## 5 Eliminating B-O LID during Cell and Module Fabrication
### LID Mitigation during Cell Fabrication  
With hydrogen distributed throughout the cell during the co-firing process, a short-illuminated process can be used to form the B-O defects and provide the required hydrogen charge state manipulation to enable the regeneration of the B-O defects. This illuminated process can be achieved during the cool-down section of the high-temperature co-firing process, or as a separate thermal process after co-firing. Several companies, including Centrotherm (Blaubeuren, Germany), Despatch Industries (Mineapolis, MN, USA), Schmid (Freudenstadt, Germany), Asia Neo Tech (Taoyuan City, Taiwan), DR Laser (Wuhan, China) and Folungwin (Dongguan Shi, China) have tools available for LID mitigation that can reduce the extent of LID by approximately 75-80% [144; 145; 146; 147]. For tools with LID mitigation processes integrated into tools for the fast-firing of the metallisation contacts, any potential changes to belt speed resulting from the requirements of future pastes could directly impact B-O LID mitigation. As a result, standalone tools for B-O LID mitigation offer more flexibility with possible future processing changes. They also allow PV module manufacturers to perform LID stabilisation processes on cells fabricated by other manufacturers, before module encapsulation. These LID mitigation tools are tending towards higher and higher illumination intensities to accelerate the process and improve performance. Currently, most tools are using intensities in the range of 5-20 suns [144; 145; 146; 147]. The illumination intensities used leads to industrial belt-furnace based tools typically requiring processing times of 25-45 s per wafer for the process. The requirement for high-throughput processes with an average of one wafer being processed every second (current industrial standard) thus means that multiple samples must be processed simultaneously. However, laser-based tools are capable of substantially higher illumination intensities, with a correlating decrease in process time. An alternative to the use of high-intensity illumination is the application of current injection to stabilise the cells. In this instance, cells are coin-stacked, with current passing through the entire stack [148]. However, this process results in additional stress and handling of the wafers and potentially an inhomogeneous heating of the wafers, which could reduce the effectiveness of the LID mitigation. One potential issue with eliminating LID at the cell level, is that during module fabrication the cells are exposed to high temperatures. During soldering localised regions of the cells are exposed to  
Figure 3: The process flow for the fabrication of industrial screen-printed PERC solar cells with a rear surface passivation dielectric stack of plasma-enhanced chemical vapour deposited (PECVD) hydrogenated aluminium oxide (AlO\({}_{x}\)H) and silicon nitride (SiN\({}_{x}\):H). Processes related to the hydrogenation of B-O defects are shown in green. Other processes are shown in blue. temperatures >230 \({}^{\circ}\)C for few seconds and the whole module is exposed to temperatures in the vicinity of 150 \({}^{\circ}\)C for up to 15 min during encapsulation. Applying additional thermal processes after an LID mitigation process can destabilise the passivation [83], allowing the material to once again degrade when exposed to illumination. Although a 15-min process at 150 \({}^{\circ}\)C is expected to destabilise less than 5% of the passivated defects, due to the large activation energy of the destabilisation reaction, any increase in temperature can lead to a significant increase in the number of defects that are destabilised. For example, increasing the temperature to 200 \({}^{\circ}\)C is expected to result in the destabilisation of approximately 20% of the passivated defects. While destabilisation after regeneration is a potential issue for long-term performance, any condition in the field that results in a heating of the modules that could enable destabilisation, typically occurs in the presence of illumination.