A progressive process that has nevertheless been utilised in other industries for several years, Print-on-Print involves the printing of one conductor grid directly on top of a previously printed and dried grid. PES asked acknowledged pioneers DEK for an in-depth explanation of this revolutionary procedure.
A simple Internet search will reveal that the photovoltaics industry is working hard on higher aspect ratio frontside conductor grids as a route to increased solar cell efficiencies. This is because the silver energy-collecting conductors that are typically screen printed onto the frontside of a silicon-based solar cell actually block out sunlight, effectively stopping it from reaching the energy converting strata below. While it is important that these conductors are optimally distributed across the surface of the cell in order to mop up all the energy available, it is clear that the more surface area they cover, the more they will hinder cell functionality. Which is why the solar industry is putting such a lot of effort into reducing conductor widths. Currently in the region of 80-120μm, these are expected to drop to 50-60µm in the immediate future, reducing the shadow effect and improving cell performances considerably, while saving on materials costs.
In doing so, however, cell manufacturers must contend with a simple law of physics, and this is that a conductor’s electrical resistance is directly proportional to its cross-sectional area. In other words, as width decreases, height must increase if the conductor is to maintain its current-carrying capacity. And this is where aspect ratio comes into its own. A simple enough equation describing a printed feature’s height/width relationship, this metric, and more specifically, high aspect ratio conductors, will become increasingly critical as the solar industry pushes the limits on cell efficiencies.
Giving finer lines more height sounds straightforward enough, but the physical characteristics of printing screens, together with the release properties of standard silver screen printing pastes, effectively put a maximum height on 50-60µm lines that is insufficient for the industry’s needs. Special materials such as high aspect silver pastes have been instrumental in taking the industry forward, and several deposition methods, described in numerous papers, also show a great deal of potential to do the same. Perhaps one of the most promising of these is a new metallisation process, already in commercial use in some leading cell manufacturers, which could deliver aspect ratios that, at between 0.4 and 0.5, were until now impossible.