High-quality passivation with intrinsic hydrogenated amorphous Si (i-a-Si:H) is essential for achieving high-efficiency Si heterojunction (SHJ) solar cells. The formation of i-a-Si:H with a high passivation quality requires strict control of the hydrogen content and film density. In this study, we report the effective discovery of i-a-Si:H deposition conditions through catalytic chemical vapor deposition using Bayesian optimization (BO) to maximize the passivation performance. Another contribution of this study to materials science is the establishment of a practical BO scheme consisting of several prediction models in order to account for the practical constraints. By applying the BO scheme, effective minority carrier lifetime (τeff) is maximized within the deposition condition range, while being constrained by the i-a-Si:H thickness and the capabilities of the experimental setup. We achieved a high passivation performance of τeff > 2.6 ms with only 8 cycles in BO, starting with 14 initial samples. Within the investigated range, the deposition conditions were further explored over 20 cycles. The BO provided not only optimal deposition conditions but also scientific knowledge. Contour plots of the predicted τeff values obtained through the BO process demonstrated that there is a band-like high τeff condition in the parameter space between the substrate temperature and SiH4 flow rate. The high void fraction and epitaxial growth were inhibited by controlling the substrate temperature and SiH4 flow rate, resulting in a high passivation quality. This indicates that the combination of the SiH4 flow rate and substrate temperature parameters is crucial to passivation quality. These results can be applied to determine the deposition conditions for a good a-Si:H layer without a high void fraction or epitaxial growth. The research methods shown in this study, practical BO scheme, and further analysis based on the optimized results will be also useful to optimize and analyze the process conditions of semiconductor processes including plasma-enhanced chemical vapor deposition for SHJ solar cells.
Keywords: Cat-CVD; amorphous silicon; bayesian optimization; passivation; silicon heterojunction.