Progress in understanding and reducing the light degradation of CZ silicon solar cells

Abstract

Solar cells made on boron-doped Czochralski (Cz) silicon show a degradation in performance when exposed to light until a stable efficiency is reached. This effect is due to the creation of metastable defects in the Cz silicon base which strongly reduce the bulk carrier lifetime. In a recent model, proposed by one of the authors, the metastable defect was tentatively identified with a boron-oxygen pair. In this paper, a newly developed lifetime spectroscopy method is applied in order to determine the electronic properties of the light-induced recombination centres. The measurements reveal that the fundamental light-induced defect centre has an energy level very different from that of the boron-oxygen pair. Together with complementary experimental results, these findings allow us to identify the fundamental light-induced recombination centre with a new boron-oxygen complex, probably of the type BOn with n » 5. In the second part of the paper, a novel, very effective method for a permanent reduction of the light degradation of boron-doped Cz silicon wafers is introduced. The method is based on the deposition of plasma silicon nitride films containing large amounts of atomic hydrogen and a subsequent short annealing step in the temperature range 700-800°C in order to enable the hydrogen to diffuse into the bulk of the wafer. It is demonstrated that the light-induced defect concentration can be reduced by a factor of up to 3.5 by the hydrogenation treatment applied.