Theoretical studies of light induced defects in Cz-Si

When a B-doped Cz-Si is used as a solar cell material, light irradiation and/or electron injection causes the solar cell conversion efficiency to be degraded. To understand the light induced degradation phenomena, the generation process of interstitial impurities induced by light irradiation is theoretically studied by using ab initio molecular-orbital calculations. The impurities at substitutional sites are stable, and they do not move to the interstitial sites even by the presence of excess electrons. However, when there are interstitial Si atoms near the substitutional impurities, such as B and C, these impurities easily become to the interstitial ones through the kick-out process. This reaction is enhanced by the injection of excess electrons. On the other hand, it is difficult to generate the interstitial Ga even with the interstitial Si and electron injection. This is the reason why B doped Cz-Si is degraded by the light irradiation, while Ga doped wafer is robust for the light induced degradation.