Impact of Fast-Firing Conditions on Light- and Elevated-Temperature-Induced Degradation (LeTID) in Ga-Doped Cz–Si

Abstract

The fast-firing step commonly applied at the end of solar cell production lines triggers “Light- and elevated-Temperature-Induced Degradation” (LeTID) effects of the carrier lifetime in Ga-doped Cz–Si wafers and solar cells made thereof. As far as the defect formation within the silicon bulk is concerned, the key parameters of the fast-firing step are the peak firing temperature (FT) and the band velocity v_band of the conveyor belt, where the latter mainly defines the cooling ramp after the firing peak. In this contribution, we show that the extent of LeTID and the dependence on the applied temperature during degradation increase strongly with increasing measured FT (from 680 °C to 800 °C), v_band (from 2.8 to 7.2 m/min), and the refractive index n of the hydrogen-rich silicon nitride layer deposited on the wafer surfaces (from 2.07 to 2.37). Through temperature-dependent degradation experiments, we determine an activation energy of E_A = (0.55 ± 0.10) eV of the LeTID mechanism in Ga-doped Cz–Si, which is independent of FT and v_band. From this observation we conclude that a single defect activation mechanism is most likely responsible for the examined LeTID effect, independent of the firing conditions. However, the concentration of recombination-active defect centers after LeTID depends critically on FT, v_band, and n, which we attribute to variations of the in-diffused hydrogen concentrations from the silicon nitride layers during firing. Our experiments hence point towards an involvement of hydrogen in the LeTID mechanism observed in Ga-doped Cz–Si.