Light and elevated Temperature Induced Degradation (LeTID) of the carrier lifetime in multicrystalline silicon

Abstract

Solar cells fabricated on multicrystalline silicon show a pronounced degradation of their energy conversion efficiency under illumination at elevated temperature. This effect is frequently denoted LeTID (’Light and elevated Temperature Induced Degradation’) in the literature. Within this thesis, the properties and the root cause of the degradation phenomenon is investigated in detail. It is shown for the first time that the degradation in efficiency is caused by a pronounced degradation of the carrier lifetime in the silicon bulk. Upon prolonged illumination at elevated temperature, a regeneration of the carrier lifetime is observed which leads to lifetime values comparable to the inital value. Based on this finding, a series of comprehensive lifetime studies is performed to elucidate the fundamental defect physics and the impact of process steps as well as the illumination conditions on the carrier lifetime degradation and regeneration. Rapid thermal annealing, which is typically applied as the last process step during solar cell fabrication, is found to have a strong impact on the degradation extent. The degradation extent strongly increases with increasing peak temperature. In contrast to that, samples which receive a phosphorus gettering treatment show a less pronounced degradation than samples without phosphorus gettering. Furthermore, the degradation extent and the regeneration rate strongly depend on the wafer thickness. Thin samples show a less pronounced degradation and the regeneration of the carrier lifetime sets in earlier. Finally, we show that the surface passivation scheme of the lifetime samples affects the LeTID effect. Only samples with hydrogen-rich silicon nitride (SiNx) films being part of the surface passivation scheme show the most pronounced degradation of the carrier lifetime. Based on these findings, the role of SiNx passivation layers on the lifetime-limiting defect is examined in detail. Hydrogen bound within the SiNx films diffuses into the silicon bulk upon rapid thermal annealing. For the first time, a direct correlation between the hydrogen concentration in the silicon bulk and the degradation extent is shown, clearly proofing that hydrogen is directly involved in the LeTID mechanism. Finally, a defect model based on the experimental findings of this thesis is derived. The defect model includes weakly or even non-recombination active metal-hydrogen complexes as the most likely precursor for the lifetime-limiting defect. The degradation is caused by a dissociation of the metal-hydrogen complexes, whereas the regeneration is caused by the diffusion of the recombination active metal to the wafer surfaces. Measurements on samples with different thicknesses point towards a diffusion coefficient of (5 ± 2) × 10−11 cm2 s−1 of the metallic species in silicon at 75°C. In conclusion, cobalt and nickel are likely candidates for the metallic species.