Pei-Chieh Hsiao

Untangling the Mysteries of Plated Metal Finger Adhesion: Understanding the Contributions From Plating Rate, Chemistry, Grid Geometry, and Sintering

Historically, busbar pull tests have been used as a measure of metal-silicon adhesion for silicon solar cells; however, such measurements cannot be easily applied to evaluate finger adhesion and the propensity of metal fingers to peel. Finger adhesion will be increasingly important as the width of fingers decrease and busbars are effectively removed from the cell metallization. In this paper, we correlate metal-plated finger dislodgement measurements, which have been obtained using a stylus-based metallization testing tool, and busbar pull test forces with nanoindentation measurements of the Youngs modulus in order to determine key determinants of strong finger adhesion. It is proposed that metal fingers with a higher Youngs modulus dislodge at lower stylus impact forces because the energy associated with the impact is less easily dissipated along the fingers and consequently remains more focused on the impact location, causing not only finger dislodgement but more extensive finger peeling as well. It is shown how plating rate, chemistry, grid geometry, and postplating annealing can all contribute to plated metal finger adhesion, therefore necessitating an understanding of these factors for reliable plated metallization.

Backsheet metallization of IBC silicon solar cells

A simplified interconnection/metallization method for interdigitated back-contact (IBC) silicon solar cells is reported. A patterned PEDOT:PSS seed layer was inkjet printed on the backsheet. After curing, the seed layer pattern was electroplated first with Cu and then with eutectic Sn-Bi. During lamination at 150 °C, the eutectic Sn-Bi alloy melts and provides good ohmic contact between the IBC solar cells and the electroplated Cu on the backsheet, thus simultaneously achieving metallization and interconnection. The method eliminates the need to form thick metal conductors on the IBC cells. Furthermore, the moderate conductivity of the PEDOT:PSS seed layer lines results in tapered Cu conductors with the finger height reducing as a function of distance from the contacted busbar. This tapered metal grid can reduce the mass of Cu required for a 1% resistive power loss by 24%.

Self-annealing behavior and rapid thermal processing of light induced plated copper fingers on silicon solar cells

Light-induced plating presents a potentially lower-cost alternative to screen-printed Ag for silicon solar cell metallization. This paper presents results of experiments that investigated the effects of bias current density and post-plating rapid thermal processing (RTP) on plated Cu finger microstructure and texture. It is shown that the Cu, if not annealed after plating, self-anneals with time resulting in grain growth and increased (200) to (111) grain texture which is associated with increased tensile stress. The rate of self-annealing is much faster with high plating current densities. Post-plating RTP annealing enables fast annealing and stable Cu fingers with a low ratio of (200) to (111) grain texture. These findings have important implications for plated finger adhesion and highlight the importance of annealing after plating for reliable Cu plated metallization.

Investigating light-induced plating of silicon solar cells using in-situ current-voltage analysis

Light-induced plating of Si solar cells was analyzed using in-situ linear sweep voltammetry using a potentiostat configured with a reference electrode. The current-voltage relationships recorded depend on the characteristics of the solar cell, the electrochemical reactions at the interface and the electrolyte resistance. Linear sweep voltammetry can be used to ensure that the solar cell operates under forward bias during plating, thereby eliminating the potential of non-uniform plating which can occur if the solar cell operates under reverse bias.

Sn-Bi light-induced plating for interconnection of silicon solar cells

In this paper, the use of plated eutectic Sn-Bi for the interconnection of Ni/Cu plated laser-doped selective-emitter cells is reported. Coatings of eutectic Sn-Bi were formed over Ni/Cu plated fingers and busbars by controlling the Sn-Bi plating current density in a light-induced plating process. During lamination at 150 °C, the eutectic Sn-Bi alloy melts and provides good ohmic contact between the front busbar of the solar cell and interconnection ribbon therefore eliminating the need for soldering. Fill factors measured after lamination were comparable to those measured after plating, indicating that low resistance interconnection can be achieved.