Boron-Oxygen Complex Responsible for Light Induced Degradation in Silicon Photovoltaic Cells: a New Insight into the Problem

Results available in the literature on minority carrier trapping and light induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J.A. Hornbeck and J.R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results which have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro-Contreras et al., J. Appl. Phys. 2019, 125, 185704). The significance of the latter work is discussed and detailed experimental results on the electronic and dynamic properties of the BsO2 complex are presented. It is shown that the BsO2 complex is a defect with negative-U properties and it is responsible for minority carrier trapping and persistent photo-conductivity in non-degraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the pre-cursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron-doped Cz-Si crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.