Fabrication of bifacial poly-Si solar cells with copper-plating metallization

Abstract

Carrier-selective passivating contacts (CSPC) have been proven to be effective in suppressing recombination losses at metal-silicon interface in high-efficiency crystalline silicon solar cells. The poly-Si-based CSPCs consisting of SiOX/poly-Si stacks are used in this thesis due to their compatibility with high thermal budgets. CSPCs with thin poly-Si layers are preferred since thicker poly-Si layers induce significant parasitic absorption at device level. The bifacial solar cell design could provide a higher energy yield than monofacial solar cells and largely reduce the consumption of metals such as silver, indium due to the contribution of reflected light. The objective of this thesis is to fabricate double-side textured copper (Cu)-plated bifacial poly-Si solar cells. It is achieved by developing thin poly-Si contacts, addressing the TCO sputtering-induced passivation damage, and optimizing the Cu-plating processes.
Firstly, the passivation of the poly-Si symmetric samples and solar cells was optimized. 16 nm-thick n+ poly-Si and 16 nm-thick p+ poly-Si symmetric samples with intrinsic layer grown through PECVD demonstrated good passivation quality indicated by their iVOC of 713 mV and 665 mV respectively. To further improve the passivation quality of the p+ poly-Si, the layer thickness and the doping gas flow ratios were varied. Best passivation was achieved for a solar cell precursor with 42 nm-thick p+ poly-Si layers. The optimum doping gas flow ratio was found to be SiH4-B2H6=20/15 sccm. The highest iVOC achieved was 692 mV.
TCO sputtering caused a severe passivation drop of ~90 mV in solar cell precursors. Post-deposition annealing treatments and 2-step TCO sputtering techniques were used to reduce such a passivation loss. However, it was challenging to reproducibly obtain solar cell precursors with good passivation quality before metallization step. The initial solar cell with double side TCO use showed cell parameters were: VOC 398 mV, FF 56.22%, JSC 30.55 mA/cm2 and η 6.85% (from n-side illumination).
To maintain a good passivation quality of the solar cell precursor, an 8 nm-thick MoOX buffer layer was introduced on top of the p+ poly-Si layer. This approach effectively reduced the passivation drop from 91 mV to 12 mV. However, the MoOX layer was observed to strongly react with the solution which was used for silver seed layer removal in Cu-plating metallization procedure. Different approaches were tested to obtain a well-plated p+ poly-Si side. As for the n+ poly-Si side, a TCO-free design was deployed. To ensure a good adhesion of the plated Cu fingers, a Ti/Ag (8 nm/192 nm) seed layer was employed before Cu-plating step. Moreover, an additional SiOX layer, which acts as the anti-reflection coating, was deposited on the n-side of a complete solar cell. With these adjustments, the solar cell performance was improved to: VOC 610 mV, FF 64.98%, JSC 36.95 mA/cm2 and η 14.64% (from n-side illumination).
Furthermore, with reducing the Ti thickness in the metal seed layer to 2 nm. The solar cell performance was further improved to VOC 611 mV, FF 69.58%, JSC 36.16 mA/cm2 and η 15.38% (from n-side illumination). The bifaciality factor is 89%.