New Materials for Metallization and Interconnection of Perovskite Cells: Low-Temperature Processing & Reduced Silver Consumption

The goal of significantly reducing CO2 emissions requires a massive expansion of PV installations worldwide. Perovskite cells offer a unique opportunity to enable this at a low consumption of precious resources and energy. This requires metallization and interconnection materials that can be processed at low temperatures. In order to avoid bottlenecks in the supply of silver for contacting and inter-connection of solar cells, the specific silver consumption in the production of solar cells and PV modules must be constantly reduced.

The capillary suspension phenomenon (Koos and Willenbacher, Science 331, 897 2011) provides a unique opportunity to design printable pastes and adhesives with high electrical conductivity at low silver consumption. The addition of a second immiscible liquid to a suspension of particles leads to the self-assembly of a strong particle network controlled by capillary forces in such ternary solid/fluid/fluid systems. This results in a drastic change of texture and flow behavior, but also reduces the percolation threshold leading to a high electrical conductivity at low volumetric particle loading in composites including conductive fillers such as silver particles.

Pastes for the metallization of temperature-sensitive Perovskite solar cells are sintered at low temperature (< 150°C) and include polymeric additives to provide cohesion of printed finger lines and sufficient adhesion to the transparent conductive oxide (TCO) surface layer. The required polymer content can be reduced and the surface properties of the conductive particles in the contact areas can be designed in such a way that high electrical conductivity and low contact resistance are achieved.

The TECC wire concept uses round copper wires for cell interconnection. These wires are coated with a thermoplastic polymer filled with conductive particles. The polymer ensures good mechanical adhesion to the cell surface and the conductive particles enable low contact resistance. Good interconnection at gentle processing conditions in terms of pressure and temperature has been achieved using thermoplastics with low silver content.

Thermoplastic busbars (patent pending) is a recently developed concept further simplifying cell interconnection and reducing resource usage. Electrically conductive, thermoplastic materials melting at low temperature are applied in the metallization process and untreated copper wires are applied in a fast, low temperature stringing process.

The capillary suspension concept has been utilized first to design printable, electrically conductive materials including silver particles but the transfer of the concept to composites including other conductive particles such as copper and silver coated copper or glass particles is straight forward.