Beneficial Impact of Low Frequency PECVD SiN(x):H-Induced Hydrogenation in High-Efficiency String Ribbon Silicon Solar Cells
Hanoka, J. I.
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PECVD SiN(x):H-induced hydrogenation of bulk defects in String Ribbon Si during RTP anneal is investigated in this study to enhance the carrier lifetime and understand the role of the plasma excitation frequency and an in-situ NH3 plasma pretreatment before SiN(x):H deposition. The results show that a low frequency SiN(x):H film with a NH3 plasma pretreatment annealed in RTP at 740°C for 60 seconds enhances the lifetime in String Ribbon Si from 5-6 μs to 90-100 μs. Secondary ion mass spectroscopy underneath SiN(x):H films deposited with deuterated ammonia (ND3) and silane shows greater deuterium incorporation in Si under the low frequency SiN(x):H film. Thus, hydrogen incorporated in Si during SiN(x):H deposition may act as an additional source that enhances hydrogen defect passivation during subsequent RTP treatments. In addition, the effect of the anneal time during RTA for hydrogenation is studied in an effort to reduce the hydrogenation time and improve the retention of hydrogen at defects in Si. The RTA time for hydrogenation is reduced to one second without loss of lifetime enhancement and leads to the fabrication of high-efficiency String Ribbon solar cells (17.9%) with photolithography-defined contacts. A rapid belt furnace contact co-firing scheme is developed based on the short RTA and produces screen-printed 4-cm2 String Ribbon solar cells with efficiencies as high as 15.9%.