Abstract:
Conventionally, CdTe solar cells are grown in superstrate configuration where the light enters the photovoltaic device through a transparent substrate. Still, efficiencies of CdTe solar cells grown in substrate configuration have so far been considerably lower than those grown in superstrate configuration. This invention discloses a photovoltaic device ( 0 ) in substrate configuration and a process of making thereof with which efficiencies approaching those of superstrate devices can be reproducibly achieved. Furthermore, long term stability is expected to be better than in state of the art devices. This method is advantageous because the growth in substrate configuration offers several advantages like the growth on metal foils and a more precise control of the junction.

Description:
TECHNICAL FIELD 
       [0001]    The present invention describes a method for production of a photovoltaic device in substrate configuration, comprising a p-semiconducting absorber layer which is deposited on a back contact coated substrate and a window layer, wherein a first treatment of the absorber layer in the form of an annealing in the presence of chlorine containing species at temperatures between 350° C. and 500° C. for time periods of minutes up to hours leading to recrystallization and grain growth of the absorber layer, a deposition of the window layer on the pretreated absorber layer, comprising n-type semiconducting material and later deposition of a transparent contact layer onto the window layer is carried out. 
         [0002]    Furthermore it describes a multi layer thin film solar cell with a p-type semiconducting absorber layer grown on a back contact coated substrate, on which a n-type semiconducting window layer is deposited in substrate configuration, comprising an amount of a metal component. 
       STATE OF THE ART 
       [0003]    Increasing oil prices are emphasizing the need of low cost solar electricity. While currently most photovoltaic electricity is produced from crystalline silicon, thin film technologies like Cu(In,Ga)Se2, amorphous Si, or CdTe are increasing their market share. CdTe comprising photovoltaic devices have proven to be one of the cheapest photovoltaic technologies due to the simplicity of the production process. The company First Solar is producing more than 1 GWp per year at prices well below 1 $/Wp. 
         [0004]    Further price reduction can be expected by producing flexible CdTe solar cells by roll-to-roll production using CdTe solar cells in substrate configuration. 
         [0005]    As disclosed in Singh et al., “Thin film CdTe-CdS herterojunction solar cells on lightweight metal substrates”, Solar Energy Materials &amp; Solar Cells 59 (1999), a photovoltaic device in substrate configuration, based on a thin metal foil as substrate was achieved. By introducing a conductive interlayer between substrate and CdTe absorber layer, before depositing the CdTe absorber layer onto the substrate, the electrical properties could be improved. The electrical contact between the substrate and the absorber layer could therewith be improved. The reachable efficiency was still not satisfying. Due to the fact that the absorber-window junction is crucial to the cell performance, Singh et al. describe the deposition of up to two CdS layers followed by annealing steps to reach optimal formation of CdTe and CdS in the region of the junction. 
         [0006]    All the produced CdTe solar cells in substrate configuration so far exhibit lower efficiencies compared to their superstrate counterpart. Up to 17.3% efficiency has been achieved in superstrate configuration (15.6% in our lab), while for example Gessert et al. [“Comparison of CdS/CdTe superstrate and substrate devices fabricated with a ZnTe:Cu contact interface”, Proceeding of IEEE PV Specialists Conference 2010, p335] report efficiencies well below 10% for CdTe solar cells in substrate configuration. It is well known by the skilled persons, that processes developed in superstrate configuration cannot be simply applied in substrate configuration. The differing deposition sequence of CdS on CdTe or CdTe on CdS, leads to differing manufacturing methods, where appropriate materials have to be used. 
         [0007]    Due to the inferior reachable efficiency of the photovoltaic devices in substrate configuration, the document of Gessert et al. could be understood as an advice against the substrate configuration, which seems to be more problematic to be controlled. 
         [0008]    On the other hand the conventional superstrate configuration, where light has to pass through the substrate in the finished device, prohibits the use of flexible metal foil substrates and leads to considerable absorption losses when cells are grown on flexible polyimide foil. 
         [0009]    In state of the art CdTe solar cells in substrate configuration, Cu is added at the back contact e.g. in the form of CuTe x  or Cu doped ZnTe with equivalent layer thicknesses of at least several nanometers. Long term stability of CdTe solar cells is strongly affected by the amount of Cu used in the back contact as excessive Cu diffusion towards the junction can deteriorate cell performance. On the one hand, enough Cu has to be added to achieve highest efficiencies, on the other hand, addition of Cu leads to inferior long term stability. Therefore, in state of the art CdTe solar cells the reproducibility is a problem and it is difficult to combine highest performance with long term stability. 
         [0010]    Another idea to reach stable CdTe solar cells with good efficiencies, in WO2011/044382 special porous back substrates are introduced formed from graphite foils. By using the porous graphite foils, metal substrates and therewith uncontrollable copper diffusion from a metal substrate into the absorber layer should be avoided. The porous substrate is chosen to modify physical and/or chemical properties of the CdTe absorber layer proximate the porous conductive substrate by introducing metallic species through the pores of the substrate. As be stated in WO2011/044382 different deposition steps or techniques are more complicated respectively the results are disadvantageous and leading also to irreproducible solar cells. The surface change of the graphite foil surface may cause cracks in the later CdTe layer, what results in degradation of the solar cell properties. 
       DESCRIPTION OF THE INVENTION 
       [0011]    The object of the present invention is to create a method for production of a photovoltaic device in substrate configuration with increased reproducibility for simple low-priced manufacturing of photovoltaic devices with high efficiencies, e.g. exceeding 13%, and fill factors above 70% on preferably flexible substrates. Flexible CdTe absorber photovoltaic devices on cheap metal or plastic foil as substrate with efficiencies approaching those on rigid glass can be achieved produced in a roll-to-roll process. 
         [0012]    In state of the art CdTe solar cells in substrate configuration, copper was added on the back contact in high amounts, improving the ohmic contact between back contact and CdTe absorber layer. We have found that the addition of much smaller amounts of conductive material at a later processing step and a subsequent diffusion of the conducting material into all underlying layer, comprising window layer, absorber layer and back contact, considerably enhances performance of the devices and reproducibility of the process. The prior art documents neither disclose nor suggest the here proposed order of deposition in substrate configuration with subsequent diffusion in order to distribute a conducting material in all underlying layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Preferred production methods and the resulting photovoltaic devices are described below in conjunction with the attached drawings. 
           [0014]      FIG. 1  shows a schematic sectional view of one embodiment of a photovoltaic device in substrate configuration. 
           [0015]      FIG. 2   a  shows a schematic sectional view of a basic method of stepwise production, while 
           [0016]      FIG. 2   b  shows a schematic view of step b′, where the metal component is deposited at first and 
           [0017]      FIG. 2   c  shows a schematic view of a varied step b″ where the n-type semiconducting material is deposited at first. 
           [0018]      FIG. 3   a  shows a schematic sectional view of another embodiment of a production method, depositing a second n-type semiconducting material layer, after a further treatment step. 
           [0019]      FIG. 3   b  shows a TEM image of a doubled window layer, after deposition of a second n-type semiconductor layer, comprising one large grained pretreated CdS layer and one nano-grained CdS resulting of the second deposition step. 
           [0020]      FIG. 3   c  shows a diagram of the depth dependant metal component (in this case Cu) distribution in different layers of the photovoltaic device after production according  FIG. 3   a ), while 
           [0021]      FIG. 3   d  is showing the copper concentration in the doubled CdS window layer magnified, comprising large grained CdS and nano-grained CdS and the difference in copper concentration. 
           [0022]      FIG. 3   e  shows a XRD pattern, showing that the first window layer comprising as deposited CdS grows in a phase which differs from the phase of the optimized double-CdS layer. 
           [0023]      FIG. 4  shows a I/V diagram where a photovoltaic device produced with the method according to  FIG. 2   a  is compared with a photovoltaic device with a double CdS layer produced with the method according to  FIG. 3   a.    
       
    
    
     DESCRIPTION 
       [0024]    A photovoltaic device  0  is shown in  FIG. 1  in a schematic cross section. The photovoltaic device  0  is a multilayer thin film device consisting of several semiconductor and metal layers.  FIG. 1  shows the photovoltaic device  0  in the form of a CdTe/CdS solar cell in substrate configuration, where the reference numerals are roughly characterizing the order of deposition. 
         [0025]    On a substrate  1  a conducting back contact  2  is deposited. Onto the back contact  2  an absorber layer  3 , here in the form of CdTe as p-type semiconducting material is added. Between the back contact  2  and the absorber layer  3  a back contact-absorber interface  23  is formed. Placed onto the absorber layer  3  on the side facing away from the back contact  2  is a window layer  4 , comprising n-type semiconducting material  41  and a metal component  40 . The junction between absorber layer  3  and window layer  4  forms the area of the absorber-window-junction or pn-junction  34 . On top of the window layer  4  a transparent contact layer  5  in form of at least a transparent oxide layer is deposited, through which light enters the finished photovoltaic device  0 . A metallic grid can optionally be deposited on top of the photovoltaic device  0  to improve the transparent contact resistance. All these layers are subsequently deposited onto each previous layer. Each layer can cover all or a part of the underlying layer. Due to preferably chosen flexible substrates  1  and very thin deposited layer  2 ,  3 ,  4  and  5 , the whole photovoltaic device  0  is flexible respectively bendable and can be used in a roll-to-roll process. 
         [0026]    Because light in this configuration does not have to pass through the substrate  1 , opaque substrates  1  like metal foils can be used. This would enable roll-to roll production, which can considerably reduce manufacturing cost of the devices  0 . It is common practice to use a multilayer back contact comprising at least a conductive layer and at least a buffer layer, which is not depicted here. Furthermore, a barrier layer can be introduced between the substrate and the conducting back contact, which is not depicted here. 
         [0027]    The substrate  1  is preferably flexible comprising a polymer or metal foil. The back contact can comprise metals like Mo, W, Al, Cr, Ni, Ti, Ag, Au or Pt, oxides like ZnO:Al, tin doped indium oxide or fluorine doped tin oxide, nitrides like MoN or TiN or combinations thereof. The p-type semiconductor absorber can be made of CdTe or a mixture comprising CdTe. The n-type semiconducting material of the window layer can be made of CdS or a mixture comprising CdS. 
         [0028]    Each underlying layer has to withstand the temperatures during the subsequent depositions. This leads to different diffusion of impurities or interdiffusion of different layers as compared to the conventional superstrate configuration. Therefore the new processes had to be invented specially for the growth in substrate configuration. 
         [0029]    In the following a method for production of a photovoltaic device  0  by means of an absorber layer  3  comprising CdTe as p-type semiconductor, a metal component  40  consisting of copper and a window layer  4  comprising CdS as n-type semiconducting material  41  only in substrate configuration is described. With treatment or pretreatment a conditioning of the topmost layers in the form of an annealing in the presence of chlorine containing species, for example CdCl 2  or Freon® gas, preferably CdCl 2  at temperatures between 350° C. and 500° C. for time periods of minutes up to hours is meant. The treatment atmosphere can contain oxygen. 
         [0030]    Annealing means the annealing of the topmost layers with at least 180° C. for time periods of minutes up to hours. The annealing atmosphere can contain oxygen. 
         [0031]    A so called diffusion treatment can be performed by the explained annealing or the explained treatment. The diffusion treatment is a tempering process which also leads to diffusion of deposited metal atoms into the underlying layer of the photovoltaic device  0 , namely the window layer  4 , absorber layer  3  and the back contact  2 . Such a diffusion is impossible in a superstrate configuration, where the CdTe layer is deposited after a copper doped CdS layer deposition and following diffusion treatment step is carried out. 
         [0032]    For deposition of a polycrystalline thin-film of CdTe as absorber layer  3  the person skilled in the art uses for example close spaced sublimation, electrodeposition, sputtering, high vacuum thermal evaporation methods, vapor transport deposition, metal organic vapor deposition, spray deposition, screen printing, nanoparticle based approach or other chemical methods. The preferred deposition method is high vacuum evaporation at a substrate temperature of about 300° C. 
         [0033]    Starting with the already deposited CdTe layer  3  typically with a thickness of a few micrometers, preferred 5 μm, a first treatment a) step of the surface of the absorber layer  3  is carried out. The first treatment a) comprises an annealing of the absorber layer  3  in the presence of CdCl 2  at temperatures of higher than 350° C. for minutes to hours. The atmosphere can contain oxygen. Preferred is a deposition of 400 nm of CdCl 2  by high vacuum evaporation, followed by an anneal for 25 minutes at 430° C. in an atmosphere containing 40% of oxygen 
         [0034]    In the following a window layer  4 , comprising a metal component  40  and n-type semiconducting material  41  will be deposited together onto the pretreated surface of the absorber layer  3  in a deposition step b). 
         [0035]    The n-type semiconducting material  41  (here CdS) and the metal component  40  can be deposited either simultaneous as shown in FIG.  2   a ) or timely staggered in different chronological order as depicted in  FIG. 2   b ) (step b′) and  2   c ) (step b″). The amount of the metal component  40  lies in all embodiments in a range equivalent to a layer of a thickness of less than 0.5 nm, preferred between 0.05-0.15 nm. Dependent of the deposition method the metal component  40  is deposited for example by physical vapour deposition, by application of a metal component containing solution or by chemical methods in a separate step or the metal is mixed up with CdS material or it can be included in the chemical bath for deposition of CdS. 
         [0036]    The deposition of CdS with or without metal atoms is for example possible by sputtering, vacuum evaporation, closed space sublimation, chemical bath deposition, spray pyrolysis, electrodeposition or other chemical methods. We preferred chemical bath deposition at 70° C. The thickness of the CdS window layer  4  can for example be between 50 and 200 nm. The achieved thickness of the window layer  3  was about 100 nm. Accordingly the relative amount of the introduced metal component  40  can be in the range of 1/4000 to 1/100 relative to the window layer  3  thickness. 
         [0037]    After the deposition step b) a diffusion treatment in form of an annealing or treatment as described above is following, leading to the diffusion of copper atoms through the window layer  4 , the absorber layer  3  to the back contact  2 . The diffusion treatment is carried out at at least 180° C. for time periods of minutes. 
         [0038]    We have found, that the deposition of small amounts of the conducting metal component  40 , for example elements of group  11  or group IB of the periodic table of elements comprising copper, gold or silver directly or in combination with the deposition of CdS onto the surface of the pretreated absorber layer  3  on the side facing away from the back contact  2  enhances the performance of the photovoltaic device  0  and reproducibility of the process. 
         [0039]    Beside the simultaneous deposition according to step b) the deposition of the metal component  40  and the n-type semiconductor  41  can be carried out timely staggered according to  FIGS. 2   b ) and  2   c ). 
         [0040]    According to step b′) in  FIG. 2   b ) the metal component  40  is deposited with an amount equivalent to a layer thickness of less than 0.5 nm directly onto the pretreated absorber layer  3 . For example it can be between 0.05 and 0.15 nm. The metal component deposition is followed by deposition of a CdS layer  41  comprising an amount of CdS material  41  for example by chemical bath deposition. After this timely staggered deposition of metal atoms and the n-type semiconducting material  41  the diffusion treatment is carried out, for example by a further treatment step at temperatures of 400° C. The atmosphere can contain oxygen. The copper atoms are diffusing through the CdS layer  4 , CdTe layer  3  and to the back contact  2  due to the diffusion process. Optional an annealing step can be carried out after copper deposition and before CdS deposition, as indicated in  FIG. 2   b . For example, the annealing step can be carried out at around 400° C. in an atmosphere that can contain oxygen. 
         [0041]    According to step b″) in  FIG. 2   c ) the chronological order of deposition is changed and at first the CdS material  41  is deposited directly onto the pretreated surface of the CdTe layer  3 . After the CdS deposition the deposition of the metal component  40  with an equivalent layer thickness of less than 0.5 nm is carried out. The metal atoms of the metal component  40  are later diffusing in the semiconductor layers due to further diffusion treatment in form of thermal conditioning. During processing, Cu diffuses and in the finished device it is distributed in several layers including CdS  4 , CdTe  3  and at the back contact-absorber interface  23  of the solar cell. 
         [0042]    For deposition of CdS we used CBD (chemical bath deposition) by which CdS is deposited in the form of nano crystals with sizes less than 30 nm. Therewith CdS layer  4  comprising nano-grained CdS structures can be achieved. Because of the use of nano crystals of CdS with crystal sizes less than 30 nm, deposited CdS layer  41  is forming a nano-grained structure. If further thermal diffusion treatments are carried out a large grained CdS layer can be formed. 
         [0043]    After deposition of the metal component  40 , the metal (in this case copper) diffuses into every layer due to diffusion treatments, which was proved by SIMS measurements, which showed higher Cu-signal levels for every layer as compared to Cu free samples. In the near of the back contact-absorber junction  23  and the window layer  4  the highest Cu-signal levels could be measured due to the highest copper concentrations. We have found, that the addition of small amounts of copper (e.g. 10̂-7 g/cm̂2) after recrystallization of the CdTe layer  3  considerably enhances performance of the devices and reproducibility of the process. 
         [0044]    We identified an influence of copper in the CdTe absorber layer  3  as it increases acceptor density and reduces space charge region. This can increase the open circuit voltage of the device and reduce a hole blocking barrier at the back contact. Furthermore, it was found that appropriate amounts of Cu in CdS window layer  4  also have a beneficial effect. For example, Cu could enhance photoconductivity of the window layer  4 . A similar effect could be achieved by other impurities like oxygen, hydrogen or combinations thereof. 
         [0045]    As much less Cu is used the newly developed process is expected to have a beneficial effect on the long-term stability of the photovoltaic device  0 , as stability issues were commonly associated with diffusion of Cu and accumulation of excessive Cu in CdS. 
         [0046]    We have tested several positions of Cu, amounts of Cu and different annealing or diffusion treatment temperatures. As specific example, good efficiencies can be obtained by adding about 0.8 Angstrom of Cu by high vacuum evaporation on top of CdTe  3  after the first treatment at around 430° C. in 40% of oxygen in the presence of 400 nm of CdCl 2  and enhancing diffusion of Cu into CdTe with an annealing at around 400° C. in an atmosphere containing 40% of oxygen, acting as diffusion treatment before the deposition of CdS. As the layer of the photovoltaic device  0  are again annealed after deposition of the next layer ( 41 , CdS), Cu also diffuses into this n-type semiconducting layer  41 . 
         [0047]    Subsequently, 100 nm of CdS is deposited, followed by a treatment, which leads to recrystallisation and grain growth of the CdS and diffusion of Cu into the CdS layer  41 . Preferrably the treatment after deposition of CdS is performed in the presence of 100 nm of CdCl 2  in an ambient containing  50 % of oxygen at around  400 ° C. Afterwards another CdS layer (second CdS layer  41 ′, e.g. 100 nm) can be deposited as described below. 
         [0048]    All produced photovoltaic devices  0  showed a depth dependant distribution with non-zero metal concentration at the back contact  2 , absorber layer  3  and window layer  4 , measured by secondary ion mass spectroscopy (SIMS). The metal component concentration in the window layer  4  was higher than in most parts (&gt;90%) of the absorber layer  3 . For example, the concentration of the metal component in the window layer  4  can be at least  10  times higher than in the absorber layer  3 . For example, the concentration of the metal component can be greater than 10 18  cm −3  in the window layer  4  and less than 10 18  cm −3  in most parts of the absorber layer. Interpretation of SIMS measurements is difficult due to matrix effects. Still we could identify two maximum metal concentrations to be located, one in the n-type semiconducting window layer  4  and one at the interface  23  between absorber layer  3  and the back contact  2 . These copper accumulations could be produced with diffusion treatment steps. Such thin film photovoltaic devices  0  with substrate configuration are reaching the wanted efficiencies. Cu distributes in the semiconductor layers and can lead to concentrations of greater than 10 18  cm −3  in CdS  4  and close to the back contact interface  23  and less than 10 18  cm −3  in CdTe layer  3 . For example, the concentration in the CdS  4  and close to the back contact interface  23  was at least one order of magnitude higher than in most parts (&gt;90%) of the absorber layer  3 . For example, the concentration in the CdS  4  was between 2*10 18  and 1*10 20  cm −3 , in most of the CdTe  3  (&gt;90%) it can be between 1*10 16  to 9*10 17  cm −3  and close to the back contact interface  23  it was between 2*10 18  and 1*10 20  cm −3 . More specifically, the concentration in the CdS  4  was around 7*10 18  cm −3  and in most of the CdTe  3  (&gt;90%) around 1*10 17  cm −3  and at the back contact interface  23  around 1*10 19  cm −3 . 
         [0049]    solar cell with second CdS layer  41 ′In lightly modified embodiment of the production method the diffusion treatment has been changed. The method is stepwise shown in  FIG. 3   a , while the results of an exemplary analysis are shown in  FIGS. 3   b  to  3   e . The achievable multi layer thin film photovoltaic device  0  possesses a window layer with one CdS 1 ,  41  and another CdS 2 ,  41 ′ layer (double layered CdS layer  4 ) with a copper depth distribution as described above. 
         [0050]    The produced photovoltaic device  0  is based on a buffer layer  20 , which is deposited as part of the back contact  2 . The buffer layer  20  can for example comprise MoO x , Sb x Te y , Bi x Te, Sb, ZnTe, CuTe x , Te, Cu doped CdTe or combinations thereof. Here, the metallic part of the back contact  2  is Mo and the buffer layer  20  is a double layer of MoO x  and Te. Preferred thicknesses of the metal back contact  2  and the buffer layer  20  are 0.5-1 μm and 50-300 nm, respectively. 
         [0051]    After the deposition of the CdTe on the buffer layer  20  and the first treatment according to step a) the deposition of metal component  40  and n-type semiconducting material  41  according to step b), b′) or b″) is carried out. A diffusion treatment in form of a second treatment c) is following using the described treatment parameters. Due to the diffusion treatment metal atoms diffuse in the layers  2 ,  20 ,  3 ,  4  and the grain size of CdS 1   41  is enlarged as can be seen in  FIG. 3   b . It leads to recrystallization of the nano-grained CdS 1 , leading to larger grain sizes and can lead to phase change of the CdS layer to preferentially wurtzite CdS ( FIG. 3   e ). 
         [0052]    After step c) a second deposition d) of a further CdS 2  layer  41 ′ is carried out, for example again using chemical bath deposition. The CdS 2  layer can grow in the same wurtzite phase as the underlying recrystallized CdS 1  layer. 
         [0053]    This procedure leads to a double layered CdS layer  4 , comprising a treated CdS 1  layer  41  with large grains and an as deposited CdS 2  layer  41 ′ with nano grains. The size of the nano grained CdS is less than 30 nanometers. After the diffusion treatment the grain sizes of the CdS 1  layer are enlarged and can reach grain sizes of greater than 30 nm. An example of a resulting grain distribution is shown in  FIG. 3   b . For example, the annealed CdS 1  layer has grain sizes of 100-500 nm, while the second CdS 2  layer has nano-crystalline grains with less than 30 nm. 
         [0054]    The transparent conductive layer  5  consists of a multilayer of highly resistive transparent oxides and conductive transparent oxides. We prefer a double layer of intrinsic ZnO with a thickness of approximately 200 nm and ZnO:Al with a thickness of approximately 800 nm. This double layer was deposited in another deposition step. 
         [0055]    As indicated in  FIG. 3   a  an optional annealing can be carried out either directly after the deposition of the double window layer  4  or after the subsequent deposition of the transparent contact layer  5  in form of at least one transparent oxide. Preferrably this annealing is carried out at temperatures between 180° C. and 300° C. 
         [0056]    In order to determine, where Cu diffuses, the depth dependant distribution of Cu was measured by secondary ion mass spectroscopy (SIMS) and an example of an overview distribution of the back contact  2  to the transparent contact layer  5  is shown in  FIG. 3   c . Compared to Cu free devices, the Cu amount in every layer increases. 
         [0057]    Even though interpretation of SIMS measurements are difficult due to the matrix effect, we could determine the concentrations of Cu in the different layers to be around 7*10 18  cm −3  in the CdS window layer  4  and around 1*10 17  cm −3  in the CdTe absorber layer  3  and around 1*10 19  cm −3  close to the back contact interface  23  or in the buffer layer  20  in an excellent device with open circuit voltage &gt;800 mV and FF &gt;70%. As can be seen copper could be detected in every layer  2 ,  20 ,  3 ,  4 , whereas the copper concentration in the window layer  4  and at the back contact  2  is significantly higher than in the absorber layer  3 . 
         [0058]    The two visible copper concentration maximums are lying in the window layer  4  and at the back contact-absorber interface  23  respectively in the buffer layer  20 , wherein the metal concentration at the back contact-absorber interface  23  and the window layer  4  is at least ten times higher than in the absorber layer  3 . 
         [0059]    As visible from the SIMS depth profile depicted in  FIG. 3   d , Cu is present in the window layer  4  comprising CdS 1  and CdS 2 . In this magnification of the copper depth distribution of the window layer  4  it can be seen that the copper in the large grained CdS 1  is higher than in the small grained CdS 2 . 
         [0060]    In one embodiment, only one obvious maximum in the depth dependant metal component distribution is observed and this maximum is located in the window layer  4 . Using a diffusion treatment with temperatures at around 200° C., a photovoltaic device  0  showing only the one maximum is achievable. The resulting metal component concentration in the window layer  4  can be at least ten times higher than in the absorber layer  3 . 
         [0061]    As can be seen in the XRD pattern of  FIG. 3   e , a CdS layer  41  as deposited CdS grows in a phase which can correspond to hexagonal CdS  002  or zincblende CdS  111 . An optimized double-CdS layer  41 ,  41 ′ can show an XRD pattern corresponding to hexagonal CdS ( 100  &amp;  101 ). 
         [0062]    The optimized doubled CdS layer  41 ,  41 ′ also has a beneficial influence on the photovoltaic device  0  performance. It increases open circuit voltage and fill factor and it reduces rollover in the first quadrant ( FIG. 4 ). Prior art CdTe solar cells in substrate configuration had lower fill factor, more pronounced roll-over and mostly lower V OC  than the devices disclosed in this invention. With the process disclosed in some of the embodiments of this invention we achieved &gt;70% fill factor, which was not achieved with previously published processes. 
         [0063]    With our new approach a step-like increase in efficiency to more than 13% efficiency was achieved and it still has good potential for further improvement. Furthermore, the newly developed process shows much better reproducibility. Further reduction in cost and increase in throughput can be expected from the use of roll-to-roll production using the disclosed method. Therefore, we see better potential of the invention for flexible foils and low cost metal (e.g. aluminum or mild steel) substrates. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           0  photovoltaic device (in substrate configuration) 
           1  substrate 
           2  back contact 
       
     
         [0067]      23  back contact-absorber interface
     3  absorber layer/p-type semiconducting layer   
 
         [0069]      34  pn-junction/absorber-window junction
     4  window layer   
 
         [0071]      40  metal component 
         [0072]      41  n-type semiconducting material
     5  transparent contact layer (TCO layer)   
 
         [0074]    a) first treatment 
         [0075]    b) deposition of n-type semiconducting material 
         [0076]    c) second treatment 
         [0077]    d) second deposition of n-type semiconducting material 
         [0078]    CdS 1  large grained CdS/ 41 ′ 
         [0079]    CdS 2  nano-grained CdS/ 41