Abstract:
The disclosure discloses a method for modifying the light absorption layer, including: (a) providing a substrate; (b) forming a light absorption layer on the substrate, wherein the light absorption layer includes a Group IB element, Group IIIA element and Group VIA element; (c) forming a slurry on the light absorption layer, wherein the slurry includes a Group VIA element; and (d) conducting a thermal process for the light absorption layer with the slurry.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 100149119, filed on Dec. 28, 2011, the entirety of which is incorporated by reference herein. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for modifying a light absorption layer, and in particular relates to a method for modifying a CIGS light absorption layer. 
     BACKGROUND 
     Description of the Related Art 
     Technological development in the solar cell industry is driven by global environmental concerns and raw material prices. Among the various solar cells developed, CIGS thin film (Cu(In, Ga)Se 2 ) solar cells have become the subject of considerable interest due to advantages of high conversion efficiency, high stability, low cost, and large area fabrication ability. 
     A CIGS compound is made by a Group IB-IIIA-VIA compound and has a chalcopyrite structure. The Group IB-IIIA-VIA compound is a direct band gap semiconductor material that is used as a light absorbing layer of a CIGS solar cell. By changing the element ratios of the CIGS compound, the band gap of the semiconductor material can be regulated. 
     In a conventional method for forming the CIGS compound, a CIGS light absorption layer is firstly formed and then a sulfurization step is conducted to increase the interface band gap. 
     However, the hydrogen sulfide (H 2 S) gas used in the sulfurization step has high toxicity and high pollution, and the cost of the hydrogen sulfide (H 2 S) gas is expensive making fabrication costs high. Furthermore, because the reaction between the hydrogen sulfide (H 2 S) gas and the CIGS light absorption layer is a solid-gas reaction, if the gas is non-uniformly distributed in the reactin chamber, a poor uniformity of the light absorption layer will be obtained. 
     SUMMARY 
     The present disclosure provides a method for modifying the light absorption layer, including: (a) providing a substrate; (b) forming a light absorption layer on the substrate, wherein the light absorption layer comprises a Group IB element, Group IIIA element and Group VIA element; (c) forming a slurry on the light absorption layer, wherein the slurry comprises a Group VIA element; and (d) conducting a thermal process for the light absorption layer with the slurry. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. This description is made for the purpose of illustrating the general principles of the disclosure and should not be taken in a limiting sense. 
     The disclosure provides a method for modifying the light absorption layer. A light absorption layer (CIGS layer) is firstly formed and then a slurry is coated on the light absorption layer. Then, a thermal process is conducted for the light absorption layer with the slurry to modify the light absorption layer. Thus, a CIGSS layer is formed from the CIGS layer. 
     The method for modifying the light absorption layer comprises step (a)-step (d). Firstly, in step (a), a substrate is provided, and the substrate comprises molybdenum (Mo), silver (Ag), aluminum (Al) or combinations thereof. 
     Then, in step (b), the light absorption layer is formed on the substrate. The light absorption layer comprises a Group IB element, Group IIIA element and Group VIA element. Thus, the light absorption is made by a Group IB-IIIA-VIA compound. 
     The Group IB element comprises copper (Cu), silver (Ag), gold (Au) or combinations thereof. The Group IIA element comprises aluminum (Al), indium (In), gallium (Ga) or combinations thereof. 
     The Group VIA element comprises a sulfur (S), selenium (Se), tellurium (Te) or combinations thereof. 
     In one embodiment, the Group compound is CuInGaSe 2 . In another embodiment, the Group IB-IIIA-VIA compound is CuInGaS 2 . 
     The light absorption layer is formed by a method comprising a vapor deposition method, sputter method, electrodeposition method or coating method. Note that the deposition methods are not limited to the above-mentioned methods, and other deposition methods are also included in the scope of the disclosure. 
     In one embodiment, an molybdenum (Mo) substrate is placed in a vapor deposition chamber, and the copper (Cu), indium (In), gallium (Ga) and selenium (Se) elements are deposited on the molybdenum (Mo) substrate by a heating system. 
     The Group IB element, Group IIIA element, and Group VIA element have a mole ratio of about (0.7-1.4):(0.7-1.4):(0.005-2.2), and preferably (0.7-1.3):(0.7-1.3):(0.006-2.2), and more preferably (0.8-1.3):(0.8-1.3):(0.008-2.2). 
     Then, in step (c), a slurry is formed on the substrate. The slurry comprises a Group VIA element and the Group VIA element comprises a sulfur (S), selenium (Se), tellurium (Te) or combinations thereof. 
     Note that the Group VIA element in the light absorption layer is different from the Group VIA element in the slurry. In one embodiment, when the light absorption layer comprises a sulfur (S) element, a slurry containing selenium (Se) is used. In another embodiment, when the light absorption layer comprises a selenium (Se) element, a slurry containing sulfur (S) is used. 
     Additionally, in yet another embodiment, when the light absorption layer is a CZTS compound containing sulfur (S), a slurry containing selenium (Se) is used. Alternatively, when the light absorption layer is a CZTS compound containing selenium (Se), a slurry containing sulfur (S) is used. 
     Furthermore, the slurry further comprises a solvent, and the solvent comprises water, alcohol-like solvent, ketone solvent, ether solvent, amine solvent, acid solvent or base solvent. 
     The alcohol-like solvent comprises methanol, ethanol, propanol, isopropanol, 1-butanol, isopentanol or ethylene glycol. The ketone solvent comprises acetone, butanone or methyl isobutyl ketone. The ether solvent comprises methyl ether, ethyl ester, methyl ethyl ether, diphenyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether or ethylene glycol monoethyl ether acetate. The amine solvent comprises ethylamine, dimethyl acetamide, triethanol amine or diethanol amine. The acid solvent comprises nitric acid, hydrochloric acid, sulfuric acid, acetic acid or pyruvic acid. The base solvent comprises sodium hydroxide (NaOH), potassium hydride (KOH), lithium hydride (LiOH), urea (CON 2 H 4 ), ammonium water (NH 3 ), sodium bicarbonate (Na 2 CO 3 ), hydrated sodium carbonate (NaHCO 3 ) or combinations thereof. 
     Note that the solvents are not limited to the type of solvents mentioned herein, as other single solvents or mixed solvents which can dissolve the above compounds may be implemented, and are within the scope of this disclosure. 
     In one embodiment, the slurry comprises water and sulfur powder. Additionally, additives may be added into the slurry according to actual applications. For example, a thickener may be added into the slurry to adjust the viscosity and adhesion of the slurry for subsequent coating processes. 
     The slurry is formed by a method comprising a capillary coating method, spin coating method, brush coating method, knife coating method, spraying method or printing method. Additionally, the slurry is formed at a temperature of 100° C.-200° C., and preferably 150° C.-170° C. The slurry has a thickness of about 10 nm-1000 nm, and preferably 300 nm-700 nm. 
     Next, in step (d), a thermal process is conducted for the light absorption layer with the slurry. The thermal process is conducted in an atmosphere which comprises air, nitrogen (N 2 ), hydrogen (H 2 ), argon (Ar), ammonia (NH 3 ), gas containing a Group IIIA element, or combinations thereof. 
     The atmosphere pressure of the thermal process was about 760 torr-10 −7  torr, and preferably 760 torr-10 −4  torr. The thermal process is conducted at a temperature of 300° C.-600° C., and preferably 450° C.-550° C. The thermal process is conducted for 10 second-8 hours, and preferably 1 minute-60 minutes. 
     Note that in prior art, the hydrogen sulfide (H 2 S) gas is used in the sulfurization step. In the disclosure, the slurry containing a Group VIA element instead of the hydrogen sulfide (H 2 S) gas is coated on the light absorption layer by a wet coating method. The light absorption layer is then modified by the thermal process. 
     Compared with prior art, the hydrogen sulfide (H 2 S) gas is not used in the sulfurization step in the disclosure, and thus high toxicity and high fabrication costs are avoided. Additionally, the coating method may be applied to a large-area, and thus the uniformity of the light absorption layer is also improved. 
     An interface band gap and the open circuit voltage (V oc ) of the solar cell containing the light absorption layer of the disclosure may be improved by the above-mentioned modifying method. Indeed, the experimental data also shows that the open circuit voltage (V oc ) of the solar cell containing the light absorption layer of the disclosure was improved. 
     EXAMPLE 
     Example 1 
     The molybdenum (Mo) was coated on a soda-lime glass (SLG) substrate by a sputter method. Then, the Mo/SLG substrate was placed in a vapor deposition chamber, and the copper (Cu), indium (In), gallium (Ga) and selenium (Se) elements were deposited on the Mo/SLG substrate by a heating system to form a CIGS precursor film. 
     Then, the slurry containing sulfur (S) element was coated on the CIGS precursor film by a capillary coating method. The CIGS precursor film was placed on a heater plate and the slurry was dipped on it. Then, a glass cover plate was put above the CIGS precursor film. The heater was heated to about 115.2° C. (higher than the melting point of sulfur (S)), and thus the slurry containing sulfur (S) element was uniformly distributed on the CIGS precursor film by capillary force and the solvent in the slurry was removed by the heater. 
     Then, after the CIGS precursor film was placed and a temperature thereof was at room temperature, the glass cover was removed. Next, a thermal process was conducted to the CIGS film at 550° C. under 10 −4  torr pressure for 10 minutes to obtain a CIGSS absorption layer. 
     Example 2 
     The CdS (as buffer layer), iZnO/AZO (as transparent conducting layer) and top electrode were sequential formed on the light absorption layer to form a solar cell. The solar cell was divided into six smaller solar cells (Cell 1-Cell 6). Table 1 shows that photoelectric efficiency test of the six solar cells. 
     Table 1 shows the open-circuit voltage (V oc ), short-circuit current (J sc ), fill factor, photoelectric conversion efficiency (%), series resistance (R s ) and shunt resistance (R sh ) of Example 1. The open-circuit voltage (V oc ) was about 0.56-0.59 V, the short-circuit current (J sc ) was about 20-24 mA/cm 2 , the fill factor was about 67-69, and the photoelectric conversion efficiency (%) of the Example 1 was about 8-9.2%. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 photoelectric 
                   
                   
               
               
                   
                 V oc   
                 J sc   
                 F.F. 
                 conversion 
                 R sh   
                 R s   
               
               
                   
                 (V) 
                 (mA/cm 2 )  
                 (%) 
                 efficiency (%) 
                 (Ohm) 
                 (Ohm) 
               
               
                   
               
             
             
               
                 Cell 1 
                 0.58 
                 20.492 
                 69 
                 8.242 
                 3595 
                 11 
               
               
                 Cell 2 
                 0.58 
                 22.274 
                 69 
                 8.910 
                 2147 
                 10 
               
               
                 Cell 3 
                 0.59 
                 20.382 
                 69 
                 8.396 
                 4346 
                 11 
               
               
                 Cell 4 
                 0.56 
                 21.839 
                 67 
                 8.309 
                 1560 
                 10 
               
               
                 Cell 5 
                 0.57 
                 23.774 
                 68 
                 9.189 
                 1588 
                 10 
               
               
                 Cell 6 
                 0.58 
                 21.542 
                 69 
                 8.596 
                 2430 
                 11 
               
               
                   
               
             
          
         
       
     
     Comparative Example 
     The molybdenum (Mo) was coated on the soda-lime glass (SLG) substrate by a sputter method. Then, the Mo/SLG substrate was placed in a co-vapor deposition chamber, and the copper (Cu), indium (In), gallium (Ga) and selenium (Se) elements were deposited on the Mo/SLG substrate by a heating system to form a CIGS light absorption layer. 
     Then, the CdS (as buffer layer), iZnO/AZO (as transparent conducting layer) and top electrode were sequential formed on the CIGS light absorption layer to form a solar cell. The solar cell was divided into six smaller solar cells. Table 2 shows that photoelectric efficiency of the solar cells (Cell 1-Cell 6). 
     The difference between Example 2 and Comparative Example is that the light absorption layer of Comparative Example was not modified. As shown in Table 1 and Table 2, the open circuit voltage (V oc ) of Example 2 is indeed higher than that of Comparative Example. Thus, the interface band gap and the open circuit voltage (V oc ) were improved by modifying the light absorption layer. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 photoelectric 
                   
                   
               
               
                   
                 V oc   
                 J sc   
                 F.F. 
                 conversion 
                 R sh   
                 R s   
               
               
                   
                 (V) 
                 (mA/cm 2 )  
                 (%) 
                 efficiency (%) 
                 (Ohm) 
                 (Ohm) 
               
               
                   
               
             
             
               
                 Cell 1 
                 0.53 
                 19.708 
                 63 
                 6.547 
                 1026 
                 12 
               
               
                 Cell 2 
                 0.55 
                 20.718 
                 64 
                 7.265 
                 1096 
                 12 
               
               
                 Cell 3  
                 0.56 
                 18.589 
                 64 
                 6.701 
                 1062 
                 13 
               
               
                 Cell 4 
                 0.54 
                 18.768 
                 64 
                 6.487 
                 1397 
                 13 
               
               
                 Cell 5 
                 0.56 
                 19.024 
                 65 
                 6.911 
                 1263 
                 14 
               
               
                 Cell 6  
                 0.56 
                 16.955 
                 65 
                 6.158 
                 1254 
                 14 
               
               
                   
               
             
          
         
       
     
     Example 3 
     The fabrication method of Example 3 was the same as that of Example 2, except that the thermal process of Example 3 was conducted under 1 torr pressure. Table 3 shows the open-circuit voltage (V oc ), short-circuit current (J sc ), fill factor, photoelectric conversion efficiency (%), series resistance (R s ) and shunt resistance (R sh ) of Example 3. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                 photoelectric 
                   
                   
               
               
                   
                 V oc   
                 J sc   
                 F.F. 
                 conversion 
                 R sh   
                 R s   
               
               
                   
                 (V) 
                 (mA/cm 2 ) 
                 (%) 
                 efficiency (%) 
                 (Ohm) 
                 (Ohm) 
               
               
                   
               
             
             
               
                 Cell 1 
                 0.55 
                 22.437 
                 65 
                 7.938 
                 2837 
                 12 
               
               
                 Cell 2 
                 0.56 
                 23.203 
                 65 
                 8.471 
                 5284 
                 12 
               
               
                 Cell 3 
                 0.55 
                 21.534 
                 64 
                 7.567 
                 3355 
                 13 
               
               
                 Cell 4 
                 0.56 
                 22.413 
                 66 
                 8.334 
                 4137 
                 11 
               
               
                 Cell 5 
                 0.57 
                 24.239 
                 66 
                 9.134 
                 4305 
                 11 
               
               
                 Cell 6 
                 0.56 
                 21.813 
                 65 
                 7.904 
                 2978 
                 12 
               
               
                   
               
             
          
         
       
     
     While the disclosure has been described by way of example and in terms of the preferred embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.