Abstract:
A chemical bath deposition method of depositing on a semiconductor substrate a layer of zinc sulfide by dipping the semiconductor substrate into an aqueous solution of zinc sulfate and thiourea and ammonia.

Description:
[0001]    Chemical bath deposition (CBD) has proved top be the most favourable method for application of a buffer layer to semiconductor substrates, for example, chalcopyrite thin-film solar cells, whereby previously cadmium sulphide (CdS) was deposited and as cadmium is a highly toxic heavy metal, alternatives have been required. According to the invention, the semiconductor substrate is dipped in a solution for approximately 10 minutes, produced by the dissolution of zinc sulphate (0.05-0.5 mol/l) and thiourea (0.2 to 1.5 mol/ml) in distilled water at a temperature being held essentially constant throughout said period. For the first time, the ZnS layer permits comparable or higher efficiencies than conventionally only achieved with toxic cadmium compounds. The method is hence much more environmentally-friendly with the same result. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention, in general, relates to a novel method of applying a zinc sulfide buffer layer to a semiconductor substrate by chemical bath deposition, and, more particularly, to the absorber layer of a thin-film solar cell. The method may also be applied, for instance, in the production of II-IV “superstrat” thin-film solar cells and chalcopyrite “superstrat” thin-film solar cells. 
         [0004]    2. The Prior Art 
         [0005]    Currently, great efforts are being made to develop cost-efficient solar cells. In this respect, chalcopyrite thin-film solar cells represent the latest state of development. In this connection, layers of copper and indium are applied to a substrate. Gallium may be added optionally. The gallium is then reacted with selenium and/or sulfur (CIS, CIGS or CIGSSe, generally CIS solar cells). Among the thin-film technologies, the CIS technology is especially interesting because of its environmental compatibility and because of its low costs. 
         [0006]    The solar cells are produced by initially applying, usually in a high vacuum, an electrically conductive layer, usually a sputtered layer of molybdenum, onto float glass or soda lime glass followed by CIS, CIGS or CIGSSe of a thickness of less than 3 μm. As a rule, a buffer layer of cadmium sulfide is applied to this absorber layer. The cover layer then is a transparent and conductive layer, the so-called window layer. For producing modules, the absorber is applied to a structured glass-molybdenum substrate and is also structured following the application of the buffer layer, resulting in an electrical connection. By contacting and framing the entire structure a thin-film solar module results which serves to convert light energy into electrical power. 
         [0007]    Because of the disadvantages of glass as a substrate various efforts have also been made to use flexible materials instead of glass. Thus, solar modules are known in which a metallic band is used as the substrate. Plastic foils have also become known for the CIS deposition. An electrolytic application of the CIS layer has also been suggested. 
         [0008]    For such thin-film solar cells to operate efficiently, a buffer layer is required between the absorber layer (e.g. Cu(In a Ga b )(S x Se y ) and the window layer (i-ZnO/ZnO:Ga). It protects the surface from damage which may result from the application of the window layer. Furthermore, it provides for surface-doping of the absorber layer as well as for passivation of the surface to yield lower surface conditions. The three usual processes, viz. sputtering, vapor deposition or (electro-)chemical deposition are used to apply this very thin buffer layer. Sputtering and vapor deposition both require considerably complex apparatus. Chemical bath deposition (CBD) up to now precipitating cadmium sulfide (CdS) has so far been most desirable. Moreover, below 50 nm, only CBD results in a thin homogenous and uniform cover. 
         [0009]    Since cadmium is an extremely toxic heavy metal attempts have been made to find alternatives. Thus, instead of cadmium sulfide, zinc sulfide (ZnS) may be used as well (see, e.g. Nakada et al., Polycrystalline (Cu(In,Ga)Se 2  Thin Film Solar Cells with Chemically Deposited Sulfide (CdS, ZnS) Buffer Layers; 2 nd  World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 6-10 Jul. 1998, Vienna. 
         [0010]    However, chemical bath deposition processes using zinc sulfide have good deposition rates only when simultaneously using hydrazine. In such a process, zinc sulfide is precipitated from a solution of zinc acetate, thiourea, ammonia and hydrazine, see e.g. Neve et al., ERDA Analysis of ZnS x (OH) y  Thin Films Obtained by Chemical Bath Deposition, Mat. Res. Symp. Proc. Vol. 668, 2001 Materials Research Society, H5.3.1- H5.3.6 or Ennaoui et al., Highly Efficient Cu(Ga,In)(S,Se) 2  Thin Film Solar Cells with Zinc-Compound Buffer Layers, Thin Solid Films 431-432 (2003) pp. 335-339. According to the tests by Neve et al. ammonia may be left out; however, hydrazine is absolutely necessary. Since, however, hydrazine is classified as toxic and carcinogenic, this process offers not much of an ecological advantage. See Herrero et al. Journal of Electrochemical Society, Vol. 142, No. 3, 1995, S. 764-770. 
       OBJECT OF THE INVENTION 
       [0011]    It is, therefore, an object of the present invention to provide a method of applying a zinc sulfide buffer layer on a semiconductor substrate which requires no toxic or environmentally hazardous additives and which offers a comparable or lesser complexity in terms of equipment and time than current cadmium sulfide application methods. 
       SUMMARY OF THE INVENTION 
       [0012]    In accordance with the invention, the object is accomplished by a method of applying a zinc sulfite layer to a semiconductor substrate by a chemical bath deposition comprising the steps of preparing a first solution from water at 70° C. to 90° C. and 0.05 to 0.5 mol/l of zinc sulfate and 0.2 to 1.5 mol/l of thiourea, adding to the first solution ammonia in a quantity of about one-third of the quantity of water to prepare a second solution and dipping the semiconductor substrate into the second solution for about ten minutes after the second solution has become clear—; 
         [0013]    Accordingly, following the dissolving of zinc sulfate in distilled water at a temperature of 70° C. to 90° C., thiourea is added. The concentration of zinc sulfate (C ZnSO4  should be 0.05-0.5 mil/l, and the concentration of thiourea (C SC(NH2)2 ) should be 0.2-1.5 mol/l. Thereafter, concentrated ammonia (25%) is added (about ⅓ of the quantity of water). A white precipitate is formed which has to be dissolved again by stirring. Before the actual precipitation, the semiconductor substrate was treated with a solution of ammonia and water. Thereafter, it was dipped for about 10 minutes into the chemical bath the temperature of which is maintained constant during this time. During the entire time, the solution is continuously stirred. 
         [0014]    It was surprisingly found that simply by another sequence of the reactions between the three participating substances zinc sulfide, thiourea and ammonia and predetermined process conditions the manner of depositing zinc sulfide changed completely, in that by comparison with the method disclosed by Nakada et al. (supra), the zinc ions initially react with the thiourea (they are complexed) and ammonia is only added thereafter which then also reacts (complexes) with the remaining zinc ions. In this reaction thiourea acts as a complexing agent and as a source of sulfide ions. 
         [0015]    A uniformly thick layer of zinc sulfide is formed. To obtain greater layer thickness, the process may be repeated several times. 
         [0016]    Advantageously, the substrates are thereafter rinsed in a solution of ammonia and water and dried in a stream of nitrogen gas. 
         [0017]    Finally, tempering takes place in air at between 100° C. and 180° C. 
         [0018]    A complete cell is obtained after application of the window layer. 
         [0019]    In respect of CuInS 2  solar cells, an efficiency of more than 10% (active range) can be reached. 
         [0000]    In connection with CuInS 2  solar cells, the following advantages are offered:
       With the ZnS layer it has become possible for the first time to obtained comparable or higher efficiencies than is currently possible with toxic cadmium compounds. At the same yield, the process is significantly more environmentally friendly.   The deposition takes place in one operational step (only a single cycle of ten minutes&#39; duration) so that the process may be automated in a simple manner.   The deposition takes no more time than does the conventional method. Rates of deposition of 1-2 nm/min are achieved. Thick layers in particular are very homogenous and compact. Thus, cell have become possible without the superimposed i-ZnO layer.   The same equipment as heretofore can be used without requiring significant restructuring. “Upscaling” is possible in a simple manner.   Compared to a cadmium sulfide layer, the zinc sulfide layer offers the advantage of a greater band gap so that it is effective in a wider spectral range and delivers a higher current yield.       
 
         [0025]    As a by-product, the method yields high-value ZnS/ZnO powder which may used for other purposes. 
     
    
     
       DESCRIPTION OF THE SEVERAL DRAWINGS 
         [0026]    The novel features which are considered to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, in respect of its structure, construction and lay-out, as well as manufacturing techniques, together with other objects and advantages thereof, will be best understood from the following description when read with reference to the drawings, in which:— 
           [0027]      FIG. 1  depicts a first test arrangement for coating a sample; 
           [0028]      FIG. 2  shows the quantum yield of a sample; and 
           [0029]      FIG. 3  shows the current-voltage-curve of a sample. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    The following layer structure was exemplarily chosen for a chalcopyrite thin-film solar cell (no module). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Substrate: 
                 Glass 
               
               
                   
                 Rear Contact Layer: 
                 Molybdenum 
               
               
                   
                 Absorber: 
                 CuInS 2   
               
               
                   
                 Buffer Layer: 
                 ZnS 
               
               
                   
                 Window Layer: 
                 i-ZnO/ZnO:Ga 
               
               
                   
                 Light-side Contact: 
                 NiAl 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    For applying ZnS to chalcopyrite absorbers the samples 1 are stored in a solution of ammonia and water prior to the chemical bath deposition. 
         [0032]      FIG. 1  depicts the structure of the test. A sample vessel  2  provided with a heater  3  is used. 
         [0033]    The chemical bath depicted in  FIG. 1  is prepared as follows:
       1) A hot solution (70-m90° C.) with 0.05 to 0.5 mol/l zinc sulfate and 0.2 to 1.5 mol/l thiourea is prepared with twice-distilled water (indicated by feed line  4 ).   2) This solution is stirred for some time.   3) NH 3  solution (25%) is then added (indicated by feed line  6 ). A white precipitate can be observed at once; it is dissolved again by continuous stirring. A complete clear solution with a pH value of 10 to 11 results.   4) The samples 1 are fixed in a sample holder and dipped. Stirring of the solution is continued during the deposition and the temperature is maintained.   5) After abut 10 minutes the samples 1 are removed; the initially clear solution begins to turn opaque.   6) The samples 1 are washed in a water/ammonia solution and dried in a stream of nitrogen gas.   7) The resulting layer thickness of the samples 1 on the absorber is about 10-15 nm.   8) The process may be repeated to provide thicker film layers. Three cycles yield a layer thickness of 60 nm. There is no need for rinsing and drying between the cycles.   9) The coated samples 1 are then tempered in air on a heating plate at 100-180° C. for a predetermined time.       
 
         [0043]    The reaction is presumed to proceed as follows: 
         [0000]      Zn 2+   +n TU=[Zn(TU) n ] 2+   (1) 
         [0044]    When adding NH 3  a second complex results from excess Zn 2+ : 
         [0000]      Zn 2+ +4NH 3 =[Zn(NH 3 ) 4 ] 2+   (2) 
         [0045]    The dissociation of the Zn complexes from reaction (1) and (2) yields free Zn cations. Thiourea acts as a complexing agent as well as a source of sulfide ions. 
         [0046]      FIG. 2  depicts the quantum yield attained with the sample (ratio of charges generated relative to the applied light energy. 
         [0047]    The efficiencies reached were at about 10.5% (in the photovoltaic active range).  FIG. 3  depicts the current-voltage-curve of a solar cell measured in a solar simulator (AM 1.5/100 mW/cm 2 ). The short-circuit current density was at about 22.7 mA/cm 2  (relative to the active range) and the open-circuit voltage V oc  at about 700 mV. The filling factor (ratio of the product of current and voltage at an operating point relative to the product of short-circuit current I sc  and open-circuit voltage V oc ) is very high at 65%.