Passivation and carrier selectivity of TiO₂ contacts combined with different passivation layers and electrodes for silicon solar cells

Titanium dioxide (TiO₂) films have previously been demonstrated to function as electron-selective contacts to silicon solar cells, and an efficiency of 21.6% has been reported for a cell featuring a full-area TiO₂ contact. However, the passivation quality of TiO₂ contacts still falls short of that possible with best-in-class contacts based on, e.g., hydrogenated amorphous silicon (a-Si:H). We investigate here the performance of a-Si:H/TiO₂ stacks as electron-selective, passivating contacts. We show that combining a-Si:H with TiO₂ can result in excellent surface passivation (lifetime close to 3 ms for textured CZ wafers), especially for 7.5-nm-thick TiO₂ capping layers. However, initial cell results show that such a-Si:H/TiO₂ stacks give poorer efficiencies than TiO₂ only, with extremely low fill factors due to S-shaped current-voltage curves. Also, the role of the rear electrode becomes apparent when substituting Al for an ITO/Ag stack: the latter has significantly lower open-circuit voltage and fill factor than the former. Combining a TiO₂/Al rear electron contact (with no a-Si:H) and an intrinsic a-Si:H/p-type a-Si:H front hole contact, we demonstrate a double heterojunction silicon solar cell with an efficiency of approximately 15%. Furthermore, a full metal-oxide heterojunction cell that combines a molybdenum oxide (MoO_x)/ITO hole contact with the TiO₂/Al electron contact achieves an efficiency of 11%.