Silicon photovoltaics (PV) are one of the most important markets globally for printed electronics. This is because each wafer carriers a small of fired screen printed silver paste. Indeed, this could be the largest market worldwide.
The chart below- from the indistry roadmap ITRPV 2022- shows the amount of Ag metallization utilized per wafer (both front and back metallization) per watt depending on the type of silicon photovoltaic (monofacial p-type, TOPCon n-type, HJT n-type, etc). It shows that today something around 25-30 tonnes is used per GW of solar for HJT n-type PV and around 12-14 tonnes for monofacial and bifacial types. Considering the size of the PV market, this translates into a 100+ tpa market!
As shown in the charts below, screen printing remains the prevelant technology for metallization, despite long-term attempts by other technologies to make even a small dent into this space. In the long term future, other technologies such as plating on seed layer or stencil printing are expected to obtain a small foothold, although we have heard this story too many times already.
There are various screen printing techniques. Single print and dual print (finger and bus bar printed seperately in two seperate steps) are the most common techniques. Double printing (print a second layer on an already screen printed already for better aspect ratio) is also popular. The advantage of dual print is that different paste types could be used for fingers and bus bars, giving optimal results.
There is of course always a trend to narrow the linewidht of the screen printed lines, whilst maintaing high aspect ratio, excellent ohmic contact, and high conductivity. This has been the direction of development for years. Today, the state of practice in production is a screen printed linewidth of around 34-35um. The industry expects this to evolve to a linewidth of 20um, which is very narrow for screen printing and would represents a real advancement of the art.
In the slide below, you can see an example from Fraunhofer ISE (2019) demonstrating a screen printed finger with a linewidth and height of 19um and 18um, respectively. This is, in my view, the state of the art, and requires close collaboration of all those involved from stainless mesh makers, to paste and particle manufacturers, to emulsion makers, and so on.
This is an incredibly important market for the printed electronics industry. Outside China, the main particle makers remain Dowa, Ames Goldsmith, Metalor, and Technic. There are many paste makers including Heraeus, DuPont, etc. Of course, given that the market is in China, the supply chain has also been moving there with Chinese suppliers rising in terms of market share as well as technology capabilities. Indeed, their powders and pastes are no longer significantly inferior to the state of the art.
To protect market share, others must evolve their particle/powder and paste technology so that it can sustain the roadmap towards ever narrow printed linewidths without a loss in efficiency. This is one of the guiding principles directing technology development.
Finally, Fraunhofer ISE publishes an excellent and very detailed annual report on the state of the global photovoltaic industry. As seen below, global production is already a staggering 140+ GW/year with 82% being produced in Asia.
To support the scale of this industry, any metallization technology requires to have excellent throughput. The ITRPV 2022 roadmap also outlines the throughput step for the backend steps. It shows that screen printing machines today handle something around 7000 wafers per hour (180 x 182 mm2). This is expected to rise to over 9000 wafers per hour in a decade. This is included here to show the scale of the challenge faced by alternative processes including non-contact technologies such as inkjet.
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