Beam trap and method for making a beam trap
FIELD OF THE INVENTION
The present invention is directed to a beam trap, particularly for improving the efficiency of solar cells, and a method for making a beam trap.
BACKGROUND OF THE INVENTION
Anti-reflex coatings are commonly applied to photovoltaic cells (solar cells) for efficiency improvements. More advanced methods include surface texturing to enhance light trapping into solar cells. Dry and wet etching methods are commonly used to obtain these surface structures.
It is the object of the invention to provide a beam trap which, for example, improves the efficiency of solar cells, as well as to provide a method for making such a beam trap.
SUMMARY OF THE INVENTION
This object is solved by the features of the independent claims. Preferred embodiments and further developments are outlined in the dependant claims.
In accordance with a first aspect, the invention provides a beam trap, particularly for improving the efficiency of solar cells, comprising: a substrate at least partially transmissive for the beams and comprising a beam ingress surface and a beam emission surface, and a plurality of beam reflectors arranged in the substrate, wherein each of the plurality of reflectors comprises, in the direction from the beam ingress surface to the beam emission surface, a cross section that first broadens and than narrows. With such a beam trap, for example light rays that are transmitted through the substrate are trapped and are prevented to travel backwards. In this way, the total reflection from, for example, a solar cell module is minimized, if the solar cell module is equipped with a beam trap. While the invention is particularly useftil in connection with solar cells, it is not limited to such applications. It clear for the person skilled in the art that the beam trap in accordance with the invention can be used in all cases where the reflection of electromagnetic beams needs to be

minimized. Therefore, it is for example also possible to equip green houses, outdoor water supply tanks etc. with the beam trap of the invention.
With preferred embodiments of the beam trap in accordance with the invention the plurality of beam reflectors comprise a diamond like shape. In such a case the reflectors can comprise the form of an octahedron. However, it is, for example, also possible that the reflectors are constituted by two frustums arranged peak to peak
It is highly preferred that the substrate is flexible. If a rigid substrate material is used for replication, the dovetail connection will prevent, or make at least very difficult, loosening of the replicated surface, i.e. the surface provided with the recesses that will form, in most cases after filling, the reflectors. However, if a flexible substrate is used, for example diamond-like structures can be replicated in the substrate.
In this connection it is for example possible that the substrate comprises a polymer material, particularly a flexible polymer like acrylates. For example, a preferred substrate or replication material is polydimethylsiloxane (Sylgard® 184 Silicone Elastomer) because of the high transmission in the deep UV wavelength range (250-400 nm wavelength). Particularly foils replicated with this material will have a better resistance against UV degradation due to exposure to sun light.
Depending on the actual field of application, the substrate preferably comprises a thickness between 500 nm and 10 cm.
With preferred embodiments of the beam reflectors comprise at least one material selected from the following group: organic dyes, PEDOT, organic materials, metallic particles. In general, every material can be used that is reflective enough for kind of beam that is to be trapped.
Every solar cell that comprises a beam trap in accordance with the invention is intended to fall into the scope of the accompanying claims.
In accordance with a second aspect the present invention provides a method
for making a beam trap, particularly a beam trap in accordance with the invention as
discussed above, the method comprising the following steps: providing a stamper with
a plurality of stamping structures, wherein each of the plurality of stamping structures comprises, in the direction of the stamper's normal vector, a cross section that first broadens and than narrows; providing a flexible substrate at least partially transmissive for the beams; and using the stamper to provide a relief structure in the substrate. With a beam trap made by this method the characteristics and advantages discussed above in connection with the beam trap of the invention can be obtained, in most cases after the relief structure was filled to
provide suitable reflectors, as discussed below. Therefore, to avoid repetitions, reference is made to the above description of the beam trap. To provide the stamper it is preferred to use one or more methods known as such from the field of making stampers for the replication of optical discs. In this connection, for example, the following six possibilities exist;
a) Some photoresists possess a positive effect: the written areas remain while the unwritten photoresist is removed during the wet etching step (the development process). The illumination of the photoresist may be followed by a heating step to induce cross-linking of the photoresist molecules. The photoresist can be directly applied to aNi shell. The heating mode makes the exposed photoresist areas rigid such that the Ni shell with created photoresist bumps/barriers can directly be used as stamper. The diamond-like barrier shape remains from the balance between optical absorption and chemical treatment (pre-development in combination with wet etching). The recording (photoresist) layer can be treated with a chemical agent prior to illumination to make the top of the photoresist layer less sensitive to the illumination. The illuminated area has a conical shape due to the absorption of the recording layer (the illuminated area reduces with increasing penetration depth). Chemical removal of the unwritten areas (positive photoresist) and partially the upper part of the exposed areas lead to the diamond-like bump/barrier.
b) Some phase-change mastering (PTM) materials possess also a positive effect: the written areas remain while the unwritten areas are removed after the wet etching step. Such a material is ZnS-SiOz (with 80-20 %, 70-30 or 60-40 %, or other mass ratio). The unwritten areas can be easily dissolved in diluted acids (like HNO3), the written areas have a much higher resistance to the etching liquid, hence resulting in a good contrast in etching rate. A substrate (for instance a Ni shell) with patterned PTM layer can directly be used as stamper. The diamond-like barrier shape remains from the balance between optical absorption, PTM layer composition and chemical treatment (wet etching). The etch rate of the inorganic PTM layer is influenced by the sputter-deposition conditions. The inorganic PTM layer can be composed of two ZnS-Si02 sub-layers, each with different etch properties, or the PTM layer can posses a gradient in selective etch properties. The illuminated area has
a conical or diamond shape due to the absorption of the recording layer. Chemical removal of the unwritten areas (positive photoresist) and partially the upper part of the exposed areas result in the diamond-like bump/barrier.
c) The recording layer comprises two different layers, for example a
photoresist layer (negative photoresist) deposited on an inorganic ZnS-Si02 layer. The
photoresist layer is illuminated with a focused laser beam to write grooves or pits. The power
is too low to induce a phase-change in the ZnS-Si02 underlayer. The illuminated areas in the photoresist layer are removed with an alkaline developer (NaOH, KOH). The ZnS-Si02 is resistant to these alkaline etch liquids. In a second wet etching step, the master substrate is exposed to an acid developer (HNOi). The patterned photoresist layer serves as masks. The ZnS-Si02 underlayer is partially dissolved. The final result is a diamond-shape pit/groove. Thermal treatment can be used to make the patterned photoresist layer sufficient rigid
d) The master substrates discussed in 1), 2) and 3) may also comprise additional layers to steer the optical contrast and the thermal response to improve the diamond-like bump/barrier shape.
e) A Ni stamper can be grown fi-om the developed master substrates. The developed master is typically provided with a 100 nm Ni-like sputter-deposited layer. Electro-chemical plating is used to grow a thick metallic stamper. The main challenge is the separation of the remainders from the grown stamper because of the involved dovetail connections. Photoresist remainders can be easily removed from the traps (for instance with UV ozone and highly concentrated alkaline solutions). Removing ZnS-SiOz remainders is more difficult. Possibilities include concentrated alkaline liquids, but the Ni will also be attacked (Ni passivation). The Nickel can be protected from passivation by lowering its electrical potential. In that case, nickel can be made immune to both acid and basic solutions thereby preventing deterioration of the Ni.
f) A conventional photoresist process (ultra, Clariant, I-line resists etc.) is used to make a conventional master. By using an additional etching method, the underlayer or substrate may be underetched.
Also in connection with the method in accordance with the invention it is preferred that the substrate comprises a polymer material. As mentioned above it is, for example, possible to use polydimethylsiloxane (Sylgard®184 Silicone Elastomer) for the substrate because of the high transmission in the deep UV wavelength range (250-400 nm wavelength).
The substrate can, for example, comprise a thickness between 500 nm, a few mm, and 10 cm.
With highly preferred embodiments of the method in accordance with the invention the method comprises the additional step of filling, at least partially, the relief structure to provide beam reflectors. For example, a non-uniform coverage process can be used to fill the relief structures (which is preferably diamond-like) of the replicated substrate
with a high-reflecting material. At least the two following methods are for example possible to fill the structures with an organic material.
a) A non-uniform coverage processes such as spin coating: the prepits/grooves get much higher layer coverage than the intermediate land areas. It is also possible to only fill the prepits/grooves.
b) Uniform deposition processes with a subsequent anisotropic patterning (wet etching) step to obtain more land than prepit erosion.
Similar as in the case of the beam trap in accordance with the invention, the beam reflectors can comprise at least one material selected from the following group: organic dyes, PEDOT, organic materials, metallic particles.
In summary, the invention provides a method to make a light trap from a replicated substrate. The replicated substrate contains, for example, diamond-like pits that are filled with a reflecting material. The foil texture is preferably obtained via optical disc replication methods. The process can comprise the following steps: a stamper is made via optical disc mastering methods, the stamper is used to replicate the foil, the foil is provided with a reflective coating, the foil is applied to the device. The shape (angles, width, height) of the light trap can be adjusted by the design of the master substrate and the process parameters. In some cases, the neck of the light trap needs to be small.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically illustrates a stamper that can be used to provide a relief structure in the form of diamond-like recesses in a flexible substrate;
Figure 2 schematically illustrates a flexible substrate provided with a relief structure in the form of diamond-like recess after treatment with the stamper of Figure 1;
Figure 3 schematically illustrates an embodiment of the beam trap in the form of the flexible substrate of Figure 2 after filling the diamond-like recesses with a suitable reflecting material; and
Figure 4 schematically illustrates the beam trap in accordance with Figure 3 attached to a solar cell module.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure I schematically illustrates a stamper 24 that can be used to provide a relief structure in the form of diamond-like recesses in a flexible substrate not shown in Figure 1. The stamper 24 is equipped with a plurality of stamping structures 26, wherein each of the plurality of stamping structures 26 comprises, in the direction of the stamper's normal vector 28, a cross section that first broadens and than narrows. In the illustrated case the stamping structures 26 have a diamond-like configuration, but this is only one of a plurality of possible configurations. The stamper 24 can be made by techniques known as such for making stampers for replicating optical discs. Some of those techniques have been described above, and a further description is omitted here to avoid repetitions.
Figure 2 schematically illustrates a flexible substrate 14 provided with a relief structure 30 in the form of diamond-like recesses made with the stamper 24 of Figure 1, for example via injection molding. The substrate 14 or replication material can, for example, be polydimethylsiloxane (Sylgard® 184 Silicone Elastomer) because of its high transmission in the deep UV wavelength range (250-400 nm wavelength).
Figure 3 schematically illustrates a beam trap 10 in the form of the flexible substrate 14 of Figure 2 after filling the diamond-like recesses 30 with a suitable reflecting material to provide reflectors 22. Each of the plurality of reflectors 22 comprises, in the direction from the beam ingress surface 18 to the beam emission surface 20 of the substrate 14, a cross section that first broadens and then narrows. For filling the diamond-like recesses 30 at least one of the following materials can be used; organic dyes, PEDOT, organic materials, metallic particles. As mentioned above, non-uniform coverage processes, such as spin coating, or a uniform deposition processes can be used to completely or partly fill the diamond-like recesses 30 and to provide the reflectors 22.
Figure 4 schematically illustrates the beam trap 10 in accordance with Figure 3 attached to a solar cell module 12. The beam trap 10 (replicated foil or thin substrate) can be bonded to the fiiel cell module 12. The solar module 12 is in that case a self-supporting substrate with a deposited thin-film stack. The thin film stack of the solar cell module 12 can be deposited directly on the replicated side of the substrate 14. The working principle of the arrangement shown in Figure 4 is as follows: Light rays or beams 16 traveling through the replicated substrate 14 will reflect on the 'hat' of the diamond-like reflectors 22. Once guided through the narrow neck, the beams 16 are trapped and will reflect back and forward between the device surface and the reflectors 22. Thereby, the efficiency of the solar cell module 12 is improved. However, as mentioned above, the invention is not limited to be used in
connection with solar cells, but it can be used in all cases where it is advantageous to reduce beam reflection.
Finally, it is to be noted that equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

CLAIMS;
1. A beam trap (10), particularly for improving the efficiency of a solar cell (12),
comprising:
a substrate (14) at least partially transmissive for the beams (16) and comprising a beam ingress surface (18) and a beam emission surface (20), and
a plurality of beam reflectors (22) arranged in the substrate (14), wherein each of the plurality of reflectors (22) comprises, in the direction from the beam ingress surface (18) to the beam emission surface (20), a cross section that first broadens and then narrows.
2. The beam trap (10) according to claim 1, wherein the plurality of beam reflectors (22) comprise a diamond like shape
3. The beam trap (10) according to claim 1, wherein the substrate (14) is flexible.
4. The beam trap (10) according to claim 1, wherein the substrate (14) comprises a polymer material.
5. The beam trap (10) according to claim 1, wherein the substrate (14) comprises a thickness between 500 nm and 10 cm.
6. The beam trap (10) according to claim 1, wherein the beam reflectors (22) comprise at least one material selected from the following group: organic dyes, PEDOT, organic materials, metallic particles
7. A solar cell (12) comprising a beam trap (10) according to any of claims 1 to 6,
8. A method for making a beam trap (10), particularly a beam in accordance with any of claims 1 to 6, the method comprising the following steps:
providing a stamper (24) with a plurality of stamping structures (26), wherein
each of the plurality of stamping structures (26) comprises, in the direction of the stamper's normal vector (28), a cross section that first broadens and than narrows;
providing a flexible substrate (14) at least partially transmissive for the beams (16); and
using the stamper (24) to provide a relief structure (30) in the substrate (14).
9. The method according to claim 8, wherein the substrate (14) comprises a polymer material.
10. The method according to claim 8, wherein the substrate (14) comprises a thickness between 500 nm and 10 cm.
11. The method according to claim 8, flirther comprising the following step:
filling, at least partially, the relief structure (30) to provide beam reflectors
(22).
12. The method according to claim 11, wherein the beam reflectors (22) comprise
at least one material selected from the following group: organic dyes, PEDOT, organic
materials, metallic particles.