Evidence for the role of hydrogen in the stabilization of minority carrier lifetime in boron-doped Czochralski silicon

This study demonstrates that the presence of a hydrogen source during fast-firing is critical to the regeneration of B-O defects and that is it not a pure thermally based mechanism or due to plasma exposure. Boron-doped p-type wafers were fired with and without hydrogen-rich silicon nitride (SiNx:H) films present during the fast-firing process. After an initial light-induced degradation step, only wafers fired with the SiNx:H films present were found to undergo permanent and complete recovery of lifetime during subsequent illuminated annealing. In comparison, wafers fired bare, i.e., without SiNx:H films present during firing, were found to demonstrate no permanent recovery in lifetime. Further, prior exposure to hydrogen-rich plasma processing was found to have no impact on permanent lifetime recovery in bare-fired wafers. This lends weight to a hydrogen-based model for B-O defect passivation and casts doubt on the role of non-hydrogen species in the permanent passivation of B-O defects in commercial-grade p-type Czochralski silicon wafers

Investigations on accelerated processes for the boron-oxygen defect in p-type Czochralski silicon

As new solar cell architectures are developed with superior surface passivation, the boron-oxygen defect becomes an increasingly significant limitation on device performance for p-type Czochralski silicon solar cells. This has led to research into methods of permanently deactivating the recombination activity associated with the defect and how these might be implemented in an industrial environment. While the ability to passivate this defect at temperatures below 500 K has been widely reported in the literature, recent results from the authors have demonstrated the ability to achieve near complete passivation of this defect at temperatures in excess of 600 K under high intensity illumination. This ability to passivate the defect at higher temperatures than previously reported may be explained by an increase in the rate of defect passivation, or alternately, by an increase in the defect formation rate. This paper explores the dependence of defect passivation upon illumination intensity, temperature and the initial state of the defects. Evidence is presented to suggest that high intensity illumination does not significantly increase the rate of passivation, but rather greatly enhances the defect formation rate. Based upon this understanding it is demonstrated how a 10 s process under high intensity illumination may be used to completely eliminate the impact of the boron-oxygen defect on solar cell performance, with no requirement for prior defect formation

Proceedings of the 6th World Conference on Photovoltaic Energy Conversion

To evaluate the long-term reliability of Ni/Cu contacts and Cu metallization under real operating conditions, we study the long-term performance of 3 BP Saturn® (BP585F) modules with buried Ni/Cu contacts in a 12-year old fixed-tilt array installed in Sydney. Electrical characterization and thermal imaging indicate a lack of interconnect failures and low overall fill-factor losses. Junction recombination losses are suggested by higher than average diode ideality factors; however, the role of Cu contamination could not be confirmed. Nonetheless, module power degradation rates are observed to be similar to that of screen-printed modules of similar vintage, suggesting that it is possible to make Ni/Cu contacts that are as durable as screen-printed contacts

Boron-Oxygen Defect Formation Rates and Activity at Elevated Temperatures

In this work the dependence of the slow boron-oxygen defect formation rate on excess carrier density is examined in p-type Cz silicon. In order to examine behavior at elevated temperatures simple models are developed to simulate the injection-level dependent lifetime of samples at a range of temperatures and active defect concentrations. These models are then verified against experimental data. Based on these models it is possible to clearly observe a quadratic dependence of defect formation rate upon total hole concentration over a range of temperatures. The implications of a hole (and hence excess carrier (Δn)) dependent defect formation rate, combined with the temperature dependence of defect activity are then discussed. It is demonstrated how a dependence of formation rate upon hole concentration increases the rate of defect formation and mitigation of carrier-induced degradation in samples with reduced saturation current density during anneals at elevated temperatures and illumination intensities

Boron-Oxygen Defect Formation Rates and Activity at Elevated Temperatures

In this work the dependence of the slow boron-oxygen defect formation rate on excess carrier density is examined in p-type Cz silicon. In order to examine behavior at elevated temperatures simple models are developed to simulate the injection-level dependent lifetime of samples at a range of temperatures and active defect concentrations. These models are then verified against experimental data. Based on these models it is possible to clearly observe a quadratic dependence of defect formation rate upon total hole concentration over a range of temperatures. The implications of a hole (and hence excess carrier (Δn)) dependent defect formation rate, combined with the temperature dependence of defect activity are then discussed. It is demonstrated how a dependence of formation rate upon hole concentration increases the rate of defect formation and mitigation of carrier-induced degradation in samples with reduced saturation current density during anneals at elevated temperatures and illumination intensities

Rapid thermal anneal activates light induced degradation due to copper redistribution

| openaire: EC/FP7/307315/EU//SOLARXWhile it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron-doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40-60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when studying other LID phenomena. Numeric simulations describing the phosphorus diffusion and the gettering process reproduce the experimental trends and elucidate the underlying physical mechanisms.Peer reviewe