Abstract:
This disclosure is related 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 result at a second distance from each other, wherein the second distance is greater that the first distance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/EP2014/056089, filed Mar. 26, 2014, designating the United States of America and published in English as International Patent Publication WO 2014/154767 A1 on Oct. 2, 2014, which claims the benefit under Article 8 of the Patent Cooperation Treaty and under 35 U.S.C. §119(e) to French Patent Application Serial No. 1352867, filed Mar. 29, 2013, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure 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. 
       BACKGROUND 
       [0003]    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 toward 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. 
         [0004]    One particularly advantageous form of photovoltaic generators consists of III-V concentrator generators. Even more specifically, multi-junction III-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. 
         [0005]    Such a standard approach is illustrated in  FIG. 6 . In particular,  FIG. 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 S 81 , a photovoltaic layer  8120  is provided. For instance, the photovoltaic layer  8120  could be grown by epitaxial growth of III-V semiconducting materials like indium gallium phosphide and gallium arsenide on, for example, a germanium wafer, leading to a common multi junction solar cell of type InGaP/GaAs/Ge. The germanium wafer could typically have a diameter of 4, 6, or 8 inches. The photovoltaic layer  8120  could further include an anti-reflective coating and metal contacts on the front and/or rear side. During a step S 82 , 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 the case of the above-mentioned InGaP/GaAs/Ge multi junction 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 multi-junction, 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 S 83 , 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 . 
         [0006]    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 WO2011/123285A1, 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. 
         [0007]    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. 
       BRIEF SUMMARY 
       [0008]    It is, therefore, an object of this disclosure 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 of size B, where B&gt;A, for instance, from 4 to 8 inches or from 6 to 12 inches. 
         [0009]    This disclosure 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. 
         [0010]    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. 
         [0011]    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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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. 
         [0017]    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 inches, 4 inches, 6 inches, or 8 inches to a respective larger size B of value 4 inches, 6 inches, 8 inches, or 12 inches. 
         [0018]    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. 
         [0019]    In some embodiments, the step of assembling can comprise bonding, in particular, direct bonding or metal bonding to the target substrate. 
         [0020]    This provides the beneficial advantage that an improved electrical and thermal connection can be achieved between the cells and the target substrate. 
         [0021]    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. 
         [0022]    This provides the beneficial advantage that an improved bonding relative to its strength and quality is obtained. 
         [0023]    In some embodiments, the target substrate can be a semiconductor substrate and can comprise contacts to the plurality of photovoltaic cells. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    In some embodiments, the manufacturing method can further comprise the step of assembling the plurality of solar cell assemblies on a heat sink. 
         [0028]    This provides the beneficial advantage that the heat generated in the cells can be dissipated. 
         [0029]    In some embodiments, the photovoltaic cells can be multi-junction III-V concentrator photovoltaic cells. 
         [0030]    This provides the beneficial advantage that efficiency of the cells can be increased. 
         [0031]    In some further embodiments, the multi junction III-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. 
         [0032]    This provides the beneficial advantage that the stretching process can be implemented in the manufacturing process of the III-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. 
         [0033]    In some further embodiments, the multi junction III-V concentrator photovoltaic cells can have a thickness of several micrometers, in particular, below 10 μm. 
         [0034]    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. 
         [0035]    Further, this disclosure 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 that separates the plurality of photovoltaic cells from each other when the stretched material is not stretched. 
         [0036]    In some embodiments, the stretching material can be made of metallic material at the position where the plurality of photovoltaic cells are attached. 
         [0037]    This provides the beneficial advantage that the thus-obtained semiconductor structure can represent a multi-junction solar cell bonded to a stretching material representing a final heat sink material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The disclosure 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: 
           [0039]      FIGS. 1 to 3  schematically illustrate steps of a manufacturing method for a plurality of photovoltaic cells in accordance with an embodiment of this disclosure; 
           [0040]      FIG. 4  schematically illustrates the operation of a stretching material in accordance with a further embodiment of this disclosure; 
           [0041]      FIG. 5  schematically illustrates the operation of an alternative stretching material in accordance with a further embodiment of this disclosure; and 
           [0042]      FIG. 6  schematically illustrates a manufacturing method for a solar cell assembly in accordance with the state of the art. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]      FIGS. 1 to 3  schematically illustrate a manufacturing method for a plurality of photovoltaic cells in accordance with an embodiment of this disclosure. 
         [0044]    Similar to what has been described with respect to  FIG. 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  FIG. 1  or even a conventional gallium arsenide, indium phosphide or germanium wafer on its own, with a typical diameter of 4 inches, 6 inches, or 8 inches. The photovoltaic layer  8120  may include an anti-reflective coating and metal contacts on the front and/or rear side. 
         [0045]    After a step S 82  is realized on the structure  8100 , as already illustrated schematically in  FIG. 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 S 13 , stretching material  1150  is applied on top of photovoltaic cells  8121 - 8124 . The stretching material  1150  is made of polymer and/or metal and/or other materials, which allow expansion by application of mechanical force, pressure or heat. For instance, one could use a thick material that 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  1150 , during a step S 14 , 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  1150 , thereby forming unstretched structure  1180 . In contrast to the standard pick and place approach described with respect to  FIG. 6 , the photovoltaic cells  8121 - 8124  have no minimum thickness required for the embodiments of this disclosure. The photovoltaic cells can, for instance, be formed on the 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 S 14 . For example, in accordance with this disclosure, in the case of an InGaP/GaAs/Ge multi junction cell, one could also remove the part of the germanium substrate wafer that is used for stiffening purposes that allowed a pick and place process. It becomes clear from  FIG. 1  that the unstretched structure  1180  can also be obtained by the application of the stretching material  1150  on the initial structure  8100 , removal of substrate  8130  and subsequent separation/cutting of the cells. In a further step, the stretching material  1150  and the unstretched structure  1180  are stretched during a stretching step S 21 , thereby resulting into stretched material  1151  and stretched structure  1190 . In particular, during the stretching step S 21 , the lateral dimension of the stretching material  1150  is increased by stretching the stretching material  1150  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  1150  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  1150 , 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  1150 . 
         [0046]    During a positioning step S 22 , 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  1151  is removed, thereby resulting into structure  8101 . In both cases, the semiconductor substrate  2160  or the heat sink  8140  acts as a target substrate for the assembling of the cells  8121 - 8124 . 
         [0047]    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, SiAl, Si on SiAl, etc. During a removal step S 23 , the stretched material  1151  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 S 24 , 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 electrically relay each of the back side contacts  2163  to each back of the photovoltaic cells. This results in 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. 
         [0048]    In both cases where the cells  8121 - 8124  are placed on a heat sink  8140  or on a semiconductor substrate  2160 , the use of the stretching material  1150  as a stretching element and as a positioning element is advantageous in simplifying the manufacturing process. In particular, the stretching material  1150  is first used as a stretching element in order to separate the cells  8121 - 8124  during stretching step S 21  and, then, further subsequently used as a structural element in positioning the cells  8121 - 8124  on their target position during positioning step S 22 . Thanks to this dual usage of the stretching material  1150 , the handling of the cells  8121 - 8124  is further simplified. 
         [0049]    As described above, positioning step S 22  can be carried out by keeping the connection between the stretched material  1151  and the cells  8121 - 8124 , while the cells are assembled onto heat sink  8140  or on semiconductor substrate  2160 . However, this disclosure is not limited thereto and positioning step S 22  can alternatively be carried out by using the stretched material  1151  for positioning the cells  8121 - 8124  onto the target substrate, removing the stretched material  1151  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 by gluing, magnetic or electrostatic attraction, or simply by friction. 
         [0050]    While  FIG. 2  schematically illustrates a cut view of the structures during the manufacturing steps S 21 -S 24 ,  FIG. 3  schematically illustrates a top view of the structures. In particular, at the top of  FIG. 3 , structure  2102  is schematically illustrated. In particular, although structure  2102  may, in some embodiments, be a circular wafer, to ease representation, only a rectangular section of it is represented in  FIG. 3 . During a cutting step S 31 , 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 S 32 , the plurality of solar cell assemblies can then be placed on top of a heat sink  8140  as illustrated by step S 32 , thereby realizing structure  3103 . In alternative embodiments, individual heat sinks are also possible for each solar cell assembly. 
         [0051]    Alternatively, or in addition, although not illustrated, each of semiconductor structures  3161 - 3164  may 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. 
         [0052]    Moreover, although the embodiment has been described with reference to structure  8100  and steps S 82 , S 13  and S 14 , any process that will result in the placement of a plurality of cells  8121 - 8124  onto a stretching material  1150  can be used instead, as a basis for the subsequent stretching step S 21  and placement of the separated cells onto a target substrate. 
         [0053]    In some embodiments, the photovoltaic cell can have lateral dimensions in a range from 1 mm 2  to 5 cm 2 , preferably a value of 2 mm 2  to 20 mm 2 , while the semiconductor structures  3161 - 3164  can have lateral dimensions in a range from 4 mm 2  to 2500 mm 2 , preferably a value of 9 mm 2  to 1 cm 2 . In some embodiments, the stretching can separate cells between 0.5 mm to 1 cm, preferably 1.5 mm to 5 mm from a wafer of size A to a wafer of size B larger than A. 
         [0054]    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. 
         [0055]      FIG. 4  schematically illustrates a possible operation of stretching material  1150  in accordance with an embodiment of this disclosure. In particular,  FIG. 4  illustrates a top view of the stretching material  1150  that, in some embodiments, could be shaped so as to overlap with a wafer of size A, for instance, a 4-inch wafer. More specifically, stretching material  1150  can be stretched into any direction. In the specific example of  FIG. 4 , the stretching material  1150  is stretched in four directions as illustrated by arrows A 1 -A 4 , resulting into stretched material  1151  after stretching step S 61 , which, in some embodiments, could be shaped so as to overlap with a wafer of size B, for instance, an 8-inch 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  1150 , and application of pressure, or by thermal expansion of the stretching material  1150  or by any other suitable stretching means. The two-dimensional stretching operation of step S 61  may, therefore, be used in step S 21  described above to stretch from wafer with size A to wafer with size B. 
         [0056]      FIG. 5  schematically illustrates the cut view and operation of a stretching material  7350  in accordance with a further embodiment of this disclosure. In particular, stretching material  7350  of  FIG. 5  can be used as an alternative to the stretching material  1150 . Stretching material  1150  or  7350  can be realized by different materials. In particular, stretching material  1150  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 reversibly deformed to the initial state, for example, by an appropriate heat treatment. 
         [0057]    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 S 71 , the stretching material  7350  is stretched along directions A 5  and A 6 , thereby resulting into partially stretched material  7353 . As can be seen in  FIG. 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 S 71  and equivalent stretching step S 72  along directions A 7  and A 8 , 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 surface  7351 . Although not illustrated, the same two-dimensional stretching operation described in connection with  FIG. 4  can also be used for stretching material  7350 . 
         [0058]    The advantage of using stretching material  7350  over stretching material  1150  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  1150 . Furthermore, since the stretching material  7350  is stretched 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. 
         [0059]    Although the stretched material  7354  is illustrated as completely flat, this disclosure is not limited thereto, and the stretching material may retain a non-flat profile at the end of the stretching process. 
         [0060]    In further embodiments, one could envisage the use of the stretching material  1150  as a final substrate, the semiconductor structure corresponding to the stretched structure  1190  obtained by the stretching of the unstretched structure  1180  corresponding to the final solar cell assemblies. This could be advantageous in case of a complete multi junction 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  1190  representing a multi junction solar cell stack on top of a metallic heat sink, thus forming a solar cell assembly that 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. 
         [0061]    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.