Patent Publication Number: US-2011048528-A1

Title: Structure of a solar cell

Description:
This application claims the benefit of Taiwan application Serial No. 98129308, filed Aug. 31, 2009, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a structure of solar cell, and more particularly to a structure of solar cell with high photoelectric conversion efficiency. 
     2. Description of the Related Art 
     Due to the energy crisis, the whole world is engaged in the pursuit of all sorts of alternative energies. Of the alternative energy sources with great development potential such as hydraulic power, wind power, solar power, terrestrial heat, sea water, temperature difference, waves, and tides, the solar power has become a mainstream of the new energies. According to estimation, the energy that the sun illuminated on the surface of the earth per year is one million times of the energy annually consumed by people on the earth. If 1% of the inexhaustible energy of solar light can be converted into electric power by solar cells, the generated energy will suffice to meet people&#39;s needs of energy. 
     When the solar light enters a conventional solar cell, a large amount of solar light will be reflected by the surface of the conventional solar cell. As the reflected solar light cannot be used for photoelectric conversion, the conversion efficiency of conventional solar cell decreases accordingly. In addition, among the generally known technologies, there is a method which increases the photoelectric conversion efficiency by etching the surface of the solar cell. However, the manufacturing process of such solar cell is costly and time consuming and not suitable for large-scale production for civil uses. 
     Besides, when the solar light moves along with the rotation of the earth, the solar light cannot be vertically illuminated on the solar cell (that is, the contained angle between the normal of the solar cell surface and the incident light is not equal to zero). Thus, conventional solar cell is configured on the solar power tracking system to position the relative location between the solar cell and the solar light to achieve vertical incidence of the solar light (that is, the contained angle between the normal line of the solar cell surface and the incident light is equal to zero). However, the cost increases significantly. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a structure of solar cell. Nanostructures are applied to coarsen a surface of the solar cell so as to increase the light absorption rate of the solar cell with respect to the incident light. 
     According to a first aspect of the invention, a structure of solar cell including a substrate, a base and a plurality of nanostructures is provided. The base is disposed on the substrate. The nanostructures are disposed on a surface of the base, so as to increase light absorption of the structure. 
     According to a second aspect of the invention, a structure of solar cell including a substrate, a first base, a second base and a plurality of nanostructures is provided. The first base is disposed on the substrate. The second base is disposed on a surface of the first base. The nanostructures are disposed on a surface of the second base, so as to increase light absorption of the structure. 
     According to a third aspect of the invention, a solar cell structure including a substrate and a base is provided. The base is disposed on the substrate, and a surface of the base has a plurality of nanostructures so as to increase light absorption of the structure. 
     According to a fourth aspect of the invention, a solar cell structure including a substrate, a first base and a second base is provided. The first base is disposed on the substrate. The second base is disposed on a surface of the first base, and a surface of the second base has a plurality of nanostructures so as to increase light absorption of the structure. 
     The above and other aspects of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a solar cell according to a first embodiment of the invention. 
         FIG. 2  shows a cross-sectional view of an example of the structure of a solar cell of  FIG. 1  having coplanar electrodes. 
         FIG. 3  shows a cross-sectional view of an example of the structure of a solar cell of  FIG. 1  having a top and a bottom electrode. 
         FIG. 4A  shows a cross-sectional view of an example of the structure of a solar cell of  FIG. 2 . 
         FIG. 4B  illustrates the bandgap distribution of a first semiconductor layer of  FIG. 4A  being a graded layer. 
         FIG. 4C  shows a first semiconductor layer of  FIG. 4A  being a super lattice layer. 
         FIG. 5  shows a plurality of nanoparticles of  FIG. 1  in a square arrangement. 
         FIG. 6  shows a plurality of nanoparticles of  FIG. 1  in a hexagonal arrangement. 
         FIG. 7  illustrates a measurement system for measuring the optical response of the solar cell of  FIG. 1 . 
         FIG. 8  shows a curve chart of normalized photocurrents of the solar cell of  FIG. 1  and a conventional solar cell, illuminated with a 500 nm incident light under different incident angles. 
         FIG. 9  shows a curve chart of the photocurrent difference of the solar cell of  FIG. 1  and a conventional solar cell, illuminated with a 500 nm incident light under different incident angles. 
         FIG. 10  shows a cross-sectional view of a solar cell according to a second embodiment of the invention. 
         FIG. 11  shows a cross-sectional view of an example of the structure of a solar cell of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention discloses a solar cell structure with nano-sized microstructures, that is, nanostructures (such as nanoparticles), disposed on a surface of a material used as solar cell absorber. The light absorption of the solar cell with respect to the incident light is enhanced through the structural relationship between the nanostructures and the absorber. Any solar cell structure can be adapted if the nanostructures can be disposed on the surface of the base of the solar cell to increase its light absorption efficiency. A number of embodiments are disclosed below for elaboration. 
     First Embodiment 
     Referring to  FIG. 1 , a cross-sectional view of a solar cell according to a first embodiment of the invention is shown. The solar cell  100  includes a substrate  10 , a base  30 , and a plurality of nanostructures  50 . The base  30  is disposed on the substrate  10 . The nanostructures  50  such as nanoparticles are disposed on a surface of the base  30 , or a surface of the base  30  has the nanostructures  50  to increase the light absorption of the entire solar cell  100 . 
     In practical applications, electrodes can be disposed according to the structure of solar cell as shown in  FIG. 1 .  FIG. 2  shows a cross-sectional view of an example of the structure of a solar cell of  FIG. 1  having coplanar electrodes. In practical application, various implementations of disposition of electrodes on the solar cell  100  can be employed. According to an implementation shown in  FIG. 2 , a portion  12  of the substrate  10  extends over the base  30  for the disposition of the electrodes such as a first electrode  70  and a second electrode  90 . For example, the first electrode  70  is disposed on a portion of the base  30 , for example, a portion of a top surface of the base  30 . The second electrode  90  is disposed on a portion  12  of a top surface  15  of the substrate  10 . 
     Referring to  FIG. 3 , a cross-sectional view of an example of the structure of a solar cell of  FIG. 1  having a top and a bottom electrode is shown. In this example, the second electrode  90  is directly disposed on a bottom surface  17  of the substrate  10 , and the first electrode  70  is disposed on a portion of the base  30 . In addition, as the implementation is not limited thereto, the solar cell structure according to the invention can be adapted in any solar cell if the nanostructures can be disposed on a surface of the base of the solar cell to increase the light absorption efficiency of the entire solar cell, and this holds true for the following embodiments. 
     The substrate  10  can be made from a low or high bandgap semiconductor material such as an N-type or P-type material, and the base  30  can be made from a high bandgap semiconductor material such as a P-type material. In another embodiment, the base  30  can be made from a high bandgap P-type material, and the substrate  10  can be made from a low bandgap semiconductor N-type material. In other examples, the substrate  10  and the base  30  can be made from a high bandgap semiconductor material and a low bandgap semiconductor respectively. Nevertheless, any solar cell can be employed for the implementation of the solar cell  100  if the junction of the substrate  10  and the base  30  forms a P—N junction according to the theory of the solar cell so as to achieve photoelectric conversion when the light is illuminated on the solar cell. 
     Referring to  FIG. 4A , a cross-sectional view of an example of the structure of a solar cell of  FIG. 2  is shown. For example, the base  30  includes a first semiconductor layer  32  and a second semiconductor layer  34 . The first semiconductor layer  32 , such as a graded layer, is disposed on the substrate  10 . The second semiconductor layer  34  is disposed on the graded layer. The arrangement of the bandgaps of the materials of the substrate  10 , the first semiconductor layer  32  (such as a graded layer) and the second semiconductor layer  34  will be exemplified by a number of implementations below. 
     Referring to  FIG. 4B , a bandgap distribution of a first semiconductor layer of  FIG. 4A  being a graded layer is illustrated. In an embodiment, the substrate  10  can be made from a low bandgap semiconductor material, and the second semiconductor layer  34  can be made from a high bandgap semiconductor material. Meanwhile, the bandgap of the graded layer (that is, the first semiconductor layer) increases with distance away from the substrate  10 , as indicated in the direction of arrow A. In another embodiment, the substrate  10  can be made from a high bandgap semiconductor material, and the second semiconductor layer  34  can be made from a low bandgap semiconductor material. In this case, the bandgap of the graded layer decreases with distance away from the substrate  10 , as indicated in the direction of arrow A. 
     In  FIG. 4A , the first semiconductor layer  32  can also be a super lattice layer, disposed on the substrate  10 , and the second semiconductor layer  34  is disposed on the super lattice layer. The super lattice layer includes at least one thin film set, wherein one thin film set includes a first and a second thin film, the first thin film is disposed on the substrate  10 , and the second thin film is disposed on the first thin film. The arrangement of the bandgaps of the substrate  10 , the first semiconductor layer  32  (such as a super lattice layer) and the second semiconductor layer  34  will be exemplified by various implementations below. 
     Referring to  FIG. 4C , a super lattice layer is taken as the first semiconductor layer  32  of  FIG. 4A . In an embodiment, the super lattice layer includes three thin film sets  35 - 37 . The substrate  10  can be made from a low bandgap semiconductor material, and the second semiconductor layer  34  can be made from a high bandgap semiconductor material. In addition, for each thin film set, the first thin films  351 - 371  can be made from high bandgap semiconductor materials, and the second thin films  352 - 372  can be made from low bandgap semiconductor materials. 
     In another embodiment, the substrate  10  is made from a high bandgap semiconductor material, and the second semiconductor layer  34  is made from a low bandgap semiconductor material. In this case, for each thin film set, the first thin films  351 - 371  can be made from low bandgap semiconductor materials, the second thin films  352 - 372  can be made from high bandgap semiconductor materials. Indeed, the number of thin films of the super lattice layer can be designed and adjusted according to requirements and the environment of application, and it is not limited to the above exemplifications. 
     In the present embodiment, the oxide semiconductor material can be, for example, zinc oxide material (ZnO), and examples of low bandgap semiconductor material include silicon (Si), germanium (Ge) or gallium arsenide (GaAs) material, and at least one material selected from the group consisting of germanium (Ge), indium (In), aluminum (Al), gallium (As), phosphorous (P) or antimony (Sb) or other alternative materials. 
     Further, the first electrode  70  and the second electrode  90  form respective ohmic contacts on the base  30  and the substrate  10  respectively. Examples of the first electrode  70  include titanium (Ti) and gold (Au). Examples of the second electrode  90  include nickel (Ni) and gold (Au). Other materials, locations or ways of forming ohmic contacts on the base and the substrate can also be employed for the implementation of the first and the second electrode such as the back electrodes of  FIG. 3  or other ways of implementation. 
     As indicated in  FIG. 1 , the shape of the nanostructures  50  can be a circular or a non-circular geometric shape. Examples of the nanostructures include oxides, organic materials, semconductors, and metallic materials. Examples of the oxides include silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ) and titanium dioxide (TiO 2 ). Examples of the metallic materials include gold (Au), silver (Ag), nickel (Ni) and titanium (Ti). Examples of the organic materials include any suitable polymers such as polystyrene. The size of the nanostructures  50  ranges from about 10 nm to about 100 μm. 
     In the present embodiment, the nanostructures  50  are exemplified by a spherical structure and the material of the nanostructure is exemplified by a silicon dioxide (SiO 2 ) material. Besides, other different structures such as elliptical, powder, polygonal or other geometric structures capable of increasing the light absorption can be regarded as embodiments of the nanostructures  50 . 
     For example, the refractive index of the nanostructures  50  (such as is 1.55) is less than the refractive index (about 3.6) of the base  30 . When the solar light transmitted through the air (the refractive index is approximately equal to 1) is illuminated on the solar cell, the difference between the refractive index of the air and that of the base of the solar cell  100  is proportional to the reflective index of the solar light. That is, the greater the refractive index difference, the greater the reflective index. In other words, when the solar light is illuminated on the solar cell  100 , a large amount of the incident light will be reflected off so that less amount of solar light can be illuminated on the solar cell  100  (that is, the photoelectric conversion efficiency deteriorates). 
     According to the solar cell of the present embodiment, the refractive index of the nanostructures  50  can be between the refractive index of the base  30  and that of the air, and the difference between the refractive index of the nanostructures  50  and that of the air is less than the difference between the refractive index of the base  30  and that of the air. Thus, the photoelectric conversion efficiency of the solar cell can be enhanced by the decrease in the reflective index of the solar light photoelectric. In addition, the nanostructures  50  are not limited to be those whose refractive indices are less than that of the base; nanostructures with refractive index equal to or greater than that of the base can also be employed to implement the present embodiment. 
     The disposition of a plurality of nanostructures  50  on a surface of the base  30  is disclosed in an exemplification below. In an example, the nanostructures  50  are nanoparticles and mixed with a solution such as isopropyl alcohol (IPA). Then, the mixed solution is dripped on the base  30 . In the present embodiment, the nanostructures  50  are coated on the base  30  by a spinner according to the spin-coating method. The nanostructures  50  and the isopropyl alcohol are mixed according to a concentration ratio such as a weight percentage of nanostructure of 1.45% and a weight percentage of the isopropyl alcohol of 98.55%. The nanostructures  50  are disposed on the base  30  according to the spin-coating method for example. Besides, the user can further adjust the rotation speed or the duration corresponding to rotation speed with a spinner. Alternatively, the nanostructures  50  are disposed on the surface of the base  30  in multiple stages at different rotation speeds. Furthermore, the spin-coating process can be performed in two stages, such as a first stage and a second stage. In the first stage and the second stage, the nanostructures  50  are disposed at a first rotation speed and a second rotation speed respectively. 
     For example, in the first stage, the first rotation speed is 1000 rpm (that is, revolution per minute) and the duration is about 10 seconds; in the second stage, the second rotation speed is 4000 rpm and the duration is about 30 seconds. In other examples, the rotation speed, the duration and the number of stages can be adjusted according to the requirement of the operator. In addition, the operator can coat the nanostructures at one or two different rotation speeds, not limited thereto. In other examples, the rotation speed can be adjusted and designed according to the mixing ratio of the nanostructures  50  and the isopropyl alcohol. 
     In addition to the spin-coating method for disposing the nanostructures  50  on a surface of the base  30 , the present embodiment can also be implemented by employing other methods, such as the etching method for disposing the nanostructures  50  on the surface of the base  30 . For example, a blocking layer (such as photoresist, oxides or other material layers capable of blocking erosive liquids or gases) can be formed on the surface of the base  30  by wet etching method or dry etching method for disposing nanostructures on the surface of the base  30 . However, the nanostructures  50  are not limited to nanoparticles, and any structures enabling the nanostructures  50  to be disposed on the surface of the base  30  to increase the light absorption efficiency of the entire solar cell can be used for implementing the present embodiment. 
     Next, after the spin-coating process is performed to the nanostructures  50 , the arrangement of the nanostructures  50  has many ways of implementation. In an example, the nanostructures  50  disposed on the base  30  are in the form of single-layer arrangement. In the present embodiment, the solar cell  100  is exemplified by a plurality of nanostructures with single-layer arrangement, but is not limited thereto. In other embodiments, the nanostructures  50  disposed on the base  30  can be in the form of multi-layer arrangement, regular arrangement or random arrangement. 
     Besides, the abovementioned multiple arrangement method can be adjusted by changing the mixing ratio of the nanostructures  50  and the isopropyl alcohol or by changing the rotation speed. 
     Besides, the arrangement of the nanostructures  50  corresponds to a two-dimensional grating vector in the vector space. The refracted light of the incident light (that is, the solar light) entering the solar cell  100  with a minimum refraction angle can be obtained by considering the wave vector of the incident light and the two-dimensional grating vector. Since the projection of the incident light on the solar cell  100  is in a one-dimensional path, the two-dimensional grating vector can be simplified to a one-dimensional path from a two-dimensional space. That is, when the incident light is illuminated on the nanostructures  50 , Bragg diffraction effect is considered in the one-dimensional path following the equation: 
       2 sin θ=[(2 m+ 1)/2 DG]λ/neff;   (Equation1)
 
     wherein θ indicates the incident angle, m denotes the order of diffraction, DG denotes the effective grating period, λ denotes the wavelength of the incident light, and neff denotes the effective refractive index of the nanostructures. Thus, the destructive interference occurs on the reflected light caused by illuminating the incident light on the nanostructures at an angle, according to equation 1, and results in the reduction of the reflected light of the solar light entered the solar cell  100  at that angle. The reflective index of the incident light on the surface of the solar cell is reduced and the photoelectricphotocurrent is increased so as to improve photoelectric conversion efficiency of the solar cell  100 . Examples of the arrangements of the nanostructures are elaborated below. 
     Referring to  FIG. 5  and  FIG. 6 ,  FIG. 5  shows a square arrangement of a plurality of nanoparticles of  FIG. 1   f .  FIG. 6  shows a hexagonal arrangement of nanoparticles of  FIG. 1 . In the basic mode (that is, m=1), the effective grating period DN will form a grating period DG 1  and a grating period DG 2  of the square and hexagonal arrangements respectively. When the wavelengths of the incident light are 500 nm, 550 nm and 600 nm, according to the theoretic calculation, the destructive interference occurs on the solar light entering the nanostructures  50  at the incident angle θ of 48°, 54°, and 62° respectively. In other words, less amount of the solar light will be reflected if the solar light enters the surface of the solar cell  100  at the angles of 48°, 54°, and 62°. 
     Thus, the incident angle corresponding to a wavelength at which destructive interference occurs can be adjusted by changing the arrangement of the nanostructures  50  or the size of the nanostructures  50 . In other words, the diffraction effect (such as destructive interference) resulting from the periodic structure of the nanostructures relates to the gain of the photocurrent generated by the incident light of the wavelength on the solar cell. In an example, the grating period DN is 116 nm. In another example, the grating period can be adjusted by changing the size or the arrangement of the nanostructures  50 . 
     Referring to  FIG. 7 , a measurement system for measuring the optical response (i.e., quantum efficiency) for the solar cell of  FIG. 1  is shown. As indicated in  FIG. 2 , the measurement system  150  includes a light source  151 , a collimator  152  and a carrier  153 . The light source  151  generates multiple incident lights with respective wavelengths, that is, to simulate the wavelengths that could be included in the solar light. The collimator  152  transforms the incident light L 1  generated and radiated by the light source  151  (such as a point light source) into a parallel incident light L 2 , which is further illuminated on the solar cell  100  to simulate the solar light at a wavelength. 
     Furthermore, the measurement system  150  generates multiple incident lights with respective wavelengths by the light source  151  and illuminates the incident lights on the solar cell  100 . The light source  151  moves from an angle A 1  to an angle A 2  (or from the angle A 2  to the angle A 1 ) along a path M to measure the photocurrent gain correspondingly generated by the solar cell  100  when the incident light L 2  is illuminated on the solar cell  100  at different angles, wherein the wavelengths of the incident lights are 500 nm, 550 nm and 600 nm respectively for example. The angles A 1  and A 2  are 90° and 0° respectively for example. The incident angle θ is defined by the contained angle between the normal Q of the solar cell  100  and the incident light L 2 . 
       FIG. 8  shows a curve chart of normalized photocurrents of the solar cell of  FIG. 1  and a conventional solar cell, illuminated with a 500 nm incident light. In  FIG. 8 , the horizontal axis denotes angle A and the verticle axis denotes the photocurrent C. As indicated in  FIG. 8 , when the wavelength of the incident light L 2  on the solar cell  100  is 500 nm, a curve S 1  shows an acceptance angle R 1  for the 500 nm incident light L 2  illuminated on the solar cell  100 , and a curve F 1  shows an acceptance angle R 2  for the 500 nm incident light L 2  illuminated on the conventional solar cell whose surface does not have nanostructures disposed thereon. The acceptance angle is defined as the incident angle of the incident light at or above 90% of the maximum photocurrent of the solar cell illuminated by the incident light. The maximum photocurrent is defined as the current generated by the incident light which is illuminated on the solar cell at 0° (that is, the angle between the normal and the incident light is 0°). 
     In an example, the acceptance angle R 1  of the solar cell  100  is 46°, and the acceptance angle R 2  of the conventional solar cell, without nanostructures disposed on its surface, is 27°. Compared with conventional solar cell, when the wavelength is 500 nm, the acceptance angle is increased by 19° for the solar cell  100  of the present embodiment. In other example, when the wavelengths are 550 nm and 600 nm, the acceptance angles are increased by 27° and 21° respectively. That is, the increase of acceptance angle of the light enables the photoelectric conversion of the solar light not vertically illuminated on the solar cell  100 , wherein vertical illumination indicates the angle between the normal Q and the incident light L 2  equal to 0°. In other words, the solar cell  100  enables the light absorption of the solar light at a wider range of incident angles, produces higher photocurrent gain, and thus enhances photoelectric conversion efficiency. 
       FIG. 9  shows a curve chart of the photocurrent difference of the solar cell of  FIG. 1  and a conventional solar cell, illuminated with a 500 nm incident light under different incident angles. In  FIG. 9 , the horizontal axis denotes angle A and the verticle axis denotes photoelectricphotocurrent C. As indicated in  FIG. 9 , when the wavelength of the incident light L 2  entering the solar cell  100  is 500 nm, the maximum difference between the photocurrents corresponding to the solar cell  100  and conventional solar cell occurs at an incident angle θ 1 , such as 52° for example. When the wavelengths of the incident light L 2  are 550 nm and 600 nm, the maximum difference of the photocurrent occurs at incident angles θ 2  and θ 3 , such as 62° and 63°, respectively, for example. 
     Thus, according to the theoretic calculation of the equation 1, when the wavelengths of the incident lights on the solar cell  100  are 500 nm, 550 nm, and 600 nm, the incident angles θ enabling the destructive interference of the reflected light of the incident light, that is, the reduced reflective index of the incident light, are 48°, 54° and 62° respectively. In other words, illuminating the incident light on the solar cell  100  at these incident angles decreases the amount of reflective light of the incident light. Thus, the incident angle at which destructive interference of the reflected light occurs can be adjusted according to relevant parameters of equation 1 for estimation. The incident angles can be determined, for example, by adjusting the size of the nanostructure to adjust the grating period and by changing the refractive index of the nanostructure. 
     Second Embodiment 
     The solar cell  100 A of the present embodiment differs from the solar cell  100  of the first embodiment in that the solar cell  100 A includes a first base and a second base, which form a P—N junction, and that the substrate  10 A is made from a transparent material, and the similarities are not repeated for the sake of brevity. To elaborate the solar cell of the present embodiment, a block diagram is disclosed below. 
     Referring to  FIG. 10 , a cross-sectional view of a solar cell according to a second embodiment is shown. The solar cell  100 A has a substrate  10 A, a first base  20 , a second base  30 A, and a plurality of nanostructures  50 . The first base  20  is disposed on the substrate  10 A. The second base  30 A is disposed on the first base  20 . The nanostructures  50  are disposed on a surface of the second base so as to increase the entire light absorption. 
     In the present embodiment, the substrate  10 A can be made from a transparent material or a soft material. The transparent material is such as glass or quartz, and the soft material is such as plastics. The substrate  10 A can also be made from a semiconductor material. 
     Referring to  FIG. 11 , a cross-sectional view of an example of the structure of a solar cell of  FIG. 10  is shown. The second base  30 A includes a first semiconductor layer  32 A and a second semiconductor layer  34 A. The first and the second semiconductor layer respectively correspond to the first semiconductor layer  32  and the second semiconductor layer  34  of the first embodiment, and their details are not repeated here for the sake of brevity. Besides, the bandgaps of the first semiconductor layer  32 A and the second semiconductor layer  34 A can be designed according to the bandgap of the first base  20 . 
     In the present embodiment, the first base  20  can be made from a low bandgap semiconductor material such as a P-type material, and the second base  30 A can be made from a high bandgap semiconductor material such as an N-type material. The low bandgap semiconductor material can be implemented according to an example of the first embodiment, and is not repeated here. In short, as is disclosed in the first embodiment, the solar cell can be implemented if the first base  20  and the second base  30 A being bonded together can achieve photoelectric conversion according to the theory of the solar cell. 
     In practical application, the disposition of electrodes on the solar cell structure  100 A of the present embodiment can be implemented in the manner of that of electrodes of the first embodiment disclosed in  FIG. 2  or  FIG. 3 , and is not repeated here. 
     In other example, the solar cell  100 A is implemented not subjected to the material of the substrate such as a glass substrate with higher hardness. For example, the substrate can be a substrate with lower hardness, such as a flexible plastics substrate, so as to increase the area of application of the solar cell. 
     Although the base of the first embodiment (or the second base of the second embodiment) is exemplified by a high bandgap semiconductor material such as an oxide semiconductor material, the base can also be implemented by using other semiconductor material with a high bandgap, compared to the substrate (or the first base of the second embodiment), or by using a semiconductor layer made from mixed materials or with multi-layer different materials. In short, any structures of solar cell can be used for implementing according to the invention if nanostructures can be disposed or included in the surface of a base of the solar cell to enhance its entire light absorption efficiency. 
     As disclosed above, the different embodiments of solar cell according to the invention lead to advantages exemplified as below: 
     (1) According to an embodiment disclosed above, the disposition of nanostructures reduces the reflective index of the incident light, and the manner of arrangement of the nanostructures improves the gain of the photocurrent generated by the incident light on the solar cell, thus increasing the acceptance angle and improving the photoelectric conversion efficiency of the solar cell. Accordingly, the solar cell can achieve improved efficiency and save cost without having to be disposed on a solar power tracking system. 
     (2) According to an embodiment disclosed above, the solar cell can be adapted in a substrate made from soft material or transparent material, so as to expand the area of application of the solar cell. 
     While the invention has been described by way of examples and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.