Axel Herguth

Influence of hydrogen on the regeneration of boron-oxygen related defects in crystalline silicon

When exposed to light, boron doped monocrystalline Czochralski grown silicon suffers from degradation of the minority carrier lifetime due to the formation of recombination active boron-oxygen related defects. The so called regeneration procedure is able to convert these recombination active defects into a new less recombination active state characterized by a higher minority charge carrier lifetime and stability under illumination. However, the exact working principle on microscopic scale is still unknown even though some influencing factors were identified. The role of hydrogen in the regeneration process is investigated in this work. We find that the characteristic regeneration time constant is subject to variation depending on the process parameters of a Plasma Enhanced Chemical Vapor Deposition a-SiNx:H deposition, namely the applied gas flows, as well as on the thermal history of the sample prior to applying the regeneration procedure. The positive effect of a short high temperature (800–900 °C) ste...

Kinetics of the boron-oxygen related defect in theory and experiment

The formation of boron-oxygen complexes in boron-doped crystalline silicon can lead to a severe reduction in the minority charge carrier lifetime. This strongly influences, e.g., solar cell efficiencies if the material is used for photovoltaic application. Recent investigations have shown that a recovery of the carrier lifetime can be achieved by a subsequent thermally enhanced reaction induced by charge carriers. A model of the reaction dynamics of the boron-oxygen complex by means of rate equations is presented in this paper. Following a mathematical description of the reactions involved, the consequences based on the calculations are presented and allow a prediction of the observable electrical parameters. The fundamental agreement with measured data is proven experimentally for different phenomena.

A New Approach to Prevent the Negative Impact of the Metastable Defect in Boron Doped CZ Silicon Solar Cells

A new reaction model concerning the boron-oxygen related degradation is presented, introducing a third recombination inactive state, that stabilizes the electrical parameters of Cz-Si solar cells, and the transition to this new inactive state is proven by experimental data. Furthermore, the stability under solar cell working conditions and the formation kinetics of this additional state are discussed

Minority charge carrier lifetime mapping of crystalline silicon wafers by time-resolved photoluminescence imaging

A camera-based method to record spatially and time-resolved photoluminescence images of crystalline silicon wafers was developed. The camera signal is modulated by a rotating shutter wheel, allowing for a wide range of camera types to be used for the measurement and easy integration into existing photoluminescence setups. The temporal resolution is sufficient to record the decay curve of photoexcited charge carriers in surface-passivated silicon wafers. A transient measurement of minority carrier lifetimes down to less than 10 μs can be obtained for each pixel individually, without the need for any external calibration.

Influence of hydrogen effusion from hydrogenated silicon nitride layers on the regeneration of boron-oxygen related defects in crystalline silicon

The degradation effect boron doped and oxygen-rich crystalline silicon materials suffer from under illumination can be neutralized in hydrogenated silicon by the application of a regeneration process consisting of a combination of slightly elevated temperature and carrier injection. In this paper, the influence of variations in short high temperature steps on the kinetics of the regeneration process is investigated. It is found that hotter and longer firing steps allowing an effective hydrogenation from a hydrogen-rich silicon nitride passivation layer result in an acceleration of the regeneration process. Additionally, a fast cool down from high temperature to around 550 °C seems to be crucial for a fast regeneration process. It is suggested that high cooling rates suppress hydrogen effusion from the silicon bulk in a temperature range where the hydrogenated passivation layer cannot release hydrogen in considerable amounts. Thus, the hydrogen content of the silicon bulk after the complete high temperature step can be increased resulting in a faster regeneration process. Hence, the data presented here back up the theory that the regeneration process might be a hydrogen passivation of boron-oxygen related defects.

Temperature and Light-Induced Changes in Bulk and Passivation Quality of Boron-Doped Float-Zone Silicon Coated With SiNx:H

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.

High Speed Regeneration of BO-Defects: Improving Long-Term Solar Cell Performance within Seconds

Boron-oxygen related defects typically limit the efficiency of solar cells made from silicon containing high concentrations of boron as well as oxygen. The detrimental effect of these defects can be eliminated by applying a Regeneration procedure that needs carrier injection at slightly elevated temperatures. The kinetics of this process is influenced by different processing steps like thermal treatment and was found to rely on a high enough hydrogen concentration in the silicon bulk. It is shown here that neither emitter formation nor the use of Al2O3/SiNx:H or SiO2/SiNx:H passivation stacks affect Regeneration in a fundamental way. By contrast, the thickness of a SiNx:H layer acting as hydrogen source during a high temperature firing step has direct influence on Regeneration confirming that better hydrogenation results in faster Regeneration reactions. Condensing different process steps that all accelerate Regeneration allows the application of a high-speed Regeneration process consisting of a combination of relatively high temperature and high carrier injection, resulting in complete Regeneration of BO defects in less than 10 s. This makes Regeneration feasible as an in-line process in solar cell production and thus solves the problem of the boron-oxygen defects. Even further acceleration is achieved by laser induced Regeneration where the substrate is heated and illuminated simultaneously.

The etchback selective emitter technology and its application to multicrystalline silicon

We have developed a simple and industrially applicable selective emitter cell process using only one diffusion step and an emitter etchback to create the high sheet resistance emitter [1, 2]. The process generates a deeper doping profile with a lower surface phosphorous concentration than a directly diffused emitter with the same sheet resistance. This results in an extremely low emitter saturation current j0E even at a moderate sheet resistance of 60–80 Ω/□. The highest independently confirmed cell efficiency on Cz-Si (146 cm2 was 18.7%. In this work the etching behavior of the acidic solution at the grain boundaries is studied by SEM imaging and high resolution LBIC measurements at 405 nm wavelength. The etchback also leads to a change in reflectivity, which is quantified by reflectance measurements. We furthermore investigate the influence of the base material quality on the gain that can be achieved by this process. Large area solar cells have been processed from solar grade and UMG mc silicon.

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

A decrease of surface passivation quality is observed in FZ, Cz, and mc-Si lifetime samples during light-induced degradation (LID) treatments. It is shown that this degradation occurs not only in samples with single SiN<italic>_x</italic>:H layers but also when using layer stacks consisting of SiO<italic>_x</italic>/SiN <italic>_x</italic>:H or AlO<italic>_x</italic>:H/SiN<italic>_x</italic>:H. Time-resolved calculation of the surface saturation current density <italic>J</italic>₀<italic>_s </italic> is shown to be a reliable method to separate changes in the bulk and at the surface of samples during LID treatments. The impact of the observed changes in passivation quality on the outcome and interpretation of LID experiments aiming at changes in the bulk of Cz or mc-Si is investigated and discussed.

Influence of Al Particle Size and Firing Profile on Void Formation in Rear Local Contacts of Silicon Solar Cells

In this paper, the influence of Al particle size and the applied firing profile on void formation in local rear contacts of wafer-based silicon solar cells is investigated. Samples with a passivated emitter and rear cell (PERC) rear, but without front metallization, were metalized with six different Al screen-printing pastes, i.e., both commercial and homemade, featuring different particle size distributions and fired in a rapid thermal processing furnace with different firing profiles. Voids were detected with scanning acoustic microscopy measurements, and the fraction of voids in rear local contacts was analyzed. It was shown that the heating phase of the firing process has the strongest influence on void formation. With slower heating, void formation could be reduced to a fraction lower than 5% of the local contact area. Furthermore, it was shown that Al pastes consisting of a mixture of small and large Al particle sizes have a positive effect on the formation of voids.