with a piezo-resistive cantilever. As a light source, a monochromatic light from tunable Ti:Al2O3 laser system was used, and this light was focused on the sample surface just under the AFM tip. The polycrystalline silicon solar cell sample was fabricated on a p-type substrate with a phosphorus doped surface n-layer of approximately 500 nm in thickness.
First, we evaluated photovoltage by subtracting the potential value in the dark condition from that under the light illumination at a certain point on the polycrystalline silicon solar cell at various incident laser intensities. As a result, we observed both the photovoltage increase and its saturation according to an increase of the light intensity which is a basic feature of photovoltage in the solar cells, and the saturation value was about 400 mV which is reasonable in typical silicon solar cells. These results clearly prove the validity of our photovoltage measurements based on KFM.
Secondly, we performed photovoltage mapping on a certain area including a grain boundary. Consequently, we confirmed photovoltage drop around the grain boundary and photovoltage difference between the different grains. We also found that this site dependence of photovoltage can be well explained from the intrinsic potential distribution obtained in the dark condition. When the intrinsic potential for electrons is low or high, the electron is attracted or repulsed, respectively, and therefore the photo-generated electrons accumulate in the low potential region rather than the high potential region. Since the photovoltage depends on the number of electrons accumulating in the surface n-type layer, the photovoltage in the low potential region should be higher than that in the high potential region, which is very consistent with the experimental results obtained by our KFM method. |