Nitin Nampalli

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-gra...

Implications of Accelerated Recombination-Active Defect Complex Formation for Mitigating Carrier-Induced Degradation in Silicon

A three-state model is used to explore the influence of the accelerated formation of recombination-active defect complexes on the mitigation of carrier-induced degradation in p-type silicon containing boron and oxygen. Defect formation is observed to be a critical factor for the speed at which carrier-induced degradation can be mitigated. Defect formation also plays a critical role in determining the effectiveness of mitigation at elevated temperatures. It is observed that under conventional conditions, at a processing temperature of 200 °C, approximately 170 s are required to form and passivate 99% of possible defects. The experimentally demonstrated improved effectiveness of carrier-induced defect passivation with a process time of 10 s at temperatures over 300 °C is consistent with a substantial acceleration of defect formation.

Influence of Hydrogen on the Mechanism of Permanent Passivation of Boron–Oxygen Defects in p-Type Czochralski Silicon

Strong evidence is provided for the critical role of hydrogen in the permanent passivation of boron-oxygen (B-O) defects in p-type Czochralski silicon. In particular, the impact of rapid thermal processing (firing), plasma exposure, and hydrogen-containing dielectrics on B-O defect passivation is explored. Importantly, no permanent passivation of B-O defects is observed in samples fired bare (both with and without exposure to a hydrogen-rich plasma prior to firing) and in nonfired samples coated with hydrogenated silicon nitride (SiNx:H). In contrast, samples with SiNx:H layers present during firing resulted in significant levels of B-O passivation, even at firing temperatures as low as ~500 °C. Increasing peak firing temperatures (Tpeak) appeared to correlate to increased B-O passivation ability; however, increasing Tpeak above a value of 670 °C resulted in suboptimal levels of surface and bulk passivation. These observations are explained within a hydrogen-based model for permanent passivation of B-O defects. Implications for nonhydrogen-based models are also discussed.

Influence of the formation- and passivation rate of boron-oxygen defects for mitigating carrier-induced degradation in silicon within a hydrogen-based model

A three-state model is used to explore the influence of defect formation- and passivation rates of carrier-induced degradation related to boron-oxygen complexes in boron-doped p-type silicon solar cells within a hydrogen-based model. The model highlights that the inability to effectively mitigate carrier-induced degradation at elevated temperatures in previous studies is due to the limited availability of defects for hydrogen passivation, rather than being limited by the defect passivation rate. An acceleration of the defect formation rate is also observed to increase both the effectiveness and speed of carrier-induced degradation mitigation, whereas increases in the passivation rate do not lead to a substantial acceleration of the hydrogen passivation process. For high-throughput mitigation of such carrier-induced degradation on finished solar cell devices, two key factors were found to be required, high-injection conditions (such as by using high intensity illumination) to enable an acceleration of defect formation whilst simultaneously enabling a rapid passivation of the formed defects, and a high temperature to accelerate both defect formation and defect passivation whilst still ensuring an effective mitigation of carrier-induced degradation.

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...

High efficiency at module level with almost no cell metallisation: Multiple wire interconnection of reduced metal solar cells

Perpendicular multiple busbar wires have proven an effective way of interconnecting standard screen printed solar cells with high reliability and low cell to module loss. The technology is also an effective way to interconnect plated solar cells, where conventional soldered interconnects may be problematic. However, the full benefits of this interconnection technology can be fully realized on plated cell structures with drastically reduced plated metallization. This paper presents a new selective emitter plated cell structure with metallization reduced to around 1 μm thickness, interconnected using perpendicular multiple busbar wires. Metal usage on the cell is reduced by more than 90% compared to conventional plated or screen print cells and the use of Ag almost eliminated, while high efficiency at the module level is achieved along with environmental durability.

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 in the permanent passivation of boron-oxygen defects in czochralski silicon

Strong evidence is provided for the critical role of hydrogen in the permanent passivation of boron-oxygen (B-O) defects in silicon. In particular, the impact of rapid thermal processing (firing), plasma exposure and hydrogen-containing dielectrics (PECVD SiNx:H) are explored. Importantly, no B-O passivation is observed in bare-fired wafers, regardless of exposure to hydrogen-rich plasma, whereas wafers with SiNx:H layers present during firing result in significant levels of B-O passivation even at firing temperatures as low as 500 °C, thereby strongly suggesting that hydrogen is directly involved in the passivation mechanism. Implications of such a mechanism are discussed.

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.

Stability of hydrogen passivated UMG silicon with implied open circuit voltages over 700mV

Interstitial hydrogen is used to passivate defects in upgraded metallurgical grade (UMG) Czockralski silicon. It is observed that the quality of the UMG material can be improved progressively, with the passivated defects appearing to be stable to subsequent light soaking. New defects however continue to form for a prolonged period of light soaking, requiring subsequent further hydrogen passivation to restore the open circuit voltages to over 700 mV for the UMG wafers. By repeatedly applying the advanced hydrogenation process, the quality and stability of UMG wafers are improved to the point of being comparable to that for Czockralski wafers produced from semiconductor grade silicon purified by the Siemens Process.