METHOD FOR PRODUCING A LAYER SYSTEM FOR THIN-FILM SOLAR CELLS HAVING A SODIUM INDIUM SULFIDE BUFFER LAYER

Abstract

A method for producing a layer system for thin-film solar cells is described, wherein a) an absorber layer is produced, and b) a buffer layer is produced on the absorber layer, wherein the buffer layer contains sodium indium sulfide according to the formula Na.sub.xIn.sub.y-x/3S with 0.063≦x≦0.625 and 0.681≦y≦1.50, and wherein the buffer layer is produced, without deposition of indium sulfide, based on at least one sodium thioindate compound.

Claims

1.-15. (canceled)

16. A method of producing a layer system for thin-film solar cells comprising the steps of: a) producing an absorber layer, and b) producing a buffer layer on the absorber layer, wherein the buffer layer comprises indium sulfide according to the formula NA.sub.xIn.sub.y-x/yS with 0.063≦x≦0.625 and 0.681≦y≦1.5, and wherein the buffer layer is produced, without deposition of indium sulfide, based on at least one sodium thioindate compound.

17. The method according to claim 16, wherein the buffer layer is produced based on a compound selected from one of a group of sodium thioindate compounds: a) NaIn.sub.3S.sub.5, b) NaIn.sub.5S.sub.8 and c) NaOnS.sub.z.

18. The method according to claim 16, wherein the buffer layer is produced in step b) using a method selected from a group consisting of: wet-chemical bath deposition, atomic layer deposition (ALD), ion layer gas deposition (ILGAR), spray pyrolysis, chemical vapor deposition (CVD), physical vapor deposition (PVD), sputtering, thermal evaporation, or electron beam evaporation, from separate sources for one or various sodium thioindate compounds.

19. The method according to claim 16, wherein the buffer layer in step b) is deposited out of a gas phase, wherein a concentration of component of a material to be deposited is reduced in its gas phase before its deposition on the absorber layer.

20. The method according to claim 19, wherein the concentration of component of the material to be deposited is reduced in its gas phase by physically and/or chemically bonding the material to be deposited to a material introduced into a deposition chamber.

21. The method according to claim 16, wherein the absorber layer is conveyed, in an in-line method or in a rotation method, past a steam beam of a sodium thioindate compound or past a plurality of steam beams of sodium thioindate compounds different from each other with completely, partially, or not overlapping steam beams.

22. The method according to claim 16, wherein the buffer layer arranged on the absorber layer comprises sodium indium sulfide according to a formula NA.sub.xIn.sub.y-x/3S with 0. 063≦x≦0.469 and 0.681≦y≦1.01.

23. The method according to claim 16, wherein the buffer layer arranged on the absorber layer comprises sodium indium sulfide according to a formula NA.sub.xIn.sub.y-x/3S with 0.13≦x≦0.32 and 0.681≦y≦0.758.

24. The method according to claim 16, wherein in the buffer layer, a ratio of mole fractions of sodium and indium is greater than 0.2.

25. The method according to claim 16, wherein the buffer layer has a mole fraction of sodium of more than 5 atom-%.

26. The method according to claim 16, wherein the buffer layer has a mole fraction of sodium of more than 7 atom-%.

27. The method according to claim 16, wherein the buffer layer has a mole fraction of sodium of more than 7.2 atom-%

28. The method according to claim 16, wherein the buffer layer contains a mole fraction of a halogen, such as chlorine, or of copper of less than 7 atom-%.

29. The method according to claim 16, wherein the buffer layer contains a mole fraction of a halogen, such as chlorine, or of copper of less than 5 atom-%.

30. The method according to claim 16, wherein the buffer layer comprises a mole fraction of oxygen of less than 10 atom-%.

31. The method according to claim 16, wherein the buffer layer has a layer thickness from 10 nm to 100 nm, wherein the buffer layer is amorphous or fine crystalline.

32. The method according to claim 16, wherein the buffer layer has a layer thickness from 20 nm to 60 nm, wherein the buffer layer is amorphous or fine crystalline.

33. The method according to claim 16, wherein the absorber layer contains a chalcopyrite compound semiconductor selected from a group consisting of: Cu.sub.2 ZnSn(S, Se).sub.4, Cu(In, Ga, Al) (S, Se).sub.2, CuInSe.sub.2, CuInS.sub.2, Cu(In, Ga)Se.sub.2, and Cu(In, Ga) (S, Se).sub.2.

34. A method for producing a thin-film solar cell: comprising the steps of: preparing a substrate, arranging a rear electrode on the substrate, producing a layer system according to the method of claim 1, wherein the layer system is arranged on the rear electrode, and arranging a front electrode on the layer system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The invention is now explained in detail using an exemplary embodiment, referring to the accompanying figures. They depict:

[0047] FIG. 1 a schematic cross-sectional view of a thin-film solar cell produced in accordance with the method according to the invention with a layer system produced in accordance with the method according to the invention;

[0048] FIG. 2A a ternary diagram for the representation of the composition of the sodium indium sulfide buffer layer of the thin-film solar cell of FIG. 1;

[0049] FIG. 2B an enlarged detail of the ternary diagram of FIG. 2A with the region claimed according to the invention;

[0050] FIG. 3A a measurement of the efficiency of the thin-film solar cell of FIG. 1 as a function of the sodium indium ratio of the buffer layer;

[0051] FIG. 3B a measurement of the efficiency of the thin-film solar cell of FIG. 1 as a function of the absolute sodium content of the buffer layer;

[0052] FIG. 4 a measurement of the bandgap of the buffer layer of the layer system of FIG. 1 as a function of the absolute sodium content of the buffer layer;

[0053] FIG. 5 a measurement of the depth profile of the sodium distribution in the buffer layer of the layer system of FIG. 1 with differently high sodium fractions;

[0054] FIG. 6 an exemplary embodiment of the process steps according to the invention using a flowchart;

[0055] FIG. 7 a schematic representation of an in-line method according to the invention for producing the buffer layer;

[0056] FIG. 8 a schematic representation of an alternative in-line method according to the invention for producing the buffer layer;

[0057] FIG. 9 a schematic representation of a rotation method according to the invention for producing the buffer layer.

DETAILED DESCRIPTION OF THE DRAWINGS

[0058] FIG. 1 depicts, purely schematically, a preferred exemplary embodiment of a thin-film solar cell 100 produced in accordance with the method according to the invention with a layer system 1 produced in accordance with the method according to the invention in a cross-sectional view. The thin-film solar cell 100 includes a substrate 2 and a rear electrode 3. A layer system 1 is arranged on the rear electrode 3. The layer system 1 includes an absorber layer 4 and a buffer layer 5. A second buffer layer 6 and a front electrode 7 are arranged on the layer system 1.

[0059] The substrate 2 is made here, for example, of inorganic glass, with it equally possible to use other insulating materials with sufficient stability as well as inert behavior relative to the process steps performed during production of the thin-film solar cell 100, for example, plastics, in particular polymers or metals, in particular metal alloys. Depending on the layer thickness and the specific material properties, the substrate 2 can be implemented as a rigid plate or flexible film. In the present exemplary embodiment, the layer thickness of the substrate 2 is, for example, from 1 mm to 5 mm.

[0060] A rear electrode 3 is arranged on the light-entry-side surface of the substrate 2. The rear electrode 3 is made, for example, from an opaque metal. It can, for example, be deposited on the substrate 2 by vapor deposition or magnetic field-assisted cathodic sputtering. The rear electrode 3 is made, for example, of molybdenum (Mo), aluminum (Al), copper (Cu), titanium (Ti), zinc (Zn), or of a multilayer system with such a metal, for example, molybdenum (Mo). The layer thickness of the rear electrode 3 is, in this case, less than 1 μm, preferably in the range from 300 nm to 600 nm, and is, for example, 500 nm. The rear electrode 3 serves as a back-side contact of the thin-film solar cell 100. An alkali barrier, made, for example, of Si.sub.3N.sub.4, SiON, or SiCN, can be arranged between the substrate 2 and the rear electrode 3. This is not shown in detail in FIG. 1.

[0061] A layer system 1 is arranged on the rear electrode 3. The layer system 1 includes an absorber layer 4, made, for example, of Cu(In, Ga) (S, Se).sub.2, which is applied directly on the rear electrode 3. The absorber layer 4 was deposited, for example, with the RTP process described in the introduction. The absorber layer 4 has, for example, a thickness of 1.5 μm.

[0062] A buffer layer 5 is arranged on the absorber layer 4. The buffer layer 5 contains Na.sub.xIn.sub.y x/3S with 0.063≦x≦0.625, 0.681≦y≦1.50, preferably 0.063≦x≦0.469, 0.681≦y≦1,01 and even more preferably 0.13≦x≦0.32, 0.681≦y≦0.78. The layer thickness of the buffer layer 5 is in the range from 20 nm to 60 nm and is, for example, 30 nm.

[0063] A second buffer layer 6 can be arranged, optionally, above the buffer layer 5. The buffer layer 6 contains, for example, non-doped zinc oxide (i-ZnO). A front electrode 7 that serves as a front-side contact and is transparent to radiation in the visible spectral range (“window layer”) is arranged above the second buffer layer 6. Usually, a doped metal oxide (TCO=transparent conductive oxide), for example, n-conductive, aluminum (Al)-doped zinc oxide (ZnO), boron (B)-doped zinc oxide (ZnO), or gallium (Ga)-doped zinc oxide (ZnO), is used for the front electrode 7. The layer thickness of the front electrode 7 is, for example, roughly 300 to 1500 nm. For protection against environmental influences, a plastic layer (encapsulation film) made, for example, of polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), or silicones can be applied to the front electrode 7.

[0064] In addition, a cover plate transparent to sunlight that is made, for example, from extra white glass (front glass) with a low iron content and has a thickness of, for example, 1 to 4 mm, can be provided.

[0065] The described structure of a thin-film solar cell or of a thin-film solar module is well known to the person skilled in the art, for example, from commercially available thin-film solar cells or thin-film solar modules and has also already been described in detail in numerous printed documents in the patent literature, for example, DE 19956735 B4.

[0066] In the substrate configuration depicted in FIG. 1, the rear electrode 3 adjoins the substrate 2. It is understood that the layer system 1 can also have a superstrate configuration, in which the substrate 2 is transparent and the front electrode 7 is arranged on a surface of the substrate 2 facing away from the light-entry side.

[0067] The layer system 1 can serve for production of integrated serially connected thin-film solar cells, with the layer system 1, the rear electrode 3, and the front electrode 7 patterned in a manner known per se by various patterning lines (“P1” for rear electrode, “P2” for contact front electrode/rear electrode, and “P3” for separation of the front electrode).

[0068] FIG. 2A depicts a ternary diagram for the representation of the composition Na.sub.xIn.sub.y-x/3S of the buffer layer 5 of the thin-film solar cell 100 of FIG. 1. The relative fractions for the components sulfur (S), indium (In), and sodium (Na) of the buffer layer 5 are indicated in the ternary diagram.

[0069] The composition region claimed according to the invention, defined by 0.063≦x≦0.625 and 0.681≦y≦1.50, is defined by the region outlined by the solid line. Data points inside the outlined composition region indicate exemplary compositions of the buffer layer 5. FIG. 2B depicts an enlarged detail of the ternary diagram with the composition region claimed according to the invention.

[0070] The straight line identified with “Ba”, which is not part of the composition region claimed by the invention, indicates a composition for a sodium indium sulfide buffer layer depicted in the publication of Barreau et al. cited in the introduction. This can be described by the molecular formula Na.sub.xIn.sub.2.33-x/S.sub.32 with 1≦x≦4. Accordingly, the straight line is marked through the starting point In.sub.2S.sub.3 and the endpoint NaIn.sub.5S.sub.8. It is characteristic here that thin-films have a maximum sodium fraction of 5 atom-% (Na/In=0.12) and that a monocrystal has a sodium fraction of 7 atom-% (Na/In=0.2). High crystallinity has been reported for these layers.

[0071] As already stated in the introduction, these buffer layers have, with a sodium content of more than 6 atom-%, a bandgap of 2.95 eV, which results in an unsatisfactory band adaptation to the absorber or to the front electrode and, thus, results in the degradation of the electrical properties such that these buffer layers are unsuitable for use in thin-film solar cells. The composition range claimed according to the invention is, according to Barreau et al., impossible.

[0072] This disadvantage is avoided according to the invention in that the sodium fraction reaches values clearly higher than Na/In=0.12 or 0.2. As the inventors were surprisingly able to demonstrate, only by means of a relatively small sulfur fraction in the buffer layer 5 is a higher sodium fraction made possible, with the satisfactory layer properties for band adaptation in the solar cells retained. For example, with the capabilities for reducing the sulfur fraction in the buffer layer described in international patent application WO 2011/104235, the composition can be selectively controlled in an indium-enriched region. Thus, it is possible to deposit the sodium indium sulfide buffer layer either amorphously or in a nanocrystalline structure (instead of crystalline), since the sodium indium sulfide phases present in the buffer layer have different crystalline structures. In this manner, an inward diffusion of copper from the absorber layer into the buffer layer can be inhibited, which improves the electrical properties of solar cells, in particular chalcopyrite solar cells. Due to alloying with sodium, the bandgap and the charge carrier concentration of the buffer layer 5 can be adjusted, by means of which the electronic transition from the absorber layer 4 via the buffer layer 5 to the front electrode 7 can be optimized. This is explained in greater detail in the following.

[0073] FIG. 3A depicts a diagram, in which the efficiency Eta (percent) of the thin-film solar cell 100 of FIG. 1 is plotted against the sodium indium fraction in the buffer layer 5. This is a corresponding projection from FIG. 2A. FIG. 3B depicts a diagram, in which the efficiency Eta (percent) of the thin-film solar cell 100 of FIG. 1 is plotted against the absolute sodium fraction (atom-%) in the buffer layer 5.

[0074] For example, the thin-film solar cell 100 used for this contains a substrate 2 made of glass as well as a rear electrode 3 made of a Si.sub.3N.sub.4 barrier layer and a molybdenum layer. An absorber layer 4 made of Cu(In, Ga) (S, Se).sub.2, which was deposited according to the above described RTP process, is arranged on the rear electrode 3. A Na.sub.xIn.sub.y-x/3S buffer layer 5 with 0.063≦x≦0.625 and 0.681≦y≦1.50 is arranged on the absorber layer 4. The layer thickness of the buffer layer 5 is 50 nm. A 100-nm-thick second buffer layer 6, which contains non-doped zinc oxide, is arranged on the buffer layer 5. A 1200-nm-thick front electrode 7, which contains n-conductive zinc oxide, is arranged on the second buffer layer 6. The area of the thin-film solar cell 100 is, for example, 1.4 cm.sup.2.

[0075] In FIG. 3A and 3B, it is discernible that through an increase of the sodium indium fraction (Na/In>0.2) or through an increase of the absolute sodium content (Na>7 atom-%) of the buffer layer 5, the efficiency of the thin-film solar cell 100 can be significantly increased compared to conventional thin-film solar cells. As already stated, such a high sodium fraction can be obtained in the buffer layer 5 only through a relatively low sulfur fraction. With the structure according to the invention, it was possible to obtain high efficiencies of up to 13.5%.

[0076] FIG. 4 depicts, for the above-described layer system 1, a measurement of the bandgap of the buffer layer 5 as a function of the sodium fraction of the buffer layer 5. Accordingly, an enlargement of the bandgap from 1.8 eV to 2.5 eV can be observed with a sodium fraction of more than 7 atom-%. By means of the buffer layer 5 according to the invention, significant improvement of the efficiency of the thin-film solar cell 100 can be obtained, without a degradation of the electrical layer properties (good band adaptation to absorber or front electrode by not excessively large bandgap).

[0077] FIG. 5 depicts a depth profile of the sodium distribution in the buffer layer 5 of the layer system 1 of FIG. 1 generated by a ToF-SIMS measurement. The normalized depth is plotted as the abscissa, the normalized signal intensity is plotted as the ordinate. The region from 0 to 1 of the abscissa marks the buffer layer 5 and the region with values greater than 1 marks the absorber layer 4. Compounds of sodium with the chalcogen sulfur (S), preferably Na.sub.2S, were used as starting materials for the sodium alloying of the indium sulfide layer. It would also be equally conceivable to use a compound of sodium with sulfur and indium, for example, NaIn.sub.3S.sub.5. In the buffer layer 5, which is applied in each case on a CIGSSe absorber layer 4, differently high sodium fractions are contained (amount 1, amount 2). An indium sulfide buffer layer not alloyed with sodium was used as a reference.

[0078] An increase of the sodium fraction discernibly develops in the layer stack due to the sodium alloy, with, despite uniform deposition of the alloy on the absorber-buffer interface, by means of diffusion mechanisms, a slight enrichment of the sodium fraction in the buffer layer 5 (“doping peak”) developing. The buffer layer 5 can, at least theoretically, be divided into two regions, namely, a first layer region adjoining the absorber layer and a second layer region adjoining the first layer region, with the layer thickness of the first layer region being, for example, equal to the layer thickness of the second layer region. Accordingly, the mole fraction of sodium has a maximum in the first layer region and decreases both toward the absorber layer 4 and also toward the second layer region. A specific sodium concentration is retained over the entire layer thickness in the buffer layer 5. The accumulation of sodium at the absorber-buffer interface is believed to be attributable to high defect density at this location.

[0079] Besides sodium, oxygen (O) or zinc (Zn) can also accumulate in the buffer layer 5, for example, by diffusion out of the TCO of the front electrode 7. Due to the hygroscopic properties of the starting materials, an accumulation of water from the ambient air is also conceivable.

[0080] Particularly advantageously, the halogen fraction in the buffer layer according to the invention is small, with the mole fraction of a halogen, for example, chlorine, being less than 5 atom-%, in particular less than 1 atom-%. Particularly advantageously, the buffer layer 5 is halogen free.

[0081] FIG. 6 depicts a flow chart of a method according to the invention. In a first step, an absorber layer 4, made, for example, of a Cu(In, Ga) (S, Se).sub.2 semiconductor material, is prepared. In a second step, the buffer layer 5 made of sodium indium sulfide is deposited. The ratio of the individual components in the buffer layer 5 is regulated, for example, by control of the evaporation rate, for example, by a screen or temperature control. In further process steps, a second buffer layer 6 and a front electrode 7 can be deposited on the buffer layer 5. In addition, wiring and contacting of the layer structure 1 to a thin-film solar cell 100 or a solar module can occur.

[0082] FIG. 7 depicts a schematic representation of an in-line method according to the invention for producing a buffer layer 5 made of sodium indium sulfide. The substrate 2 with rear electrode 3 and absorber layer 4 is conveyed, in an in-line method past the steam beams 11, 12 of a first sodium thioindate source 8, for example, NaIn.sup.5S.sub.8, as well as a second sodium thioindate source 9, for example, NaInS.sub.2. The transport direction is indicated by an arrow with the reference character 10. The steam beams 11, 12 do not overlap. In this manner, the absorber layer 4 is coated first with a thin layer of NaIn.sub.5S.sub.8, then, with a thin layer of NaInS.sub.2, which blend. Both sodium thioindate sources 8, 9 are, for example, effusion cells, from which sodium thioindate is thermally evaporated. Especially simple process control is enabled by the non-overlapping sources. It would also be conceivable for the both sources 8, 9 to contain the same sodium thioindate compound, for example, only NaIn.sub.3S.sub.5 or only NaIn.sub.5S.sub.8, or for only one single sodium thioindate source to be used, for example, the sodium thioindate source 8. Alternatively, any other form of generating steam beams 11,12 is suitable for depositing the buffer layer 5. Alternative sources are, for example, boats of linear evaporators or crucibles of electron-beam evaporators.

[0083] FIG. 8 depicts an alternative apparatus for performance of the method according to the invention, wherein only the differences relative to the apparatus of FIG. 7 are explained and, otherwise, reference is made to the above statements. Accordingly, the substrate 2 is conveyed, in an in-line method, past the steam beams 11,12 of two sodium thioindate sources 8, 9, wherein, in this case, the steam beams 11, 12 overlap partially. It would also be conceivable for the steam beams to overlap completely.

[0084] FIG. 9 depicts another alternative embodiment of the method according to the invention using the example of a rotation method. The substrate 2 with rear electrode 3 and absorber layer 4 is arranged on a rotatable sample carrier 13, for example, on a sample carousel. Alternatingly arranged sources 8, 9 of sodium thioindate, for example, a first source 8 with NaIn.sub.5S.sub.8 and a second source 9 with NaInS.sub.2 are situated below the sample carrier 13.

[0085] During the deposition of the buffer layer 5, the sample carrier 13 is rotated. Thus, the substrate 2 is moved into the steam beams 11, 12 and coated.

[0086] From the above assertions, it has become clear that by means of the present invention, the disadvantages of previously used CdS buffer layers could be overcome in thin-film solar cells, with the efficiency and the stability of the solar cells produced therewith also very good or better. At the same time, the production method is, through the use of at least one sodium thioindate compound technically relatively simple, economical, effective, and environmentally safe. It has been demonstrated that with the layer system according to the invention, comparably good solar cell characteristics can be obtained as are present with conventional CdS buffer layers.

LUST OF REFERENCE CHARACTERS

[0087] 1 layer system

[0088] 2 substrate

[0089] 3 rear electrode

[0090] 4 absorber layer

[0091] 5 buffer layer

[0092] 6 second buffer layer

[0093] 7 front electrode

[0094] 8 first sodium thioindate source

[0095] 9second sodium thioindate source

[0096] 10 transport direction

[0097] 11 first steam beam

[0098] 12 second steam beam

[0099] 13 sample carrier

[0100] 100 thin-film solar cell