The performance of boron-doped CZ Si solar cell is degraded by the light irradiation and/or the minority carrier (electron) injection. To understand these phenomena, ab initio molecular orbital calculations are carried out using a cluster model. The theoretical results indicate that the activation energy for kicking out a substitutional B by the interstitial Si is decreased due to the injected electron. Therefore,the interstitial B is easily produced by the minority carrier injection and then diffuses in the Si crystal, resulting in the generation of the interstitial B and interstitial O defect complex. (C) 2002 Elsevier Science B.V. All rights reserved.