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The formation mechanism and properties of the boron-oxygen center responsible for the degradation of Czochralski-grown Si(B) solar cells during operation is investigated using density functional calculations. We find that boron traps an oxygen dimer to form a bistable defect with a donor level in the upper half of the band gap. The activation energy for its dissociation is found to be 1.2 eV. The formation of the defect from mobile oxygen dimers, which are shown to migrate by a Bourgoin mechanism under minority carrier injection, has a calculated activation energy of 0.3 eV. These energies and the dependence of the generation rate of the recombination center on boron concentration are in good agreement with observations.

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The description of the diffusion mechanism of O 2i in the caption of Fig. (3) contained an error in the original article. It should have read: ''A configuration-coordinate diagram for the oxygen dimer. Arrows show the proposed thermally assisted Bourgoin diffusion mechanism with a thermal barrier of 0.3 eV. O sq 2i at A first captures a photogenerated or injected electron and, after overcoming a 0.2 eV barrier, changes its configuration to O st 2i . It then traps a hole becoming O st 2i , and executes a diffusion jump to O sq 2iat B after overcoming a thermal barrier of 0.3 eV.'' This mechanism was described correctly in the main text.

It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity--vacancy pair)--one of the most studied defects in semiconductors--has only one energy level in the band gap: namely, the acceptor level at about 0.45 eV below the conduction band. We now demonstrate that it has a second level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this level, having a donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.