Patent Publication Number: US-2023146964-A1

Title: Photovoltaic cell device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of PCT Application No. PCT/JP2021/024609, filed Jun. 29, 2021 and based upon and claiming the benefit of priority from Japanese Patent Application No. 2020-119969, 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. Although the dye-sensitized photovoltaic cell transmits part of visible light, a constituent material of the cell absorbs some wavelength ranges. Thus, there is a problem in which transmitted light is colored. 
    
    
     
       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 a first liquid crystal layer  31  constituting the optical element  3 . 
         FIG.  5    is a cross-sectional view schematically showing the optical element  3  according to a modified example of embodiment 1. 
         FIG.  6    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 2. 
         FIG.  7    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 3. 
         FIG.  8    is a cross-sectional view schematically showing an optical element  3  according to a modified example of embodiment 3. 
         FIG.  9    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 4. 
         FIG.  10    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 4. 
         FIG.  11 A  is a plan view schematically showing an example of the infrared reflective layer RI which can be combined with a first photovoltaic cell  51  according to embodiment 4. 
         FIG.  11 B  is a plan view schematically showing an example of an ultraviolet reflective layer RU which can be combined with a second photovoltaic cell  52  according to embodiment 4. 
         FIG.  12    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 5. 
         FIG.  13    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 5. 
         FIG.  14    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 6. 
         FIG.  15    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 6. 
         FIG.  16    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 1 of embodiment 6. 
         FIG.  17    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2 of embodiment 6. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a photovoltaic cell device comprises an optical waveguide comprising a first main surface, a second main surface facing the first main surface, and a side surface, an optical element facing the second main surface, and a photovoltaic cell facing the side surface. The optical element comprises a first liquid crystal layer which comprises a cholesteric liquid crystal, reflects, of visible light incident on the first main surface, circularly polarized light of one of first circularly polarized light and second circularly polarized light rotating in an opposite direction of the first circularly polarized light toward the optical waveguide and the photovoltaic cell, and transmits the other circularly polarized light. The visible light includes a plurality of wavelength ranges. The first liquid crystal layer reflects one of the first circularly polarized light and the second circularly polarized light of part of the wavelength ranges. 
     According to another embodiment, a photovoltaic cell device comprises an optical waveguide comprising a first main surface, a second main surface facing the first main surface, and a side surface, an optical element facing the second main surface, and a first photovoltaic cell facing the side surface and comprising polycrystalline silicon. The optical element comprises an infrared reflective layer which comprises a cholesteric liquid crystal and reflects, of infrared light incident on the first main surface, at least one of first circularly polarized light and second circularly polarized light rotating in an opposite direction of the first circularly polarized light toward the optical waveguide and the first photovoltaic cell. 
     Embodiments described herein can provide a photovoltaic cell device which can generate electricity without coloring. 
     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 A1. A direction parallel to the Y-axis is referred to as a second direction A2. A direction parallel to the X-axis is referred to as a third direction A3. The first direction A1, the second direction A2 and the third direction A3 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  and a photovoltaic cell  5 . 
     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, the term “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) LT 1  of a first wavelength range (for example, 400 to 500 nm), the second component (green component) LT 2  of a second wavelength range (for example, 500 to 600 nm), and the third component (red component) LT 3  of a third wavelength range (for example, 600 to 700 nm). Invisible light LT 4  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 a 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 A1. The side surface F 3  is a surface extending in the first direction A1. In the example shown in  FIG.  1   , the side surface F 3  is a surface substantially parallel to an X-Z plane. The 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 A1. 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 first 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. Each of the first circularly polarized light and the second circularly polarized light includes the first component LT 1 , the second component LT 2  and the third component LT 3  described above. 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. 
     In embodiment 1, the first liquid crystal layer  31  comprises a first layer L 1 , a second layer L 2  and a third layer L 3 . In the example of  FIG.  1   , 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 A1. 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.  1   . 
     For example, each of the first layer L 1 , the second layer L 2  and the third layer L 3  is a liquid crystal layer configured to reflect the first circularly polarized light and transmit the second circularly polarized light which rotates in the opposite direction of the first circularly polarized light. The first layer L 1  is a layer which mainly reflects, of the first component LT 1 , the first component LT 11  of the first circularly polarized light. The second layer L 2  is a layer which mainly reflects, of the second component LT 2 , the second component LT 21  of the first circularly polarized light. The third layer L 3  is a layer which mainly reflects, of the third component LT 3 , the third component LT 31  of the first circularly polarized light. 
     The photovoltaic cell  5  faces the side surface F 3  of the optical waveguide  1  in a second direction A2. 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 photovoltaic cell  5  is directly or indirectly connected to the optical waveguide  1 . For example, the photovoltaic cell  5  is directly or indirectly connected to the side surface F 3  of the optical waveguide  1 . When the photovoltaic cell  5  is indirectly connected to the side surface F 3  of the optical waveguide  1 , for example, a transparent layer or an optical component (lens, etc.,) is interposed between the photovoltaic cell  5  and the side surface F 3  of the optical waveguide  1 . 
     Now, in embodiment 1 of  FIG.  1   , the operation of the photovoltaic cell device  100  is explained. 
     The light LTi which enters the first main surface F 1  of the optical waveguide  1  is, for example, solar light. Light LTi includes invisible light LT 4  in addition to the first, second and third components LT 1 , LT 2  and LT 3  of visible 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. In embodiment 1, the light LTr reflected on the optical element  3  is equivalent to the first circularly polarized light of visible light. The light LTt which passes through the optical element  3  includes the second circularly polarized light of visible light. In this specification, circularly polarized light may be strict circularly polarized light or may be circularly polarized light which approximates elliptically polarized light. 
     More specifically, in the optical element  3 , the first layer L 1  reflects the first component LT 11  of the first circularly polarized light, and transmits the first component LT 12  of the second circularly polarized light, and in addition, transmits the second component LT 2 , the third component LT 3  and invisible light LT 4 . 
     The second layer L 2  reflects the second component LT 21  of the first circularly polarized light, and transmits the first and second components LT 12  and LT 22  of the second circularly polarized light, and in addition, transmits the third component LT 3  and invisible light LT 4 . 
     The third layer L 3  reflects the third component LT 31  of the first circularly polarized light, and transmits the first, second and third components LT 12 , LT 22  and LT 32  of the second circularly polarized light, and in addition, transmits invisible light LT 4 . 
     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 optical element  3  reflects each of the first component LT 11 , the second component LT 21  and the third component LT 31  toward the optical waveguide  1  at an entering angle θwhich 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 side surface F 3  and generates electricity. 
     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 and invisible light LT 4 . 
     According to this embodiment 1, the optical element  3  reflects approximately 50% of circularly polarized light toward the photovoltaic cell  5  with respect to each of the first (blue), second (green) and third (red) components which are the main components of visible light, and transmits the other approximately 50% of circularly polarized light. In this way, approximately 50% of visible light can be used for electric generation, and the coloring of the light which passes through the photovoltaic cell device  100  can be prevented. 
     Further, the light of substantially the entire wavelength range of visible light can be introduced into the photovoltaic cell  5 , and the amount of the received light of the photovoltaic cell  5  per unit time can be increased. In this way, the electric generation efficiency of the photovoltaic cell  5  can be improved. 
     The above embodiment 1 is explained regarding the example in which each of the first layer L 1 , the second layer L 2  and the third layer L 3  reflects the first circularly polarized light and transmits the second circularly polarized light. However, the configuration is not limited to this example. Each of the first layer L 1 , the second layer L 2  and the third layer L 3  may reflect one of the first circularly polarized light and the second circularly polarized light and transmit the other. 
       FIG.  2    is a cross-sectional view schematically showing the structure of the optical element  3 . Here, as a representative of the first to third layers of the first liquid crystal layer  31  constituting the optical element  3 , the first layer L 1  is shown. It should be noted that the second layer L 2  and the third layer L 3  are configured in the same manner as the first layer L 1 . The second layer L 2  and the third layer L 3  are shown by alternate long and short dash lines. 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 A1. 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 A1. 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 A1 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 first layer L 1  of 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 first layer L 1  of the optical element  3  and the second layer L 2 . 
     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’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 φof 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 A2 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 A2 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 φof 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  shows 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 λ, 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. 
     In  FIG.  2   , the light LTr reflected by the helical structures  311  of the first layer L 1  is the first component LT 11  of the first circularly polarized light. The light LTt which passes through the first layer L 1  includes the first component LT 12  of the second circularly polarized light, and in addition, the second and third components LT 2  and LT 3  of visible light and invisible light LT 4 . 
     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”. 
     In the first layer L 1  shown in  FIG.  2   , the helical pitch P of the helical structures  311  and refractive indices ne and no of liquid crystal molecules as the elements  315  are set so as to reflect the first component LT 1 . Similarly, in the second layer L 2 , the helical pitch P and refractive indices ne and no are set so as to reflect the second component LT 2 . Similarly, in the third layer L 3 , the helical pitch P and refractive indices ne and no are set so as to reflect the third component LT 3 . In some cases, the helical pitch of the first layer L 1  is called a first helical pitch P 1 , and the helical pitch of the second layer L 2  is called a second helical pitch P 2 , and the helical pitch of the third layer L 3  is called a third helical pitch P 3 . When the first layer L 1 , the second layer L 2  and the third layer L 3  consist of the same elements  315 , the first helical pitch P 1 , the second helical pitch P 2  and the third helical pitch P 3  are different from each other. 
     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 . In  FIG.  3   , the optical waveguide  1  is shown by alternate long and two short dashes lines, and the optical element  3  are shown by solid lines, and the helical structures  311  are shown by dotted lines, and the photovoltaic cell  5  is shown by alternate long and short dash lines. 
       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 A2, 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 A2. 
     To the contrary, regarding the helical structures  311  arranged in a third direction A3, 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 A3. 
     In particular, regarding the helical structures  311  arranged in the second direction A2, 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 A2. Thus, the spacial phase linearly varies with the helical structures  311  arranged in the second direction A2. 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 A2 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 first liquid crystal layer  31  constituting the optical element  3 . Here, 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, for example, reflect the first circularly polarized light. 
     In the first layer L 1 , the helical structure  311  comprises the 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 the 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 the 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; P2 &lt; P3) . 
     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 embodiment 1, 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. 
     Modified Example 
       FIG.  5    is a cross-sectional view schematically showing the optical element  3  according to a modified example of embodiment 1. Here, as a representative of the first to third layers of the first liquid crystal layer  31  constituting the optical element  3 , the first layer L 1  is shown. It should be noted that the second layer L 2  and the third layer L 3  are configured in the same manner as the first layer L 1 . 
     The modified example 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 the modified example 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 the modified example are the same as the helical structures  311  of embodiment 1. 
     In this modified example, the optical element  3  reflects light LTr which is part of the incident light LTi 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, effects similar to those of the above embodiment 1 are obtained. 
     Embodiment 2 
       FIG.  6    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 2. The embodiment 2 shown in  FIG.  6    is different from the above embodiment 1 in respect that a first liquid crystal layer  31  constituting an optical element  3  is a single-layer body. Here, as a helical structure  311  in the first liquid crystal layer  31 , a cholesteric liquid crystal which twists in a single direction is schematically shown. 
     In the first liquid crystal layer  31 , the helical pitch P of the helical structure  311  continuously changes in a first direction A1. The helical structure  311  comprises a first portion  31 A comprising a first helical pitch P 1  for reflecting a first component LT 11 , a second portion  31 B comprising a second helical pitch P 2  for reflecting a second component LT 21 , and a third portion  31 C comprising a third helical pitch P 3  for reflecting a third component LT 31 . In other words, each of the first portion  31 A, the second portion  31 B and the third portion  31 C is part of the helical structure  311  which twists in the same direction. 
     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; P2 &lt; P3) . 
     In this embodiment 2, effects similar to those of embodiment 1 are obtained. 
     Embodiment 3 
       FIG.  7    is a cross-sectional view schematically showing a photovoltaic cell device  100  according to embodiment 3. The embodiment 3 shown in  FIG.  7    is different from the above embodiment 2 in respect that an optical element  3  comprises a second liquid crystal layer  32  overlapping a first liquid crystal layer  31 . In the example shown in  FIG.  7   , the first liquid crystal layer  31  is provided between an optical waveguide  1  and the second liquid crystal layer  32 . However, the second liquid crystal layer  32  may be provided between the optical waveguide  1  and the first liquid crystal layer  31 . It should be noted that the first liquid crystal layer  31  may be a single-layer body like embodiment 2 or may be a stacked layer body of a plurality of layers like embodiment 1. 
     The second liquid crystal layer  32  comprises cholesteric liquid crystals which twist in a single direction as helical structures  311  in the same manner as the first liquid crystal layer  31 . Here, a cholesteric liquid crystal in the second liquid crystal layer  32  is schematically shown. The second liquid crystal layer  32  is configured to reflect, of the incident light LTi which passed through the optical waveguide  1 , invisible light LT 4  of first circularly polarized light or second circularly polarized light. 
     For example, in the second liquid crystal layer  32 , the helical structure  311  comprises a fourth helical pitch P 4  so as to reflect invisible light LT 41  of the first circularly polarized light. The fourth helical pitch P 4  is different from each of the first helical pitch P 1 , the second helical pitch P 2  and the third helical pitch P 3  shown in  FIG.  4   , etc. When invisible light LT 4  is ultraviolet light, the fourth helical pitch P 4  is less than the first helical pitch P 1 . When invisible light LT 4  is infrared light, the fourth helical pitch P 4  is greater than the third helical pitch P 3 . 
     In this embodiment 3, 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 reflected on the reflective surfaces  321  of the first liquid crystal layer  31 , and invisible light LT 41  of the first circularly polarized light reflected on the reflective surface  321  of the second liquid crystal layer  32 . 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, and invisible light LT 42 . 
     In this embodiment 3, effects similar to those of embodiment 1 are obtained. Further, in addition to the light of substantially the entire wavelength range of visible light, invisible light can be introduced into a photovoltaic cell  5 . Thus, the electric generation efficiency of the photovoltaic cell  5  can be further improved. 
     Modified Example 
       FIG.  8    is a cross-sectional view schematically showing the optical element  3  according to a modified example of embodiment 3. The modified example shown in  FIG.  8    is different from the embodiment 3 shown in  FIG.  7    in respect that the second liquid crystal layer  32  consists of a stacked layer body of a fourth layer L 4  and a fifth layer L 5 . 
     In the second liquid crystal layer  32 , each of the fourth layer L 4  and the fifth layer L 5  comprises cholesteric liquid crystals which twist in a single direction as the helical structures  311 . Here, a cholesteric liquid crystal in each of the fourth layer L 4  and the fifth layer L 5  is schematically shown. In the fourth layer L 4  and the fifth layer L 5 , the cholesteric liquid crystals twist in opposite directions. These fourth layer L 4  and fifth layer L 5  are configured to reflect invisible light LT 4  of the incident light LTi which passed through the optical waveguide  1 . 
     For example, in the fourth layer L 4 , the helical structure  311  comprises the fourth helical pitch P 4  so as to reflect invisible light LT 41  of the first circularly polarized light. In the fifth layer L 5 , the helical structure  311  comprises a fifth helical pitch P 5  so as to reflect invisible light LT 42  of the second circularly polarized light. The fourth helical pitch P 4  and the fifth helical pitch P 5  are substantially equal to each other. 
     The fourth helical pitch P 4  and the fifth helical pitch P 5  are different from each of the first helical pitch P 1 , the second helical pitch P 2  and the third helical pitch P 3  shown in  FIG.  4   , etc. When invisible light LT 4  is ultraviolet light, the fourth helical pitch P 4  and the fifth helical pitch P 5  are less than the first helical pitch P 1 . When invisible light LT 4  is infrared light, the fourth helical pitch P 4  and the fifth helical pitch P 5  are greater than the third helical pitch P 3 . 
     In this modified example, 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 reflected on the reflective surfaces  321  of the first liquid crystal layer  31 , invisible light LT 41  of the first circularly polarized light reflected on the reflective surface  321  of the fourth layer L 4  of the second liquid crystal layer  32 , and invisible light LT 42  of the second circularly polarized light reflected on the reflective surface  321  of the fifth layer L 5 . Thus, 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. 
     In this modified example, effects similar to those of embodiment 3 are obtained. In addition, the invisible light of the first circularly polarized light and the invisible light of the second circularly polarized light can be introduced into the photovoltaic cell  5 . Thus, the electric generation efficiency of the photovoltaic cell  5  can be further improved. 
     In the embodiments 1 to 3 described above, the first liquid crystal layer  31  of the optical element  3  is configured to reflect one of the first circularly polarized light and the second circularly polarized light of at least part of a plurality of wavelength ranges. Further, the first liquid crystal layer  31  is configured to reflect one of the first circularly polarized light and the second circularly polarized light in at least two wavelength ranges of the first, second and third wavelength ranges described above. 
     Embodiment 4 
       FIG.  9    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 4. The photovoltaic cell device  100  comprises an optical waveguide  1 , an optical element  3 , a first photovoltaic cell  51  and a second photovoltaic cell  52 . Each of the first photovoltaic cell  51  and the second photovoltaic cell  52  is a silicon-based photovoltaic cell. It should be noted that the first photovoltaic cell  51  comprises polycrystalline silicon and the second photovoltaic cell  52  comprises amorphous silicon. 
     When polycrystalline silicon is compared with amorphous silicon, the peaks of the respective absorption wavelengths are different from each other. The peak of the absorption wavelength of amorphous silicon is approximately 450 nm. The peak of the absorption wavelength of polycrystalline silicon is approximately 700 nm. In other words, polycrystalline silicon has a higher absorptance for infrared light than amorphous silicon. Thus, the first photovoltaic cell  51  is suitable for electric generation by infrared light. Amorphous silicon has a higher absorptance for ultraviolet light than polycrystalline silicon. Thus, the second photovoltaic cell  52  is suitable for electric generation by ultraviolet light. It should be noted that the first photovoltaic cell  51  may be a compound-based photovoltaic cell and may be, for example, a gallium arsenide-based photovoltaic cell. 
     The first photovoltaic cell  51  and the second photovoltaic cell  52  face a side surface F 3  at different positions. In the example shown in  FIG.  9   , the first photovoltaic cell  51  and the second photovoltaic cell  52  are arranged in a third direction A3. 
       FIG.  10    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 4. Here, the illustrations of the first photovoltaic cell  51  and the second photovoltaic cell  52  are omitted. 
     The optical element  3  comprises an infrared reflective layer RI, and an ultraviolet reflective layer RU overlapping the infrared reflective layer RI. These infrared reflective layer RI and ultraviolet reflective layer RU are equivalent to the second liquid crystal layer  32  which is provided to reflect invisible light and is explained in embodiment 2 and embodiment 3. In the example of  FIG.  10   , the infrared reflective layer RI is provided between the optical waveguide  1  and the ultraviolet reflective layer RU. However, the ultraviolet reflective layer RU may be provided between the optical waveguide  1  and the infrared reflective layer RI. 
     Each of the infrared reflective layer RI and the ultraviolet reflective layer RU is a liquid crystal layer comprising cholesteric liquid crystals which twist in a single direction as helical structures  311 . Here, a cholesteric liquid crystal in each of the infrared reflective layer RI and the ultraviolet reflective layer RU is schematically shown. In the infrared reflective layer RI and the ultraviolet reflective layer RU, the cholesteric liquid crystals twist in the same direction. However, they may twist in opposite directions. 
     For example, in the infrared reflective layer RI, the helical structure  311  comprises a sixth helical pitch P 6  so as to reflect the infrared light I 1  of first circularly polarized light. The sixth helical pitch P 6  is greater than the third helical pitch P 3  described above. 
     In the ultraviolet reflective layer RU, the helical structure  311  comprises a seventh helical pitch P 7  so as to reflect the ultraviolet light U 1  of the first circularly polarized light. The seventh helical pitch P 7  is less than the first helical pitch P 1  described above. 
     In this embodiment 4, the light LTr reflected on the optical element  3  includes the infrared light I 1  of the first circularly polarized light reflected on the reflective surface  321 A of the infrared reflective layer RI, and the ultraviolet light U 1  of the first circularly polarized light reflected on the reflective surface  321 B of the ultraviolet reflective layer RU. The light LTt which passes through the optical element  3  includes a first component LT 1 , a second component LT 2  and a third component LT 3 , the infrared light I 2  of second circularly polarized light, and ultraviolet light U 2 . 
     In this embodiment 4, as most of the visible light passes through the photovoltaic cell device  100 , the coloring of the light which passes through the photovoltaic cell device  100  can be prevented. Further, infrared light and ultraviolet light as the invisible light of solar light can be used for electric generation. 
       FIG.  11 A  is a plan view schematically showing an example of the infrared reflective layer RI which can be combined with the first photovoltaic cell  51  according to embodiment 4. The infrared reflective layer RI is configured to condense infrared light I 1  toward the first photovoltaic cell  51 . In order to facilitate understanding of propagation of the infrared light I 1  reflected on the infrared reflective layer RI,  FIG.  11 A  shows the wavefronts WF of infrared light I 1 . 
     In  FIG.  11 A , the section of the infrared reflective layer RI along the  a   1 - a   1  line, the section of the infrared reflective layer RI along the  b   1 - b   1  line and the section of the infrared reflective layer RI along the  c   1 - c   1  line are similar to the section of the first layer L 1  shown in  FIG.  2    or the section of the first layer L 1  shown in  FIG.  5   . 
     In other words, the reflective surfaces  321 A of the infrared reflective layer RI shown in  FIG.  10    are inclined surfaces which incline so as to reflect infrared light I 1  toward the first photovoltaic cell  51  at respective positions in an X-Y plane. The infrared light I 1  reflected on the reflective surfaces  321 A propagates through the optical waveguide  1  toward the first photovoltaic cell  51 . 
       FIG.  11 B  is a plan view schematically showing an example of the ultraviolet reflective layer RU which can be combined with the second photovoltaic cell  52  according to embodiment 4. The ultraviolet reflective layer RU is configured to condense ultraviolet light U 1  toward the second photovoltaic cell  52 .  FIG.  11 B  shows the wavefronts WF of the ultraviolet light U 1  reflected on the ultraviolet reflective layer RU. 
     In  FIG.  11 B , the section of the ultraviolet reflective layer RU along the  a   2 - a   2  line, the section of the ultraviolet reflective layer RU along the  b   2 - b   2  line and the section of the ultraviolet reflective layer RU along the  c   2 - c   2  line are similar to the section of the first layer L 1  shown in  FIG.  2    or the section of the first layer L 1  shown in  FIG.  5   . 
     In other words, the reflective surfaces  321 B of the ultraviolet reflective layer RU shown in  FIG.  10    are inclined surfaces which incline so as to reflect ultraviolet light U 1  toward the second photovoltaic cell  52  at respective positions in the X-Y plane. The ultraviolet light U 1  reflected on the reflective surfaces  321 B propagates through the optical waveguide  1  toward the second photovoltaic cell  52 . 
     Thus, as the reflective surfaces  321 A of the infrared reflective layer RI are inclined surfaces different from the reflective surfaces  321 B of the ultraviolet reflective layer RU, infrared light I 1  propagates toward the first photovoltaic cell  51 , and ultraviolet light U 1  propagates toward the second photovoltaic cell  52 . Thus, the amount of the received light of the first photovoltaic cell  51  and the second photovoltaic cell  52  per unit time can be increased. In this way, the electricity generated in the photovoltaic cell device  100  can be increased. 
     Embodiment 5 
       FIG.  12    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 5. The embodiment 5 shown in  FIG.  12    is different from the embodiment 4 shown in  FIG.  9    in respect that a first photovoltaic cell  51  faces a second photovoltaic cell  52  across an intervening optical waveguide  1  in a second direction A2. In the example shown in  FIG.  12   , of side surfaces F 3 , the first photovoltaic cell  51  faces a side surface F 31  on the right side of the figure, and the second photovoltaic cell  52  faces a side surface F 32  on the left side of the figure. 
     The first photovoltaic cell  51  may face the second photovoltaic cell  52  in a third direction A3. For example, the first photovoltaic cell  51  may face a side surface F 33  and the second photovoltaic cell  52  may face a side surface F 34 . The first photovoltaic cell  51  may face the side surface F 34  and the second photovoltaic cell  52  may face the side surface F 33 . 
       FIG.  13    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 5. 
     The reflective surface  321 A of an infrared reflective layer RI is an inclined surface different from the reflective surface  321 B of an ultraviolet reflective layer RU. In other words, the reflective surface  321 A inclines so as to reflect, of the incident light LTi which passed through the optical waveguide  1 , infrared light I 1  toward the first photovoltaic cell  51 . The reflective surface  321 B inclines so as to reflect, of the incident light LTi which passed through the optical waveguide  1 , ultraviolet light U 1  toward the second photovoltaic cell  52 . 
     In this embodiment 5, effects similar to those of the above embodiment 4 are obtained. 
     Embodiment 6 
       FIG.  14    is a plan view schematically showing a photovoltaic cell device  100  according to embodiment 6. The photovoltaic cell device  100  comprises an optical waveguide  1 , an optical element  3 , a first photovoltaic cell  51  and a phosphor layer  10 . The first photovoltaic cell  51  is, for example, a silicon-based photovoltaic cell comprising polycrystalline silicon. However, the first photovoltaic cell  51  may be a compound-based photovoltaic cell such as a gallium arsenide-based photovoltaic cell. The phosphor layer  10  is a wavelength conversion layer which converts ultraviolet light U into infrared light I. The phosphor layer  10  is in contact with a side surface F 3  and is provided between the optical waveguide  1  and the first photovoltaic cell  51 . 
     The photovoltaic cell device  100  of embodiment 6 does not comprise the second photovoltaic cell  52  explained in the above embodiment 4 or 5. 
       FIG.  15    is a cross-sectional view schematically showing the photovoltaic cell device  100  according to embodiment 6. The reflective surface  321 A of an infrared reflective layer RI inclines so as to reflect, of the incident light LTi which passed through the optical waveguide  1 , infrared light I 1  toward the first photovoltaic cell  51 . The reflective surface  321 B of an ultraviolet reflective layer RU inclines so as to reflect, of the incident light LTi which passed through the optical waveguide  1 , ultraviolet light U 1  toward the first photovoltaic cell  51 . 
     The infrared light I 1  reflected on the reflective surface  321 A propagates inside the optical waveguide  1 , is emitted from the side surface F 3 , and subsequently, passes through the phosphor layer  10  and is received in the first photovoltaic cell  51 . The ultraviolet light U 1  reflected on the reflective surface  321 B propagates inside the optical waveguide  1 , is emitted from the side surface F 3 , and subsequently, is converted into infrared light in the phosphor layer  10  and is received in the first photovoltaic cell  51 . 
     In this embodiment 6, in addition to the infrared light I 1  reflected on the infrared reflective layer RI, the ultraviolet light U 1  reflected on the ultraviolet reflective layer RU can be used for electric generation after it is converted into infrared light. In addition, compared to embodiments 4 and 5, as it is unnecessary to prepare different types of photovoltaic cells, the cost can be reduced. 
     Modified Example 1 
       FIG.  16    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 1 of embodiment 6. The modified example 1 shown in  FIG.  16    is different from the embodiment 6 shown in  FIG.  14    in respect that the phosphor layer  10  is provided for all of the side surfaces F 3 . 
     In this modified example 1, effects similar to those described above are obtained. 
     Modified Example 2 
       FIG.  17    is a plan view schematically showing the photovoltaic cell device  100  according to modified example 2 of embodiment 6. The modified example 2 shown in  FIG.  17    is different from the embodiment 6 shown in  FIG.  15    in respect that the phosphor layer  10  is provided over substantially the whole surfaces of a first main surface F 1  and a second main surface F 2 . It should be noted that the phosphor layer  10  may be provided in one of the first main surface F 1  and the second main surface F 2 . The phosphor layer  10  may be provided in all of the first main surface F 1 , the second main surface F 2  and the side surfaces F 3  so as to cover the entire optical waveguide  1 . 
     In this modified example 2, ultraviolet light U is converted into infrared light I by the phosphor layer  10  provided in the first main surface F 1  or the phosphor layer  10  provided in the second main surface F 2 . Thus, the optical element  3  can be configured as a single-layer body of the infrared reflective layer RI without comprising the ultraviolet reflective layer RU. 
     In this modified example 2, effects similar to those described above are obtained. 
     In the embodiments 4 to 6 described above, the optical element  3  may further comprise a first liquid crystal layer  31  which reflects visible light in a manner similar to that of embodiments 1 to 3. 
     Each of the infrared reflective layer RI and the ultraviolet reflective layer RU may be configured to reflect both the first circularly polarized light and the second circularly polarized light toward the optical waveguide  1 . Specifically, in a manner similar to that of the embodiment 3 shown in  FIG.  8   , the infrared reflective layer RI should consist of a stacked layer body of at least two layers such that the cholesteric liquid crystal of one of the layers and the cholesteric liquid crystal of the other layer comprise substantially the same helical pitch and twist in opposite directions. The ultraviolet reflective layer RU may be configured in a manner similar to that of the infrared reflective layer RI. 
     Further, the embodiments 1 to 6 described above can be combined with each other as needed. 
     Each of the embodiments explained above can provide a photovoltaic cell device which can generate electricity without coloring. 
     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.