Abstract:
Sputtered zinc oxide layer is used to improve and control the crystalline properties of a molybdenum back contact used in photovoltaic cells. Optimum thicknesses for the zinc oxide layer are identified.

Description:
BACKGROUND OF THE INVENTION 
     The invention is concerned with photovoltaic devices, particularly with improving the quality of molybdenum contacts used in such devices. 
     The use of photovoltaic cells for the generation of electrical energy is well known. The cells contain semiconductor materials exhibiting the photovoltaic effect and these are typically realised in solar panels comprising a number of cells. Solar energy represents a clean, environmentally friendly source of electricity and, although at present it accounts for a small fraction of global energy consumption, it is also a rapidly expanding industry. Of the thin film photovoltaic technologies in mass production at this time, CuIn 1-x Ga x Se 2-y Sy (CIGS) and CuInS 2  (CIS) have demonstrated the highest efficiencies. It is generally accepted that as device efficiencies increase to high levels, incremental improvements become of greater importance. 
     Molybdenum is used for the back contact in CIGS/CIS cells due to its excellent adhesion and low contact resistance with the materials concerned. Molybdenum is also inert and stable during the aggressive CIGS deposition process. 
     Additionally an intermediate MoSe 2  layer is formed during CIGS deposition, which provides an ohmic contact between the absorber and the back contact. Therefore the main focus when depositing Mo has been on providing a material with low sheet resistance and good adhesion. In order to achieve both of these properties it has been shown by Scofield et al, that deposition of a Mo bi-layer is necessary (Scofield et al,  Thin Solid Films,  260, (1995), 26.). This involves deposition of a high pressure Mo layer to give a well adhered film under tensile stress, followed by deposition of a Mo layer at lower pressure to give low sheet resistance (this film is under compressive stress). 
     Attainment of CIGS material of the desired texture for highly efficient devices is heavily dependent on the structural properties of the Mo layer (Contreras et al,  Thin Solid Films,  361-362, (2000), 167.). Properties such as morphology, grain size, and stress state are variables that are likely to affect the CIGS nucleation and growth process. Work by NREL demonstrated that it was desirable for the Mo to be crystalline and in the (110) orientation, the so-called “fish-like” morphology (Dhere, NREL report, FSEC CR1416-03, (2003)). Work by Assmann et at confirms this (Assmann et al,  Applied Surface Science,  246, (2005), 159.). 
     As CIGS devices are grown in the substrate configuration, the Mo effectively serves as a growth layer for the rest of the device and so a high quality Mo layer is essential to achieve high efficiency devices. Improvement of the (110) orientation of the Mo and the ability to control its crystalline structure without adversely affecting other properties would represent significant advances in the art. This ability allows for tuning of the Mo layer properties to device requirements, which is important as small variations in the crystalline structure of CIGS can strongly affect the series resistance and the fill factor of devices (Siebentritt et al,  Prog Photovolt Res  &amp;  Apps,  18, (2010), 390.). Furthermore it is likely that for different CIGS deposition processes, the optimum Mo texture is likely to be different and so being able to tailor morphology for an individual process is important. 
     One final function that Mo can perform is in the control of sodium diffusion into the CIGS material. In order to obtain the highest efficiency devices, the presence of sodium is necessary when the CIGS material is deposited (Hedström et al.,  Proc. of the  23 rd IEEE Photovoltaic Specialists Conference , (1993), 364., Niki et al.,  Prog. Photovolt: Res. Appl.,  18, (2010), 453.). In laboratory cells this sodium is generally supplied by diffusion from a soda lime glass substrate, through the Mo layer and into the CIGS material. Therefore the microstructure of the Mo can be tailored to control this sodium diffusion. However for an industrial process, close control of sodium content is essential to achieve consistent high performance and efficiency and so relying on diffusion from the glass would be considered difficult. Sodium content and mobility in glass can vary depending on glass origin, composition, age and storage conditions. To prevent uncontrolled sodium diffusion from the glass, a sodium barrier layer is used between the glass and the molybdenum coating. Sodium can then be introduced in a controlled manner via deposition of a sodium compound, such as sodium fluoride (N. G. Dhere,  Solar Energy Materials and Solar Cells,  95, (2011), 277.). Examples of common sodium barrier layers would be SiO 2  or SiN. When using barrier layers to restrict sodium diffusion it is desirable that Mo is grown with a densely packed grain structure to give greater protection from uncontrolled Na diffusion and with as few pinholes as possible, i.e. a high quality film is needed. 
     Control of Mo growth can be achieved by varying the deposition parameters; however this invariably changes other properties of the material. A means of enhancing the orientation of the material without affecting other properties would therefore be desirable. As mentioned previously, for high efficiency CIGS devices the Mo should be in the (110) orientation. By the invention, this (110) orientation is enhanced and can be tuned as desired. 
     Barreau and Bommersback (Proceeding of EUPVSEC, Valencia, (2010), 3BV.2.23) discuss the improvement of Mo orientation by using ZnO, however they use rather thick layers &gt;100 nm to improve growth. According to this disclosure, increasing the ZnO thickness from 0 to 50 nm, gives an increase in the intensity of the (110) signal by a factor of ˜1.5. A further increase to 150 nm ZnO leads to a 3-fold increase after which the effect levels off. Overall the ZnO layer gives a 4.5-fold increase in (110) intensity. No data is presented concerning ZnO layers having a thickness between 0 and 50 nm. The implication is that the data obtained for other thicknesses may be extrapolated over this range. 
     SUMMARY OF THE INVENTION 
     According to the invention, a method of controlling the crystal orientation of a molybdenum layer on a substrate comprises the steps set out in claim  1  attached hereto. 
     Throughout the following description of the invention, references to ZnO include reference to materials having a substantial fraction of their composition comprised of ZnO. For example, reference to ZnO includes material commonly represented as ZnO:Al—a material comprising largely ZnO but including some Al. Layers of such materials may be deposited by sputter coating using commercially available target materials. 
     The inventors have also shown that an additional primer layer may be included with deposition of the ZnO layer being done directly thereon. This assists growth of the ZnO layer. The primer layer may comprise, for example ZnSnO x  or TiO 2 . Preferably the primer layer has a thickness of between 5 and 50 nm. 
     In various embodiments, the substrate may be formed in glass; metal such as stainless steel or polymer. 
     A preferred material for the substrate is glass, in which case a sodium barrier layer may be included, between the glass and any other layer. Preferably, the barrier layer comprises at least one of Si x N y , SiO 2 , SnO 2 , SiCO and TiO 2 . 
     Where the barrier layer is SiO 2 , preferably 8 nm&lt;t&lt;30 nm, more preferably 12 nm&lt;t&lt;18 nm. 
     Where the barrier layer comprises a layer of Si x N y , preferably 0 nm&lt;t&lt;30 nm, more preferably 0 nm&lt;t&lt;15 nm, most preferably 2 nm&lt;t&lt;8 nm. 
     The barrier layer may have a thickness of between 5 and 200 nm and may be deposited by chemical vapour deposition. 
     The primer layer may be ZnSnO x  having a thickness of between 5 and 30 nm. 
     According to a second aspect of the invention, a substrate comprises the features set out in claim  19  attached hereto. 
     The substrate may further comprise an additional primer with the ZnO layer being located thereon. The primer layer may comprise, for example ZnSnO x  or TiO 2 . The primer layer preferably has a thickness of between 5 and 50 nm. 
     The substrate may be comprised of glass, in which case a sodium barrier layer may be located between the substrate and the other layer or layers. 
     Preferably, the barrier layer comprises at least one of: Si x N y , SiO 2 , SnO 2 , SiCO and TiO 2 . 
     Where the barrier layer comprises SiO 2 , preferably 8 nm&lt;t&lt;30 nm, more preferably 12 nm&lt;t&lt;18 nm. 
     Where the barrier layer comprises Si x N y , preferably 0 nm&lt;t&lt;30 nm, more preferably 0 nm&lt;t&lt;15 nm, most preferably 2 nm&lt;t&lt;8 nm. 
     The barrier layer may have a thickness of between 5 and 200 nm. 
     The primer layer may comprise ZnSnO x  having a thickness of between 5 and 30 nm. 
     A substrate according to the invention is preferably incorporated in a photovoltaic cell. 
     The inventors have shown that, contrary to previous teaching, the Mo crystallinity and (110) presence may be enhanced by use of thin ZnO layers. X-ray diffraction (XRD) studies have shown an increase in the intensity of the (110) reflection by up to 60 times when using thin ZnO layers. Moreover the ZnO thickness can be used to tune the Mo to the required orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by non-limiting example, with reference to the following figures in which: 
         FIG. 1  is a schematic representation of a photovoltaic device according to the invention; 
         FIG. 2  shows an X-Ray Diffraction (XRD) pattern obtained from samples of Mo layers grown on glass substrates, the Mo layers having thicknesses of 500 nm and 100 nm respectively; 
         FIG. 3  shows a comparison of XRD data obtained from samples of Mo grown on a glass substrates having a sodium barrier layer and samples having the same barrier layer along with various realisations of a ZnO growth layer according to the invention; 
         FIGS. 4 and 5  shows a comparison of texture measurement data obtained for Mo layers produced according to the prior art and Mo layers produced on ZnO growth layers according to the invention; 
         FIG. 6  illustrates the variation of (110) peak intensity with ZnO thickness for a range of samples of Mo grown according to the invention on a SiO 2  barrier layer; 
         FIG. 7  shows the XRD pattern for a selection of the samples used to provide the data in  FIG. 6  and 
         FIG. 8  illustrates the variation of (110) peak intensity with ZnO thickness for a range of samples of Mo grown according to the invention on a Si x N y  barrier layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , according to the invention a substrate  1  is processed by deposition of a ZnO layer  2  and a Mo layer  3 . As will be shown below, the morphology and crystal structure on the Mo layer  3  is heavily dependent on the thickness of the ZnO layer  2  and this thickness is selected according to the desired characteristics of the Mo layer  3 . 
     A stack having only the ZnO layer  2  between the substrate and the Mo layer  3  represents the simplest embodiment of the invention. However, the quality of the ZnO layer  2  (and the extent of its effect on the characteristics of the Mo layer  3 ) is enhanced by inclusion of a primer layer  4  of, for example ZnSnO x , located between the substrate  1  and the ZnO layer  2 . 
     Moreover, while the invention has applicability to substrates of a range of materials, a preferred material is glass. Where a glass substrate is used, a sodium barrier layer  5  may also be included. 
     Preferably, the substrate thus processed is incorporated in a photovoltaic cell by further including a photoactive region  6  comprising CuIn 1-x Ga x Se 2-y Sy or CuInS 2  which forms a heterojunction with an Al doped ZnO layer  7 , typically buffered by a thin layer  8  of CdS and a layer  9  of intrinsic ZnO. As previously noted, a layer of MoSe 2  10 may be included between the Mo layer  3  and the photoactive region  6  in order to provide an improved ohmic contact. 
     Thicknesses illustrated in  FIG. 1  are not to scale. 
     EXAMPLES 
     In the following examples, Molybdenum growth was carried out by sputtering a molybdenum target in Argon gas and the deposited films had a thickness of 500 nm unless otherwise stated. At 500 nm, sheet resistance was ˜0.3Ω/□ and films were durable, surviving the scotch tape test. As discussed previously, the stress of the films could be controlled through use of deposition pressure. 
     The examples involve deposition of layers on a float glass substrate but this feature should not be seen as limiting. The invention has applicability where molybdenum is provided on any substrate (including other types of glass and other materials such as metals or polymers) and where the crystal orientation/morphology of the molybdenum layer is important. 
     Examples 1 and 2 
     Two Mo films, having thicknesses of 500 nm and 1000 nm (examples 1 and 2 respectively) were grown on soda-lime-silica glass substrates (referred to below as “float”). Since both films were deposited under the same deposition conditions, the crystalline orientation of the films is similar and a doubling in thickness results in an increase in the XRD intensity by a factor of around two in line with expectations. 
     The X-Ray Diffraction patterns for the examples 1 and 2 are shown in  FIG. 2  as solid lines and broken lines respectively. For the major part of the graph, the two lines are difficult to distinguish as the XRD intensity is similar. As can be seen however, the (110) orientation of the films is dominant (indicating that these films would be suitable for use in CIGS devices) and the XRD intensity in this region of the graph is for the 500 nm sample is approximately double that for the 1000 nm sample. 
     Examples 3 to 7 
     Growth of Mo on a variety of barrier layers was explored. A 500 nm Mo layer was grown on each of the examples described in table 1. Figures in parenthesis indicate the thickness of the preceding layer in nm. Samples were subjected to XRD analysis of the Mo layer and the results are summarised. C in strain indicates compressive stress and T tensile stress. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Selected properties of Mo layers on different 
               
               
                 substrates from XRD analysis 
               
             
          
           
               
                 Exam- 
                   
                 (110) 
                 Crystallite 
                 Strain in 
               
               
                 ple 
                 Description 
                 Intensity 
                 size (nm) 
                 Film (%) 
               
               
                   
               
               
                 1 
                 Float 
                 17 
                 30 
                 0.17C 
               
               
                 3 
                 Float./Si x N y (50) 
                 15 
                 32 
                 0.13C 
               
               
                 4 
                 Float/SiO2 (30) 
                 16 
                 31 
                 0.23C 
               
               
                 5 
                 Float/SnO2 (25)/ 
                 13 
                 31 
                 0.17C 
               
               
                   
                 SiO2 (25) 
                   
                   
                   
               
               
                 6 
                 Float/SiCO (40) 
                 16 
                 34 
                 0.23C 
               
               
                 7 
                 Float/SiO2 (30)/ 
                 15 
                 35 
                 0.20C 
               
               
                   
                 TiO2 (15) 
               
               
                   
               
             
          
         
       
     
     The data shown in table 1 indicates that Mo growth on all of these substrates is quite similar. 
     Examples 8 and 9 
     Further samples comprising a thin ZnO growth layer were prepared. Example 8 comprised a ZnO layer on the SiO 2  barrier layer. Example 9 was similar to example 8 but further included a ZnSnO x  primer layer on the barrier layer to improve the ZnO growth. 
     Table 2 summarises these sample structures (including the float/SiO2 example shown in table 1 for ease of comparison) and shows the results of XRD analysis. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Selected properties of Mo layers grown 
               
               
                 on ZnO with and without primer layer 
               
             
          
           
               
                 Exam- 
                   
                 (110) 
                 Crystallite 
                 Strain in 
               
               
                 ple 
                 Description 
                 Intensity 
                 size (nm) 
                 Film (%) 
               
               
                   
               
             
          
           
               
                 4 
                 Float/SiO2 (30) 
                 16 
                 31 
                 0.23C 
               
               
                 8 
                 Float/SiO2 (30)/ 
                 520 
                 31 
                 0.08C 
               
               
                   
                 ZnO (8) 
                   
                   
                   
               
               
                 9 
                 Float/SiO2 (30)/ 
                 1044 
                 30 
                 0.05C 
               
               
                   
                 ZnSnO x ) (10)/ZnO (8) 
               
               
                   
               
             
          
         
       
     
       FIG. 3  further illustrates the respective degrees of (110) orientation exhibited by the samples having a ZnO growth layer. 
     As can be seen from table 2 and  FIG. 3 , the crystallinity of the samples has improved considerably with the signal for the preferred (110) orientation increasing by a factor of around 65. Sheet resistance of the Mo films was ˜0.3Ω/□ and all samples passed the Scotch tape test, confirming that durability had not been compromised. In addition, this effect was observed on Mo deposited over a range of pressures. 
     Table 2 shows a marked improvement in the properties of Mo grown on ZnO and a further marked improvement when the ZnO is grown on a primer layer. 
     Texture measurement is a technique that provides a measure of the strength of the columnar orientation of a sample. By this procedure, the angular distribution of a selected hkl plane is measured. 
     Texture measurement was performed on Mo layers grown on Float glass and on a Barrier/ZnSnOx/ZnO (8) growth layer. The net Mo (110) intensities at 0 tilt and 5 deg tilt were summed and the summed total expressed as a percentage of the total net intensity in the pole figure. The pole figures and intensities of these samples are shown in  FIGS. 4 and 5  respectively. The sample on the ZnO growth layer was shown to have significantly greater columnar orientation than the sample on float. 
     Example 10 
     A further surprising aspect of this invention is that the crystallinity of the sample and intensity of the (110) peak are very much dependent on the ZnO thickness and that as this was increased beyond an optimum point, the intensity actually dropped. This was demonstrated by preparing a series of samples comprising a Float/SiO2 (30)/ZnSnO x  (5)/ZnO/Mo (500) structure, wherein the thickness of the ZnO layer was varied across the series. The results of analysis are illustrated in  FIGS. 6 and 7 . 
       FIG. 6  shows little improvement of the (110) peak intensity for coating thicknesses of 0 nm or 60 nm and above. However, a significant improvement is shown for any value between these two extremes and it is clear that the extent of the improvement depends heavily on the actual value within the range. 
     In particular, the steep region of the graph as the thickness increases from zero indicates a significant improvement for any coating thickness above zero. The improvement is especially marked between values of about 8 nm and 30 nm. The optimum thickness is in the range 12-18 nm. 
     Selected XRD analysis data of these samples are shown in  FIG. 7  (for clarity, only data for ZnO thicknesses of 0, 15 and 120 nm are shown). 
     Sheet resistance and durability remained constant as described previously and so the ZnO layer can be used as a tuning layer to obtain the optimum morphology of Mo for the relevant CIGS deposition process. 
     Example 11 
     In order to demonstrate that the benefits of the invention are not restricted to systems having an SiO 2  barrier layer, a series of samples comprising Float/SixNy (20)/ZnSnOx (5)/ZnO/Mo (500) was prepared. The variation of (110) peak intensity with ZnO thickness is shown in  FIG. 8 . 
     Again a significant improvement in (110) peak intensity is shown for thicknesses between 0 and 60 nm, with the degree of improvement depending heavily on the actual value of the thickness. Comparison of  FIGS. 6 and 8  further reveals that the optimum thickness for the ZnO layer depends on the type of barrier layer: in  FIG. 8 , the improvement is especially marked for thicknesses between 0 and 30 nm, more so between values of between 0 and 15 nm. The optimum thickness is in the range 2-8 nm. 
     So, while the optimum thickness for the ZnO layer may vary according to the barrier layer used, it is clear that using ZnO layers that are thinner that the 150 nm or so suggested by the prior art will give rise to improvements in the Mo layer characteristics. Moreover, these characteristics may be finely tuned by selection of the precise thickness of the ZnO layer. 
     Examples 12-14 
     As previously noted, circumstances may occur in which a certain degree of sodium diffusion from a glass substrate is desirable. In examples 12-14, ZnSnOx layers of various thicknesses were deposited directly on the glass substrates followed by deposition of a ZnO layer. 
     As with previous samples, a 500 nm Mo layer was deposited on each of these and the results of analysis are shown in table 3. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Properties of Mo layers grown on substrates 
               
               
                 having no sodium barrier layer 
               
             
          
           
               
                 Exam- 
                   
                 (110) 
                 FWHM 
                 Crystallite 
                 Strain in 
               
               
                 ple 
                 Description 
                 Intensity 
                 (deg) 
                 size (nm) 
                 Film (%) 
               
               
                   
               
             
          
           
               
                 12 
                 Float/ 
                 721 
                 0.35 
                 29.3 
                 0    
               
               
                   
                 ZnSnOx(5)/ 
                   
                   
                   
                   
               
               
                   
                 ZnO(8) 
                   
                   
                   
                   
               
               
                 13 
                 Float/ 
                 1938 
                 0.3031 
                 33.8 
                 0.07T 
               
               
                   
                 ZnSnOx(10)/ 
                   
                   
                   
                   
               
               
                   
                 ZnO(8) 
                   
                   
                   
                   
               
               
                 14 
                 Float/ 
                 1270 
                 0.3573 
                 28.8 
                 0.04T 
               
               
                   
                 ZnSnOx(20)/ 
                   
                   
                   
                   
               
               
                   
                 ZnO(8) 
               
               
                   
               
             
          
         
       
     
     Table 3 indicates (inter alia) a high degree of (110) orientation among the Mo crystal, demonstrating that the invention works well with samples that do not include a sodium barrier layer. 
     Sample 12 shows an increased (110) presence for a ZnSnOx layer of 5 nm and this increases further and markedly for sample 13, which has a 10 nm ZnSnOx layer. For sample 14 (20 nm ZnSnOx), the (110) intensity is still high, but lower than that of sample 13. 
     So the data shown in table 3 indicates that deposition of Mo in the (110) orientation is still enhanced when no barrier layer is present. Moreover, an optimum thickness for a ZnSnOx layer (in terms of deposition of Mo in the (110) orientation) lies somewhere between 5 nm and 20 nm.