LAYER SYSTEM FOR THIN-FILM SOLAR CELLS

Abstract

A layer system (1) for thin-film solar cells (100), comprising an absorber layer (4), which contains a chalcogenide compound semiconductor, and a buffer layer (5), which is arranged on the absorber layer (4), wherein the buffer layer (5) has a semiconductor material of the formula A.sub.xIn.sub.yS.sub.z, where A is potassium (K) and/or cesium (Cs), with 0.015x/(x+y+z)0.25 and 0.30y/(y+z)0.45.

Claims

1. Layer system (1) for thin-film solar cells (100), comprising: an absorber layer (4), which contains a chalcogenide compound semiconductor, a buffer layer (5), which is arranged on the absorber layer (4), wherein the buffer layer (5) has a semiconductor material of the formula A.sub.xIn.sub.yS.sub.z, where A is potassium (K) and/or cesium (Cs), with 0.015x/(x+y+z)0.25, and 0.30y/(y+z)0.45.

2. Layer system (1) according to claim 1, wherein in the buffer layer (5) 0.05x/(x+y+z)0.20 and 0.35y/(y+z)0.45.

3. Layer system (1) according to claim 1, wherein in the buffer layer (5) with A=K 0.05x/(x+y+z)0.15 and 0.35y/(y+z)0.45.

4. Layer system (1) according to claim 1, wherein in the buffer layer (5) with A=Cs 0.05x/(x+y+z)0.12 and 0.35y/(y+z)0.45.

5. Layer system (1) according to one of the preceding claims 1 through 4, wherein the buffer layer (5) has at least one halogen, selected in particular from the group consisting of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

6. Layer system (1) according to claim 5, wherein a halogen content of the buffer layer (5) corresponds to an alkali content of the buffer layer (5).

7. Layer system (1) according to one of the preceding claims 1 through 6, wherein a second buffer layer (6) is arranged on an A.sub.xIn.sub.yS.sub.z-containing first buffer layer (5), wherein the second buffer layer (6) contains, in particular, non-doped zinc oxide (ZnO) and/or non-doped zinc magnesium oxide (ZnMgO).

8. Layer system (1) according to one of the preceding claims 1 through 7, wherein the buffer layer (5) contains zinc (Zn), wherein the zinc content is a maximum of 15 atom-%.

9. Layer system (1) according to one of the preceding claims 7 and 8, wherein the zinc content of the first buffer layer (5) increases to the second buffer layer (6).

10. Thin-film solar cell (100), comprising: a substrate (2), a rear electrode (3), which is arranged on the substrate (2), a layer system (1) according to one of claims 1 through 9, which is arranged on the rear electrode (3), and a front electrode (7), which is arranged on the layer system (1).

11. Method for producing a layer system (1) according to one of the preceding claims 1 through 9, wherein a) an absorber layer (4), which contains a chalcogenide compound semiconductor, is prepared, and b) a buffer layer (5) is arranged on the absorber layer (4), wherein the buffer layer has a semiconductor material of the formula A.sub.xIn.sub.yS.sub.z with 0.015x/(x+y+z)0.25 and 0.30y/(y+z)0.45, where A is potassium and/or cesium, wherein the buffer layer (5) is produced on the basis of at least one potassium compound and indium sulfide and/or on the basis of at least one cesium compound and indium sulfide, in particular by atomic layer deposition (ALD), ion layer gas deposition (ILGAR), spray pyrolysis, chemical vapor deposition (CVD), or physical vapor deposition (PVD), sputtering, thermal evaporation, or electron beam evaporation, in particular from separate sources for the potassium compound and/or cesium compound and indium sulfide.

12. Method according to claim 11, wherein the absorber layer (4) is conveyed, in an in-line method or a rotation method, past at least one steam beam (11) of a potassium compound and/or a cesium compound and at least one steam beam (12) of indium sulfide, in particular with at least partially overlapping steam beams (11, 12).

13. Method according to claim 11 or 12, wherein the buffer layer (5) is produced on the basis of at least one potassium halide and/or at least one binary potassium compound, in particular potassium sulfide, and/or at least one ternary potassium compound, in particular at least one ternary potassium indium sulfur compound, for example, KInS.sub.2, KIn.sub.3S.sub.5, KIn.sub.5S.sub.6, KIn.sub.5S.sub.7, and/or KIn.sub.5S.sub.8, and/or at least one cesium halide and/or at least one binary cesium compound, in particular cesium sulfide, and/or at least one ternary cesium compound, in particular at least one ternary cesium indium sulfur compound, for example, CsInS.sub.2, CsIn.sub.3S.sub.5, CsIn.sub.5S.sub.6, CsIn.sub.5S.sub.7, and/or CsIn.sub.5S.sub.8.

14. Method for producing a layer system (1) according to one of the preceding claims 1 through 9, wherein a) an absorber layer (4), which contains a chalcogenide compound semiconductor, is prepared, and b) a buffer layer (5) is arranged on the absorber layer (4), wherein the buffer layer has a semiconductor material of the formula A.sub.xIn.sub.yS.sub.z with 0.015x/(x+y+z)0.25 and 0.30y/(y+z)0.45, where A is potassium and/or cesium, wherein the buffer layer (5) is produced on the basis of at least one ternary potassium indium sulfur compound, for example, KInS.sub.2, KIn.sub.3S.sub.5, KIn.sub.5S.sub.6, KIn.sub.5S.sub.7, and/or KIn.sub.5S.sub.8, and/or on the basis of at least one ternary cesium indium sulfur compound, for example, CsInS.sub.2, CsIn.sub.3S.sub.5, CsIn.sub.5S.sub.6, CsIn.sub.5S.sub.7, and/or CsIn.sub.5S.sub.8, in particular by atomic layer deposition (ALD), ion layer gas deposition (ILGAR), spray pyrolysis, chemical vapor deposition (CVD), or physical vapor deposition (PVD), sputtering, thermal evaporation, or electron beam evaporation.

15. Method according to claim 14, wherein, in step b), the buffer layer (5) is deposited out of the gas phase, wherein the concentration of at least one component of the material to be deposited is reduced by selective bonding in its gas phase and thus, before its deposition on the absorber layer (4).

16. Method according to claim 14 or 15, wherein the absorber layer (4) is conveyed, in an in-line method or a rotation method, past a steam beam (11) of a ternary potassium indium sulfur compound and/or past a steam beam (11) of a ternary cesium indium-sulfur compound, in particular with at least partially overlapping steam beams (11, 12).

Description

[0055] The invention is explained in detail in the following with reference to the accompanying figures. They depict:

[0056] FIG. 1 a schematic cross-sectional view of a thin-film solar cell according to the present invention;

[0057] FIG. 2 a measurement of the efficiency of a thin-film solar cell with the structure of FIG. 1 as a function of the potassium fraction in the buffer layer;

[0058] FIG. 3 a measurement of the efficiency of a thin-film solar cell with the structure of FIG. 1 as a function of the cesium fraction in the buffer layer;

[0059] FIG. 4 a measurement of the bandgap of K.sub.xIn.sub.yS.sub.z buffer layers or Na.sub.xIn.sub.yS.sub.z buffer layers as a function of the alkali fraction;

[0060] FIG. 5 a measurement of the optical transmittance of Cs.sub.xIn.sub.yS.sub.z buffer layers as a function of the cesium fraction;

[0061] FIG. 6 a measurement of the efficiency of thin-film solar modules of a thin-film solar cell with cesium-containing or sodium-containing indium sulfide buffer layers;

[0062] FIG. 7 a measurement of the short-circuit current of the thin-film solar modules of FIG. 6;

[0063] FIG. 8 a schematic representation of an in-line method for producing the buffer layer of the layer system according to the invention;

[0064] FIG. 9 a schematic representation of a rotation method for producing the buffer layer of the layer system according to the invention.

[0065] FIG. 1 depicts, purely schematically, an exemplary embodiment of a thin-film solar cell 100 according to the invention with a layer system 1 according to the invention in a cross-sectional view. The thin-film solar cell 100 includes a substrate 2 and a rear electrode 3. The layer system 1 according to the invention is arranged on the rear electrode 3. The layer system 1 according to the invention comprises an absorber layer 4, a first buffer layer 5, as well as, optionally, a second buffer layer 6. A front electrode 7 is arranged on the layer system 1.

[0066] 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.

[0067] The 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), 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 m, 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.

[0068] The layer system 1 according to the invention 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 and has, for example, a thickness of 1.5 m.

[0069] The first buffer layer 5 is arranged on the absorber layer 4. The first buffer layer 5 contains a semiconductor material of the formula K.sub.xIn.sub.yS.sub.z or Cs.sub.xIn.sub.yS.sub.z with

0.05x/(x+y+z)0.25 and 0.30y/(y+z)0.45, preferably 0.05x/(x+y+z)0.20 and 0.35y/(y+z)0.45.

[0070] For K.sub.xIn.sub.yS.sub.z, the following preferably applies: 0.05x/(x+y+z)0.15 and 0.35y/(y+z)0.45. For Cs.sub.xIn.sub.yS.sub.z, the following preferably applies:

0.05x/(x+y+z)0.12 and 0.35y/(y+z)0.45.

[0071] As was already mentioned in the introduction, the first buffer layer 5 advantageously has, in addition to a halogen, if potassium or cesium was deposited in a halogen compound, no significant fraction (1 atom-%) of other elements. The layer thickness of the buffer layer 5 is preferably in the range from 15 nm to 60 nm and is, for example, 30 nm.

[0072] The second buffer layer 6 is arranged above the first buffer layer 5. The second buffer layer 6 is optional, i.e., it does not necessarily have to be present in the layer system 1. The buffer layer 6 contains, for example, non-doped zinc oxide (i-ZnO).

[0073] The 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 DNP can be applied to the front electrode 7.

[0074] 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.

[0075] The basic structure of a thin-film solar cell or 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.

[0076] 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.

[0077] 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).

[0078] FIG. 2 depicts a diagram of the efficiency of thin-film solar cells 100 according to the structure of FIG. 1 as a function of the potassium fraction in the first buffer layer 5, which has a semiconductor material of the formula K.sub.xIn.sub.yS.sub.z. The thin-film solar cell 100 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. The first buffer layer 5 is arranged on the absorber layer 4. The layer thickness of the first buffer layer 5 is 30 nm. A 100-nm-thick second buffer layer 6, which contains non-doped zinc oxide, is arranged on the first 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.

[0079] As indicated in FIG. 2, indium and sulfur are present in the first buffer layer 5 in the following molar ratio: 0.394y/(y+z)0.421. The potassium fraction of the first buffer layer 5 varies from 0 atom-% to 16 atom-%. The measurements yielded a maximum efficiency of ca. 12.5%, by means of which it is discernible that the efficiency is clearly above the efficiency (ca. 9%) of the comparable solar cell without potassium in the first buffer layer 5.

[0080] FIG. 3 depicts a diagram of the efficiency of thin-film solar cells 100 according to the structure of FIG. 1 as a function of the cesium fraction in the first buffer layer 5, which has a semiconductor material of the formula Cs.sub.xIn.sub.yS.sub.z. The solar cells of the structure of FIG. 3 correspond to those of FIG. 2, with the exception that the indium sulfide-containing semiconductor material of the first buffer layer 5 was provided with cesium.

[0081] As indicated in FIG. 3, indium and sulfur are present in the first buffer layer 5 in the following molar ratio: 0.400y/(y+z)0.413. The cesium fraction of the first buffer layer 5 varies from ca. 1.5 atom-% to ca. 14 atom-%. The measurements yielded a maximum efficiency of ca. 16%, with the efficiency up to a cesium fraction of ca. 8 atom-% clearly above 14% and dropping off after a cesium fraction of ca. 10 atom-%.

[0082] FIG. 4 depicts a measurement of the optical bandgap E.sub.g in electron volts (eV) as a function of the potassium content of K.sub.xIn.sub.yS.sub.z(:Cl) buffer layers produced by co-evaporation of KCl and InS. As a comparative measurement, the optical bandgap E.sub.g is shown as a function of the sodium content of Na.sub.xIn.sub.ySz(:Cl) buffer layers produced by co-evaporation of NaCl and InS. Das the quantity ratio y/(y+z) was ca. 0.39 to 0.42. The bandgap was determined using ellipsometry, the potassium content, using x-ray fluorescence analysis (XRFA) via the chlorine content.

[0083] As emerges from FIG. 4, the bandgap increases with a potassium content in the range from 0 to 10.5 atom-% nearly linearly from ca. 2 eV to almost 2.5 eV, which results, in a thin-film solar cell, in a corresponding increase of the short-circuit current due to reduced absorption. The bandgaps of the Na-containing indium sulfide buffer layers are clearly under those of the K-containing indium sulfide buffer layers.

[0084] FIG. 5 depicts a measurement of the optical transmittance of Cs.sub.xIn.sub.yS.sub.z buffer layers on glass as a function of the cesium fraction. The respective cesium content in the buffer layer is indicated in the diagram, with the lowest curve referring, as a reference measurement, to a cesium-free indium sulfide buffer layer.

[0085] In FIG. 5, it is clearly discernible that with an increasing cesium fraction, the beginning of transmittance shifts to smaller wavelengths, which is caused by a widening of the bandgap. Overall, an improvement of transmittance can be observed with increasing cesium content.

[0086] FIG. 6 depicts a comparison of the efficiencies of thin-film solar modules with an In.sub.yS.sub.z:CsCl buffer layer (left) and an In.sub.yS.sub.z:NaCl buffer layer (right). Obviously, clearly higher efficiencies can be obtained with a buffer layer with a cesium-containing indium sulfide semiconductor material than with a buffer layer with a sodium-containing indium sulfide semiconductor material.

[0087] In FIG. 7, the associated short-circuit current Isc (mA/cm.sup.2) is shown. A significant increase of the short-circuit current is observed for solar modules with a buffer layer with a cesium-containing indium sulfide semiconductor material compared to a buffer layer mit a sodium-containing indium sulfide semiconductor material. The introduction of CsCl into the buffer layer thus improves the transmittance.

[0088] FIG. 8 is a schematic representation of an in-line method for producing a buffer layer 5 made of A.sub.xIn.sub.yS.sub.z with A=potassium or cesium. The substrate 2 with rear electrode 3 and absorber layer 4 is conveyed past the first steam beam 11 of a first source 8 and past a second steam beam 12 of a second source 9. The transport direction is indicated by an arrow with the reference character 10. As illustrated schematically, the steam beams 11, 12 of the two sources 8, 9 overlap partially.

[0089] Potassium compounds or cesium compounds and indium sulfide can be used as starting materials for producing a semiconductor material of the formula K.sub.xIn.sub.yS.sub.z or Cs.sub.xIn.sub.yS.sub.z. Accordingly, the first source 8 is a potassium and/or cesium source 8; and the second source 9, an indium sulfide source. For example, compounds of potassium or cesium with the chalcogen sulfur, in particular K.sub.2S or Cs.sub.2S, can be used. Alternatively, halogen salts of potassium or cesium can be used, for example, potassium fluoride (KF) or cesium fluoride (CsF), potassium chlorid (KCl) or cesium chloride (CsCl), or corresponding bromides or iodides. This offers advantages, particularly through easier handling of these relatively harmless materials in a production environment. In this manner, the absorber layer 4 is coated with thin layers of a halogen or sulfide of potassium or cesium and indium sulfide, which blend. The two sources 8, 9 are, for example, effusion cells, from which the respective substance is thermally evaporated. Alternatively, any other form of generating steam beams 11, 12 is suitable for depositing the buffer layer 5, so long as it enables establishing the required ratio of the mole fractions of potassium or cesium as well as indium and sulfur. Alternative sources are, for example, boats of linear evaporators or crucibles of electron-beam evaporators. Since the potassium or cesium compound is deposited before the indium sulfide on the absorber layer 4, electrical passivation of the interface can be obtained, by means of which the efficiency of the solar cell is improved.

[0090] Alternatively, for performance of the method according to the invention, one or a plurality of sources of potassium or cesium 8 and one or plurality of sources of indium sulfide 9 can be alternatingly arranged one- or two-dimensionally such that [0091] two or more potassium or cesium layers are deposited alternatingly with one or plurality of indium sulfide layers, or [0092] two or more indium sulfide layers are deposited alternatingly with one or plurality of potassium or cesium layers,
on the absorber layer 4. Thus, a very homogeneous buffer layer 5 can be produced, by which means the efficiency of the solar cell is increased. It is particularly advantageous for a potassium or cesium layer to be applied as the first layer on the absorber layer 4, in order to obtain electrical passivation of the interface for further improvement of efficiency.

[0093] Alternatively, it is possible to use only at least one ternary potassium indium sulfur compound, for example, KInS.sub.2, KIn.sub.3S.sub.5, KIn.sub.5S.sub.6, KIn.sub.5S.sub.7, and/or KIn.sub.5S.sub.8, as starting materials for producing a semiconductor material of the formula K.sub.xIn.sub.yS.sub.z, and to use only at least one ternary cesium indium sulfur compound, for example, CsInS.sub.2, CsIn.sub.3S.sub.5, CsIn.sub.5S.sub.6, CsIn.sub.5S.sub.7, and/or CsIn.sub.5S.sub.8, as starting materials for producing a semiconductor material of the formula Cs.sub.xIn.sub.yS.sub.z. The use of indium sulfide is dispensed with here. Accordingly, the first source 8 is a source for a ternary potassium indium sulfur compound and the second source 9 is a source for a ternary cesium indium sulfur compound. It is also possible for the first source 8 to be a source for a first ternary potassium indium sulfur compound and the second source 9 to be a source for a second ternary potassium indium sulfur compound different therefrom. Similarly, it is also possible for the first source 8 to be a source for first ternary cesium indium sulfur compound and the second source 9 to be a source for a second ternary cesium indium sulfur compound different therefrom. It is also conceivable, in each case, to use more than two sources 8, 9. As already stated, the use of the ternary potassium indium sulfur compound as well as the ternary cesium indium sulfur compound bring with them significant process technology advantages. It is conceivable for the two sources to contain the same ternary potassium indium sulfur compound or the same ternary cesium indium sulfur compound. It is also conceivable for only one source 8 with only one ternary potassium indium sulfur compound or only one ternary cesium indium sulfur compound to be present.

[0094] FIG. 9 depicts an 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 of potassium or cesium 8 and indium sulfide 9 are situated below the sample carrier 13. 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.

[0095] Both in the lab process with a rotating substrate 2 on a rotatable sample carrier 13 and also in the industrial in-line method with linear feed of the substrate 2, the evaporation rates of sources 8,9 can be selected such that the fraction of potassium and/or cesium varies such that a potassium and/or cesium gradient can be generated in the buffer layer 5.

[0096] Alternatively, with regard to the sources 8, 9 of FIG. 9, the first source 8 can be a source for a ternary potassium indium sulfur compound or a ternary cesium indium sulfur compound, and the second source 9 can be a source for a ternary potassium indium sulfur compound or a ternary cesium indium sulfur compound.

[0097] 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. This is been achieved by means of an indium sulfide buffer layer provided with potassium or cesium. Through the introduction of potassium or cesium, different crystal structures are formed, which results in a fine crystalline layer during layer growth. In addition, the relatively large atomic radius disrupts the formation of the indium sulfide crystal structure and, thus, also reduces the crystallinity. Compared to sodium, potassium and cesium have a larger ionic radius and thus have a reduced diffusion tendency. By means of potassium and cesium, the optical bandgap of the indium sulfide buffer layer can be widened. Compared to the sodium-containing indium sulfide buffer layers, better band adaptation, reduced light absorption, as well as a significant increase in the short-circuit current and improved transmittance, as well as, overall, higher efficiency can be obtained. At the same time, the production method is economical, effective, and environmentally safe.

[0098] 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. With the structure according to the invention, it was possible to obtain very high efficiencies up to 16%. This was unexpected and surprising for the person skilled in the art.

LIST OF REFERENCE CHARACTERS

[0099] 1 layer system [0100] 2 substrate [0101] 3 rear electrode [0102] 4 absorber layer [0103] 5 first buffer layer [0104] 6 second buffer layer [0105] 7 front electrode [0106] 8 first source [0107] 9 second source [0108] 10 transport direction [0109] 11 first steam beam [0110] 12 second steam beam [0111] 13 sample carrier [0112] 100 thin-film solar cell