Abstract:
Novel structures of photovoltaic cells (also known as solar cells) are provided. The Cells are based on the nanometer-scaled wire, tubes, and/or rods, which are made of the electronics materials covering semiconductors, insulator or metallic in structure. These photovoltaic cells have large power generation capability per unit physical area over the conventional cells. These cells can have also high radiation tolerant capability. These cells will have enormous applications such as in space, in commercial, residential and industrial applications.

Description:
PARENT CASE TEXT 
       [0001]    This is a divisional of application(s) Ser. No. 11/161,840 filed on Aug. 18, 2005. 
     
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
       [0002]    This application claims the benefits of U.S. Provisional Application No. 60/522,134 filed Aug. 19, 2004. 
       FIELD OF INVENTIONS 
       [0003]    This patent specification relates to structures of photovoltaic cells (also solar cells). More specifically, it relates to structures of photovoltaic cells comprising numerous nanometer-scale wires, rods and/or tubes to have large power generation capability per unit area. The photovoltaic cells have also highly radiant tolerant, necessary for space applications. These photovoltaic cells can be used in commercial, residential, and also industrial application for power generation. 
       BACKGROUND OF THE INVENTIONS 
       [0004]    Photovoltaic cells where light is converted into electric power to be fed to external loads electrically connected to the photovoltaic cells have been prevailing in a wide range of application fields such as consumer electronics, industrial electronics and space exploration. In consumer electronics, photovoltaic cells that consist of materials such as amorphous silicon are choices for a variety of inexpensive and low power applications. Typical conversion efficiency, i.e. the solar cell conversion efficiency, of amorphous silicon based photovoltaic cells ranges between ˜10% [Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, Okamoto T, Nakajima A.  Thin film poly - Si solar cell on glass substrate fabricated at low temperature . Presented at MRS Spring Meeting, San Francisco, April 1998.]. Although the fabrication processes of amorphous silicon based photovoltaic cells are rather simple and inexpensive, one notable downside of this type of cell is its vulnerability to defect-induced degradation that decreases its conversion efficiency. 
         [0005]    In contrast, for more demanding applications such as industrial solar power generation systems, either poly-crystalline or single-crystalline silicon is the choice because of more stringent requirements for better reliability and higher efficiency than the applications in consumer electronics. Photovoltaic cells consisting of poly-crystalline and single-crystalline silicon generally offer the conversion efficiency ranging ˜20% and ˜25% [Zhao J, Wang A, Green M, Ferrazza F.  Novel  19.8%  efficient ‘honeycomb’ textured multicrystalline and  24.4%  monocrystalline silicon solar cell. Applied Physics Letters  1998; 73: 1991-1993.] respectively. As many concerns associated with a steep increase in the amount of the worldwide energy consumption are raised, further development in industrial solar power generation systems has been recognized as a main focus. 
         [0006]    Group II-VI compound semiconductors, for example CdTe and CdS, have been investigated in the context of having industrial solar power generation systems manufactured at a lower cost with maintaining a moderate conversion efficiency, resulted in a comparable conversion efficiency ˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A, Gessert T A, Asher S, Levi D H, Sheldon P. 16 — 5%- efficient CdS/CdTe polycrystalline thin - film solar cell. Proceedings of the  17 th European Photovoltaic Solar Energy Conference , Munich, 22-26 October 2001; 995-1000.] to those for the single crystalline silicon photovoltaic devises, however toxic natures of these materials are of great concerns for environment. 
         [0007]    Group I-III-VI compound semiconductors, such as CuInGaSe 2 , have been also extensively investigated for industrial solar power generation systems. This material can be synthesized potentially at a much lower cost than its counterpart, single crystalline silicon, however conversion efficiency, ˜19%, comparable to that of single crystalline silicon based cells can be obtained, so far, by only combining with the group II-VI compound semiconductor cells [Contreras M A, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R.  Progress toward  20%  efficiency in Cu ( In,Ga ) Se polycrystalline thin - film solar cell. Progress in Photovoltaics: Research and Applications  1999; 7: 311-316.], which again raise issues associated with toxic natures of these materials. 
         [0008]    A type of photovoltaic cells designed for several exclusive applications where the main focus is high conversion efficiency generally consists of group III-V semiconductors including GaInP and GaAs. Synthesis processes of single crystalline group III-V are in general very costly because of substantial complications involved in epitaxial growth of group III-V single crystalline compound semiconductors. Typical conversion efficiency of group III-V compound semiconductor based photovoltaic cells, as these types of photovoltaic cells are intended to be, can be as high as ˜34% when combined with germanium substrates, another very expensive material [King R R, Fetzer C M, Colter P C, Edmondson K M, Law D C, Stavrides A P, Yoon H, Kinsey G S, Cotal H L, Ermer J H, Sherif R A, Karam N H.  Lattice - matched and metamorphic GaInP/GaInAs/Ge concentrator solar cells. Proceedings of the World Conference on Photovoltaic Energy Conversion  ( WCPEC -3), Osaka, May 2003; to be published.]. 
         [0009]    All types of photovoltaic cells in the prior arts described above, no matter what materials a cell is made of, essentially falls into one specific type of structure as in  FIG. 1 . Shown in  FIG. 1  is a photovoltaic cell comprising a thick p-type semiconductor layer  101  and a thin n-type semiconductor layer  102  formed on an electrically conductive substrate  100 . A pn-junction  103  is formed at the interface between the p-type semiconductor layer  101  and the n-type semiconductor layer  102 . Incident light  104  entering the cell generate electron-hole pairs after being absorbed by the p- and also n-type semiconductor layers  101  and  102 . The incident light generates electrons  105   e  and also holes  105   h  in the region near the pn-junction  103  and also  106   e  and  106   h  in the region far from the pn-junction  103 . The photo generated electrons (and holes)  105   e  and  106   e  (hereafter considering only electronics, i.e. minority carriers in p-type semiconductors, and the same explanation is applicable for holes, minority carriers in n-type semiconductors, also) diffusing toward the pn-junction  103  and entering the pn-junction  103  contribute to photovoltaic effect. This is also vice versa for the holes, existing as minority carriers in n-type semiconductor  102 . The two key factors that substantially impact the conversion efficiency of this type of photovoltaic cell are photo carrier generation efficiency (PCGE) and photo carrier collection efficiency (PCCE). 
         [0010]    The PCGE is the percentage of the number of photons entering a cell and contributing to the generation of photo carriers, which needs to be, ideally, as close as ˜100%. On the other hand, the PCCE is the percentage of the number of photo-generated electrons  105   e  and  106   e  reaching the pn-junction  103  and contributing to the generation of photocurrent. For a monochromatic light, the PCGE of ˜100% can be achieved by simply making the p-type layer  101  thicker, however, electrons  106   e  generated at the region far away from the pn-junction  103  cannot be collected efficiently due to many adverse recombination processes that prevent photo generated carriers from diffusing into the pn-junction  103 , thus the basic structure of current photovoltaic cells has its own limitation on increasing the conversion efficiency. Both PCGE and PCCE are mainly dependent on material and structure of the photovoltaic cells, and today&#39;s photovoltaic cells are structured in such a way that (a) wide ranges of solar spectrum cannot be absorbed due to its material limitation, and (b) photo carrier&#39;s collection efficiency is lower due to its inherent structure. Besides, today&#39;s solar cell material is not highly radiation-tolerant. In space application specially, photovoltaic cells should have a structure and material systems, which could generate high-power per unit area and also to highly radiation tolerant. 
         [0011]    For both commercial and space applications, therefore, it would be desirable to have photovoltaic cell structures where both the PCGE and the PCCE can be increased simultaneously by having a photo absorption region that is thick enough to capture all the photons entering the cell and a pn-junction that is located at as close to the photo absorption region as possible. It would be further desirable to have, with maintaining ideal PCGE and PCCE, different materials having photo responses at different spectrum to efficiently cover a wide range of spectrum of light that enters a photovoltaic cell. It would be further desirable to have a large junction area within a given volume of a photovoltaic cell so that generated electric power that is proportional to the junction area can be maximized. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    Accordingly, it is an object of the invention to provide the structures of the photovoltaic cells, which could have the high power generation capability per unit area over conventional counterpart, mentioned as the prior arts. 
         [0013]    According to this invention, the photovoltaic cell can be made to highly radiation tolerant. 
         [0014]    Structures of photovoltaic cells comprising single or plurality of nanometer(s)-scale wires, rods or tubes consisting of various electronic materials are described. The surfaces of the nanometer(s)-scale wires, rods or tubes formed on a supporting substrate are connected to another electronic material or several different electronic materials, forming a large area of pn- or Schottky junctions on the surfaces of the nanometer(s)-scale wires, rods or tubes. The created large area pn- or Schottky junctions on the surface of the plurality of nanometer(s)-scale wires, rods or tubes form built-in potential by which photo generated electrons and holes are swept away, leading to photovoltaic effect. 
         [0015]    According to this invention, the nanometers-scaled wires, rods or tubes are made of electronic materials such as semiconductors, insulator or metals, or their combination, and they are formed on base substrates with geometries in which the axial direction of nanometers-scaled wires, rods or tubes are either in perpendicular or parallel with respect to the surface normal of the base substrates. According to this invention, the nanometer-scaled wires, rods or tubes could be made of any types, elementary or compound, of semiconductors such as Si, Ge, C, ZnO, BN, Al 2 O 3 , AlN, Si:Ge, CuInSe, II-VI and III-V, etc., or their combinations. The nanometer(s)-scaled tube could be made of semiconductor, insulator, or metallic type tubes such as carbon nano-tubes. 
         [0016]    It is also another object of this invention to provide the structures of the photovoltaic cells based on the carbon-nanotubes or semiconductor wires or rods which could provide more junction area per unit physical area, which results in increasing the power generation per unit area over conventional photovoltaic cells. 
         [0017]    This invention offers to generate power 100 times per unit area ad beyond over conventional photovoltaic cells. Also, the proposed photovoltaic cells are highly radiation tolerant, necessary in the space application. The main advantages of these inventions are that today&#39;s highly matured semiconductor process technologies can be used to fabricate the photovoltaic cell which has the power generation capability a few order and beyond as compared with that of conventional photovoltaic cell. 
         [0018]    Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be explained in more detail in conjunction with the appended drawings wherein: 
           [0020]      FIG. 1  is the schematic showing the cross-sectional view of a conventional photovoltaic cell structure. This is the explanatory diagram showing the prior-art of today&#39;s photovoltaic cell. 
           [0021]      FIG. 2  is the schematic showing the cross-sectional view of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, or rods, vertically arranged, in the first embodiment in accordance to the present invention. 
           [0022]      FIG. 3  is the schematic showing the cross-sectional view of a photovoltaic cell structure consisting of the nanometer(s)-scale tubes vertically arranged, in the second embodiment in accordance to the present invention. 
           [0023]      FIG. 4  is the schematic showing the cross-sectional view of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, rods or tubes and multi-layered semiconductors, bandgaps of which are relates to the different spectrum of the solar spectrum, in the third embodiment in accordance to the present invention. 
           [0024]      FIG. 5  is the schematic showing the cross-sectional view of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, rods or tubes, formed on pyramid or triangular shaped surface to achieve large junction area, in the fourth embodiment in accordance to the present invention. 
           [0025]      FIGS. 6A and 6B  are the schematic showing the side-view and cross-sectional views of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, rods or tubes, arranged horizontally, parallel to the substrate to achieve large junction area, in the fifth embodiment in accordance to the present invention. 
           [0026]      FIG. 7  is the schematic showing the cross-sectional view of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, rods or tubes formed on the semiconductor/insulator self-assembled dots or islands, arranged vertically to the substrate to achieve large junction area, in the sixth embodiment in accordance to the present invention. 
           [0027]      FIG. 8  is the schematic showing the side-view of a photovoltaic cell structure consisting of the nanometer(s)-scale wires, rods or tubes, in the seventh embodiment in accordance to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    According to a preferred embodiment illustrated in  FIG. 2 , shown is a photovoltaic cell comprising plurality of nanometer(s)-scale wires or rods  201  electrically connected to an electrode  200 . The nanometer(s)-scale wires or rods  201  can have metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The nanometer(s)-scale wires or rods are further surrounded by an electronic material  202  having metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The electronic material  202  is further electrically connected to an electrode  203 . The electrode  200  has direct electrical contact to neither the electrical material  202  nor the electrode  203 . As described above, the electrode  200  is intended to serve as a common electrode that connects all wires or rods  201 . The electrode  203  is provided for the electronic material  202 . The interface between the nanometer scale wires or rods  201  and the electronic material  202  form pn- or Schottky junctions where built-in potential for both electrons and holes is generated. 
         [0029]    According to this invention, alternatively the nanometer(s)-scale wires or rods  201  can be formed on separate substrate (not shown here), and the electrode  203  can be formed on the substrate to have common contact for each nanometer(s)-scale rods or tubes  201 , necessary for creating junction. In way of an example not way of limitation, the nanometer(s)-scale wires or rods  201  can be made of n-type semiconductor and the electric material  202  that surrounds the nanometer(s)-scale wires or rods  201  can be made of p-type semiconductor. Incident light  204  enters the photovoltaic cell through either the electrode  203  or the electrode  200  (In  FIG. 2 , the incident light enters the photovoltaic cell through the electrode  200 ). As the incident light  204  travels through the electronic material  202 , a numerous number of electrons  205  in the region near the electrode  200  and electrons  206  in the region far from the electrode  200  are generated. It should be pointed out that electrons are apparently generated all over the region along the thickness of the electric material  202 . In addition, as the incident light  204  travels through the nanometer(s)-scale wires or rods  201 , a numerous number of holes  207  in the region near the electrode  200  and holes  208  in the region far from the electrode  200  are generated. It also should be pointed out that holes are apparently generated all over the region along the thickness of the nanometer(s)-scale wires or rods  201 . Photo-generated electrons  205  and  206  in the electronic material  202  made of p-type semiconductor and photo-generated holes  207  and  208  in the nanometer(s)-scale wires or rods  201  made n-type semiconductor, then diffuse toward pn-junctions, created at the interface between the nanometer(s)-scale wires rods  201  and the electronic material  202 . At the pn-junctions, the electrons and the holes are swept away by built-in potential, thus photovoltaic effects set in. 
         [0030]    Apparent advantage of this invention over conventional photovoltaic cells is directly associated with the fact that, unlike conventional photovoltaic cells, pn-junctions are almost parallel to the direction to which incident light  204  travels, i.e., for all photo generated carriers in the electronic material  202 , no matter where they are generated, the distance the photo generated carriers have to diffuse to reach the pn-junctions is within the range of the distance between two nanometer(s)-scale wires or rods existing next to each other and independent to the location where they are generated. Furthermore, for all photo generated carriers in the nanometer(s)-scale wires or rods  201 , no matter where they are generated, the distance the photo generated carriers have to diffuse to reach pn-junctions is within the range of the diameter of the nanometer(s)-scale rods  201 . On the other hand, as explained in the description for the prior art shown in  FIG. 1 , in conventional photovoltaic cells where pn-junctions are perpendicular to the direction to which incident light travels, the photo generated carriers generated in region far away from pn-junctions need to diffuse much longer distance (diffusion-length) than that for the photo generated carriers generated near the pn-junctions, thus they have a greater chance to recombine without contributing to photovoltaic effects. Therefore in this invention, PCCE is expected to be much higher than that in conventional photovoltaic cells. In addition, it is evident that the total effective area that contributes to photovoltaic effect in this invention can be increased by 25 times and beyond for a given cell size with realistic assumptions on the dimension of the nanometer(s)-scale structures in the cell. 
         [0031]    In an alternative preferred embodiment shown in  FIG. 3 , a photovoltaic cell comprises plurality of nanometer(s)-scale tubes  301  are electrically connected to an electrode  300 . The nanometer(s)-scale tubes  301  can have metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The nanometer(s)-scale tubes are further surrounded by an electronic material  302  having metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The inside of the nanometer(s)-scale tubes  301  can be either empty or filled up with an electronic material  303  having metallic electrical conduction, p-type or n-type semiconductor electrical conduction. Two electronic materials  302  and  303  are further electrically connected to an electrode  304 . The electrode  304  has direct electrical contact to neither the electrode  300  nor the nanometer(s)-scale tubes  301 . The electrode  304  is intended to serve as a common electrode that connects all materials inside of the tubes  303  and outside of the tubes  302 . The interface between the nanometer(s)-scale tubes  301  and the electronic materials  302 / 303  form pn- or Schottky junctions, thus there are pn- or Schottky junctions on both sides, inside and outside, of the nanometer(s)-scale tubes  301 . 
         [0032]    According to this invention, alternatively the nanometer(s)-scale tubes  301  can be formed on the substrate (not shown here), and the electrode  304  can be made on the substrate to have a common contact for each nanometer(s)-scale rods or tubes  301 , necessary for creating junction. 
         [0033]    In way of an example not way of limitation, the nanometer(s)-scale tubes  301  can be made of metal and the electronic material  302  that surrounds the nanometer(s)-scale tubes  301  and the electronic material  303  that fills up the inside of the nanometer(s)-scale tubes  301  can be made of p-type semiconductor, thus a sandwich structure  302 / 301 / 303  forms two Schottky junctions on both sides of the metallic nanometer(s)-scale tubes  301 . Incident light  305  enters the photovoltaic cell through either the electrode  304  or the electrode  300  (In  FIG. 3 , the incident light enters the photovoltaic cell through the electrode  300 ). As the incident light  305  travels through the electronic material  302  and  303 , numerous numbers of electrons  306  and  307  (of electron-hole pairs) are generated. It should be pointed out that electrons (of electron-hole pairs) are apparently generated all over the region along the thickness of the nanometer(s)-scale tubes  301 . Photo-generated electrons in the electronic material  302  and the electric materials  303  made of p-type semiconductor, then diffuse toward Schottky junctions in the sandwich structure  302 / 301 / 303 . At the Schottky junctions, the diffused electrons are swept away by built-in potential, thus photovoltaic effects set in. 
         [0034]    In addition to the common advantages already described for the photovoltaic cell in  FIG. 2 , since, in this invention, both inside and outside of the nanometer(s)-scale tubes  301  form junctions, the effective area that contributes to photovoltaic effects are roughly double the area provided by the cell in  FIG. 2 , thus the total electric power can be increased 50 times and beyond for a given cell size compared to conventional photovoltaic cells. 
         [0035]    In an alternative preferred embodiment illustrated in  FIG. 4 , a photovoltaic cell comprises plurality of nanometer(s)-scale wires or rods  401  are electrically connected to an electrode  400 . It is obvious for a person of ordinary skill in the art to recognize the following description applies to a photovoltaic cell comprises plurality of nanometer(s)-scale tubes, instead of wires or rods, as in  FIG. 3 . The nanometer(s)-scale wires or rods  401  can have metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The nanometer(s)-scale wires or rods are further surrounded by multiple layers of different electronic materials  402 ˜ 404  having metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The number of layers shown in  FIG. 4  is not the limitation, and it is apparent that the number of layers can be a wide range of numbers depending primarily on the thickness of each layer and the height of the nanometer(s)-scale wires or rods  401 . The multiple layers can be dissimilar semiconductors having different bandgaps appropriately tuned to cover a wide range of spectrum of the light entering the cell as described later. The electronic material  404  is further electrically connected to an electrode  405 . The electrode  400  has direct electrical contact to neither the electrical material  402  nor the electrode  405 . The electrode  400  is intended to serve as a common electrode that connects all wires or rods  401 . The electrode  405  is provided for the electronic material  404 . The interface between the nanometer(s)-scale wires or rods  401  and the electronic material  402 ˜ 404  form pn- or Schottky junctions where built-in potential for both electrons and holes is generated. 
         [0036]    In way of an example not way of limitation, the nanometer(s)-scale wires or rods  401  can be made of n-type semiconductor and the electronic material  402 ˜ 404  that surrounds the nanometer(s)-scale wires or rods  401  can be made of three different types of p-type semiconductors having different bandgaps. Incident light  406  that contains a broad-spectrum range enters the photovoltaic cell through either the electrode  405  or the electrode  400  (In  FIG. 4 , the incident light enters the photovoltaic cell through the electrode  400 ). As the incident light  406  travels through the electronic material  402 ˜ 404 , a specific spectrum range in the incident light  406  is absorbed in a specific layer in the multiple layers of electronic materials  402 ˜ 404 , in that, short, middle and long wavelengths in the incident light  406  can be absorbed subsequently in the layers  402 ,  403  and  404  respectively, then numerous number of electron  407  (and holes) are generated in each layers. Photo-generated electrons in the electronic material  402 ˜ 404  made of p-type semiconductor (and vice versa for the holes (not shown here)), then diffuse toward the pn-junctions, created at the interface between the nanometer(s)-scale wires or rods  401  and the multi layered electronic material  402 ˜ 404 . At the pn-junctions, the electrons  407  are swept away by built-in potential, thus photovoltaic effects set in. Apparently, in addition to the common advantages already described in  FIG. 2˜FIG .  3  over conventional cells in  FIG. 1 , the additional advantage of the cell in  FIG. 4  over the photovoltaic cells described in  FIG. 2˜FIG .  3  is to have a capability of covering a wide range of spectrum contained in incident light and converting a wide range of spectrum to photo generated carriers. Dozens of different layers could be stacked to catch photons at all energies, to make absorb wide band of solar spectrum, from lower wavelengths (as low as X-ray) to longer wavelength (e.g. long infrared). Addition of the multiple junction of different materials which could absorb wide solar spectrum plus the increasing of the junction area with using of the rod, wires, or tubes help to increase the electrical power energy 50 times and beyond as compared with the conventional solar cell of same size. According to this invention, dozens of materials, which could absorb wide solar spectrum, may or may not require the lattice mismatch with the rod, wires, or tubes. Lattice matched material could increase further increase of the power generation due to reduction of the recombination. 
         [0037]    According to this invention, the rods, or wires could be GaN materials (n or p type) and the dozens of the materials could be In 1-x Ga x N (p or n type, opposite to GaN rods). With increasing of the Ga contents, the band-gap of InGaN can be increased to close to ˜3.4 eV which is same as that of the GaN. With increasing of the In contents in InGaN, the band gap can be reduced to ˜0.65 eV. Photons with less energy than the band gap slip right through. For example, red light photons are not absorbed by high-band-gap semiconductors. While photons with energy higher than the band gap are absorbed—for example, blue light photons in a low-band gap semiconductor—their excess energy is wasted as heat. 
         [0038]    According to this invention, alternatively the rods, or wires could be III-V based materials (n or p type) for example InP and the dozens of the materials could be III-V based material for example In 1-x Ga x As (p or n type, opposite to InP rods). In this case, with adjusting of In contents, band gap can be tuned and thereby the wide spectrum of the solar energy can be absorbed. 
         [0039]    According to this invention, alternatively the rods, or wires could be II-V based materials (n or p type) for example CdTe and the dozens of the materials could be II-VI based material for example CdZnS (p or n type, opposite to CdTe rods). In this case, with adjusting of Zn contents, band gap can be tuned and thereby the wide spectrum of the solar energy can be absorbed. 
         [0040]    According to this invention, alternatively the rods, or wires could be Si (or amorphous Silicon materials (n or p type) and the dozens of the materials could be Si: Ge alloy (p or n type, opposite to Si rods). In this case, with adjusting of Ge contents, band gap can be tuned and thereby the wide spectrum of the solar energy can be absorbed. 
         [0041]    According to this invention, alternatively the rods, or wires could be Si, InP, or CdTe (n or p type) and the dozens of the materials, could be different material which could make the junction with the rods (wires or tubes) and each type of material has the specific band gap for absorbing the specific range of solar spectrum. In this way also wide range of solar spectrum can be absorbed, and with increasing of the junction area (due to use of the rods, wires, or tubes), the electrical power generation could be increased tremendously 50 times and beyond. 
         [0042]    According to this invention, alternatively the nanometer(s)-scale rods  401  can be formed on the substrate (not shown here), and the electrode  405  can be made on the substrate to have a common contact for each nanometer(s)-scale rods or tubes  401 , necessary for creating junction. 
         [0043]    According to this invention, alternatively the nanometer(s)-scale tubes (not shown here) can be formed instead of nanometer(s)-scale rods  401  and furthermore those nanometer(s)-scale tubes can be either empty or filled up with an electronic material having metallic electrical conduction, p-type or s-type semiconductor electrical conduction. 
         [0044]    In another preferred embodiment shown in  FIG. 5 , plurality of photovoltaic cells comprising plurality of nanometer(s)-scale wires or rods  501  are randomly and electrically connected to an electrode that has arbitrary shapes  500  (In  FIG. 5 , triangular shape is illustrated, however any arbitrary shapes can be applicable here) further connected an electrode  502 . It is obvious for a person of ordinary skill in the art to recognize the following description applies to a photovoltaic cell comprises plurality of nanometer(s)-scale tubes, instead of wires or rods, as in  FIG. 3 . The nanometer(s)-scale wires or rods  501  can have metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The nanometer(s)-scale wires or rods are further surrounded by an electronic material  503  having metallic electrical conduction, p-type or n-type semiconductor electrical conduction. The electronic material  503  is further electrically connected to an electrode  504 . The electrodes  500  and  502  have direct electrical contact to neither the electrical material  503  nor the electrode  504 . The interface between the nanometer(s)-scale wires or rods  501  and the electronic material  502  form pn- or Schottky junctions where built-in potential for both electrons and holes is generated. The way this photovoltaic cell operates is just the same way in other photovoltaic cells illustrated in  FIG. 2˜FIG .  4 , therefore, the unique characteristics for this photovoltaic cell in  FIG. 5  is the fact that the nanometer(s)-scale wires, rods or tubes, are connected to electrodes that can have, instead of planar surface, three-dimensionally arbitrary shape. This structure helps to increase the effective junction area and PCCE is expected to be much higher than in photovoltaic cells described in  FIGS. 2 to 4 . 
         [0045]      FIGS. 6A and 6B , another preferred embodiment, are schematics showing the side and cross-sectional view of a photovoltaic cell structure consisting of nanometer(s)-scale wires, rods or tubes, arranged horizontally, parallel to a substrate to achieve large junction area with respect to the incident light entering the cell perpendicular to the substrate surface. As in  FIG. 6A , a substrate  600  that has metallic, or n-type or p-type semiconductor electrical conduction, has plurality of vertical walls  601  made of a material that has metallic, or n-type or p-type semiconductor electrical conduction. Electrodes  606  are provided to the substrate  600  for electrical connection to the photovoltaic cell. Metal or semiconductor nanometer(s)-scale wires or rods  602  are formed perpendicular to the vertical walls  601 , bridging two adjacent vertical walls  601 . It should be noted that the metallic or semiconductor wires, rods or tubes  602  could also be tubes rather than wires or rods. The nanometer(s)-scale rods  602  can be made of either a similar or a dissimilar material to that of the vertical walls  601 . The nanometer(s)-scale wires or rods  602  are further surrounded by an electronic material  603  made of a semiconductor material. The electronic material  603  is electrically connected to an electrode  604 , however the electrode  604  is electrically isolated from the substrate  600  by an insulator  605  and from the vertical walls  601 . 
         [0046]    In way of an example not way of limitation, in  FIG. 6B , the nanometer(s)-scale rods  602  can be made of n-type semiconductor and the electronic material  603  that surrounds the nanometer(s)-scale wires or rods  602  can be made of p-type semiconductors, thus forming large area pn-junction  607  at the interface between the nanometer(s)-scale wires or rods  602  and the electronic material  603 . As the incident light  608  enters the cell, carriers are generated both in the nanometer(s)-scale rods  602  and the electronic material  603 , diffusing into the pn-junction  607  and contributing to photovoltaic effects. 
         [0047]    According to this invention, the surrounding material  603  can be single or plurality layers of materials having different band gaps, which correspond to the different absorption wavelength, contained in the incident light, as described in  FIG. 4 . 
         [0048]      FIG. 7 , another preferred embodiment, is a schematic showing the cross-sectional view of a photovoltaic cell structure consisting of plurality of nanometer(s)-scale wires, rods or tubes  701  formed on semiconductor/insulator self-assembled dots or islands  702 , arranged vertically to a substrate  700 . The nanometer(s)-scale wires, rods or tubes  701  can be The nanometer(s)-scale wires, rods or tubes can be made of a variety of electronic materials such as metals and semiconductors (Elemental semiconductors such as Si, Ge and C and compound semiconductors).  701  can also be directly formed on the electronic material substrate  700  (not shown here). In the case of nanometer(s)-scale tubes, the tubes can be empty or filled up by the electronic materials (not shown here), as described in  FIG. 3 . The nanometer(s)-scale wires, rods or tubes  701  are further connected to an electrode  704 . The nanometer(s)-scale wires, rods or tubes  701  are also surrounded by a semiconductor  703  for which electrode  705  are provided. 
         [0049]    In way of an example not way of limitation, in  FIG. 7 , the nanometer(s)-scale wires, rods or tubes  701  can be made of n-type semiconductor and the electronic material  703  that surrounds the nanometer(s)-scale wires, rods or tubes  701  can be made of p-type semiconductors, thus forming large area pn-junction  706  at the interface between the nanometer(s)-scale rods  701  and the electronic material  703 . As the incident light  707  and  708  enter the cell, carriers are generated both in the electronic material  703  and the nanometer(s)-scale wires, rods or tubes  701 , diffusing into the pn-junction  706  and contributing to photovoltaic effects. 
         [0050]    In another preferred embodiment,  FIG. 8  illustrates a side view of a photovoltaic cell comprising of nanometer(s)-scale wires, rods or tubes, arranged horizontally, parallel to a substrate to achieve large junction area with respect to the incident light entering the cell along the surface normal of the substrate  800 . As in  FIG. 8 , a substrate  800  that has metallic, or n-type or p-type semiconductor electrical conduction, is covered with an electrical insulation layer  801 . Pluralities of nanometer(s)-scale tubes  802  are formed horizontally on the insulating layer  801 . The nanometer(s)-scale tubes  802  are further surrounded by an electronic material  803  that has metallic, or n-type or p-type semiconductor electrical conduction. An electrode  804  is provided on the electronic material  803 . The both ends of the plurality of nanometer(s)-scale tubes  802  are connected to electrodes  805 , thus the plurality of nanometer(s)-scale tubes  802  are electrically accessible from the electrodes  805 , and the interface between the plurality of nanometer(s)-scale tubes  802  and the electronic material  803  forms either pn-junction of Schottky junctions. 
         [0051]    In way of an example not way of limitation, the nanometer(s)-scale tubes made of carbon atoms  802  that have metallic electrical conduction can be surrounded by the electronic material  803  made of p-type semiconductors, thus forming Schottky-junction at the interface between the nanometer(s)-scale tubes  802  and the electronic material  803 . As the incident light  807  enters the cell, electrons are generated in the electronic material  803 , and then the photo-generated electrons diffusing into the Schottky-junction contribute to photovoltaic effects. 
         [0052]    According to this invention, the nanometer(s)-scale wires, rods or tubes, mentioned in the preferred embodiments, can be any kinds of electronics materials covering semiconductor, insulator or metal. 
         [0053]    According to this invention, the nanometer sized rods, wire or tubes can be made from the semiconductors such as Si, Ge, or compound semiconductors from III-V or II-VI groups. As an example for rods, wire, or tubes, InP, GaAs, or GaN III-V compound semiconductor can be used and they can be made using standard growth process for example, MOCVD, MBE, or standard epitaxial growth. According to this invention, the self-assembled process can also be used to make wires, rods, or tubes and their related pn-junction to increase the junction area. These rods, wire, or tubes can be grown on the semiconductors (under same group or others), polymers, or insulator. Alternatively, according to this invention, these rods, wire, or tubes, can be transferred to the foreign substrate or to the layer of foreign material. The foreign substrate or the layer of material can be any semiconductor such as Si, Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can cover also all kinds of polymers or ceramics such as AlN, Silicon-oxide etc. 
         [0054]    According to this invention, the nanometer sized rods, wire or tubes based on II-VI compound semiconductor can also be used. As an example CdTe, CdS, Cdse, ZnS, or ZnSe can also be used, and they can be made using standard growth process for example, sputtering, evaporation, MOCVD, MBE, or standard epitaxial growth. According to this invention, the self-assembled process can also be used to make wire, rods, or tubes and their related pn-junction to increase the junction area. These rods, wire, or tubes can be grown on the semiconductors (under same group or others), polymers, or insulator. Alternatively, according to this invention, these rods, wire, or tubes, can be transferred to the foreign substrate or to the layer of foreign material. The foreign substrate or the layer of material can be any semiconductor such as Si, Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can cover also all kinds of polymers or ceramics such as AlN, Silicon-oxide etc. 
         [0055]    According to this invention, the nanometer sized rods, wire or tubes can be made from the carbon type materials (semiconductor, insulators, or metal like performances) such as carbon nano-tubes, which could be single, or multiple layered. They can be made using standard growth process for example, MOCVD, MBE, or standard epitaxial growth. According to this invention, the self-assembled process can also be used to make wires, rods, or tubes and their related pn-junction to increase the junction area. These tubes can be grown on the semiconductors (under same group or others), polymers, or insulator. Alternatively, according to this invention, these rods, wire, or tubes, can be transferred to the foreign substrate or to the layer of foreign material. The foreign substrate or the layer of material can be any semiconductor such as Si, Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can cover also all kinds of polymers or ceramics such as AlN, Silicon-oxide etc. 
         [0056]    Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, reference to the details of the preferred embodiments is not intended to limit their scope. Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth. 
         [0057]    Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth. 
         [0058]    The present invention is expected to be found practically use in the novel photo-voltaic cells which as higher power generation capability (25 times and beyond) as compared with that of the conventional cells. The proposed invention can be used for fabricating wide solar panel for both commercial and space applications.