Layer structure for a thin-film solar cell and production method

Abstract

A layer structure for a thin-film solar cell and production method are provided. The layer structure for the thin-film solar cell includes a photovoltaic absorber layer doped, at least in a region which borders a surface of the photovoltaic absorber layer, with at least one alkali metal. The layer structure has an oxidic passivating layer on the surface of the photovoltaic absorber layer, which is designed to protect the photovoltaic absorber layer from corrosion.

Claims

1. A layer structure for a thin-film solar cell, comprising: a photovoltaic absorber layer including a first region at a first end of the photovoltaic absorber layer that has been converted from a surface portion of the photovoltaic absorber layer into an oxidic passivating region and a second region at a second end of the photovoltaic absorber layer that is opposite from the first end, the photovoltaic absorber layer being doped, at least in the first region, with at least one alkali metal, wherein the oxidic passivating region is corrosion-resistant, wherein the photovoltaic absorber layer comprises Cu(In.sub.1-xGa.sub.x)(Se.sub.1-yS.sub.y).sub.2, where 0≤x,y≤1, and Cu.sub.2ZnSn(Se.sub.1-xS.sub.x).sub.4, where 0≤x≤1, and the oxidic passivating region further comprises at least one of (In,Ga).sub.2O.sub.3 and M.sub.x(In,Ga).sub.yO.sub.z, where M=K, Rb, Cs and where 0<x, y, z, or the photovoltaic absorber layer comprises Cu.sub.2ZnSn(Se.sub.1-xS.sub.x).sub.4, where 0≤x≤1, and the oxidic passivating region further comprises SnO.sub.x, where x=1 to 2.

2. The layer structure as claimed in claim 1, wherein an upper end of the second region of the photovoltaic absorber layer is doped such that it exhibits a p-n inversion.

3. The layer structure as claimed in claim 1, wherein the at least one alkali metal is at least one of rubidium and cesium.

4. The layer structure as claimed in claim 1, wherein the oxidic passivating region comprises at least one of (In,Ga).sub.2O.sub.3 and M.sub.x(In,Ga).sub.yO.sub.z, where M=K, Rb, Cs and where 0<x, y, z, and the oxidic passivating region further comprises Al.sub.2O.sub.3, ZnO, and SnO.sub.x where x=1 to 2.

5. The layer structure as claimed in claim 1, wherein a thickness of the oxidic passivating region is in the range from 1 nm to 50 nm.

6. The layer structure as claimed in claim 1, wherein the oxidic passivating region bears an applied buffer layer.

7. The layer structure as claimed in claim 1, wherein the oxidic passivating region further comprises ZnO.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Advantageous embodiments of the invention are set out in the drawings and are described below. In these drawings:

(2) FIG. 1 shows a schematic cross-sectional view through a thin-film solar cell, and

(3) FIG. 2 shows a flow diagram of a method for producing the thin-film solar cell of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) FIG. 1 shows a schematic cross-sectional view through a thin-film solar cell 1 according to an advantageous embodiment. In a flow diagram, FIG. 2 shows the essential steps that are of interest here in an exemplary production method for the thin-film solar cell 1. In a first step I, a substrate 2 made of glass, for example, is provided. In a second step II a Mo layer 3 (molybdenum), which is intended to serve as back contact for the thin-film solar cell 1, is applied by cathodic sputtering under reduced pressure to the glass substrate 2, in a thickness of around 500 nm, for example.

(5) Thereafter, in a co-vaporization process (step III, ax), the elements Cu, In, Ga, and Se are applied by vapor deposition at a temperature of the glass substrate 2 of around 600° C., for example. In this way a photovoltaic absorber layer 5, consisting of the compound semiconductor material Cu(In,Ga)Se.sub.2 (CIGS), grows on the Mo layer 3, in a thickness of 2 μm to 3 μm (micrometers), for example. At this point in time, the photovoltaic absorber layer 5 is slightly p-doped everywhere, owing to intrinsic defects in the CIGS material and owing to a possible extrinsic doping with sodium and potassium from the glass substrate, if present there.

(6) After the photovoltaic absorber layer 5 has been generated, RbF and/or CsF is applied by vapor deposition under reduced pressure from a vaporization crucible at crucible temperatures of at least about 400° C. onto a surface 6 of the photovoltaic absorber layer 5 (step IV). At slightly elevated temperature of the glass substrate 2, of around at least 350° C., the at least one alkali metal diffuses into the photovoltaic absorber layer 5 (step V, az). In this procedure, on account of their greater ionic radius, Rb and/or Cs atoms/ions irreversibly displace Cu atoms from the surface 6 in the direction of the molybdenum layer 3 into the volume of the photovoltaic absorber layer 5. Because of this exchange, the maximum alkali metal concentration is found close to the surface in the photovoltaic absorber layer 5. This leads to a p-n inversion 9 in the photovoltaic absorber layer 5 in the vicinity of the surface 6. The Rb or Cs penetrates into the volume of the photovoltaic absorber layer 5, preferably along grain boundaries in the CIGS material, up to the molybdenum layer 3, where it may partially displace sodium and potassium, if present. Steps IV and V in this case are PDT processes. In an alternative embodiment, RbF and/or CsF may be applied by vapor deposition under an Se atmosphere and/or S atmosphere. Alternatively or additionally, the at least one alkali metal may diffuse into the photovoltaic absorber layer under an Se atmosphere and/or S atmosphere.

(7) Thereafter an oxidic passivating layer 8 is generated on the surface 6 of the photovoltaic absorber layer 5 by annealing of the layer structure generated so far, in ambient air (step VI, b), at around 200° C., for example. In this process, oxygen on the surface 6 is built into the alkali-metal-doped CIGS material; the photovoltaic absorber layer 5 is converted on the surface 6. The oxidic passivating layer 8 in that case, for example, comprises the materials (In,Ga).sub.2O.sub.3 and M.sub.x(In,Ga).sub.yO.sub.z where M=Rb and/or Cs and where 0<x, y, z. The oxidic passivating layer 8 extends over a thickness D of, for example, around 3 nm to 5 nm into the photovoltaic absorber layer 5 in FIG. 1 downwardly. The oxidic passivating layer 8 protects the major remaining part of the photovoltaic absorber layer 5 from corrosion, especially during the further production of the thin-film solar cell 1.

(8) Subsequently a CdS buffer layer 10 around 30 nm to 50 nm thick is deposited or grown, in a chemical deposition bath, on the surface 6 of the photovoltaic absorber layer 5 and/or of the oxidic passivating layer 8 (step VII). Because of a change in the surface polarity of the absorber layer 5, the oxidic passivating layer 8 promotes the growth of the CdS buffer layer 10 and enhances its quality.

(9) There follows an undoped i-ZnO layer 11, around 50 nm to 100 nm thick, by sputtering under vacuum (step VIII), and a ZnO:Al layer 12 around 150 nm to 250 nm thick, heavily n-doped with aluminum (step IX). The CdS buffer layer 10 and the i-ZnO layer 11 ensure band matching between the CIGS material of the photovoltaic absorber layer 5 and the ZnO:Al layer 12. The ZnO:Al layer 12 is transparent and conductive and is intended to serve as the front contact of the thin-film solar cell 1.

(10) Applied to the ZnO:Al layer 12 are an Ni—Al—Ni grid structure 13 (nickel Ni) approximately several μm thick (step X) and an antireflection layer 14 of MgF (step XI).

(11) Efficiencies of up to 21.7% could be measured on a thin-film solar cell 1 produced as described.

(12) In the embodiment shown, the photovoltaic absorber layer comprises CIGS material. Additionally or alternatively, the photovoltaic absorber layer may comprise other compound semiconductor materials or may consist thereof, more particularly Cu(In.sub.1-xGa.sub.x)(Se.sub.1-yS.sub.y).sub.2 and/or Cu.sub.2ZnSn(Se.sub.1-xS.sub.x).sub.4, which can be produced by co-vaporization of the stated elements or selenization/sulfurization of precursor layers onto an Mo-coated substrate (e.g., glass, metal, ceramic, plastic).

(13) Furthermore, the photovoltaic absorber layer need not be completely doped or have a p-n inversion; at least one region, which borders the surface of the photovoltaic absorber layer, is doped with the at least one alkali metal. Moreover, the doping of the photovoltaic absorber layer of at least one alkali metal may be generated, in at least one region bordering the surface of the photovoltaic absorber layer, by other conventional processes rather than the aforementioned PDT process.

(14) Alternatively or additionally to RbF and/or CsF, moreover, the at least one alkali metal may be provided by means of a different alkali metal fluoride and/or different alkali metal compounds, examples are alkali metal oxides, alkali metal carbonates, alkali metal selenides and/or other alkali metal halides, such as RbCl, CsCl, RbBr, CsBr, RbI and/or CsI.

(15) The oxidic passivating layer, alternatively or additionally to the materials (In,Ga).sub.2O.sub.3 and M.sub.x(In,Ga).sub.yO.sub.z where M=Rb and/or Cs, may comprise or consist of the material Al.sub.2O.sub.3, and Al.sub.2O.sub.3 can be applied to the surface of the photovoltaic absorber layer. Further alternatively or additionally, particularly for a photovoltaic absorber layer which consists of or comprises a kesterite material, the oxidic passivating layer may comprise ZnO and/or SnO.sub.x where x=1 to 2, and/or a mixture of both materials, or may consist of at least one of these materials, in which case alkali metal fractions may also be present as a result of diffusion from the surface or interface.

(16) Alternatively or additionally to CdS, the buffer layer applied to the oxidic passivating layer may comprise or consist of a different material, more particularly Zn(O,S) and/or In.sub.2S.sub.3, which may be deposited in a chemical deposition bath or through a gas-phase process.

(17) Alternatively or additionally to i-ZnO material, the layer applied to the CdS buffer layer may comprise i-Zn.sub.1-xMg.sub.xO where 0≤x≤0.4, or may consist of this material, which can be applied by cathodic sputtering similarly to the i-ZnO.

(18) By optimizing the thickness and the optoelectronic properties of the passivating oxide layer (combination of materials, doping, band gap, etc.), it may not only meet chemical requirements (e.g., protecting the photovoltaic absorber layer from corrosion) but may also, furthermore, meet physical requirements, such as band matching between the material of the photovoltaic absorber layer and the layer of the front contact, for example. Accordingly it is possible to omit the subsequent buffer layer and/or the i-ZnO and/or ZnMgO layer. There have already been multiple demonstrations of conventional CIGS solar cells in which, on the basis of optimization of the thickness and of deposition process of the CdS and/or Zn(O,S) buffer layer, it has been possible to omit the subsequent i-ZnO and/or i-ZnMgO layer.

(19) As the embodiments shown and those explained above make clear, the invention provides an advantageous layer structure for a thin-film solar cell, comprising, on a surface of a photovoltaic absorber layer, an oxidic passivating layer which is designed so as to protect the photovoltaic absorber layer from corrosion, thereby allowing the efficiency of the thin-film solar cell to be increased.