Moonyong Kim

Dynamic inverse obstacle problems with electrical impedance tomography

Electrical impedance tomography (EIT) is a relatively new imaging modality in which the internal resistivity distribution is reconstructed based on the known sets of injected currents and measured voltages on the surface of the object.In this paper, a dynamic inverse obstacle problem is considered based on the electrical impedance tomography. The considered situation here is for the case where the shape and location of the obstacle are known a priori whereas the resistivity of the obstacle changes rapidly in time. The inverse problem is treated as nonlinear state estimation problem and the unknown time-varying state (resistivity) is estimated on-line with the aid of the extended Kalman filter. The reconstruction performance is enhanced considerably by taking into account the first- or second-order time-derivative of the resistivity change in the obstacle.

Modulating the extent of fast and slow boron-oxygen related degradation in Czochralski silicon by thermal annealing: Evidence of a single defect

The fast and slow boron-oxygen related degradation in p-type Czochralski silicon is often attributed to two separate defects due to the different time constants and the determination of different capture cross section ratios (k). However, a recent study using high lifetime samples demonstrated identical recombination properties for the fast and slow degradation and proposed an alternative hypothesis that these were in fact due to a single defect. The study presented in this article provides further experimental evidence to support the single defect hypothesis. Thermal annealing after light soaking is used to investigate the behaviour of subsequent boron-oxygen related degradation. Modifying the temperature and duration of dark annealing on pre-degraded samples is observed to alter the fraction of fast and slow degradation during subsequent illumination. Dark annealing at 173 °C for 60 s is shown to result in almost all degradation occurring during the fast time-scale, whereas annealing at 155 °C for 7 h c...

Rapid mitigation of carrier-induced degradation in commercial silicon solar cells

We report on the progress for the understanding of carrier-induced degradation (CID) in p-type mono and multi-crystalline silicon (mc-Si) solar cells, and methods of mitigation. Defect formation is a key aspect to mitigating CID. Illuminated annealing can be used for both mono and mc-Si solar cells to reduce CID. The latest results of an 8-s UNSW advanced hydrogenation process applied to industrial p-type Czochralski PERC solar cells are shown with average efficiency enhancements of 1.1% absolute from eight different solar cell manufacturers. Results from three new industrial CID mitigation tools are presented, reducing CID to 0.8–1.1% relative, compared to 4.2% relative on control cells. Similar advanced hydrogenation processes can also be applied to multi-crystalline silicon passivated emitter with rear local contact (PERC) cells, however to date, the processes take longer and are less effective. Modifications to the firing processes can also suppress CID in multi-crystalline cells during subsequent illumination. The most stable results are achieved with a multi-stage process consisting of a second firing process at a reduced firing temperature, followed by extended illuminated annealing.

Impact of interstitial iron on the study of meta-stable B-O defects in Czochralski silicon: Further evidence of a single defect

We explore the influence of interstitial iron (Fei) on lifetime spectroscopy of boron-oxygen (B-O) related degradation in p-type Czochralski silicon. Theoretical and experimental evidence presented in this study indicate that iron-boron pair (Fe-B) related reactions could have influenced several key experimental results used to derive theories on the fundamental properties of the B-O defect. Firstly, the presence of Fei can account for higher apparent capture cross-section ratios (k) of approximately 100 observed in previous studies during early stages of B-O related degradation. Secondly, the association of Fe-B pairs can explain the initial stage of a two-stage recovery of carrier lifetime with dark annealing after partial degradation. Thirdly, Fei can result in high apparent k values after the permanent deactivation of B-O defects. Subsequently, we show that a single k value can describe the recombination properties associated with B-O defects throughout degradation, that the recovery during dark annea...

Role of hydrogen: Formation and passivation of meta-stable defects due to hydrogen in silicon

Hydrogen is well known for its ability to effectively passivate defects in silicon. However, it has been recently proposed that it can also be detrimental, with susepected involvement in light and elevated temperature induced degradation (LeTID). The role of rapid firing processes in the passivation of B-O defects and activation of LeTID indicate a similar involvement of hydrogen in the passivation of B-O defects and LeTID related reactions. While B-O passivation rate is suppressed by dark annealing in Czochalski mono-crystalline silicon, both mono-crystalline silicon and multi-crystalline wafer types showed a similar formation rate of LeTID. It has been recently demonstrated that interstitial hydrogen can migrate to the surface by annealing. Thermal annealing suppressed B-O passivation rate significantly while the LeTID formation rate was not changed significantly. The implication is that as the B-O defect is present uniformly in the bulk of silicon, while hydrogen must either be readily present in bulk, or diffuse to the entire bulk region to passivate B-O defect, the LeTID may not necessarily occur throughout the bulk like B-O passivation. During the first 300 hours of light soaking on mc-Si samples, Cz-Si samples showed improvement in lifetime, which can be explained by the passivation of the B-O defect. However, the full recovered lifetime was similar to the lifetime after firing. The lifetime curve showed no shockley-read-hall component when the lifetime was fully recovered, which confirms the LeTID, which is independent defect to B-O defect, was no longer present. This implies that while B-O defect was passivated, the LeTID defect may have also been recovering at the same rate. It may be that hydrogen initially contributing to the recombination as LeTID, is contributing to B-O passivation, which leads to a recovery in lifetime with a reduction in LeTID and B-O defect.Hydrogen is well known for its ability to effectively passivate defects in silicon. However, it has been recently proposed that it can also be detrimental, with susepected involvement in light and elevated temperature induced degradation (LeTID). The role of rapid firing processes in the passivation of B-O defects and activation of LeTID indicate a similar involvement of hydrogen in the passivation of B-O defects and LeTID related reactions. While B-O passivation rate is suppressed by dark annealing in Czochalski mono-crystalline silicon, both mono-crystalline silicon and multi-crystalline wafer types showed a similar formation rate of LeTID. It has been recently demonstrated that interstitial hydrogen can migrate to the surface by annealing. Thermal annealing suppressed B-O passivation rate significantly while the LeTID formation rate was not changed significantly. The implication is that as the B-O defect is present uniformly in the bulk of silicon, while hydrogen must either be readily present in bulk,...

Direct transitions between states A and C in the boron-oxygen defect system — Fact or fiction?

Potential transitions in the boron-oxygen defect system are investigated using kinetic modeling and experimental data. Strong evidence is shown that no direct pathway occurs for passivation from state A and to C, and therefore, defects require formation prior to passivation. Whilst it is more difficult to rule out a possible destabilisation reaction as occurring from state C to state A, the system kinetics can adequately be described by a transition from C to B, with a subsequent dissociation of the defect. These conclusions are in agreement with the experimentally determined effectiveness of passivation as a function of temperature.