Miniaturization Redux Print E-mail
User Rating: / 0
PoorBest 
Written by Tom Falcon   
Saturday, 05 March 2011 00:21

Like SMT, solar is benefitting from – and challenged by – smaller conductor widths.

Much of the experience gained from SMT experience has proven invaluable for the solar industry as we have worked to reduce the width of the front-side conductor grids that we typically screen-print onto the front side of solar cells.

There are two very good reasons for wanting to do this. The first is, as they are made of silver paste, the conductors prevent sunlight from reaching the light energy-converting strata below. The second is that the more widespread they are, the more of the cell’s converted energy will be harvested. The solution is to provide adequate coverage by printing numerous conductors fine enough to permit the sun to play its part in the energy cycle. Materials, tools, equipment and techniques have accordingly been developed that reduce conductor widths to a current industry standard of between 80 and 120 µm. Some higher end products take this still further with 60 µm-wide features, and 40 µm capabilities are firmly on the radar for a number of solar manufacturing pioneers.

Now, the selective emitter process, described in our last column (December 2010, http://circuitsassembly.com/cms/component/content/article/159/10708-solar-technologies), and which is due to impact this year, places an imperative on such ultra-fine features. This is because, while it improves the solar cell’s reactivity to light at the blue end of the spectrum, selective emitter also increases the resistance between the conductors (sheet resistance) and therefore makes it more difficult for the energy released within the wafer to reach the grid. To mitigate this loss the conductors must be printed at higher densities, which potentially means more shading, and added silver costs – unless conductor widths are reduced.

Another process under development that may well drive the need for miniaturization is the seed and plate (S&P) technique, whereby a very fine (30-40 µm) silver seed line is printed onto the wafer, then overplated using silver in an electrolytic process. In the Fraunhofer Institute’s Light-Induced Plating process, this second pass uses light to activate the cell, which in turn drives the plating process.1 The advantage of S&P is that the superior conductivity of the plated silver combines with the connectivity of the seed line to offer much improved cell performance. The downside is that it adds a further and somewhat complex manufacturing process to the solar cell manufacturing line.

As the electronics industry knows well, one of the keys to ultra fine-line printing is the print screen itself. At DEK, we are studying a special two-stage high-aspect-ratio metallization process developed by the Netherlands-based Energy Research Centre (ECN) that uses stencils for the production of ultra-fine conductors.
A great deal of work is also ongoing to manufacture finer wires for screen manufacture. Standard solar industry print screens are currently built using 20-25 µm wires, while top-end commercial production may use finer 18 µm products. This looks poised to drop still further to a tiny 11 µm thanks to special alloys now under development – an achievement that will yield the finest screens yet, with over 800 wires per inch. In parallel with these advances and in order to get the most from them, ultra-fine screen printing emulsions incorporating new chemistries and finer particulate must, and are, being designed to improve resolution capabilities to well below the current 40 µm limit.

Paste vendors, too, have done a remarkable job enabling cell manufacturers to print the narrower, higher-aspect ratio conductors fast entering the industry’s mainstream. As features continue to shrink, however, they face the added challenge of increased conductor contact resistance. This is because, unlike a soldered joint, the conductor/wafer interface lacks an intermetallic phase – so apart from a few scattered crystals that penetrate slightly into the wafer surface, standard silver paste tends to sit on top of the silicon wafer. This creates a less-than-optimum electrical contact, which may get in the way of the flow of electrons from the wafer to the conductors. As features become finer, this contact is reduced further, making the energy harvest even more of a challenge. Recognizing this, paste vendors have already developed a number of products designed specially to optimize electrical contact, and work is ongoing for the future.

It is easy to see from this brief outline that solar cell manufacture includes an onerous task: that of maintaining a delicate balance between several competing and interdependent factors – a balance that becomes increasingly critical as feature size decreases. Ongoing developments in higher throughput, high-accuracy printing equipment and screen printing materials and tools promise to introduce a paradigm shift in high-end screen printing, reconfirming its status as the technology of choice for solar cell metallization, and taking the capabilities for ultra-fine screen printing well into the future. This will widen the process window for solar cell manufacturers and ease their task, even as the solar industry follows its roadmap to improved efficiencies through miniaturization.

References

1.  A. Mette, C.Schetter, D. Wissen, S. Lust, S. W. Glunz and G. Willeke, “Increasing the Efficiency of Screen-Printed Silicon Solar Cells by Light-Induced Silver Plating,” 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, May 7-12, 2006.

Tom Falcon is a senior process development specialist at DEK Solar (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 07 March 2011 15:59
 

Columns

Eastern-US: China’s New Competitor?

Parity emerges among EMS Factories from Asia, Mexico and the US.

For the first time in years we see parity in the Eastern US among EMS factories from Asia, Mexico and the US. This EMS market condition will permit American OEMs (the EMS industry refers to OEMs as customers) to have more EMS pathways to choose from. Now more than ever, such EMS assignments will require deeper investigation relating to the OEMs’ evaluation of manufacturing strategies.

Read more...
 
The Human Touch

For those who count on the electronics industry for big feats, it’s been a remarkable couple of years.

Read more...
 

Features

Advances in Concentration Monitoring and Closed-Loop Control

Contaminated bath water skews refractive index results. New technology can accurately measure aqueous cleaning agent concentration.

Read more...
 
Circuits Disassembly: Materials Characterization and Failure Analysis

A systematic approach to nonconventional methods of encapsulant removal.

Read more...
 

Search

Search

Login

CB Login

Language

Language

English French German Italian Portuguese Russian Spanish
 

Products

Panasonic Debuts PanaCIM Maintenance with Augmented Reality
PanaCIM Maintenance with Augmented Reality software provides instant communication and information to factory technicians -- when and where it is needed -- so they can respond to factory needs more...