• Energy Research

Abstract 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.

Abstract 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.

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

Abstract This paper reports a new approach for exposing materials, including solar cell structures, to atomic hydrogen. This method is dubbed Shielded Hydrogen Passivation (SHP) and has a number of unique features offering high levels of atomic hydrogen at low temperature whilst inducing no damage. SHP uses a thin metallic layer, in this work palladium, between a hydrogen generating plasma and the sample, which shields the silicon sample from damaging UV and energetic ions while releasing low energy, neutral, atomic hydrogen onto the sample. In this paper, the importance of the preparation of the metallic shield, either to remove a native oxide or to contaminate intentionally the surface, are shown to be potential methods for increasing the amount of atomic hydrogen released. Excellent, damage free, surface passivation of thin oxides is observed by combining SHP and corona discharge, obtaining minority carrier lifetimes of 2.2 ms and J0 values below 5.47 fA/cm2. This opens up a number of exciting opportunities for the passivation of advanced cell architectures such as passivated contacts and heterojunctions.

Abstract This paper reports a new approach for exposing materials, including solar cell structures, to atomic hydrogen. This method is dubbed Shielded Hydrogen Passivation (SHP) and has a number of unique features offering high levels of atomic hydrogen at low temperature whilst inducing no damage. SHP uses a thin metallic layer, in this work palladium, between a hydrogen generating plasma and the sample, which shields the silicon sample from damaging UV and energetic ions while releasing low energy, neutral, atomic hydrogen onto the sample. In this paper, the importance of the preparation of the metallic shield, either to remove a native oxide or to contaminate intentionally the surface, are shown to be potential methods for increasing the amount of atomic hydrogen released. Excellent, damage free, surface passivation of thin oxides is observed by combining SHP and corona discharge, obtaining minority carrier lifetimes of 2.2 ms and J0 values below 5.47 fA/cm2. This opens up a number of exciting opportunities for the passivation of advanced cell architectures such as passivated contacts and heterojunctions.

Abstract In this work, we demonstrate a form of minority carrier degradation on n-type Cz silicon that affects both the bulk and surface related lifetimes. We identify three key behaviors of the degradation mechanism; 1) a firing dependence for the extent of degradation, 2) the appearance of bulk degradation when wafers are fired in the presence of a diffused emitter and 3) a firing related apparent surface degradation when wafers are fired in the absence of an emitter. We further report a defect capture cross-section ratio of σn/σp = 0.028 ± 0.003 for the defect in n-type. Utilizing our understanding of light and elevated temperature induced degradation (LeTID) in p-type silicon, we demonstrate that the degradation behaviors in both n-type and p-type silicon are closely correlated. In light of numerous reports on the involvement of hydrogen, the potential role of a hydrogen-induced degradation mechanism is discussed in both p- and n-type silicon, particularly in relation to the diffusion of hydrogen and influence of hydrogen-dopant interactions.