Patent Publication Number: US-2011056534-A1

Title: Semitransparent photovoltaic film

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98130062, filed on Sep. 7, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a light, thin, and flexible semitransparent photovoltaic film. 
     2. Description of Related Art 
     Solar energy is one of the renewable energy sources free of pollution. While crisis caused by the usage of fossil fuels such as pollutions, global warming, and shortage of supply emerge worldwide, attentions have been focused on effective utilize of solar energy. Being capable of transferring solar energy into electrical energy, the photovoltaics have received considerable effort to make this technology grow in global energy markets. 
     However, the primary factor impeding widespread utilization of photovoltaics is its relatively high cost of energy generation when compared with other electricity generation techniques. Therefore, many researchers have sought to decrease the cost by developing new materials and fabrication techniques. Many research studies have shown that a flexible PV may have the advantages of ease of storage and rapid roll to roll mass production due to its flexibility. In addition, using of flexible substrate advantages the flexible photovoltaics to the property of high energy to weight ratio, thus it may be suitable for use as a portable energy source. Moreover, if the conversion efficiency and the product lifespan are sufficient, the flexible photovoltaics could also compete in applications of Building Integrated Photovoltaics-BIPV such as solar roofing and facade systems based on flexibility (a PV truly integrated into building materials) and on cost. 
     From a market standpoint, products possessing special applications often demand a high premium. For instance, if solar cells possess characteristics of light and flexibility, the profit would be increased when applying them to portable electronics. If a “semitransparent” flexible photovoltaics could be developed and new applications such as a heat resistance and electricity generating thin film could be explored, it is expectable that a specific market (i.e. photovoltaic adiabatic paper) would be created and easily separated from other conventional photovoltaics by such particular product characteristics. 
     Recent researches, for example U.S. Pat. Nos. 6,180,871, 6,320,117, and 6,509,204 have proposed transparent solar cell structures constructed by using polycrystalline silicon thin films and transparent positive/negative electrodes. However, such proposals suffer not only high fabrication cost on polycrystalline silicon thin film forming, but also serious color shift of transmitting light. Moreover, a decrease in light absorption is not the only factor contributing to device efficiency loss. Other factors causing further device deterioration along with efficiency loss include not having a multiple reflecting surface structure while having thinner absorption layers. 
     Therefore, other researchers have suggested using high band gap (i.e. no visible light absorption) semiconductor materials (e.g. metal oxides mostly) to fabricate transparent solar cells. For example, US Patent Publication No. 2008/0053518 disclosed this technique to obtain non color shifting transparent solar cells. However, 51.8% of solar radiation lies in visible region, while only approximately 6% lies in the ultraviolet part absorbed by the aforementioned proposal. Hence, the decrease of solar power absorption of the high band gap semiconductor layer of the device results in deficient electric power generation. 
     In addition, other works, for example U.S. Pat. Nos. 4,137,098, 5,221,363, and 5,258,076, as well as US Patent Publication No. 2008/0257403 have proposed the novel design of solar window structures involving assembling PV components as strip-like horizontal slats into a module. Each slat in the module has an angle with the vertical surface, so that it can shield sunlight and generate electricity at the same time; however, having an enormous volume along with a poor aesthetic appearance and complex installation process, this semitransparent solar window is seldom applied in modern building constructions. 
     SUMMARY 
     The disclosed semitransparent photovoltaic film comprises a flexible substrate integrating a plurality of first planar portions and a plurality of second planar portions, and a plurality of photovoltaic cells. The second planar portions are coupled with the first planar portions to form an angle. The photovoltaic cells are formed on a plurality of surfaces of the first planar portions of the flexible substrate. 
     Another embodiment of the semitransparent photovoltaic film comprises a support substrate, a flexible substrate, and a plurality of photovoltaic cells. The aforementioned first support substrate has a first zigzag surface. The aforementioned flexible substrate integrates a plurality of first planar portions and a plurality of second planar portions. The second planar portions are coupled with the first planar portions to form an angle. The photovoltaic cells are formed on a surfaces of each of the first planar portions of the flexible substrate. The flexible substrate is further laminated on the first support substrate to form the photovoltaic film, on condition that a first planar surface is laminated on the first zigzag surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the embodiment, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the embodiment and, together with the description, serve to explain the principles of the embodiment. 
         FIG. 1  is a schematic three-dimensional view of a semitransparent photovoltaic film in accordance with a first embodiment. 
         FIG. 2  illustrates an alternative embodiment to the first embodiment. 
         FIG. 3  is a schematic three-dimensional view of a semitransparent photovoltaic film in accordance with a second embodiment. 
         FIG. 4  is a schematic three-dimensional view of a semitransparent photovoltaic film in accordance with a third embodiment. 
         FIG. 5  is a schematic three-dimensional view of a semitransparent photovoltaic film in accordance with a fourth embodiment. 
         FIG. 6A  illustrates a flexible substrate of the semitransparent photovoltaic film in accordance with some embodiments. 
         FIG. 6B  illustrates a support substrate in accordance with some embodiments. 
         FIG. 6C  is a schematic diagram illustrating a rapid integration process of the flexible substrate depicted in  FIG. 6A  and the support substrate depicted in  FIG. 6B . 
         FIGS. 7A-7B  are schematic diagrams illustrating another fabrication process of the semitransparent photovoltaic film in accordance with some embodiments. 
         FIG. 8  is an exploded three-dimensional view of a semitransparent photovoltaic film in accordance with a fifth embodiment. 
         FIG. 9  is a diagram illustrating a relation between sunlight exposure and the semitransparent photovoltaic film in accordance with some embodiments. 
         FIG. 10  is a diagram illustrating a relationship of sunlight collection efficiency (solid line) and horizontal light transmittance (dotted line) versus the α angle. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
       FIG. 1  is a schematic three-dimensional view of a semitransparent photovoltaic film in accordance with a first embodiment. 
     Referring to  FIG. 1 , a semitransparent photovoltaic film  100  of the embodiment includes a flexible substrate  102  and a plurality of photovoltaic cells  104 . The flexible substrate  102  integrates a plurality of first planar portions  106  and a plurality of second planar portions  108 . Moreover, the second planar portions  108  and the first planar portions  106  are coupled to each other so as to form an angle α. In other words, the first planar portions  106  and the second planar portions  108  are formed on the aforementioned flexible substrate  102 . For example, a coupling side is formed by a side of the first planar portions  106  for coupling to the second planar portions  108 , so as to separate the other sides of the first planar portions  106  from the second planar portions  108 . Thereafter, by using the coupling side as an axis, the first planar portions  106  and the second planar portions  108  form the above-mentioned angle. In addition, the second planar portions  108  may form a rectangular frame structure having a plurality of first planar portions  106  configured therein. In the embodiments described hereinafter, the second planar portions are similar. The first planar portions  106  can be rectangular. Using  FIG. 1  as an example, the first planar portions  106  are formed by each of the photovoltaic cells  104  arranged as a parallel fan and assembled in the second planar portions  108 . 
     Referring again to  FIG. 1 , the flexible substrate  102  can be a light transmitting or an opaque substrate. The transparent substrate is plastic or glass, or the like, for example. Moreover, the opaque substrate is a metal substrate (e.g., aluminum substrate, stainless steel, molybdenum substrate, or the like) or an opaque plastic substrate (e.g., polyimide substrate or the like). The photovoltaic cells  104  are formed on the surfaces  106   a  of the first planar portions  106  of the flexible substrate  102 . Depending on different incident light directions, the structure of a photovoltaic cell can be categorized into a superstrate structure and a substrate structure. The superstrate structure starts with coating a transparent electrode (e.g., transparent conductive oxide (TCO)) under the substrate, and thereafter coating the photoelectric conversion cell and the opaque electrode (e.g., metal conductive layer) in turns. On the other hand, the substrate structure starts with coating the opaque electrode above the substrate, then coating in turns the photoelectric conversion cell and finally the transparent electrode. Since light is incident on the photovoltaic cells  104  side, the above-described photovoltaic cells  104  are a substrate structure photovoltaic cell. Therefore, the flexible substrate  102  can be a light transmitting substrate or an opaque substrate. The photovoltaic cells  104  include an opaque electrode  110 , a light transmitting electrode  112  disposed on the opaque electrode  110 , and a photoelectric conversion cell  114  disposed between the opaque electrode  110  and the transparent electrode  112 . 
     Continuing reference to  FIG. 1 , the material of the aforementioned opaque electrode  110  includes metals (e.g., aluminum, silver, etc.) or alloys (e.g., silver-aluminum alloy etc.). For instance, the photoelectric conversion cell  114  is an amorphous silicon thin film photovoltaic cell, a CuInGaSe 2  (CIGS) thin film photovoltaic cell, an organic photovoltaic cell, a CdTe thin film photovoltaic cell, or the like. For example, the photoelectric conversion cell  114  can be an amorphous silicon thin film photovoltaic cell formed by buffer layers (e.g., ZnO), n-i-p amorphous layers, and transparent conductive oxide (TCO) materials (e.g., JO, TO, ZO, ITO, or IZO). Additionally, the photoelectric conversion cell  114  can also be a CIGS thin film photovoltaic cell formed by molybdenum electrodes and materials such as CIGS, CdS (or other suitable materials) and ZnO. Furthermore, the photoelectric conversion cell  114  can also be an organic photovoltaic cell formed by buffer layers, polymer blends or p/n bilayers, and buffer layer and TCO materials (e.g., JO, TO, ZO, ITO, or IZO). The photoelectric conversion cell  114  can also be a CdTe thin film photovoltaic cell formed by CdTe, CdS, and TCO materials (e.g., JO, TO, ZO, ITO, or IZO). 
     Referring again to  FIG. 1 , each of the second planar portions  108  has at least a light transmitting opening  120  to allow light to pass, when the flexible substrate  102  is an opaque substrate. Moreover, each of the second planar portions  108  can also have an adhesive surface  108   a  in order to widen an application surface of the semitransparent photovoltaic film  100 . For instance, the semitransparent photovoltaic film  100  can be adhered to structures such as the outer windows of buildings. The outer windows can be a glass curtain (or wall), on which sunlight can be absorbed but the aesthetic appearance of the building is not affected. Since the semitransparent photovoltaic film  100  of the first embodiment can absorb sunlight incident from above while not obstructing a horizontal light  122  and a light beneath  124 , the semitransparent photovoltaic film  100  visually appears transparent because the film can absorb most of the incident sunlight and further allow the horizontal light and the light underneath to penetrate. 
     Continuing reference to  FIG. 1 , the above-described semitransparent photovoltaic film  100  also has a plurality of conductive lines  116  and  118  formed on the flexible substrate  102 . In order to increase conductivity, the conductive lines  116  are coupled to the light transmitting electrode  112  of each of the photovoltaic cells  104 . Since two neighboring (i.e. located above and below each other) light transmitting electrodes  112  on the first planar portions  106  are coupled to each other through the conductive lines  116 , and the opaque electrodes  110  are coupled to each other through the conductive lines  118 , a parallel configuration coupling electrodes of the same polarity is formed. 
     Besides, an alternative embodiment to the first embodiment is illustrated in  FIG. 2 . Except for the location of the conductive lines  202 , the rest of the structure of a semitransparent photovoltaic film  200  depicted in  FIG. 2  is the same as depicted in  FIG. 1 . In order to turn on the photovoltaic cells  104 , the conductive lines  202  depicted in  FIG. 2  are coupled to the light transmitting electrodes  112  of the photovoltaic cells  104  as well as the opaque electrodes  110  of the next one of the photovoltaic cells  104 . Therefore, a serial configuration of opposite polarity electrodes can be formed. 
       FIG. 3  is a schematic cross-sectional view illustrating a semitransparent photovoltaic film in accordance with a second embodiment. Same reference numerals as those according to the first embodiment are used to represent same components. 
     Referring to  FIG. 3 , a photovoltaic cell  302  in a semitransparent photovoltaic film  300  of the embodiment is a superstrate structure photovoltaic cell. The superstrate structure refers to light being incident on the substrate side. Therefore, a light transmitting substrate is required for the flexible substrate  102 . If this design is adopted, the light transmitting electrodes  112 , the photoelectric conversion cell  114 , and the opaque electrode  110  are sequentially formed on the surfaces  106   b  of the first planar portions  106  of the flexible substrate  102  (light transmitting substrate). The second planar portions  108  and the first planar portions  106  are coupled to each other so as to form an angle α. Moreover, the horizontal light  122  and the light beneath  124  are not shielded. Most of the incident sunlight can be absorbed while further allowing the horizontal light and the light beneath to pass though the semitransparent photovoltaic film  300 , thereby achieving a transparent visual effect. In addition, the photovoltaic cell  302  depicted in  FIG. 3  uses the conductive lines  202  of  FIG. 2  to form a serial configuration, although the disclosure is not limited thereto. 
       FIG. 4  is a schematic three-dimensional view illustrating a semitransparent photovoltaic film in accordance with a third embodiment. Same reference numerals as those according to the first embodiment are used to represent same components. 
     Referring to  FIG. 4 , a difference between a semitransparent photovoltaic film  400  of the embodiment and the semitransparent photovoltaic film  100  of the first embodiment is that extra conductive lines are not required (e.g., conductive lines  116  and  118  of  FIG. 1  and conductive lines  202  of  FIG. 2 ). By contrast, the opaque electrode  110 , the photoelectric conversion cell  114 , and the light transmitting electrode  112  are sequentially formed on the surface  106   a  of the first planar portions as well as a surface  108   b  of the second planar portions  108 . Moreover, the opaque electrode  110  and the transparent electrode  112  on the surface  108   b  of the second planar portions  108  are used as conductive lines to directly couple the photovoltaic cells  104  on different first planar portions  106 . Besides, the above-described photovoltaic cells  104  are substrate structure photovoltaic cells. Therefore, the flexible substrate  102  can be a light transmitting substrate or an opaque substrate. 
       FIG. 5  is a schematic three-dimensional view illustrating a semitransparent photovoltaic film in accordance with a fourth embodiment. Same reference numerals as those according to the second embodiment are used to represent same components. 
     Referring to  FIG. 5 , a difference between a semitransparent photovoltaic film  500  of the embodiment and the semitransparent photovoltaic film  200  of the second embodiment is that extra conductive lines are not required (e.g., conductive lines  202  of  FIG. 3 ). By contrast, the light transmitting electrode  112 , the photoelectric conversion cell  114 , and the opaque electrode  110  are sequentially formed and completely cover the surface  106   a  of the first planar portions as well as the surface  108   b  of the second planar portions  108 . Moreover, the opaque electrode  110  and the transparent electrode  112  on the surface  108   b  of the second planar portions  108  are used as conductive lines to directly couple the photovoltaic cell  302  on different first planar portions  106 . 
     For fabrication of the above-described semitransparent photovoltaic film depicted in  FIGS. 1-5 , currently available techniques can be used. For example, as shown in  FIG. 6A , a required photovoltaic cell (not shown) can be formed on a flexible substrate  600 , and a plurality of planar sections  602  can be formed by laser cutting or mechanical cutting. Areas outside the first planar sections  602  form a plurality of second planar sections  604 . The plastic molding of the flexible substrate can be formed by a heating process or a pressurizing process so that the semitransparent photovoltaic film depicted in  FIGS. 1-5  is fabricated. Alternatively, as shown in  FIG. 6A , by using an ultraviolet (UV) molding technique, a support substrate  608  including a zigzag surface  608   a  is fabricated. Thereafter, as shown in  FIG. 6C , the flexible substrate  600  and the support substrate  608  can be rapidly integrated by using a roll to roll process. 
     Another technique to fabricate the semitransparent photovoltaic film of the disclosure is illustrated in  FIG. 7A-7B . In  FIG. 7A , after the flexible substrate  600  is cut, the flexible substrate  600  is integrated with the plastic molded support substrates  700  and  702 . As shown in  FIG. 7B , after integration, a tightly encapsulated semitransparent photovoltaic film is formed. A total thickness T of the completed semitransparent photovoltaic film is between 1 mm and 15 mm. 
       FIG. 8  is an exploded three-dimensional view of a semitransparent photovoltaic film in accordance with a fifth embodiment. 
     Referring to  FIG. 8 , a semitransparent photovoltaic film  800  of the embodiment includes a first support substrate  802 , a flexible substrate  804 , and a photovoltaic cell  806 . The aforementioned first support substrate  802  has a first zigzag surface  802   a . The flexible substrate  804  is laminated on the zigzag surface  802   a  of the first support structure  802 . After lamination, the flexible substrate  804  of the fifth embodiment can have a zigzag shape allowing the flexible substrate  804  to be disposed on the zigzag surface  802   a  of the first support substrate  802 . The aforementioned flexible substrate  804  integrates a plurality of first planar portions  808  and a plurality of second planar portions  810 . The second planar portions  810  and the first planar portions  808  are adjacent and coupled to each other so as to form an angle α. By configuring the second planar portions  810  and the first planar portions  808  to indirectly couple to each other, a zigzag structure is formed. The photovoltaic cell  806  is formed on a surface  808   a  of the first planar portions  808  of the flexible substrate  804 . For the material and the configuration of the flexible substrate  804  and the photovoltaic cell  806  of the embodiment, since points of reference can be directed to the above-described embodiments, no further description is provided hereinafter. Moreover, the first support substrate  802  of the embodiment further includes a first planar surface  802   b  opposing the first zigzag surface  802   a . For example, the first planar surface  802   b  is an adhesive surface that helps to widen the application surface of the semitransparent photovoltaic film  800 . By adhering the film to structures such as the outer windows of buildings, the aesthetic appearance of the buildings can be preserved while sunlight is absorbed. Moreover, each of the second planar portions  810  has at least a light transmitting opening  812  to allow light to penetrate, specifically when the flexible substrate  804  is an opaque substrate. 
     Besides, in the fifth embodiment, the aforementioned semitransparent photovoltaic film  800  further includes a second support substrate  814 . The second support substrate has a second zigzag surface  814   a  complementing the first zigzag surface  802   a  of the first support substrate  802 . Moreover, the flexible substrate  804  is laminated between the first zigzag surface  802   a  of the first support substrate  802  and the second zigzag surface  814   a  of the second support substrate  814 . The second support substrate can also have a second planar surface  814   b  opposing the second zigzag surface  814   a . Furthermore, the aforementioned second planar surface  814   b  is an adhesive surface, for example, that helps widen the application surface of the semitransparent photovoltaic film. In the embodiment of the disclosure, the above-described first support substrate  802  and the second support substrate  814  can be made of soft materials, such as plastic or glass. In addition, one of the first support substrate  802  or the second support substrate  814  is, for example, disposed on a light receiving surface of the semitransparent photovoltaic film  800  to be light transmitting. 
     According to embodiments of the disclosure, the design principles for the semitransparent photovoltaic film need to consider the sunlight collection efficiency as well as the light transmittance. Note that the sunlight collection efficiency is defined as the fraction of sunlight incident on the first planar portions when sun moves from the horizon to the zenith.  FIG. 9  illustrates the relationship between incident sunlight and the semitransparent photovoltaic film as embodied herein. The parameter L represents the length of the first planar portions, H represents the vertical distance between two first planar portions, Ha represents the length of the shadow cast by the first planar portions on the second planar portions (vertical plane), α represents the angle formed between the first planar portions and the second planar portions, and θ represents the angle between the incident light and the second planar portions. 
       FIG. 10  exhibits the calculated results of the sunlight collection efficiency (solid line) and horizontal light transmittance (dashed line) of the semitransparent photovoltaic film versus α. The calculations considered not only the variation of sunlight incident angle θ [determined as a function of air mass (AM=cos −1 θ)], but also the change of global irradiance I G  according to the following experimentally determined formula (reference: Meinel A. B., and Meinel M. P.,  Applied Solar Energy , Addison Wesley Publishing Co., 1976): 
         I   G =1.1·1.353·0.7 AM     0.678   .
 
     Note that the calculation results are applicable to the case that the surfaces of the first planar portions are curved. In the curved surface case, L values would rather be defined as the distance between the edge and the end of the first planar portions than be defined as the length of the first planar portions. 
     When α=0°, the values of (H-L)/H are substantially the fractions of the areas of the light transmitting opening to the total area of the flexible substrate. As the value of α increases, the horizontal light transmittance increases monotonically (the dashed lines), whereas the sunlight collection efficiency has a maximum value (the solid lines). The value of α corresponding to the maximum sunlight collection efficiency shifts toward zero as the value of L/H increases. When L/H=1, the sunlight is most efficiently collected at α=0°. It is noted that each design of the semitransparent photovoltaic film with a L/H value should consider two particular characteristic α values. One design benefits the semitransparent photovoltaic film by maximizing the sunlight collection efficiency while the improvement of light transmittance is limited; the other one maximize the light transmittance while keeping the value of sunlight collection efficiency the same as that at α=0°. Therefore, the design range of the semitransparent photovoltaic film of the disclosure as embodied herein includes these two characteristic points and the range within, unless other considerations are factored. 
     Referring to  FIG. 10  and using L/H=0.7 as an example, when α=0 (named as conventional design in the later description), the sunlight collection efficiency is 70% and the light transmittance is 30%. When α=28.8 °, the maximum sunlight collection efficiency design is reached and shows the value of 87.1%, whereas the light transmittance is 38.7%. The increment of sunlight collection efficiency is as high as 17.1% compare to that of the conventional design. When α=75.6°, the maximum light transmittance design is reached and show the value of 82.6%, whereas the sunlight collection efficiency is still 70.7%. Without sacrificing the sunlight collection efficiency, the design of the semitransparent photovoltaic film allows the light transmittance to be increased by the increment of 52.6%. In other words, when L/H=0.7, a preferable design range of a lies approximately within 28.8° to 75.6°. 
     In light of the foregoing, according to the disclosure as embodied herein, the semitransparent photovoltaic film formed by an integrated flexible substrate absorbs most part of the sunlight for generating electricity, as well as transmits the relatively weak horizontal light and the light underneath allowing the human vision to see through it, thereby achieving the transparent visual effect. Having the characteristics of light weight, thinness, and flexibility, the semitransparent photovoltaic film disclosed herein can be designed as a flexible solar film capable of generating electricity and shielding the sunlight at the same time, thereby making the semitransparent photovoltaic film suitable for mass production and applicable on BIPV. 
     A semitransparent photovoltaic film is provided which is characterized by the properties of light weight, thinness and flexibility. It can be designed as a flexible solar film capable of generating electric energy and shielding the sunlight, which makes the film applicable to adhere on the outer window surface of modern building constructions. 
     The first support substrate of the photovoltaic film is required to be a light transmitting substrate if the first support substrate is disposed on a light receiving side of the semitransparent photovoltaic film. It is noted that transparency is defined herein as a visible light transmittance, whereas light transmitting is defined herein as a light transmittance according to the absorption spectrum of the photovoltaic cells. 
     In summary, the sunlight has its nature of high intensity at top-incident angle. On the other hand, collecting the horizontal light and the light underneath passing through a vertical object by eyes, people can see-through the vertical object and have a transparent visual experience. Therefore, the semitransparent photovoltaic film of the embodiment efficiently absorbs most part of the sunlight for generating electricity, as well as transmits the relatively weak horizontal light and the light underneath allowing the human vision to see through it, thereby achieving the transparent visual effect. 
     Although the embodiment has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the embodiment. Accordingly, the scope of the embodiment will be defined by the attached claims not by the above detailed descriptions.