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THE PATH TO VOLUME PRODUCTION FOR CPV OPTICS 1 1 Thomas Luce and Joel Cohen Eschenbach Optik GmbH, Nuremberg, Germany 1 ABSTRACT A crucial prerequisite for a commercial success of largescale CPV is the capability for mass production of the manufacturing processes. For the optical components (primary and secondary lenses) there do exist various production concepts with different potential for cost efficient mass production. The most promising concept is (compression) injection molding, a process technology for optical components already proven in mature industries like Automotive or Semiconductor. While the capability of the injection molding process for a real mass production is already well established, it needs to be proven that also the high quality requirements in CPV can be met. Injection molded PMMA Fresnel lenses can be manufactured with peak-to-valley deviations less than 10 µm, showing nearly no inner stress and very low and homogeneous edge radii in the order of 5 µm. A second approach for primary Fresnel lenses, namely injection molding of Silicone on Glass allows to take advantage of the mass production capability of injection molding also for this 2 component lens. The inherent advantage of this production method is clearly the achievable lower cycle time (= lower cost) and the high quality, e.g. no bubbles or intrusions. For volume production, this process can be extended to a fully automated production line with standard production equipment. For secondary CPV lenses, the requirements for the material are significantly higher, especially for larger concentration ratios. Since the conditions are comparable to lenses for high-brightness LEDs, the materials and technologies used there can be transferred to CPV secondary lens applications. Transparent silicone lenses have been proven to withstand the radiation density as well as the high temperatures. Thus, the use of injectionmoldable silicone lenses for CPV is obvious. INTRODUCTION The basic idea of Concentrated Photovoltaic (CPV) concepts is the replacement of expensive PV chip area by cheap optical elements, to reduce the cost per generated electrical power. Various concepts for this idea exist, most of them using a one- or two-lens approach. A prerequisite for the concept to work is of course, that the added cost e.g. for trackers and more efficient PV cells are compensated by the savings due to the use of optical elements. While during the starting phase of CPV the focus has been on feasibility, now manufacturability and especially cost efficiency are key for a fast market penetration of CPV systems. Both primary and secondary optics require mass manufacturing processes allowing scaling of the quantities up to several million parts per year, while keeping quality and cost at the best possible level. A promising approach is to look into industries where the transition from low to high volume production has already been accomplished. In the automotive lighting and semiconductor (LED) area, the problems with optical elements are quite similar; target there is as well to achieve high quality and quantity at the lowest possible cost, and to meet the lifetime requirements even under harsh environmental conditions. As the best suited production technology for optics, injection molding is established. If the process is controlled accordingly, the quality of injection molded lenses is outstanding, with very good surface quality and high transparency. Eschenbach Optik as a leader in the field of high precision optics molding has developed large scale production technologies to answer these demands. The main cost driver for injection molded parts is the cycle time. Due to the faster processing of the polymer by injection molding (the material is already molten in the injection unit before starting the cycle) and the multi-cavity option, the part price drops significantly with larger quantities. Figure 1 Cost comparison of parts made with injection molding compared to parts made with hot embossing ([1], for single cavity molds). The break-even point depends strongly on the type of part. For multi-cavity tools, the cost for injection molding should be even lower. The graph suggests that for mass production processes, the price potential for injection molding is much better than for hot embossing. INJECTION MOLDING OF OPTICS From a manufacturing point of view, the large quantities of Fresnel lenses for CPV power-plants require a fully automated production process at short cycle times. While for low (prototype) and medium size quantities, hot embossing is a good choice for these lenses, a real mass production requires a process which can be automated more easily. Injection molding has been proven to be capable of a mass production of optical parts in high quality. It uses standard production equipment, and the cycle times are typically significantly lower than e.g. with hot embossing. A typical comparison showing the advantage of injection molding over hot embossing is shown in Fig. 1. It demonstrates that the cost for small numbers is lower for hot embossing, but for high quantities somewhere above 100.000 parts, injection molding is more competitive. Replication quality of injection molding To quantify the achievable quality of injection molded Fresnel lenses, Eschenbach has performed several investigations on microstructure replication using different transparent thermoplastic materials. As sample geometry, edge shaped structures of 50 µm step size and 2 µm radius were used (Fig.3 top). These structures were manufactured using diamond turning machine and a mono crystalline diamond tool (Fig.3 bottom). A replaceable optical insert was cut using this diamond tool, the optical insert was then measured and its shape used as reference for comparison with the injection molded parts. To define a metric to rate the results, the optimum parameters for a flat plate without structure were recorded. Then the optical insert was mounted, and the first shots were done using the parameters optimal for the flat plate. The profile depth (peak-to-valley) was measured and used as metric for the quality of the replication. It turned out that it matched quite well with the overall impression of the structure quality (e.g. edge radius, planarity of the shoulder). Figure 5. Non-optimized replication results using standard injection parameters and standard material, using the optical insert of Fig.4. The edge has a radius about 8µm, a degradation of the PV-value of about 11% relatively to the optical insert. Figure 3. 50µm-groove structure or the measurement sample (top) and geometry of the diamond tool for cutting the 50µm grooves (bottom). Figure 6. Replicated structure with injection molded PMMA using the optical insert from Fig. 4. By optimization of material and process parameters, nearly the same precision as the tool can be achieved in the part. The edge radius is less than 3 µm. Injection compression molding can improve the results slightly. Figure 4. Tool insert shape with Fresnel type structure of 50µm depth and 2µm edge radius. The groove was cut with the diamond tool shown in Fig.3. As an example, Fig.5 shows results without optimization (i.e. using the parameters for the flat plate), The replication performance is reasonable, also some degradation with respect to the tool insert is visible. The PV-value is 89% of the tool insert value. An optimization procedure improves the PV-value to 98%. The edge radius is comparable to the tool insert; only very slight deviations are measureable. This is shown in Fig.6, where an edge radius less than 3mm is measured. As a conclusion from these investigations, it can be stated that injection molding is capable of nearly 100% replication of the tool inserts. Of course, this makes necessary careful optimization and control of the process parameters, requiring significant experience in injection molding of optical parts. good PV-deviation less than 10 µm. Cycle times for such a lens can be as low as one minute. CPV FRESNEL LENSES Beside approaches using mirrors as primary optics, the most popular options for CPV primary optics are Fresnel lenses. The best suited thermoplastic material for this application is PMMA. It is a low-cost, high performance transparent material offering superior optical properties (high Abbe number, transmission rates (see Fig. 7), Figure 8. The peak-to-valley deviation of the Fresnel structure for injection molded PMMA-lens is less than 5 µm! Figure 7. Transmission data of 3mm thick PMMA. For VIS and IR, very good transmission data are achieved, in the visible area 92,03%. (Source: Evonik Industries) good scratch resistance and excellent UV stability. PMMA has been used since decades in many outdoor applications, e.g. glass roofs or car rear lamp lenses, so that a lot of experience is already available. PMMA fulfills the so-called Florida weathering test for automotive rear lamps. Using thicknesses of 3 to 4 mm, it also withstands the IEC 6108 hail tests mandatory for CPV Systems. A further benefit from a production point of view is the easy application of PMMA in an injection molding process. A second approach is to use silicone on glass (SoG). A flat (float-) glass is coated with a thin layer of transparent silicone, which forms the Fresnel structure. Due to the two components and the more expensive base material, typically SoG Fresnel lenses are more costly than PMMA Fresnel lenses. PMMA Fresnel Lenses In Fig. 8 results of the peak-to-valley (PV) deviation of an injection-molded PMMA lens is given. It shows a very Figure 9. Microscopic section of the Fresnel structures of a injection molded Fresnel lens. Typical radii are about 5 µm and are varying only slightly over the whole Fresnel lens. The replication of the Fresnel structures even for these low cycle times is very homogeneous all over the part, the radii of the Fresnel edges are in the order of 3 µm (see Fig. 9) and have been achieved with a 2µm radius diamond tool. Thus, the replication quality is very good, with only slight deterioration of the edge sharpness. can be seen, the PMMA lens shows little stress only in the injection gate area (top of the picture). Optical Efficiency For CPV application, the optical efficiency is a crucial parameter for achieving the module price goals. Using a transmission measurement, we investigated the influence of the radii and the insert material on the efficiency. Results for injected parts from an aluminum insert (1), from an NiP-coated insert with 25 µm radius (2), from an NiP-coated insert with 3 µm edge radius (3) and from a directly turned part (i.e. not injected)(4) are displayed in Table 1. The results clearly suggests that injection molded parts replicate very well with an (surface weighted) efficiency of 83% and a reduction of only 3% compared to a directly turned part. It also shows that aluminum is not suited to provide high efficiency, and that by using lower edge radii, the efficiency can be increased by approx. 10%. We also found differences with different PMMA types, the best suited ones can improve the results by 4%. Figure 10. Optical birefringence measurement to identify inner stress in an injection molded Fresnel lens. In the gate area (top of the picture), low stress is visible, whereas the main area of the Fresnel lens is almost free of stress. This can further be reduced by optimization of the injection process parameters and the injection gate shape. Except for this area, the lens is free of stress, thus proving that injection molding is capable of providing very high quality Fresnel lenses. Another option to reduce even more the stress is to use injection compression tools. While the tool itself is more expensive due to the additional mechanics for the compression stroke, it removes nearly completely stress, resulting in a homogeneous polarization image. SILICONE ON GLASS (SOG) FRESNEL LENSES Table 1. Comparison of the transmission efficiency of Fresnel lenses produced with different methods. optical inserts. Best results are achieved with direct turned PMMA (column 4), but an injected production sample with NiP-coated optical inserts turned with 2µm diamond tip radius (3) gives transmission > 83%. Also the PMMA material can influence significantly the results. A further quality aspect is the nonexistence of inner stress which weakens the parts mechanically and may result in early failures. Inner stress is visible under inspection with polarized light. Fig. 5 shows a qualitative birefringence measurement of an injection molded Fresnel lens. As it A second approach for primary Fresnel lenses for CPV is silicone on glass. The transparency of silicone is outstanding, and also the temperature and UV stability are excellent. Combining the advantages of the injection molding process and the experience Eschenbach already acquired with the mass manufacturing of injection molded silicone lenses (esp. for High-Brightness LEDs), Eschenbach has developed an injection molding process for SoG optics. A glass plate is inserted automatically into an injection tool, the tool closes and the silicone is injected on one side of the glass. The achievable maximum size depends mainly on the maximum clamping force of the injection molding machine and the injection pressure needed to form the part. Due to the low viscosity of the Silicone resin the injection pressure needed is much lower than for a PMMA part. E.g. for a 100 x 100 mm glass plate, a clamping force of about 50t is necessary (for PMMA about twice the pressure is needed). The replication quality of very small structures by the SoG injection molding process such as Fresnel structures is even better than for PMMA. This is due to the very low viscosity of the silicone resin before the polymerization starts. Thus, also very fine structures are replicated accurately, posing very high demands on the optical inserts and a very accurate mold manufacturing. The lenses have virtually no air bubbles or other defects. Cycle time can be as low as one minute, depending on the silicone layer thickness, its polymerization properties and the handling system used. Mechanically, the surface is very difficult to measure, due to the elasticity and softness of the silicone. Thus, optical measurement means have to be employed. Fig. 11 shows an example of a SoG Fresnel lens manufactured with this injection molding process, using a simplified semi automated process. It demonstrates that this manufacturing technology is ready for mass production to take advantage of the high output capability of the injection molding process. candidates for CPV secondary optics. Additionally, the price pressure for LEDs has already sorted out solutions which are not cost competitive. Since most High Power LEDs today have silicone domes or silicone encapsulations, we investigated the application of silicone optics for CPV secondary optics. Again, the flexibility of the possible designs and mass production capability of the injection molding process is a clear advantage of this manufacturing method. Multi cavity tools with up to 64 cavities for optical parts are feasible and are already existing, allowing the production of tens of millions of lenses per year from only one injection molding tool. Figure 12. Multi-cavity injection-molded silicone lens. 32 lenses are injected simultaneously, allowing an annual production of several million lenses per year. Figure 11. Sample of a silicone on glass Fresnel lens. No bubbles are visible, and the replication is nearly as good as the original, showing all details of the optical inserts. SECONDARY OPTICS To improve the performance and reduce the sensitivity to positioning tolerances, secondary optics on the PV chip are more and more under consideration. For this application, especially the high energy densities pose a challenge for all transparent materials. Concentration factors of about 1000 suns result in transmitted radiation per square millimeter of 1 W or more. These are energy densities comparable to those of high brightness LEDs at the chip, and thus it is obvious that materials already proven for high brightness LED optics are also good Fig. 12 shows a mass production lens with silicone optics, with 32 cavity tool. Of course, due to the high number of cavities and using a fully automated process, very high part quantities at competitive prices are feasible. Special care needs to be taken for the fixation of the optics on the chip. A mechanical solution is often possible, e.g. using the elasticity of silicone. Most common is the use of an index matched silicone glue, which allows an off shore production of the optics at the lens manufacturer site and an assembly on the production line of the CPV manufacturer. Another option can be the in-line overmolding of the lens directly onto the chip. Although this is feasible in principle by inserting the PCB into the mold, the PCB design has to be adapted to withstand the heat and mechanical stress during the injection molding process. Especially wire bonds need to be protected, and circuit paths have to be placed in a way to allow the correct tightening of the cavity and to avoid being damaged during the process. Also the closing of the mold has to be very accurate, to prevent silicone from draining, which results in flashes and pollution of the board surface. Once these challenges are solved, the overmolding process is very efficient. and depending on the needs of the system, can be manufactured in very high quantities and superior quality. Also for secondary optics, injection molding can be used, utilizing transparent silicone with its superior UV and temperature stability. For both applications, not only high quality optics with very tight tolerances is possible, but also the integration of mechanical elements for fixation and positioning can be realized easily. This reduces furthermore the cost of the optics and improves the overall performance of the CPV system. REFERENCES Figure 13. Examples of secondary optics prototypes from transparent silicone. These parts are made from a prototype mold, showing the design freedom available for this material. Lenses from silicone have been successfully tested already in HALT tests for an automotive Xenon headlamp application [2] OUTLOOK From the experience in manufacturing high precision optical parts for mass markets, the most suitable process for high volume and cost efficient production of optics for CPV is injection molding and its variants. Fully automated processes are key for achieving low cost, high quality optical elements. Here injection molding is ahead of other replication technologies. To improve the quality further while reducing cycle time, additional options can be a highly dynamic temperature control of the cavities, which results in better replication of precise structures by heating and cooling closely to the mold cavity. PMMA or silicone on glass are both good candidates to fulfill all technical requirements for a CPV primary optics application, while PMMA has a significant cost advantage over SoG. For secondary optics, the process development of inserting and overmolding PV cells PCB’s with transparent silicone could result in significant cost advantages due to elimination of handling and gluing process steps. SUMMARY CPV is a technology requiring mass production means and processes. Therefore, a close look at already well developed mass markets like semiconductor (LED) and Automotive industry shows that injection molding is the most suited manufacturing technology to satisfy the needs of such an industry. Experiences acquired in these sectors should be transferred also to the CPV optical component production, to achieve the best quality per cost ratio. For primary optics, injection molding of thermoplastic (PMMA) and Silicone on Glass Fresnel lenses both are feasible, 1. R. Zengerle: Vorlesung Mikrosystemtechnik, Kap. 12: Heissprägen & Spritzgiessen; Institut für Mikrosystemtechnik, Uni Freiburg (2005) 2. S. Bäumer (Ed.), Handbook of Plastic Optics 2 edition, WileyVCH (2010) nd 3. T. Luce, E. Schalle, N. Ziegler, „The Advent of Polymer Headlamp Lenses“, in Proceedings of ISAL 2009 (2009) 4. T. Luce, E. Schalle “Kunststoff-Optiken für industrielle Präzisionsanwendungen”, Photonik (4/2008) 5. T. Luce, “Silicone optics”, LEDs Magazine, May/June issue (2008) 6. T. Luce, J.Cohen “Mass production challenges for CPV primary and secondary optics”, CPV-6 conference proceedings, Freiburg (2010)