Patent Publication Number: US-2009223564-A1

Title: Solar cell and method for making same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly-assigned copending applications: Ser. No. 11/967,008, entitled “SOLAR CELL WITH FLEXIBLE SUBSTRATE” (attorney docket number US 14906); Ser. No. 11/967,009, entitled “SOLAR CELL WITH FLEXIBLE SUBSTRATE” (attorney docket number US 14910); Ser. No. 11/933,941, entitled “FLEXIBLE SOLAR CELL” (attorney docket number US 15052); and Ser. No. 12/002,129, entitled “CARBON NANOTUBE FILM STRUCTURE AND METHOD FOR FABRICATING THE SAME”. Disclosures of the above-identified applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to solar cells and methods for manufacturing the same. 
     2. Description of Related Art 
     Generally, a flexible solar cell includes a flexible substrate, a back metal contact layer, a P-type semiconductor layer, an N-type semiconductor layer, and a transparent conductive oxide (TCO) film subsequently formed on the substrate. Indium tin oxide (ITO) has been the most commonly used material for TCO film. 
     However, ITO has an inferior flexibility and abrasion resistance due to its brittle nature. In addition, the indium component of ITO is rapidly becoming scarce, and therefore is becoming an increasingly expensive commodity, which has fueled demand for lower-cost solutions in recent years. 
     Therefore, a new solar cell is desired to overcome the above described shortcomings. 
     SUMMARY 
     An exemplary solar cell includes a substrate having a surface, a back metal contact layer formed on the surface of the substrate, a first type semiconductor layer formed on the back metal contact layer, a second type semiconductor layer formed on the first type semiconductor layer, and a CNT film formed on the second type semiconductor layer. The CNT film includes a plurality of successive CNT bundles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Many aspects of the embodiments can be better understood with references to the following drawing. The components in the drawing are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawing, like reference numerals designate corresponding parts throughout the several views. 
       The drawing FIGURE is a schematic, cross-sectional view of a solar cell according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments will now be described in detail below with reference to the drawing. 
     Referring to  FIG. 1 , a solar cell  100 , according to an exemplary embodiment, is shown. The solar cell  100  includes a substrate  101  with a surface  1012 . A back metal contact layer  102 , a first type semiconductor layer (e.g., a P-type semiconductor layer  103 ), an active layer  104 , a second type semiconductor layer (e.g., an N-type semiconductor layer  105 ), a carbon nanotube (CNT) layer  106 , and a front metal contact layer  107  are formed on the surface  1012  of the substrate  101  in the order written. In the present embodiment, the first type semiconductor layer is the P-type semiconductor layer  103 , and the second type semiconductor layer is the N-type semiconductor layer  105 . It is to be understood that the first type semiconductor layer can be an N-type semiconductor layer while the second type semiconductor layer can be a P-type semiconductor layer. 
     The substrate  101  is flexible and so the solar cell  10  is also flexible. The substrate  101  can be made of polymer or stainless steel. The polymer can be, for example, polymide, polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), or arton (Norbornene). A thickness of the substrate  101  is in an approximate range from 10 microns to 100 microns. 
     The back metal contact layer  102  can be made of silver, copper, molybdenum, aluminum, copper aluminum alloy, silver copper alloy, or copper molybdenum alloy. The back metal contact layer  102  can be formed on the substrate  101  using any of a variety of common techniques including, but not limited to, sputtering. 
     The P-type semiconductor layer  103  can be made of P-type amorphous silicon (P-a-Si), particularly, P-type amorphous silicon with hydrogen (P-a-Si:H). Also, the P-type semiconductor layer  103  can be made of III-V group compound semiconductors or II-VI group compound semiconductors, particularly above semiconductors doped with aluminum, gallium, or indium, e.g., aluminum gallium nitride (AlGaN), aluminum gallium arsenide (AlGaAs). The P-type semiconductor layer  103  can be formed by plasma enhanced chemical vapor deposition (PECVD). 
     The active layer  104  can be made of III-V or I-III-VI group compound semiconductors, e.g., cadmium telluride (CdTe), copper indium diselenide (CuInSe 2 , CIS). Also, The active layer  104  can be made of copper indium gallium diselenide (CuIn 1-x GaSe 2 , CIGS). The active layer  104  can be formed on the P-type semiconductor layer using any of a variety of common techniques including, but not limited to, chemical vapor deposition, or sputtering. 
     The N-type semiconductor layer  105  can be made of N-type amorphous silicon (N-a-Si), particularly, N-type amorphous silicon with hydrogen (N-a-Si:H). Also, the N-type semiconductor layer  105  can be made of III-V group compound semiconductors or II-VI group compound semiconductors, particularly above semiconductors doped with nitrogen, phosphorus, arsenic, e.g., gallium nitride (GaN), indium gallium phosphide (InGaP). The N-type semiconductor layer  105  can be formed by, for example, PECVD. 
     The CNT layer  106  functions as a transparent electrically conductive layer of the solar cell  10 . The CNT layer  106  can be a single CNT film or a plurality of stacked CNT films. A method for making such CNT film or stacked CNT films are taught in a commonly-assigned copending application: Ser. No. 11/967,008, entitled “Carbon nanotube film structure and method for fabricating the same”, which is incorporated herein by reference in its entirety. If the CNT layer  106  is a single CNT film, the CNT film includes a plurality of successive CNT bundles. All the CNT bundles are aligned in the same direction. If the CNT layer  106  includes two overlapped CNT films, the two CNT films are aligned along different directions. The angle between the aligned directions of the two CNT films is 90°. The two CNT films are combined by van de Warrls attractive force to form a stable layer structure. Each CNT film includes a plurality of successive CNT bundles, all of which are aligned in the same direction. After being treated with ethanol, the CNTs compact/shrink to bundles, and a space/distance is formed between every two adjacent bundles in each CNT film. Bundles in two films cross with each other to form a microporous structure. The diameter of the respective micropores is in an approximate range of 10 nanometers to 10 microns. It should be noted that the CNT layer  106  can include more than two overlapped CNT films. The total number of the CNT films is arbitrary and depends on the actual needs/use. 
     In the present embodiment, the CNT layer  106  is a single CNT film. The thickness of the CNT layer  106  can be in an approximate range from 10 nm to 100 nm. The CNT layer  106  is light pervious with a transmittance of more than 75%. 
     The front metal contact layer  107  can be made of silver, copper, molybdenum, aluminum, copper aluminum alloy, silver copper alloy, or copper molybdenum alloy. The front metal contact layer  107  can be formed on the CNT layer  106  using any of a variety of common techniques including, but not limited to, sputtering. The front metal contact layer  107  has a high electrical conductivity. The front metal contact layer  107  can be formed by, for example, sputtering. 
     One or more anti-reflective coatings (not shown) can be applied on the front metal contact layer  107  to improve the solar cell&#39;s  10  ability to collect incident light. 
     In order to improve the waterproofing ability of the solar cell  10 , a protective layer (not shown) can be formed on the front metal contact layer  107 . The protective layer can be made of resin. 
     The CNT layer  106  is substantially more mechanically robust than ITO films. Furthermore, the CNT layer  106  is chemically resistant and is manufactured from carbon, which is one of the most abundant elements on Earth. Therefore, the cost of the CNT layer  106  is relatively low. 
     A method for making the solar cell  10  is also provided. The method includes the steps of: (a) forming the back metal contact layer  102  on the surface  1012  of the substrate  101  by, for example, sputtering; (b) forming the P-type semiconductor layer  103  on the back metal contact layer  102  using methods such as, chemical vapor deposition (CVD); (c) forming the active layer  104  on the P-type semiconductor layer  103  using, for example, CVD; (d) forming the N-type semiconductor layer  105  on the active layer  104  using, for example, CVD; (e) forming the CNT layer  106  on the N-type semiconductor layer; and (f) forming the front metal contact layer  107  on the CNT layer  106  by methods, such as, screen printing, sputtering, and so on. 
     The step (e) can further include the substeps of: (e1) providing an array of CNTs, quite suitably, providing a super-aligned array of CNTs; (e2) pulling out a CNT film from the array of CNTs, by using a tool (e.g., adhesive tape or another tool allowing multiple CNTs to be gripped and pulled simultaneously); and (e3) directly disposing the CNT film on the surface of the N-type semiconductor layer  105 , thus forming the CNT layer  106 . 
     In step (e1), the CNT array can be a single-walled CNT array or a multi-walled CNT array. A given super-aligned array of CNTs can be formed by the substeps of: providing a substantially flat and smooth substrate; forming a catalyst layer on the substrate; annealing the substrate with the catalyst at a temperature in the approximate range from 700° C. to 900° C. in air for about 30 to 90 minutes; heating the substrate with the catalyst at a temperature in the approximate range from 500° C. to 740° C. in a furnace with a protective gas therein; and supplying a carbon source gas into the furnace for about 5 to 30 minutes and growing a super-aligned array of the CNTs from the substrate. 
     In step (e2), the first CNT film can be pulled out from the array of CNTs by the substeps of: selecting a plurality of CNT segments having a predetermined width; and pulling the CNT segments at an even/uniform speed to form the CNT film. 
     During the pulling process, as the initial CNT segments are drawn out, other CNT segments are also drawn out end to end, due to the van der Waals attractive force between ends of the adjacent segments. This process of drawing ensures a successive CNT film can be formed. 
     In the present embodiment, because the CNTs obtained from the super-aligned array of CNTs are very pure and the specific surface areas of the CNTs are extremely high, the CNTs are quite viscous. Accordingly, the CNT film can be directly adhered to the surface of the N-type semiconductor layer  105 . 
     While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims.