Abstract:
A chemical bath deposition method and a system are presented to prepare different thin films on continuous flexible substrates in roll-to-roll processes. In particular, they are useful to deposit CdS or ZnS buffer layers in manufacture of thin film solar cells. This method and the deposition system deposit thin films onto vertically travelling continuous flexible workpieces delivered by a roll-to-roll system. The thin films are deposited with continuously spraying the reaction solutions from their freshly mixed styles to gradually aged forms until the designed thickness is obtained. The substrates and the solutions are heated to a reaction temperature. During the deposition processes, the front surfaces of the flexible substrates are totally covered with the sprayed solutions but the substrate backsides are remained dry. The reaction ambience inside the reactor can be isolated from the outside atmosphere. The apparatus is designed to generate a minimum amount of waste solutions for chemical treatments.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a chemical bath deposition (CBD) apparatus and a method that can be used to deposit semiconductor thin films onto flexible substrates continuously delivered through a roll-to-roll or reel-to-reel process. It is particularly useful for deposition of cadmium sulfide (CdS) or zinc sulfide (ZnS) semiconductor films for fabrication of thin film solar cells. 
       BACKGROUND 
       [0002]    Photovoltaic technology has been developed under a background of global warming and exhausting of fossil fuels. The substitutes of traditional energy sources include nuclear energy and renewable energy. Among them, the nuclear energy will also be exhausted in the future. Moreover, the potential radiation contamination arising from the nuclear energy may bring serious problems to our environment, especially after the recent accident in a nuclear power station of Japan. Therefore, our future may greatly rely on renewable energy. The photovoltaic devices, or solar cells, are playing a leading role in the renewable energy. This enormous future demand has dramatically pushed the development of photovoltaic technology. At present, the second generation photovoltaic devices, thin film solar cells, have appeared in the global market. They currently include three main types: amorphous silicon, CIGS and CdTe. In this thin film solar cell family, an amorphous silicon cell has a low conversion efficiency that may reach up to 13% for a triple junction design. Besides, it has a problem of power degradation with initial illumination, but its technology is relatively mature. By contrast, a CIGS solar cell possesses the highest conversion efficiency that is as high as 20%, higher than 17% efficiency of the CdTe ones. In the periodic table of the elements, the elements of a CIGS absorber are located in Group IB-IIIA-VIA and the ones of a CdTe absorber in Group IIB-VIA. These absorber materials all belong to multi-component p-type semiconductors. For such a semiconductor material, the distribution of different components and stoichiometry may determine the quality of the material. 
         [0003]    Both of CIGS and CdTe solar cells contain a stack of absorber/buffer thin film layers to create an efficient photovoltaic heterojunction. A metal oxide window containing a highly resistive layer, which has a band gap to transmit the sunlight to the absorber/buffer interface, and a lowly resistive layer to minimize the resistive losses and provide electric contacts, is deposited onto the absorber/buffer surface. This kind of design significantly reduces the charge carrier recombination in the window layer and/or in the window/buffer interface because most of the charge carrier generation and separation are localized within the absorber layer. In general, CIGS solar cell is a typical case in Group IB-IIIA-VIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) elements of the periodic table. These elements are excellent absorber materials for thin film solar cells. In particular, compounds containing Cu, In, Ga, Se and S are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se) 2  or CuIn 1-x Ga x (S y Se 1-y ) n , where 0≦x≦1, 0≦y≦1 and n is approximately 2, and have already been applied in the solar cell structures that gave rise to conversion efficiencies approaching 20%. Here, Cu(In,Ga)(S,Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. It should be noted that the molar ratios of Ga/(Ga+In) and Cu/(Ga+In) are very important factors to determine the compositions and the conversion efficiencies of the CIGS solar cells. In general, a good CIGS solar cell requires a ratio of Cu/(Ga+In) between 0.75 and 0.95, and Ga/(Ga+In) between 0.3 and 0.6. In comparison with CIGS, the composition of a CdTe solar cell is much simple. In general, the content of Cd is close to 50% in the CdTe films. However, the Cd content may change after the deposition of a CdS layer and the subsequent annealing procedure. Close to the interface of the p-n-junction, for example, a CdS x Te 1-x  layer is formed with x usually not exceeding 0.06. However, x has a range changing from 0 to 1, which results in a compound from CdTe (x=0) to CdS (x=1). 
         [0004]    Both CIGS and CdTe films have to be annealed to form a uniform stoichiometric compound. A CIGS film is usually annealed at a temperature between 350 and 600° C. in a typical two-stage fabrication procedure. For a CdTe solar cell, a CdS film may firstly be annealed in a superstrate configuration and a CdS/CdTe bilayer may be annealed in a substrate configuration. After annealing, an n-type semiconductor buffer layer such as CdS, ZnS, or In 2 S 3  should be deposited onto a CIGS semiconductor absorber. After then, transparent conductive oxide (TCO) materials, i.e., ZnO, SnO 2 , and ITO (indium-tin-oxide), should be deposited to form the solar cells. For a CdTe solar cell, CdS may be deposited onto the surface of CdTe or TCO, depending on its substrate or superstrate configuration. However, a superstrate configuration may not be applicable on a flexible workpiece on the basis of current technology. Therefore, CdS should be deposited onto a CdTe surface to fabricate a CdTe thin film solar cell on a flexible substrate. The CdS thickness requirement is similar to a CIGS and a CdTe cell. They usually require a thin layer of CdS with about 100 nm or thinner. 
         [0005]    There are some different technologies to deposit CdS, such as vacuum sputtering and evaporation, spray pyrolysis and chemical bath deposition. Among them, the chemical bath deposition (CBD) seems to produce the best result. In comparison with the vacuum methods, the CBD process is simple and requires cheap equipments. The disadvantage is that it produces lots of cadmium wastes and the post-treatment may be a heavy work. 
         [0006]    A common CdS chemical deposition bath contains a cadmium salt and a thiourea solution in ammonia medium. The ammonia medium in the solution plays two roles. On one hand, it provides OH −  ions to hydrolyze thiourea, which slowly releases S 2−  into the solution. On the other hand, it controls the amount of Cd 2+  cations in the bath through the generation of its tetra-ammonium complexes that decompose to slowly release Cd 2+  into the solution. The slowly created Cd 2+  and S 2−  ions synthesize the CdS, some of which deposits onto the substrate surfaces. The reaction mechanism may be shown below: 
         [0000]      a. [Cd(NH 3 ) 4 ] 2+ ═Cd 2+ +4NH 3   (1)
       the instability constant of [Cd(NH 3 ) 4 ] 2+ : K i =7.56×10 −8 .       
 
         [0000]      b. (NH 2 ) 2 CS+2OH − →CH 2 N 2 +2H 2 O+S 2−   (2)
 
         [0000]      c. Cd 2+ +S 2− →CdS  (3)
 
         [0000]    The stability product of CdS is: K sp =1.4×10 −29  in a strong alkaline solution.
 
There is a possible side reaction below:
 
         [0000]      Cd 2+ +2OH − →Cd(OH) 2   (4)
 
         [0000]    The solubility product constant of Cd(OH) 2  is: K sp =7.2×10 −15 . However, Cd(OH) 2  will dissolve in the ammonia alkaline solution to become [Cd(NH 3 ) 4 ] 2+  complex cations and not affect the quality of the CdS product. 
         [0008]    The CdS precipitation can take place either in the bulk solution to form colloids or at the immersed substrate surface to generate a layer of thin film. According to a main theory, the layer deposition may take three stages: 1). Induction with the ion adsorption and formation of nucleation centers; 2) layer growth with an “ion by ion” mechanism; and 3) layer growth with a “cluster by cluster” mechanism. 
         [0009]    The key technology is that the solution must be freshly prepared to make the initial nucleation of CdS to take place directly on the surface of a substrate. Otherwise, the CdS deposition on the substrate surface may be powdery because the initial induction mainly takes place in the bulk solution and the deposition on the substrate may be directly from the “ion-by-ion” or even “cluster-by-cluster” stages. As reported in a U.S. Pat. No. 7,846,489 B2, which is incorporated herein by reference, the formation of CdS in a reaction solution follows two directions, i.e., a homogeneous particle formation in the solution and a heterogeneous film growth on a substrate surface. No matter which kind of mechanism is involved in the CdS formation in an alkaline solution, i.e., ion-by-ion, hydroxide cluster, or complex-decomposition mechanism, as discussed in literatures, a uniform growth of a CdS thin film on a substrate requires interaction between the substrate surface and un-reacted species firstly. A freshly prepared solution can meet this requirement very well. The reaction starting from a freshly prepared solution is not difficult to achieve in a laboratory work. The sample can be simply immersed into a freshly mixed solution at a certain reaction temperature. In fact, there are lots of literature reports relating to the CdS chemical bath deposition in laboratory works, including some patents such as U.S. Pat. No. 6,537,845 B1 describing a CdS chemical surface deposition onto glass substrates coated with CIGS films. However, it is difficult in an industrial manufacture process, especially for a continuously traveling flexible substrate in a roll-to-roll or reel-to-reel process. How to freshly mix a reaction solution on a moving flexible substrate and remain this mixed solution on the substrate surface to react for a period of time at a constant enhanced temperature is a great challenge for a manufacture design. 
         [0010]    Although there are plenty of patents and publications related to CdS deposition via CBD methods, the apparatus or equipments for industrially manufacturing CdS or ZnS buffer layers in solar cells with a CBD process are seldom reported. Recently, some CBD apparatus and methods were described in U.S. Pat. No. 7,541,067 B2, US. Pat. Appl. Pub. Nos. 2011/0039366 A1, 2009/0255461 A1 and 2009/0246908 A1, which are incorporated herein by references. In these patent and applications, the CdS deposition apparatus were designed for roll-to-roll manufacture processes in fabrications of thin film CIGS solar cells. In these apparatuses, the freshly mixed solutions were delivered onto the surfaces of travelling flexible substrates and remained there during the periods of deposition reactions to grow up the CdS thin films. 
         [0011]    The apparatuses presented in the patents described above require some special designs to remain the solutions on the surfaces of the moving flexible substrates in a roll-to-roll process. For example, the substrates might need to be bent using some magnetic or mechanical forces to hold the solutions on the tops. These applied forces may affect smooth movement of the flexible substrates. It might be difficult to remain the surfaces flat, especially for a wide web. It might always be a great challenge to remain backside of the substrates drying. The present invention may solve these problems. The apparatus and the related method presented here do not have to bend the flexible substrates and fit to different widths. The equipment is very simple, inexpensive but effective. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a roll-to-roll or reel-to-reel chemical bath deposition apparatus and the related method to fabricate semiconductor thin films in preparation of solar cells. It possesses a unique design to vertically deliver a flexible foil through the whole apparatus. The deposition solution is continuously sprayed onto the substrate surface through a whole process from a freshly mixed to an aged stage. During the whole process, the backside of a substrate remains dry. The structure of this equipment is very simple and inexpensive. In addition, the preparation of semiconductor thin films with this apparatus is economical. It also saves volumes of the waste solution to greatly reduce expenses of the waste treatment. 
         [0013]    The present invention can be used for preparing thin film materials onto the surfaces of flexible substrates via a CBD method. It is particularly useful to fabricate CdS or ZnS films in preparation of Group IB-IIIA-VIA and Group IIB-VIA thin film solar cells. In comparison with previous inventions, the presently invented apparatus and method should provide a more convenient and economical way in preparation of thin films through a CBD process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a front view of an apparatus for a CBD deposition with a continuous flexible workpiece through a roll-to-roll process. 
           [0015]      FIG. 2  shows a top view of an apparatus for a CBD deposition with a continuous flexible workpiece through a roll-to-roll process. 
           [0016]      FIG. 3  illustrates a schematic diagram of air-solution spray nozzles. 
           [0017]      FIG. 4  is a schematic diagram illustrating the chemical bath deposition of a CdS buffer layer onto a CIGS solar cell absorber layer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The present invention provides an apparatus and a method for CBD deposition of CdS or ZnS thin films to fabricate Group IB-IIIA-VIA or Group IIB-VIA thin film solar cells in a roll-to-roll process. Different from other previous inventions, the present invention delivers the continuous flexible foils in a vertical position. The whole CBD deposition process is performed with the present apparatus. This process starts from cleaning the substrate, spraying a freshly mixed solution to the substrate surface, continuously spraying the aged solution to the substrate, washing away the reaction solution from the substrate surface while a desired thickness of the film is obtained, rinsing the freshly deposited thin film, and eventually to drying the film at the end. The length of this system depends on the required thickness of thin films. The whole line can be designed with assembled modular sections. The middle sections can be added or removed to adjust the film thickness. There are several more advantages for this CBD reactor. Firstly, the apparatus design is simple to result in an inexpensive fabrication. One does not have to worry how to hold a reaction solution on the substrate surface with the development of a thin film. This reaction style on the substrate top surface requires a complicated mechanical design and significantly increases the cost to fabricate the reactor, especially for a wide substrate. Secondly, the present invention can remain backside of the substrate dry and clean, which may greatly reduce amount of the waste solutions for chemical treatments and make the deposition easier. Thirdly, the present invention separates the waste compartment from the rinsing sections. This also significantly reduces the waste amount and amount of the rinsing water because it can be reused in the waste compartment. 
         [0019]      FIG. 1  shows the front view diagram for a presently invented apparatus. The flexible substrate  100  is loaded into an unwinding section as a roll  100 A, vertically delivered through the entire reactor and ended as a product roll  100 B in the winding section.  101 A and  101 B are guide rollers controlling the substrate movement. In particular,  101 B should be combined with alignment and tension control capability, but the details are not drawn here. Between the rollers  101 A and  101 B, there are a series of idle rollers  101  inside the reactor chamber to hold the substrate vertically flat at correct positions. As shown in  FIG. 1 , the substrate and the rollers are drawn in dashed lines and the substrate surface to be deposited is facing back. The substrate is delivered from left to right along the arrow direction. 
         [0020]    When the roll enters the reactor, it firstly passes a narrow slit on the left wall into the apparatus. Outside the narrow slit, are there a column of wind knifes  102 A to gently blow preheated air into the chamber to avoid the atmosphere inside the chamber coming out. Similarly, a column of wind knifes  102 B are arranged on the other side out of the chamber, as shown in  FIG. 1 . After the roll enters the chamber, it is firstly washed by preheated DI water introduced through the valve  112  and sprayed out of the spray nozzles  105 . The DI water will cover the whole surface from the top to the bottom to clean the surface and wet it before CdS deposition. It also helps to heat the substrate to a reaction temperature. This DI water washing chamber has been separated from the main reaction chamber with a board  117 . The waste water flows through a solution outlet  110  to exhaust without any special treatment. 
         [0021]    The substrate then moves into the CBD deposition section. As illustrated in  FIG. 1 , there is a solution mixing vessel  104  on top of the reactor. Three solution inlets through the valves  112 ,  113  and  114 . Valve  112  delivers the preheated DI water, Valve  113  carries one preheated reaction solution such as Cd 2+  cation ammonia solution, and Valve  114  delivers another preheated solution such as thiourea solution. All of these three solutions are sprayed into the mixing vessel  104 . This mixing vessel possesses a oval or pear shape. The sprayed solutions swirl down around the round internal wall and pass a small opening on the bottom of the vessel. During this process, the solutions have been well mixed in just a couple of seconds. The bottom opening of the vessel connected to a pipe  107  with some spray nozzles  105  to spray the mixed solution onto the substrate surface from the top to the bottom. In order to increase the pressure to spray the solution, an air tube can be introduced to the nozzles as demonstrated on  FIG. 3 . In this device, the solution pipe  302  is incorporated inside an air tube  301 . The solution flows from the top to the bottom, runs out of the nozzles  304  and is sprayed by the compressed air nozzles  303 . This device carries preheated compressed air to help the solution spray, similar to a spray pyrolysis process. The preheated air helps to remain a constant temperature inside the reaction chamber. 
         [0022]    When the freshly mixed solution is sprayed onto the substrate surface, [Cd(NH 3 ) 4 ] 2+  and (NH 2 ) 2 CS will start to adsorb and nucleate on the surface at an induction stage. The used solution flows down into a groove under the substrate and slowly flows to the right direction. Then this used solution is pumped up by a pump  106  and sprayed onto the substrate surface again through the pipe  107 . In this case, the top end of the spray pipe is blocked and the solution is sprayed out with pressure out of the nozzles  105  from the top to the bottom. The reacted solution flows back to the groove again and is pumped up again with next pump, as illustrated in  FIG. 1 . With the substrate movement to the right direction, its surface is covered with the solutions pumped and sprayed from a series of the pump-spray pipe combinations. The substrate surface is continuously reacted from the freshly mixed solution to the gradually aged solution until end of the CBD process. The deposited layer will go through the induction and film growth processes to achieve a high quality thin film. During the whole CBD process, density of the pumps and the spray pipes should be arranged in a way of covering and wetting the full substrate surface. The length of the reaction chamber can be determined by the delivery speed and the reaction time. If a substrate delivery speed is 1 meter per minute and the reaction time is 10 minutes, for instance, a 10 meter long reaction chamber is necessary. 
         [0023]    In  FIG. 1 , the dashed line  119  represents a series of heating elements underneath the solution groove  108 . These heating elements can be fully program-controlled to heat the solution to a constant temperature. In front of the top edge of the substrate, there is a board  111  crossover the whole reaction chamber. It should be designed to block the solution from spraying to the backside. Although this may waste 1-2 cm top edge of the substrate, the edge is usually not useful to fabricate a solar cell. In back of the bottom edge of the substrate, there is a row of wind knifes crossover the whole chamber. It gently blows the preheated air to the back edge of the substrate to avoid the solution wetting onto the back edge. These top and bottom protection may remain the entire backside dry during the whole CBD process. 
         [0024]    At the end of the reaction chamber as separated with a board  116 , the substrate surface is washed with the used DI water through a valve  115 . The DI water here has been used at the next rinsing stage but preheated before it is used in the reaction chamber. Also, the preheated compressed air may be used here to help spraying if necessary. The aged solution and this washing solution are combined here and flow out of the waste outlet  109  on the equipment bottom. This waste solution contains cadmium, sulfur, ammonium and other chemicals. It needs to be seriously treated. 
         [0025]    When the substrate goes through the slit on a separation board  116  into the rinsing chamber, perhaps 99% of the residue of the reaction solution has been washed away in the previous washing stage. In this chamber, the substrate surface is further rinsed twice with clean DI water delivered through the valves  112  to make the deposited film totally clean. The rinsed water is collected through the water outlet  110  on the equipment bottom, and a part of it is reused to wash the substrate in the previous reaction chamber. The cleaned substrate is now travelling out of the reactor. When it goes through the slit in the end wall, it is pre-dried by the wind knifes  102 B and further dried through a heating device  103  before it is winded as a product roll  100 B in the winding section. 
         [0026]    Within the reactor, the atmosphere is controlled at a constant temperature with the heating elements on the bottom and the preheated air, DI water and the solutions. The waste gas containing ammonium is exhaused through the top outlet  118 . The whole process is further demonstrated in  FIG. 2  as a top view. Here the workpiece roll and the rollers are more clearly illustrated. The wind knifes  102 A and  102 B are clearly shown.  120  is the row of wind knifes in back of the bottom edge of the substrate to avoid the solution wetting to the back edge of the substrate, as described above. The width of the aged solution groove  108  is illustrated as well. 
       Example 
     A CdS Film Deposition for Fabrication of a CIGS Solar Cell 
       [0027]    The present invention can be demonstrated with an example to deposit a CdS thin film on the surface of a CIGS absorber layer, as shown in  FIG. 4 . This figure is a schematic diagram illustrating the deposition of a CdS buffer  404  onto a CIGS absorber layer  403  that is coated on a piece of stainless steel substrate  401  isolated with a Mo layer  402 . As an example, a 1000 meter long and 1 meter wide stainless steel roll coated with CIGS absorber layer may be used and delivered at a speed of 1 meter per minute. The main reaction chamber between the separation boards  117  and  116  is designed to be 12 meters long to meet a 12 minutes of reaction time. If the CIGS surface is assumed to be covered with a 3 mm thick solution during the deposition, 3 liters of solution shall be consumed in one minute and totally 3 m 3  of solution has to be used for the whole roll. To increase the efficiency of the spray, one may want to increase the solution consumption to 6 liters per minute. 10 spray nozzles  105  can be arranged from the top to the bottom. In this way, every spray nozzle covers the area of 10×10 cm 2  with 10 ml of solution spraying out every second. However, a solution consumption of 6 liters per minute may not be necessary. One can control the top spray nozzles with more solution volume and gradually reduce the sprayed amount from the top to the bottom because the solution flows down from the top to the bottom. The sprayed solution particle sizes and distribution can be well controlled by controlling the air pressure within the nozzles for the freshly mixed solution and the pump pressures for the aged one. To be careful, one can select a spray rate of 4 liters per minute. The total volume of the reaction solution used to complete the entire roll should be about 4 m 3 . Two starting solutions should be prepared in two mixing tanks and then stored in two storage tanks with heating capabilities. One starting solution is cadmium cation in concentrated ammonium hydroxide. The other starting solution is a concentrated thiourea solution. A small tank such as 500 liters should be enough to mix and store the thiourea solution, but a tank volume larger than 1 m 3  is necessary to mix and store the cadmium ammonium solution. In addition, a large DI water container with a volume of 3 m 3  or more is required for the solution mixing. Two starting solutions and DI water are preheated and delivered at a quantitative ratio and a total volume more than 4 liters per minute to be mixed inside the vessel  104  and sprayed onto the travelling substrate. Turn on all of the wind knifes at a constant air temperature between 70 and 80° C. Meanwhile, the heating elements underneath the solution groove are adjusted to a temperature higher than 80° C. to remain a constant solution temperature between 70 and 80° C. as required. Turn on all the spray pumps  106  to remain the reaction under control. 
         [0028]    At the end of the reaction chamber, the substrate is washed with the preheated DI water that has been used to rinse the substrate surface in next stage. In order to remove the reaction residues from the CdS surface as more as possible, one may use 6 liter of DI water per minute. As a result, 10 liters of the waste solution per minute are poured into the waste outlet  109 . For the whole roll, 10 m 3  of the waste solution will be generated. It suggests that a waste treatment capability of 0.7 m 3  per hour may be necessary. By optimizing the washing process, one could reduce consumption of the washing solution to 4 liters per minute to minimize the waste treatment capability down to 0.5 m 3  per hour. The substrate surface can then be further washed, dried and winded to be a product roll. After completion of the production, one can use some diluted hydrochloric acid to clean the pumps and the spray pipes including the nozzles. 
         [0029]    As the art described and exampled above, the present invention including the system and the process can be used to fabricate many different deposition materials. These deposition materials include but do not limit to: Ag 2 S, Ag 2 Se, AgO, Ag 2 O, Al 2 O 3 , As 2 S 3 , BaO, Bi 2 S 3 , Bi 2 Se 3 , CdS, CdO, CdSe, CdTe, CdZnS, CeO 2 , CoS, CoSe, CoO, CrO 2 , CuBiS 2 , CuGaSe 2 , Cu(In,Ga)Se 2 , CuInS 2 , CuInSe 2 , Cu 2 S, CuS, Cu 2 Se, CuSe, Cu 2 O, CuO, FeO(OH), Fe 2 O 3 , Fe 3 O 4 , Fe 2 S 3 , Fe 3 S 4 , GaAs, GaN, Ga 2 O 3 , GaP, Ga 2 S 3 , GeO 2 , HfO 2 , HgS, HgSe, InGaAs, InAs, In 2 O 3 , InP, In 2 S 3 , In 2 Se 3 , La 2 O 3 , MgO, MnS, MnO 2 , MoS 2 , MoSe 2 , NbO 2 , NbS 2 , NiS, NiSe, NiO, PbHgS, PbS, PbSe, PbTe, PbO 2 , Re 3 , RhO 2 , RuO 2 , Ru 2 O 3 , Sb 2 S 3 , Sb 2 Se 3 , SiGe, SiO 2 , SnS, SnS 2 , SnSe, SnO 2 , TiO 2 , TlS, TlSe, Tl 2 O 3 , VO 2 , WO 3 , Y 2 O 3 , ZnO, ZnS, ZnSe, ZrO 2 , or combinations thereof. 
         [0030]    In particular, this apparatus and process are very useful to deposit CdS, ZnS, In 2 S 3  or CdZnS window layers in manufacture of Group IB-IIIA-VIA and Group IIB-VIA solar cells onto the continuous flexible substrates with different widths. The equipment is easy to make, the process is easy to control, and the waste solutions are less.