Patent Publication Number: US-9905714-B2

Title: High efficiency photovoltaic cells

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/766,575 filed Jan. 28, 2006, and also patent application Ser. No. 11/626,826, filed on Jan. 24, 2007. 
    
    
     FIELD OF INVENTIONS 
     This patent specification relates to structures of photovoltaic cells (hereafter mentioned also as “solar cells”). More specifically, it relates to photovoltaic cells comprising structures that increase the junction area and absorb a broad solar spectral spectrum in order to increase power generation capability per unit area. This also relates to photovoltaic cells containing nano-scaled blocks. These photovoltaic cells can be used in commercial, residential, and also industrial applications for power generation. 
     BACKGROUND OF THE INVENTIONS 
     Photovoltaic cells where light is converted into electric power to be fed to external loads, which are electrically connected to the photovoltaic cells, have been prevailing in a wide range of applications such as consumer electronics, industrial electronics, and space exploration. In consumer electronics, photovoltaic cells that consist of materials such as amorphous silicon are used 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 is in the range of ˜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. 
     In contrast, for more demanding applications such as residential and industrial solar power generation systems, either poly-crystalline or single-crystalline silicon is typically used because there are more stringent requirements of better reliability and higher efficiency than those in consumer electronics. Photovoltaic cells consisting of poly-crystalline and single-crystalline silicon generally offer conversion efficiencies in the range of ˜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 for an alternative energy source. However, due to the high cost ($3 to $5/Watt) of today&#39;s Si-based solar cell, it is not yet widely accepted as an alternative energy source solution. 
     Group II-V compound semiconductors, for example CdTe and CdS, have been considered for the purpose of creating industrial solar power generation systems, manufactured at a lower cost and maintaining a moderate conversion efficiency. This approach resulted in a comparable conversion efficiency of ˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A, Cessert 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 Oct. 2001; 995-1000.]. This conversion efficiency is comparable to those for the single crystalline silicon photovoltaic devises; however, the toxic nature of these materials is of great concern for environment. 
     Group I-III-VI compound semiconductors, such as CuInGaSe 2 , have also been extensively investigated for industrial solar power generation systems. This material can potentially be synthesized at a much lower cost than its counterpart, single crystalline silicon. However, a conversion efficiency of ˜19%, is comparable to that of single crystalline silicon based cells and can be obtained, thus far, only by 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: 31-316.], which again raises issues associated with the toxic nature of these materials. 
     Photovoltaic cells designed for several exclusive applications, where the main focus is high conversion efficiency, generally consist of group III-V semiconductors, including GaInP and GaAs. In general, synthesis processes of single crystalline group III-V are very costly because of substantial complications involved in epitaxial growth of group III-V single crystalline compound semiconductors. Typical conversion efficiencies 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.], usually more than 10 times as expensive as the conventional Si-solar cell. 
     All photovoltaic cells in the prior art described above, regardless of what materials the cell is made from, essentially fall into one specific type of structure, which usually limits its power generation capability. Usually a flat pn-junction structure is used in conventional solar cells ( FIG. 1A ). Shown in  FIG. 1A  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 generates electron-hole pairs after being absorbed by the p- and also n-type semiconductor layers  101  and  102 . The incident light  104  generates electrons  105   e  and holes  105   h  in the region near the pn-junction  103  and also electrons  106   e  and holes  106   h  in the region far from the pn-junction  103 . The photogenerated electrons  105   e  and  106   e  (and holes) (hereafter considering only electronics, i.e. minority carriers in p-type semiconductors, although the same explanation is applicable for holes, minority carriers in n-type semiconductors) diffusing toward the pin-junction  103  and entering the pn-junction  103  contribute to photovoltaic effect. 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). 
     The PCGE is the percentage of photons entering a cell which contribute to the generation of photo carriers, which needs to be, ideally, 100%. On the other hand, the PCCE is the percentage of photogenerated electrons  105   e  and  106   e  that reach the pn-junction  103  and contribute to the generation of photocurrent. For a monochromatic light, a 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 pr-junction  103  cannot be collected efficiently due to many adverse recombination processes that prevent photogenerated 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. As the minority carriers travel through the semiconductors, the longer the life-time, the less recombination, which increases the conversion efficiency. Usually, a thicker and higher quality wafer is used to increase the conversion efficiency of the conventional solar cell. However, this makes the solar cell costly and heavier. In addition to increasing the collection efficiency, the absorption of a wide range of the solar spectrum will also increase the conversion efficiency. It is highly desirable to have the solar cell structure in which (a) the increase of the PCCE is independent of the substrate thickness and (b) the ability to absorb a wide range of the solar spectrum is possible. 
       FIG. 1B  shows typical monochromatic light intensity behavior  108  inside the semiconductor. As illustrated in  FIG. 1B , the light intensity behavior  108  inside the bulk semiconductor is exponential. The light intensity p at certain depth x can be expressed as p(x)=P o exp(−αx), where P o  is the peak intensity at the surface and α is the absorption co-efficient of the semiconductor in which light is entering. Carriers (not shown here) generated due to light flux  112  absorbed by the pn-junction  103  is only drifted by the junction field and can be collected efficiently, whereas, carriers  106   e  and  106   h  generated due to absorption of light-flux  110  by semiconductor region  101  are diffused in all directions. Only those carriers  105   e  and  105   h  which are generated closer (a distance equal to or less than the diffusion-length of the semiconductor) to the pn-junction  103 , can be collected. Those carriers  106   e  and  106   h  which are generated far away (a distance longer than the diffusion-length of the semiconductor) from the pn-junction  103  are recombined and lost. The light flux  112  is usually lost either by leaving out or being absorbed by the substrate. 
     Both PCGE and PCCE are largely dependent on the material and structure of the photovoltaic cells. Today&#39;s photovoltaic cells are structured in such a way that (a) wide ranges of the solar spectrum cannot be absorbed due to material limitations, and (b) PCCE is low due to its inherent structure. For example, the typical conversion efficiency of today&#39;s crystal-Si based solar cell is ˜18%. Wavelengths of the solar spectrum spread from &lt;0.1 μm to 3.5 μm, but Si can only absorb ˜0.4 μm to 0.9 μm of light. ˜50% of light belonging to the solar spectrum cannot be absorbed by Si, due to its inherent material properties. The remaining 32% is lost due to (i) recombination of photogenerated carriers and (ii) loss of light, which is represented by  112  in  FIG. 1B ; these two factors are structure dependent. If we could reduce these two factors, ˜50% conversion efficiency could be achieved, even in a Si-based solar cell. If we could capture different wavelengths of light belonging to the solar spectrum by utilizing different material systems or nano-material systems, we could increase the conversion efficiency ideally to 100%. Furthermore, if the solar cell (photovoltaic cell) detection capability could be extended to the infrared-spectrum, then the cell could produce electrical energy not only during the day (while sun is present), but also at night (hereafter defined by when the sun is not out). Additionally, today&#39;s solar cell material is not highly radiation-tolerant. Specifically, in space applications, photovoltaic cells should be highly radiation tolerant and have structure and material systems which can generate high-power per unit area. In order to increase the conversion efficiency (ideally to 100%), it would be desirable to have photovoltaic cell structures which have (a) larger surface area to volume ratios to capture all the photons (at specific wavelength) entering the cell, (b) a pn-junction that is located as close to the photo absorption region as possible, and (c) photo responses at different spectrums in order to efficiently cover a wide range of spectrums of light that enter a photovoltaic cell. It would be further desirable to have solar cells which could generate electric power in both day and night. 
     In addition to conversion efficiency, cost-effective manufacturing is another important factor which needs to be taken into consideration. In today&#39;s solar cell, the high-cost is one of the main concerns in addition to the issue of achieving low conversion efficiencies. It is found that more than 93% of solar cells are silicon (Si) based, meaning a silicon (Si) wafer is the base material, and the rest are thin-film based solar cells. In manufacturing Si-based solar cells, more than 85% of the cost comes from the Si wafer cost, the remaining comes from other processing costs. It is highly desirable to reduce the wafer cost and at the same time increase the conversion efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide structures of photovoltaic cells, which have high power generation capability per unit area, with respect to the conventional counterparts, mentioned as the prior art. 
     Accordingly, it is an object of this invention to reduce the recombination of carriers and increase the absorption of light, which effectively increases the photogenerated carriers. 
     Accordingly, it is an object of this invention to increase the absorption bandwidth of the solar spectrum in order to increase the conversion efficiency. 
     It is an object of this invention to provide solar cell structures based on nano-scaled blocks structures which are formed on the base substrate. The pn- or Schottky junctions are formed with nano-blocks, generating built-in potential by which photogenerated electrons and holes are swept away, leading to photovoltaic effect. 
     It is an object of this invention to provide solar cell structures based on nano-blocks, such as rods or wires or nanoparticles, formed on the supporting substrate or formed on the electronic materials which are formed on the base substrate. The pn- or Schottky junctions are formed with nano-blocks, generating built-in potential by which photogenerated electrons and holes are swept away, leading to photovoltaic effect. 
     According to this invention, the supporting substrate can be Si, CdTe, Cu, GaAs, InP, GaN, glass, polymer, ceramics, Ge, C, ZnO, BN, Al 2 O 3 , AlN, Si:Ge, CuInSe, II-VI, III-V, etc. 
     It is an object of this invention to have electronic materials on which nano-blocks (rods, wires, or tube) or nanoparticles can be formed and said electronic materials can be formed on the base substrate, made from, for example, Si, Ge or glass to lower the cost. 
     It is an object of this invention to provide structures of photovoltaic cells which can capture most of the wavelengths belonging to the solar spectrum and can provide &gt;80% conversion efficiency. 
     It is an object of this invention to provide structures of photovoltaic cells which can generate electric power in both day and night. 
     It is an object of this invention to provide low-cost manufacturing processes for manufacturing the novel photovoltaic cells. 
     This invention proposes to achieve &gt;60% conversion efficiency utilizing Si-materials and &gt;80% conversion efficiency for other materials. The main advantage of these inventions are that today&#39;s matured process technologies allow fabrication of the photovoltaic cell which has power generation capabilities a few orders or more greater than that of conventional photovoltaic cells. 
     Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail in conjunction with the appended drawings wherein: 
         FIG. 1A  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.  FIG. 1B  shows the light intensity behavior inside prior-art semiconductor materials. 
         FIG. 2A  is the schematic showing the nanoparticle structures which illustrate the benefits of achieving a significantly high ratio of surface area to volume used in the simulation.  FIG. 2B  is the simulation results, showing the ratio of the surface area T to the volume A as a function of diameter of the nanoparticle d with the gap between the particles gas the parameter. 
         FIGS. 3A and 3B  are the schematics showing the cross-sectional views of a photovoltaic cell structure comprising the nanoparticles in the first embodiment, in accordance with the present invention. 
         FIGS. 4A, 4B, and 4C  are the schematics showing the cross-sectional views of photovoltaic cell structures comprising nanoparticles in the second embodiment, in accordance with the present invention. 
         FIGS. 5A and 58  are the schematics showing the cross-sectional views of a photovoltaic cell structures comprising layers of nanoparticles of different sizes and types in the third embodiment, in accordance with the present invention. 
         FIG. 6  is the schematic showing the cross-sectional view of a photovoltaic cell structure based on the nanoparticles layer and the cylindrical structure, in the fourth embodiment, in accordance with the present invention. 
         FIG. 7  is the schematic showing the cross-sectional view of a photovoltaic cell structure based on the nano-scaled cylindrical blocks, in the fifth embodiment, in accordance with the present invention. 
         FIG. 8  is the schematic showing the cross-sectional view of a photovoltaic cell structure based on the nano-scaled blocks embedded into the polymer, in the sixth embodiment, in accordance with the present invention. 
         FIGS. 9A, 9B, 9C, and 9D  are the schematics showing the fabrication process flow for flexible photovoltaic cells comprising a nanoparticle layer as the absorption layer, in the seventh embodiment, in accordance with the present invention. 
         FIGS. 10A, 10B, 10C, and 10D  are the schematics showing an alternate fabrication process flow for flexible photovoltaic cells comprising nanoparticles embedded into the polymer layer, in the eighth embodiment, in accordance with the present invention. 
         FIGS. 11A, 11B, 11C, 11D, and 11E  are the schematics showing an alternate fabrication process flow for photovoltaic cells comprising nano-scaled wires/blocks formed on the dielectric or semiconductor substrate, in the ninth embodiment, in accordance with the present invention. 
         FIGS. 12A and 12B  are the schematics showing the formation of the semiconductor-metal nanoparticles, as required by  FIG. 11  for fabricating the photovoltaic cell. 
     
    
    
     DETAILED DESCRIPTION 
     According to the present invention it is our object to provide several photovoltaic cell structures that increase the surface area, which would increase the junction area, and also cover a wide range of the solar spectrum in order to increase conversion efficiency to as high as &gt;60%. Before proceeding to give the detailed explanation of the photovoltaic cell structures and their manufacturing, simulation results are given to show the benefits of increasing the surface area and using the nanoparticles in the photovoltaic cell. 
       FIGS. 2A and 21  are the schematics representing the layer  202  formed with the nanoparticles. The layer  202  represents the absorption layer in the photovoltaic cell.  FIG. 2B  shows the simulation results as function of the nanoparticle diameter d with gap g between two nanoparticles as the parameter in  FIG. 2A , n×n number of nanoparticles are arranged in a×a sized layers with thickness t. T/A is the ratio of the surface area of the total nanoparticles T to the volume (a×a×t=A). It is assumed that if we could make the n×n number of nanoparticles  204  on the a×a-area surface. Increase of the ratio indicates the increasing increment of the surface area Tin the proposed cell, as compared to the conventional photovoltaic cell, which is usually flat. As depicted in  FIG. 2B , by reducing the gap g and the nanoparticle diameter d, the ratio increases significantly. For example, if the particle gap g is kept to 5 nm and the particle diameter d is kept to 5 nm, over 750 times the surface area can be achieved as compared to a conventional 5 cm×5 cm photovoltaic cell. 
     According to the preferred embodiment illustrated in  FIGS. 3A and 3B , a photovoltaic cell comprising a nanoparticle based layer  301  acts as the absorption layer, sandwiched by top metal contact  303   a  and bottom metal contact  303   b , formed on the supporting substrate  300 . The nanoparticles  302  can be formed as the layer (not shown here) acting as the absorption layer. The nanoparticles  302  can be embedded into the electronic material  304 , creating electrical conduction. The electronic material  304  can be semiconductor material or conductive polymer material, which may create the junction (n or p type junction) with the nanoparticles  302 . A single or plurality of layers may be require to make the junction (not shown here). The electronic material  304  can be a separate material or electronic materials of p or n type. The electronic material  304  and the supporting substrate  300  are further electrically connected to electrodes  303   a  and  303   b , respectively. The electrodes  303   a  and  303   b  are intended to serve as common electrodes which connect all nanoparticles  302 . The electrode  303   a  is provided for the electronic material  304  or junction. The electrode  303   a  can be transparent (not shown here) and can be formed on the electronic material  304  or junction. The interface between the nanoparticle  302  and the electronic material (or junction)  304  form pn- or Schottky junctions where built-in potential for both electrons and holes is generated. The nanoparticles could be made of any kind of semiconductor material of a few nanometers in size, which could create the quantum confinement effect. Absorption of a broad range of the solar spectrum is possible by utilizing different sizes and different semiconductor nanoparticles. Substrate  300  could be semiconductor, glass, or polymer. 
     According to this invention, alternatively the nanoparticle based layer  301  can be formed various ways as shown in  FIGS. 4A, 4B, and 4C . As shown, the nanoparticle based layer  401  ( 401   a ,  401   b , and  401   c ) acting as the absorption layer, can be formed by stacking nanoparticles  402  which are either of same size ( 402   a ), different size ( 402   b ), or different type ( 402   c ), Alternatively, the nanoparticles  402  ( 402   a ,  402   b , and  402   c ) can be embedded into the electronic conducting material (not shown here). In way of an example not way of limitation, the nanoparticles  402  can be made of semiconductor material (n- or p-type) and the electric conduction material (not shown here) that is on or surrounds the nanoparticles  402  can be made of p-type semiconductor. Incident light  406  ( FIG. 4C ) enters the photovoltaic cell through either the electrode  403   a  or on the material or junction substrate  400 . (In  FIG. 4C , the incident light  406  enters the photovoltaic cell through the electrode  403   a ). The incident light  406  travels through the nanoparticle based layer  401 , and the substrate  400 . The light absorbed by both nanoparticles  402  and the electronic material (not shown here) generates numerous electrons (not shown here). Each nanoparticle  402  helps form the junction all over the surface of the electronic material (not shown here), and helps to collect the carriers without allowing recombination. Utilizing the nanoparticles  402  helps to collect the carriers with less or even no recombination by increasing the junction area. It should be pointed out that electrons are apparently generated all over the region along the thickness of the electric material and the nanoparticles  402 . It also should be pointed out that holes are apparently generated all over the region along the thickness of the absorption layer  401 . 
     Unlike a conventional solar cell, the solar cells shown in  FIGS. 4A, 4B, and 4C  have junctions all over the nanoparticles  402 , which create an equivalent to multiple junctions across the thickness of the absorption layer  401 . Increasing the thickness would help to absorb photon flux perpendicular to the front contact. As the particle size is nano-scaled, absorption of a broad range of the spectrum can be possible and almost all carriers can be collected, as the junctions are formed a few nanometers apart. It is apparent that utilizing the solar cell shown in  FIGS. 3 and 4  can (i) reduce the recombination, (ii) absorb all photo flux, and (iii) cover a broad range of the spectrum, thereby increasing the conversion efficiency. 
     According to the preferred embodiment illustrated in  FIGS. 5A and 5B , a photovoltaic cell comprising a plurality of absorption layers  501  based on nanoparticles of different types  508 ,  510 , and  512  are formed on the bottom contact  503   b  which is on the substrate  500 . The only difference here, when compared to  FIGS. 3 and 4 , is that the absorption layer  501  consists of multiple layers of nanoparticles of different type and different sizes  508 ,  510 ,  512 . Similar to  FIGS. 3 and 4 , the surface area of the junction for receiving light  506  is also increased which causes a reduction of the photogenerated carrier recombination and the absorption of all photo-flux incident on the surface, thereby increasing the conversion efficiency. Note here that by utilizing multiple layers of nanoparticles of different type and sizes, absorption of the full solar spectrum can be possible and, ideally, the conversion efficiency can be increased to 100%. 
     Apparent advantages of this invention over conventional photovoltaic cells are directly associated with the fact that, unlike conventional photovoltaic cells, multiple discrete junctions are created for collecting all photogenerated carriers created in the absorption layer  501 , regardless of where they are generated. According to this invention, recombination can be eliminated (ideally) and all photon flux can be absorbed (ideally), and the conversion efficiency can be ˜100% and still &gt;50% when using Si. Conventionally, as explained in the description of the prior art shown in  FIG. 1 , a conventional photovoltaic cell, where electrical-junctions are perpendicular to the direction to which incident light travels, the photogenerated carriers generated in the region far away from electrical-junctions need to diffuse much longer distances (diffusion-length) than those photogenerated carriers generated near the 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 of conventional photovoltaic cells. In addition, it is evident that the total effective area that contributes to the photovoltaic effect in this invention can be increased significantly by a few orders (&gt;3000) considering a 5 nm diameter with a gap of 0 nm for a 5 cm×5 cm size cell. 
     According to this invention, in way of an example not way of limitation, the supporting substrate  500  can be ceramics, glass, polymer or any kind of semiconductor on which transparent or nontransparent metal contact  503   b  is made. Alternatively, supporting substrate  500  can be metal which also acts the metal contact. For this case, copper, stainless steel, Aluminum, or alloyed metal can be used. According to this invention, the nanoparticles  508 ,  510 ,  512  can be any kind of semiconductor or compound semiconductors, having absorption capabilities in the desired spectrum region. By utilizing the quantum confinement effect, which is dependent on the size of the nanoparticle, the absorption range can be extended in the blue and red-shift of their parent bulk type materials. For nanoparticles, Si, Ge, InP, GaAs, CdSe, CdS, ZnO, ZnTe, ZnCdTe, CuInSe, CuSe, InGaAs, etc. can be used. 
     According to this invention, in a way of an example not way of limitation, the nanoparticles  508 ,  510 ,  512  can be stacked to form the absorption layer  501 . Alternatively, the electronic conduction material can be used to embed the nanoparticles  508 ,  510 ,  512 . The electrical conduction material can be the sol-gel layer or any conductive polymer. The top metal contact  503   a  can be transparent or non-transparent metal. Indium-tin-oxide (ITO) can be used as the transparent metal contact. Alternatively, the electrical conduction layer can be formed onto the absorption layer  501  to create the junction. 
     In an alternative preferred embodiment shown in  FIG. 6 , photovoltaic cell comprises a substrate  600  with three-dimensional protrusions wherein the cross-sectional shape of the three-dimensional protrusions is cylindrical, absorption layer  601  based on the nanoparticles  602 , electrical conduction layer  604 , and top and bottom metal contacts  603   a  and  503   h , respectively. The supporting substrate  600  can be flexible or rigid substrate formed by molding, casting, or etching. The substrate  600  can be polymer, semiconductor or ceramic, or glass type material. Alternatively, supporting substrate  600  can be metal, which can also work as the bottom contact (not shown here). The nanoparticles  602  and the electrical conduction material  604  form the absorption layer  601 . The incident beam  606  is absorbed by both the electrical conduction material  604  and the nanoparticles  602 , which generate the electrons (and holes), which are in turn collected by the metal contacts  603   a  and  603   b . The junction created by the electrical conduction material  604  and nanoparticles  602  helps sweep away the generated carriers to the contacts  603   a  and  603   b . The electronic material  604  is further electrically connected to electrodes  603   a  and  603   b , which are intended to serve as common electrodes that connect all cylindrical shaped electrical junctions. The electrode  603   a  is on the electronic material  604 . The interface between the nanoparticles  602  and the electronic material  604  form pn- or Schottky junctions, which can be assumed to be multiple discrete junctions inside the conduction material  604 . 
     Photogenerated electrons in the electronic material  604 , made of p- and n-type semiconductor or conductive polymer, then diffuse toward the junction created by conduction material  604  and nanoparticles  602 . At the junctions, the diffused electrons are swept away by built-in potential, thus photovoltaic effects set in. Common advantages already described for the photovoltaic cells in  FIGS. 3, 4, and 5 , can be achieved in this embodiment as well. The only differences in  FIG. 6  are that the supporting substrate  600  shape is cylindrical and single or multiple layered nanoparticles are used to increase the combined surface to volume ratio significantly. 
     According to this invention, in way of an example not way of limitation, the supporting substrate  600  can be semiconductors such as Ge, GaAs, GaN, InP, GaN, CdTe, or ZnO or polymer or metal. 
     In the preferred embodiment shown in  FIG. 7 , a photovoltaic cell comprises a plurality of nanometer(s) scaled cylinders  714  which are electrically connected to a substrate  700 . The nano-meter(s) scaled cylinders  714  are surrounded by an electronic material  716 , having metallic electrical conduction and forming the junction in the interface of  716  and  714 . The electronic material  716  and the supporting substrate  700  are further electrically connected to electrodes  703   a  and  703   b . The electronic conduction material  716 , used to make the junction with nano-meter (s) scaled rods/wires  714 , is electrically isolated with the bottom contact  703   b  by dielectric  718 . The planarization dielectric (polymer)  720  is on which the metal electrical contact  703   a  is formed. The electrode  703   a  is intended to serve as a common electrode that connects all rods  714 . The electrode  703   a  is provided for the electronic material  716 . The interface between the nanometer(s)-scale rods  714  and the electronic material  716  form pn- or Schottky junctions, thus there are pn- or Schottky junctions outside of the nanometer(s)-scale rods  714 . 
     In an alternative preferred embodiment shown in  FIG. 8 , a photovoltaic cell comprises a plurality of nanometer(s)-scaled rods (or wires)  822  embedded into the electrical conduction material  804  acting as the absorption layer  801 , which is electrically connected to a substrate  800 . The micrometer(s) or nanometer(s)-scaled rods (or cylinders)  822  are surrounded by an electronic material  804  having electrical conduction. The electronic material  804  can be a separate material in which nanometer-scaled rods or wires  822  are embedded. The electronic material  804  and the supporting substrate  800  are further electrically connected to electrode  803   a  on the non-substrate side and electrode  803   b  on the substrate side. The interface between the nanometer(s)-scale rods  822  and the electronic material  804  form junctions, thus creating built-in-potential for collecting photogenerated carriers and transferring them to the electrodes. The main difference between the photovoltaic cell shown in  FIG. 8  and others of  FIGS. 3, 4, 5, and 6 , is that the nanometer(s) scaled rods or wires  822  are separately synthesized and are embedded into the electronic material  804  formed on the supporting substrate  800 . For example, the nanorods or wires  822  can be semiconductors like, Si, Ge, InP, GaAs, Cds, Cdse, CdTe, ZnO, Znse, ZnS, etc., or combinations thereof, separately synthesized either by epitaxial growth in a vacuum deposition system or chemically formed from the solutions. 
     According to this invention, in way of an example not way of limitation, the supporting substrate  800  can be Ge, GaAs, GaN, CdTe, ZnO, Cu, Al 2 O 3 , AlN, glass, polymer, metal, etc. The electronic material  804  can be conductive polymer or a sol-gel based semiconductor. Note here that use of the nanometer-scaled rods or wires  822  helps not only increase the junction area but also helps transfer the generated carriers to the electrodes  803   a  and  803   b  before recombination. The rods or wires  822  inside the conductive polymer help to transfer the carriers through the rods. By varying the size (diameter) of the nanometer scaled rods (or wires)  822 , a quantum confinement effect can be created and thereby absorption of broad spectral ranges can be possible. 
       FIGS. 9A, 9B, 9C and 9D  are the schematics showing the fabrication process of the flexible photovoltaic cell based on nanoparticles contained in the absorption layer, according to this invention, wherein the similar numerals in  FIG. 9  represents the similar parts in  FIGS. 3, 4, 5, and 6 , so that similar explanations are omitted. According to this invention, supporting substrate  900  can be semiconductor such as Si, Ge, GaAs, InP, etc., polymer, or glass. The metal  903   b  acting as the contact in the photovoltaic cells, is formed onto the substrate  900 . The metal  903   b  can be pressed and a rolling technique can be used, utilizing the roller  930 . To make a firm connection with the substrate, epoxy or glue can be used. The absorption layer  901 , comprising the nanoparticles embedding into the conducting polymer, is formed on the metal contact  903   b . To form the absorption layer  901 , spin coating or press and rolling techniques can be used. Alternatively, a printing technique or ink-jettable technique can also be used to form the absorption layer  901 . With proper heat treatment, the top contact  903   a  can be formed. It is noted here that the instead of using the substrate  900 , alternatively the absorption layer  901  can be directly formed onto the metal  903   b  by using the printing or ink-jettable technique. The advantages of this process are that low-cost and high efficiency flexible photovoltaic cells can be fabricated. 
       FIGS. 10A, 10B, 10C, and 10D  are the schematics showing the fabrication process of the photovoltaic cell based on nanoparticles contained in the absorption layer  1001 , according to this invention, wherein the similar numerals in  FIG. 10  represent similar parts in  FIGS. 3, 4, 5, and 6 , so that similar explanations are omitted. According to this invention, a layer of nanoparticles  1002  is formed on the substrate  2000  by chemical synthesis. A thin layer (not shown here) may be required in between substrate  2000  and the nanoparticle  1002 , to increase the sticking coefficient. The layer of nanoparticles  1002  then transferred to the foreign substrate  1000  with metal electrode  1003   b , on which the photovoltaic cell is to be made. The transfer of the nanoparticles  1002  to the foreign substrate  1000 , can be done by heat or imprinting treatment. According to this invention, foreign substrate  1000  can be a semiconductor such as Si, Ge GaAs, InP, etc., polymer, or glass. Alternatively, the layer of nanoparticles  1002  can be directly transferred to the metal which could act as the substrate and also as the contact for the photovoltaic cell. Alternatively, without using the transferring process described in  FIGS. 10A and 10B , the nanoparticle  1002  layer can be formed using printing or ink-jettable printing processes. The absorption layer  1001  comprising the nanoparticles  1002  embedding into the conducting polymer  1004  is formed on the metal contact  1003   b . To form the absorption layer, spin coating or press and rolling techniques, as mentioned earlier, can be used. Alternatively, printing or ink-jettable techniques can also be used to form the absorption layer  1001 . After proper heat treatment, the top contact  1003   a  is formed. It is noted here that the instead of using the substrate  1000 , alternatively the absorption layer  1001 , can be directly formed onto the metal  1003   b , using printing or ink-jettable techniques. The advantages of this process are that low-cost and high efficiency flexible photovoltaic cells can be fabricated. 
       FIGS. 11A, 11B, 11C, 11D, and 11E  are the schematics showing the fabrication process of the photovoltaic cell based on nanoscaled rods or wires  1114  formed on the supporting substrate  1100 , according to this invention, wherein the similar numerals in  FIG. 11  represent similar parts in  FIG. 7 , so that similar explanations are omitted. According to this invention, nano-meter sized metal-semiconductor compounds  2100  are formed on the substrate  1100  with the metal electrode  1103   b . The metal-semiconductor compounds can be formed either on the substrate  1100  by chemical synthesis, or transferred to the substrate  1100  by imprinting or heat treatment processes. According to this invention, the metal-semiconductor compounds  2100  have semiconductor and metal attached together. The semiconductor portion is attached to the substrate and the metal portion acts as the catalytic and is on top of the semiconductor. They are periodically organized onto the substrate  1100  with metal contact  1103   b  by imprinting or lithography processes. With chemical vapor deposition, the nano-scaled rods  1114  are formed. A dielectric layer, such as silicon-oxide or silicon nitride layer  1118 , is used to isolate the junction formed by the electronic material  11116  and nano-rods  1114 . 
     To make the pn-junctions of dissimilar type (p or n), electronic material  1116  is used. For example, if the rod-material  1114  is n-type Si, then p-type Si is to be formed as the electronic material  1116 . This can be formed by the diffusion of p-dopants into the n-type substrate. The interface of  1114 / 1116  forms the junction which has the built-in-potential to create the photovoltaic effect on a large surface area. Finally, a passivation or conformal layer of dielectric or polymer  1120  is formed on the electronic material  1116  after proper chemical mechanical processes. The final stage is to make the planarization using insulator layer  1120  and contact  1113   a.    
       FIGS. 12A and 12B  are the schematics showing the formation of the semiconductor-metal interface onto the desired substrate  1200 , for example glass for fabricating the photovoltaic cell as explained in  FIG. 11 , according to this invention, Here, the nano-silicon particles are formed uniformly onto the desired substrate  1200  with the metal contact  1203   b . Nano-silicon particles made separately by chemical synthesis can be used for this purpose. This is followed by the nanometal attachment on to the nano-silicon particles, as shown in  FIG. 12B . The nanometal onto the nano-silicon acts as the catalytics for forming the nano-rods, as explained in  FIG. 11B . 
     According to this invention, the absorption layer formed by the electronic material and the nanoparticles or nanometer(s) scaled rods, explained in  FIGS. 3  thru  11  is a single layer used to form the junctions. By varying the diameter of the nanoparticles or the nano-rods used in the absorption layer, absorption of a wide range of the solar spectrum is possible, which increases the power generation. 
     According to this invention, as explained in  FIGS. 3  thru  11 , the nanoparticles or nano-rods are used to increase the junction area, so the junction can be extended closer to the region where the photogenerated carriers are formed. The 3-dimensional structures can be used as part of the electronic material to form the junctions. This structure can be formed utilizing a suitable substrate. The substrate can be used to form the structure where junctions are formed, utilizing the other electronic materials formed on to the 3D structures. 
     According to this invention, the nanoparticles or rods could be GaN materials (n or p type) and the dozens of materials could be In 1-x Ga x N (p or n type, opposite the GaN rods). By increasing the Ga contents, the band-gap of InGaN can be increased to ˜3.4 eV, which is the same as that of GaN. By increasing the in content in InGaN, the band gap can be reduced to ˜0.65 eV. Photons with less energy than the band gap will slip right through. For example, red light photons are not absorbed by high-band-gap semiconductors, while photons with an 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. 
     According to this invention, alternatively the nanoparticles or rods 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 like In 1-x Ga x As (p or n type, opposite the InP rods). In this case, by adjusting the In contents, the band gap can be tuned and thereby a wide spectrum of the solar energy can be absorbed. 
     According to this invention, alternatively the nanoparticles or rods 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 like CdZnS (p or n type, opposite the CdTe rods) or Zn(Cd)Te/ZnS based materials. In this case, by adjusting the Zn contents, the band gap can be tuned and thereby a wide spectrum of the solar energy can be absorbed. 
     According to this invention, alternatively the nanoparticles or rods 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 the Si rods). In this case, by adjusting the Ge contents, the band gap can be tuned and thereby a wide spectrum of solar energy can be absorbed. 
     According to this invention, alternatively the nanoparticles or rods could be Si, InP, or CdTe (n or p type) and dozens of different materials could make the junction with the rods (wires or tubes) and each type of material would have a specific band gap for absorbing a specific range of the solar spectrum. In this way a wide range of the solar spectrum can be absorbed, and by increasing the junction area (due to use of the rods, wires, or tubes), the electrical power generation could be increased tremendously (50 times and beyond). 
     According to this invention, the nanoparticles or nanometer(s)-scale wires, mentioned in the preferred embodiments, can be any kind of electronic materials, covering semiconductor, insulator, or metal. 
     According to this invention, the nanometer sized nanoparticles or rods can be made from semiconductors such as Si, Ge, or compound semiconductors from III-V or II-VI groups. As an example for rods, wires, or tubes, InP, GaAs, or GaN III-V compound semiconductors can be used and they can be made using standard growth processes, 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 in order to increase the junction area. These rods, wires, or tubes can be grown on the semiconductors (under same group or others), polymers, or insulators. Alternatively, according to this invention, these rods, wires, 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 also cover all kinds of polymers or ceramics such as AlN, Silicon-oxide, etc. 
     According to this invention, the nanometer sized nanoparticles or rods, based on an II-VI compound semiconductor can also be used. As an example CdTe, CdS, Cdse, ZnS, or ZnSe can be used, and they can be made using standard growth processes, for example, sputtering, evaporation, MOCVD, MBE, or standard epitaxial growth or chemical synthesis. According to this invention, the self-assembled process can also be used to make nanoparticles or wires, and their related pn-junctions to increase the junction area. These rods, wires, or tubes can be grown on the semiconductors (under same group or others), polymers, or insulators. Alternatively, according to this invention, these rods, wires, 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 also cover all kinds of polymers or ceramics such as AlN, Silicon-oxide, etc. 
     According to this invention, the nanometer sized rods, wires, or tubes can be made from carbon type materials (semiconductors, insulators, or metal like performances), such as carbon nano-tubes, which could be single or multiple layered. They can be made using standard growth processes, 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 in order to increase the junction area. These rods, wires, or tubes can be grown on the semiconductors (under same group or others), polymers, or insulators. Alternatively, according to this invention, these rods, wires, 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 also cover all kinds of polymers or ceramics such as AlN, Silicon-oxide, etc. 
     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 embodiments 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 occur to one skilled in the art which fairly fall within the basic teaching here is set forth. 
     The present invention is expected to be found practically useful in that the novel photo-voltaic cells have higher power generation capability (25 times and beyond) when compared with that of the conventional cells. The proposed invention can be used for fabricating wide solar panels for both commercial and space applications.