Abstract:
Solar cells provided with color modulation and a method for fabricating the same are disclosed. The solar cell includes a photoelectric conversion layer and a color-modulating layer provided over the photoelectric conversion layer. The photoelectric conversion layer is employed for generating electrical energy from incident light and the color-modulating layer is used to modulate colorful appearance. The color-modulating layer is composed of at least one dielectric layer which is free of granules.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation application and claims priority of U.S. patent application Ser. No. 12/468,606, filed on May 19, 2009, which claims the benefit of U.S. provisional application No. 61/088,779, filed Aug. 14, 2008, and the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The present inventions relates to photovoltaic cells capable of converting solar radiation into usable electrical energy. More specifically, the present invention relates to solar cells provided with color modulation and a method for fabricating the same. 
       2. Description of the Prior Art 
       [0003]    Solar cells or photovoltaic cells are devices that convert light energy of sunlight into electrical energy by means of photoelectric conversion mechanism. From the view point of global environmental conservation, the solar cell is highly expected to generate electricity and actively developed for widespread commercialization in recent years. Buildings, vehicles and other objects may be covered in part with solar cells to maximize the use of solar energy. For decorative or aesthetic reasons, solar cell units may be required to have different colors. As an example, when the solar cells are employed to cover roofs or walls of buildings, different colors maybe required for being integrated into the color (s) of the buildings or surrounding environment in consideration of design choice or aesthetic appearance. 
         [0004]    Conventional approaches, such as U.S. Pat. Nos. 5,725,006 and 6,049,035, for providing solar cells with different colors may require additional manufacturing process or may deteriorate the photoelectric conversion efficiency of the solar cells. Therefore, it is desirable to provide solar cells with variable colors without complicated designs or processes or without too much impact on the solar power conversion efficiency thereof. 
       SUMMARY OF THE INVENTION 
       [0005]    One objective of the present invention is to provide solar cells provided with color modulation and a method for fabricating the same. The solar cell includes a photoelectric conversion layer and a color-modulating layer provided over the photoelectric conversion layer. The photoelectric conversion layer is employed for generating electrical energy from incident light and the color-modulating layer is used to modulate colorful appearance. 
         [0006]    One embodiment of the present invention discloses solar cell comprising:
       a photoelectric conversion layer for generating electrical energy from incident light;   at least one first electrode and at least one second electrode formed over the photoelectric conversion layer for outputting the electrical energy; and   a color-modulating layer provided over the photoelectric conversion layer to modulate colorful appearance thereof.       
 
         [0010]    The solar cell in accordance with the present invention further comprises a protective layer formed over the color-modulating layer and a transparent layer formed over the protective layer. 
         [0011]    Another embodiment of the present invention discloses a method for fabricating a solar cell comprising the steps of:
       providing a photoelectric conversion layer;   forming at least one first electrode and at least one second electrode over the photoelectric conversion layer; and   forming a color-modulating layer over the photoelectric conversion layer to modulate colorful appearance thereof.       
 
         [0015]    The method in accordance with the present invention further comprises the steps of forming a protective layer over the color-modulating layer and forming a transparent layer over the protective layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other features and advantages of the present invention will be apparent from the detailed description of the invention that follows, taken in conjunction with the accompanying drawings of which: 
           [0017]      FIGS. 1-5  schematically illustrate a process for fabricating solar cells in accordance with one preferred embodiment of the present invention in cross-sectional views of partial presentation; 
           [0018]      FIG. 6  illustrates the reflective spectrum of a solar cell as exemplified in Example I; 
           [0019]      FIG. 7  illustrates the refractive index vs. wavelength curve of a color-modulating layer in Example II; 
           [0020]      FIG. 8  illustrates the reflective spectrum of a solar cell as exemplified in Example II; 
           [0021]      FIG. 9  illustrates the refractive index vs. wavelength curve of a color-modulating layer in Example III; 
           [0022]      FIG. 10  illustrates the reflective spectrum of a solar cell as exemplified in Example III; and 
           [0023]      FIG. 11  illustrates the reflective spectrum of a solar cell as exemplified in Example IV. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Certain terms are used through the description and following claims to refer to particular elements. As one skilled in the art will appreciate, solar cell manufacturers may refer to an element by different names. This document does not intend to distinguish between elements that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . .” Also, the term “formed on” or formed over” are intended to mean either indirect or direct contact between two layers. Accordingly, if an upper layer is “formed on” or “formed over” a lower layer, two layers maybe direct contact with each other, or an intermediate layer may be inserted or deposed between the two layers. 
         [0025]      FIGS. 1 through 5  schematically illustrates the process flow for fabricating a solar cell unit  1  according to one preferred embodiment of the present invention in cross-sectional views of partial representation. Referring to  FIG. 1 , an n-type semiconductor layer  12  is formed on a p-type semiconductor substrate  10  so as to form a p-n junction  14  therebetween. As such, an electric field can be established at the p-n junction  14 . Light striking on this electric field may separate the positive charge carriers and the negative charge carriers, thus creating an electrical current passing through the p-n junction  14 , which is so-called photoelectric conversion mechanism. Generally speaking, the combination of the p-type semiconductor substrate  10  and the n-type semiconductor layer  12  constitutes a photoelectric conversion layer  11  which is employed to generate electrical energy from incident light. The p-type semiconductor substrate  10  may be a p-type silicon substrate such that the n-type semiconductor layer  12  can be conformably deposited over the p-type semiconductor substrate  10  or formed by means of doping n-type impurities into the p-type semiconductor substrate  10 . Alternately, an n-type semiconductor substrate in combination of a p-type semiconductor layer can be utilized to constitute the photoelectric conversion layer  11  as well. Generally speaking, the photoelectric conversion layer  11  may be made of one or more semiconductor materials, such as single crystalline, polycrystalline, amorphous state of semiconductor material such as silicon, germanium or the like. 
         [0026]    As shown in  FIG. 2 , the transparent anti-reflection layer  16  is formed over the photoelectric conversion layer  11  and may be made of silicon nitride by means of an evaporation method, a sputtering method, a print screen method, a CVD method or any other methods that are known to the persons skilled in the art. The anti-reflection layer is employed to protect the solar cell unit  1 , serving as a passivation layer, and also decreases reflective loss on the unit surface. Preferably, the anti-reflection layer  16  has a thickness ranging from 1 nm to 500 nm. 
         [0027]    Conductive layers  18  and  20  are thereafter formed over opposite surfaces of the photoelectric conversion layer  11  by an evaporation method, a sputtering method, a print screen method, a CVD method or any other methods that are known to the persons skilled in the art. As shown in  FIG. 3 , the conductive layer  18  is formed over the front surface of the photoelectric conversion layer  11  and, therefore, on the anti-reflection layer  16 . The conductive layer  20  is formed over the back surface of the photoelectric conversion layer  11  in contact with the p-type substrate  10 . The conductive layer  18  or  20  may be made of metal or alloy, for example, gold, silver, aluminum, copper, or platinum or the like, and could be made of transparent conductive oxide (TCO) layer such as ITO film or a ZnO film as well. 
         [0028]    The conductive layer  18  can be subject to heat treatment such that conductive material contained in the conductive layer  18  can pass through the anti-reflection layer  16  to be in contact with the n-type semiconductor layer  12  by means of spiking effect. In addition, the conductive layers  18  and  20  can be patterned into parallel lines to form front electrodes  22  and back electrodes  24  respectively. As shown in  FIG. 4 , the front electrodes  22  are electrically connected with the n-type semiconductor layer  12  and the back electrodes  24  are electrically connected to the p-type semiconductor substrate  10 . Accordingly, the front electrodes  22  and the back electrodes  24  are formed to become two electrical terminals for the photoelectric conversion layer  11 . In other words, the electrodes  22  and  24  are used to charge or discharge the electrical energy generated from the photoelectric conversion layer  11  if the solar cell unit  1  is subject to light of sunlight. 
         [0029]    According to the present invention, the color-modulating layer  26  is formed over the anti-reflection layer  16  so as to provide the solar cell unit  1  with variable colors. The color-modulating layer  26  may be composed of one or more dielectric material over the anti-reflection layer  16  under a vacuum or near-vacuum environment by a coating method, an evaporation method (such as e-gun), a sputtering method, a CVD method or other methods if suitable and feasible. 
         [0030]    Various dielectric materials or combination of thereof may be utilized. In some examples, materials such as oxides (SnO 2 , Al 2 O 3 , SiO 2 , ZnO, Y 2 O 3 , Ta 2 O 5 , TiO 2 , Cr 2 O 3 , etc.), fluorides (MgF 2 , Na 3 AlF 6 , etc.), sulphides (ZnS, PbS, CdS, etc.), nitrides (Si 3 N 4 , AlN, AlO x N y , etc.), tellurides (CdTe, etc.) and selenides (PbSe), and/or the like. In various examples, the thickness of the color-modulating layer  26  may range from 1 nm or less to 5000 nm depending on various applications. 
         [0031]    By providing color-modulating layer  26  over the anti-reflection layer  16 , desirable visual effect may be achieved without suffering from conversion efficiency loss and using complicated manufacturing methods. 
         [0032]    Thereafter, a protective layer  28  and a transparent layer  30  are sequentially formed to cover the color-modulating layer  26 . The protective layer  28  is a transparent film made of, preferably, ethylene vinyl acetate (EVA) or polyvinyl butyral (PVB) in order to prevent the solar cell unit from direct exposure to sun and rain or subject to humidity. The transparent layer  30  is preferably made of treated or nontreated glass. 
         [0033]    It is noted that the step sequence of the aforementioned embodiment can be modified in consideration of practical use. Therefore, the exemplified embodiment cannot be used to interpret the scope of claims in limiting sense. 
         [0034]    There are some examples are provided for reference as follows. 
       EXAMPLE I 
       [0035]    The photoelectric conversion layer  11  is made of a silicon layer of a first conductivity type formed in/on a silicon substrate of a second conductivity type. If the first conductivity type is p-type, the second conductivity type is n-type. To the contrary, the second conductivity type is p-type if the first conductivity type is n-type. As an example, the photoelectric conversion layer  11  is formed of silicon has a refractive index (n) in the range of 3.4˜3.6 and has thickness in the range of 140˜250 μtm. The anti-reflective layer  16  is formed of silicon nitride having a refractive index (n) in the range of 1.8˜2.2 and a thickness in the range of 60˜120 nm. It is noted that no color-modulating layer  26  is formed to overlie the underlying layers to be compared with Examples II, III and IV. Accordingly, the reflective spectrum thereof is measured and illustrated in  FIG. 6 . The CIE L*a*b* values thereof are measured to be 34.92, 1.73 and −29.49, respectively. 
       EXAMPLE II 
       [0036]    The photoelectric conversion layer  11  is made of a silicon layer of a first conductivity type formed in/on a silicon substrate of a second conductivity type. If the first conductivity type is p-type, the second conductivity type is n-type. To the contrary, the second conductivity type is p-type if the first conductivity type is n-type. As an example, the photoelectric conversion layer  11  is formed of silicon has a refractive index (n) in the range of 3.4˜3.6 and has thickness in the range of 140˜250 μm. The anti-reflective layer  16  is formed of silicon nitride having a refractive index (n) in the range of 1.8˜2.2 and a thickness in the range of 60˜120 nm. The color-modulating layer  26  is made of a material having a thickness of about 1,600˜2,000 Å and a refractive index vs. wavelength curve as shown in  FIG. 7 . As such, the reflective spectrum thereof is measured and illustrated in  FIG. 8 . The CIE L*a*b* values are measured to be 56.65, −18,54 and 23.76, respectively. 
       EXAMPLES III 
       [0037]    The photoelectric conversion layer  11  is made of a silicon layer of a first conductivity type formed in/on a silicon substrate of a second conductivity type. If the first conductivity type is p-type, the second conductivity type is n-type. To the contrary, the second conductivity type is p-type if the first conductivity type is n-type. As an example, the photoelectric conversion layer  11  is formed of silicon has a refractive index (n) in the range of 3.4˜3.6 and has thickness in the range of 140˜250 μm. The anti-reflective layer  16  is formed of silicon nitride having a refractive index (n) in the range of 1.8˜2.2 and a thickness in the range of 60˜120 nm. The color-modulating layer  26  is made of a material having a thickness of about 800˜1,200 Å and a refractive index vs. wavelength curve as shown in  FIG. 9 . As such, the reflective spectrum thereof is measured and illustrated in  FIG. 10 . The CIE L*a*b* values are measured to be 22, 14.41 and −8.29, respectively. 
       EXAMPLES IV 
       [0038]    The photoelectric conversion layer  11  is made of a silicon layer of a first conductivity type formed in/on a silicon substrate of a second conductivity type. If the first conductivity type is p-type, the second conductivity type is n-type. To the contrary, the second conductivity type is p-type if the first conductivity type is n-type. As an example, the photoelectric conversion layer  11  is formed of silicon has a refractive index (n) in the range of 3.4˜3.6 and has thickness in the range of 140˜250 μm. The anti-reflective layer  16  is formed of silicon nitride having a refractive index (n) in the range of 1.8˜2.2 and a thickness in the range of 60˜120 nm. The color-modulating layer  26  is composed of multiple layers; that is, three layers are provided in this example. In the example, a first layer is provided with a refractive index (n1) in the range of 2.15˜2.55 and a thickness in the range of 750˜1100 Å; a second layer is provided with a refractive index (n2) in the range of 3.6˜4.0 and a thickness in the range of 1,550˜1,950 Å; a third layer is provided with a refractive index (n3) on the range of 2.15˜2.55 and a thickness in the range of 960˜1360 Å. The first, second and third layers are stacked sequentially from bottom to top. Therefore, the reflective spectrum thereof is measured and illustrated in  FIG. 11 . The CIE L*a*b* values are measured to be 47.05, 28.63 and ˜13.77, respectively. 
         [0039]    The examples given hereinbefore show that the present invention provides those skilled in the art with the means to design solar cells with color-modulating layer having the most simple structure possible and sufficient efficiency, while exhibiting a predetermined color, so that they are well suited to serve as building material or whatever aesthetic appearance of which is an important requirement. 
         [0040]    Although the invention has been described above by the embodiment and the examples, the invention is not limited to the foregoing embodiments and examples but can be variously modified. The material of the color modulation is not always limited to any of the materials in the lists but can be freely sets as long as the external color of the solar cell can be adjusted by using color modulation property of the color-modulating layer  26 . More specifically, the material of the color-modulating layer  26  may be, for example, oxides, fluorides, sulphides, nitrides, tellurides and selenides of a kind other than the kinds listed above, or a material other than oxides, fluorides, sulphides, nitrides, tellurides and selenides. 
         [0041]    Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of appended claims, the invention may be practiced otherwise than as specifically described.