The present invention relates to the field of photovoltaic cells. In particular, it relates to a manufacturing method for a plurality of photovoltaic cells such that the cells can be moved from a first position in which they are separated from each other by a first distance, to a second position in which they are separated from each other by a second distance greater than the first distance, in a fast and economical manner.

In recent years, due to the constant increase of energy prices dictated, in particular, by the constant increase in the price of fossil fuels, more and more interest has been shown towards renewable energy sources. One particularly appreciated form of renewable energy consists of photovoltaic energy, which can be installed virtually anywhere in sizes ranging from a few square centimeters to photovoltaic parts covering several square kilometers.

One particularly advantageous form of photovoltaic generators consists of lll-V concentrator generators. Even more specifically, multi-junction l ll-V concentrator photovoltaic cells are preferred since they can achieve much higher efficiencies compared to the standard Silicon technology. In the concentrator photovoltaic module, the size of the photovoltaic cells is reduced thanks to the use of a lens focusing light from a larger area to a smaller area, corresponding to the smaller size of the photovoltaic cells. This implies that the photovoltaic cells do not cover the entire area of the module, but are rather spaced from each other. Depending on the concentration of the light the use of a heat sink might be necessary in order to avoid efficiency losses due to increased temperature of the photovoltaic cells. Therefore, when realizing the module, it is a standard approach to pick and place each of the cells, at a certain distance from each other, for instance on top of the heat sink as part of the concentrator photovoltaic module.

Such a standard approach is illustrated in Figure 6. In particular, Figure 6 illustrates a plurality of cut views of the product during different steps of the manufacturing process. A substrate 8130 is provided on which, during a step S81 , a photovoltaic layer 8120 is provided. For instance, the photovoltaic layer 8120 could be grown by epitaxial growth of lll-V semiconducting materials like indium gallium phosphide and gallium arsenide on for example a Germanium wafer, leading to a common multijunction solar cell of type InGaP/GaAs/Ge. The Germanium wafer could typically have a diameter of 4", 6" or 8". The photovoltaic layer 8120 could further include an anti- reflective coating and metal contacts on the front and/or rear side. During a step S82 the photovoltaic layer 8120 is cut so as to realize a plurality of photovoltaic cells 8121 -8124 separated from each other. The substrate 8130 has to serve as a stabilizing support for the photovoltaic layer 8120. In case of the above mentioned InGaP/GaAs/Ge multijunction layer the photovoltaic layer 8120 comprising the Germanium wafer and the epitaxially grown semiconducting layers are provided on an adhesive foil, in particular a typically blue foil with adhesive which reduces adhesion after UV exposure, acting as substrate 8130 in order to maintain the positions of the photovoltaic cells 8121-8124 after cutting/separation. The Germanium wafer has to have a minimum thickness in order to allow a subsequent pick and place of the cells. Part of that Germanium wafer forms the bottom junction of the InGaP/GaAs/Ge multijunction, whereas the rest of the Germanium substrate acts as stiffener for the separated photovoltaic cells. Such a separation or cut could be achieved by etching, diamond cut, sawing, laser separation, or any other technique used in the field of photovoltaic manufacturing and, more generally, semiconductor technology. During a step S83, the photovoltaic cells 8121-8124 are picked and placed, one by one, from substrate 8130 onto a heat sink 8140, thereby realizing structure 8101 . Thereby the application of conductive glue or solder 8131 -8134 is necessary in order to secure each individual photovoltaic cell on the heat sink 8140. One could also imagine several individual heat sinks 8140 for each individual photovoltaic cell 8121 -8124. Heat sinks could be made of copper, aluminum or other metals and contain further elements like bypass diode or contact pads. Structure 8101 can thereafter be placed into a module, with the addition of a lens layer (not illustrated), above the cells 8121- 8124. Such an approach is, however, slow and expensive, mainly due to the fact that the photovoltaic cells 8121-8124 have to be singularly, one by one, picked and placed from substrate 8130 to heat sink 8140. The pick and place process can be manual, or automated, but it remains a serial process, which is complicated and slow. The process does not allow all cells on one wafer to be processed simultaneously. Further a sort of pick and place process has been developed by Semprius, as disclosed in WO201 1/123285A1 , and can be used to transfer a multiple number of cells by selectively bonding widely spaced cells from one wafer and releasing them to another structure at their initial separation distance. In this process not all the cells from the starting wafer are processed simultaneously neither a change of their distance separating them from each other is disclosed. Additionally, the pick and place approach is not satisfactory since the cells have to be manufactured with a certain thickness to provide sufficient stiffness in order to be manipulated. Further in case of the common InGaP/GaAs/Ge cells the initial thick Germanium substrate is rather expensive. Further, thick layers of the photovoltaic layer 8120 can cause losses due to high resistance for current and/or heat conduction. Thin layers with sufficient conductivity are preferred to minimize such losses. A thick layer of solder or conductive glue 8131-8134 is used to connect the semiconductor layers 8121-8124 to the heat sink 8140. Also this layer causes additional resistance losses and leads to concerns of reliability due to different thermal expansion and corrosion which can damage the interconnect over time. It is therefore, an object of the present invention to provide a solution to these problems, thereby lowering costs and/or increasing production speed of photovoltaic modules. In other terms, it is an objective to provide an automated method for expanding distributed cells from a wafer of size A to a distributed configuration on a wafer with size B, where B>A, for instance from 4" to 8" or from 6" to 12". The present invention can relate to a manufacturing method for a plurality of photovoltaic cells comprising the steps of: obtaining a plurality of photovoltaic cells, placed at a first distance from each other; attaching a stretching material to the plurality of photovoltaic cells; and stretching the stretching material such that the plurality of photovoltaic cells results at a second distance from each other, wherein the second distance is greater than the first distance. This provides the beneficial advantage that the cells can be moved away simultaneously from each other to a second distance, with a simple and parallel process.

In some embodiments the manufacturing method can further comprise the step of positioning the plurality of photovoltaic cells onto a target substrate, while the cells are still attached to the stretching material, after the step of stretching. This provides the beneficial advantage that the stretching material can be used not only for displacing the cells away from each other, but also for carrying them onto the target substrate and for precisely positioning them onto the target substrate.

In some embodiments the manufacturing method can further comprise the step of assembling the plurality of photovoltaic cells to the target substrate, after the step of positioning; and removing the stretched material from the plurality of photovoltaic cells, after the step of assembling.

This provides the beneficial advantage that the cells can be firmly kept in place and stabilized mechanically, during the assembling step, by the stretching or stretched material. In some embodiments the manufacturing method can further comprise the step of removing the stretched material from the plurality of photovoltaic cells, after the step of positioning; and assembling the plurality of photovoltaic cells to the target substrate, after the step of removing.

This provides the beneficial advantage that the assembling is facilitated, since access to the cells is improved by the removing of the stretched stretching material before assembling

In some embodiments the plurality of photovoltaic cells can be obtained from a wafer of size A and the target substrate can be a wafer of size B greater than size A, and the plurality of photovoltaic cells can be attached to the stretching material and stretched from the initial size A of value 2" or 4" or 6" or 8" to a respective larger size B of value 4" or 6" or 8" or 12". This provides the beneficial advantage that the manufacturing of solar cell assemblies is facilitated using standard sizes of wafer for both the initial substrate and the target substrate, commonly used in semiconductor technology and, thus, easily implemented in currently existing production lines.

In some embodiments the step of assembling can comprise bonding, in particular direct bonding or metal bonding, to the target substrate.

This provides the beneficial advantage that an improved electrical and thermal connection can be achieved between the cells and the target substrate.

In some embodiments the assembling step can further comprise bond preparation steps, in particular the deposition of conductive glue or the deposition of adhesive and/or conductive intermediate layer on at least one of the plurality of photovoltaic cells or the target substrate, or surface preparation by a plasma treatment or a chemical mechanical polishing, prior to bonding.

This provides the beneficial advantage that an improved bonding relative to its strength and quality is obtained.

In some embodiments, the target substrate can be a semiconductor substrate and can comprise contacts to the plurality of photovoltaic cells.

This provides the beneficial advantage that subsequent connections to the cell can be done via connections to the target substrate, on which bigger contacts may be provided, than on the cells themselves, thereby facilitating the realization of connections such as wirebond connections. In some embodiments the manufacturing method can further comprise the step of cutting the target substrate so as to realize a plurality of solar cell assemblies, each solar cell assembly comprising one of the pluralities of photovoltaic cells.

This provides the beneficial advantage that the solar cell assemblies can be subsequently placed onto another structure, such as a heat sink or a base plate of a module. Since the solar cell assemblies have bigger dimensions compared to the photovoltaic cells, handling of the former is easier compared to handling of the latter.

In some embodiments the manufacturing method can further comprise the step of assembling the plurality of solar cell assemblies on a heat sink. This provides the beneficial advantage that the heat generated in the cells can be dissipated.

In some embodiments the photovoltaic cells can be multi-junction lll-V concentrator photovoltaic cells.

This provides the beneficial advantage that efficiency of the cells can be increased.

In some further embodiments the multijunction lll-V concentrator photovoltaic cells can be provided or formed on a semiconductor substrate, in particular GaAs, InP, Ge or Si, and the manufacturing method can further comprise removal of the substrate and separation of the photovoltaic cells.

This provides the beneficial advantage that the stretching process can be implemented in the manufacturing process of the lll-V concentrator photovoltaic cells, the removal of the mostly expensive substrates permits to reduce costs as these substrates could be reused for subsequent manufacturing processes of other cells.

In some further embodiments the multijunction lll-V concentrator photovoltaic cells can have a thickness of several micrometer, in particular below 10 μηη.

This provides the beneficial advantage that even very thin photovoltaic cell layers can be transferred, reducing the electrical losses due to normally rather thick substrates kept for stiffening reasons and, thus, also further reducing costs concerning the providing of such substrates.

Further, the present invention can relate to a semiconductor structure comprising a stretched material attached to a plurality of photovoltaic cells such that the plurality of photovoltaic cells result at a second distance from each other, wherein the second distance is greater than a first distance which separates the plurality of photovoltaic cells from each other when the stretched material is not stretched.

In some embodiments the stretching material can be made of metallic material at the position where the plurality of photovoltaic cells are attached.

This provides the beneficial advantage that the thus obtained semiconductor structure can represent a multijunction solar cell bonded to a stretching material representing a final heat sink material.

The invention will now be described in more detail by way of example hereinafter, using advantageous embodiments and with reference to the drawings. The described embodiments are only possible configurations in which the individual feature may, however, as described above, be implemented independently of each other, or may be omitted, or may be combined between different embodiments. Equal elements illustrated in the drawings are provided with equal reference signs. Part of the description relating to equal elements illustrated in the different drawings may be left out. In the drawings:

Figures 1 to 3 schematically illustrate steps of a manufacturing method for a plurality of photovoltaic cells in accordance with an embodiment of the present invention;

Figure 4 schematically illustrates the operation of a stretching material in accordance with a further embodiment of the present invention; - Figure 5 schematically illustrates the operation of an alternative stretching material in accordance with a further embodiment of the present invention; and

Figure 6 schematically illustrates a manufacturing method for a solar cell assembly in accordance with the state of the art.

Figures 1 to 3 schematically illustrate a manufacturing method for a plurality of photovoltaic cells in accordance with an embodiment of the present invention.

Similar to what has been described with respect to Figure 6, a photovoltaic layer 8120 is provided on a substrate 8130, thereby forming the structure 8100. The substrate may be an adhesive foil as illustrated schematically in Figure 1 or even a conventional Gallium Arsenide, Indium Phosphide or Germanium wafer on its own, with a typical diameter of 4", 6" or 8". The photovoltaic layer 8120 may include an anti-reflective coating and metal contacts on the front and/or rear side.

After a step S82 realized on the structure 8100, as already illustrated schematically in Figure 6, the photovoltaic layer 8120 which has been provided on the substrate 8130, is cut, thereby resulting in the plurality of photovoltaic cells 8121-8124 available on top of substrate 8130. During a placement step S13, stretching material 1 150 is applied on top of photovoltaic cells 8121-8124. The stretching material 1 150 is made of polymer and/or metal and/or other materials which allow to be expanded by application of mechanical force, pressure or heat. For instance one could use a thick material which expands for example thermally by heating. The stretching material is attached to the photovoltaic cells 8121 -8124 by using a non-permanent adhesive like UV-glue, thermal glue or electrostatic force. After application of the stretching material 1 150, during a step S14, the substrate 8130 is removed, thereby leaving photovoltaic cells 8121-8124 placed at a first distance from each other attached to the bottom side of stretching material 1 150, thereby forming unstretched structure 1 180. In contrast to the standard pick and place approach described with respect to Figure 6, the photovoltaic cells 8121-8124 have no minimum thickness required for the embodiments of the present invention. The photovoltaic cells can for instance be formed on above mentioned types of Gallium Arsenide (GaAs), Indium Phosphide (InP) or Germanium (Ge) wafers as substrate 8130, for instance by epitaxial growth or bonding techniques, and where the substrate 8130 is finally removed in step S14. For example in accordance with our invention, in case of a InGaP/GaAs/Ge multijunction cell, one could also remove the part of the Germanium substrate wafer which is used for stiffening purposes which allowed a pick and place process. It becomes clear from Figure 1 that the unstretched structure 1 180 can also be obtained by the application of the stretching material 1 150 on the initial structure 8100, removal of substrate 8130 and subsequent separation/cutting of the cells. In a further step the stretching material 1 150 and the unstretched structure 1 180 are stretched during a stretching step S21 , thereby resulting into stretched material 1 151 and stretched structure 1 190. In particular, during the stretching step S21 , the lateral dimension of the stretching material 1 150 is increased by stretching the stretching material 1 150 into at least one direction. This results in the distance between each of the cells 8121 -8124 increasing in the at least one direction in which the stretching material 1 150 is stretched. Thanks to this step, the cells can be moved apart from each other, at a predetermined second distance in accordance with the amount of lateral stretching exerted on the stretching material 1 150, in a parallel and simultaneous manner, without having to move each single cell independently in a serial manner. Thanks to such an approach, the manufacturing method described above results in a faster production and/or in a more economical production of a photovoltaic module. In other words, instead of the serial processing of picking and placing each of the cells one by one, from a first position in which they are close to each other to a second position in which they are separated from each other, such separation is achieved, in parallel, for all cells at once, thanks to the usage of stretching material 1 150. During a positioning step S22 the plurality of photovoltaic cells 8121 -8124 are positioned on top of an optional semiconductor substrate 2160. Alternatively, at this step, the cells 8121-8124 could be placed on top of heat sink 8140, secured by means of gluing and/or welding, and the stretched material 1 151 is removed, thereby resulting into structure 8101 . In both cases, the semiconductor substrate 2160 or the heat sink 8140 act as a target substrate for the assembling of the cells 8121-8124.

If the cells 8121 -8124 are placed on optional semiconductor substrate 2160, they can be assembled by means of gluing, soldering and/or welding, in particular laser welding, and/or bonding, in particular direct bonding or metal bonding, and/or any other suitable process. Semiconductor substrate 2160 could be any of a Silicon Molybdenum wafer, or Si, SiGe, Ge, Si on Mo, SiAI, Si on SiAI etc. During a removal step S23, the stretched material 1 151 is removed e.g. by UV exposure, heat, chemicals, solvents, mechanical force, releasing electrostatic charge and the cells 8121 -8124 are left assembled to the semiconductor substrate 2160, forming structure 2101 . During a patterning step S24, contacts 2161 -2163 as well as wirebond connection 2164 are realized on semiconductor substrate 2160 as well as the plurality of cells 8121-8124. In particular, a front side contact 2161 is connected to a front cell contact 2162 placed on top of each of cells 8121-8124 via wirebond connection 2164, while a back side contact 2163 is electrically connected to the backside of each of cells 8121-8124 via a conducting path through the semiconductor substrate 2160. In case of other types of target substrates, the back contact could be formed by metal deposition of a contact pad prior to positioning and assembling of the photovoltaic cells in order to relay electrically each of the back side contacts 2163 to each back of the photovoltaic cells. This results into structure 2102, thereby containing a plurality of cells 8121 -8124 with the respective electrical contacts 2161- 2163 to the front and to the back side of each of the cells.

Both in case where the cells 8121-8124 are placed on a heat sink 8140 or on a semiconductor substrate 2160, the use of the stretching material 1 150 as a stretching element and as a positioning element is advantageous in simplifying the manufacturing process. In particular, the stretching material 1 150 is first used as a stretching element, in order to separate the cells 8121 - 8124 during stretching step S21 and then further subsequently used as a structural element in positioning the cells 8121-8124 on their target position, during positioning step S22. Thanks to this dual usage of the stretching material 1 150, the handling of the cells 8121 -8124 is further simplified.

As described above, positioning step S22 can be carried out by keeping the connection between the stretched material 1 151 and the cells 8121 -8124 while the cells are assembled onto heat sink 8140 or on semiconductor substrate 2160. However, the present invention is not limited thereto and positioning step S22 can alternatively be carried out by using the stretched material 1 151 for positioning the cells 8121 -8124 onto the target substrate, removing the stretched material 1 151 , before assembling the cells onto the target substrate, while leaving the cells on their respective position on the target substrate, and then proceeding to securing the cells onto the target substrate. In the latter case, the cells can be kept in position by several ways such as gluing, magnetic or electrostatic attraction, or simply by friction.

While Figure 2 schematically illustrates a cut view of the structures during the manufacturing steps S21 -S24, Figure 3 schematically illustrates a top view of the structures. In particular, at the top of Figure 3, structure 2102 is schematically illustrated. In particular, although structure 2102 may, in some embodiments, be a circular wafer, for easiness of representation only a rectangular section of it is represented in Figure 3. During a cutting step S31 , semiconductor substrate 2160 is cut, thereby resulting in a plurality of semiconductor structures 3161-3164, each supporting a single photovoltaic cell, respectively cells 8121-8124, and the corresponding electrical connections. The semiconductor structures 3161-3164 therefore act as solar cell assemblies, providing a mechanical support for the cells 8121 -8124, as well as electrical connection to the front and back side of the cells. During an assembling step S32, the plurality of solar cell assemblies can then be placed on top of a heat sink 8140 as illustrated by step S32, thereby realizing structure 3103. In alternative embodiments, individual heat sinks are also possible for each solar cell assembly.

Alternatively, or in addition, although not illustrated, each of semiconductor structures 3161-3164 can further comprise other circuits or functionalities for and connections to the photovoltaic cell on the semiconductor structure and to the neighboring structures. For instance, any of semiconductor structures 3161 -3164 could further comprise a bypass diode for the photovoltaic cell. Alternatively, or in addition, front side contact 2161 could be realized directly on top of the photovoltaic cell, and not on the semiconductor structure and, thus, front cell contact 2162 is replaced. Further alternatively, or in addition, front side contact 2161 of a given cell could be connected to back side contact 2163 of the neighboring cell, so as to realize a series connection between the two photovoltaic cells. A parallel connection between two photovoltaic cells can be obtained connecting the respective front side contacts 2161 of each cell.

Moreover, although the embodiment has been described with reference to structure 8100 and steps S82, S13 and S14, any process which will result in the placement of a plurality of cells 8121-8124 onto a stretching material 1 150 can be used instead, as a basis for the subsequent stretching step S21 and placement of the separated cells onto a target substrate.

In some embodiments, the photovoltaic cell can have lateral dimensions in a range from 1 mm ² to 5cm ² , preferably a value of 2mm ² to 20mm ² , while the semiconductor structures 3161-3164 can have lateral dimensions in a range from 4mm ² to 2500mm ² , preferably a value of 9mm ² to 1 cm ² . In some embodiments the stretching can separate cells between 0.5mm to 1 cm, preferably 1.5mm to 5mm from wafer of size A to wafer of size B larger than A.

Additionally, the contacts 2161-2163 are only schematically represented and could be realized with several shapes. For instance, front cell contact 2162 on the cells 8121-8123 could be advantageously realized as a contact grid with minimum shadowing in the middle of the cell. Figure 4 schematically illustrates a possible operation of stretching material 1 150 in accordance with an embodiment of the present invention. In particular, Figure 4 illustrates a top view of the stretching material 1 150 which, in some embodiments, could be shaped so as to overlap with a wafer of size A, for instance a 4" wafer. More specifically, stretching material 1 150 can be stretched into any direction. In the specific example of Figure 4, the stretching material 1 150 is stretched in four directions as illustrated by arrows A1 -A4 resulting into stretched material 1 151 after stretching step S61 , which, in some embodiments, could be shaped so as to overlap with a wafer of size B, for instance a 8" wafer. The stretching could be carried out by clamps pulling with appropriate strength into the desired stretching direction, not illustrated, clamping the sides of the stretching material 1 150, and application of pressure, or by thermal expansion of the stretching material 1 150 or by any other suitable stretching means. The two dimensional stretching operation of step S61 can therefore be used in step S21 described above to stretch from wafer with size A to wafer with size B.

Figure 5 schematically illustrates the cut view and operation of a stretching material 7350 in accordance with a further embodiment of the present invention. In particular, stretching material 7350 of Figure 5 can be used as an alternative to the stretching material 1 150. Stretching material 1 150 or 7350 can be realized by different materials. In particular, stretching material 1 150 or 7350 could be made of metal and/or polymer: In the case of stretching material 7350 being made of metal, appropriate cuts can be placed at the fold edges to be able to maintain folding in the horizontal and vertical direction of the stretching material, that is, in the two planar directions. In case of stretching material 7350 being made of polymer an extra amount of matter placed at the fold edges leads to elasticity in the vertical and horizontal direction for instance, thereby keeping the part of the polymer in contact with the cells unstretched. Both configurations, metal or polymer material used for the stretching material 7350, are advantageously of use whenever stress on the cell has to be avoided. This is specifically the case if the semiconductor layer 8120 is a thin and brittle layer. In alternative embodiments a combination of several materials could be envisaged, for instance metallic plates interconnected with polymer material. The cells are attached to the metallic plates and the polymer interconnection provides the elasticity necessary for the stretching operation. One can choose polymers that, after deformation due to stretching, can be reversible deformed to the initial state for example by an appropriate heat treatment.

Stretching material 7350 schematically shows a plurality of vertical surfaces 7351 and a plurality of horizontal surfaces 7352 on which the plurality of cells 8121-8124 are placed. The vertical and horizontal surfaces can be connected by joints, or can be realized from a single, bent, element, or a combination of those two approaches can be used. During a stretching step S71 , the stretching material 7350 is stretched along directions A5 and A6 thereby resulting into partially stretched material 7353. As can be seen in the Figure 5, the vertical surfaces 7351 of stretching material 7350 move apart from each other, thereby resulting in the space between cells 8121 - 8141 to increase. During a plurality of stretching steps such as S71 and equivalent stretching step S72 along directions A7 and A8, the stretching material 7350 is stretched to a final position corresponding to stretched material 7354 in which the vertical surfaces 7351 have become horizontal, thereby becoming a prolongation of original horizontal surfaces 7352 with defined distance of the cells twice as large as the vertical surface7351. Although not illustrated, the same two dimensional stretching operation described in connection with Figure 4 can also be used for stretching material 7350.

The advantage of using stretching material 7350 over stretching material 1 150 consists in the fact that the stretching material 7350 does not need to be made of elastic material, thereby lowering costs and allowing for use of a wider range of material for the stretching material 1 150. Furthermore, since the stretching material 7350 is stretching by moving vertical surfaces 7351 , the lateral dimensions of horizontal surfaces 7352 remain unchanged. This provides the beneficial advantage of the contact surface between horizontal surfaces 7352 and cells 8121- 8124 is not subjected to any stress due to the potential stretching of horizontal surfaces 7352 providing better support for fragile solar cell layers.

Although the stretched material 7354 is illustrated as completely flat, the present invention is not limited thereto and the stretching material may retain a non-flat profile at the end of the stretching process.

In further embodiments one could envisage the use of the stretching material 1 150 as a final substrate, the semiconductor structure corresponding to the stretched structure 1 190 obtained by the stretching of the unstretched structure 1 180 corresponding to the final solar cell assemblies. This could be advantageous in case of a complete multijunction solar cell stack obtained on substrate 8130 which has been provided in an inverted manner so as to have the lowest band gap junction exposed on which the stretching material could be applied. Application of a metallic type of stretching material, at least at the positions used to attach the cells, would lead to the stretched structure 1 190 representing a multijunction solar cell stack on top of a metallic heat sink, thus forming a solar cell assembly which could be used and connected further in a solar cell module. In particular embodiments, the folds or joints of the metallic stretching material 7350 could be used as guidance for a subsequent separation step to individualize these solar cell assemblies.

Moreover, in alternative embodiments the positions of the folds or joints could be used for alignment purposes with respect to the final positioning and assembling on the target substrate.