Patent Publication Number: US-2020279694-A1

Title: Photovoltaic element

Description:
TECHNICAL FIELD 
     The present invention relates to photovoltaic elements. 
     BACKGROUND ART 
     So-called solar cells and various other types of elements and devices have been devised as photovoltaic elements that convert optical energy into electric energy. The photovoltaic elements are roughly classified into two; those using silicon-based material and those using compound-based material as the material for exerting photovoltaic effect. 
     Elements that use monocrystalline silicon, polycrystalline silicon, heterojunction model, amorphous silicon and thin-film polycrystalline silicon are typical examples of elements that use silicon-based material. Meanwhile, elements using group III-V compounds, CIS (using copper (Cu), indium (In) and selenium (Se) as main components), CIGS (using copper (Cu), indium (In), gallium (Ga) and selenium (Se) as main components), CdTe, organic thin film and dye-sensitized material are examples of elements that use compound-based material. 
     In addition to the above-described photovoltaic elements, there are elements using silicon dioxide, which is an insulator, as power generating material. This is based on a finding by the present inventors that silicon dioxide itself exerts photo electrolysis effect and photovoltaic effect. 
     The present inventors have found that synthetic quartz and fused quartz, which are silicon dioxides, exert photovoltaic effect, and proposed a silicon dioxide solar cell as photoelectrode material and photocell material (Patent Literatures 1 and 2). 
     With reference to  FIG. 7 , we will describe a tandem-type power generation element using two photovoltaic layers formed of silicon dioxide (SiO 2 ) and titanium oxide (TiO 2 ) as a prior art example. 
     In  FIG. 7 , reference numbers  1  and  2  denote glass substrates, and  3  and  4  denote FTO (fluorine-doped tin oxide) layers. 
     A porous titanium dioxide layer  6  hardened by sintering is formed on the FTO layer  3  on the side from which incident light enters. The porous titanium dioxide layer  6  carries titania particles on which are adsorbed ruthenium complex dye as sensitized dye. Further, a platinum film  5  is formed on the FTO layer  4 . 
     A silicon dioxide layer  7  composed of silicon dioxide particles is formed on the platinum film  5 , so that the layer  7  has a thickness of 0.15-0.20 mm in the height direction. 
     Moreover, the distance between the titanium dioxide layer  6  and the silicon dioxide layer  7  in the height direction is 0.2 mm or greater, and electrolyte  9  is sealed in a space surrounded on four sides by a sealing member  8 . 
     As illustrated in  FIGS. 1, 2, 4 and 7 , the direction perpendicular to the substrate surface of the photovoltaic element is referred to as the height direction, and thickness of layers and films is described by the distance thereof, 
     The silicon dioxide layer  7  serving as the photovoltaic layer is composed of silicon dioxide particles, which are formed by immersing particles of glass and the like containing silicon dioxide in a 5-10% hydrofluoric solution, washing the particles with water, drying, and pulverizing the same so that the particle size is 0.2 mm or smaller. 
     As described, individual shapes of the pulverized silicon dioxide particles may be approximately spherical, but nonspherical particles as illustrated in  FIG. 8  also exist. 
     The individual silicon dioxide particles  10  have various shapes. In the present specification, as illustrated in  FIG. 8 , a maximum elongation direction of the individual silicon dioxide particles  10  is referred to as a major axis L, and the average major axis is used to denote the shape of the silicon dioxide particles used in the photovoltaic layer and a first photovoltaic layer  17 . In the prior art example illustrated in  FIG. 7 , a material having an average major axis L of 500-800 nm is used. 
     The tandem-type photovoltaic element described here characterizes in using silicon dioxide as the photovoltaic layer. As illustrated in  FIG. 9 , it is confirmed that silicon dioxide has higher quantum efficiency than titanium dioxide even in the ultraviolet region, and that it also absorbs light in the infrared region of 2500 nm and higher. Therefore, silicon dioxide exerts photovoltaic effect in a wider wavelength region compared to titanium dioxide and realizes an extremely high power generation efficiency. According to such tandem-type photovoltaic element, the inventors of the present invention have achieved a maximum output of 28.00 μW/cm 2  per unit area in an illumination of 1000 lux. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] International Publication of International Patent Application WO 2011/049156 A1 
     [PTL 2] International Publication of International Patent Application WO 2012/124655 A1 
     SUMMARY OF INVENTION 
     Technical Problem 
     The photovoltaic elements disclosed in PTL 1 and PTL 2 can be manufactured using a low-cost material compared to prior art solar cells, and the energy conversion effect thereof is extremely high compared to other photovoltaic elements. However, even further enhancement of energy conversion effect is desired in photovoltaic elements. 
     Solution to Problem 
     According to one typical photovoltaic element for solving the above-described problem, a photovoltaic layer of the photovoltaic element is composed of a silicon dioxide particle that has an average major axis of 100 nm or smaller. 
     According to another typical photovoltaic element, the photovoltaic layer of the photovoltaic element is composed of a silicon dioxide particle, and a thickness of the first photovoltaic layer in a height direction is formed to be smaller than three times the average major axis of the silicon dioxide particle. 
     According to yet another typical photovoltaic element, the photovoltaic layer of the photovoltaic element is composed of a silicon dioxide particle, and the silicon dioxide particle is arranged on a charge exchange layer that has a roughness in the height direction. Further, the roughness of the charge exchange layer in the height direction is 50 nm or greater, and preferably 100 nm or greater. 
     Advantageous Effects of Invention 
     The photovoltaic element described above significantly improves the power generation output per unit area compared to the prior art photovoltaic element. 
     The problems, configurations and effects other than those described above will become apparent from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a tandem-type photovoltaic element according to a first embodiment. 
         FIG. 2  is a cross-sectional view of a tandem-type photovoltaic element according to a second embodiment. 
         FIG. 3  is an enlarged view of portion A of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of a tandem-type photovoltaic element according to a third embodiment. 
         FIG. 5  is an enlarged view of portion B of  FIG. 4 . 
         FIG. 6  is a schematic diagram in which a first conductive film according to the third embodiment is illustrated from bird&#39;s eye view. 
         FIG. 7  is a cross-sectional view of a tandem-type photovoltaic element according to a comparative example. 
         FIG. 8  is a view illustrating an example of a silicon dioxide particle. 
         FIG. 9  is a measurement chart of quantum efficiency of the photovoltaic element composed of TiO 2  and the photovoltaic element including SiO 2  in a light wavelength region. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described with reference to the drawings. At first, matters common to the first, second and third embodiments are described. 
       FIG. 1  is a tandem-type photovoltaic element according to a first embodiment,  FIG. 2  is a cross-sectional view of a tandem-type photovoltaic element according to a second embodiment, and  FIG. 4  is a cross-sectional view of a tandem-type photovoltaic element according to a third embodiment wherein matters described below are common to  FIGS. 1, 2 and 4 , so they are described with reference to  FIG. 1  as the representative drawing. 
       FIGS. 1, 2 and 4  all illustrate a tandem-type photovoltaic element comprising two photovoltaic layers, which are a first photovoltaic layer and a second photovoltaic layer. 
     In  FIG. 1 , among a first substrate  12  and a second substrate  11 , at least the second substrate arranged on a side from which incident light enters is composed of a transparent material, and preferably, both substrates are composed of transparent material Glass is a popular transparent material, but resin, such as plastic, can be used instead of glass. 
     A transparent second conductive film  13  is formed on the second substrate. The second conductive film  13  is preferably composed of FTO (fluorine-doped fin oxide), but other than the FTO layer, an indium-tin complex oxide (IOT) may be used, for example. 
     A second photovoltaic layer  16  is formed on the second conductive film  13 . A typical example of the second photovoltaic layer  16  is an oxide semiconductor layer, and specifically, oxide semiconductors such as TiO 2 , SnO, ZnO, WO, Nb 2 O, In 2 O 3 , ZrO 2 . Ta 2 O 5  and TiSrO 3  are preferable. A porous titanium dioxide layer hardened by sintering is even further preferable. 
     Sulfide semiconductors such as CdS, ZnS, In 2 S, PbS, Mo 2 S, WS 2 , Sb 2 S 3 , Bi 2 S 3 , ZnCdS 2  and CuS 2  may be used. Moreover, metal chalcogenide such as CdSe, In 2 Se 2 , WSe 2 , PbSe and CdTe are also applicable. 
     Even further, elemental semiconductors such as GaAs, Si, Se and InP may be used. 
     Further, a composite of two or more substances described above, such as a composite of SnO and ZnO or a composite of TiO 2  and Nb 2 O 5 , may also be used. 
     The varieties of semiconductors are not restricted to those described above, and a mixture of two or more substances may also be used. 
     The thickness of the second photovoltaic layer  16  in the height direction should preferably be 3-30 μm, and more preferably, 6-20 μm. 
     Further, the above-described second photovoltaic layer  16  may carry sensitized dye. Various dyes that exert sensitization can be applied as the dye carried by the second photovoltaic layer  16 , and for example, N3 complex, N719 complex (N719 dye), Ru complex such as Ru terpyndine complex (black dye) and Ru diketonate complex, organic dyes such as coumarin dye, merocyanine dye and polyene dye, metal porphyrin dye and phthalocyanine dye are applicable. Among these dyes, the Ru complex is preferable, and specifically. N719 dye and black dye are especially preferable since they exert a wide absorption spectrum in the visible light range. 
     The dye can be used alone, or two or more dyes can be used in a mixture. 
     The above-described matters are common to the first, second and third embodiments and  FIGS. 1, 2 and 4 . In the following description, matters common to the first to third embodiments but have different reference numbers assigned in the drawings will be described by referring to the different reference numbers in the drawings. 
     A first conductive film ( 14  in  FIGS. 1 and 2 ;  22  in  FIG. 4 ) is formed on an upper surface of the first substrate  12 . The first conductive film is preferably FTO (fluorine-doped tin oxide), but other than the FTO layer, for example, an indium-tin complex oxide (ITO) may be used. 
     A charge exchange layer ( 15  in  FIGS. 1 and 2 ;  23  in  FIG. 4 ) Is formed on the first conductive film. A platinum (Pt) film is preferable as the charge exchange layer, but carbon electrode and conductive polymer may also be used instead of the platinum (Pt) film. 
     A first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) is formed on the charge exchange layer. 
     In any of the first to third embodiments, a first photovoltaic layer is composed by dispersing silicon dioxide particles  10  as a first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) on the charge exchange layer ( 15  in  FIGS. 1 and 2 ;  23  in  FIG. 4 ). 
     The silicon dioxide particles  10  that constitute the first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) use glass particles formed for example of synthetic quartz, fused quartz glass, soda-lime glass, non-alkali glass or borosilicate glass, which are immersed in a solution of 5-10% hydrofluoric acid or hydrochloric acid, washed with water and dried, and pulverized so that a major axis L of the particles is 20 to 100 nm. The first to third embodiments use synthetic quartz particles, which are crystalline of silicon dioxide, which are immersed in 10% hydrofluoric solution, washed with water and dried, and pulverized so that a major axis L of the particles is 20-100 nm. 
     Electrolyte  19  is enclosed between the first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) and the second photovoltaic layer  16 , in a space that is surrounded by a sealing member  18  on four sides. The electrolyte  19  is used in the prior-art dye-sensitized solar cells, and it can be of any of the following states; liquid, solid, coagulated and ordinary temperature molten salt. 
     The electrolyte can be, for example, a combination of metal iodide, such as lithium iodide, sodium iodide, potassium iodide and cesium iodide, and iodine; a combination of iodine salt of quaternary ammonium compound, such as tetraalkylammonium iodide, pyridinium iodide and imidazolium iodide, and iodine; a combination of bromine compound—bromine instead of the aforementioned iodine and iodine compound; or a combination of cobalt complex. 
     If the electrolyte is an ionic liquid, there is no need to use a solvent. The electrolyte may be a gel electrolyte, a high polymer electrolyte or a solid electrolyte, and an organic charge transport material may be used instead of the electrolyte. 
     If the electrolyte  19  is in a state of a solution, the solvent may be, for example, nitrile-based solvent such as acetonitrile, methoxyacetonitrile and propionitrile, carbonate-based solvent such as ethylene carbonate, and ether-based solvent. 
     Specifically, the electrolyte  19  used in the first to third embodiments is formed by adding 0.1 mol LiI, 0.05 mol I 2 , 0.5 mol 4-tetra-butylpyridine and 0.5 mol tetrabutylammonium iodide in acetonitrile solvent 
     The distance between the first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) and the second photovoltaic layer  16  in the height direction should preferably be as short as possible, since transfer of charge becomes easier if the distance is shorter. 
     In the first to third embodiments, the thickness of the electrolyte  19  portion in the height direction, that is, the distance between the first photovoltaic layer ( 21  in  FIG. 1 ;  17  in  FIG. 2 ;  24  in  FIG. 4 ) and the second photovoltaic layer  16  in the height direction, is 200 μm or smaller. 
     Method for evaluating the maximum output value per unit area according to the present specification is as described below. 
     An LED light (manufactured by Cosmotechno Co., Ltd.) was used to irradiate light from the second substrate side, and light corresponding to 1000 lux by illuminometer DT-1309 manufactured by CEM Corporation was irradiated to the photovoltaic element being the target for measurement. A digital multimeter was used to measure the I-V characteristics of the photovoltaic element as the target for measurement, by which values of short circuit current, open circuit voltage and form factor ff were acquired, and the maximum output value per unit area was derived. 
     Hereafter, characteristics of the present embodiments will be described with reference to the drawings. The other portions are similar to the description regarding the matters common to the first to third embodiments described above. 
     First Embodiment 
       FIG. 1  is a view illustrating a first embodiment In the first embodiment, silicon dioxide particles having an average major axis L of 20-100 nm are used as the silicon dioxide particles  10  used in the first photovoltaic layer  21 . These silicon dioxide particles  10  are dispersed in an overlapped manner on a flat first conductive film  14  (FTO layer) and a similarly flat charge exchange layer  15  (Pt layer) formed thereon, by which the first photovoltaic layer  21  having a thickness of 300 to 500 nm in the height direction is composed. 
     Other conditions are as described as matters common to the first to third embodiments. 
     As a result, the embodiment realizes a significant improvement of photovoltaic efficiency compared to the prior art example described in the background art. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                   
                 Maximum 
               
               
                   
                   
                   
                   
                 output 
               
               
                   
                   
                   
                 FTO layer 
                 per unit 
               
               
                   
                 L 
                 t 
                 roughness 
                 area 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Prior Art 
                 500~800 nm 
                 0.15~0.20 mm 
                 Very little 
                 28.00 μW/cm 2   
               
               
                   
                   
                   
                 surface 
               
               
                   
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                 First 
                  20~100 nm 
                     300~500 nm 
                 Very little 
                 35.00 μW/cm 2   
               
               
                 Embodiment 
                   
                   
                 surface 
               
               
                   
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                   
               
               
                 L: Average major axis of silicon dioxide particles 
               
               
                 t: Silicon dioxide layer thickness 
               
            
           
         
       
     
     In the first embodiment, the average major axis of the silicon dioxide particles  10  is small compared to the prior art, which is considered effective in increasing the surface area of the silicon dioxide particles  10  in the first photovoltaic layer  21  and raising the photovoltaic efficiency. 
     Second Embodiment 
       FIG. 2  is a view illustrating a second embodiment. The second embodiment uses the same materials and the like used in the first embodiment. However, in the second embodiment, a first photovoltaic layer  17  is composed so that the silicon dioxide particles  10  are arranged on a flat first conductive film  14  and a similarly flat charge exchange layer  15  disposed thereon, so that the thickness thereof in the height direction is 300 nm or smaller. 
     That is, the thickness of the first photovoltaic layer in the height direction is reduced compared to the first embodiment. 
       FIG. 3  is an enlarged view of portion A of  FIG. 2 , wherein the silicon dioxide particles  10  constituting the first photovoltaic layer  17  are dispersed on the flat first conductive film  14  (FTO layer) and the similarly flat charge exchange layer  14  (Pt layer) formed thereon, in a state where there is small overlap of particles. 
     As a result, the embodiment realizes a significant improvement of photovoltaic efficiency compared to the prior art example described in the background art. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                   
                 Maximum 
               
               
                   
                   
                   
                   
                 output 
               
               
                   
                   
                   
                 FTO layer 
                 per unit 
               
               
                   
                 L 
                 t 
                 roughness 
                 area 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Prior Art 
                 500~800 nm 
                 0.15~0.20 mm 
                 Very little 
                 28.00 μW/cm 2   
               
               
                   
                   
                   
                 surface 
               
               
                   
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                 Second 
                  20~100 nm 
                 300 nm or less 
                 Very little 
                 45.48 μW/cm 2   
               
               
                 Embodiment 
                   
                   
                 surface 
               
               
                   
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                   
               
               
                 L: Average major axis of silicon dioxide particles 
               
               
                 t: Silicon dioxide layer thickness 
               
            
           
         
       
     
     In the second embodiment, the overlapping of the silicon dioxide particles  10  in the first photovoltaic layer  17  is reduced, according to which the property of charge transfer near the first photovoltaic layer  17  is enhanced, by which the photovoltaic efficiency is considered to be increased. 
     Therefore, it is important not to arrange too much silicon dioxide particles  10  on the upper surface of the charge exchange layer  15  in order to improve the photovoltaic efficiency. That is, it has been confirmed that the photovoltaic amount is increased if the silicon dioxide particles  10  are not excessively overlapped and sufficient space is formed therebetween. 
     Therefore, the thickness of the first photovoltaic layer  17  in the height direction should preferably be equal to or smaller than three times the average major axis L of the silicon dioxide particles. 
     The silicon dioxide particles  10  should preferably be arranged on the surface of an upper layer of the charge exchange layer  15  in a dispersed manner with spaces formed therebetween. This arrangement is to prevent the silicon dioxide particles  10  from being arranged in an overcrowded manner and hindering conductivity between the charge exchange layer  15 , the silicon dioxide particles  10  and the electrolyte  19 . It is preferable that the charge exchange layer  15 , the silicon dioxide particles  10  and the electrolyte  19  are arranged with sufficient allowance, so that the total sum of contact surface areas of the charge exchange layer  15 , the silicon dioxide particles  10  and the electrolyte  19  that perform charge exchange is maximized. 
     Therefore, the photovoltaic amount can be increased by arranging the silicon dioxide particles  10  in the first photovoltaic layer  17  such that the charge exchange layer  15  is visible through the spaces between the silicon dioxide particles  10  when the first substrate  12  is viewed from the second substrate  11  side. 
     Third Embodiment 
       FIG. 4  is a view illustrating a third embodiment. The third embodiment uses the same materials and the like as the first embodiment. However, in the third embodiment, a first conductive film  22  (FTO layer) and a charge exchange layer  23  (Pt layer) that constitute a base on which the silicon dioxide particles  10  are arranged are not flat. As illustrated in  FIG. 4 , the first conductive film  22  has an uneven surface (roughness or asperity), with a height difference of approximately 50 nm formed on the surface. The charge exchange layer  23  formed on the first conductive film  22  also has a roughness on the surface, influenced by the height difference formed on the first conducive film  22 . 
       FIG. 5  is an enlarged view of portion B of  FIG. 4 . The silicon dioxide particles  10  constituting the first photovoltaic layer  24  are dispersed on the first conductive film  22  that has a roughness on the surface and the charge exchange layer  23  formed thereon and having a similar roughness, in a state where there is small overlap of particles. 
     The difference of height of the surface roughness of the first conductive film  22  should be 50 nm or greater, and more preferably, 100 nm or greater. Further, it is preferable that the charge exchange layer  23  formed on the first conductive film  22  is formed in a manner maintaining the shape of the roughness on the surface of the first conductive film  22  without burying the surface roughness of the first conductive film  22 . 
     As a result, the embodiment realizes an even further significant improvement of photovoltaic efficiency compared to the prior art example described in the background art. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                   
                 Maximum 
               
               
                   
                   
                   
                   
                 output 
               
               
                   
                   
                   
                 FTO layer 
                 per unit 
               
               
                   
                 L 
                 t 
                 roughness 
                 area 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Prior Art 
                 500~800 nm 
                 0.15~0.20 mm 
                 Very little 
                 28.00 μW/cm 2   
               
               
                   
                   
                   
                 surface 
               
               
                   
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                 Third 
                  20~100 nm 
                 300 nm or less 
                 Surface 
                  70.8 μW/cm 2   
               
               
                 Embodiment 
                   
                   
                 height 
               
               
                   
                   
                   
                 difference 
               
               
                   
                   
                   
                 approx. 
               
               
                   
                   
                   
                 50 nm 
               
               
                   
               
               
                 L: Average major axis of silicon dioxide particles 
               
               
                 t: Silicon dioxide layer thickness 
               
            
           
         
       
     
     The arrangement of the silicon dioxide particles  10  dispersed on the charge exchange layer  23  formed on the first conductive film  22  is influenced by the surface roughness of the first conductive film  22  and charge exchange layer  23  as base layers. 
     Thanks to the surface roughness of the base layers, the silicon dioxide particles  10  are arranged in a thinly dispersed manner. Thereby, the silicon dioxide particles  10  are arranged with appropriate spatial allowance without excessive overlap, and therefore, the increase of photovoltaic amount is confirmed. 
       FIG. 6  is a schematic diagram in which the first conductive film  22  is illustrated from bird&#39;s eye view. The shape of the surface roughness of the first conductive film  22  is not only risen steeply, as illustrated in  FIG. 5 , but may also include a structure  25  where the surface is somewhat rounded, as illustrated in  FIG. 6 . Further, the roughness does not have to be random, as illustrated in  FIGS. 5 and 6 , and the roughness can be regularly arranged shapes, such as structural cones, trigonal pyramids, quadrangular pyramids and other pyramid shapes. 
     The present invention is not restricted to the above-described first to third embodiments, and various modifications are possible. For example, the optimum average major axis of the silicon dioxide particles  10  may vary according to the distribution of size and shape of the silicon dioxide particles  10  constituting the first photovoltaic layer. Similarly, the optimum value of thickness of the first conductive film in the height direction may vary according to the distribution of size and shape of the silicon dioxide particles  10 . 
     Further, various optimum combinations of height difference of unevenness in the height direction of the first conductive film and/or the charge exchange layer, the shape of the roughness, and the distribution of the roughness in a direction parallel to the first substrate may be adopted in response to the distribution of size and shape of the silicon dioxide particles  10 . 
     Of course, a portion of the respective embodiments may be added to, deleted from or replaced with other materials and configurations. 
     REFERENCE SIGNS LIST 
     
         
           10  silicon dioxide particle 
           11  second substrate 
           12  first substrate 
           13  second conductive film 
           14  first conductive film 
           15  charge exchange layer 
           16  second photovoltaic layer 
           17  first photovoltaic layer 
           18  sealing member 
           19  electrolyte 
           21  first photovoltaic layer 
           22  first conductive film 
           23  charge exchange layer 
           24  first photovoltaic layer