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Standard solar cells vs. next-gen cells: which is winning? Overcapacity is stalling the rate of improvement, but only for so long.

IN MANY MANUFACTURING decisions the question is whether to pursue the higher-tech or the lower-cost option. Build it better, or make it cheaper?

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The answer is largely determined by what the market will bear, of course. Such is the case with the current photovoltaic (PV) manufacturing market. Overcapacity and cuts to feed-in-tariff (FIT) rates have dramatically impacted the solar market in the past couple of years, resulting in a decrease in demand and, therefore, a lowering of module--and, by default, cell--prices. And, while manufacturing lower-cost cells--what I refer to as standard cells--is winning the current high-tech vs. low-cost battle, steady continuous technology improvements are still reaping gains.

For example, 17.5% conversion efficiency is now the norm for multi-crystalline solar cells, with over 18% efficiency possible if certain improvements are implemented. For mono-crystalline cells, the average efficiency sits at 18.5%, with the possibility of 19%. There are many factors for the average 0.5% potential efficiency gains with each type of silicon cell. These factors include the quality of the poly silicon, incorporation of lightly doped emitters, the reduced contact resistance and enhanced conductivity in metallization pastes, printer accuracy, novel print techniques such as dual print and print-on-print, as well as advances in screens technology for finer-line printing. Any one or more of these added onto a standard solar cell are what I call "standard extra" cell manufacturing.

Beyond standard and standard extra cell technologies, there are also next-generation solar cells. Next-generation cell concepts are effectively different cell architectures, so require alternative manufacturing processes and capital investment. These include Metal Wrap Through (MWT) and Emitter Wrap Through (EWT), which are two classifications of back contact cells. With MWT, holes are drilled in the wafers and conduct current from the front side of the cell to the back side of the cell. EWT cells also incorporate laser drilled holes, but far more than that of MWT and the holes are doped simultaneously with the front side of the cell, which then enables conductivity from front to back. While there is certainly evidence that both MWT and EWT can offer incremental efficiency gains (approximately 0.4% and 1%, respectively) over traditional standard cell architectures, there are also large investment considerations alongside complete process changes.

Due to current solar market conditions, next-generation higher efficiency cell projects are being sidelined in favor of standard or standard extra cells. As one can imagine, it's quite difficult for manufacturers to plan ROI on new capital expenditures when cell price forecasting is highly unpredictable. For that reason, the majority of solar cell manufacturers are maintaining current standard cell designs, while refining the process to make it more cost-effective. And, there are a smaller percentage of solar cell production firms that are implementing standard extra techniques such as selective emitter, fine-line screen printing, dual print and print-on-print to realize the extra 0.5% possible efficiency gains mentioned previously. Even though the market is challenging at the moment, research and development departments are still evaluating these processes, and, in fact, our company has high-volume production trials underway. I firmly believe that when the solar market rebounds, which many predict will be in the next 12 to 18 months, we will see a significant rise in implementation of standard extra technologies. Until then, standard cells will maintain their dominance.

That's not bad news, particularly for those of us involved in screen printing solutions for the solar market. Screen printing continues to be the most widely implemented and cost-effective form of metallized paste deposition for standard cells and is also the key enabler for many of the standard extra technologies mentioned previously. In fact, both traditional and advanced screen printing methods have been and will remain the paste transfer techniques of choice by a wide margin well into the future. This is, quite simply, because screen printing offers the throughput, yield, accuracy and reliability not matched by other methods. Indeed, this is good news for us and the marketplace, as even greater-efficiency screen printing innovation developments are occurring today and will no doubt be adopted as solar market conditions continue to improve.

So, standard solar cells are winning out for now. But next-generation cell architectures are proven technologies, and the sun certainly hasn't set on them either. The next few years will be interesting, to say the least.

TOM FALCON is a senior process development specialist at DEK Solar (dek.com): tfalcon@dek.com. His column runs bimonthly.
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Title Annotation:SOLAR TECHNOLOGIES
Author:Falcon, Tom
Publication:Printed Circuit Design & Fab
Date:Jun 1, 2013
Words:740
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