DE102013113585A1 - Passivation layer with point contacts for thin-film solar cells - Google Patents

Abstract

The invention relates to a passivation layer with point contacts for thin-film solar cells comprising a porous layer (pS) of a dielectric material, such as e.g. Al2O3 or VxOy, having a layer thickness in the range of 1 nm to 50 nm. In the pores of the porous layer (pS) nanorods (NS) of a conductive metal oxide are grown with a minimum length corresponding to the layer thickness and which at a length which is greater than the layer thickness of the porous layer and can be up to 1000 nm, stick out of the layer on one side. The nanorods (NS) terminate flush with the passivation layer on the back of the passivation layer. The interfaces between the nanorods and an absorber into which they protrude form the point contacts.

Description

Technical area

The invention relates to a passivation layer with point contacts for thin-film solar cells.

State of the art

Solar cells essentially consist of a front electrode, an absorber layer and a back electrode.

The basic structure of a thin-film solar cell comprises at least one substrate (eg glass or foil), a back contact layer (back electrode, eg made of molybdenum), an absorber layer (for example made of Cu (In, Ga) Se ₂ ), as a rule a buffer layer (eg from CdS ) and a transparent front electrode made, for example, of a transparent oxide conductor (TCO) such as indium tin oxide (ITO) or aluminum doped zinc oxide.

As already mentioned, the back contact layer can be formed from a metal as well as from a transparent conductive material (TCM, transparent conductive material), in particular from a transparent conductive oxide (TCO). It serves the photocurrent generated in the thin-film solar cell as an electrode and may be formed as an anode or as a cathode.

In order for the generated photocurrent to be as efficient as possible, i. With as few losses as possible, it is possible, especially for absorbers with reduced thickness, to suppress, among other effects, recombinations on the rear surface of the absorber layer.

The suppression of recombinations occurs, for example, with a passivation layer which renders most of the interface between the absorber layer and the back contact layer for charge carriers impermeable, as described in article 1 of FIG G. Dingemanns et al. (Status and prospect of Al2O3-based surface passivation systems for silicon solar cells, Journal of Vacuum Science and Technology A 30 (4), Jul / Aug 2012, S. 040802-1-040802-27) for silicon-based solar cells is described.

In order to reduce the recombinations on the surface of the absorber layer, the recombination rate of charge carriers resulting from surface states of the entire surface of the absorber must be reduced. With a positive or negative surface charge density of a dielectric layer (e.g., alumina) contacting the absorber surface, charge carriers are displaced from the semiconductor surface (absorber) by the formation of inversion zones in the barrier layers. The semiconductor surface is then largely passivated due to field effects.

So that the charge carriers in the absorber e.g. reach the back contact layer, the passivation layer at local points (point contacts) is made continuous for the charge carriers, wherein the density of the point contacts is adapted to the carrier diffusion lengths on the semiconductor surface.

In essay 1 of G. Dingemanns et al. Also, the necessity of contacting the metal back contact layers with the absorber layer through the passivation layer disposed therebetween is demonstrated. Contacting can, as set forth in article 1, by e.g. Laser bombardment or photolithography process can be achieved. In the case of laser bombardment, this is done subsequently through the passivation layer or through the metallic back contact layer and the passivation layer. In the case of photolithography process several additional steps (photoresist apply, exposure, etching, developing, etc.) are necessary.

In the US 4,395,583 there is disclosed a passivation layer system in which one of the layers is an extrinsic semiconductor layer in contact with the backside of an absorber layer. The thickness of the extrinsic semiconductor layer is smaller than the diffusion length of the minority carriers in this layer whose effective surface recombination speed is low. This layer is followed by a non-metallic layer having a plurality of micropores. The non-metallic layer is terminated by a back contact layer of metal, the metal filling the micropores and thus making point contacts to the passivation layer. The micropores are introduced by etching into the non-metallic layer.

The reverse side is the side facing away from the incidence of light in classical architecture. This does not exclude the possible in both the so-called "bifacial configuration" exposure from both sides of the solar cell.

In the DE 38 15 512 A1 discloses a solar cell having at the back of the absorber layer, between this and the back contact layer, an insulating layer which is provided over the entire surface with openings. These openings are filled with a contact material (eg aluminum), which forms an ohmic contact to the absorber layer. Starting from the openings, p + doped regions extend into the absorber layer. The openings in the insulating layer are produced by a photolithography and etching process. Alternatively, the contact material aluminum can also be vapor-deposited through a mask onto a compact insulation layer.

Subsequent annealing produces limited, doped regions in the insulation layer.

The application of the layers takes place in the DE 38 15 512 A1 like in the US 4,395,583 and in the procedures described in article 1, process-dependent from front to back.

In the essay 2 of JC Shin et al. (Heterogeneous Integration of InGaAs Nanowires on the rear Surface of Si Solar Cells for Efficiency Enhancement, ACSNano, Vol. 6 (12), 2012, pp. 11074-11079) describes a passivation layer for a silicon solar cell, which also acts as a back reflector. In the layer are InGaAs nanowires that are in contact with the silicon absorber and back contact layer. In the interstices of the nanowires is Epoxidgießharz. The method for producing this passivation layer comprises the steps of applying the nanowires by metal organic vapor phase epitaxy on a prepared silicon wafer surface, covering the nanowires with SiO ₂ , annealing, removing the SiO ₂ layer, applying a SiN _x layer, filling the epoxy resin in the Interspaces of the nanowires, planarization of the back with an O ₂ plasma and removal of the SiN _x layer and is therefore also very expensive.

One way to make porous alumina is anodization, as described in Essay 3 of PH Lu et al. (Anodic Aluminum Oxide Passivation For Silicon Solar Cells, IEEE Journal of Photovoltaics, Vol. 3 (1), 2013, pp. 143-151) is described. The resulting layers, however, with several hundred nanometers unfavorable for the optical properties and need a lot of material. In addition, in the case of anodic aluminum oxide, the pores in the layer are sometimes not completely open.

task

Based on the disadvantages of the known prior art, the object of the present invention is to provide a passivation layer with point contacts for thin-film solar cells, which is to carry out with little effort, has a small layer thickness and additionally in the back region of an absorber layer diffused light scattering and / or antireflective and thereby achieves a better light coupling.

The object is achieved by the features of claim 1.

The passivation layer with point contacts consists at least of a porous layer of a dielectric material whose thickness is in the range of 1 nm to 50 nm.

By forming inversion or accumulation zones at the interface with the absorber caused by the dielectric material of the porous passivation layer, recombinations on the back surface of the absorber layer are suppressed, thus increasing the efficiency of the solar cell.

The small layer thickness of the passivation layer, from 1 nm to 50 nm, causes a reduced in material and time production. In addition, it guarantees good transparency, even with otherwise less transparent materials.

The pores of the passivation layer are open on both sides and filled with nanorods formed of a conductive oxide. The nanorods terminate flush with one side of the passivation layer. This page corresponds to the classical structure of a solar cell of the side facing away from the light, ie. the back. In other, non-classical solar cell structures, however, this side may also face the light. The nanorods have a length which extends from the thickness of the passivation layer up to 1000 nm, and at lengths which are greater than the layer thickness of the passivation layer, can protrude from this layer into a layer applied in the direction of light incidence (in the case of conventional solar cell assembly), for example in an absorber layer.

The ratio of the length of the nanorods to the layer thickness of the passivation layer is therefore between 1000: 1 and 1: 1.

The shape of the nanorods includes all possible shapes with parallel or oblique side surfaces, such. As conical, prismatic, cylindrical or pyramidal forms and mixed forms thereof.

The surface of the nanorods, which protrude into an absorber or have contact therewith, forms the contact from the absorber to the back contact layer through the passivation layer and is referred to as point contact. These point contacts take over the charge transfer between absorber and back contact layer.

By light-reflecting properties, the nanorods in the rear part of the absorber layer into which they protrude, antireflecting and / or cause a diffuse light scattering. This increases the light coupling and thus also the efficiency of the solar cell.

The passivation layer according to the invention with point contacts can, in addition to the arrangement between the back contact layer and the absorber layer, also be applied at other locations, e.g. be arranged between the absorber layer and the front electrode in a solar cell.

A substrate, for example made of glass or a flexible film, with a film of a conductive transparent material, for example a transparent conductive oxide (such as FTO-SnO ₂ : F) or a metal, is to be used advantageously for applying the passivation layer according to the invention.

In addition to the use of the passivation layer according to the invention in inorganic thin-film solar cells, use in other solar cells, e.g. hybrid or organic solar cells, possible.

The passivation layer in one embodiment consists of a dielectric material containing at least one of the metals aluminum, vanadium, indium, niobium, tantalum, chromium, molybdenum, tungsten, silicon or titanium. The passivation layer is further formed in one embodiment from a metal oxide, metal sulfide or a metal nitride. The metal oxide is particularly advantageously alumina (Al ₂ O ₃ ), vanadium oxide, especially with a stoichiometry close to V ₂ O ₃ , ^{or SiO} 2 ^{, as} it corresponds to a specific embodiment.

According to one embodiment, the pores of the passivation layer according to the invention have a diameter which is in the range of 30 nm to 500 nm. The pore density in the layer is between 4 × 10 ⁷ ppi and 250 × 10 ⁸ ppi (pores per inch ² ). This density range has the effect of preserving both the passivation property of the porous layer from which the passivation layer is formed, and allowing a sufficiently large current flow from the absorber layer into the back contact for a number of absorber materials. The adaptation takes place taking into account the different diffusion and recombination lengths of the absorber materials.

Since, as already mentioned, the length of the nanorods lies in a range from 1 nm to 1000 nm, the ratio of the length of the nanorods to their diameter is accordingly between 1000: 30 and 50: 500. The rods can thus also have a stocky shape.

The conductive oxide from which the nanorods are formed is, for example, ZnO, doped zinc oxide, titanium dioxide (TiO ₂ ) or tin dioxide (SnO ₂ ), according to one embodiment. Doped ZnO or TiO ₂ can here mean both extrinsic ZnO or TiO ₂ doped with boron or aluminum, as well as intrinsically doped ZnO or TiO ₂ .

In one embodiment, the nanorods are at least partially surrounded by an intermediate layer. The intermediate layer can be embodied both as a buffer layer and as a layer which influences the light-reflecting properties of the nanorods by means of a refractive index adapted to the refractive index of the nanorods.

The buffer layer may also contribute to band matching between nanorods and subsequent layers or to stabilize the nanostructures.

In the case where the intermediate layer is conductive, the point contacts are formed from the interface between the nanorods and the intermediate layer and optionally additionally from areas of direct contact of nanorods and absorbers. If the intermediate layer is not conductive, the point contacts only consist of the remaining direct contact surfaces between nanorods and absorber, which are consequently imperative for charge transport.

In the embodiment of the intermediate layer as a buffer layer, which completely covers the nanorods in the region of the absorber, it allows charge transfer through the buffer layer between the absorber and the nanorods by means of corresponding line properties.

In a variation of this buffer layer embodiment, the buffer layer cap-covers the nanorod protruding portions of the passivation layer. In the lower area of the nanorods, complete coverage of the nanorods with the buffer layer may also result in partial coverage of the passivation layer.

In a further variation of the buffer layer embodiment, the buffer layer covers the nanoparticle protruding out of the passivation layer and the surface of the porous layer coherently and completely.

A next variation of the buffer layer embodiment has the contiguous covering of the entire surface of the nanorods with a buffer layer above and below (FIG. ingrown parts of the nanorods) of the passivation layer. The buffer layer is thus also below the passivation layer and covers the substrate coherently.

In a further embodiment of the buffer layer, it has a layer thickness in the range from a monolayer to about 100 nm and is formed from organic materials. The organic materials are in particular PEDOT: PSS or hydroquinone / methanol mixtures.

In an alternative embodiment, the buffer layer is formed from inorganic materials, especially from MoSe _x or MoO _x (for example, where _x = 2 or 3), CdS, Zn (O, S) or In ₂ S ₃ , ^{and likewise has a layer thickness in Range} from a monolayer to about 100 nm.

An adaptation of the optical properties by the intermediate layer is also possible. This is achieved by a layer on the nanorods, which has a different refractive index than that of the nanorods, as it corresponds to one embodiment. If the refractive index of the intermediate layer is higher than that of the nanorods and the intermediate layer envelops the nanorods on their side surfaces, they function as concentrators for the incidence of light from the rear side (through the backside electrode) by reflection in the nanorods. This is especially the case for a conical habit of the nanorods. If the refractive index is lower than that of the nanorods, a diffuse scattering of the light is promoted. The intermediate layer can also be used as a (non-transparent) opaque, structured reflector, e.g. in one embodiment, as a metal, the light reflects much more diffuse than a planar reflector.

The advantage of the passivation layer according to the invention with point contacts is, in addition to the advantages already mentioned above, on the one hand in the manufacturing process that takes place from the back to the front and can be easily incorporated into an industrial manufacturing process. On the other hand, the passivation layer with point contacts can be easily adapted to the properties of the absorber materials of the solar cell and thereby increase the efficiency of the solar cell. In addition, the nanorods projecting into an absorber layer offer the possibility of an antireflective and a light-scattering effect.

embodiment

The invention will be explained in more detail in the following embodiments and with reference to figures.

The 1 to 4 These show in each case a cross section of a schematically illustrated section of an embodiment of the passivation layer according to the invention with point contacts, which lies between a back contact layer and an absorber layer, for a classical solar cell structure.

The 1 schematically shows a section of an embodiment of the invention Passivierungsschicht with point contacts in cross section. On a back contact layer R, a porous layer pS of a dielectric material is formed, which forms the passivation layer. The porous layer pS has nanorods NS, which protrude into an absorber layer A applied to this porous layer pS.

The 2 corresponds to another embodiment and differs from the 1 by the fact that the parts of the nanorods NS, which protrude from the porous layer pS (the tips), are encased with an intermediate layer ZS, in the form of a buffer layer, in the shape of a cap. At the lower ends of the nanorods NS, the intermediate layer ZS also covers parts of the surface of the porous layer pS in this embodiment.

The 3 corresponds to a further embodiment and differs from the 1 by the fact that the nanorods NS, which protrude from the porous layer pS (the tips) and the surface of the porous layer pS, are covered contiguously with an intermediate layer ZS in the embodiment as a buffer layer.

The 4 also corresponds to an embodiment and differs from the 1 by the fact that the side surfaces of the nanorods NS are covered contiguously with an intermediate layer ZS, in the embodiment as a layer with refractive index different from the nanorods NS, above (protruding part) and below (in the porous layer ingrown part) of the porous layer pS , The surface of the back contact layer R is thereby also covered by the intermediate layer ZS. The ends of the nanorods remain uncovered.

The passivation layer of dielectric material may be applied to a substrate by both spray pyrolysis and spin coating. The nanorods of conductive metal oxide are grown or grown with electrode position. The intermediate layer of organic materials or inorganic materials is applied by standard methods.

In the spray pyrolysis for the preparation of the passivation layer, together with the dielectric material of the designated porous layer, a further additional material, for example Zinc oxide (ZnO) can be produced with in the layer. Starting materials in the acid solution to be sprayed for the dielectric material and the additional material are, for example, the respective corresponding acetylacetonates or other complexes which are decomposable at temperatures which are below the decomposition temperature of the materials of a substrate used. The material additionally produced in the spray pyrolysis, for example ZnO, is subsequently removed by etching, for example with hydrochloric acid. It is important to ensure that the solution properties (soluble or insoluble in acid, alkali and / or water) of the end products of spray pyrolysis are coordinated so that the desired material remains. By removing the additional material, the continuous pores are formed in the layer. The concentration of the materials in the solution and the duration of spraying must be adapted to the desired layer thickness as well as the pore density and the pore diameter. With spray pyrolysis it is possible to produce oxides, sulfides and nitrides.

In the spin coating for the production of the passivation layer, colloidal nanoparticles of the dielectric material of the passivation layer are used in a solution. The solvent evaporates without residue under test conditions (temperature, pressure). With the spin coating, porous passivation layers of all materials, which are in the form of nanoparticles, can be produced.

The nanorods of a conductive oxide are grown by means of electrode position in the pores of the porous layer or grown on the substrate. The localization of the nanorods in the pores of the porous layer is effected by the conductivity, which is given only in the pores by the here exposed back contact layer. The electrode position is served by a standard three-electrode reactor. Designs for eg ZnO or TiO ₂ are in the EP 2 252 728 B1 (ZnO) or the essay 4 (TiO ₂ ) of Chen et al. (Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications; Chemical Reviews, Vol. 107, 2007, pp. 2891-2959) specified.

A buffer layer of inorganic materials such as MoSe _x , MoO _x , CdS, Zn (O, S) or In ₂ S ₃ ^{is applied by standard coating methods} (particularly suitable are chemical vapor deposition and physical vapor deposition), as well as a buffer layer from organic materials such as PEDOT: PSS or hydroquinone: methanol mixtures, as described in the article 5 of Po et al. (The role of buffer layers in polymer solar cells; Energy and Environmental Science Vol. 4 (2), 2011, pp. 285-310) is described.

For two exemplary embodiments, which have a porous layer of aluminum oxide and nanorods of zinc oxide, two different concrete preparation paths are described below, which lead to two different embodiments.

The structure of a device for performing a spray pyrolysis corresponds to that of an ILGAR (Ion Layer Gas Reaction) construction without the use of the gas reaction step. The starting materials are aluminum acetylacetonate (Al (acac) ₃ ) and zinc acetylacetonate (Zn (acac) ₂ ) dissolved in an aqueous acetic acid solution having a pH of 3. The ratio of Al (acac) ₃ to Zn (acac) ₂ in the Solution is 1: 1.5 and the concentration of Al (acac) ₃ in the solution is 10 mol·l ^-1 .

The solution is atomized with the ultrasonic generator at a frequency of 1138 kHz, transported with nitrogen at a gas flow of 5 l · min ^-1 to the furnace and sprayed there via a nozzle. The substrate consists of a glass slide (2.5 cm × 2.5 cm) with a back contact layer made of FTO on it and is located at a distance of 15 cm from the nozzle at an angle of 45 ° to the gas flow. The substrate is heated to 500 ° C. The spraying process has a duration of 5 min.

Since the layer produced in the spray pyrolysis contains not only aluminum oxide (Al ₂ O ₃ ) but also zinc oxide (ZnO), this is etched out after completion of the spray pyrolysis with hydrochloric acid with a pH of 0.5 from this layer.

The aluminum oxide porous layer produced by the spray pyrolysis has a thickness of 5 nm, a pore density of 7 × 10 ⁸ ppi, and the pores have an average diameter of 300 nm.

The alternative spin coating of a porous Al ₂ O ₃ passivation layer according to the invention is carried out in a standard apparatus for spin coating. An isopropanol solution containing colloidal Al ₂ O ₃ nanoparticles having a diameter ≤ 50 nm and a concentration of 0.2 mol·l ^{-1 is} applied to a substrate consisting of a glass substrate and a back contact layer made of FTO thereon , On an area of 3 cm ² substrate 1 ml of solution is added at room temperature. Subsequently, the substrate is rotated at room temperature at 5000 rpm for 0.5 min.

The spinel-coated porous aluminum oxide layer has an average thickness of 20 nm, a pore density of 2 × 10 ¹⁰ ppi, and the pores have an average diameter of 65 nm.

ZnO nanorods are then grown into the pores of the alumina layer with electrodeposition in a standard three-electrode reactor and beyond their layer thickness.

For this purpose, an aqueous solution of 5 × 10 ^-3 mol·l ^-1 zinc nitrate (Zn (NO ₃ ) ₂ ) and 5 × 10 ^-3 mol·l ^-1 ammonium nitrate (NH ₄ NO ₃ ) is used. The pH is 5 and the temperature is 75 ° C. It is operated with a constant potential of -1.4 V against the platinum pseudo reference electrode. The average deposition current density is 0.15 mA · cm ^-2 for the produced by means Spraypyrolse and 0.22 mA · cm ^-2 by spin coating of Al ₂ O ₃ layer. It is deposited over a period of 8.3 min (porous layer of spray pyrolysis) or 6 min (porous layer of spin coating).

The ZnO nanorods thus produced have a length of 550 nm for the aluminum oxide layer produced by spray pyrolysis and a length of 200 nm for the oxide layer produced by spin coating.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US Pat. No. 4,395,583 [0010, 0014]
• DE 3815512 A1 [0012, 0014]
• EP 2252728 B1 [0054]

Cited non-patent literature

• G. Dingemanns et al. (Status and prospect of Al2O3-based surface passivation schemes for silicon solar cells, Journal of Vacuum Science and Technology A30 (4), Jul. Aug. 2012, pp. 040802-1-040802-27) [0006]
• JC Shin et al. (Heterogeneous Integration of InGaAs Nanowires on the Rear Surface of Si Solar Cells for Efficiency Enhancement, ACSNano, Vol. 6 (12), 2012, pp. 11074-11079) [0015]
• PH Lu et al. (Anodic Aluminum Oxide Passivation For Silicon Solar Cells, IEEE Journal of Photovoltaics, Vol. 3 (1), 2013, pp. 143-151) [0016]
• Chen et al. (Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications; Chemical Reviews, Vol. 107, 2007, pp. 2891-2959) [0054]
• Po et al. Energy and Environmental Science Vol. 4 (2), 2011, pp. 285-310) [0055] The role of buffer layers in polymer solar cells.

Claims (14)

Passivation layer with point contacts for thin-film solar cells, at least having A porous layer (pS) of a dielectric material having a layer thickness in the range of 1 nm to 50 nm, the pores of the layer being open on both sides of the layer, and Conductive oxide nanorods (NS) arranged in the pores of the porous layer (pS) to form the point contacts, the nanorods (NS) having a length of at least the thickness of the porous layer (pS) up to 1000 nm and on one side of the porous layer (pS) flush with this. Passivation layer with point contacts according to claim 1, characterized in that the dielectric material contains at least one of the metals aluminum, molybdenum, vanadium, indium, niobium, tantalum, chromium, tungsten, silicon or titanium. Passivation layer with point contacts according to claim 1, characterized in that the porous layer (pS) consists of a dielectric metal oxide, metal sulfide or metal nitride. Passivation layer with point contacts according to Claim 3, characterized in that the metal oxide consists of aluminum oxide (Al ₂ O ₃ ), vanadium oxide, in particular with a stoichiometry close to V ₂ O ₃ , ^{or SiO} 2 ^. Passivation layer with point contacts according to claim 1, characterized in that the pores of the porous layer (pS) have a diameter of 30 nm to 500 nm and the pore density is 0.4 × 10 ⁸ ppi to 250 × 10 ⁸ ppi. Passivation layer with point contacts according to claim 1, characterized in that the nanorods (NS) of ZnO, doped ZnO, TiO ₂ or SnO _{2 are} formed. Passivation layer with point contacts according to claim 1, characterized in that the nanorods (NS) are at least partially covered by an intermediate layer (ZS). Passivation layer with point contacts according to claim 7, characterized in that the intermediate layer (ZS) is formed as a buffer layer. Passivation layer with point contacts according to Claim 8, characterized in that the intermediate layer (ZS) formed as a buffer layer covers the parts of the nanorods (NS) projecting from the porous layer (pS) in the shape of a cap. Passivation layer with point contacts according to claim 8, characterized in that the buffer layer formed as an intermediate layer (ZS) of the porous layer (pS) protruding parts of the nanorods (NS) and the surface of the porous layer (pS) coherently and completely covered. Passivation layer with point contacts according to claim 8, characterized in that the buffer layer formed as intermediate layer (ZS) covers the nanorods (NS) and below the porous layer (pS) is formed contiguous. Passivation layer with point contacts according to claim 8, characterized in that the buffer layer formed as intermediate layer (ZS) consists of organic material, in particular from PEDOT: PSS or a hydroquinone: methanol mixture and has a layer thickness ranging from monomolecular coverage to 100 nm. Passivation layer with point contacts according to claim 8, characterized in that the buffer layer formed as an intermediate layer (ZS) of inorganic material, especially of CdS, Zn (O, S) or In ₂ S ₃ , is formed and a layer thickness in the range of monomolecular coverage to 100 nm. Passivation layer with point contacts according to claim 7, characterized in that the intermediate layer (ZS) is formed of a material having a refractive index greater or lesser than the refractive index of the nanorods and the nanorods covered at their side surfaces, otherwise otherwise continuous coverage below the porous layer.

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