Growth layer for photovoltaic applications

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.

Claims

1. A substrate bearing a stack of layers as the back contact in a molybdenum photovoltaic device, said back contact comprising in order from the substrate: a barrier layer comprising at least one of: Si.sub.xN.sub.y, SiO.sub.2, SnO.sub.2, SiCO and TiO.sub.2; a primer layer; a layer of ZnO; and a layer of molybdenum, wherein the molybdenum is deposited directly on the layer of ZnO; the ZnO layer having a thickness, t, of 0 nm<t<50 nm; wherein the ZnO layer is deposited directly on the primer layer; and the primer layer is deposited directly on the barrier layer and comprises TiO.sub.2 or ZnSnO.sub.x.

2. The substrate according to claim 1, wherein the primer layer has a thickness of between 5 and 50 nm.

3. The substrate according to claim 1, wherein the substrate is glass.

4. The substrate according to claim 1, wherein the barrier layer comprises a sodium barrier layer.

5. The substrate according to claim 4, wherein the barrier layer has a thickness of between 5 and 200 nm.

6. The substrate according to claim 1, wherein the barrier layer comprises SiO.sub.2.

7. The substrate according to claim 6, wherein 8 nm<t<30 nm.

8. The substrate according to claim 7, wherein 12 nm<t<18 nm.

9. The substrate according to claim 1, wherein the barrier layer comprises Si.sub.xN.sub.y.

10. The substrate according to claim 9, wherein 0 nm<t<30 nm.

11. The substrate according to claim 10, wherein 0 nm<t<15 nm.

12. The substrate according to claim 11, wherein 2 nm<t<8 nm.

13. The substrate according to claim 1, wherein the primer layer comprises a layer of ZnSnO.sub.x having a thickness of between 5 and 30 nm.

14. The substrate according to claim 1, wherein the ZnO layer comprises a component of Al.

15. The substrate according to claim 1, incorporated in a photovoltaic cell.

16. A substrate bearing a stack of layers as the back contact in a molybdenum photovoltaic device, said back contact comprising in order from the substrate: a barrier layer; a layer of ZnO; and a layer of molybdenum, wherein the molybdenum is deposited directly on the layer of ZnO; the ZnO layer having a thickness, t, of 0 nm<t<50 nm; wherein the ZnO layer is deposited directly on the barrier layer; and the barrier layer comprises at least one of: SnO.sub.2, SiCO and TiO.sub.2.

17. The substrate according to claim 16, incorporated in a photovoltaic cell.

18. A method of controlling the crystal orientation of a molybdenum layer on a substrate comprising the steps of: depositing a barrier layer comprising at least one of: Si.sub.xN.sub.y, SiO.sub.2, SnO.sub.2, SiCO and TiO.sub.2 on the substrate, depositing a primer layer comprising TiO.sub.2 or ZnSnO.sub.x directly on the barrier layer, depositing a layer of ZnO directly on the primer layer, and depositing a layer of molybdenum directly on the layer of ZnO; wherein a thickness, t, for the ZnO layer is selected according to the desired crystal orientation wherein 0 nm<t<50 nm.

19. The method according to claim 18, wherein the primer layer has a thickness of between 0 and 50 nm.

20. The method according to claim 18, wherein the substrate is glass.

21. The method according to claim 20, wherein the barrier layer comprises a sodium barrier layer on the glass prior to deposition of any other layer.

22. The method according to claim 21, wherein a thickness is selected for the barrier layer of between 5 and 200 nm.

23. The method according to claim 21, wherein the barrier layer is deposited by chemical vapour deposition.

24. The method according to claim 18, wherein the barrier layer comprises a layer of SiO.sub.2.

25. The method according to claim 24, wherein 8 nm<t<30 nm.

26. The method according to claim 25, wherein 12 nm<t<18 nm.

27. The method according to claim 18, wherein the barrier layer comprises a layer of Si.sub.xN.sub.y.

28. The method according to claim 27, wherein 0 nm<t<30 nm.

29. The method according to claim 28, wherein 0 nm<t<15 nm.

30. The method according to claim 29, wherein 2 nm<t<8 nm.

31. The method according to claim 18, wherein the primer layer comprises a layer of ZnSnO.sub.x having a thickness of between 5 and 30 nm.

32. The method according to claim 18, wherein the ZnO layer comprises a component of Al.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described, by non-limiting example, with reference to the following figures in which:

(2) FIG. 1 is a schematic representation of a photovoltaic device according to the invention;

(3) 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;

(4) 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;

(5) 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;

(6) 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.sub.2 barrier layer;

(7) FIG. 7 shows the XRD pattern for a selection of the samples used to provide the data in FIG. 6 and

(8) 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.sub.xN.sub.y barrier layer.

DETAILED DESCRIPTION OF THE INVENTION

(9) 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.

(10) 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.sub.x, located between the substrate 1 and the ZnO layer 2.

(11) 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.

(12) Preferably, the substrate thus processed is incorporated in a photovoltaic cell by further including a photoactive region 6 comprising CuIn.sub.1-xGa.sub.xSe.sub.2-ySy or CuInS.sub.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.sub.2 10 may be included between the Mo layer 3 and the photoactive region 6 in order to provide an improved ohmic contact.

(13) Thicknesses illustrated in FIG. 1 are not to scale.

EXAMPLES

(14) 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.

(15) 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

(16) 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.

(17) 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

(18) 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.

(19) TABLE-US-00001 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.sub.xN.sub.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)

(20) The data shown in table 1 indicates that Mo growth on all of these substrates is quite similar.

Examples 8 and 9

(21) Further samples comprising a thin ZnO growth layer were prepared. Example 8 comprised a ZnO layer on the SiO.sub.2 barrier layer. Example 9 was similar to example 8 but further included a ZnSnO.sub.x primer layer on the barrier layer to improve the ZnO growth.

(22) 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.

(23) TABLE-US-00002 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.sub.x) (10)/ZnO (8)

(24) FIG. 3 further illustrates the respective degrees of (110) orientation exhibited by the samples having a ZnO growth layer.

(25) 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.

(26) 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.

(27) 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.

(28) 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

(29) 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.sub.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.

(30) 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.

(31) 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.

(32) 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).

(33) 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

(34) In order to demonstrate that the benefits of the invention are not restricted to systems having an SiO.sub.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.

(35) 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.

(36) 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

(37) 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.

(38) 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.

(39) TABLE-US-00003 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)

(40) 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.

(41) 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.

(42) 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.