LOW RECOMBINATION n~+ REGIONS CREATED BY n~+ c-Si EPITAXIAL LAYERS AND LASER PROCESSING OF PHOSPHORUS-DOPED SiC_X FILMS.

摘要

Solar cells with locally contacted rear surface exhibit a trade off between surface passivation and ohmic losses. In order to reduce the penalty in Fill Factor (FF) when long distances between contacts are used, we explore the use of a low temperature epitaxially grown n~+ c-Si layer at the rear surface of n-type c-Si solar cells. Additionally, the passivation and contact formation are also developed at low temperature using phosphorus-doped silicon carbide films (SiC_x) and laser processing. Firstly, we obtain an effective surface recombination velocity (S_(eff)) on 3.5 W cm n-type c-Si surface as low as 3 cm/s for SiC_x films deposited at 225 °C. Next, we determine laser parameters that yield good quality n~(++) regions ready to be contacted. These regions exhibit a surface recombination velocity of 1500 cm/s that results in a final S_(eff) value well below 100 cm/s even for short pitches. Regarding the epitaxial silicon layer (epi-Si), an electrically active donor density of 1.2×10~(20) cm~(-3) is determined. The combination of the epi-Si layer with the SiC_x film leads to S_(eff) of ～300 cm/s. This relatively high S_(eff) value can be attributed to the high donor density that increases bulk recombination within the epi-Si layer. Finally, we compare c-Si heterojunction solar cells with and without the epi-Si layer. The introduction of an epi-Si layer improves FF from 72.5 % to 77.3 % with the same distance between contacts. However, open-circuit voltage significantly decreases due to a higher rear surface recombination velocity.