Formation and annihilation of the metastable defect in boron-doped Czochralski silicon

This paper aims at elucidating the physical mechanism responsible for the light-induced efficiency degradation of solar cells fabricated on boron-doped Czochralski silicon (Cz-Si). Lifetime measurements on Cz-Si wafers at defined injection levels show that the concentration of the light-induced metastable defect increases linearly with the substitutional boron concentration. Moreover, a quadratic increase with the interstitial oxygen density is measured. The defect generation rate is examined as a function of temperature at constant illumination intensity. Our measurements clearly prove that the defect generation process is thermally activated with a relatively low energy barrier of E_gen = 0.4 eV. The activation energy of the defect annihilation process is determined to be independent of the boron doping level at E_ann = 1.8 eV. On the basis of our experimental findings, we introduce a new defect reaction model. In this model, fast-diffusing oxygen dimers O_2i are captured by substitutional boron B_s, to form a complex B_s-O_2i, acting as highly effective recombination center. Promising new strategies for an effective reduction of the light degradation are derived from the proposed model.