Investigation of the degradation of crystalline silicon solar cells

Solar cells made from crystalline silicon (mc-Si, CZ-Si) show a degradation in performance when exposed to light. From literature it is known, that the degradation of solar cells is correlated to an increase of recombination centers in the bulk when the solar cells are exposed to sun light, that means that the bulk diffusion length decreases. From the Shockley-Read-Hall-theory of recombination it is known, that the recombination at a trap level depends on the doping level of the material. Therefore, in order to reduce the degradation we first investigated BSF cells of lower doped bulk material. We found, that within the typical resistivity range of 1-3 ohm cm of solar grade CZ-silicon, BSF cells of standard thickness 230µm show no significant difference in the degradation of about 4% relative. For very low doped electronic grade CZ we observed a much lower degradation of only about 1% relative. Another approach we investigated in order to reduce degradation was the reduction of the cell thickness. Thin cells show a higher diffusion length to cell thickness ratio. Cells which are thin enough compared to the final stable diffusion length after degradation are expected to show no degradation. For BSF cells of 100µm thickness of solar grade CZ we found indeed less than 1% efficiency degradation. Due to the high quality of the BSF, the efficiency of our BSF cells increases with decreasing cell thickness