Stability of silicon surface passivation under damp heat and light soaking

The reliable and stable performance of crystalline silicon solar cells is vital in mitigating climate change, as they contribute to approximately 95% of all produced solar cells. During operation, solar cells are subjected to intense illumination, high temperatures, and high ambient humidity, which can be especially harmful for their surface passivation. This thesis investigated the stability of silicon surface passivation under damp heat and light soaking as well as potential mechanisms behind the resulting degradation. Particularly, the influence of the spatial atomic layer deposition (SALD) mode, the passivation layer thickness, the properties of the interface between substrate and the layer, and nanotextured surfaces on the stability were considered. The thesis demonstrated the potential of using SALD AlOx for the surface passivation of nanotextured silicon. 20-nm-thick SALD AlOx films provided similar performance as AlOx deposited with conventional ALD, but with over 9 times higher deposition rate. Additionally, the damp heat stability of AlOx films deposited with both SALD and thermal ALD was studied, and film thickness was observed to have a more significant impact on the stability than the surface nanotexture. 20-nm-thick SALD AlOx films were stable for up to 1000 h of exposure, but degradation was observed with thinner films. Interestingly, loss of chemical passivation explained the degradation in the case of conventional ALD, while a reduction of field-effect passivation was observed with SALD AlOx. Under light soaking, film thickness above 5 nm provided stable passivation. An important result was that a thin SiOx film resulting from an RCA2 clean underneath the AlOx layer enhanced the passivation stability under both conditions, making this interfacial oxide one of the key parameters for the stability. The degradation under light soaking could not be linked to the charge of the AlOx film or the density of interface defects, due to which a separate degradation mechanism linked to copper impurities was considered. Indeed, a dramatic increase of surface recombination was observed in copper-contaminated silicon, indicating that also copper can contribute to surface recombination. Additionally, it was demonstrated that bulk degradation caused by copper impurities could be mitigated in solar cells using dark anneals. However, this procedure might lead to increased surface recombination as a trade-off. In conclusion, this thesis brings insights on the stability of silicon surface passivation, which provides means to enhance the in-field performance of high-efficiency solar cells. These results are also applicable in the microelectronics and photodetector industries, in which strict process and impurity control to achieve stable surface passivation are vital for device performance.