Analysis of local Al-doped back surface fields for high efficiency screen-printed solar cells

In this paper, we investigate the surface recombination of local screen-printed aluminum contacts applied to rear passivated solar cells. We measure the surface recombination velocity by microwave-detected photoconductance decay measurements on test wafers with various contact geometries and compare two different aluminum pastes. The aluminum paste which is optimized for local contacts shows a deep and uniform local back surface field that results in Smet = 600 cm/s on 1.5 Ωcm p-type silicon. In contrast, a standard Al paste for full-area metallization shows a non-uniform back surface field and a Smet of 2000 cm/s on the same material. We achieve an area-averaged rear surface recombination velocity Srear = (65 ± 20) cm/s for line contacts with a pitch of 2 mm. The application of the optimized paste to screen-printed solar cells with dielectric surface passivation results in efficiencies of up to 19.2% with a Voc = 655 mV and a Jsc = 38.4 mA/cm2 on 125 × 125 mm2 p-type Cz silicon wafers. The internal quantum efficiency analysis reveals Srear = (70 ± 30) cm/s which is in agreement with our lifetime results. Applying fine line screen-printing, efficiencies up to 19.4% are demonstrated.