Enhanced silicon solar cell performance by rapid thermal firing of screen-printed metals

Abstract

Rapid thermal processing (RTP) of screen-printed (SP) Al on the back and silver (Ag) grid on the front produced significant improvement in back surface field (BSF) of n/sup +/-p-p/sup +/ float-zone (FZ) Si solar cells. Two-step firing was found to form more effective BSF than co-firing, resulting in 0.6-1.0% increase in absolute cell efficiency. In addition, RTP was found to be more effective than the beltline processing (BLP), resulting in 0.5-1.0% increase in absolute cell efficiency. Although the Al-BSF formed by the BLP was inferior to the RTP, the difference between the two is virtually eliminated during the subsequent RTP contact firing. Internal quantum efficiency (IQE) analysis of the solar cells gave effective back surface recombination velocities (S/sub eff/) of >5000 cm/s and /spl sim/1500 cm/s for co-firing in the BLP and the RTP, respectively. Two-step firing produced S/sub eff/ of /spl sim/1500 cm/s and /spl sim/700 cm/s in the BLP and the RTP, respectively. However, S/sub eff/ for the two-step firing, involving BLP BSF formation followed by RTP contact firing, was found to be /spl sim/700 cm/s, which indicates that RTP contact firing with a faster ramp-up (100/spl deg/C/s) restores the poor-quality BLP BSF. On the other hand, BLP contact firing with a slow ramp-up (<10/spl deg/C/s) degrades the high-quality RTP BSF, increasing S/sub eff/ from /spl sim/700 cm/s to /spl sim/1500 cm/s.