Abstract:
A solar battery unit is proposed, including: a first electrode; a nano rough layer formed on the first electrode; a semiconductor active layer formed on the nano rough layer; and a second electrode formed on the semiconductor active layer, thereby enabling the nano rough layer formed on the first electrode to fully absorb solar energy not completely absorbed by the semiconductor active layer so as to allow solar energy to be fed back to the semiconductor active layer with a view to maximizing absorption of solar energy.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to solar energy elements, and more particularly, to a solar battery unit. 
         [0003]    2. Description of the Prior Art 
         [0004]    At present, organic semiconductor materials, of which solar energy devices are fabricated, are flexible, lightweight, thin, cheap to manufacture, and environmentally friendly. Organic semiconductors have lower carrier (electrons and holes) mobility rate than inorganic semiconductors and thus their electrons and holes have an extremely short drift distance, that is, less than 100 nanometers. Given a drift distance of greater than 100 nanometers, recombination of electrons and holes occurs readily to thereby cause a waste of absorbed solar energy. Although it is necessary for a solar energy device to be thin, but the solar energy device may be too thin to take in solar energy thoroughly. 
         [0005]    According to the prior art, to prevent recombination of electrons and holes, it is necessary to use nanocarbon tubes or form holes by laser, and then fill the holes with an electron- or hole-conveying material so as to lower the chance of recombination of electrons and holes. However, the prior art of forming the aforesaid holes is limited by difficulty in controllably attaining nanoscale size and depth of the aforesaid holes and difficulty in forming deep said holes so as to prevent organic materials from being filled therein. 
         [0006]    To increase the amount of solar energy taken in, that is, light absorption efficiency, it is necessary to use a metal plated film as a reflection layer and use a periodic grating so as to increase the rate of utilization of incident light by organic materials. However, in a laboratory setting, a metal membrane functioning as a reflection layer has a much lower rate of utilization of incident light than metallic nanoparticles functioning as a rough electrode surface. Also, there are plenty of restrictions on a periodic grating; for example, incident light requires a specific incident angle or polarization direction, otherwise absorption of light energy is rarely efficient. 
         [0007]    Hence, it is imperative to solve the problems facing the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    In light of the aforesaid drawbacks of the prior art, it is an objective of the present invention to provide a solar battery unit, comprising: a first electrode; a nano rough layer disposed on the first electrode for absorbing and recycling solar energy; a semiconductor active layer disposed on the nano rough layer; and a second electrode disposed on the semiconductor active layer. 
         [0009]    Regarding the solar battery unit, a material of which at least one of the first and second electrodes is made is a transparent material, and a material of which the other electrode is made is a metallic material. The first electrode has a convoluted surface on which the nano rough layer is disposed. 
         [0010]    Regarding the solar battery unit, the nano rough layer comprises a plurality of metallic nanoparticles stacked up, and the metallic nanoparticles is of a dimension ranging between 10 nm and 800 nm. 
         [0011]    Regarding the solar battery unit, the nano rough layer comprises a metal membrane and a plurality of metallic nanoparticles disposed on the first electrode and covered with the metal membrane. The metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0012]    Regarding the solar battery unit, the first electrode and the second electrode are connected to an external circuit. Once sunlight falls on the first electrode or the second electrode to thereby enter the solar battery unit, the solar battery unit will convert absorbed solar energy into electrical energy. The electrical energy thus generated is available for use by the external circuit. The semiconductor active layer is made of an organic or inorganic material. 
         [0013]    The solar battery unit further comprises an electron or hole transport layer disposed between the nano rough layer and the semiconductor active layer or between the semiconductor active layer and the second electrode. The electron or hole transport layer is made of an organic or inorganic material. The solar battery unit further comprises an optical modulation layer disposed between the nano rough layer and the electron or hole transport layer. 
         [0014]    The solar battery unit further comprises an electron or hole barrier layer disposed between the nano rough layer and the semiconductor active layer or between the semiconductor active layer and the second electrode. The solar battery unit further comprises an optical modulation layer disposed between the nano rough layer and the electron or hole barrier layer. 
         [0015]    The present invention further discloses a solar battery unit, comprising: a substrate; a nano rough structure disposed on the substrate for absorbing and recycling solar energy; a first electrode disposed on the nano rough structure; a semiconductor active layer disposed on the first electrode; and a second electrode disposed on the semiconductor active layer. 
         [0016]    Regarding the solar battery unit, the first electrode is made of an elemental metal or an alloy, and the second electrode is made of a transparent material. 
         [0017]    The nano rough structure is a convoluted structure formed on the substrate. The difference between the highest peak and the lowest trough of the convoluted structure ranges between 3 nm and 500 nm. The difference in height between a peak and a trough adjacent thereto of the convoluted structure ranges between 1 nm and 500 nm. Alternatively, the nano rough structure comprises a plurality of metallic nanoparticles stacked up, and the metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0018]    Regarding the structure, sunlight falls on the second electrode to thereby enter the solar battery unit whereby absorbed solar energy is converted into electrical energy for use by an external circuit connecting the first electrode and the second electrode. 
         [0019]    Regarding the solar battery unit, the first electrode is made of a transparent material, and the second electrode is made of an elemental metal or an alloy. 
         [0020]    Regarding the structure, the nano rough structure comprises a plurality of metallic nanoparticles stacked up, and a metal membrane is disposed between the first electrode and the semiconductor active layer. The metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0021]    Regarding the structure, sunlight falls on the first electrode to thereby enter the solar battery unit whereby absorbed solar energy is converted into electrical energy for use by an external circuit connecting the first electrode and the second electrode. 
         [0022]    Regarding the solar battery unit, the semiconductor active layer is made of an organic or inorganic material. 
         [0023]    The solar battery unit further comprises an electron or hole transport layer disposed between the first electrode and the semiconductor active layer or between the semiconductor active layer and the second electrode. The electron or hole transport layer is made of an organic or inorganic material. 
         [0024]    The solar battery unit further comprises an electron or hole barrier layer disposed between the first electrode and the semiconductor active layer or between the semiconductor active layer and the second electrode. 
         [0025]    The solar battery unit further comprises an optical modulation layer disposed between the first electrode and the electron or hole transport layer. 
         [0026]    The present invention further discloses a method for fabricating a solar battery unit, comprising the steps of: a method for fabricating a solar battery unit, comprising the steps of: providing a first electrode; forming a nano rough layer on the first electrode; forming a semiconductor active layer on the nano rough layer; and forming a second electrode on the semiconductor active layer. Sunlight falls on the first electrode or the second electrode to thereby enter the solar battery unit whereby absorbed solar energy is converted into electrical energy for use by an external circuit connecting the first electrode and the second electrode. 
         [0027]    Regarding the method, one of the first and second electrodes is made of a transparent material, and the other one of the first and second electrodes is made of a metallic material. The surface of the first electrode is a convoluted surface on which the nano rough layer is disposed. 
         [0028]    Regarding the method, the nano rough layer comprises a plurality of metallic nanoparticles stacked up, and the metallic nanoparticles are of a dimension ranging between 10 nm and 800 nm. 
         [0029]    Regarding the method, the nano rough layer comprises a metal membrane and a plurality of metallic nanoparticles disposed on the first electrode and covered with the metal membrane. The metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0030]    The method further comprises connecting the first electrode and the second electrode to an external circuit such that sunlight falls on the first electrode or the second electrode to thereby enter the solar battery unit whereby absorbed solar energy is converted into electrical energy for use by the external circuit, wherein the semiconductor active layer is made of an organic or inorganic material. 
         [0031]    The method further comprises forming an electron or hole transport layer between the nano rough layer and the semiconductor active layer or between the semiconductor active layer and the second electrode, wherein the electron or hole transport layer is made of an organic or inorganic material. The method further comprises forming an optical modulation layer between the nano rough layer and the electron or hole transport layer. 
         [0032]    The method further comprises forming an electron or hole barrier layer between the nano rough layer and the semiconductor active layer or between the semiconductor active layer and the second electrode. The method further comprises forming an optical modulation layer between the nano rough layer and the electron or hole barrier layer. 
         [0033]    The present invention further discloses a method for fabricating a solar battery unit, comprising the steps of: a method for fabricating a solar battery unit, comprising the steps of: providing a substrate; forming a nano rough structure on the substrate; forming a first electrode on the nano rough structure to cover the nano rough structure; forming a semiconductor active layer on the first electrode; and forming a second electrode on the semiconductor active layer. Sunlight falls on the second electrode to thereby enter the solar battery unit whereby absorbed solar energy is converted into electrical energy for use by an external circuit connecting the first electrode and the second electrode. 
         [0034]    Regarding the method, the first electrode is made of an elemental metal or an alloy, and the second electrode is made of a transparent material. 
         [0035]    Regarding the method, the nano rough structure is a convoluted structure formed on the substrate, and the convoluted structure is formed by a patterning process performed by a chemical or physical means. The difference between a highest peak and a lowest trough of the convoluted structure ranges between 3 nm and 500 nm. The difference in height between a peak and a trough adjacent thereto of the convoluted structure ranges between 1 nm and 500 nm. The nano rough structure comprises a plurality of metallic nanoparticles stacked up, and the metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0036]    Regarding the method, the first electrode and the second electrode are connected to an external circuit. Once sunlight falls on the second electrode, the external circuit can use electrical energy generated by the solar battery unit through conversion of energy. 
         [0037]    Regarding the method, the first electrode is made of a transparent material, and the second electrode is made of an elemental metal or an alloy. 
         [0038]    Regarding the method, the nano rough structure comprises a plurality of metallic nanoparticles stacked up, and a metal membrane is formed between the first electrode and the semiconductor active layer. The metallic nanoparticles are of a dimension ranging between 1 nm and 500 nm. 
         [0039]    Regarding the method, the first electrode and the second electrode are connected to an external circuit. Once sunlight falls on the first electrode, the external circuit can use electrical energy generated by the solar battery unit through conversion of energy. 
         [0040]    Regarding the method, the semiconductor active layer is made of an organic or inorganic material. 
         [0041]    The method further comprises forming an electron or hole transport layer between the first electrode and the semiconductor active layer or between the semiconductor active layer and the second electrode. The electron or hole transport layer is made of an organic or inorganic material. 
         [0042]    The method further comprises forming an electron or hole barrier layer between the first electrode and the semiconductor active layer or between the semiconductor active layer and the second electrode. 
         [0043]    The method further comprises forming an optical modulation layer between the first electrode and the electron or hole transport layer. 
         [0044]    Hence, the present invention teaching forming a nano rough layer on electrodes randomly and by a variable means, or forming a nano rough structure randomly distributed across the substrate by a processing process performed by a variable means, so as to maximize utilization of residual solar energy left behind after absorption of solar energy by the semiconductor active layer and feed back the energy to the semiconductor active layer with a view to optimizing the recycling of solar energy and absorption of solar energy. 
         [0045]    Where the solar battery unit is made of an inorganic semiconductor material, the semiconductor active layer of a lesser thickness can work efficiently, because solar energy is effectively recycled in the presence of the rough surfaces of randomly distributed nanoparticles. Also, a desirable thickness of the semiconductor active layer can be controllably attained because of the electron or hole transport layer selectively formed between the nano rough layer and the semiconductor active layer. 
         [0046]    In addition, the nano rough layer/structure is conducive to the increase in the contact surface between electrodes and a semiconductor material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIGS. 1A through 1D  are schematic cross-sectional views of a method for fabricating a solar battery unit in a first embodiment according to the present invention; 
           [0048]      FIGS. 2A through 2D  are schematic cross-sectional views of the method for fabricating a solar battery unit in a second embodiment according to the present invention; 
           [0049]      FIGS. 3A through 3D  are schematic cross-sectional views of the method for fabricating a solar battery unit in a third embodiment according to the present invention; FIG.  3 D′ is a cross-sectional view of another embodiment of the method illustrated with  FIG. 3D ; 
           [0050]      FIGS. 4A through 4D  are schematic cross-sectional views of the method for fabricating a solar battery unit in a fourth embodiment according to the present invention; 
           [0051]      FIGS. 5A through 5D  are schematic cross-sectional views of the method for fabricating a solar battery unit in a fifth embodiment according to the present invention; and 
           [0052]      FIGS. 6A through 6D  are schematic cross-sectional views of the method for fabricating a solar battery unit in a sixth embodiment according to the present invention; FIG.  6 D′ is a cross-sectional view of another embodiment of the method illustrated with  FIG. 6D . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0053]    The present invention is herein illustrated with specific embodiments, so that one skilled in the pertinent art can easily understand other advantages and effects of the present invention from the disclosure of the invention. 
       First Embodiment 
       [0054]    Referring to  FIGS. 1A through 1D , there are shown schematic cross-sectional views of a method for fabricating a solar battery unit  1  in a first embodiment according to the present invention. 
         [0055]    Referring to  FIG. 1A , a first electrode  11  is provided, and the first electrode  11  is disposed on a substrate  10 . The material of which the substrate  10  is made is a transparent material, paper, glass, a polymeric material, or a metallic material. 
         [0056]    In this embodiment, the first electrode  11  is formed by applying a metallic material to the substrate  10 , using sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction. The metallic material is Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. In this embodiment, the material of which the first electrode  11  is made can also be a non-metallic material. 
         [0057]    In this embodiment, the first electrode  11  has a convoluted surface  11   a . The convoluted surface  11   a  is formed by creating randomly distributed nano-convolution on the surface of the first electrode  11  according to different fabrication parameters, or by creating a randomly distributed nanoscale convoluted rough surface on the first electrode  11  by means of dry-etching after plating. The extent of convolution of the first electrode  11  is adjustably set to between 1 nm and 500 nm according to different fabrication parameters. 
         [0058]    Referring to  FIG. 1B , a nano rough layer  12  is formed on the convoluted surface  11   a  of the first electrode  11 . In this embodiment, the nano rough layer  12  comprises a plurality of metallic nanoparticles  120  stacked up. 
         [0059]    The plurality of metallic nanoparticles  120  is stacked up by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction (such as Tollens&#39; test, also known as silver-mirror test) and thereby randomly distributed across the first electrode  11 . The metallic nanoparticles  120  is made of Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. The dimensions of the metallic nanoparticles  120  are controllably set to between 10 nm and 800 nm by adjustment and processing according to different fabrication parameters, so as to alter the absorption wavelength of the metallic nanoparticles  120 . The thickness of the nano rough layer  12  is subject to changes as needed, so as to enhance the performance thereof. 
         [0060]    The nano rough layer  12  of the present invention is effective in increasing particle dimensions and variety, enhancing surface roughness, and enhancing utilization of spectral energy. 
         [0061]    Referring to  FIG. 1C , a semiconductor active layer  13  is formed on the nano rough layer  12 , and the semiconductor active layer  13  is formed from an organic or inorganic material. The semiconductor active layer  13  and the metallic nanoparticles  120  are equal in absorption wavelength. 
         [0062]    In an embodiment of the present invention, an electron or hole transport layer  14   a  is selectively formed between the nano rough layer  12  and the semiconductor active layer  13  so as to enhance performance thereof. Alternatively, an electron or hole transport layer  14   b  is selectively formed on the semiconductor active layer  13  to allow an electrode layer to be subsequently disposed thereon, so as to enhance performance thereof. In this embodiment, two said electron or hole transport layers  14   a ,  14   b  are disposed in the solar battery unit  1 . 
         [0063]    Referring to  FIG. 1D , a second electrode  15  is formed on the electron or hole transport layer  14   b  above the semiconductor active layer  13 . The material of which the second electrode  15  is made is a transparent material. The first electrode  11  and the second electrode  15  are connected to an external circuit  3 . Once sunlight falls on the second electrode  15  to thereby enter the solar battery unit  1 , the solar battery unit  1  will convert absorbed solar energy into electrical energy. The external circuit  3  is configured to use the electrical energy thus generated. 
         [0064]    In this embodiment, the material of which the first electrode  11  is made is a transparent material, and both the first electrode  11  and the second electrode  15  are made of a transparent material. 
         [0065]    The electron or hole transport layers  14   a ,  14   b  of the solar battery unit  1  are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form an optical modulation layer  16  (an optical spacer layer) between the nano rough layer  12  and the electron or hole transport layer  14   a  (or an electron or hole barrier layer), so as to enhance utilization of spectral energy, as shown in  FIG. 1D . 
       Second Embodiment 
       [0066]    Referring to  FIGS. 2A through 2D , there are shown schematic cross-sectional views of the method for fabricating a solar battery unit  1 ′ in a second embodiment according to the present invention. The difference between the second embodiment and the first embodiment is that, in the second embodiment, the first electrode  11 ′ and the second electrode  15 ′ are made of different material. 
         [0067]    Referring to  FIG. 2A , the first electrode  11 ′ is provided, and the first electrode  11 ′ is disposed on the substrate  10 . In this embodiment, the material of which the first electrode  11 ′ is made is a transparent material, and thus the substrate  10  is also made of a transparent material. The first electrode  11 ′ has a flat surface. 
         [0068]    Referring to  FIG. 2B , the nano rough layer  12  is formed on the first electrode  11 ′, and the nano rough layer  12  comprises a plurality of metallic nanoparticles  120  stacked up. The plurality of metallic nanoparticles  120  is stacked up by spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction (such as Tollens&#39; test, also known as silver-minor test). As a result, the metallic nanoparticles  120  are randomly distributed across the first electrode  11 ′. The dimensions of the metallic nanoparticles  120  are controlled by adjustment and processing according to different fabrication parameters. 
         [0069]    Referring to  FIG. 2C , the electron or hole transport layer  14   a , the semiconductor active layer  13 , and the electron or hole transport layer  14   b  are formed on the nano rough layer  12  in a bottom-to-top order. 
         [0070]    Referring to  FIG. 2D , the second electrode  15 ′ is formed on the electron or hole transport layer  14   b  above the semiconductor active layer  13 . The material from the second electrode  15 ′ is made is a metallic material. The first electrode  11 ′ and the second electrode  15 ′ are connected to the external circuit  3 . Once sunlight falls on the first electrode  11 ′ to thereby enter the solar battery unit  1 ′, the solar battery unit  1 ′ will convert absorbed solar energy into electrical energy. The external circuit  3  is configured to use the electrical energy thus generated. 
         [0071]    In this embodiment, the material of which the second electrode  15 ′ is made is a non-metallic material. Likewise, the material of which the second electrode  15 ′ is made is a transparent material, and thus both the first electrode  11 ′ and the second electrode  15 ′ are made of a transparent material. 
         [0072]    The electron or hole transport layers  14   a ,  14   b  of the solar battery unit  1 ′ are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form an optical modulation layer  16  between the nano rough layer  12  and the electron or hole transport layer  14   a  (or an electron or hole barrier layer), so as to enhance utilization of spectral energy, as shown in  FIG. 2D . 
       Third Embodiment 
       [0073]    Referring to  FIGS. 3A through 3D , there are shown schematic cross-sectional views of the method for fabricating a solar battery unit  1 ″ in a third embodiment according to the present invention. The difference between the third embodiment and the second embodiment is that, in the third embodiment, a nano rough layer  12 ′ takes on a new structure. 
         [0074]    Referring to  FIG. 3A , the first electrode  11 ′ is provided, and the first electrode  11 ′ is disposed on the substrate  10 . The material of which the first electrode  11 ′ and the substrate  10  are made is a transparent material. 
         [0075]    Referring to  FIG. 3B , the nano rough layer  12 ′ is formed on the first electrode  11 ′, and the nano rough layer  12 ′ comprises a metal membrane  121  and a plurality of metallic nanoparticles  120 ′ disposed on the first electrode  11 ′ and covered with the metal membrane  121 . 
         [0076]    There is no limitation on the material of which the metallic nanoparticles  120 ′ are made, though the material is preferably a transparent material. The metallic nanoparticles  120 ′ are formed on the first electrode  11 ′ by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction. As a result, the metallic nanoparticles  120  are randomly distributed across the first electrode  11 ′. The dimensions of the metallic nanoparticles  120 ′ are controllably set to between 1 and 500 nm by adjustment and processing according to different fabrication parameters, so as to alter the absorption wavelength of the metallic nanoparticles  120 ′. 
         [0077]    The metal membrane  121  is made of Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. The metallic nanoparticles  120 ′ are covered with the metal membrane  121  by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction. 
         [0078]    The nano rough layer  12 ′ of the present invention is effective in increasing particle dimensions and variety, enhancing surface roughness, and augmenting utilization of spectral energy. 
         [0079]    Referring to  FIG. 3C , the electron or hole transport layer  14   a , the semiconductor active layer  13 , and the electron or hole transport layer  14   b  are formed on the nano rough layer  12 ′ in a bottom-to-top order. The semiconductor active layer  13  and the metallic nanoparticles  120 ′ are equal in absorption wavelength. 
         [0080]    Referring to  FIG. 3D , the second electrode  15 ′ is formed on the electron or hole transport layer  14   b  above the semiconductor active layer  13 . The material of which the second electrode  15 ′ is made is a metallic material. The first electrode  11 ′ and the second electrode  15 ′ are connected to the external circuit  3 . Once sunlight falls on the first electrode  11 ′ to thereby enter the solar battery unit  1 ″, the solar battery unit  1 ″ will convert absorbed solar energy into electrical energy. The external circuit  3  is configured to use the electrical energy thus generated. 
         [0081]    In this embodiment, the electron or hole transport layers  14   a ,  14   b  of the solar battery unit  1 ″ are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form the optical modulation layer  16  between the nano rough layer  12 ′ and the electron or hole transport layer  14   a  (or an electron or hole barrier layer), so as to enhance utilization of spectral energy, as shown in FIG.  3 D′. 
         [0082]    In the aforesaid three embodiments, the present invention further provides the solar battery unit  1 ,  1 ′,  1 ″ comprising: the first electrodes  11 ,  11 ′; the nano rough layers  12 ,  12 ′ formed on the first electrodes  11 ,  11 ′; the semiconductor active layer  13  formed above the nano rough layers  12 ,  12 ′; and the second electrodes  15 ,  15 ′ formed above the semiconductor active layer  13 . 
         [0083]    One of the first and second electrodes  11 ,  11 ′,  15 ,  15 ′ is made of a transparent material. The other one of the first and second electrodes  11 ,  11 ′,  15 ,  15 ′ is made of a metallic material. The first electrodes  11 ,  11 ′ and the second electrodes  15 ,  15 ′ are connected to an external circuit. Once sunlight falls on the first electrode  11 ′ or the second electrode  15 , absorbed solar energy will be converted into electrical energy. The external circuit is configured to use the electrical energy thus generated. The first electrodes  11 ,  11 ′ have the convoluted surface  11   a  on which the nano rough layers  12 ,  12 ′ are disposed. 
         [0084]    The material of which the semiconductor active layer  13  is made is an organic or inorganic material. The nano rough layer  12  comprises a plurality of metallic nanoparticles  120  stacked up. The metallic nanoparticles  120  are of a dimension ranging between 10 nm and 800 nm. Alternatively, the nano rough layer  12 ′ comprises the metal membrane  121  and the plurality of metallic nanoparticles  120 ′ disposed on the first electrode  11 ′ and covered with the metal membrane  121 . The metallic nanoparticles  120 ′ are of a dimension ranging between 1 nm and 500 nm. 
         [0085]    The solar battery unit  1 ,  1 ′,  1 ″ further comprises the electron or hole transport layers  14   a ,  14   b  disposed between the nano rough layers  12 ,  12 ′ and the semiconductor active layer  13  and between the semiconductor active layer  13  and the second electrodes  15 ,  15 ′, respectively. The material of which the electron or hole transport layers  14   a ,  14   b  are made is an organic or inorganic material. 
       Fourth Embodiment 
       [0086]    Referring to  FIGS. 4A through 4D , there are shown schematic cross-sectional views of the method for fabricating a solar battery unit  2  in a fourth embodiment according to the present invention. 
         [0087]    Referring to  FIG. 4A , a substrate  20  is prepared, and a nano rough structure  22  is disposed on the substrate  20 . The material of which the substrate  20  is made is paper, glass, a polymeric material, or a metallic material. In this embodiment, the nano rough structure  22  is a convoluted structure formed on the substrate  20 . 
         [0088]    The nano rough structure  22  is formed by a patterning process performed by a chemical or physical means, such as molding, dry-etching, wet-etching, mechanical polishing, photolithography, scanning-beam lithography, or printing, so as to form on the substrate  20  a convoluted structure characterized by randomly distributed nanoscale roughness. The maximum peak-to-trough height h of the convoluted structure ranges between 3 nm and 500 nm, which is the difference between the highest peak and the lowest trough on the substrate  20 . The reference surface L shown in  FIG. 4A  is the original surface of the substrate  20 . The contiguous peak-to-trough height s of the convoluted structure ranges between 1 nm and 500 nm, which is the difference in height between a peak and a trough adjacent thereto on the substrate  20 . 
         [0089]    The nano rough structure  22  of the present invention is effective in increasing particle dimensions and variety, enhancing surface roughness, and augmenting utilization of spectral energy. 
         [0090]    Referring to  FIG. 4B , after the substrate  20  is rinsed and dried, a first electrode  21  is formed on the nano rough structure  22  to thereby cover the nano rough structure  22 . The first electrode  21  is formed by applying an elemental metal or an alloy to the substrate  20  by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction. The first electrode  21  is made of Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. 
         [0091]    Randomly distributed nano-convolution is formed on the surface of the first electrode  21  according to different fabrication parameters. Alternatively, a randomly distributed nanoscale convoluted rough surface is formed on the first electrode  21  by means of dry-etching after plating. The extent of convolution of the first electrode  21  is adjustably set to between 1 nm and 500 nm according to different fabrication parameters. 
         [0092]    The thickness of the first electrode  21  is subject to changes as needed. 
         [0093]    Referring to  FIG. 4C , a semiconductor active layer  23  is formed on the first electrode  21 , and the material of which the semiconductor active layer  23  is made is an organic or inorganic material. 
         [0094]    In an embodiment of the present invention, an electron or hole transport layer  24   a  is selectively formed between the first electrode  21  and the semiconductor active layer  23  so as to enhance performance thereof. Alternatively, an electron or hole transport layer  24   b  is selectively formed on the semiconductor active layer  23  to allow an electrode layer to be subsequently disposed thereon, so as to enhance performance thereof. In this embodiment, two said electron or hole transport layers  24   a ,  24   b  are disposed in the solar battery unit  2 . 
         [0095]    Referring to  FIG. 4D , a second electrode  25  is formed on the electron or hole transport layer  24   b  above the semiconductor active layer  23 . The material of which the second electrode  25  is made is a transparent material. The first electrode  21  and the second electrode  25  are connected to the external circuit  3 . Once sunlight falls on the second electrode  25  to thereby enter the solar battery unit  2 , the solar battery unit  2  will convert absorbed solar energy into electrical energy. The external circuit  3  is configured to use the electrical energy thus generated. 
         [0096]    In this embodiment, the electron or hole transport layers  24   a ,  24   b  of the solar battery unit  2  are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form an optical modulation layer  26  between the first electrode  21  and the electron or hole transport layer  24   a  (or an electron or hole barrier layer), so as to enhance utilization of spectral energy, as shown in  FIG. 4D . 
       Fifth Embodiment 
       [0097]    Referring to  FIGS. 5A through 5D , there are shown schematic cross-sectional views of the method for fabricating a solar battery unit  2 ′ in a fifth embodiment according to the present invention. The difference between the fifth embodiment and the fourth embodiment is that the nano rough structure  22 ′ in the fifth embodiment assumes a new structure. 
         [0098]    Referring to  FIG. 5A , the substrate  20  is provided, and the nano rough structure  22 ′ is formed on the substrate  20 . In this embodiment, the nano rough structure  22 ′ comprises a plurality of metallic nanoparticles  220  stacked up. 
         [0099]    There is no limitation upon the material of which the metallic nanoparticles  220  are made, though the material is preferably a transparent material. The metallic nanoparticles  220  are stacked up by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction, before being randomly distributed across the substrate  20 . The metallic nanoparticles  220  are processed or adjusted by different fabrication parameters so as for the dimensions of the metallic nanoparticles  220  to range between 1 nm and 500 nm with a view to varying the absorption wavelength of the metallic nanoparticles  220 . 
         [0100]    The nano rough structure  22 ′ of the present invention is effective in increasing particle dimensions and variety, enhancing surface roughness, and augmenting utilization of spectral energy. 
         [0101]    Referring to  FIG. 5B , the first electrode  21  is formed on the nano rough structure  22 ′ to thereby cover the nano rough structure  22 ′. 
         [0102]    Referring to  FIG. 5C , the electron or hole transport layer  24   a , the semiconductor active layer  23 , and the electron or hole transport layer  24   b  are formed on the first electrode  21  in a bottom-to-top order. 
         [0103]    Referring to  FIG. 5D , the second electrode  25  is formed on the electron or hole transport layer  24   b  above the semiconductor active layer  23 , and both the first electrode  21  and the second electrode  25  are connected to the external circuit  3 . Once sunlight falls on the second electrode  25  to thereby enter the solar battery unit  2 ′, the solar battery unit  2 ′ will convert absorbed solar energy into electrical energy. The electrical energy thus generated is available for use by the external circuit  3 . 
         [0104]    In this embodiment, the electron or hole transport layers  24   a ,  24   b  of the solar battery unit  2 ′ are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form the optical modulation layer  26  between the first electrode  21  and the electron or hole transport layer  24   a  (or an electron or hole barrier layer), so as to enhance utilization of spectral energy, as shown in  FIG. 5D . 
       Sixth Embodiment 
       [0105]    Referring to  FIGS. 6A through 6D , there are shown schematic cross-sectional views of the method for fabricating a solar battery unit  2 ″ in a sixth embodiment according to the present invention. The difference between the sixth embodiment and the fifth embodiment lies in the material of which the first and second electrodes  21 ′,  25 ′ are made and a metal membrane  221  in the sixth embodiment. 
         [0106]    Referring to  FIG. 6A , the substrate  20  is provided, a nano rough structure  22 ′ is formed on the substrate  20 . The nano rough structure  22 ′ comprises a plurality of metallic nanoparticles  220  stacked up. 
         [0107]    The nano rough structure  22 ′ of the present invention is effective in increasing particle dimensions and variety, enhancing surface roughness, and augmenting utilization of spectral energy. 
         [0108]    Referring to  FIG. 6B , a first electrode  21 ′ is formed on the nano rough structure  22 ′ to cover the nano rough structure  22 ′, and then the metal membrane  221  is formed on the first electrode  21 ′. The material of which the first electrode  21 ′ is made is a transparent material. 
         [0109]    The metal membrane  221  is made of Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. The metallic nanoparticles  120 ′ are covered with the metal membrane  221  by sputtering, evaporation, spin coating, immersion, spraying, drying after dripping, organic metal chemical vapor deposition (MOCVD), electroplating, or a chemical reaction. 
         [0110]    Referring to  FIG. 6C , the electron or hole transport layer  24   a , the semiconductor active layer  23 , and the electron or hole transport layer  24   b  are formed on the metal membrane  221  in a bottom-to-top order. 
         [0111]    Referring to  FIG. 6D , the second electrode  25 ′ is formed on the electron or hole transport layer  24   b  above the semiconductor active layer  23 . The material of which the second electrode  25 ′ is made is an elemental metal or an alloy. The elemental metal is Al, Au, Cu, Ag, Cr, Pt, Co, Ni, or Ti. The first electrode  21 ′ and the second electrode  25 ′ are connected to the external circuit  3 . Once sunlight falls on the substrate  20  and the first electrode  21 ′ to thereby enter the solar battery unit  2 ″, the solar battery unit  2 ″ will convert absorbed solar energy into electrical energy. The external circuit  3  can use the electrical energy thus generated. 
         [0112]    In this embodiment, the electron or hole transport layers  24   a ,  24   b  of the solar battery unit  2 ″ are replaceable by an electron or hole barrier layer (not shown). Also, it is feasible to form the optical modulation layer  26  between the first electrode  21 ′ and the metal membrane  221 , so as to enhance utilization of spectral energy, as shown in FIG.  6 D′. 
         [0113]    In the aforesaid three embodiments, the present invention further provides the solar battery unit  2 ,  2 ′,  2 ″ comprising: the substrate  20 ; the nano rough structure  22 ,  22 ′ formed on the substrate  20 ; the first electrode  21 ,  21 ′ formed on the nano rough structures  22 ,  22 ′; the semiconductor active layer  23  formed on the first electrodes  21 ,  21 ′; and the second electrodes  25 ,  25 ′ formed on the semiconductor active layer  23 . 
         [0114]    If the first electrode  21  is made of an elemental metal or an alloy, the second electrode  25  will be made of a transparent material. The nano rough structure  22  is a convoluted structure formed on the substrate  20 . The maximum peak-to-trough height h of the convoluted structure ranges between 3 nm and 500 nm. The contiguous peak-to-trough height s of the convoluted structure ranges between 1 nm and 500 nm. Alternatively, the nano rough structure  22 ′ comprises a plurality of metallic nanoparticles  220  stacked up, and the metallic nanoparticles  220  are of a dimension ranging between 1 nm and 500 nm. The first electrode  21  and the second electrode  25  are connected to the external circuit  3 . Once sunlight falls on the second electrode  25 , the external circuit  3  will use electrical energy generated. 
         [0115]    If the first electrode  21 ′ is made of a transparent material, the second electrode  25 ′ will be made of an elemental metal or an alloy. The nano rough structure  22 ′ comprises a plurality of metallic nanoparticles  220  stacked up. The metal membrane  221  is formed between the first electrode  21 ′ and the semiconductor active layer  23 . The metallic nanoparticles  220  are of a dimension ranging between 1 nm and 500 nm. The first electrode  21 ′ and the second electrode  25 ′ are connected to the external circuit  3 . Once sunlight falls on the substrate  20  and the first electrode  21 ′, the external circuit  3  will use electrical energy generated. 
         [0116]    The material of which the semiconductor active layer  23  is made is an organic or inorganic material. The solar battery unit  2 ,  2 ′,  2 ″ further comprises the electron or hole transport layers  24   a ,  24   b  disposed between the first electrode  21 ,  21 ′ and the semiconductor active layer  23  and between the semiconductor active layer  23  and the second electrodes  25 ,  25 ′. The electron or hole transport layers  24   a ,  24   b  are made of an organic or inorganic material. 
         [0117]    In conclusion, the present invention teaches forming a nano rough layer on electrodes randomly or forming a randomly distributed nano rough structure by processing the substrate using a variable means, so as to maximize utilization of residual solar energy left behind after absorption of solar energy by the semiconductor active layer and then feed back the energy to the semiconductor active layer with a view to optimizing the recycling of solar energy and absorption of solar energy. 
         [0118]    Where the solar battery unit is made of an inorganic semiconductor material, the semiconductor active layer of a lesser thickness can work efficiently, because solar energy is effectively recycled in the presence of the rough surfaces of randomly distributed nanoparticles. Also, a desirable thickness of the semiconductor active layer can be controllably attained because of the electron or hole transport layer selectively formed between the nano rough layer and the semiconductor active layer. 
         [0119]    In addition, the nano rough layer/structure is conducive to the increase in the contact surface between electrodes and a semiconductor material. 
         [0120]    The foregoing descriptions of the detailed embodiments are provided to illustrate and disclose the features and functions of the present invention and are not intended to be restrictive of the scope of the present invention. It should be understood by those in the art that many modifications and variations can be made according to the spirit and principle in the disclosure of the present invention and still fall within the scope of the invention as set forth in the appended claims.