## 2 Alternative Silicon Materials to Avoid Boron-Oxygen (B-O) Light-Induced Degradation (LID)
### Decreasing the Interstitial Oxygen Concentration  
A negative side effect of Cz growing technique is a quite strong level of oxygen contamination. Silicon feedstock is melted in a quartz (SiO\({}_{2}\)) crucible from which the crystal is pulled with rather low growth rates. As molten silicon is highly reactive, it partially dissolves the crucible during growth thus continuously enriching the melt with oxygen, which then is incorporated unintentionally into the growing crystal. Several alternative crystallisation approaches exist that can reduce the interstitial oxygen concentration in silicon wafers. When using mono-crystalline silicon one approach to reduce the [O\({}_{\rm i}\)] is to use Float-zoned (FZ) silicon. FZ silicon typically has an [O\({}_{\rm i}\)] two orders of magnitude lower than that in Cz material [31], which suppresses B-O related LID [21]. However, FZ silicon wafers are significantly more expensive than those produced from Cz silicon and hence not preferred by solar cell manufacturers. FZ silicon can also be subject to various other defects, such as nitrogen related defects or LID effects that can significantly reduce bulk lifetimes [32; 33; 34]. For Cz silicon, [O\({}_{\rm i}\)] may be reduced by applying magnetic fields during crystal growth, manipulating convective flows in the melt [35; 36]. However, this technique has proven challenging for the current size of industrial Cz ingots and throughput/cost constraints for solar cell fabrication. Alternatively, coatings such as silicon nitride can be used on the wall of the crucible [37] to reduce [O\({}_{\rm i}\)] by up to one order of magnitude [38; 39]. This method is commonly used by the industry. Control of the crystallisation parameters such as crucible and crystal rotation, pressure and ambient atmosphere can also reduce [O\({}_{\rm i}\)] [37]. Alternatively, casting methods can be used to reduce the [O\({}_{\rm i}\)] in the silicon wafers [40]. However, the casting method results in multi-crystalline crystal growth with large structural defects such as dislocation clusters and grain boundaries. Also, a range of contaminants is often introduced into the silicon ingot that typically originates from less pure crucible material. Both structural defects and contaminations reduce carrier lifetime [41; 42] and thus solar cell efficiency, especially when a cell architecture like PERC (passivated emitter and rear cell) is used which benefits from high carrier lifetimes. BP Solar developed a promising casting approach to eliminate grain boundaries by seeding the growth of mono-crystalline silicon [43]. Unfortunately, this material suffers from significant quality variations throughout the ingot due to the generation of high dislocation densities [44] and unintended partial multi-crystalline crystal growth. As a result, the industry has not adopted this approach to date [45]. An alternative casting method seeds the crystal growth to deliberately reduce the grain size, therefore reducing dislocation densities, to produce \"high-performance multi\" [46]. This material is rapidly being adopted by the industry to replace conventional cast silicon material [5]. However, these wafers suffer from a different form of degradation that can reduce cell performance by as much as 12%relative [7; 47; 48; 49; 50], which has posed a significant challenge for solar cell manufacturers with the transition to the PERC cell technology [5]. In general, while cast silicon material is cheaper than Cz silicon and less susceptible to B-O LID, the casting methods introduce more severe recombination mechanisms into the silicon bulk. At the end of 2017 (6 December) wafer prices were US$0.629 for high-performance multi-crystalline silicon and US$0.716 for mono-crystalline silicon and Al-BSF cell prices were US$0.894 for multi-crystalline silicon and US$1.005 for mono-crystalline silicon [19].