Atomistic Insight into the Defect Structure and Mechanism of Light- and Elevated-Temperature-Induced Degradation and Regneration in Ga-Doped Cz Si

Light- and elevated-temperature-induced degradation (LeTID) is a decrease in the minority carrier lifetime in <i>p</i>-type Si which occurs under simultaneous heat-exposure and carrier injection. It is well accepted that the defect structure and degradation mechanism are tied to hydrogen that is injected into the Si bulk during the firing of the Si paste to make contact to the solar cell. There are three characteristic stages of LeTID: an annealed state where defects are metastable and non-recombination-active, a degraded state where defects are recombination active, and a stabilized state where defects are permanently non-recombination-active. Even though there have been many publications on LeTID over the past 10 years, the fundamental defect structure and degradation mechanism of LeTID is largely speculative. Empirical studies relate H to the defect structure and degradation mechanism, but atomistic evidence is lacking. If the exact structure of LeTID would be discovered, mitigation strategies that are industrially feasible could be quickly studied, discovered, and implemented, allowing for increased reliability of conventional solar cells.<br/>In this work we utilize electron paramagnetic resonance (EPR) spectroscopy and minority carrier lifetime spectroscopy to study the structure of the LeTID defect and the degradation mechanism. We report on the EPR spectra of Ga-doped Cz Si in different stages of LeTID and correlate these results to lifetime spectroscopy measurements. Firstly, we show that the light- and elevated-temperature fully degraded shows a strong Si-dangling bond EPR signature at <i>g </i>= 2.0055, compared to samples in the annealed state, where the Si-dangling bond EPR signature has a decreased intensity. When we regenerate the samples, the Si-dangling bond EPR signature decreases again, this time to a minimum intensity. Thus, the Si-dangling bond EPR signature correlates to the stage of LeTID the sample is in. Additionally, in the fully degraded state of LeTID, we observe hyperfine splitting EPR signatures that we attribute to H that is injected into the Si bulk during fast firing and which has been long suspected to play an integral role in the degradation and defect structure of LeTID. We confirm that the hyperfine splitting EPR signatures originate from H with isotope experiments. We observe a direct correlation between the minority carrier lifetime and the peak-to-peak intensity of the Si-dangling bond EPR signature further supporting the involvement of a Si-dangling bond in the LeTID defect.<br/>Finally, in control samples that did not undergo a hydrogenation step, we do <i>not </i>observe an increase in the Si-dangling bond EPR signature as LeTID progresses, hyperfine splitting EPR signatures due to H, or a decrease in the minority carrier lifetime upon simultaneous heat and light exposure. Thus, for the first time, EPR has been used to show manipulation of the LeTID defect during simultaneous heat and light exposure and confirm at the atomistic level that H is involved in the degradation mechanism and defect structure of LeTID.