Improving Solar Conversion Efficiency Print E-mail
User Rating: / 1
PoorBest 
Written by Tom Falcon   
Monday, 09 May 2011 16:21

Greater photovoltaic efficiency may require a step back.

With the growth of the photovoltaic market in the past decade have come significant advances in manufacturing techniques to improve cell efficiency. For traditional crystalline-silicon (c-Si) cell structures, developments in novel cell architectures and process alterations have yielded big efficiency gains. In fact, last summer, SunPower Corp. announced it had reached an industry milestone of 24.2% conversion efficiency for large area silicon wafers1 – a percentage that significantly surpasses that of traditional c-Si cells, which produce somewhere in the range of 15% to 18% efficiency.

But, in many cases, higher efficiency cells may mean higher production costs. There is most certainly a balance (and arguments both ways, to be sure!) between greater efficiency cells and more cost-efficient production techniques. For cell metallization, line speed, accuracy and repeatability are key to delivery of greater throughput (and, consequently, lower cost) processes. In addition to lowering costs through high wafer per hour (wph) processing capability, advances in metallization techniques have also delivered notable efficiency improvements through novel technology developments such as print-on-print, for example.

The print-on-print technique has been addressed in this space previously, but I’ll describe it here again briefly. Conventional conductor patterns (H-pattern collector fingers and busbars) cause shadowing on the front side of the c-Si cell – an effect that may block light from reaching as much as 7% to 9% of the cell. So, there is a delicate balance between having enough printed conductors to collect all the current, but not so many covering the silicon surface that they prevent sunlight absorption. Print-on-print technology uses specialized screens in combination with proven high-volume, lower cost mainstream print processes to improve the aspect ratio of printed silver conductors. Higher, narrower collectors effectively reduce the shadowing impact and can improve cell efficiency by 0.2% to 0.5%, significant in the photovoltaic market.

That said, for more widespread adoption of solar technology, efficiencies must be greater still, and at a cost per watt price point palatable to the average consumer. In addition to technologies such as print-on-print, another way of improving conversion efficiency and mitigating the efficiency losses associated with the traditional c-Si cell front-side metallization is to remove as much metal as you can from the front side and put it on the backside in what is known as a back contact solar cell.
The back contact solar cell is not new. In fact, the technology has been in existence for well over 30 years. But, the indisputable need for higher-efficiency alternative energy sources has spurred further development of back contact cells and here, as with traditional cell structures, print processes are delivering the speed, throughput and accuracy required for high-volume, low-cost production (more on that later). Just as with conventional c-Si cells, there are a multitude of approaches and cell architectures for back contact cells. There are interdigitated, metal wrap through (MWT) and emitter wrap through (EWT) back contact cells – all with pros and cons from a production, cost and efficiency standpoint. Arguably, though, MWT and EWT have emerged as the go-forward technologies, at least for now.

The MWT back contact cell basically just removes the busbars (the wider lines used for connecting the cells together) from the front side of the cell. The current collecting conductors are still present on the front side and feed into the laser-drilled holes that then carry current through the cell to the backside. The collector patterns can be complex – often similar to a floral design – and precise deposition of the silver paste and alignment to the through hole is critical. This is where the precision of the print process is essential. In some MWT designs, there is a requirement to pull the conductive paste through the hole in the cell to form a conductive path, much like a pin-in-paste process. For this step, the print system requires additional vacuum capabilities – a technology our company has been developing for some time.

Developers of MWT technology have not only evaluated improvements in cell efficiency, but have also analyzed simplification and cost reductions for module manufacturing. Interestingly enough, the MWT module assembly process is much like the surface mount technology (SMT) process and offers similar throughput advantages as well. A back contact sheet, basically a simplified printed circuit board, is printed with a conductive adhesive, and an encapsulant layer (EVA) is placed on top of the back contact sheet. Cells are then placed via a pick-and-place machine onto the EVA layer; a second layer of EVA is placed on top, and the module is laminated2. Like SMT assembly, the MWT back contact module buildup process is highly automated, which enables very high-volume production and precise handling of the delicate cells.

Moving contact points to the back of the cell, however, isn’t the only area where efficiencies can be gained with MWT: Combining multiple available technologies could improve conversion efficiency exponentially. For example, print-on-print processes are also viable with MWT cells, making the collection fingers taller and narrower and achieving similar improvements in cell efficiency. Add to this selective emitter technology (reference previous column, “Which Way for Selective Emitter,” January 2011, Circuits Assembly), which provides for different levels of phosphorus doping underneath and between the collector fingers, and the MWT cell becomes even more appealing from an efficiency point of view. I’m not suggesting that merging all these available technologies would be an easy feat, as this would be challenging. But, with highly accurate and repeatable metallization systems capable of precise deposition (exactly on the highly doped collector areas for selective emitter and then again for print-on-print), this is certainly achievable.

I must admit that I believe MWT is really an intermediate stage on the way to EWT, which is more difficult but more rewarding technically. Conceptually, EWT is similar to MWT in that there are multiple through holes in the wafer, and cell connection occurs on the backside only. But, with EWT, there are many, many times more holes in the cell than with MWT. Just to give you a sense for how many holes there are, I recently went to an institute in Germany where I saw an EWT wafer with 60,000 holes in it – all done in the span of about 2.5 sec. on a laser machine! With EWT, the phosphorus doping is not only done on the top side, it is also done through the holes as well. So, similar to other cell types, the bulk layer of the EWT cell is still p-type silicon and then the phosphorus, or n-type, is on the top surface and through the holes. Then, all that is required is printing of the two different metallizations – silver for the n-type and aluminum for the p-type. The rear side of an EWT cell has two sets of gridlines. One set is for the silver, which makes contact with the wrapped through emitter, and the second set is for the aluminum, which makes contact with the p-type bulk. Paste deposition accuracy is critical, as space between the gridlines is minimal and any cross-contamination would result in a local short-circuit, thus reducing overall efficiency. Because the phosphorus doping of the hole provides the conductivity, no metal is required on the top surface of the cell, leaving all of the area unoccupied by holes available for capturing sunlight. As one can imagine, this has a profound impact on conversion efficiency, with EWT cells producing efficiencies in excess of 19% for screen-printed EWT cells. Medium-term roadmaps forecast over 20% cell efficiency for rear-passivated EWT cells.

The winning back contact technology remains to be determined. While I’m of the opinion that ultimately EWT will win out, it is a bit harder to produce at the moment, and MWT is further down the line in terms of its development and ability to be easily implemented from a manufacturing point of view. Time will tell. What is clear, however, is that whether it’s traditional front side cells or backside cells, high wph metallization systems will be required to accommodate the accuracy and speed needed for high-volume, lower cost production.

References
1. “SunPower Announces New World Record Solar Cell Efficiency,” June 23, 2010, sunpowercorp.com.
2. T. Adcock and A. Henckens, “Lower Cost, Greater Efficiency Drive Development of Back-contact Solar Modules,” PV World, May/June 2011.

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, 09 May 2011 17:12
 

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

Yincae Rolls Out ACP120 Solderable Coating
ACP 120 series anti-oxidation solderable coatings replace nanofilm gold. Are for anti-corrosion and anti-rusting on solderable surfaces on which solder paste, solder preform or a solder ball can be...