Influence of Tabula Rasa on Process- and Light-Induced Degradation of Solar Cells Fabricated From Czochralski Silicon

• Published in: IEEE journal of photovoltaics 2020-11, Vol.10 (6), p.1557-1565
• Main Authors: Meyer, Abigail R., LaSalvia, Vincenzo, Nemeth, William, Xu, Wanxing, Page, Matthew, Young, David L., Agarwal, Sumit, Stradins, Paul
• Format: Article
• Language: English
• Subjects: Annealing; Bonding strength; Boron; Budgets; Carrier lifetime; Chemical precipitation; Degradation; Emitters; Gettering; High temperature; Impurities; Minority carriers; Open circuit voltage; Oxygen atoms; oxygen precipitation; P-n junctions; Photodegradation; Photoluminescence; photovoltaic; Photovoltaic cells; Precipitates; Pretreatment; Silicon; silicon solar cell; Solar cells; SOLAR ENERGY; Tabula Rasa
• ISSN: 2156-3381; 2156-3403
• DOI: 10.1109/JPHOTOV.2020.3020214

Monocrystalline Si solar cells are fabricated from Czochralski (Cz) Si, which contains 10 17 -10 18 cm −3 oxygen atoms. Cz Si undergoes degradation during high-temperature thermal processing steps, such as dopant diffusion to form the p - n junction. This degradation in the bulk minority carrier lifetime can be related to the formation of oxygen precipitates. We found that a high-temperature annealing process known as tabula rasa (TR) not only mitigates process-induced degradation via oxygen precipitate nuclei dissolution, but also modifies subsequent light-induced degradation. We report on the bulk carrier lifetime of n - and p -type Cz Si after TR, which homogenizes the interstitial oxygen in the bulk Si to its monoatomic form in either an N 2 or O 2 environment. A control sample, which was not subjected to a TR processing step, experienced severe process-induced degradation during a boron emitter thermal budget as oxygen precipitates were formed in the Si bulk. These precipitates could be imaged using photoluminescence. Additionally, samples that underwent a TR processing step prior to the boron emitter thermal budget show efficient gettering of metallic impurities compared to the control sample, which showed a decline in the implied open-circuit voltage after the gettering step. Furthermore, modification of the interstitial oxygen bonding by TR had a strong effect on the light-induced degradation kinetics. Instead of a typically observed monotonic decrease, minority carrier lifetime increases first, followed by a nonmonotonic decrease over a ∼20 h period. We conclude that by modifying the interstitial oxygen bonding via TR pretreatment, p -type Cz Si wafers become substantially resistant to harsh solar cell processes and strongly modified light-induced degradation, which would open alternative ways to mitigate this degradation mechanism.