Cell manufacturers worldwide, are in the process of transitioning to advanced cell structures in order to boost cell efficiencies and drive down the cost per watt. The photovoltaic (PV) learning curve has traditionally seen a 20% decline in module prices for every doubling of total installed modules. While this rate of reduction has primarily been driven by scale, we are now at a point where cell efficiency will become crucial in accelerating cost reduction.

Traditional screen-printed cells have been around since the mid-1970s. The scaling has allowed much of the cost reduction to occur in what we call conventional cells, and several years ago the optimization of line-widths on the front of the cell was improved by double printing, by being able to make narrower lines that stand up taller, and minimize how much light is blocked.

Today’s technology for crystalline silicon (c-Si) efficiency improvement — selective emitter (SE), allows for the tuning of the field emitter to achieve higher cell current and voltage. How is this achieved? The diagram below shows how we are able to separate the cell grid contacting requirement from the constrained field emitter design by using selective emitter. By tuning the field emitter to a lower surface dopant concentration and reducing the junction depth, we are able to increase cell current and voltage. The SE emitter surface area needs to be controlled to a small area at the front of the cell in order to lower light absorption and reduce surface recombination. This combined allows for a low cost and effective solar cell efficiency gain without large changes to the solar cell production process.

There are several approaches in the market today for producing selective emitter cells namely screen printing, laser doping, etch back and ion implant. However, the winning solution must have the following characteristics:

• easy to implement and compatible with existing equipment

• stable and repeatable with high process yields

• maximum efficiency gain on different wafers (both mono and multi)

• lowest total cost of ownership

We believe that out of these approaches; screen printing offers the most proven, straightforward and least risky path to implementation as has been demonstrated by several of our customers.

Why Screen Printing?

Screen printing is a production-proven technology. It's low risk, quick and easy to implement, using equipment that is already present in large numbers in conventional lines, which requires just one additional step. This additional step involves screen printing a dopant line on the wafer followed by diffusion. The only hardware required to implement screen printed selective emitter technology is an additional printer and dryer. The total cost of ownership for the additional step is among the lowest on the market with a low capital impact and the solution serves both mono- and multi-crystalline structures. Lastly, there is consensus that scaling this technology could bring an additional competitive advantage versus alternative solutions that could potentially provide a slightly lower TCO, although with a significantly lower gain in terms of efficiency.

Applied Materials’ Baccini Cell Systems division has been the market and technology leader in screen printing for several decades. Baccini platforms are ideally suited for screen printing the selective emitter paste as well as for metallization. A common platform for both selective emitter formation and metallization reduces variables for optimum cell performance and binning resulting in a tighter yield, and higher performance. Further, customers benefit from commonality of spare parts and services and from the familiarity that operators and technicians have with the platform having used it extensively for standard cell metallization.

When it comes to precision printing based on advanced pattern recognition, Esatto Technology from Baccini comprises advanced vision systems and software algorithms that enable detection of the difficult to see SE pattern for accurate alignment and printing. Esatto is capable of providing high accuracy metallization on top of any SE pattern, regardless of the SE formation method (and not just screen printing) and is available as an upgrade for existing Baccini printers.

Accurate alignment is essential to achieve the maximum benefit from differentiated emitter profiles; for this reason Esatto Technology’s pattern based alignment for metallization provides the following benefits to customers:

• robust and repeatable metal deposition with tight process distribution and higher average solar cell efficiency

• savings from eliminating the need for high precision on the previous SE formation process step (a process drift can be tolerated and would not adversely affect cell performance)

• possibility to optimize the SE formation step by reducing the width of the heavily doped regions which should be minimized ideally only to the area right below the metal finger (further enhancement of cell efficiency)

In conclusion, screen printing is the proven, low risk, fast path to SE implementation. And Applied Materials offers both equipment as well as service solutions for the implementation of this technology.

To learn more about selective emitter technology, the various paths to implementation and why screen printing is the only way to go visit our web site and listen to the replay of a recent web cast with key Applied Materials experts.