DEKs Senior Process Development Specialist presented the PoP paper to around 200 industry professionals, customers and academics. The two-day event in Germany was designed to showcase Research & Development from universities, institutes and industry in the field of metallization for crystalline silicon solar cells.
Commenting on the significance of his presentation, Falcon explains: Print-on-Print itself is not a new idea; but the implementation of this technique in a high volume manufacturing environment requires much more than an accurate printing machine. It requires an optimised process with each print operation finely tuned for maximum performance, both individually and in combination. Whats more, this must be repeatable over many thousands of wafers. Our new research outlines the development of a successful PoP process to show how we can deliver a measurable and consistent improvement over a single print for dramatically improved productivity.
The DEK Solar paper describes the development of a PoP process for high aspect ratio frontside conductors, with in-depth results spanning the testing of various screen types, aperture widths, emulsion thickness, paste types and process parameters to empirically determine optimisation.
Conclusions reached include the fact that paste choice is critical to maximise performance and that quality screens are essential for alignment. Finer meshes and moderate emulsion are identified as delivering optimal prints while screen life is proven to stand at around 7.5K wafers.
Ultimately, DEK Solar evidenced that around a 40% increase in conductor aspect ratio is possible and sustainable in high volumes over a single print. Ideal for solar manufacturers looking to enhance cell efficiency and drive down cost-per-watt, print-on-print is designed to give finer grid lines more height, printing the lines twice over to enhance current-carrying capacity without shadowing underlying silicon substrate.
Repeatable accuracy is the enabler here since even the slightest misalignment can result in printing paste flooding out beneath the screen thereby reducing cell efficiency. As such, along with process engineering expertise and global support capabilities, DEK Solars capacity to drive new advances in this area are also boosted by its PV3000 metallization technology, achieving print-on-print capability (PoP) – +/- 12.5 microns at 2 Cpk. Delivering six-sigma repeatability for capability in advance of current solar cell requirements, the platforms inherent accuracy and repeatability is ideal for progressive techniques such as print-on-print.