Type of presentation: Poster

MS-14-P-3391 Intermetallic phase formation in SmartWires: novel concept of interconnection technology for solar cells

Hessler-Wyser A.^{1, 3}, Faes A.², Cattin J.^1,3, Baumgartner Y.^1,3, Levrat J.², Escarré J.², Champliaud J.², Despeisse M.², Gattaneo G.¹, Ufheil J.⁴, Papet P.⁵, Yao Y.⁶, Söderström T.⁶, Ballif C.^1,2

¹Laboratory of Photovoltaics and Thin-Film Electronics, Ecole Polytechnique Fédérale de Lausanne, 2000 Neuchâtel, Switzerland, ²PV-Center, Centre Suisse d'Electronique et de Microtechnique, 2000 Neuchâtel, Switzerland, ³Centre Interdisciplinaire de Microscopie Electronique, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, ⁴Somont, 79224 Umkirch, Germany, ⁵Roth&Rau Research AG, 2068 Hauterive, Switzerland, ⁶Division Module, Meyer Burger AG, Gwatt, Switzerland

aicha.hessler@epfl.ch

Standard busbars and ribbons are the commonly used contacting technology for crystalline silicon solar cells. For costs and efficiency constraints, alternative contacting solutions with less shadowing effects are developed. The innovative SmartWire Contacting Technology (SWCT) offers excellent perspectives as it combines several advantages: (1) reduction of production steps, (2) efficiency improvement by lowering ohmic losses in the existing metallization, (3) a reduced consumption of the costly raw materials by 85%, (4) enhancement of the module reliability and (5) improved aesthetics.

This work presents material characterization of those SmartWires when contacting c-Si heterojunction solar cells by scanning and/or transmission electron microscopy (SEM/(S)TEM), combined with analytical X-ray dispersive sepctroscopy (EDX), for as-fabricated and degraded modules. SWCT consists of polymer foils supporting copper wires coated with InSn alloy that has a low melting point (117 °C) and melts during the module lamination process (T = 160 °C). This leads to solder contacts to the solar cell silver metallization thanks to interdiffusion of metallic species and intermetallic phase formation occurring both at front and back contact of the cell. Degradation tests like thermo-cycling and damp heat IEC tests show a strong effect on electrical properties of the contacts. Therefore a detailed investigation of the microstructure evolution of the contacts and the formed phase is needed.

SEM-EDX was performed on an FEI xlf30 equipped with a Si-drift detector (Oxford), whereas TEM observations were carried out on FEI Technai Osiris with the dedicated ChemiSTEM technology. TEM lamellae were extracted by focussed ion beam (Zeiss, Nvision) from embedded samples. Simultaneously, diffusion tests were performed on In-Sn coated wires alone covered by silver paste, then analysed by SEM. In all cases, three ternary phases of Cu-In-Sn were found (Fig.1). Surprisingly, no silver was found in the tin phase that is directly in contact with silver paste, but significant silver amounts (up to 12 at%) were measured in the In-rich intermetallics (Cu₂In₃Sn). Degraded cell (80°C in dry air during 1500 h) shows that silver back contact disappears when in contact with the SmartWire, and that silver content of the In-rich phase raises up to 20 at%. Furthermore, silver grains segregate at the In-rich/Cu-rich phase interface (Fig. 2). Those results explain the disappearance of silver at the cell back surface.

The authors thank F. Bobard and D. Laub for sample preparation.

Fig. 1: SEM BSE micrograph (left) of a wire outer layer with the three phases differentiation (right). The wire was coated with silver paste (visible in the upper part of the image) and annealed at 160°C.

Fig. 2: SEM micrograph of the extracted TEM lamella (left), EDS mapping of the region (middle) and silver segregation at the inferface between Cu2(In,Sn) and Cu2In3Sn.