Influencing Light and Elevated Temperature Induced Degradation and Surface-Related Degradation Kinetics in Float-Zone Silicon by Varying the Initial Sample State
2020-01, Hammann, Benjamin, Engelhardt, Josh, Sperber, David, Herguth, Axel, Hahn, Giso
Light and elevated temperature induced degradation (LeTID) kinetics in float-zone silicon are investigated by varying the initial sample state, composed of different base material, base doping, SiN x :H films, and subsequent firing, and/or annealing steps. The approach of deliberately changing the initial sample state is shown to allow for specific studies of influences of LeTID kinetics. Bulk- and surface-related degradations are examined separately and the influence on the kinetics of bulk- and surface-related degradation is illustrated by a four-state and three-state model, respectively. In case of bulk-related degradation, an increase in defect density because of the firing step is shown, whereas the annealing step has an inverse effect. Both temperature steps—individually and combined—influence the transition rates of bulk-related degradation and regeneration by presumably changing the distribution of a defect precursor. For surface-related degradation, the firing step reduces the transition rate from the initial to the degraded state. In addition, the influence of a comparably humid atmosphere and the absence of UV light are found to be negligible.
A Detailed Study on Light-Induced Degradation of Cz-Si PERC-Type Solar Cells : Evidence of Rear Surface-Related Degradation
2018-09, Herguth, Axel, Derricks, Christian, Sperber, David
A light-induced degradation phenomenon of unknown origin in p-type Cz-Si passivated emitter and rear cell (PERC)-type solar cells is thoroughly investigated by collating results from different measurement techniques and predictions from various simulations. The observed degradation manifests in slight losses in jsc, strong losses in Voc, and devastating losses in FF, and thus massively impacts efficiency. It is found that the series resistance degrades significantly due to a degradation of front contact resistance. This, however, does not explain losses in Voc and jsc, which are attributed to the degradation of a different cell component. Neither a degradation by defect formation in the space charge region, nor the emitter, nor the bulk is found to consistently explain the observations. Only a rear surface-related degradation mechanism explains consistently all experimental findings.
Temperature and Light-Induced Changes in Bulk and Passivation Quality of Boron-Doped Float-Zone Silicon Coated With SiNx:H
2017-03, Sperber, David, Heilemann, Adrian, Herguth, Axel, Hahn, Giso
In this study, it is observed that boron-doped float-zone silicon coated with hydrogenated silicon nitride shows strong instabilities in effective minority carrier lifetime after a fast firing step and subsequent treatment at elevated temperatures and illumination. During such a treatment, both degradation and recovery features are visible over time scales from minutes to months. To further investigate the observed behavior, corona charging series, capacitance voltage measurements, and chemical repassivation methods are applied. It is shown that a first fast degradation and recovery is associated with changes in the bulk lifetime, and it is observed that the fast firing step strongly influences this bulk instability. A subsequent slower degradation and recovery reflects changes in the effective surface recombination velocity that can be attributed to changes in the chemical passivation quality. It can be concluded that care has to be taken when boron-doped float-zone silicon is used as a supposedly stable high lifetime reference material after a fast firing step. Additionally, it can be stated that a silicon nitride related passivation may be far from stable at elevated temperatures and illumination after a fast firing step.
Bulk and Surface-Related Degradation in Lifetime Samples Made of Czochralski Silicon Passivated by Plasma-Enhanced Chemical Vapor Deposited Layer Stacks
2018-12, Sperber, David, Schwarz, Anton, Herguth, Axel, Hahn, Giso
Significant bulk-related degradation (BRD) is followed by surface-related degradation (SRD) of effective excess charge carrier lifetime in lifetime samples made of Czochralski silicon during illuminated treatment at 80–150°C. Samples are passivated with either AlOx:H/SiOxNy:H/SiNx:H or SiOxNy:H/SiNx:H stacks stemming entirely from plasma-enhanced chemical vapor deposition. Samples show strong variations in BRD depending on passivation stacks and treatment conditions, and a potential link to light and elevated temperature‐induced degradation (LeTID) is discussed. All samples are fired in a belt furnace, and variations of firing temperature and belt speed are shown to influence SRD slightly. SRD is furthermore accelerated with increasing treatment temperature and an apparent activation energy Eapp=1.07±0.02eV is determined in SiOxNy:H/SiNx:H passivated samples. Interpretation of Eapp is, however, difficult as both changes in interfacial defect and fixed charge density occur in parallel during SRD.
Enhanced stability of passivation quality on diffused silicon surfaces under light-induced degradation conditions
2018, Sperber, David, Schwarz, Anton, Herguth, Axel, Hahn, Giso
Significant surface related degradation (SRD) is observed in samples passivated with either SiNx:H or AlOx:H/SiNx:H during treatment at 150 °C and 1 sun equivalent illumination intensity. Degradation of SiNx:H passivation is caused by a decrease of chemical passivation quality whereas degradation of AlOx:H/SiNx:H is caused by a decrease of fixed charge density. SRD is, however, strongly suppressed on highly doped silicon surfaces resulting from a diffusion step. Device simulations indicate that this cannot only be explained by reduced sensitivity to changes at the silicon surface due to the diffused region, and implications for defect formation are discussed.
A 3-state defect model for light-induced degradation in boron-doped float-zone silicon
2017-03, Sperber, David, Herguth, Axel, Hahn, Giso
We report on a light-induced bulk defect activation and subsequent deactivation in boron doped float-zone silicon that can be described by a 3-state model. During treatment at elevated temperature and illumination, a sample first converts from an initial high lifetime state into a degraded low lifetime state and then shows a recovery reaction leading to a third high lifetime state that is then stable under degradation conditions. Furthermore, it is shown that reverse reactions into the initial state appear to be possible both from the degraded as well as the regenerated state. An injection dependent analysis of lifetime data yields a defect capture cross section ratio of ∼20 suggesting a positively charged defect.
On improved passivation stability on highly-doped crystalline silicon and the long-term stability of regenerated Cz-Si
2018-10, Sperber, David, Herguth, Axel, Hahn, Giso
Different surface passivation approaches are applied on Cz-Si and FZ-Si samples and long-term stability is investigated during treatments at 60–80 °C and up to 1 sun equivalent illumination intensity. It is shown that SiNx:H and AlOx:H/SiNx:H surface passivation show a much more stable passivation quality when deposited on P-diffused and B-diffused surfaces, respectively. Long-term measurements lead to the conclusion that Cz-Si samples fired at measured peak temperatures up to 750 °C are very stable after regeneration of bulk defects. Samples fired at 850 °C show much stronger bulk-related degradation potentially linked to light and elevated temperature induced degradation (LeTID). Furthermore, Cz-Si samples fired at 850 °C express an instable behavior after a regeneration treatment.
Instability of Dielectric Surface Passivation Quality at Elevated Temperature and Illumination
2016, Sperber, David, Herguth, Axel, Hahn, Giso
Hydrogenated silicon nitride and aluminum oxide passivation layers were deposited on boron doped floatzone silicon wafers that underwent a high temperature firing step. The passivation quality was monitored during thermal treatment at 75°C, 150°C and 250°C in darkness or under illumination. It was found that the passivation quality of the specific layers under investigation is far from stable in the course of time showing both deterioration and improvement features on a time scale of minutes to weeks. Furthermore, it was found that these changes occur in both darkness and under illumination, whereupon (stronger) illumination accelerates the changes. Via corona charging and capacitance voltage experiments it could be shown that the observed changes in the short term are mainly caused by changes in the chemical passivation quality.