Zn as the protective layer for Cu electrode in wafer-Si solar cells

Abstract

Zn is proposed as the protective layer for the Cu electrode in wafer-Si solar cells to replace today's Ag front electrode. Zn provides a lower material cost, a lower resistivity and more abundant material reserve than Sn. The thermal stability of the Zn/Cu/Ni stack is examined by annealing it in air and the Zn/Cu/Ni stack is advantageous over the Sn/Cu/Ni stack in thermal stability. This is attributed to the better coverage of electroplated Zn on Cu and the higher melting point of Zn over Sn. The sheet resistance of the Zn/Cu/Ni stack is also lower than the Sn/Cu/Ni stack due to the lower resistivity of Zn. XRD measurements before and after annealing confirms that the increased sheet resistance upon annealing is due to oxidation and alloying of the Cu layer. For solderability, a Sn/Zn/CulNi stack with reduced Sn thickness is demonstrated by sequential electroplating.

Light-Induced Al Plating on Si for Fabrication of an Ag-Free All-Al Solar Cell

“…Our previous experiments demonstrate a resistivity of 3 μΩ-cm for electroplated Cu. 5 As discussed later in this paper, we have achieved an Al resistivity of 3.1 μΩ-cm using light-induced Al plating. These values are similar despite the different bulk resistivities as shown in in Table I.…”

“…6 We have demonstrated Zn as the capping layer for Cu. 5 Zn has a few advantages over Sn which include a lower resistivity (5.9 μΩcm vs 11 μΩ-cm), a higher melting point (420 °CC vs 232 °C), and a lower price ($2.66 kg −1 vs $18.92 kg −1 as of November 18, 2020). 1 In contrast, Al is not a deep level impurity in Si and can be placed in direct contact with the Si cell without the need for a barrier layer.…”

“…Nicotinic acid yields the lowest film resistivity of 3.1 μΩ-cm at 0.25 wt%, which is comparable to electroplated Cu. 5 The cross-sectional SEM image (Fig. 9) reveals that nicotinic acid exhibits a good throwing power and allows Al to plate deep into to the valleys between pyramids on the textured Si surface.…”

“…Therefore the suitable candidates for substituting Ag in Si cells are Cu and Al. 3 While most of the research efforts have so far focused on a Cu front electrode, 5,6 our group reported an Al front electrode on both ptype and n-type Si cells by Al electroplating. [7][8][9] Due to the highlyresistive solar Si wafer, direct Al plating on Si is difficult and a Ni seed layer is necessary to reduce the wafer resistance and facilitate Al plating.…”