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THE PATH TO VOLUME PRODUCTION FOR CPV OPTICS
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Thomas Luce and Joel Cohen
Eschenbach Optik GmbH, Nuremberg, Germany
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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,
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R. Zengerle: Vorlesung Mikrosystemtechnik, Kap. 12:
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&
Spritzgiessen;
Institut
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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)
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