Sperber, David

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On the impact of substrate contact resistance in bifacial MIS-type lifetime structures

2022, Herguth, Axel, Kostrzewa, Fabian, Sperber, David

Bifacial metal-insulator-semiconductor (MIS) structures are used to intentionally manipulate field-effect passivation of dielectric layers on crystalline silicon substrates during lifetime measurements by external biasing. It is found that the impact of biasing on surface passivation depends on the quality of the substrate contact, in particular in asymmetric front/rear biasing conditions. A mathematical model is presented and used to demonstrate the problems arising from a high substrate contact resistance.

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Bulk and Surface Related Degradation Phenomena in Monocrystalline Silicon at Elevated Temperature and Illumination

2019, Sperber, David

This work investigates the stability of lifetime samples made of monocrystalline silicon at elevated temperature and illumination. It is shown that samples made of Czochralski (Cz-Si) and float-zone silicon (FZ-Si) are affected by significant bulk related degradation (BRD) and surface related degradation (SRD) phenomena. SRD is observed in samples passivated with SiNx:H, SiO2/SiNx:H, SiOx:H/SiNx:H, SiOxNy:H/SiNx:H, and AlOx/SiNx:H. SRD furthermore occurs after variation of wet-chemical and most other processing steps and thus appears to present a general phenomenon in samples made of crystalline silicon. However, SRD is much weaker on highly doped silicon surfaces, and different degrees of changes in chemical passivation and field effect passivation are observed depending on choice of passivation layers. Frequently, SRD is followed by a recovery of surface passivation quality. A large number of samples is furthermore affected by BRD and subsequent regeneration phenomena. While BRD is expected in Cz-Si samples due to boron oxygen related degradation, even samples made of FZ-Si may show severe BRD. An in-depth analysis of degradation properties leads to the conclusion that BRD in FZ-Si is very likely related to light and elevated temperature induced degradation (LeTID) as originally observed in multicrystalline silicon. It is furthermore shown that samples made of Cz-Si may likely suffer from LeTID in addition to boron oxygen related degradation. Finally, the long-term stability of samples is investigated after a regeneration process. It is shown that regeneration of boron oxygen defects is very stable whereas samples affected by strong LeTID may suffer from a bulk related long-term instability.

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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.

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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.

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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.

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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.

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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.

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Does LeTID occur in c-Si even without a firing step?

2019, Sperber, David, Furtwängler, Florian, Herguth, Axel, Hahn, Giso

It is shown that a non-fired B-doped floatzone silicon sample coated with SiNx:H may show severe bulk related degradation and regeneration during illuminated treatment at elevated temperature. It is discussed that the likely cause is light and elevated temperature induced degradation (LeTID) in the silicon bulk. Firing is found to modulate the extent of LeTID so that degradation may either be weaker or stronger compared to the non-fired sample depending on firing parameters. A sample which was annealed instead of fired is found to be stable for up to 1,000 h of treatment time.

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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.

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Degradation of Surface Passivation on Crystalline Silicon and Its Impact on Light-Induced Degradation Experiments

2017-11, Sperber, David, Graf, Alexander, Skorka, Daniel, Herguth, Axel, Hahn, Giso