1. Patent Search
  2. Details

P-Type Solar Cell and the Production Thereof

20170133541 · 2017-05-11

    Inventors

    Cpc classification

    International classification

    Abstract

    A P-type solar cell comprises a layer stack with: a back electrode, a p-type semiconductor absorber layer disposed on the back electrode, a crystalline cadmium sulfide (CdS) layer disposed on the absorber layer, and a front electrode disposed on the side of the layer stack opposite of the back electrode. The CdS layer has Cu-doping and a layer thickness between 50 and 300 . A method for producing a p-type solar cell comprises: providing a p-type photoactive semiconductor absorber layer, etching the surface of the absorber layer such that crystallographic unevenness and pinholes are reduced, depositing a CdS layer on the absorber layer, with a layer thickness between 50 and 200 , heating at least the CdS layer to recrystallize the CdS layer, and optionally placing on the absorber layer a Cu-containing layer different from the CdS layer, either after etching or after the application of the CdS layer.

    Claims

    1. P-type solar cell comprising a layer stack (10) with: a rear electrode (14), a p-type semiconductor absorber layer (11) disposed on the rear electrode (14), a crystalline n-type connector layer (12) disposed on the absorber layer (11), a front electrode (15) disposed on the side of the layer stack (10) opposite of the rear electrode (14), characterized in that the n-type layer (12) comprises a dopant and a layer thickness in the range of 50 to 250 .

    2. P-type solar cell according to claim 1, characterized in that the n-type layer (12) is selected from the group of chalcogenides, in particular cadmium sulfide.

    3. P-type solar cell according to claim 1, characterized in that the n-type layer (12) has a proportion of 30-80 ppm, preferably in the range of 40 to 80 ppm, particularly 60 ppm, of a dopant.

    4. P-type solar cell according to claim 1, characterized in that the dopant of the n-type layer (12) is an element from the group of metals of the transition group elements, in particular selected from the group silver, gold or copper.

    5. Method for producing a p-type solar cell, comprising the following steps in the specified order, or in the reverse order: providing a p-type photoactive semiconductor absorber layer (11), etching the surface of the absorber layer (11) such that crystallographic unevenness and pinholes are reduced, applying an n-type layer (12) on the p-type absorber layer (11), with a layer thickness in the range of 50 to 200 , applying heat to at least the n-type layer (12) for recrystallizing the n-type layer (12), as well as optionally placing on the absorber layer (11) a dopant-containing layer different from the n-type layer (12), either after etching or after application of the n-type layer (12).

    6. Method according to claim 5, characterized in that the n-type layer (12) is deposited on the absorber layer (11) by vapor deposition of a phase.

    7. Method according to claim 5, characterized in that etching is performed by using an etching solution comprising hydrochloric acid and a solvent, in particular glycerol.

    8. Method according to claim 5, characterized in that heat is applied at a temperature of at least 350 C., in particular in the range of 350 to 500 C., preferably in the range of 350 to 450 C.

    9. Method according to claim 5, characterized in that heat is applied for a period in the range of 0.5 to 4 h, especially in the range of 0.5 to 2 h.

    10. P-type solar cell comprising a layer stack (10) with: a rear electrode (14), a p-type semiconductor absorber layer (11) disposed on the rear electrode (14), a crystalline cadmium sulfide (CdS) layer (12) disposed on the absorber layer (11), a front electrode (15) disposed on the side of the layer stack (10) opposite of the rear electrode (14), characterized in that the CdS layer (12) is Cu-doped and has a layer thickness in the range of 50 to 300 .

    11. P-type solar cell according to claim 10, characterized in that the CdS layer (12) has a layer thickness in the range of 80 to 200 , in particular in the range of 100 to 180 , preferably 150 .

    12. P-type solar cell according to claim 10, characterized in that the doped CdS layer (12) has a proportion of 30-80 ppm, preferably in the range of 40 to 80 ppm, particularly 60 ppm, of a dopant.

    13. P-type solar cell according to claim 10, characterized in that the dopant of the CdS layer (12) is copper.

    14. Method for producing a p-type solar cell according to claim 10, comprising the following steps in the specified order, or in the reverse order: providing a p-type photoactive semiconductor absorber layer (11), etching the surface of the absorber layer (11) such that crystallographic unevenness and pinholes are reduced, depositing a CdS layer (12) on the absorber layer (11), with a layer thickness in the range of 50 to 200 , applying heat to at least the CdS layer to recrystallize the CdS layer (12), as well as optionally placing on the absorber layer (11) a Cu-containing layer different from the CdS layer, either after etching or after the application of the CdS layer (12).

    15. Method according to claim 14, characterized in that the CdS layer (12) is applied on the absorber layer (11) by vapor deposition of a CdS phase.

    16. Method according to claim 14, characterized in that etching is performed by using an etching solution comprising hydrochloric acid and a solvent, in particular glycerol.

    17. Method according to claim 14, characterized in that heat is applied at a temperature of at least 350 C., in particular in the range of 350 to 500 C., preferably in the range of 350 to 450 C.

    18. Method according to claim 14, characterized in that heat is applied for a duration in the range of 0.5 to 4 h, especially in the range of 0.5 to 2 h.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0041] The invention will now be described with reference to exemplary embodiments illustrated in the drawings, which show in:

    [0042] FIG. 1 a schematic diagram of a layer stack in a preferred embodiment of the invention, and

    [0043] FIG. 2 a diagram of the band structure at the interface between the absorber layer and the p-doped CdS layer by forming a high-field domain in a preferred embodiment of the invention.

    DETAILED DESCRIPTION

    [0044] FIG. 1 shows a schematic diagram of a layer stack 10 in a preferred embodiment of the invention. Shown is the schematic structure of the layer sequence in an exemplary rudimentary form.

    [0045] The layer stack 10 includes a back electrode 14, on which an electrically conductive absorber layer 11 is arranged. The absorber layer 11 includes a semiconductor material exhibiting an internal photo effect, such as CdTe. Alternatively, the absorber material may include a semiconductor material from the group of CIS absorbers (copper indium (gallium)-sulfide (selenide)).

    [0046] A copper-doped CdS layer 12 is disposed on the side of the absorber layer 11 facing away from the back electrode 14. The CdS layer 12 has a thickness in the range of 80 to 200 , preferably in the range of 100 to 180 , in particular 150 . The CdS layer 12 is also p-doped and is preferably doped with copper. The dopant concentration is in the range of 30-90 ppm, preferably in the range of 40 to 80 ppm, in particular 60 ppm. The concentration is distributed within the CdS-layer 12 as uniformly as possible in relation to a plane parallel to the layers boundaries. The concentration may also be uniformly distributed In relation to the layer thickness of the CdS layer 12, or may in a preferred embodiment have a gradient, in which case the concentration decreases starting from the interface between the absorber layer 11 and the CdS layer 12.

    [0047] The CdS layer 12 is in a crystalline state, i.e. is preferably not amorphous, preferably in the form of microscopic crystallites or macroscopic crystallites.

    [0048] A high-field domain is formed within the CdS layer 12. Depending on the layer thickness, the doping level and the degree of crystallization of the CdS layer, the high-field domain extends through and beyond the layer thickness of the CdS.

    [0049] The layer stack 10 is part of a photovoltaic thin layer cell which is generally encapsulated and connected (not shown). The function results in particular from the internal photoelectric effect of the absorber material. When light is incident, a photocurrent is generated within the absorber layer 11 due to the separation by the light excitation of charge carrier pairs that are generated in the space charge region, i.e. in the p/n-junction. The excited charge carriers are carried away by the electrodes that are electrically conductively connected to the absorber material. The inventive design of the cell, in particular the optimization of the transition between the absorber layer 11 and CdS layer 12, enables utilization of the largest possible proportion of the generated charge carriers, i.e. the theoretical efficiency of the cell is to the most part achieved. It turned out that the effect is to a large part due to the p-type nature of the CdS layer 12. This layer has a space-charge-free high-field domain, with a density of free charge carriers (holes) of up to 10.sup.10 cm.sup.3.

    [0050] Therefore, the cell according to the invention has a high open-circuit voltage V.sub.oc, with an only slightly increased series resistance.

    [0051] Such cells can be produced with the method according to the invention.

    Preferred Exemplary Embodiment

    [0052] In a particularly preferred exemplary embodiment, a p-type solar cell according to the invention has a CdTe absorber layer with a layer thickness in the range of 1.5 to 5 m, preferably 2 m. A Cu-doped CdS layer is disposed on the surface the absorber layer facing a front electrode. The dopant concentration in the CdS layer is in a range from 30 to 90 ppm, especially with 60 ppm Cu. The CdS layer has a layer thickness of 130 to 200 , preferably 150 .

    [0053] A band structure as shown in FIG. 2 could be determined for a p-type solar cell according to the preferred exemplary embodiment. Shown are the conduction band and the valence band of the cell in the region absorber layer/CdS layer.

    [0054] The employed CdTe absorber material has at a temperature of 0K a band gap of 1.45 eV, a photocurrent with a short-circuit current density j.sub.SC of 26 mA/cm.sup.2 and an electric field of 100 kV/cm. A hole density of p(CdS)=5.1.Math.10.sup.10 cm.sup.3 was determined, resulting in a band discontinuity between valence band and conduction band E.sub.FpE.sub.v=0.48 eV in CdS. The hole density for the absorber layer is diffusion-limited in the region of the interface. In the described preferred embodiment, the hole density p(CdTe) is 1.25.Math.10.sup.10 cm.sup.3 and the band discontinuity at the interface CdTe/CdS is E.sub.FpE.sub.v=0.54 eV. Based on these values, a Debye length L.sub.D of 15,000 m was determined. This is based on the following calculation of the Debye length

    [00001] L D = .Math. .Math. .Math. k B .Math. T q 2 .Math. N d .Math. .Math. ( generic .Math. .Math. formula ) to L = [ .Math. .Math. .Math. T .Math. .Math. 10 15 ( 10 300 .Math. .Math. p ) ] 1 / 2

    [0055] This means in turn that the field in the thin layer cell according to the invention with the very small CdS layer thickness is constant within the CdS layer over the entire layer thickness. The discontinuity to the Fermi level of the metal is so small that the free charge carriers p can tunnel through the junction without experiencing a significant loss in the current.

    LIST OF REFERENCE NUMBERS

    [0056] 10 Layer stack [0057] 11 Absorber layer [0058] 12 CdS layer with high-field domain [0059] 14 Back electrode [0060] 15 Front electrode