Fabrication of double- and triple-junction solar cells with hydrogenated amorphous silicon oxide (a-SiOx:H) top cell

• The Voc×FF of a-SiOx:H solar cells decreases as i-layer thickness increases.
• Various multi-junction solar cells are investigated using a-SiOx:H top cells.
• Current mismatching can be reduced with thin a-SiOx:H top cells for triple cells.
• 12.58% a-SiOx:H/a-Si:H/nc-Si:H triple cell is demonstrated.
• An efficiency of the triple cell degrades 18.6% after 720-h light soaking.

In this study we investigate the potential of intrinsic a-SiOx:H, with a band gap of about 2.07 eV, as absorber material in thin-film silicon solar cells. We found that single junction a-SiOx:H cells with an i-layer thickness of 100–200 nm can have a high Voc and FF of up to 1.04 V and 0.74, respectively. However, when the i-layer thickness is increased further to obtain higher Jsc, the Voc×FF product decreases significantly and limits the conversion efficiency to around 7.2%. Although the efficiency of a single-junction device is limited, the high Voc×FF product makes this device interesting for application as the first sub-cell in multi-junction devices. To investigate this, first double-junction (2J) solar cells were fabricated with the second sub-cell of either a-Si:H (Eg≈1.7 eV) or nc-Si:H (Eg≈1.1 eV), reaching efficiencies of 10.25% and 10.92%, respectively. Although these are decent efficiencies, both 2J cells are limited by the a-SiOx:H first sub-cell and so far have no advantage over the conventional a-Si:H/nc-Si:H cell. Secondly, we fabricate two types of triple-junction (3J) solar cells, which consist of the above mentioned 2J solar cells with the additional nc-Si:H third sub-cell at the bottom. In both 3J solar cells current mismatching can be reduced with a relatively thin a-SiOx:H first sub-cell (≈100 nm) such that its high Voc×FF product can be fully used. An initial efficiency as high as 12.58% was obtained (Voc: 2.37 V, Jsc: 7.27 mA/cm2 and FF: 0.73) for the a-SiOx:H/a-Si:H/nc-Si:H 3J solar cell. This efficiency is competitive with the efficiency of other types of 3J solar cells and demonstrates that a-SiOx:H opens up new routes towards high efficiency 3J thin-film silicon solar cells.

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