## 2 Alternative Silicon Materials to Avoid Boron-Oxygen (B-O) Light-Induced Degradation (LID)
### Decreasing the Boron Doping Concentration  
Another alternative to fabricating wafers that are less susceptible to B-O LID is the use of wafers with reduced boron doping concentration and hence higher bulk resistivity. Decreasing the boron doping concentration not only reduces the concentration of B-O defects but has the added benefit of reducing the sensitivity to Shockley-Read-Hall (SRH) recombination [51; 52]. However, for PERC solar cells, this increases bulk resistance in the device for the lateral flow of majority carriers to the localised p-type contacts. Hence, there is a trade-off between reducing the effects of reducing SRH recombination in general and B-O LID in particular and increasing series resistance within the bulk. As a result, mid-range doping around 2-5 \(\Omega\)cm is mostly used in industrial applications.  
Boron can also be substituted with other dopants. If retaining a p-type substrate, typically gallium is used. The benefit of using gallium-doped wafers is that they are free of any B-O related LID effects. However, like boron-doped wafers, they are susceptible to the formation of acceptor-iron pairs [53; 54]. A weakness of gallium-doped ingots is the substantially lower segregation coefficient for gallium (\(8\times 10^{-3}\)) than that for boron (0.8) [55; 56]. The low segregation coefficient results in a large range of wafer resistivities throughout a Cz ingot that can lead to significant variations in the electrical parameters and potentially reduce the usable portion of the ingot. Other p-type dopants such as aluminium or indium can also be used. The drawback is that these have even smaller segregation coefficients than gallium of \(2\times 10^{-3}\) and \(4\times 10^{-4}\), respectively [55; 56]. Furthermore, aluminium is susceptible to the formation of recombination active defects with O\({}_{\text{i}}\)[57] and indium doped Cz wafers are also susceptible to LID [58].  
Phosphorus-doped n-type wafers can also be used to eliminate B-O defects, provided that the material is not compensated with boron dopants [25]. Two strengths of n-type wafers are that they are less susceptible to intrinsic recombination [59] as well as to SRH recombination from impurities such as iron [60]. Besides, for a given resistivity, n-type wafers have a lower dark saturation current than their p-type counterparts. As a result, n-type wafers have higher lifetimes than p-type wafers. On the other hand, n-type wafers are more susceptible to SRH recombination from other metallic impurities such as nickel, copper, chromium and cobalt that are more difficult to better than iron [60]. The minority carrier mobility is also lower in n-type silicon than in p-type silicon.  
The use of n-type wafers can require significant processing and equipment changes for industrial solar cell fabrication. The processing sequence used can be more thermally and energy intensive and is typically more complicated than that required for p-type substrates. For example, if using a conventional homo-junction device structure, extended high-temperature diffusion processes are required for boron-doped emitter formation in n-type substrates as compared to phosphorous-doped emitter formation in p-type substrates due to the substantially lower diffusion coefficient of boron than that of phosphorus [55]. The high thermal budget required can induce the formation of other oxygen-related defects such as oxygen precipitates [61] that can be more severe than B-O related degradation [62; 63]. Phosphorus also has a smaller segregation coefficient (0.35) [55; 56] than that of boron, leading to larger variations in the bulk doping throughout the ingot. More recently, other LID effects have also been observed in the seed-end of n-type Cz ingots that can reduce performance by 10%relative [64].