Improved string ribbon silicon solar cell performance by rapid thermal firing of screen-printed contacts

Al-enhanced SiN_x-induced hydrogenation is implemented to improve the minority carrier lifetime in string ribbon Si. Rapid cooling after the hydrogenation anneal is found to increase the spatially averaged relative lifetime enhancement by over 160% for string ribbon Si samples with a spatially averaged as-grown lifetime of 2.9 μs. Partial coverage of back surface by Al eliminates wafer bowing in 100 μm thick substrates, but reduces the spatially averaged lifetime enhancement to below 100% because vacancy generation at the back surface is decreased. Rapid thermal Firing (RTF) of screen-printed contacts, with high heating and cooling rates, is found to improve string ribbon solar cell efficiency by an average of 1.2% absolute over lamp heated belt furnace contact firing. Light beam-induced current (LBIC) mapping and light biased or differential internal quantum efficiency (IQE) analysis show that the enhancement in cell performance is primarily due to an improved effective diffusion length and diffusion length uniformity, which are both a result of the improved retention of hydrogen at defects achieved during rapid cooling after contact firing. Screen-printed string ribbon cells with independently confirmed efficiencies as high as 14.7% are achieved through an understanding and implementation of hydrogen passivation of defects.