Bistable character of a deep level in polycrystalline Si substrate for solar cell

Electronic levels in polycrystalline Si (pc-Si) substrates for solar cells were studied by means of deep level transient spectroscopy (DLTS). A broad peak was observed at around 250 K when samples with grain boundaries (GBs) were thermally treated. The origin of this peak was investigated and we conclude that it is attributed to Cu contaminants gathered around GBs. We found interesting character of this peak. The peak intensity became small by annealing with reverse-biased voltage on the Schottky junction and it was recovered after keeping the sample at room temperature for several days. We explained this character as bistability of the center depending on its charge state. From the application viewpoint, we tried remote hydrogen-plasma treatment and could annihilate the peak.