Carrier lifetime instabilities in the bulk and at the surfaces of crystalline silicon solar cells triggered by fast-firing processes

Abstract

Carrier lifetime instabilities in crystalline silicon solar cells under illumination affect the long-term stability of photovoltaic modules in view of the steadily increasing conversion efficiency of the solar cells used in state-of-the-art modules. This thesis examines carrier lifetime degradation and regeneration – both in the bulk and at the surfaces of different crystalline silicon materials and solar cells made thereof – after a fast-firing step has been applied in a conveyor belt furnace. It is shown that fast-firing triggers degradation effects under illumination. However, fast-firing is also a key step in the contact formation of screen-printed contacts, which is the dominating metalization technique applied today in the solar cell production. Independent of fast-firing in a standard conveyor belt furnace, the bulk carrier lifetime of boron-doped Czochralski-grown silicon (Cz-Si:B) is dominated by the light-induced activation of a boron-oxygen (BO) defect. Gallium-doped Czochralski-grown silicon (Cz-Si:Ga) and boron-doped float-zone silicon (FZ-Si:B) – both not prone to BO-related degradation –, however, show a strongly temperature-dependent degradation of the bulk lifetime activated under simultaneous illumination. The equilibrium state establishing between activated (recombination-active) and deactivated (recombination-inactive) state of the defect is dependent on the temperature and the activation and deactivation of the defect are reversible processes. It is shown that the in-diffusion of hydrogen from hydrogen-rich silicon nitride layers (SiNy:H), used for the surface passivation and as antireflection coating, into the silicon bulk during fast firing plays a key role for the so-called ”light- and elevated-temperature-induced degradation”(LeTID) in all three materials – namely Cz-Si:B, Cz-Si:Ga, and FZ-Si:B. Experiments on the very defect-lean FZ-Si:B material furthermore suggest a second participant in the defect reaction of LeTID besides hydrogen. LeTID is also observed on ”polycrystalline silicon on oxide” (POLO) backjunction (BJ) solar cells fabricated on Cz-Si:Ga. The maximum relative degradation extent of the conversion efficiency η of only 2%rel at 140 °C and 1 sun, however, shows that POLO BJ cells are remarkably stable regarding light-induced degradation. Through prolonged illumination at elevated temperatures a partial permanent deactivation of the LeTID defect is possible. In addition to bulk degradation, thermally-induced instabilities of the phosphorus-doped POLO passivation layers after fast-firing are discovered. The degradation and the following regeneration of the surface passivation quality – described by the saturation current density J0 – is correlated with high probability with the hydrogen passivation of interface states.