Effective reduction of the metastable defect concentration in boron-doped Czochralski silicon for solar cells

Different approaches to reduce the light-induced degradation of Czochralski silicon (Cz-Si) solar cells are investigated. In the first part of this paper the very promising possibility of using overcompensated oxygen-rich n-type silicon with residual boron as solar cell substrate material is demonstrated. Stable bulk carrier lifetimes in the millisecond range are achievable in this material. The second part of this work deals with new technological approaches to reduce the concentration of the metastable defect responsible for the light-induced carrier lifetime degradation in boron-doped Czochralski silicon. A permanent reduction of the defect concentration by a factor of up to 3.5 is achieved with an optimized emitter diffusion process at 850°C in a conventional quartz tube furnace using fast ramping conditions. Long-term annealing treatments performed at low temperature (450°C) are shown to reduce the metastable defect concentration by a factor of up to 3.3 and, at the same time, lead to the formation of thermal donors (TDs). These results are in good agreement with our new defect model which assumes a fast-diffusing oxygen dimer as part of the Cz-specific defect.