Patent Publication Number: US-2023136958-A1

Title: Photovoltaic cell device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of PCT Application No. PCT/JP2021/024611, filed Jun. 29, 2021 and based upon and claiming the benefit of priority from Japanese Patent Application No. 2020-119970, filed Jul. 13, 2020, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a photovoltaic cell device. 
     BACKGROUND 
     Recently, various types of transparent photovoltaic cells have been suggested. For example, a display device comprising a transparent dye-sensitized photovoltaic cell on the surface of the display device has been suggested. In such a photovoltaic cell device, a technique which can increase the size at low cost has been required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 1. 
         FIG.  2    is a cross-sectional view schematically showing the structure of an optical element  3 . 
         FIG.  3    is a plan view schematically showing the photovoltaic cell device  100 . 
         FIG.  4    is a cross-sectional view schematically showing an example of the optical element  3 . 
         FIG.  5    is a cross-sectional view schematically showing the optical element  3  according to modified example 1 of embodiment 1. 
         FIG.  6    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2. 
         FIG.  7    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 2. 
         FIG.  8    is a cross-sectional view of the photovoltaic cell device  100  along the A-B line of  FIG.  7   . 
         FIG.  9    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 1 of embodiment 2. 
         FIG.  10    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2 of embodiment 2. 
         FIG.  11    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 3 of embodiment 2. 
         FIG.  12    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 4 of embodiment 2. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a photovoltaic cell device comprises a first optical waveguide comprising a first main surface, a second main surface facing the first main surface, and a first side surface, an optical element facing the second main surface, comprising a cholesteric liquid crystal, and reflecting at least part of light incident on the first main surface toward the first optical waveguide, and a first photovoltaic cell facing the first side surface. The first photovoltaic cell is attached to the first side surface by a transparent first adhesive layer. 
     Each embodiment can provide a photovoltaic cell device whose size can be increased at low cost. 
     Embodiments will be described hereinafter with reference to the accompanying drawings. The disclosure is merely an example, and proper changes in keeping with the spirit of the invention, which are easily conceivable by a person of ordinary skill in the art, come within the scope of the invention as a matter of course. In addition, in some cases, in order to make the description clearer, the widths, thicknesses, shapes, etc., of the respective parts are illustrated schematically in the drawings, rather than as an accurate representation of what is implemented. However, such schematic illustration is merely exemplary, and in no way restricts the interpretation of the invention. In addition, in the specification and drawings, structural elements which function in the same or a similar manner to those described in connection with preceding drawings are denoted by like reference numbers, detailed description thereof being omitted unless necessary. 
     In the drawings, in order to facilitate understanding, an X-axis, a Y-axis and a Z-axis orthogonal to each other are shown depending on the need. A direction parallel to the Z-axis is referred to as a first direction A 1 . A direction parallel to the Y-axis is referred to as a second direction A 2 . A direction parallel to the X-axis is referred to as a third direction A 3 . The first direction A 1 , the second direction A 2  and the third direction A 3  are orthogonal to each other. The plane defined by the X-axis and the Y-axis is referred to as an X-Y plane. The plane defined by the X-axis and the Z-axis is referred to as an X-Z plane. The plane defined by the Y-axis and the Z-axis is referred to as a Y-Z plane. 
     Embodiment 1 
       FIG.  1    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 1. The photovoltaic cell device  100  comprises an optical waveguide  1 , an optical element  3 , a photovoltaic cell  5  and an adhesive layer (first adhesive layer)  7 . 
     The optical waveguide  1  consists of a transparent member which transmits light, for example, a transparent glass plate or a transparent synthetic resinous plate. For example, the optical waveguide  1  may consist of a transparent synthetic resinous plate having flexibility. The optical waveguide  1  could have an arbitrary shape. For example, the optical waveguide  1  may be curved. For example, the refractive index of the optical waveguide  1  is greater than that of air. The optical waveguide  1  functions as, for example, window glass. 
     In this specification, light includes visible light and invisible light. For example, the wavelength of the lower limit of a visible light range is greater than or equal to 360 nm but less than or equal to 400 nm. The wavelength of the upper limit of a visible light range is greater than or equal to 760 nm but less than or equal to 830 nm. Visible light includes the first component (blue component) of a first wavelength range (for example, 400 to 500 nm), the second component (green component) of a second wavelength range (for example, 500 to 600 nm), and the third component (red component) of a third wavelength range (for example, 600 to 700 nm). Invisible light includes ultraviolet light having a wavelength range in which the wavelength is shorter than the first wavelength range, and infrared light having a wavelength range in which the wavelength is longer than the third wavelength range. 
     In this specification, the term “transparent” should preferably mean “colorless and transparent”. However, the term “transparent” may mean “semitransparent” or “colored and transparent”. 
     The optical waveguide  1  is shaped like a flat plate parallel to an X-Y plane and comprises a first main surface F 1 , a second main surface F 2  and an external side surface F 3 . The first main surface F 1  and the second main surface F 2  are surfaces substantially parallel to the X-Y plane and face each other in a first direction A 1 . The external side surface F 3  is a surface extending in the first direction A 1 . In the example shown in  FIG.  1   , the external side surface F 3  is a surface substantially parallel to an X-Z plane. The external side surface F 3  includes a surface substantially parallel to a Y-Z plane. 
     The optical element  3  faces the second main surface F 2  of the optical waveguide  1  in the first direction A 1 . The optical element  3  reflects at least part of the light LTi which entered the first main surface F 1  toward the optical waveguide  1 . For example, the optical element  3  comprises a liquid crystal layer  31  which reflects, of the incident light LTi, at least one of first circularly polarized light and second circularly polarized light which rotates in the opposite direction of the first circularly polarized light. The first circularly polarized light and the second circularly polarized light may be visible light including the first, second and third components described above or may be invisible light. In this specification, reflection in the optical element  3  is accompanied by diffraction inside the optical element  3 . 
     It should be noted that, for example, the optical element  3  may have flexibility. Further, the optical element  3  may be in contact with the second main surface F 2  of the optical waveguide  1 . Alternatively, a transparent layer such as an adhesive layer may be interposed between the optical element  3  and the optical waveguide  1 . It is preferable that the refractive index of the layer interposed between the optical element  3  and the optical waveguide  1  should be substantially equal to that of the optical waveguide  1 . The optical element  3  is configured as, for example, a film. 
     The photovoltaic cell  5  faces the external side surface F 3  of the optical waveguide  1  in a second direction A 2 . The photovoltaic cell  5  receives light and converts the energy of the received light into electricity. Thus, the photovoltaic cell  5  generates electricity by the received light. The type of the photovoltaic cell is not particularly limited. The photovoltaic cell  5  is, for example, a silicon-based photovoltaic cell, a compound-based photovoltaic cell, an organic photovoltaic cell, a perovskite photovoltaic cell or a quantum dot photovoltaic cell. The silicon-based photovoltaic cell includes a photovoltaic cell comprising amorphous silicon, a photovoltaic cell comprising polycrystalline silicon, etc. 
     The adhesive layer  7  is transparent and attaches the photovoltaic cell  5  to the external side surface F 3 . The refractive index of the adhesive layer  7  is substantially equal to that of the optical waveguide  1 . Here, the phrase “substantially equal to” means that the difference between the refractive index of the adhesive layer  7  and the refractive index of the optical waveguide  1  is less than or equal to 0.1 in the wavelength of causing reflection and diffraction, and should be preferably less than or equal to 0.05. 
     Now, in the embodiment 1 shown in  FIG.  1   , the operation of the photovoltaic cell device  100  is explained. 
     The light LTi incident on the first main surface F 1  of the optical waveguide  1  is, for example, solar light. 
     In the example of  FIG.  1   , in order to facilitate understanding, light LTi is assumed to enter the optical waveguide  1  so as to be substantially perpendicular to the optical waveguide  1 . It should be noted that the incident angle of light LTi with respect to the optical waveguide  1  is not particularly limited. For example, light LTi may enter the optical waveguide  1  at a plurality of incident angles different from each other. 
     Light LTi proceeds into the optical waveguide  1  through the first main surface F 1  and enters the optical element  3  via the second main surface F 2 . The optical element  3  reflects light LTr which is part of light LTi toward the optical waveguide  1  and the photovoltaic cell  5  and transmits the other light LTt. Here, a light loss such as absorption in the optical waveguide  1  and the optical element  3  is ignored. The light LTr reflected on the optical element  3  is equivalent to the first circularly polarized light having a predetermined wavelength. The light LTr which passes through the optical element  3  includes the second circularly polarized light having a predetermined wavelength and light having a wavelength different from the predetermined wavelength. In this specification, circularly polarized light may be strict circularly polarized light or may be circularly polarized light which approximates elliptically polarized light. 
     The optical element  3  reflects the first circularly polarized light toward the optical waveguide  1  at an entering angle θ which satisfies the optical waveguide conditions in the optical waveguide  1 . Here, the entering angle θ is equivalent to an angle greater than or equal to a critical angle θc which causes total reflection inside the optical waveguide  1 . The entering angle θ indicates an angle with respect to a perpendicular line orthogonal to the optical waveguide  1 . 
     Light LTr proceeds into the optical waveguide  1  through the second main surface F 2  and propagates inside the optical waveguide  1  while repeating reflection in the optical waveguide  1 . 
     The photovoltaic cell  5  receives the light LTr emitted from the external side surface F 3  and generates electricity. 
     The photovoltaic cell  5  is required to efficiently use the received light for electric generation. For example, in the silicon-based photovoltaic cell  5 , if an antireflective film is formed or an antireflective structure is formed on a silicon surface as a technique of preventing light reflection on a silicon surface, the cost may be increased. 
     In embodiment 1, the photovoltaic cell  5  is attached to the external side surface F 3  of the optical waveguide  1  by the transparent adhesive layer  7 , and receives the light reflected on the optical element  3  via the optical waveguide  1 . When the optical element  3  reflects light toward the optical waveguide  1 , the optical element  3  can control the direction of the reflection by the helical structures  311  described later. Thus, the incident angle of light on the photovoltaic cell device  5  can be controlled in the optical waveguide  1 , thereby preventing reflection on a silicon surface. 
     Further, in embodiment 1, the photovoltaic cell  5  is attached to the external side surface F 3  of the optical waveguide  1  by the transparent adhesive layer  7 . The refractive index of the adhesive layer  7  is substantially equal to that of the optical waveguide  1 . Thus, even if light is reflected on the photovoltaic cell device  5 , the light can be guided to the optical waveguide  1  again with a very little loss and can be reused for electric generation. In other words, the adhesive layer  7  secures the photovoltaic cell  5  to the optical waveguide  1  and forms an optical path in which the loss is low between the optical waveguide  1  and the photovoltaic cell  5 . 
     Thus, compared to a case where a technique of forming an antireflective film or forming an antireflective structure is applied, an additional component for securing the photovoltaic cell  5  to the optical waveguide  1  is not needed. In this way, the photovoltaic cell device  100  in which the loss is low can be provided at low cost. 
       FIG.  2    is a cross-sectional view schematically showing the structure of the optical element  3 . The optical waveguide  1  is shown by alternate long and two short dashes lines. 
     The optical element  3  comprises a plurality of helical structures  311 . Each of the helical structures  311  extends in the first direction A 1 . In other words, the helical axis AX of each of the helical structures  311  is substantially perpendicular to the second main surface F 2  of the optical waveguide  1 . The helical axis AX is substantially parallel to the first direction A 1 . Each of the helical structures  311  has a helical pitch P. The helical pitch P indicates one pitch (360 degrees) of the helix. Each of the helical structures  311  includes a plurality of elements  315 . The elements  315  are helically stacked in the first direction A 1  while twisting. 
     The optical element  3  comprises a first interface  317  facing the second main surface F 2 , a second interface  319  on the opposite side of the first interface  317 , and a plurality of reflective surfaces  321  between the first interface  317  and the second interface  319 . The light LTi emitted from the second main surface F 2  after passing through the optical waveguide  1  enters the first interface  317 . Each of the first interface  317  and the second interface  319  is substantially perpendicular to the helical axis AX of each helical structure  311 . Each of the first interface  317  and the second interface  319  is substantially parallel to the optical waveguide  1  (or the second main surface F 2 ). 
     The first interface  317  includes the element  315  which is located in an end portion e 1  of the both end portions of each helical structure  311 . The first interface  317  is located in the boundary between the optical waveguide  1  and the optical element  3 . The second interface  319  includes the element  315  which is located in the other end portion e 2  of the both end portions of each helical structure  311 . The second interface  319  is located in the boundary between the optical element  3  and an air layer. 
     In embodiment 1, the reflective surfaces  321  are substantially parallel to each other. Each reflective surface  321  inclines with respect to the first interface  317  and the optical waveguide  1  (or the second main surface F 2 ) and has substantially a plane shape extending in a certain direction. Each reflective surface  321  applies selective reflection to light LTr of the light LTi which entered the first interface  317  in accordance with the Bragg&#39;s law. Specifically, each reflective surface  321  reflects light LTr such that the wavefront WF of light LTr is substantially parallel to the reflective surface  321 . More specifically, each reflective surface  321  reflects light LTr based on the inclination angle φ of the reflective surface  321  with respect to the first interface  317 . 
     The reflective surfaces  321  can be defined as follows. The refractive index sensed by the light (for example, circularly polarized light) which is selectively reflected in the optical element  3  and has a predetermined wavelength gradually changes as the light travels inside the optical element  3 . Thus, the Fresnel reflection gradually occurs in the optical element  3 . In the helical structures  311 , a position at which the change in the refractive index sensed by light is the largest exhibits the strongest Fresnel reflection. In other words, each reflective surface  321  is equivalent to a surface which exhibits the strongest Fresnel reflection in the optical element  3 . 
     Of the helical structures  311 , the alignment directions of the elements  315  of the helical structures  311  which are adjacent to each other in the second direction A 2  are different from each other. Further, of the helical structures  311 , the spacial phases of the helical structures  311  which are adjacent to each other in the second direction A 2  are different from each other. Each reflective surface  321  is equivalent to a surface in which the alignment directions of the elements  315  are uniform, or a surface in which spacial phases are uniform. In other words, each of the reflective surfaces  321  inclines with respect to the first interface  317  or the optical waveguide  1 . 
     It should be noted that the shape of each reflective surface  321  is not limited to the plane shape shown in  FIG.  2   , and may be a curved shape such as a concave shape or a convex shape, and thus, is not particularly limited. Part of each reflective surface  321  may be uneven. The inclination angles φ of the reflective surfaces  321  may not be uniform. The reflective surfaces  321  may not be regularly aligned. The reflective surfaces  321  may be configured to have arbitrary shapes based on the distribution of the spacial phases of the helical structures  311 . 
     In the present embodiment, the helical structures  311  are cholesteric liquid crystals. Each of the elements  315  is equivalent to a liquid crystal molecule. In  FIG.  2   , in order to simplify the figure, each element  315  represents a liquid crystal molecule which faces an average alignment direction as a representative of the liquid crystal molecules located in the X-Y plane. 
     Cholesteric liquid crystals which are the helical structures  311  reflect circularly polarized light which is light having a predetermined wavelength λ included in a selective reflection range Δλ and which rotates in the same rotation direction as the twist directions of the helices of the cholesteric liquid crystals. For example, when the twist direction of the cholesteric liquid crystal is right-handed, of the light having the predetermined wavelength A, the cholesteric liquid crystal reflects right-handed circularly polarized light and transmits left-handed circularly polarized light. Similarly, when the twist direction of the cholesteric liquid crystal is left-handed, of the light having the predetermined wavelength λ, the cholesteric liquid crystal reflects left-handed circularly polarized light and transmits right-handed circularly polarized light. 
     When the pitch of the helix of cholesteric liquid crystals is defined as P, and the refractive index of liquid crystal molecules with respect to extraordinary light is defined as ne, and the refractive index of liquid crystal molecules with respect to ordinary light is defined as no, in general, the selective reflection range Δλ of cholesteric liquid crystals with respect to normal incident light is shown by “no*P to ne*P”. Specifically, the selective reflection range Δλ of cholesteric liquid crystals changes based on the inclination angle φ of the reflective surfaces  321 , the incident angle on the first interface  317 , etc., with respect to the range “no*P to ne*P”. 
     When the optical element  3  consists of cholesteric liquid crystals, for example, the optical element  3  is formed as a film. The optical element  3  as a film is formed by, for example, polymerizing a plurality of helical structures  311 . Specifically, the optical element  3  as a film is formed by polymerizing the elements (liquid crystal molecules)  315  contained in the optical element  3 . For example, a plurality of liquid crystal molecules are polymerized by emitting light to the liquid crystal molecules. 
     Alternatively, the optical element  3  as a film is formed by, for example, controlling the alignment of polymer liquid crystal materials showing a liquid crystalline state at a predetermined temperature or a predetermined concentration so as to form a plurality of helical structures  311  in a liquid crystalline state and subsequently causing them to transition to a solid while maintaining the alignment. 
     By polymerization or transition to a solid, in the optical element  3  as a film, adjacent helical structures  311  are bound together while maintaining the alignment of the helical structures  311 , in other words, while maintaining the spacial phases of the helical structures  311 . As a result, in the optical element  3  as a film, the alignment direction of each liquid crystal molecule is fixed. 
       FIG.  3    is a plan view schematically showing the photovoltaic cell device  100 . 
       FIG.  3    shows an example of the spacial phases of the helical structures  311 . Here, the spacial phases are shown as the alignment directions of, of the elements  315  contained in the helical structures  311 , the elements  315  located at the first interface  317 . 
     Regarding the helical structures  311  arranged in the second direction A 2 , the alignment directions of the elements  315  located at the first interface  317  are different from each other. In other words, the spacial phases of the helical structures  311  at the first interface  317  differ in the second direction A 2 . 
     To the contrary, regarding the helical structures  311  arranged in a third direction A 3 , the alignment directions of the elements  315  located at the first interface  317  are substantially coincident with each other. In other words, the spacial phases of the helical structures  311  at the first interface  317  are substantially coincident with each other in the third direction A 3 . 
     In particular, regarding the helical structures  311  arranged in the second direction A 2 , the alignment direction varies with each element  315  by a certain degree. In other words, at the first interface  317 , the alignment direction linearly varies with the elements  315  arranged in the second direction A 2 . Thus, the spacial phase linearly varies in the second direction A 2  with the helical structures  311  arranged in the second direction A 2 . As a result, like the optical element  3  shown in  FIG.  2   , the reflective surfaces  321  which incline with respect to the first interface  317  and the optical waveguide  1  are formed. Here, the phrase “linearly vary” means that, for example, the amount of variation in the alignment directions of the elements  315  is shown by a linear function. 
     Here, as shown in  FIG.  3   , the interval between two helical structures  311  when the alignment directions of the elements  315  vary by 180 degrees in the second direction A 2  at the first interface  317  is defined as pitch T of the helical structures  311 . In  FIG.  3   , DP indicates the twist direction of each element. The inclination angle φ of each reflective surface  321  shown in  FIG.  2    is arbitrarily set based on pitch T and the helical pitch P. 
       FIG.  4    is a cross-sectional view schematically showing an example of the optical element  3 . In the example shown in  FIG.  4   , the optical element  3  comprises a first layer L 1  which mainly reflects the first component LT 1 , a second layer L 2  which mainly reflects the second component LT 2  and a third layer L 3  which mainly reflects the third component LT 3 . The first layer L 1 , the second layer L 2  and the third layer L 3  are stacked in this order in the first direction A 1 . The first layer L 1  faces the second main surface F 2 . It should be noted that the order in which the first layer L 1 , the second layer L 2  and the third layer L 3  are stacked is not limited to the example shown in  FIG.  4   . 
       FIG.  4    shows, as the helical structures  311  in the first layer L 1 , the second layer L 2  and the third layer L 3 , cholesteric liquid crystals which twist in a single direction are schematically shown. The helical structures  311  in the first layer L 1 , the second layer L 2  and the third layer L 3  twist in the same direction, and are configured to reflect, for example, the first circularly polarized light. 
     In the first layer L 1 , the helical structure  311  comprises a first helical pitch P 1  so as to reflect the first component LT 11  of the first circularly polarized light. 
     In the second layer L 2 , the helical structure  311  comprises a second helical pitch P 2  so as to reflect the second component LT 21  of the first circularly polarized light. The second helical pitch P 2  is different from the first helical pitch P 1 . 
     In the third layer L 3 , the helical structure  311  comprises a third helical pitch P 3  so as to reflect the third component LT 31  of the first circularly polarized light. The third helical pitch P 3  is different from the first helical pitch P 1  and the second helical pitch P 2 . 
     The second helical pitch P 2  is greater than the first helical pitch P 1 , and the third helical pitch P 3  is greater than the second helical pitch P 2  (P 1 &lt;P 2 &lt;P 3 ). 
     Here, this specification explains a case where the incident light LTi which passed through the optical waveguide  1  includes the first component LT 1 , the second component LT 2  and the third component LT 3 . 
     On the reflective surface  321  of the first layer L 1 , the first component LT 11  of the first circularly polarized light is reflected. In addition to the first component LT 12  of the second circularly polarized light, the second component LT 2  and the third component LT 3  pass through the reflective surface  321  of the first layer L 1 . 
     On the reflective surface  321  of the second layer L 2 , the second component LT 21  of the first circularly polarized light is reflected. In addition to the first and second components LT 12  and LT 22  of the second circularly polarized light, the third component LT 3  passes through the reflective surface  321  of the second layer L 2 . 
     On the reflective surface  321  of the third layer L 3 , the third component LT 31  of the first circularly polarized light is reflected. The first, second and third components LT 12 , LT 22  and LT 32  of the second circularly polarized light pass through the reflective surface  321  of the third layer L 3 . 
     Thus, the light LTr reflected on the optical element  3  includes the first, second and third components LT 11 , LT 21  and LT 31  of the first circularly polarized light. The light LTt which passes through the optical element  3  includes the first, second and third components LT 12 , LT 22  and LT 32  of the second circularly polarized light. 
     It should be noted that the helical structures  311  of one of the layers may twist in a direction different from the helical structures  311  of the other layers. In this case, circularly polarized light rays in opposite directions are reflected. 
     In the example shown in  FIG.  4   , the first layer L 1 , the second layer L 2  and the third layer L 3  are individually formed. In the first layer L 1 , the first helical pitch P 1  of the helical structures  311  undergoes very little change and is constant. Similarly, in the second layer L 2 , the second helical pitch P 2  is almost constant, and further, in the third layer L 3 , the third helical pitch P 3  is almost constant. 
     It should be noted that the optical element  3  may be a liquid crystal layer of a single-layer body, and the helical pitch P may continuously change in the first direction A 1 . 
     The optical element  3  may include a layer which reflects invisible light. 
     Modified Example 1 
       FIG.  5    is a cross-sectional view schematically showing the optical element  3  according to modified example 1 of embodiment 1. 
     The modified example 1 shown in  FIG.  5    is different from the above embodiment 1 in respect that the helical axis AX of each helical structure  311  inclines with respect to the optical waveguide  1  or the second main surface F 2 . In modified example 1 here, the spacial phases of the helical structures  311  at the first interface  317  or the X-Y plane are substantially coincident with each other. The other properties of the helical structures  311  of modified example 1 are the same as the helical structures  311  of the embodiment 1 described above. 
     In this modified example 1, the optical element  3  reflects light LTr which is part of the incident light LTi which passed through the optical waveguide  1  at a reflective angle based on the inclination of the helical axis AX, and transmits the other light LTt. 
     In this modified example 1, effects similar to those of the above embodiment 1 are obtained. 
     Modified Example 2 
       FIG.  6    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2. Modified example 2 is different from the above embodiment and modified example 1 in respect that the optical element  3  is configured to condense light toward the photovoltaic cell device  5 . In order to facilitate understanding of propagation of the light LTr reflected on the optical element  3 ,  FIG.  6    shows the wavefronts WF of light LTr. 
     In  FIG.  6   , the section of the photovoltaic cell device  100  along the IIIa-IIIa line, the section of the photovoltaic cell device  100  along the IIIb-IIIb line and the section of the photovoltaic cell device  100  along the IIIc-IIIc line are similar to the section of the photovoltaic cell device  100  shown in  FIG.  1   . 
     The section of the optical element  3  along the IIIa-IIIa line, the section of the optical element  3  along the IIIb-IIIb line and the section of the optical element  3  along the IIIc-IIIc line are similar to, for example, the section of the optical element  3  shown in  FIG.  2    or the section of the optical element  3  shown in  FIG.  5   . 
     In other words, as explained with reference to  FIG.  2    or  FIG.  5   , the reflective surfaces  321  of the optical element  3  incline so as to reflect light toward the photovoltaic cell  5  at respective positions in the X-Y plane. The light LTr reflected on the optical element  3  propagates through the optical waveguide  1  toward the photovoltaic cell  5 . 
     In this modified example 2, effects similar to those of the above embodiment 1 are obtained. 
     Further, in modified example 2, the optical element  3  comprises the reflective surfaces  321  which incline so as to condense light toward the photovoltaic cell  5 , and the reflected light LTr propagates toward the photovoltaic cell  5  in the optical waveguide  1 . Thus, the amount of light received in the photovoltaic cell  5  per unit time can be increased. This configuration can reduce the size of the photovoltaic cell  5  and increase the electricity generated in the photovoltaic cell  5 . 
     Embodiment 2 
       FIG.  7    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 2. Here, the illustration of an optical element  3  is omitted. Embodiment 2 is different from embodiment 1 in respect that the photovoltaic cell device  100  is configured by attaching a plurality of optical waveguides  1  arranged in a second direction A 2 . In the example shown in  FIG.  7   , the photovoltaic cell device  100  comprises, as the optical waveguides  1 , an optical waveguide (first optical waveguide)  1 A and an optical waveguide (second optical waveguide)  1 B. The optical waveguide  1 A and the optical waveguide  1 B are attached to each other by an adhesive layer (second adhesive layer)  8 . The optical waveguide  1 A and the optical waveguide  1 B are arranged in the second direction A 2 . However, they may be arranged in a third direction A 3 . 
     Each of the optical waveguide  1 A and the optical waveguide  1 B is shaped like a flat plate parallel to an X-Y plane. The optical waveguide  1 A and the optical waveguide  1 B are formed of the same transparent material and have the same refractive index. 
     The optical waveguide  1 A comprises an external side surface (first side surface) F 3 A and a side surface (second side surface) F 31  different from the external side surface F 3 A. The optical waveguide  1 B comprises an external side surface F 3 B and a side surface (third side surface) F 32  different from the external side surface (fourth side surface) F 3 B. The side surface F 31  and the side surface F 32  are surfaces extending in the third direction A 3 . The side surface F 31  faces the side surface F 32  in the second direction A 2 . 
     The adhesive layer  8  is transparent and attaches the optical waveguide  1 A to the optical waveguide  1 B between the side surface F 31  and the side surface F 32 . The refractive index of the adhesive layer  8  is substantially equal to that of the optical waveguide  1 A and the optical waveguide  1 B. For example, the difference between the refractive index of the adhesive layer  8  and the refractive index of the optical waveguide  1 A and the difference between the refractive index of the adhesive layer  8  and the refractive index of the optical waveguide  1 B are less than or equal to 0.1 in the wavelength of causing reflection and diffraction, and should be preferably less than or equal to 0.05. 
     The photovoltaic cell device  100  shown in  FIG.  7    comprises a plurality of photovoltaic cells  5 A and  5 B. The photovoltaic cell (first photovoltaic cell)  5 A is attached to the external side surface F 3 A of the optical waveguide  1 A by an adhesive layer  7 A. The photovoltaic cell (second photovoltaic cell)  5 B is attached to the external side surface F 3 B of the optical waveguide  1 B by an adhesive layer  7 B. The refractive indices of the adhesive layers  7 A and  7 B are equal to the refractive index of the adhesive layer  8 . For example, the adhesive layers  7 A and  7 B are formed of the same material as the adhesive layer  8 . It should be noted that the adhesive layers  7 A and  7 B and the adhesive layer  8  may be formed of different materials as long as the materials are transparent and have substantially the same refractive index. 
     The photovoltaic cell device  100  may comprise only one of the photovoltaic cells  5 A and  5 B or may comprise three or more photovoltaic cells  5 . 
       FIG.  8    is a cross-sectional view of the photovoltaic cell device  100  along the A-B line of  FIG.  7   . The adhesive layer  8  has thickness T equal to the thicknesses of the optical waveguides  1 A and  1 B and has width W equal to the interval between the side surface F 31  and the side surface F 32 . Thickness T is greater than width W (T&gt;W). Thickness T is the length in a first direction A 1 . Width W is the length in the second direction A 2 . The adhesive layer  8  forms part of a first main surface F 1  and part of a second main surface F 2 . In other words, each of the first main surface F 1  and the second main surface F 2  is a surface formed by the optical waveguide  1 A, the optical waveguide  1 B and the adhesive layer  8 . 
     The optical element  3  comprises a first element  3 A facing the optical waveguide  1 A, and a second element  3 B facing the optical waveguide  1 B. The first element  3 A is spaced apart from the second element  3 B. In the example shown in  FIG.  8   , neither the first element  3 A nor the second element  3 B is provided at a position facing the adhesive layer  8  in the first direction A 1 . 
     The reflective surface  321 A of the first element  3 A is an inclined surface different from the reflective surface  321 B of the second element  3 B. In other words, the reflective surface  321 A inclines so as to reflect the incident light LTi which passed through the optical waveguide  1  toward the photovoltaic cell  5 A. The reflective surface  321 B inclines so as to reflect the incident light LTi which passed through the optical waveguide  1  toward the photovoltaic cell  5 B. 
     In this embodiment 2, effects similar to those of the above embodiment 1 are obtained. In addition, as a plurality of optical waveguides  1  are attached, a large photovoltaic cell device  100  can be easily provided. 
     Further, as the adhesive layer  8  which attaches the optical waveguide  1 A to the optical waveguide  1 B has the same refractive index as each optical waveguide, the adhesive layer  8  forms an optical path in which the loss is low between the optical waveguide  1 A and the optical waveguide  1 B. In this configuration, for example, the light which propagates inside the optical waveguide  1 A can be transmitted to the adhesive layer  8  and the optical waveguide  1 B, and further, the light which propagates inside the optical waveguide  1 B can be transmitted to the adhesive layer  8  and the optical waveguide  1 A. 
     Moreover, since thickness T is greater than width W in the adhesive layer  8 , even if the optical element  3  facing the adhesive layer  8  is not provided, light leakage in the adhesive layer  8  is prevented. 
     In embodiment 2, the explanation of the details of the optical element  3  is omitted. However, as explained with reference to  FIG.  2    in embodiment 1, the inclined reflective surfaces  321 A and  321 B may be formed by adjusting the spacial phase. Alternatively, as explained with reference to  FIG.  5   , the inclined reflective surfaces  321 A and  321 B may be formed by inclining the helical axis AX. 
     Modified Example 1 
       FIG.  9    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 1 of embodiment 2. 
     The modified example 1 shown in  FIG.  9    is different from the embodiment 2 shown in  FIG.  8    in respect that the optical element  3  faces the optical waveguide  1 A and the optical waveguide  1 B across the adhesive layer  8 . In other words, the optical element  3  is formed as a single sheet and is provided over substantially the entire second main surface F 2 . 
     In this modified example 1, effects similar to those of the above embodiment 2 are obtained. In addition, the number of components can be reduced. 
     Modified Example 2 
       FIG.  10    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2 of embodiment 2. Here, the illustration of the optical element  3  is omitted. The modified example 2 shown in  FIG.  10    is different from the embodiment 2 shown in  FIG.  7    in respect that a plurality of optical waveguides arranged in the second direction A 2  and the third direction A 3  are attached. 
     The photovoltaic cell device  100  comprises the optical waveguide  1 A, the optical waveguide  1 B, an optical waveguide  1 C, an optical waveguide  1 D and the adhesive layer (second adhesive layer)  8 . The optical waveguide  1 A and the optical waveguide  1 B are arranged in the second direction A 2 . The optical waveguide  1 C and the optical waveguide  1 D are arranged in the second direction A 2 . The optical waveguide  1 A and the optical waveguide  1 C are arranged in the third direction A 3 . The optical waveguide  1 B and the optical waveguide  1 D are arranged in the third direction A 3 . 
     Each of the optical waveguide  1 A, the optical waveguide  1 B, the optical waveguide  1 C and the optical waveguide  1 D is shaped like a flat plate parallel to the X-Y plane. The optical waveguide  1 A, the optical waveguide  1 B, the optical waveguide  1 C and the optical waveguide  1 D are formed of the same transparent material and have the same refractive index. 
     The optical waveguide  1 A comprises the side surface F 31  different from the external side surface F 3 A. The optical waveguide  1 B comprises the side surface F 32  different from the external side surface F 3 B. The optical waveguide  1 C comprises a side surface F 33  different from an external side surface F 3 C. The optical waveguide  1 D comprises a side surface F 34  different from an external side surface F 3 D. The side surface F 31 , the side surface F 32 , the side surface F 33  and the side surface F 34  are surfaces having an L-shape in the X-Y plane. 
     The adhesive layer  8  is transparent and attaches the optical waveguide  1 A, the optical waveguide  1 B, the optical waveguide  1 C and the optical waveguide  1 D to each other in the side surface F 31 , the side surface F 32 , the side surface F 33  and the side surface F 34 . The refractive index of the adhesive layer  8  is substantially equal to that of the optical waveguide  1 A, etc. 
     The photovoltaic cell  5 A is attached to the external side surface F 3 A of the optical waveguide  1 A by the adhesive layer  7 A. The photovoltaic cell  5 B is attached to the external side surface F 3 B of the optical waveguide  1 B by the adhesive layer  7 B. A photovoltaic cell  5 C is attached to the external side surface F 3 C of the optical waveguide  1 C by an adhesive layer  7 C. A photovoltaic cell  5 D is attached to the external side surface F 3 D of the optical waveguide  1 D by an adhesive layer  7 D. 
     In this modified example 2, effects similar to those of the above embodiment 2 are obtained. 
     Modified Example 3 
       FIG.  11    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 3 of embodiment 2. 
     The modified example 3 shown in  FIG.  11    is different from the above embodiment 2 in respect that the photovoltaic cell device  100  further comprises a transparent protective layer  9  which covers the optical waveguide  1 A and the optical waveguide  1 B. The protective layer  9  covers substantially the entire first main surface F 1  and the entire second main surface F 2 . In other words, the protective layer  9  is in contact with the first main surface F 1  and the second main surface F 2 . The protective layer  9  is also in contact with the adhesive layer  8 . On the second main surface F 2  side of the optical waveguide  1 A and the optical waveguide  1 B, the protective layer  9  is provided between the optical waveguide  1 A and the optical element  3  and between the optical waveguide  1 B and the optical element  3 . 
     The protective layer  9  is formed of, for example, the same material as the adhesive layer  8 . The refractive index of the protective layer  9  is substantially equal to that of the optical waveguide  1  (or the refractive indices of the optical waveguide  1 A and the optical waveguide  1 B). 
     In this modified example 3, effects similar to those of the above embodiment 2 are obtained. In addition, the damage to the optical waveguide  1  can be prevented. 
     Modified Example 4 
       FIG.  12    is a cross-sectional view of the photovoltaic cell device  100  according to modified example 4 of embodiment 2. 
     The modified example 4 shown in  FIG.  12    is different from the modified example 3 shown in  FIG.  11    in respect that the transparent protective layer  9  covers the optical waveguide  1 A, the optical waveguide  1 B and the optical element  3 . The protective layer  9  covers substantially the entire surface of each of the first main surface F 1  and the optical element  3 . On the second main surface F 2  side of the optical waveguide  1 A and the optical waveguide  1 B, the optical element  3  is provided between the optical waveguide  1 A and the protective layer  9  and between the optical waveguide  1 B and the protective layer  9 . In other words, the protective layer  9  is in contact with each of the first main surface F 1  and the second interface  319  of the optical element  3 . 
     In this modified example 4, effects similar to those of the above modified example 3 are obtained. 
     It should be noted that, in modified example 3 and modified example 4, the optical element  3  may comprise the first and second elements  3 A and  3 B spaced apart from each other as shown in  FIG.  8   , or may be formed as a single sheet as shown in  FIG.  9   . 
     The protective layer  9  may further cover the external side surfaces F 3 A and F 3 B. In this case, the protective layer  9  may be replaced by the adhesive layers  7 A and  7 B. In other words, the photovoltaic cells  5 A and  5 B may be attached by the protective layer  9 . The protective layer  9  and the adhesive layer  7 A may be interposed between the photovoltaic cell  5 A and the external side surface F 3 A. The protective layer  9  and the adhesive layer  7 B may be interposed between the photovoltaic cell  5 B and the external side surface F 3 B. 
     The embodiments 1 and 2 described above can be combined with each other. 
     As explained above, the embodiments can provide a photovoltaic cell device in which the loss is low at low cost. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.