Backside PassivationThe photovoltaic effect relies on light gener-ating free electrons from the photovoltaicmaterial. Any loss of these electrons on theirpath to the electrical contacts reduces theefficiency of the device. Surface recombina-tion, where electrons can combine with dan-gling bonds at the silicon surface, is one suchmechanism and becomes more important toreduce as devices get thinner. Passivation ofthe surfaces ties up all the dangling bondsand provides a more neutral electrical sur-face to minimize electrical recombination.

One material being studied as a passivationlayer is aluminum oxide deposited by chem-ical vapor deposition (CVD) using tri-methylaluminum (TMA). Results show that thisprocess can lead to a one percent improve-ment in cell efficiency. TMA is routinelyused for LED manufacture, and so the han-dling challenges are known but will be newto PV manufacturers.

N-Type WafersTraditionally p-type wafers as used by thesemiconductor industry have been the sub-

strate used for crystalline silicon (Si) solarcells. With the solar wafer market becomingimportant in its own right, manufacturers arenow looking at what sort of wafers would bebest for PV devices. P-type wafers are bulkdoped with boron, but boron forms defectswith oxygen present in the bulk materialwhen exposed to light (a process known aslight induced degradation) causing the effi-ciency of the cell to drop over time. Borondoped devices are also more sensitive tometal impurities, which can also causerecombination. N-type wafers, which arebulk doped with phosphorus, do not exhibitthe same light induced degradation and, dueto the difference in carrier type (holes ratherthan electrons), there is much less recombina-tion due to metal impurities. The use of n-type wafers can lead to an increase in cellefficiency of up to one percent.

N-type wafers require the deposition of ap-type emitter, and boron tribromide (BBr3)

June 2012 This report is reproduced with the permission of CryoGas International

New Challenges in Photovoltaic ManufacturingBy Andreas Weisheit, Jean-Charles Cigal, and Greg Shuttleworth of Linde Electronics

Innovative gas technologies from Linde help PV manufacturers increase their solar cell efficiency.

Manufacturers of photovoltaic (PV) devices are under constant pressure to improve the costper watt in order to drive market growth and compete with other forms of energy. Reducingmanufacturing costs has enabled significant market growth, but continuing with thisapproach alone will meet with diminishing returns and profitability for manufacturers.Increasing the number of watts from the same device will also reduce the cost per watt, sothere is a lot of recent attention on increasing efficiency. New materials and new applica-tions are required to achieve these gains and this article describes some of the approachesbeing taken.

This report was originally published in the June 2012 issue of CryoGas International (www.cryogas.com).

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New Challenges in Photovoltaic Manufacturing

is one material currently being studied as theboron source. This has similar properties tothe phosphorus oxychloride (POCl3) tradi-tionally used for the emitter with p-typewafers, and so it is only a material changerather than a material and process typechange. Equipment manufacturers alreadyoffer the possibility to use either BBr3 orPOCl3 with the same equipment, without orwith only minor hardware modifications.

Dry ProcessingLarge tanks of wet chemicals have tradition-ally been used for cleaning, etching, textur-ing, and saw damage (from the wire sawingprocess used to slice silicon ingots intowafers) removal processes. The semiconduc-tor industry has shown that dry processing,particularly as device dimensions reduce,offers more control and uses significantlyless chemical materials, which makes safetreatment and disposal less of an environ-mental concern. As wafer thicknessdecreases, wafer handling in chemical bathsbecomes more problematic. Dry processingsolutions already exist for thin single wafermechanical handling from the chip packag-ing industry.

Wet processes also affect both sides of thewafer—as the devices become more com-plex, single-sided processing may berequired. Thus companies are beginning toexplore replacing wet processing with dry,and some have already implemented fluori-nated gases for the texturing process. Clearlythis is a significant change in both the processequipment and the materials required to man-ufacture PV devices, and, while this may notlead to a reduction in cost per watt in the cur-rent generation of devices, it is seen as anenabling technique for future high perform-ance devices.

Selective EmitterTraditional devices contain a uniform emitterlayer on which contacts are deposited. Here,a balance needs to be reached between thehigh conductivity required at the contacts andthe low conductivity that is ideal elsewhere.By selectively heavily doping only the areawhere the contacts will go, the cell efficiencycan be increased by up to one percent. Thisdoes require a more complex process, andthere are a range of techniques under investi-gation, including printing, laser treatment, orion implantation.

Opportunities for Material SuppliersWhile reducing manufacturing costs hasenabled PV devices to reach a wider marketand get closer to grid parity, a higher cell effi-ciency will also help lower the cost per watt.A wide range of new techniques to improveefficiencies are under investigation. Materialchanges and handling challenges are requiredfor some of these, while for others, new tech-niques or a complete change in process tech-nique is needed. Not all of the approachesdescribed above may make it to mass produc-tion, however, the challenges of developingand delivering the required materials and theapplication know-how to better support theimplementation of these processes providemany opportunities for material suppliers. ■

Andreas Weisheit is Head of Global PV atLinde; Jean-Charles Cigal is OEM Pro-gram Manager at Linde; and Greg Shuttle-worth is Equipment Product Manager atLinde. For more information on Linde Elec-tronics’ innovative gas and chemical tech-nologies for PV manufacturing, visitwww.linde-gas.com/photovoltaics.

June 2012 This report is reproduced with the permission of CryoGas International

Linde is committed to developing new gas technologies to improve PV efficiencies.

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