Abstract:
The present invention relates to a method for manufacturing silicon thin-film solar cells, including: providing a substrate; forming a first electrode on the substrate; forming a first doped semiconductor layer on the first electrode by chemical vapor deposition; forming an intrinsic layer on the first doped semiconductor layer by chemical vapor deposition, where the intrinsic layer includes a plurality of amorphous/nanocrystalline silicon layers, and the intrinsic layer has various energy bandgaps formed by varying average grain sizes of the amorphous/nanocrystalline silicon layers; forming a second doped semiconductor layer on the intrinsic layer by chemical vapor deposition, where one of the first doped semiconductor layer and the second doped semiconductor layer is a p-type amorphous silicon layer and the other is an n-type amorphous/nano-microcrystalline silicon layer; and forming a second electrode on the second doped semiconductor layer. Accordingly, the present invention can achieve broadband absorption in a single junction structure.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefits of the Taiwan Patent Application Serial Number 100101245, filed on Jan. 13, 2011, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for manufacturing silicon thin-film solar cells, more particularly, to a method for manufacturing silicon thin-film solar cells with graded bandgaps. 
         [0004]    2. Description of Related Art 
         [0005]    Due to the finite nature of conventional energy such as petroleum and coal, alternative forms of energy have been developed to replace the conventional energy. Among all the forms of alternative energy, solar energy is abundant and environmentally friendly, so research about solar cells has been vigorously pursued. The solar cell is a photovoltaic device to convert light into electricity, and the structurally simplest solar cell is a single junction solar cell. As shown in  FIG. 1 , a single junction solar cell includes: a substrate  11 , a first electrode  12 , a p-type semiconductor layer  13 , an intrinsic layer  14 , an n-type semiconductor layer  15 , and a second electrode  16 . Accordingly, when sunlight passes through the substrate  11  and the first electrode  12 , the PIN junction would absorb light and thus separates electrons and holes. Subsequently, under the intrinsic electric field, the obtained electrons and holes would move to the n-type and p-type semiconductor layers respectively to contribute current. Finally, the current is derived out through electrodes to form electricity for usage or storage. 
         [0006]    Silicon solar cells are the major commercially used solar cells. Among them, amorphous silicon solar cells can be fabricated in lower cost, but exhibit lower photoelectric conversion efficiency than monocrystalline or polycrystalline silicon solar cells and have the drawback of photodegradation. In order to resolve the problems, microcrystalline silicon solar cells with inhibited photodegradation have been developed. 
         [0007]    Additionally, in order to achieve broadband absorption and improved photoelectric conversion efficiency, tandem solar cells have been suggested, such as double junction and triple junction solar cells.  FIG. 2  shows a cross-sectional view of a conventional double junction silicon solar cell. As shown in  FIG. 2 , in the double-junction silicon solar cell, the second microcrystalline silicon PIN junction unit (including a second p-type semiconductor layer  23 ′, a second intrinsic layer  24 ′ and a second n-type semiconductor layer  25 ′) is formed on the first amorphous silicon PIN junction unit (including a first p-type semiconductor layer  23 , a first intrinsic layer  24  and a first n-type semiconductor layer  25 ) on the first electrode  22 . Accordingly, the first amorphous silicon PIN junction unit can absorb light of 350 nm to 800 nm, while the second microcrystalline silicon PIN junction unit can absorb light of 350 nm to 1200 nm, resulting in broadband absorption and enhanced photoelectric conversion efficiency. Moreover, in order to further enhance the photoelectric conversion efficiency, an intermediate layer  27  may be formed between the first amorphous silicon PIN junction unit and the second microcrystalline silicon PIN junction unit to increase the reflection of light. 
         [0008]    Although broadband absorption can be achieved by stacking PIN junction units of different bandgaps, the choice of material used in the tandem solar cells is limited in consideration of current matching, and the method for fabricating the tandem solar cells is more complex. Additionally, the manufacture of the intermediate layer for the enhancement of the photoelectric conversion efficiency is difficult and causes an increase in manufacturing cost. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide a method for manufacturing a silicon thin-film solar cell, which has an absorption layer with graded bandgaps and can achieve broadband absorption in a single-junction structure. 
         [0010]    To achieve the object, the present invention provides a method for manufacturing a silicon thin-film solar cell, including: providing a substrate; forming a first electrode on the substrate; forming a first doped semiconductor layer on the first electrode by chemical vapor deposition; forming an intrinsic layer on the first doped semiconductor layer by chemical vapor deposition, where the intrinsic layer includes a plurality of amorphous/nanocrystalline silicon layers, and the intrinsic layer has various energy bandgaps formed by varying average grain sizes of the amorphous/nanocrystalline silicon layers; forming a second doped semiconductor layer on the intrinsic layer by chemical vapor deposition, in which one of the first doped semiconductor layer and the second doped semiconductor layer is a p-type amorphous silicon layer and the other is an n-type amorphous/nano-microcrystalline silicon layer; and forming a second electrode on the second doped semiconductor layer. 
         [0011]    According to the present invention, an intrinsic layer with graded bandgaps is formed by varying average grain sizes of the amorphous/nanocrystalline silicon layers, such that solar cells with broadband absorption and enhanced photoelectric conversion efficiency can be obtained. Compared to tandem solar cells, the present invention can achieve broadband absorption in a single-junction structure, and thereby the issue of current matching necessarily considered in tandem solar cells is non-existent. In particular, the present invention has the advantages of simple process and low cost. 
         [0012]    In the present invention, examples of the chemical vapor deposition include, but are not limited to, electron cyclotron plasma-enhanced chemical vapor deposition (ECR-CVD), plasma-enhanced chemical vapor deposition (PECVD), hot-wire chemical vapor deposition (HW-CVD), low pressure chemical vapor deposition (LPCVD), and very high frequency-plasma enhanced chemical vapor deposition (VHF-PECVD). 
         [0013]    In the present invention, preferably, the amorphous/nanocrystalline silicon layers are formed by ECR-CVD, and the preferred conditions for forming them are set as follows: the deposition temperature of about 60° C. to 350° C., the microwave power of about 500 W to 1500 W (more preferably, about 700 W to 1500 W), the deposition pressure of about 1 mTorr to 30 mTorr (more preferably, about 10 mTorr to 30 mTorr), and the flow ratio of H 2 /(silicon source+H 2 ) of about 0% to 98% (more preferably, about 90% to 98%). Herein, the silicon source may be SiH 4 . Preferably, the H 2  flow ranges from about 0 sccm to 300 sccm (more preferably, about 45 sccm to 200 sccm; the flow of the silicon source ranges from about 5 sccm to 30 sccm (more preferably, about 5 sccm to 10 sccm); and the flow of the carrier gas (such as He, Ar) ranges from about 10 sccm to 30 sccm. 
         [0014]    In the present invention, the variation of the amorphous/nanocrystalline silicon layers in average grain sizes can be achieved by varying at least one process parameter, for example, flow ratio of H 2 /(silicon source+H 2 ), microwave power, both flow ratio of H 2 /(silicon source+H 2 ) and deposition temperature under CF 4  flow (the CF 4  flow may be about 2 sccm to 5 sccm), both microwave power and deposition temperature, or both microwave power and deposition pressure. In addition, after growing each amorphous/nanocrystalline silicon layer, a process of rapid thermal annealing (RTA) may be optionally performed to stabilize grain size and uniformity. Herein, the RTA process may be performed at about 650-850° C. for about 1-2 minutes. 
         [0015]    In the present invention, the bandgaps of the intrinsic layer may vary in an increasing trend, a decreasing trend, or a decreasing and then increasing trend from the p-type amorphous silicon layer to the n-type amorphous/nano-microcrystalline layer. In the aspect of the gradually decreasing trend and the aspect of the gradually increasing trend, all amorphous/nanocrystalline silicon layers can have the same thickness. Meanwhile, in the aspect of the gradually decreasing and then gradually increasing trend, the amorphous/nanocrystalline silicon layer adjacent to the p-type amorphous silicon layer may have a larger bandgap than the amorphous/nanocrystalline silicon layer adjacent to the n-type amorphous/nano-microcrystalline silicon layer; or the amorphous/nanocrystalline silicon layer adjacent to the p-type amorphous silicon layer may have a smaller bandgap than the amorphous/nanocrystalline silicon layer adjacent to the n-type amorphous/nano-microcrystalline silicon layer; or the amorphous/nanocrystalline silicon layer adjacent to the p-type amorphous silicon layer may have the same bandgap as the amorphous/nanocrystalline silicon layer adjacent to the n-type amorphous/nano-microcrystalline silicon layer. Herein, in the aspect of the decreasing and then increasing trend, all amorphous/nanocrystalline silicon layers may have the same thickness (i.e. the bandgaps are varied in an approximate V-shaped trend); or the amorphous/nanocrystalline silicon layer with the lowest bandgap may have the largest thickness and other amorphous/nanocrystalline silicon layers have the same thickness (i.e. the bandgaps are varied in an approximate U-shaped trend). Preferably, the bangap of the intrinsic layer is varied in an approximate U-shaped trend. 
         [0016]    In the present invention, preferably, the intrinsic layer includes at least three amorphous/nanocrystalline silicon layers, where each amorphous/nanocrystalline silicon layer may range from about 500 Å to 5000 Å in thickness. Accordingly, the intrinsic layer may have a thickness from about 1500 Å to 15000 Å, preferably from about 2500 Å to 8500 Å. 
         [0017]    In the present invention, the p-type amorphous silicon layer may be doped with group III elements, and preferably is formed by ECR-CVD, in which the preferred conditions are set as follows: the deposition temperature of about 60° C. to 450° C., the microwave power of about 500 W to 1500 W, the deposition pressure of about 1 mTorr to 30 mTorr, the flow ratio of H 2 /(silicon source+H 2 ) of about 0% to 98% (the silicon source may be SiH 4 ), and the dopant (such as B 2 H 6 )-containing gas flow of about 5 sccm to 30 sccm (preferably 5 sccm to 15 sccm). Herein, the dopant-containing gas preferably is a carrier gas (such as He, Ar) containing dopant in about 90%-98%. Additionally, the total flow of the silicon source and H 2  gas preferably ranges from 50 sccm to 250 sccm, while the flow of the carrier gas (such as He, Ar) preferably ranges from 10 sccm to 20 sccm. Preferably, the p-type amorphous silicon layer has a thickness from about 50 Å to 500 Å. 
         [0018]    In the present invention, the n-type amorphous/nano-microcrystalline silicon layer may be doped with group V elements, and preferably is formed by ECR-CVD, in which the preferred conditions are set as follows: the deposition temperature of about 60° C. to 450° C., the microwave power of about 500 W to 1500 W, the deposition pressure of about 1 mTorr to 30 mTorr, the flow ratio of H 2 /(silicon source+H 2 ) of about 0% to 98% (the silicon source may be SiH 4 ), and the dopant (such as PH 3 )-containing gas flow of about 5 sccm to 30 sccm (preferably 5 sccm to 15 sccm). In addition, after forming the n-type amorphous/nano-microcrystalline silicon layer, a RTA process may be optionally performed to stabilize grain size and uniformity. Herein, the RTA process may be performed at about 650-850° C. for about 1-2 minutes. The dopant-containing gas preferably is a carrier gas (such as He, Ar) containing dopant in about 0.5%-2% (more preferably about 0.5%-1%). Additionally, the total flow of the silicon source and H 2  gas preferably ranges from 200 sccm to 250 sccm, while the flow of the carrier gas (such as He, Ar) preferably ranges from 10 sccm to 20 sccm. Preferably, the n-type amorphous/nano-microcrystalline silicon layer has a thickness from about 50 Å to 500 Å. 
         [0019]    In the present invention, the p-type amorphous silicon layer may have a bandgap from about 1.5 ev to 1.9 ev. 
         [0020]    In the present invention, each amorphous/nanocrystalline silicon layers of the intrinsic layer may be a layer of an amorphous silicon phase embedded with nanocrystalline silicon particles with an average grain size from about 20 Å to 200 Å (preferably about 80 Å to 180 Å), and may have a bandgap from about 1.3 ev to 1.7 ev. 
         [0021]    In the present invention, the n-type amorphous/nano-microcrystalline silicon layer may be a layer of an n-type amorphous silicon phase embedded with nano-microcrystalline silicon particles with an average grain size from about 20 Å to 400 Å, and may have a bandgap from about 1.1 ev to 1.9 ev. 
         [0022]    In the present invention, the substrate, the first electrode and the second electrode are not particularly limited, and may be any suitable conventional substrate and electrode. For example, when the silicon thin-film solar cell of the present invention is constructed in a superstate structure, the first doped semiconductor layer may be a p-type amorphous silicon layer; the second doped semiconductor layer may be an n-type amorphous/nano-microcrystalline silicon layer; the substrate may be a transparent substrate (such as a glass substrate, a plastic substrate); the first electrode may use a transparent conductive material, therewith the first electrode preferably being a first transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) to allow incident light to pass through the substrate and the first electrode; and the second electrode may be a second transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) or a metal layer (such as a silver layer, an aluminum layer, a copper layer), or the second electrode preferably includes a second transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) and a metal layer (such as a silver layer, an aluminum layer, a copper layer), therewith the second transparent conductive oxide layer being formed between the metal layer and the n-type amorphous/nano-microcrystalline silicon layer. In the case of the silicon thin-film solar cell according to the present invention being constructed in a substrate structure, the first doped semicondutctor layer may be an n-type amorphous/nano-microcrystalline silicon layer; the second doped semiconductor layer may be a p-type amorphous silicon layer; the substrate may be an opaque substrate (such as a stainless steel substrate, a metal substrate); the first electrode may be a first transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) or a metal layer (such as a silver layer, an aluminum layer, a copper layer), or the first electrode preferably includes a first transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) and a metal layer (such as a silver layer, an aluminum layer, a copper layer), therewith the first transparent conductive oxide layer being formed between the metal layer and the n-type amorphous/nano-microcrystalline silicon layer; and the second electrode may use a transparent conductive material, therewith the second electrode preferably being a second transparent conductive oxide layer (such as a zinc oxide layer, a tin oxide layer, an indium tin oxide layer) to allow incident light to pass through the second electrode. 
         [0023]    As above-mentioned, in the present invention, an intrinsic layer with graded bandgaps is formed by varying average grain sizes of the amorphous/nanocrystalline silicon layers, such that solar cells with broadband absorption and enhanced photoelectric conversion efficiency can be obtained. Compared to tandem solar cells, the present invention can achieve broadband absorption in a single junction structure, and thereby the issue of current matching necessarily considered in tandem solar cells is non-existent. In particular, the present invention has the advantages of simple process and low cost. Moreover, in the present invention, the p-type amorphous silicon layer, the amorphous/nanocrystalline silicon layers and the n-type amorphous/nano-microcrystalline silicon layer preferably are formed by ECR-CVD, which has the advantages of low temperature deposition, high plasma density and high deposition rate. Compared to PECVD, by which nano-microcrystalline silicon is grown at an excessively low rate, multilayered thin film is difficult to obtain, and the interface between thin films cannot be easily controlled, the silicon thin-film solar cell according to the present invention can be manufactured by ECR-CVD in low cost, high efficiency and high throughput. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a cross-sectional view of a conventional single junction solar cell; 
           [0025]      FIG. 2  shows a cross-sectional view of a conventional double junction solar cell; 
           [0026]      FIGS. 3A to 3G  show cross-sectional views for illustrating a method for manufacturing a silicon thin-film silicon solar cell according to one preferred example of the present invention; 
           [0027]      FIGS. 4A to 4E  show cross-sectional views for illustrating a method for manufacturing a silicon thin-film silicon solar cell according to another preferred example of the present invention; 
           [0028]      FIGS. 5A to 5C  show cross-sectional views for illustrating a method for manufacturing a silicon thin-film silicon solar cell according to yet another preferred example of the present invention; and 
           [0029]      FIGS. 6A to 6C  show cross-sectional views for illustrating a method for manufacturing a silicon thin-film silicon solar cell according to still another preferred example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Hereafter, examples will be provided to illustrate the embodiments of the present invention. Other advantages and effects of the invention will become more apparent from the disclosure of the present invention. Additional various aspects also may be practiced or applied in the invention, and various modifications and variations can be made without departing from the spirit of the invention based on various concepts and applications. 
         [0031]    In the following examples, a p-type amorphous silicon layer, amorphous/nanocrystalline silicon layers, and an n-type amorphous/nano-microcrystalline silicon layer were formed by electron cyclotron resonance chemical vapor deposition (ECRCVD). Herein, the amorphous/nanocrystalline silicon layers were formed in the condition as follows: the substrate temperature of about 60° C. to 350° C., the microwave power of about 500 W to 1500 W, the deposition pressure of about 1 mTorr to 30 mTorr, the He flow of about 10 sccm to 30 sccm, the SiH 4  flow of about 5 sccm to 30 sccm, and the H 2  flow of about 0 sccm to 300 sccm (i.e. H 2 /SiH 4 +H 2  of about 0% to 98%). Additionally, in the following examples, the variation of the amorphous/nanocrystalline silicon layers in average grain sizes can be achieved by altering (1) the ratio of silicon source to hydrogen, (2) the microwave power, (3) the ratio of silicon source to hydrogen and deposition temperature under CF 4  flow of about 2 sccm to 5 sccm, (4) microwave power and deposition temperature, or (5) microwave power and deposition pressure, such that an intrinsic layer with graded bandgaps can be formed. Moreover, the following examples used an alpha-step profilometer (veeco dektak 6M) to measure the thickness of each thin film and a Raman spectrometer (Raman Imaging Microscope System 1000, available from RENISHAW Co.) to qualitatively analyze the crystal quality and atomic bonding of each silicon film. A board peak can be found at about 480 cm −1  for amorphous silicon phase, while a sharp peak would be found at about 510-520 cm −1  for nanocrystalline or microcrystalline phase. Furthermore, the following examples used an X-ray diffractometer (D &amp; DISCOVER X-ray Diffractometer, available from BRUKER Co.) with a Cu Kα (1.54056 Å) to observe the crystal and to determine its grain size by Scherrer&#39;s equation. 
       EXAMPLE 1 
       [0032]      FIGS. 3A to 3G  show cross-sectional views for illustrating the method for manufacturing a silicon thin-film solar cell according to the present example. 
         [0033]    As shown in  FIG. 3A , a substrate  31  was first provided and a first electrode  32  was formed on the substrate  31 . In the present example, a transparent glass substrate was utilized as the substrate  31 , and transparent ITO material was coated on the substrate  31  by sputtering to form a first transparent conductive oxide layer as the first electrode  32 . Accordingly, incident light can pass through the substrate  31  and the first electrode  32 . 
         [0034]    Next, the present example utilized electron cyclotron resonance chemical vapor deposition (ECRCVD) to grow the subsequent silicon thin films. 
         [0035]    As shown in  FIG. 3B , a first doped semiconductor layer  33  was formed on the first electrode  32  by ECRCVD. In detail, a p-type amorphous silicon layer was grown as the first doped semiconductor layer  33  in the condition as follows: the substrate temperature of about 250° C., the microwave power of about 1000 W, the deposition pressure of about 20 mTorr, the He flow of about 20 sccm, the H 2  flow of about 80 sccm, the SiH 4  flow of about 10 sccm, and the B 2 H 6 :He (98:2) flow of about 5-15 sccm. Accordingly, the present example manufactured a first doped semiconductor layer  33  (i.e. a p-type amorphous silicon layer) of about 500 Å in thickness and about 1.7 eV in bangap. 
         [0036]    Subsequently, as shown in  FIGS. 3C to 3E , the present example utilized ECRCVD and modified the ratio of SiH 4 :H 2  to form an intrinsic layer  34  with graded bandgaps on the first doped semiconductor layer  33 . In detail, a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342  and a third amorphous/nanocrystalline silicon layer  343  with increasing average grain sizes were formed on the p-type amorphous silicon layer in sequence by increasing the ratio of H 2 /SiH 4 +H 2  in a constant condition as follows: the substrate temperature of about 250° C., the microwave power of about 1200 W, the deposition pressure of about 20 mTorr and the He flow of about 16 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of SiH 4  and H 2  respectively being about 5 sccm and 45 sccm (i.e. H 2 /SiH 4 +H 2  being 90%); the second amorphous/nanocrystalline silicon layer  342  of about 1000 Å in thickness, about 100-120 Å in average grain size and about 1.4-1.5 eV in bandgap was grown in the condition of SiH 4  and H 2  respectively being about 5 sccm and 47 sccm (i.e. H 2 /SiH 4 +H 2  being 90.4%); and the third amorphous/nanocrystalline silicon layer  343  of about 1000 Å in thickness, about 150-180 Å in average grain size and about 1.3 eV in bandgap was grown in the condition of SiH 4  and H 2  respectively being about 5 sccm and 50 sccm (i.e. H 2 /SiH 4 +H 2  being 91%). In addition, after forming the first amorphous/nanocrystalline silicon layer  341 , the second amorphous/nanocrystalline silicon layer  342  and the third amorphous/nanocrystalline silicon layer  343 , a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an intrinsic layer  34  with gradually decreasing bandgaps from the side of the p-type amorphous silicon layer in a thickness of about 3000 Å. 
         [0037]    Then, as shown  FIG. 3F , a second doped semiconductor layer  35  was formed on the intrinsic layer  34  by ECRCVD. In detail, an n-type amorphous/nano-microcrystalline silicon layer of about 300 Å in thickness was grown as the second doped semiconductor layer  35  in the condition as follows: the substrate temperature of about 250° C., the microwave power of about 1500 W, the deposition pressure of about 20 mTorr, the He flow of about 20 sccm, the H 2  flow of about 60 sccm, the SiH 4  flow of about 5 sccm, and the PH 3 :He (1:98) flow of about 5-15 sccm. In addition, after forming the n-type amorphous/nano-microcrystalline silicon layer, a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an n-type amorphous/nano-microcrystalline silicon layer of about 300 Å in thickness, about 1.2 eV in bandgap and 250 Å in average grain size as the second doped semiconductor layer  35 . 
         [0038]    Finally, as shown in  FIG. 3G , a second electrode  36  was formed on the second doped semiconductor layer  35 . Herein, ITO (i.e. a second transparent oxide layer  361 ) and Al (i.e. a metal layer  362 ) were coated on the second doped semiconductor layer  35  by sputtering to form as the second electrode  36 . 
       EXAMPLE 2 
       [0039]      FIGS. 4A to 4E  show cross-sectional views for illustrating the method for manufacturing a silicon thin-film solar cell according to the present example. 
         [0040]    As shown in  FIG. 4A , a substrate  31  was first provided and a first electrode  32  was formed on the substrate  31 . In the present example, a stainless steel substrate was utilized as the substrate  31 , and Ag (i.e. a metal layer  321 ) and ZnO (i.e. a first transparent oxide layer  322 ) were coated on the substrate  31  by sputtering to form the first electrode  32 . 
         [0041]    Next, the present example utilized electron cyclotron resonance chemical vapor deposition (ECRCVD) to grow the subsequent silicon thin films. 
         [0042]    As shown in  FIG. 4B , a first doped semiconductor layer  33  was formed on the first electrode  32  by ECRCVD. In detail, an n-type amorphous/nano-microcrystalline silicon layer of 300 Å in thickness was grown as the first doped semiconductor layer  33  in the condition as follows: the substrate temperature of about 250° C., the microwave power of about 1500 W, the deposition pressure of about 20 mTorr, the He flow of about 20 sccm, the H 2  flow of about 60 sccm, the SiH 4  flow of about 5 sccm, and the PH 3 :He (1:98) flow of about 5-15 sccm. In addition, after forming the n-type amorphous/nano-microcrystalline silicon layer, a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an n-type amorphous/nano-microcrystalline silicon layer of about 300 Å in thickness, about 1.2 eV in bandgap and 250 Å in average grain size as the first doped semiconductor layer  33 . 
         [0043]    Subsequently, as shown in  FIG. 4C , the present example utilized ECRCVD and modified the microwave power to form an intrinsic layer  34  with gradually decreasing bandgaps from the side of the n-type amorphous/nano-microcrystalline silicon layer (i.e. gradually increasing bandgaps from the side of the subsequent p-type amorphous silicon layer) on the first doped semiconductor layer  33 . In detail, a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342  and a third amorphous/nanocrystalline silicon layer  343  were formed on the p-type amorphous silicon layer in sequence by modifying the microwave power in a constant condition as follows: the substrate temperature of about 250° C., the deposition pressure of about 20 mTorr, the He flow of about 16 sccm, the H 2  flow of about 45 sccm and the SiH 4  flow of anout 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown under the microwave power of about 700-900 W; the second amorphous/nanocrystalline silicon layer  342  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown under the microwave power of about 1000-1100 W; and the third amorphous/nanocrystalline silicon layer  343  of about 1000 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown under the microwave power of about 1300 W. In addition, after forming the first amorphous/nanocrystalline silicon layer  341 , the second amorphous/nanocrystalline silicon layer  342  and the third amorphous/nanocrystalline silicon layer  343 , a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an intrinsic layer  34  with gradually increasing bandgaps from the side of the p-type amorphous silicon layer in a thickness of about 3000 Å. 
         [0044]    Then, as shown  FIG. 4D , a second doped semiconductor layer  35  was formed on the intrinsic layer  34  by ECRCVD. In detail, a p-type amorphous silicon layer of about 300 Å in thickness was grown as the second doped semiconductor layer  35  in the condition as follows: the substrate temperature of about 250° C., the microwave power of about 1000 W, the deposition pressure of about 20 mTorr, the He flow of about 20 sccm, the H 2  flow of about 80 sccm, the SiH 4  flow of about 10 sccm, and the B 2 H 6 :He (98:2) flow of about 5-15 sccm. Accordingly, the present example manufactured a p-type amorphous silicon layer of about 500 Å in thickness and about 1.7 eV in bandgap as the second doped semiconductor layer  35 . 
         [0045]    Finally, as shown in  FIG. 4E , a second electrode  36  was formed on the second doped semiconductor layer  35 . Herein, ITO (i.e. a second transparent oxide layer) was coated on the second doped semiconductor layer  35  as the second electrode  36  by sputtering. 
       EXAMPLE 3 
       [0046]      FIGS. 5A to 5C  show cross-sectional views for illustrating the method for manufacturing a silicon thin-film solar cell according to the present example. 
         [0047]    First, a structure as shown in  FIG. 5A  was fabricated by the process illustrated in  FIGS. 3A to 3B . 
         [0048]    Subsequently, as shown in  FIG. 5B , the present example utilized ECRCVD and modified the microwave power and the substrate temperature to form an intrinsic layer  34  with bandgaps varied in an approximate V-shaped trend from the side of the p-type amorphous silicon layer on the first doped semiconductor layer  33 . In detail, a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342 , a third amorphous/nanocrystalline silicon layer  343  and a fourth amorphous/nanocrystalline silicon layer  344  were formed on the p-type amorphous silicon layer in sequence by modifying the substrate temperature and the microwave power in a constant condition as follows: the deposition pressure of about 20 mTorr, the He flow of about 16 sccm, the H 2  flow of about 45 sccm and the SiH 4  flow of about 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the microwave power being about 1000 W and the substrate temperature being 300° C.; the second amorphous/nanocrystalline silicon layer  342  of about 1000 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown in the condition of the microwave power being about 1300 W and the substrate temperature being 250° C.; the third amorphous/nanocrystalline silicon layer  343  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the microwave power being about 1100 W and the substrate temperature being 300° C.; and the fourth amorphous/nanocrystalline silicon layer  344  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the microwave power being about 1000 W and the substrate temperature being 300° C. In addition, after forming the first amorphous/nanocrystalline silicon layer  341 , the second amorphous/nanocrystalline silicon layer  342 , the third amorphous/nanocrystalline silicon layer  343  and the fourth amorphous/nanocrystalline silicon layer  344 , a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate V-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 4000 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  344  adjacent to the n-type amorphous/nano-microcrystalline silicon layer had the same bandgap as the first amorphous/nanocrystalline silicon layer  341  adjacent to the p-type amorphous silicon layer. 
         [0049]    Finally, as shown in  FIG. 5C , a second doped semiconductor  35  and a second electrode  36  were formed on the intrinsic layer  34  in sequence by the process illustrated in  FIGS. 3F to 3G  according to Example 1. 
       EXAMPLE 4 
       [0050]    The manufacturing method according to the present example was almost the same as that illustrated in Example 3, except that the first amorphous/nanocrystalline silicon layer  341  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the microwave power being about 1100 W and the substrate temperature being 300° C.; the second amorphous/nanocrystalline silicon layer  342  of about 1000 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown in the condition of the microwave power being about 1300 W and the substrate temperature being 250° C.; the third amorphous/nanocrystalline silicon layer  343  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the microwave power being about 1100 W and the substrate temperature being 300° C.; and the fourth amorphous/nanocrystalline silicon layer  344  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the microwave power being about 1000 W and the substrate temperature being 300° C. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate V-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 4000 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  344  adjacent to the n-type amorphous/nano-microcrystalline silicon layer had a larger bandgap than the first amorphous/nanocrystalline silicon layer  341  adjacent to the p-type amorphous silicon layer, and the first amorphous/nanocrystalline silicon layer  341  had the same bandgap as the third amorphous/nanocrystalline silicon layer  343 . 
       EXAMPLE 5 
       [0051]      FIGS. 6A to 6C  show cross-sectional views for illustrating the method for manufacturing a silicon thin-film solar cell according to the present example. 
         [0052]    First, a structure as shown in  FIG. 4A  was fabricated by the process illustrated in  FIGS. 4A to 4B  according to Example 2. 
         [0053]    Subsequently, as shown in  FIG. 6B , the present example utilized ECRCVD and modified the microwave power and the deposition pressure to form an intrinsic layer  34  with bandgaps varied in an approximate V-shaped trend from the side of the p-type amorphous silicon layer on the first doped semiconductor layer  33 . In detail, a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342 , a third amorphous/nanocrystalline silicon layer  343  and a fourth amorphous/nanocrystalline silicon layer  344  were formed on the p-type amorphous silicon layer in sequence by modifying the deposition pressure and the microwave power in a constant condition as follows: the substrate temperature of about 250° C., the He flow of about 16 sccm, the H 2  flow of about 45 sccm and the SiH 4  flow of about 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the microwave power being about 1200 W and the deposition pressure being 15 mTorr; the second amorphous/nanocrystalline silicon layer  342  of about 1000 Å in thickness, about 150-180 Å in average grain size and about 1.3 eV in bandgap was grown in the condition of the microwave power being about 1500 W and the deposition pressure being 10 mTorr; the third amorphous/nanocrystalline silicon layer  343  of about 1000 Å in thickness, about 100 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the microwave power being about 1200 W and the deposition pressure being 15 mTorr; and the fourth amorphous/nanocrystalline silicon layer  344  of about 1000 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the microwave power being about 1000 W and the deposition pressure being 20 mTorr. In addition, after forming the first amorphous/nanocrystalline silicon layer  341 , the second amorphous/nanocrystalline silicon layer  342 , the third amorphous/nanocrystalline silicon layer  343  and the fourth amorphous/nanocrystalline silicon layer  344 , a process of rapid thermal annealing (RTA) can be optionally performed to stabilize the grain size and uniformity. In RTA, the temperature was set in the range from about 650° C. to 850° C., and the process time was about 1.5 minutes. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate V-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 4000 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  344  adjacent to the p-type amorphous silicon layer had a larger bandgap than the first amorphous/nanocrystalline silicon layer  341  adjacent to the n-type amorphous/nano-microcrystalline silicon layer, and the first amorphous/nanocrystalline silicon layer  341  has the same bandgap as the third amorphous/nanocrystalline silicon layer  343 . 
         [0054]    Finally, as shown in  FIG. 6C , a second doped semiconductor  35  and a second electrode  36  were formed on the intrinsic layer  34  in sequence by the process illustrated in  FIGS. 4D to 4E  according to Example 2. 
       EXAMPLE 6 
       [0055]    The manufacturing method according to the present example was almost the same as that illustrated in Example 3, except that an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer on the first doped semiconductor layer  33  by modifying the ratio of SiH 4 :H 2  and the substrate temperature under constant CF 4  flow. 
         [0056]    In detail, as shown in  FIG. 5B , a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342 , a third amorphous/nanocrystalline silicon layer  343  and a fourth amorphous/nanocrystalline silicon layer  344  were formed on the first doped semiconductor layer  33  in sequence by modifying the ratio of SiH 4 :H 2  and the substrate temperature in a constant condition as follows: the microwave power of about 1150 W, the deposition pressure of about 20 mTorr, the He flow of about 16 sccm and the CF 4  flow of about 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 700 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 300° C.; the second amorphous/nanocrystalline silicon layer  342  of about 2100 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 5 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 97.6%) and the substrate temperature being 250° C.; the third amorphous/nanocrystalline silicon layer  343  of about 700 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 300° C.; and the fourth amorphous/nanocrystalline silicon layer  344  of about 700 Å in thickness, about 80 Å in average grain size and about 1.7 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 10 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 95.2%) and the substrate temperature being 350° C. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 4200 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  344  adjacent to the n-type amorphous/nano-microcrystalline silicon layer had a larger bandgap than the first amorphous/nanocrystalline silicon layer  341  adjacent to the p-type amorphous silicon layer, and the first amorphous/nano crystalline silicon layer  341  has the same bandgap as the third amorphous/nanocrystalline silicon layer  343 . 
       EXAMPLE 7 
       [0057]    The manufacturing method according to the present example was almost the same as that illustrated in Example 1, except that an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer on the first doped semiconductor layer  33  by modifying the ratio of SiH 4 :H 2  and the substrate temperature under constant CF 4  flow. 
         [0058]    In detail, as shown in  FIGS. 3C to 3E , a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342  and a third amorphous/nanocrystalline silicon layer  343  were formed on the first doped semiconductor layer  33  in sequence by modifying the ratio of SiH 4 :H 2  and the substrate temperature in a constant condition as follows: the microwave power of about 1150 W, the deposition pressure of about 20 mTorr, the He flow of about 16 sccm and the CF 4  flow of about 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 700 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 300° C.; the second amorphous/nanocrystalline silicon layer  342  of about 2100 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 5 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 97.6%) and the substrate temperature being 250° C.; and the third amorphous/nanocrystalline silicon layer  343  of about 700 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 300° C. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 3500 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  343  adjacent to the n-type amorphous/nano-microcrystalline silicon layer had the same bandgap as the first amorphous/nanocrystalline silicon layer  341  adjacent to the p-type amorphous silicon layer. 
       EXAMPLE 8 
       [0059]    The manufacturing method according to the present example was almost the same as that illustrated in Example 5, except that an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer on the first doped semiconductor layer  33  by modifying the ratio of SiH 4 :H 2  and the substrate temperature under constant CF 4  flow. 
         [0060]    In detail, as shown in  FIG. 6B , a first amorphous/nanocrystalline silicon layer  341 , a second amorphous/nanocrystalline silicon layer  342 , a third amorphous/nanocrystalline silicon layer  343  and a fourth amorphous/nanocrystalline silicon layer  344  were formed on the first doped semiconductor layer  33  in sequence by modifying the ratio of SiH 4 :H 2  and the substrate temperature in a constant condition as follows: the microwave power of about 1150 W, the deposition pressure of about 20 mTorr, the He flow of about 16 sccm and the CF 4  flow of about 5 sccm. Herein, the first amorphous/nanocrystalline silicon layer  341  of about 700 Å in thickness, about 80-100 Å in average grain size and about 1.6 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 300° C.; the second amorphous/nanocrystalline silicon layer  342  of about 2100 Å in thickness, about 100-120 Å in average grain size and about 1.4 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 5 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 97.6%) and the substrate temperature being 250° C.; the third amorphous/nanocrystalline silicon layer  343  of about 700 Å in thickness, about 90 Å in average grain size and about 1.5 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 8 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 96%) and the substrate temperature being 275° C.; and the fourth amorphous/nanocrystalline silicon layer  344  of about 700 Å in thickness, about 80 Å in average grain size and about 1.6-1.7 eV in bandgap was grown in the condition of the SiH 4  flow and the H 2  flow respectively being about 10 sccm and 200 sccm (i.e. H 2 /SiH 4 +H 2  being 95.2%) and the substrate temperature being 350° C. Accordingly, the present example manufactured an intrinsic layer  34  with bandgaps varied in an approximate U-shaped trend from the side of the p-type amorphous silicon layer in a thickness of about 4200 Å. Herein, the fourth amorphous/nanocrystalline silicon layer  344  adjacent to the p-type amorphous silicon layer had a larger bandgap than the first amorphous/nanocrystalline silicon layer  341  adjacent to the n-type amorphous silicon layer. 
         [0061]    Accordingly, the silicon thin-film solar cell manufactured by the present invention can exhibit broadband absorption from UV to IR, improved photoelectric conversion efficiency, enhanced open-circuit voltage (Voc), increased short-circuit current (Jsc), higher filler factor (FF) and longer lifetime. In particular, the solar cell with bandgaps varied in an approximate U-shaped trend according to the present invention can exhibit the best conversion efficiency, high Jsc, reduced recombination of carriers and enhanced lifetime. 
         [0062]    The above examples are intended for illustrating the embodiments of the subject invention and the technical features thereof, but not for restricting the scope of protection of the subject invention. The scope of the subject invention is based on the claims as appended.