Gettering and Passivation of Impurities and Defects in High Efficiency Silicon Solar Cells

Impurities and defects continue to play an important role in limiting the efficiency of both laboratory and industrial silicon solar cells. In this work, we review recent progress in reducing the impact of some of the most important of these limiting defects through impurity gettering and passivation steps, which occur coincidentally during device fabrication. Since the dominant defects in a material depend very strongly on the crystal growth conditions, we categorise them by the three common silicon crystal growth techniques – Czochralski-grown mono-crystalline silicon, Cast-grown silicon (including both multicrystalline and cast-mono silicon), and Float-Zone silicon.<br/><br/>Czochralski-grown monocrystalline silicon (Cz-Si)<br/>The primary defects in Cz-si, whether p- or n-type, are oxygen-related. In boron-doped p-type silicon, the very well-known Light-Induced Degradation (LID) caused by boron-oxygen defects dominates. Although in principle it is not affected by gettering, hydrogen is widely thought to play a key role in the permanent regeneration of this defect, which is a crucial step in stabilising the high efficiency p-type PERC cells that now dominate the PV industry.<br/>In n-type Cz-Si, which is the preferred substrate for IBC, SHJ and poly-silicon passivating contact solar cells, an important class of defects that sometimes occur during high-temperature processing are the oxygen-related ring defects. These consist of oxygen precipitates, and sometimes their associated dislocations and decorating metallic impurities. Under certain conditions, they can be partially mitigated by impurity gettering and hydrogenation steps. These ring defects can also occur in p-type Cz-Si wafers.<br/>Industrial Cz-Si wafers often also contain traces of dissolved metals such as Fe, which can significantly affect the carrier lifetime, if not removed during processing by gettering. Such gettering can be achieved by phosphorus or boron diffusion, aluminium alloying, doped-poly-silicon layers, and even deposited dielectric films such as silicon nitride and aluminium oxide.<br/><br/>Cast-grown silicon (HP mc-Si and cast-mono Si)<br/>Cast high-performance multicrystalline silicon (HP mc-Si), by design, contains many grain boundaries. Fortunately the recombination activity of these grain boundaries can be significantly reduced after hydrogenation, such as via fired silicon nitride films during cell metallisation. Cast-monocrystalline silicon (cast-mono Si), on the other hand, tends to be dominated by dislocation clusters. These are much more difficult to negate through hydrogenation, which reduces the achievable efficiency and yield for devices made on such wafers. Both HP mc-Si and cast-mono wafers also contain significant quantities of metallic impurities, some of which are precipitated at structural defects, whilst others remain dissolved. Under the right conditions, they can be gettered to a significant extent during phosphorus or boron diffusions, or by deposited films. This provides a convenient method to identify the metallic species present in these materials.<br/><br/>Float-Zone silicon (FZ-Si)<br/>Although not used in industrial solar cells, Float-Zone silicon (FZ-Si) is often used in research laboratories, partly due to its immunity to oxygen-related LID and ring defects, and very low metallic impurity concentrations. However, due to the addition of nitrogen during crystal growth to control dislocations, they are subject to the formation of nitrogen-vacancy complexes. Once formed, these defects are highly recombination active. However they can be partly mitigated by hydrogenation.<br/><br/>Finally, although hydrogen is helpful in passivating some defects in silicon, it can also cause recombination-active defects when present in excess, such as the widely-researched Light and elevated-Temperature Induced Degradation (LeTID). Since these defects are caused by firing hydrogen-containing dielectric films, they can be found in all three types silicon wafer described above, whether n- or p-type.