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The susceptibility to light-induced degradation (LID) is one of the critical barriers preventing the use of commercial-grade p-type silicon wafers in silicon heterojunction (SHJ) solar cells fabrication, which instead uses high-quality n-type Czochralski-grown silicon (Cz-Si) wafers. Despite intensive research on boron-oxygen LID (BO-LID) and light- and elevated temperature- induced degradation (LeTID) on conventional silicon solar cells, little work has been done investigating such degradation phenomena in silicon heterojunction solar cells. This thesis aims to fill this knowledge gap, investigating the challenges associated with LID in p-type SHJ solar cells and demonstrating defect engineering approaches to address these challenges. Experimental work reveals for the first time that SHJ solar cells can be susceptible to LeTID if hydrogenated p-type multicrystalline silicon wafers are used, which is brought about by the thermal annealing during SHJ solar cell fabrication. P-type Cz-Si solar cells are found to be very sensitive to BO-LID, experiencing severe open-circuit voltage (VOC) losses up to 30 mV during the first few seconds under carrier injection, which could occur during unwanted short exposure to room illumination between cell fabrication and characterization. These results may provide evidence as to why the development of p-type SHJ solar cells based on commercial-grade p-type silicon wafers might have failed in achieving high-VOC values in the past. The role of gettering and hydrogenation in the stabilization of BO defects in p-type SHJ solar cells is investigated in detail. Gettering is found to be critical in reducing the susceptibility of SHJ solar cells to BO-LID and enabling high VOC values after BO stabilization processes. The typical industrial SHJ fabrication sequence is found to provide enough bulk hydrogen for rapid stabilization of BO defects in p-type boron-doped Cz-Si SHJ solar cells via a 10-second long advanced hydrogenation process (AHP), which resulted in a record stable VOC > 734 mV for a solar cell fabricated with a commercial-grade p-type silicon wafer. Using p-type gallium-doped wafers yielded stable VOC values of 734 mV (and stable conversion efficiency of 22.6%) in SHJ solar cells, despite the recent lifetime instabilities observed in such wafers in passivated emitter and rear cell (PERC) solar cells. The stable VOC values demonstrated here for defect-engineered p-type boron- and gallium-doped SHJ solar cells are higher than the VOC values of record p-type silicon solar cells with conversion efficiencies over 26%, which were fabricated with float zone silicon wafers. These results unravel the potential of using commercial-grade p-type silicon wafers with passivating contacts in next-generation solar cells with VOC values well above PERC. Comparing the findings from this thesis work with the existing literature showed that the same defect engineering approaches developed for p-type SHJ solar cells can also benefit n-type SHJ solar cells. This comparison provides the basis for a new proposed SHJ solar cell fabrication sequence incorporating defect engineering. |
