abstract
In the last few years, a transition has taken place in the silicon solar cell production from p-type to n-type Czochralski-grown silicon (Cz-Si), the latter showing the advantage of significantly higher bulk carrier lifetimes. On Ga-doped p-type Cz-Si, ‘Light- and elevated-Temperature-Induced Degradation’ (LeTID), has been attributed to the introduction of hydrogen into the silicon bulk during the fast-firing step. The question remains whether the results obtained on Ga-doped Cz-Si are also applicable to n-type Cz-Si, by now the dominant material in PV production. Hence, in this contribution we investigate the impact of fast-firing on phosphorus-doped n-type Cz-Si wafers as typically used in the production of e.g. TOPCon solar cells. We observe an increasing degradation extent with increasing peak-firing temperature ϑpeak, which we attribute to the increasing hydrogen concentration in the silicon bulk. Reducing ϑpeak, as in the application of LECO contacts, is hence highly recommended. The lifetime in the fully degraded state increases from 2 to 8 ms when reducing ϑpeak from 805 to 740 ◦C. A significant impact is also observed for the cooling rate ϑ’ cool after the firing peak. A reduction in ϑ’ cool from 70 to below 50 ◦C/s increases the lifetime from 3 to 9 ms. Varying the refractive index n of hydrogen-rich SiNy layers reveals a peak-like behavior with the highest LeTID extent at n = 2.19. Hence, controlling the hydrogen incorporated in the silicon bulk is crucial to minimize LeTID and adapting the fast-firing profile can virtually eliminate LeTID in modern n-type silicon solar cells.