Barrier properties of Cu/TiW/ITO electrode for Si heterojunction solar cell under low temperature thermal aging

The front side metallisation of silicon heterojunction (SHJ) solar cells was conducted via copper electroplating. The layer structure of the front electrode was Sn/electroplated Cu/seed Cu/TiW/ITO/Si, where the TiW layer served as a diffusion barrier. The stability of the TiW diffusion barrier during low temperature thermal aging (85 °C for 1000 h) was investigated. The barrier stability of the TiW layer was evaluated in three layer structures, namely TiW/ITO, seed Cu/TiW/ITO, and Sn/electroplated Cu/seed Cu/TiW/ITO/Si (i.e. full electrode structure). Further, three TiW layer thicknesses (20, 50, and 100 nm) were investigated. The TiW/ITO and Sn/electroplated Cu/seed Cu/TiW/ITO/Si structures were stable. However, the seed Cu/TiW/ITO structure exhibited various physical and chemical changes due to oxidation of the seed Cu layer. Specifically, a Cu2O(110) phase formed on Cu(111) during thermal aging, which triggered interdiffusion of TiW and ITO toward the seed Cu layer due to the Kirkendall effect. The Cu2O(110) formation was more severe when a thicker TiW layer was applied. Cu cations moved and reacted with absorbed oxygen, which was enhanced by the increased electric field between the p-type Cu2O and Cu/TiW metal with a thicker TiW layer.