Abstract:
A method of creating a High efficiency solar cell with a Triangular or Sinusoidal parallel Ridge above the surface, below the surface, buried under the surface and also back of the cell to improve capture cross section is described in this invention.

Description:
[0001]    This application claims a priority according to pending U.S. patent application Ser. Nos. 61/214,979, 61/214,941, and 61/914,942 filed on Apr. 29, 2009 by the same Applicant of this Application, the benefits of the filing date of Apr. 29, 2009 are hereby claimed under Title 35 of the United States Code. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to the configurations and methods for manufacturing photovoltaic cells for converting optical energy into electric energy. More particularly, this invention relates to configurations and methods to manufacture photovoltaic cells on semiconductor substrate with expanded energy absorbing surface areas and substantially eliminating optical reflection from the surface of the solar cells thus increasing efficiency of current generation from the photovoltaic cells. 
         [0004]    2. Description of the Prior Art 
         [0005]    Even with wide ranges of research efforts and design creativities devoted to increase the photovoltaic cell efficiency and to expand the photon capture and light utilization areas of solar cells, conventional technologies of manufacturing semiconductor photovoltaic cells are still confronted with a physical limitation that the light capture areas and surface utilization of the solar cells cannot be further increased. Specifically, the solar cells are formed on a semiconductor substrate with the solar cells disposed as with light capture surface disposed on the semiconductor substrate along a horizontal orientation in parallel to the top and bottom surfaces of the substrate. The total surface area of the substrate typically is the maximum area that can be exposed to the sun and utilizable to capture the solar energy into the solar cells. 
         [0006]      FIGS. 1A and 1B  are a top view and a cross sectional view respectively of a conventional solar cell device supported on a semiconductor substrate. The solar cell device is formed on top a silicon or poly silicon substrate layer  100 . The Epitaxial layer  105  is optional and is required only for Silicon cells. The epitaxial layer  105  is not necessary for a poly silicon substrate. An opposite conduction layer  110  is formed on top of the substrate layer; either by diffusion of by implantation followed by anneal process. Thus PN junction is formed in the substrate layer with opposite conduction to the substrate type on top of the substrate. Then an antireflection (AR) layer  120  is formed on the top surface covering the top layer  110 . An electrical grid layer  130  and an electrode layer  140  are formed on the top and bottom surface respectively. The photons  170  are pass through the solar cell device, pairs of electron  150  and hole  160  are generated throughout the semiconductor region and electrons are collected in the N-layer and holes in the P layer respectively. The collected electrons  150  and the holes  160  thus accumulated and separated by the PN junction, generate a voltage difference between the top and bottom PN Junction thus converting the optical energy transmitted through the photons into electrical energy. 
         [0007]    There are several disclosures related solar cell devices including Patent Publications 20040221886, 20090151637, 20090000656, 20080157106, 20080155908, and 20100006139. These Patent Application Publications disclose various improved configurations in attempt to improve the photovoltaic cell efficiency of the solar energy devices. However, these disclosures are related only to configurations and layout of solar cell modules and assemblies. The techniques and device configurations as disclosed do not provide an effective solution to overcome the limitation intrinsically imposed on the light capture areas due to the physical dimension of the flat surface of the semiconductor substrate. 
         [0008]    There was a publication By Stanford University, which deals with crating uneven surface by etching the top surface to produce lots os overlapping Pyramid structures, thereby increasing the capture cross section and also increasing the photon absorption. There is another work by University of Southwales, Australia, essentially trying to achieve the same idea using an inverted Pyramid structure on the top of the substrate. 
         [0009]    Therefore, a need still exists in the art of solar cell device design and manufacture to provide new manufacturing method and device configuration in forming the solar cell devices with new and improved configurations such that the above discussed problems and limitations can be resolved. 
       SUMMARY OF THE PRESENT INVENTION 
       [0010]    A major aspect of this invention is to provide solar panel comprises solar cells manufactured with improved configurations and methods to make the solar cell with larger capture cross sectional area for energy absorbing surface configured to have triangular, rectangular or sinusoidal ridges, which runs almost to the full length of the surface, which can be on the top, buried or at the bottom of the silicon or poly silicon substrate. With this new and improved configuration, the solar cells are made with larger surface cross sectional area, with the same wafer surface. Compared with the conventional photovoltaic solar panels extended over a same horizontal area, the solar panels of this invention that extends across a same horizontal area can produce higher current because the photons now incident onto expanded absorption cross sectional areas. 
         [0011]    Another aspect of this invention is to provide solar panel comprises solar cells manufactured with the energy absorbing surface configured to have triangular, rectangular or sinusoidal ridges and with these ridges formed on the top, buried or at the bottom of the silicon or poly silicon substrate. With this new and improved configuration, the solar cells are made with larger surface cross sectional area by using wafers of the same wafer surface thus providing multiple reflection of the photons at the surface of the structure that also produce more electron-hole pairs than the standard conventional solar cells. 
         [0012]    Another aspect of this invention is to form ridges buried at the bottom surface of the substrate. The bottom ridge configuration further expands the bottom contact area thus improving the capture rates of holes or electrons depending on the bottom material as P or N type. This configuration also improves the proximity of the portion of the bottom electrode closer to the PN junction, thus increasing current transmission efficiency because of the reduced loss with less substrate resistance. 
         [0013]    Briefly, an embodiment of this invention includes a solar cell device. The solar cell device comprises multiple semiconductor layers formed with different conductivity type to form a PN junction in a semiconductor substrate, wherein at least one of the semiconductor layers having a non-flat surface comprises a parallel ridges extend along substantially a same direction. In another embodiment, the non-flat surface comprises the parallel ridges having a sinusoidal ridge shape. In another embodiment, the non-flat surface comprises the parallel ridges having a triangular ridge shape. In another embodiment, the non-flat surface comprises the parallel ridges having a rectangular ridge shape. In another embodiment, the semiconductor substrate is composed of a III/IV semiconductor compound or a IV/V semiconductor compound. In another embodiment, the parallel ridges are disposed on a top surface or formed as a buried junction wherein the parallel ridges are configured to perform as a multiple junction device and to provide a surface capacitance or a junction capacitance for improving a capacitance per unit area, and improving resistance/conductance per unit area characteristics of the semiconductor solar device. In another embodiment, the solar device further comprises an antireflection (AR) layer disposed on a top surface of the semiconductor substrate wherein the AR layer includes a single layer or multiple layers of AR films. In another embodiment, the parallel ridges are formed as a buried junction wherein the parallel ridges are configured to perform as multiple junctions. In another embodiment, the multiple semiconductor layers formed with different conductivity types to form the PN junction in the semiconductor substrate are doped with dopant concentrations for improving absorption of photons projected onto the semiconductor layers. 
         [0014]    These advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1A and 1B  are a top view and a cross sectional view respectively of a conventional solar cell device supported on a semiconductor substrate. 
           [0016]      FIGS. 2A and 2B  are a cross sectional view and top view respectively of an array of photovoltaic cells supported on a semiconductor chip of this invention. 
           [0017]      FIGS. 3A to 3I  are a series of cross sectional views for illustrating the processing steps to manufacture the array of photovoltaic cells of  FIGS. 2A and 2B . 
           [0018]      FIGS. 4A to 4   c  are a cross sectional view, top view and bottom view respectively of an array of photovoltaic cells supported on a semiconductor chip as an alternate embodiment of this invention. 
           [0019]      FIGS. 5A to 5D  are a series of cross sectional views for illustrating the processing steps to manufacture the array of photovoltaic cells of  FIGS. 4A to 4C . 
           [0020]      FIGS. 6A and 6B  are a cross sectional view and top view respectively of an array of photovoltaic cells supported on a semiconductor chip as another embodiment of this invention. 
           [0021]      FIGS. 7A to 7G  are a series of cross sectional views for illustrating the processing steps to manufacture the array of photovoltaic cells of  FIGS. 6A and 6B . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Embodiment 
     Buried Ridge Case 
       [0022]    Referring to  FIGS. 2A and 2B  for a top view and a cross sectional view respectively of a solar cell device of the present invention supported on semiconductor substrate  200 . The substrate  200  may be formed as an N or P type substrate, it may also be a single crystal silicon or poly crystal silicon. For a single crystal silicon substrate, an epitaxial layer  205  of the same conductivity type as that of the substrate layer may be formed as an optional layer for the starting substrate. With a silicon substrate layer  200 , conduction layer  210  of opposite conductivity type may be formed on the top of silicon layer  200  using either a diffusion or implantation method (in case of epitaxial layer  205 ) thus forming a PN junction between these two layers. A top surface  210  of an opposite conductivity type from the conductivity type of the substrate  200  is then formed and then covered with an antireflection layer  220 . A top electrical contact grid  230  is formed on top of the antireflection layer  220 . An electrode is formed on the bottom surface of the substrate  200  to form another ohmic contact for the solar cell device. 
         [0023]    As the photons  270  projected onto the top surface, these photons  270  are transmitted through the antireflection layer  220  and pass through the semiconductor regions. Upon reaching the PN junction between the layers of different conductivities types, the irradiation of these photons generates pairs of electron  250  and holes  260  throughout the silicon layers. Then the electrons are separated to N-type layer  210  and holes to P-type layer  205  formed with the PN junction between these two layers. The electron-hole pairs accumulate at these respective layers creating a potential difference and voltage across the PN junction. 
         [0024]      FIGS. 3A to 3I  are a series of cross sectional views for illustrating the processing steps for manufacturing the solar cell device of  FIGS. 2A and 2B .  FIG. 3A  shows the manufacturing processes start with a substrate  200  (single crystal silicon N or P type with or without an optional epitaxial layer  205  of the same type as the substrate layer silicon or a poly silicon layer). In  FIG. 3B , an oxide layer  202  is added on the on top of the substrate layer  200  (or on epi layer  205  which is optional) either by deposition or by oxidation process. In  FIG. 3C , a photo resist layer  204  is formed on top of the oxide layer  202 . In  FIG. 3D , the photo resist layer  204  is patterned to produce a triangular or sinusoidal ridge shape photo resist layer  204 . 
         [0025]    In  FIG. 3E , an etch process is carried out to etch the oxide layer  202 , and the photo resist layer  204  is removed. In  FIG. 3F , an opposite type conduction layer to substrate is formed by either diffusion and anneal, For example; if the substrate is P type, N type doping material would be used to produce N type layer  210  on the top so as to produce a PN junction, thus forming a PN junction having a triangular or sinusoidal ridge shape. The oxide layer  202  is removed from the top and bottom surface followed by a process of depositing an antireflection (AR) layer  220  on the top surface and the formation of ohmic contact layer  230  on the top and bottom surface of the solar cell device with the top contact layer forming a contact grid in electrical connection with the Top layer  210 . A bottom contact layer  240  is formed for bottom contact 
         [0026]      FIGS. 3G to 3I  show the alternate processing steps. In  FIG. 3G , an ion implant of a dopant is carried out followed the processing step shown in  FIG. 3D  to form an opposite type region  210  on top of substrate layer  200 , thus forming a sinusoidal, triangular or even step junctions into the substrate  200 . In  FIG. 3I , the photo resist layer  204  and the oxide layer  202  are removed and the junction is formed after an anneal step followed by depositing the antireflection layer  220  and the formation and patterning the ohmic contact grid  230  on the top and bottom electrode  240  at the bottom surface respectively as described above in  FIG. 3F . 
       Second Embodiment 
     Bottom Ridge Case 
       [0027]    Referring to  FIGS. 4A to 4C  for a cross sectional view, a top view and a bottom view respectively of a solar cell device of the present invention supported on a semiconductor substrate either N type or P type layer  300 . In an embodiment, the substrate can be either a single crystal silicon or poly silicon layer. (With a silicon substrate layer  300 , an Epitaxial layer  305  on the top of the same conduction type as the substrate can be optional in some cases). An opposite type layer  310  is formed on top of the substrate layer, thus forming a PN junction between these two layers. The substrate layer  300  is a Bottom Ridge (BR) configuration which runs almost all the way horizontally from one end to another. The top surface of the N-type dopant layer  310  is then covered with an antireflection layer  320 . A top ohmic contact grid  330  is formed on top of the antireflection layer  320 . An electrode is formed on the bottom surface of the substrate  300  to form another ohmic contact for the solar cell device. 
         [0028]      FIGS. 5A to 5D  are a series of cross sectional views for illustrating the processing steps for manufacturing the solar cell device of  FIGS. 4A to 4C .  FIG. 5A  shows the manufacturing processes start with a substrate. The substrate may be either an N or P Type or a poly silicon substrate  300 . An optical epitaxial layer  305  of the same type as the substrate is formed only for a single crystal silicon substrate  300 . A conduction layer  310  of an opposite conductivity type from the substrate layer  310  is formed over the substrate by carrying out a diffusion process or by implanting and followed by an anneal process. An oxide layer  302  is either deposited or grown at the bottom of the substrate layer. In  FIG. 5B , a photo resist layer  304  is formed on the bottom surface below the oxide layer  302 . In  FIG. 5C , the photo resist layer  304  is patterned by masking, exposing, and developing processes. Then an etch process is carried out to etch the back side oxide layer  302  the wafer is ready for performing a partial etch step on the layer  300 . 
         [0029]      FIG. 5D  shows the cross section after the layer  200  is partially etched either wet or dry or combination of the two methods to a depth “a” which is in 10s of microns and is stopped just few microns from the back of the PN Junction, then the photo layer and the back side oxide layer is removed. The width of the “valley of the ridge is designated by a parameter “b” that is also in 10s of microns or even in millimeters or even centimeters. This creates the inverted ridge on the bottom surface of the solar cell device. The pitch between adjacent ridges is designated by a parameter “c” and the length of “c” could be in microns, 10s of microns or in millimeters or even in centimeters.  FIG. 5D  shows the finished wafer cross section. Layer  320  is the Anti reflection (AR) coating. Layer ( 330 ) is the top side ohmic grid contact either on the top of layer  320 , to contact layer  310 . The bottom ohmic contact is made all across the bottom layer including the valley portions of the ridge in the reverse ridge bottom surface. This produces an increased contact area and also some of the contact area closer to the bottom of the PN junction thus reducing the mean free path for the electrons or holes can be collected by the bottom electrode. 
       Third Embodiment 
     Above Ridge Case 
       [0030]    Referring to  FIGS. 6A and 6B  for a top view and a cross sectional view respectively of a solar cell device of the present invention supported on a silicon substrate  400  which has the above surface ridge which extends horizontally across almost the full length of the cell. The ridges can be triangle or sinusoidal shapes too. In an embodiment, the substrate can either be a single crystal silicon or poly silicon layer, N or P type. With a silicon substrate layer  400 , an epitaxial layer  405  can be added which is optional is only for single crystal silicon. An opposite conduction layer  410  is formed on top of the substrate layer thus forming a PN junction between these two layers. 
         [0031]    An antireflection (AR) layer  420  covering the top surface is deposited over layer  410 . A top contact grid  430  is formed on top of the antireflection layer  420 , to form an ohmic contact. An electrode is also formed on the bottom surface of the substrate  400  to form another ohmic contact for the solar cell device. The electron-hole pairs shown as  450  and  460  are generated and accumulated in layers  405  and  410  of alternate conductivities thus create an electrical potential to conduct a current between the electrodes  430  and  440  disposed on the top and bottom surface of the substrate  400 . 
         [0032]      FIGS. 7A to 7G  are a series of cross sectional views for illustrating the processing steps for manufacturing the solar cell device of  FIGS. 6A and 6B .  FIG. 7A  shows the manufacturing processes start with a silicon substrate or a polysilicon layer  400  which can be either a P or N type. An oxide layer  402  is grown on top of the substrate layer  400  and at the bottom surface of the substrate layer  400 . In  FIG. 7B , a photo resist layer  404  is formed on top of the oxide layer  202 . In  FIG. 7C , the photo resist layer  404  is patterned and in  FIG. 7D , the masking, developing and etching processes are carried out to pattern the photo resist layer  404  and the oxide layer  402  ready to carry out an etch process as described below. In  FIG. 7E , a silicon etch process is carried out. A wet or dry or wet/dry combination etch process is performed to produce the triangular or sinusoidal ridges on the top surface of the substrate layer  400 . Depending on how the mask is designed, a top surface with the triangular or sinusoidal ridges may be formed over all top surface areas as shown in this figure. Alternatively, the triangular or sinusoidal ridges may be etched below the flat or partially above and partially below the top surface. The pitch of the triangular base designated as a parameter “a” and the height of the triangular ridges designated as parameter “b” in  FIG. 7E  are in 10s of microns or even 100 micron+depending on the objective. In  FIG. 7F , the top oxide layer  402  and photo resist layer  402  may still remain on top as shown in  FIG. 7E  are then removed and the top surface is cleaned. The bottom oxide layer  402  is kept. An optional P type epitaxial layer  405  is grown for a single crystal silicon substrate  400 . This is only an optional step. The epitaxial layer  405  is not necessary when the substrate layer  400  is a polysilicon layer. Then the opposite conduction layer  410  is formed on the layer  400  (or  405  as the case may be) is formed by carrying out a diffusion process or by implant followed by an anneal process. 
         [0033]      FIG. 7G  the back side oxide  402  is removed followed by depositing an antireflection layer  420  on the top surface. Then, an ohmic contact grid  430  either on the top surface  420  for contacting top layer  410  through the AR layer  420 . Then an electrode layer  440  is formed on the backside at the bottom surface to create an ohmic contact at the bottom of the substrate layer  400 . 
         [0034]    With this proposed improved configuration, the solar cells are made with larger surface cross sectional area, with the same wafer surface. With the light absorbing areas formed with triangular or sinusoidal ridge configurations, greater cell efficiency is achieved because of the expanded absorption cross sectional areas. Compared to conventional solar cell devices, higher currents are generated by using the solar cells of this invention formed on the silicon substrate that has a same surface area. 
         [0035]    Another advantage of the solar cells of this invention formed with the ridge configuration is the multiple reflections of the photons at the tilted surface of the ridge structure. The photons are prevented from reflected out of the surface and not captured by the light absorbing layers. Therefore, compared to the flat surface devices, more electron hole pairs are generated and greater light utilization is achieved with the improved solar cells of this invention. 
         [0036]    Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.