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

Abstract

Al-enhanced SiN-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 s. Partial coverage of back surface by Al eliminates wafer bowing in 100 m 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. Index Terms-Hydrogen passivation, multicrystalline silicon (mc-Si), rapid thermal processing (RTP), ribbon silicon, silicon nitride. I. INTRODUCTION T HE U.S. Photovoltaics Industry Roadmap calls for the development of 18% manufacturable cells on low-cost crystalline silicon materials within the next eight to ten years [1]. To meet this target, the as-grown minority carrier lifetime in low-cost Si materials must be increased to over 20 s, and integrated with a high-quality surface passivation scheme, light trapping, a selective emitter, and high quality metallization, all achieved with manufacturable processing technologies.

High-efficiency solar cells on edge-defined film-fed grown (18.2%) and string ribbon (17.8%) silicon by rapid thermal processing

“…5 In the past, high-efficiency screen-printed silicon ribbon solar cells have been achieved by optimizing rapid thermal processing ͑RTP͒ to improve the plasma-enhanced chemical vapor deposited SiN x -induced hydrogenation of bulk defects and quality of the Al-doped back surface field ͑Al-BSF͒. 6,7 Recently, record high efficiency EFG ͑16.7%͒ and string ribbon ͑17.7%͒ cells were reported by Hahn and Geiger 8 using photolithography for front contacts, thermal oxidation for front surface passivation, Al gettering for 30 min, ZnS/MgF 2 for double layer antireflection coating, and 60 min. microwave-induced remote hydrogen plasma for defect passivation.…”

Generalized procedure to determine the dependence of steady-state photoconductance lifetime on the occupation of multiple defects

“…The most easily observable experiment in which a varying f n can be detected is through photoconductance measurements of multicrystalline silicon. 4,[16][17][18][19] In some cases, eff app ͑⌬n app ͒ deviates so greatly from eff ͑⌬n͒ that it prevents any room-temperature assessment of recombination. Dubbed photoconductance "trapping," this artifact, which manifests as a sharp rise in eff app ͑⌬n app ͒ with decreasing ⌬n app , cannot be explained with a single defect but requires the existence of two defects: a "recombination" defect and a trapping defect.…”

New Crystalline Si Ribbon Materials for Photovoltaics

“…Experiments using rapid thermal processing (RTP) in combination with PECVD SiN x layers revealed that higher lifetimes can be achieved by using fast cool-down ramps [78,79].…”