Novel printing technologies for silicon solar cell manufacturing

There is a tremendous opportunity for increasing performance and reducing cost in solar cell production by adopting manufacturing processes from other industries. In this paper I will present several technologies developed at PARC for crystalline silicon solar cell manufacturing, based on PARCs more than 30 years of experience in document and industrial printing. In order to overcome the very resistive contact formed by the conventional screen printed fire-through silver paste process, PARC is currently developing various improved metallization approaches. One of our methods uses a blanket sputtered nickel film as the contact layer and screen printed silver lines as an etch mask to pattern the underlying nickel film, avoiding costly photolithographic processing. Through forming a silicide contact the resulting specific contact resistance is about two orders of magnitude lower than that of fire-through silver contacts. A further improvement is to use inkjet-printed nickel nanoparticle inks instead of the sputtered nickel to form the contact layer, enabling a very low-cost inline process that can be easily implemented in current solar cell production lines. This technology could enable up to 0.9% absolute efficiency improvement while reducing cell cost per watt, which will be discussed in detail in the paper. An extremely low cost process to fabricate high efficiency IBC (Interdigitated Back Contact) cells is also being developed at PARC, based on inkjet printing and laser patterning. A key step in this technology is to inkjet print boron dopant and use laser ablated grooves to physically separate the diffused p+ and n+ regions. In this way the alternating p+ emitter/n+ back surface field structure on the backside, and the n+ front side surface field, can all be formed in a single tube furnace process. We will show that this is a very promising approach to achieve the $0.50/W module cost, the goal set by the US Department of Energy. Finally, I will discuss a novel extrusion printing process which can replace traditional screen printing for metallization. This is a fast, non-contact printing technology and can produce high aspect ratio silver gridlines, and hence is very suitable for processing thin silicon wafers and also increases the cell efficiency. PARC is now working with other companies to commercialize the technology.