IMEC Reaches 18.4% Efficiency With Cu-Plated Contacts
IMEC has developed a large-area solar cell with a conversion efficiency of 18.4%. Significant factors in the development include a shallow emitter and copper front metallization, which offers a more sustainable solution.
Aaron Hand, Executive Editor -- PV Society, 9/21/2009
IMEC (Leuven, Belgium) unveiled at the European Photovoltaic Solar Energy Conference (PVSEC) in Hamburg, Germany, today a large-area solar cell with a conversion efficiency of 18.4%, compared with previous best results of 17.5-18%. Significant factors in the development include the shallow emitter compared with a standard i-PERC cell process, as well as the copper plating used in the front metallization rather than silver.
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IMEC's i-PERC cell with shallow emitter and copper metallization has reached a conversion efficiency of 18.4%. |
Classical emitters are relatively deep and highly doped, causing the current to decrease with a loss in minority carriers, explained Jef Poortmans, IMEC's photovoltaics program director. This is particularly the case with blue light, which is absorbed primarily in the first micron of silicon. "So this means if you lose a lot of these carriers because of recombination, you have a bad response in the blue part of the spectrum."
The shallow emitter, on the other hand, enables the reduction of recombination losses, thereby increasing the current output of the cell. In combination with the current output, IMEC was also able to obtain a very good open-circuit voltage (VOC). "It was above 650 mV, meaning that we had a quite good surface passivation, in combination of course with the rear side, which was also very well passivated," Poortmans said.
The IMEC researchers were able to achieve their efficiency results copper front contacts, which is significant in terms of sustainability. "If PV continues to grow as it has been doing in previous years," Poortmans said, "then at a certain moment in time you might start to have supply issues with silver front contacts. And then it makes a lot of sense to have copper solutions which are more sustainable from a point of view of supply, and which can, in comparison with the classical silver contacts, also provide you with low line resistance and contact resistance — if you do it in the right way." Although a similar conversion efficiency could be obtained with screen-printed silver contacts, the key benefit to copper is its long-term sustainability.
The 18.4% results that IMEC has been able to achieve with copper contacts was a first result, obtained without any optimization, meaning there is likely room for improvement, Poortmans said. "So this is just the start, which could bring us much further going into the direction of 19.5-20%."
Also, the fact that the results were obtained on large-area cells (125 cm2) proves that the process is viable at industrial scales. Although in principle crystalline silicon solar cells have been able to reach efficiencies as high as 25%, going to larger sizes generally means losing in terms of fill factor and VOC, Poortmans explained. "So, in that sense, obtaining such an efficiency on a large area is definitely not that straightforward," he said. "Because in principle there is often a difference between large-area and small-area cells of 3-4%."
The next step will be to scale up the copper plating to the point where a few thousand wafers per hour can be processed. This will be a challenge ultimately for the equipment developers, Poortmans said, but IMEC's latest achievement shows that it can work, and can deliver high efficiencies.
