Photovoltaics Supplement

Harnessing sustainable energy. A look into the field of photovoltaic and solar cell energy production in the use of electronics manufacturing. With contributions from IMEC, Rena Sondermaschinen and Roth & Rau.

What the world needs now

Photovoltaic technology has been with us since the mid 20th century but development progress has always been limited as it has been difficult to manufacture at a rate that provides easy profits like oil has for so long. Since then the world has come to understand that the reliance of oil based fuels has a consequence that needs to be responded to. Hence there has been an increase in development in photovoltaics, as a prime candidate to pick up the slack when oil is reduced.

Despite the promise there is still a way to go for solar cells to go if they are to be competitive or reliable enough for everyday use but the concerted effort is seeing new developments in leaps and bounds.

In fact the growth in manufacturing of photovoltaic devices is so great that there is a potential lack of silicon in the near future. Possibly in 2007. In using the same substrate as semiconductors, the photovoltaic (PV) growth has provided many companies with new opportunities for their services, tools and processes. Europe is crrently leading the world in PV development providing many opportunities in ensuring the right tools and processes are available in this region.

To achieve the quality required to provide more energy than manufacturing requires means a concerted effort in research and Europe is doing very well there. The article from IMEC demonstrates their co-operative capability. A number of companies are becoming world leaders in their niche after changing focus from semiconductor companies.

Researchers are having to surpass physical expectations of PV devices to meet the exacting demands of energy users. As with semiconductor engineers before them, it will be amazing to watch the constant pushing of the envelope until photovoltaics deliver on the promise of many years.
David Ridsdale
Editor-in-Chief

RENA presents a solution in solar cell production

The world is patiently waiting for alternative energy sources to offset the damages caused by fossil fuels. While many people imagine photovoltaics will provide all the answers, those in the know understand that there is more work required in efficiency and manufacturing. German company RENA presents results of their work in improving the manufacturability of photovoltaics.

Newly developed fully automated InOxSide inline process line integrates edge isolation with phosphor glass etching and offering solar cell producers decisive cost and quality advantages. This leads to reduced breakage due to simplified handling.

Pure silicon is a raw material that is becoming scarcer and more expensive worldwide. Our analysis shows that the market requires a powerful process automation solution, particularly for edge and phosphor glass etching, in order to be able to effectively meet rising demands for more efficient solar cell production. This necessitates rethinking the entire process chain. Previous production processes required individual and separate process steps, for which process tasks such as saw damage etching and texturing of the wafers took place via automated roller transport. The process step of edge isolation has repeatedly proven impossible to solve via roller transport. After diffusion, each individual solar cells had to be removed, transported, stacked and fed to an equipment plant. The edge etching with plasma, carried out in individual batches, also takes place under mechanical pressure, representing an additional potential source of defects – and therefore rejects. Consequentially each interruption of the production chain not only means a high handling effort – and more costs – but also high breakage rates.

The solution
In close collaboration with our customers we have achieved a completely new solution for edge isolation. Whereby the cells are transported horizontally on rollers in parallel processing lines. During the edge etching process step the entire wafer bottom, as well as the sides, are etched in a chemical bath – until the approx. 1 µm thick ndoped silicon layer has been completely removed. Only in this way can electrical insulation be ensured between the n-doped and p-doped sides of the cell. A central requirement hereby is that the diffused upper surface of the solar wafer, which is only approx. 200 µm thick, cannot come into contact with the etching liquid in the bath. As a result of the precise wafer transport, and physically effects, the wafer's upper surface remains dry while the bottom side is etched. Thus RENA can guarantee that the emitter is not damaged.

The benefits
With this solution, RENA has succeeded in automating the important process step of edge isolation using wet-chemical inline processing – and thus considerably minimised the transport effort between the process steps of phosphor glass etching and the subsequent edge etching process. Combining edge etching with the oxide process in one inline equipment plant reduces the breakage rate thanks to simplified handling, and allows optimum integration of the process in the complete production line. The process, developed in just 8 months and awarded the Innovation Prize by the state of Baden-Württemberg, sets a new technological standard in solar cell production.

For cell producers it opens up the possibility of exploiting important cost savings and rationalisation advantages.

Overview of advantages

• Reduced breakage thanks to fully automated cell production (no stacking and manual wafer handling)
• Integration of two steps
• Greater efficiency
• Improved cost of ownership
• Reproducible process results, greater throughput and assured product quality

InOxSide technical data
Edge Isolation & Phosphor glass etching

• 8-lane equipment for 156 mm wafers
• Gross throughput: up to 3,000/h
  (156 mm wafers)
• Applicable for very thin wafers
  (from 300 µm > 120 µm)
• Length of process line: 7350 mm
• Depth of process line: 2450 mm
• Complete process and equipment development

A sunnier side to sustainable energy sources

One of the greatest challenges for the global economy is energy and the search for alternatives to reduce the reliance on coal and gas. Photovoltaics provide a promising avenue for energy. Jeff Poortmans, G. Beaucarne, T. Aernouts, G. Flamand of IMEC discuss recent research advances in photovoltaics

A applicable to all energy field scenarios for the coming decades, leading institutions such as (White Paper European Commission), energy agencies (i.e. International Energy Agency) and R&D-institutions have indicated that the part of renewable energy sources within future energy generation schemes is expected to grow. The number of solar cells being produced yearly (in terms of Watts produced by these solar cells under a standardised spectrum) has increased consistently in the last 15 years with growth rates between 15 and 60% as shown in Figure 1.

Photovoltaic energy generation will become a relevant part of the total energy generation after 2030 and, eventually, photovoltaic generation could be supplying 20-30% of the global electrical energy demand in 2100.

The present cost of a photovoltaic system is between 4 and 6 €/Watt, which in North-Western Europe translates in a cost of 0.5-0.6 Euro/KWh, whereas in Southern Europe and regions with an equivalent amount of sunshine, this cost can be divided by a factor of two [1]. Reducing the cost by a factor 4 to 5 can be done by economies of scale (larger plants) but will also require technological breakthroughs to reduce the cost of materials to manufacture cells and modules, reduction of energy input to realise these components and an increase of the energy conversion efficiency.

At IMEC this is translated into the four photovoltaic technologies the PV-Program is working on: thin crystalline Si solar cells, organic solar cells, high-efficiency photovoltaic stacks for terrestrial application and thermophotovoltaics for co-generation of electricity.

In parallel with the appearance of distributed electricity generation, one predicts a similar evolution for electronic systems, an evolution often described by the term "ambient intelligence". Most of these electronic systems contain a sensing part associated with data processing capability as well as RF-features for data communication. Within this vision, ensuring the energy autonomy of freestanding and (or) portable circuits, is a crucial task. It turns out that, even at low illumination levels of typically 0.1-1% of standard sunlight, photovoltaic cells are the most obvious means to ensure this required energy autonomy with lowest area or volume requirements [2]. For this purpose high-efficiency backside-contacted Si solar cells and flexible organic solar cells represent attractive solutions.

Crystalline Si solar cell
Crystalline Si solar cells have been and still are the workhorse of the photovoltaic industry with a market share of > 90% of the total world solar cell production in 2005. IMEC has developed an evolutionary roadmap which allows a gradual transition from thin crystalline Si solar cells towards monolithic thin-film crystalline Si solar cell modules on a low-cost carrier [3]. The development activities within the crystalline Si solar cell program are oriented towards the development of solar cell processes for very thin Si-substrates (< 200 Ìm down to 80 Ìm), advanced backside-contacted solar cell technologies, the implementation of industrial cell techniques for epitaxial thin-film crystalline Si solar cells and innovative approaches for thin-film polycrystalline Si solar cells which. All this should reduce the cost of crystalline Si solar cells by a factor of 3.

One of the IMEC-highlights of the last years is the development of a new industrial process perfectly tailored to thin crystalline Si substrates. This process is named the i-PERC process and relies on local rearside contacts and a dielectric passivation stack in between the contacts as shown in fig. 2a. All the steps are compatible with the high throughputs needed for industrial PVproduction. Using the i-PERC process an efficiency of 17.6% was recently obtained on thin large-area multicrystalline Si solar cells (thickness: 180 Ìm), which is the best ever for such thin cells with screenprinted contacts (see fig. 2b) [4].

IMEC is also developing a proprietary process for thin-film crystalline Si solar cells, epitaxially grown on very low-cost Si substrates based on metallurgical grade Si. The inclusion of the buried Bragg reflector based on porous Si (see fig. 3a and b) allows substantial current increases allowing large-area epitaxial cells with efficiencies between 13 and 14% [5].

Also very encouraging results were obtained in polycrystalline Si layers on ceramic and hightemperature glass. By an innovative emitter approach the effectiveness of the hydrogen passivation of the polycrystalline Si layer is increased which resulted in open-circuit voltages ≅540 mV, the highest ever reported for this type of material, and efficiencies near 8% [6].

Organic solar cells
Active layers of organic solar cells are typically in the order of 100 nm to several Ìm's. The low material consumption and the fact that the technologies to deposit these layers (printing) are compatible with extremely high production throughputs (1 to 2 orders of magnitude in comparison with the present solar cell technologies) could result in costs a factor 5 to 10 lower than the present solar cell technologies. One of the most promising concepts in the field of organic solar cells is that of the bulk donor/acceptor heterojunction [7]. Here, the active layer consists of an intimate mixture of two different conjugated organic materials sandwiched between metallic electrodes.

Investigations on this device concept at IMEC are focused along two main routes. The first one considers polymer-based organic solar cells in which the active layer can be processed from solution. Typically spin coating is used, though successful steps have already been made towards the introduction of printing technology like screen printing. Secondly, active organic layers can be deposited by vacuum evaporation if small conjugated molecules are considered. The upscaling of the evaporation of small conjugated molecules is pursued by means of the organic vapour-phase deposition (OVPD) technique.

On the route of polymer-based solar cells, a lot of effort has initially gone into the improvement and understanding of bulk donor-acceptor heterojunction cells based on blends of PPVpolymers (poly-para-phenylene vinylene) with a soluble derivative of C60. This resulted in efficiencies near 3.5% [8]. High-resolution electrical characterisation of such organic photovoltaic films is performed using novel techniques like EFM (Electrostatic Force Microscopy) and C-AFM (Conductive Atomic Force Microscopy), as shown in Fig. 4.

Gradually, the focus of the organic solar cell research has moved to P3HT (poly-3- hexylthiophene) as donor material because it has a higher absorption coefficient close to the maximum photon flux in the solar spectrum. Solar cells based on the P3HT:PCBM system normally require a short thermal anneal to increase their performance. The crucial parameter for the thermal anneal turned out to be the evaporation speed of the solvent [9]. In more slowly evaporating solvents the P3HT polymer segments remain mobile for a longer time during the film formation, allowing for the creation of crystalline P3HT-network. This was experimentally evidenced by testing solvents with a higher boiling temperature. Careful optimization of the solvent and the evaporation conditions of the solvent allowed to realise P3HT:PCBM solar cells with efficiencies of 4.5%, which is comparable to the best conversion efficiencies reported for polymerbased solar cells [10 ].

Besides single cell optimization and efficiency improvement, the development of a monolithic organic solar cell module is also under investigation. Screen printing was investigated for the deposition of the active layer of a bulk donoracceptor heterojunction solar cell using a PPVdonor and PCBM as acceptor material. This technique has the advantage that the active layer can be patterned and that material losses during deposition are lower as compared to spin coating. The versatility of this production process offers the possibility to use glass as well as flexible foils as substrates, as shown in Fig. 5. [11].

The second route that is strongly investigated is that of the small molecule based bulk heterojunction solar cells. Vacuum evaporation offers the possibility to control the growth and deposition of the photo-active layer in a more appropriate way by e.g. the evaporation rate of the different compounds and the substrate temperature. The technique can therefore result in model systems to study the potentials of the bulk heterojunction approach. By growing OPV5:C60 mixtures the performance was well optimized, resulting in an efficiency of about 2% [12].

One drawback of organic photoactive materials is their narrow absorption window compared to solar cells based on inorganic semiconductors. A possible way to extend the spectral sensitivity over a broader wavelength region is stacking different solar cells on top of each other. At first, the subcells were based on OPV5:C60 mixtures. By making a very short evaporation of a submonolayer of Au to serve as tunneljunction in between topand bottomcell, it was proven for the first time that two subcells based on these materials could be effectively connected, resulting in an open-circuit voltage higher than the one of the subcells. To optimise the current output there's a necessity to combine subcells with complementary absorption spectra in such multijunction devices. Other small molecule donor materials like pentacene are therefore also investigated. Organic vapour phase deposition (OVPD) is considered as a more costeffective alternative to the vacuum evaporation technique. A carrier gas transports the organic material in its vapour phase through a showerhead, requiring only a rather low vacuum environment. Thereby, it allows much faster, yet controlled, thin film growth of small molecular weight organic semiconductors. First experiences have been building up towards deposition of pentacene for organic transistor purposes [13]. This research is currently being extended to develop also organic solar cells.

Photovoltaic stacks for terrestrial concentrators
The primary goal of this activity is the development of an innovative technology to produce 4-terminal high-efficiency mechanical stacks capable of efficiencies up to 35%. This activity comprises the manufacturing of thin-film InGaP/GaAs topcells and Ge-bottomcells. Highlights are the realisation of an MOCVD-grown world record GaAs solar cell on Ge with an efficiency of 24.5% [14]. The InGaP/GaAs topcell requires a tunnel diode for current transfer from one cell to the other. A high bandgap material is preferred for this tunnel diode in order to minimise the optical losses; therefore we have chosen to realise this diode in AlxGa1- xAs. Using carbon doping for the p-type material (resulting in a 7.1019cm-3 carrier concentration), an excellent AlGaAs tunnel diode with a peak current of 17 A/cm2 and peak-to-valley ratio of 11,2 was realised. Using this tunnel diode, we have realised a dual-junction In0.49Ga0.51P/GaAs solar cell, grown by metal-organic chemical vapour deposition. Although current matching between top and bottom cell has not yet been optimized at this point, we obtained a good conversion efficiency of 26,3% (AM1.5G).

Present state-of-the-art multijunction solar cells use an In0.49Ga0.51P and GaAs top and middle cell, which can be grown lattice-matched on a Ge substrate. Although excellent solar cell efficiencies > 30% have been obtained in this way, these materials do not offer an optimum bandgap combination for photovoltaic conversion; an In0.65Ga0.35P/In0.17Ga0.83As/Ge cell offers a theoretical efficiency increase of 4%. However, as in this configuration top and middle cell are no longer lattice-matched to the substrate, misfit dislocations are formed in these cells during the epitaxial growth, which reduces the minoritycarrier lifetime and hence the device performance. The dislocation density in the active layers of these so-called metamorphic solar cells can be limited by the growth of a good buffer layer in between the Ge substrate and the active layers. As illustrated by figure 1, this has resulted in the realisation of a buffer structure that effectively blocks threading dislocations, limiting their density in the active layers to the 106cm-2 range. Using this buffer layer system a single-junction In0.17Ga0.83As solar cell has been realised with a conversion efficiency of 16.7% [15].

Low-bandgap cells for thermophotovoltaic application
Low-bandgap, stand-alone Ge solar cells have been under investigation at IMEC in the past couple of years. These cells offer numerous application possibilities, e.g. as bottom cell in a mechanically stacked multijunction solar cell or as converter in a thermophotovoltaic or co-generation system.

Whereas solar cells convert solar radiation to electricity, thermophotovoltaic cells are optimized to convert the radiation from heat sources which are at lower temperature as compared to the sun. This requires the use of materials with a lower bandgap as compared to silicon. For this purpose, several low-bandgap III-V compounds are being investigated in a number of institutes worldwide. Principally, germanium is also suited because of its low bandgap but problems related to proper surface passivation were hindering the further development. The significantly lower cost of germanium as compared to the low-bandgap III-V alternatives incited IMEC to tackle the issue again. This was done in collaboration with Umicore, a manufacturer of germanium wafers. The combination of improved surface passivation and novel contacting technologies led recently to Gecells with an open-circuit voltage over 270 mV, an AM1.5 efficiency above 8% and a broad spectral response from 400 to 1700 nm, values which exceed significantly the values reported by other groups [16].

References
[i] See e.g. the Strategic Research Agenda of the European PV Technology Platform http://ec.europa.eu/research/energy/pdf/visionreport- final.

[2]J. F. Randall, "On the use of photovoltaic ambient energy sources for powering indoor electronic devices", Ph. D Thesis Swiss Fededal Institute of Technology in Lausanne (2003)

[3] Beaucarne, G.; Agostinelli, G.; Carnel, L.; Choulat, P.; Dekkers, H.; Depauw, V.; Dross, F.; Duerinckx, F.; Gong, C.; Gordon, I.; Kuzma Filipek, I.; Ma, Y.; Posthuma, N.; Van Gestel, D.; Van Kerschaver, E.; Van Nieuwenhuysen, K.; Vermarien, E. and Poortmans, J., "Thin, thinner, thinnest: an evolutionary vision of crystalline silicon technology ", Proceedings of the 21st European Solar Photovoltaic Conference and Exhbition, Dresden, 4-8 september 2006

[4]Agostinelli, G.; Choulat, P.; Dekkers, H.; Vermarien, E. and Beaucarne, G., "Rear surface passivation for industrial solar cells on thin substrates", Proceedings of the 21st European Solar Photovoltaic Conference and Exhbition, Dresden, 4-8 september 2006

[5] Duerinckx, F.; Kuzma Filipek, I.; Van Nieuwenhuysen, K.; Beaucarne, G.; Poortmans, J.; Leloup, F.; Versluys, J. and Hanselaer, P., "Optical path length enhancement for > 13% screenprinted thin film silicon solar cells", Proceedings of the 21st European Solar Photovoltaic Conference and Exhbition, Dresden, 4-8 september 2006

[6] Beaucarne, G.; Gordon, I.; Van Gestel, D.; Carnel, L. and Poortmans, J., "Thin-film polycrystalline silicon solar cells: an emerging photovoltaic technology ", Proceedings of the 21st European Solar Photovoltaic Conference and Exhbition, Dresden, 4-8 september 2006

[7] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, "Photoinduced electron transfer from a conducting polymer to buckminsterfullerene", Science, 258, 1474 (1992) and C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, "Plastic Solar Cells", Adv. Funct. Mater., 11, 15 (2001).

[8] T. Aernouts, W. Geens, J. Poortmans, P. Heremans, S. Borghs, R. Mertens, "Extraction of Bula and Contact Components of the Series Resistance in Organc Bula Donor-Acceptor Heterojunctions", Thin Solid Films vol.403-404 (2002) 297-301

[9] Vanlaeke, P.; Swinnen, A.; Haeldermans, I.; Vanhoyland, G.; Aernouts, T.; Cheyns, D.; Deibel, C.; D'Haen, J.; Heremans, P.; Poortmans, J. and Manca, J. "P3HT/PCBM bulk heterojunction solar cells: relation between morphology and electro-optical characteristics". Solar Energy Materials & Solar Cells. Vol. 90: (14) 2150-2158; 2006.

[ii0] Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. McCulloch, C.-S. Ha, M. Ree, "A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells", Nature Materials 5 (2006) 197

[1iii] Aernouts, T.; Vanlaeke, P.; Poortmans, J. and Heremans, P. "Polymer solar cells: screen-printing as a novel deposition technique". In: Organic Optoelectronics and Photonics. SPIE, 2004. pp.252-260; (26-30 April 2004; Strasbourg, France.)(Proceedings of SPIE; Vol. 5464)

[iv2] W. Geens, T. Aernouts, J. Poortmans, G. Hadziioannou; "Organic co-evaporated films of a PPV-pentamer and C60: model systems for donor/acceptor polymer blends"; Thin Solid Films 403-404 (2002) 438-443

[v3] C. Rolin, S. Steudel, K. Myny, D. Cheyns, S. Verlaak, J. Genoe, P. Heremans, "Pentacene devices and Logic Gates Fabricated by Organic vapour Phase Deposition", Applied Physics Letters 89 (2006) 203502

[vi4] Flamand, G.; Moerman, I.; Derluyn, J.; Dessein, K. and Van Bavel, M., "A safer, efficient MOCVD process for the growth of GaAs solar cells, Photovoltaics Bulletin, issue 8, 7-9, 2002

[vii5] Y. Mols, "Comparison of buffers for metamorphic In0.17Ga0.83As solar cells on germanium", Proceedings of the 11th European Workshop on MOVPE, 499-501, 2005

[viii6] Posthuma, N.; van der Heide, J.; Flamand, G. and Poortmans, J., "Recent progress in the development of a stand-alone germanium solar cell", Proceedings of the 21st European Solar Photovoltaic Conference and Exhbition, Dresden, 4-8 september 2006.

Advanced Solar Cell Manufacturing

With a growing need to manufacture reliable photovoltaic devices there is an industry need to take advantage of experience in this emerging technology. German company Roth and Rau discuss the advances they have been involved with in their 15 years of working in the field. Specialising in plasma technology they have developed a long history that enables them to assist companies reach productivity and cost targets

What do solar cells, mobile phones, flat panel displays, hard disc drives and even cars have in common? They are all made using modern plasma technology, which offers a broad variety of methods to structure, coat, or modify surfaces. Among the suppliers for plasma process equipment is Roth & Rau AG from Germany with more than 15 years of experience in advanced plasma technology. Specialising in plasma technology for photovoltaics, Roth & Rau offers equipment for anti-reflection coating of crystalline silicon solar cells and several dry etching techniques, tailor-made equipment solutions for thin film solar module manufacture, and tools for solar research & development. Currently Roth & Rau employs 140 people and turned over round about 40 million Euro in 2006.

In 1999 Roth & Rau entered the high-growth photovoltaic market and started up the pilot line of the first industrial-scale in-line PECVD equipment SiNA for antireflective coating of crystalline silicon solar cells with a 70-80 nm thick silicon nitride layer at the Energy Research Centre of the Netherlands (ECN). In the following years Roth & Rau gained comprehensive knowledge in the area of silicon nitride deposition and experiences in successfully setting up this technology in production environments.

Five system sizes are available in order to meet throughput requirements ranging from production of prototypes up to fully automatic mass production of solar cells. The biggest SiNA has a yearly production capacity of more than 50 MWp which means more than 2.000 wafers an hour. The quality of the silicon nitride coating plays a significant role in the solar cell's production sequence as the optical and passivation properties of the layers do dramatically affect the solar cell efficiency which is the cell's most important quality and price criterion.

Roth & Rau offers turnkey solutions for modern solar cell production facilities. The basic concept has been designed to provide a continuously operating production line with un-interrupted material flow, high performance in terms of throughput, yield and cell quality at simultaneously low running costs. An integrated manufacturing equipment and technology package provides the customer with a technical equipment park, technological support and knowledge transfer with respect to the manufacturing process itself as well as to facility requirements issues and cost aspects. Roth & Rau will support the installation on site and will take care of after sales issues as training and service. The tool package provides the possibility to produce crystalline silicon solar cells on a high performance level using silicon wafers as incoming material. The specifically used material depends on the customer needs. The concept is absolutely flexible on used materials (mono as well as multi crystalline wafers), wafer sizes and shapes. The proposed manufacturing lines include the equipment for the solar cell production as well as metrology equipment and automation tools on a certain level. Production monitoring systems can be provided as an option.

To produce silicon solar cells from silicon wafers a couple of process steps have to be carried out. To do so a wide range of equipment can be used. Depending on the specific requisites of the customer this equipment is chosen on performance and cost, with a preference for in-line systems. However, the necessary process steps do not vary in their function. After material of a certain quality is inserted into the line the material is unpacked and checked for basic properties. Afterwards it will be transferred to the first process tool. This tool will be a wet chemical silicon etching tool, which etches off the so-called saw damaged surface. As an option this process step can be combined with a texturisation in order to structure the surface in a certain way by a further silicon etching process. The effect of this step is an increase of the absorption of the solar cell, which increases its efficiency as a consequence. The next step provides the emitter or so-called pn-junction, which is necessary to get the solar cell functioning. This junction will lead to the separation of generated electrons and hence their collection in the metal contacts on front and rear side of the cell. The junction is generated by applying a phosphorous dopant to the boron doped base material. In a high temperature step the phosphorous is diffused into the silicon wafer. While that diffusion an oxide, the so-called phosphorous glass (PSG), is formed at the surface which needs to be removed in the next step. This step is carried out in a further wet bench containing HF acid. As an option additional cleaning can be implemented in such a system, which provides an even improved cleaning as well as a smooth removal of the top layer of the emitter. This effect results in an improved cell efficiency due to better contacting. After this cleaning step the silicon nitride anti-reflection coating is applied at the front side of the cell. This step is performed in a SiNA PECVD reactor applying a plasma of ammonia and silan. After silicon nitride deposition metal contacts are formed at the front and rear side of the cell. These contacts are finally formed in a firing step. The metal contacts are screen printed and fired through the silicon nitride layer in an in-line high temperature furnace. When finish an edge isolation by laser cutting is applied to separate the front contact (emitter) from the back (base material) before the cells can be measured and sorted by their efficiency. Between the different process equipment and at the in- and outcome of the single equipment certain automation is required to reach a high throughput. Roth & Rau usually gets one automation company to automate the whole line. This way provides advantages regarding service and spare part management. In addition a PMS (production management system) can be applied collecting process data and measurement values from the different process steps. Single recipes can be loaded remotely, machine states and throughputs can be visualized using such a system. For process optimization more tools are necessary to collect data within the line or off-line. Roth & Rau proposes to use a certain metrology park to be able to ensure quality and outcome of the line as well as further process optimization. Roth & Rau also takes care in training of operating labour and technology staff on site during start-up and the time thereafter.

The initial optimization of the line with respect to the target throughput of either 30 MWp or 50 MWp, yield and similar parameters of the full line plus efficiency level is carried out by Roth & Rau´s well experienced process engineers. Roth & Rau has worked together with ECN for several years in developing silicon nitride layers and optimizing front and back end processes of solar cell lines. A technology transfer contract enables Roth & Rau AG to offer ECN services and technologies. Both partners are engaged in R&D projects for developing technologies for next generation solar cell lines. In result of their R&D work ECN published outstanding results for multi crystalline silicon solar cells with an efficiency of more than 16.5%. The 156x156 mm2 large multi-crystalline silicon solar cells are made with a homogeneous 70 Ohm/sq phosphorus emitter, a single layer antireflection coating, and simple screen-printed contacts on front and back, all made with ECN's inline pilot production line. ECN's strategy has always been to work on low-cost solar cell processing.

The 16.8% peak value is higher than any published result for such a large and simple cell. This specific licensed cell manufacturing process is offered together with the equipment and technology package as an option.

As a consequence the turn-key concept of Roth & Rau provides a cost-effective factory, latest state of the art equipment with absolutely production proofed systems, high end technology by high quality performance processes and high automation level, leading to a high yield at simultaneously low running costs.