Solar installations are rocketing worldwide as solar electricity becomes cheaper than electricity from fossil fuels – its already happened in 105 countries. This is the tipping point where economics takes over from altruism and solar PV becomes a serious part of the global energy mix.
The steady drop in cost-per-watt is great news for the end user and is enabled by simultaneous increases in cell efficiency and lower manufacturing costs.
If you’re a cell manufacturer, though, dropping prices are a double-edged sword: strong demand is good, but you must continuously lower costs by improving your manufacturing processes in order to be profitable.
So, in a fiercely competitive industry, how is this done? A great example of how cell manufacturers can boost profitability is an emerging technique called “double printing.” I first wrote about double printing three years ago, when the technique was in its infancy. Today, it’s rapidly being adopted by cell manufacturers around the world.
More efficient cells are a good thing, naturally, as this histogram graphic shows. More efficient cells are worth more, and help a manufacturer stand out in a crowded marketplace. Changing the design of the cells to make them more efficient can yield immediate benefits, as long as implementing the change doesn’t cost more than the extra revenue.
Repeatability is also a strong lever: every time you make a substandard cell the bottom line suffers. After all, it costs the same to make a cell whether it works well or not. And obviously, minimizing the amount of raw material in each cell is beneficial.
Let’s take a look at a typical cell. The vast majority of cells made today have a network of conducting lines on the front which carry electricity away from the cell. They’re laid down using a screen printing process, where silver paste is forced through a stencil onto the wafer and then cured in an oven. Today’s state-of-the-art lines are 60-80 micrometers wide.
After the silicon wafer itself, the largest material cost is silver paste, about 140mg of which is used to form the network of contact lines on the front of most cells today. Clearly, using less paste is a good thing. But the contacts are expensive in another way: by taking up real estate on the front of the wafer that would otherwise be harvesting light energy, they also cost efficiency.
(It would be great to remove the front side lines entirely, and there are cell architectures that do just that, moving all the contacts to the back side of the wafer. However, today, the extra manufacturing complexity prevents them from being economically viable in most cases.)
Sadly, you can’t just print thinner lines: thinner lines have higher resistance and they’re difficult to print, which can result in broken lines, called “interrupts” which result in lower efficiency cells that will probably be scrapped.
What you can do, though, is print narrower, taller lines by a technique called “double printing,” where a second set of lines is printed on top of the first. This isn’t an easy thing to do: it requires precise alignment of the wafer and printing head to ensure the second set of lines is exactly on top of the first.
But with a sophisticated machine vision system and some hefty computer processing, this can be done quickly and accurately enough to keep up with a screen printer that can spit out a finished cell every two seconds. This allows users to double print lines just 50μm wide with much better repeatability, leading to better yield. In addition, double printing reduces paste consumption by 20% and boosts absolute cell efficiency by as much as 0.2% (e.g. from 17% to 17.2% efficiency). That might not sound like a lot, but combined, these benefits can add up to a gross profit increase of a whopping 15%.
The proof is in the field: three of the world’s top ten module manufacturers are converting their production lines to add double printing. Pretty soon, double-printed lines will be everywhere.
We predict double-printed lines will be everywhere in the near future, helping to fulfill the increasingly-important need for low-cost, rapidly implemented techniques that offer an immediate boost to the bottom line.
Dr. Giorgio Cellere is a Research and Development Manager for Baccini Cell Systems at Applied Materials. He previously worked on developing biochips and electronics for the space and satellite industry at Padua University where he received his Ms.D. and Ph.D. He has received several awards for advanced scientific research, and has been widely published in leading scientific journals and books. He is a senior member of the IEEE.