WO2017035603A1 - A photovoltaic cell and a method of forming a photovoltaic cell - Google Patents

A photovoltaic cell and a method of forming a photovoltaic cell Download PDF

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WO2017035603A1
WO2017035603A1 PCT/AU2016/050835 AU2016050835W WO2017035603A1 WO 2017035603 A1 WO2017035603 A1 WO 2017035603A1 AU 2016050835 W AU2016050835 W AU 2016050835W WO 2017035603 A1 WO2017035603 A1 WO 2017035603A1
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layer
light
absorbing
photovoltaic cell
conductive material
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PCT/AU2016/050835
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French (fr)
Inventor
Hongtao Cui
Xiaojing Hao
Fangyang LIU
Xiaolei Liu
Xu Liu
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Newsouth Innovations Pty Limited
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Priority to AU2015903608A priority patent/AU2015903608A0/en
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Publication of WO2017035603A1 publication Critical patent/WO2017035603A1/en

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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/078Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier including different types of potential barriers provided for in two or more of groups H01L31/062 - H01L31/075
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264Inorganic materials
    • H01L31/028Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic System
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264Inorganic materials
    • H01L31/032Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0326Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising AIBIICIVDVI kesterite compounds, e.g. Cu2ZnSnSe4, Cu2ZnSnS4
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/072Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN heterojunction type
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B82NANOTECHNOLOGY
    • B82YSPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00Nanotechnology for materials or surface science, e.g. nanocomposites
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/50Photovoltaic [PV] energy
    • Y02E10/541CuInSe2 material PV cells
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/50Photovoltaic [PV] energy
    • Y02E10/547Monocrystalline silicon PV cells

Abstract

The present disclosure provides a photovoltaic device comprising a copper-based light-absorbing material. The copper-based light-absorbing material is formed on an intermediate carbon layer arranged to reduce the formation of voids at the rear of the device and reduce minority carrier. In some instances, the carbon layer is a graphene layer. Embodiments are also directed to a tandem photovoltaic device comprising a top cell, having a copper-based light-absorbing material, and a bottom cell having an absorber material with a bandgap lower than the copper-based light-absorbing material. The top and the bottom cells are interconnected by an intermediate carbon layer.

Description

A PHOTOVOLTAIC CELL AND A METHOD OF FORMING A PHOTOVOLTAIC
CELL
Field of the Invention
The present invention generally relates to a photovoltaic cell and a method of forming a photovoltaic cell, such as a photovoltaic cell comprising a copper based chalcogenide light-absorbing material.
Background of the Invention
Thin film solar cells with a copper based chalcogenide light-absorbing material represent an important
advancement in thin film photovoltaics technology.
Kesterite (CZTS, CZTSe or CZTSSe) based thin film solar cells, for example, use earth abundant materials and inexpensive fabrication techniques. Kesterite is a quaternary compound constituted by copper (Cu) , zinc (Zn), tin (Sn) and sulphur (S) or selenium (Se) . Kesterite has the chemical formula Cu2ZnSn(S, Se)4. Depending on whether the last element is sulphur or selenium the acronyms CZTS or CZTSe are used for Kesterite. By mixing CZTS and CZTSe, absorbers with a direct bandgap, tunable between ~1. OeV and ~1.5eV and a large absorption coefficient can be formed. These properties are ideal for a thin film solar cell absorber.
Current kesterite solar cells are realised on soda lime glass substrates coated with a molybdenum (Mo) layer which functions as a back contact. Generally, a CZTS(Se) absorber layer is formed by annealing a material
containing the precursor elements for CZTS(Se). This material is usually deposited using PVD, CVD techniques or solution techniques . A front contact consisting of a ZnO/AZO, ITO, BZO layer and a metallic material is normally realised on the absorber layer. Generally, kesterite solar cells also have a CdS intermediate layer between the absorber layer and the front contact.
Although it is widely acknowledged that kesterite solar cells could potentially perform better than other thin film photovoltaic technologies, the current performance of these devices is still below the market average. Record efficiencies of kesterite based solar cells have been reported between 8% and 12.6% compared to, for example, 21.7% for Cu(In, Ga)Se2 (CIGSe) thin film solar cells.
Some of the causes of the reduced performance of kesterite solar cells are related to structural, chemical and electrical properties of the region between the light- absorbing layer, the back contact and the influence of these properties on the light-absorbing layer.
The propagation of defects from the Mo back contact, to the kesterite absorber layer may reduce the conversion of efficiency of photons into collectable electrical
carriers. The Mo layer is generally deposited on the soda lime glass substrate using inexpensive techniques, such as sputtering. Sputtered Mo layers have a high density of structural defect which propagate into the structure affecting the performance of the solar cells .
The chemical instability of the interface between the Mo back contact and the kesterite absorber layer may also be detrimental for the cell performance. This instability can cause decomposition of the absorbing layer or formation of MoS2 and/or MoSe2 during the annealing of the precursor elements for CZTS(Se) . The formation of MoS2 and/or MoSe2 generally creates defects deep in the bandgap of the light-absorbing material, causing large current losses.
There is a need in the art for kesterite based solar cells with improved properties of the region between the light- absorbing layer and the back contact.
Summary of the Invention
In accordance with the first aspect, the present invention provides a photovoltaic cell comprising: a substrate; a copper based light-absorbing material; a first conductive material disposed between the substrate and the light-absorbing material, the first conductive material being arranged so as to be
electrically coupled to the light-absorbing material; an intermediate material arranged between the light-absorbing material and the first conductive
material, the intermediate material comprising carbon; a second conductive material arranged so as to be electrically coupled to the light-absorbing material; wherein the intermediate material is arranged to at least partially cover inner walls of voids in the region between the light absorbing material and the first conductive material. In an embodiment, the intermediate material is arranged cover at least 50% of inner walls of voids disposed in a region between the light absorbing material and the first conductive material.
The intermediate material may aggregate on inner walls of voids disposed in a region between the light absorbing material and the first conductive material during a sulphurisation or selenisation manufacturing step of the photovoltaic cell. Advantageously, the intermediate material aggregated on inner walls of voids may reduce the electrical resistance between the light absorbing material and the first conductive material. The intermediate material may also be arranged to reduce the electrical resistance between the light-absorbing material and metallic sulphide compounds or metallic selenide compounds in the region between the light-absorbing material and the first conductive material.
In an embodiment, the intermediate material is arranged to facilitate formation of metallic sulphide compounds or metallic selenide compounds in the region between the light-absorbing material and the first conductive
material .
In an embodiment, the intermediate material is further arranged to reduce formation of void regions between the light-absorbing material and the first material.
In an embodiment, the intermediate material is disposed onto a surface of the first conductive material and conforms to the morphology of the surface .
The intermediate material may consists of thin film layer of amorphous carbon; it may be uniformly distributed on the first material. The layer may have a thickness comprised between 15 nm and 35 nm and a resistivity below 100 Ω/D.
Advantageously, the introduction of the intermediate material may not affect the crystallinity and the chemical composition of the light-absorbing material of the light- absorbing material. The intermediate material may have a surface morphology similar to the morphology of the first conductive layer.
In some embodiments, the structure comprising the
substrate and the first material is annealed during formation of the photovoltaic cell. In other embodiments, the structure comprising the substrate, the first material and the intermediate material is annealed during formation of the photovoltaic cell.
Advantageously, the annealing step allows improving some properties of the first material or the intermediate material .
In embodiments, the copper based light-absorbing layer is a kesterite based layer and may comprise a copper-zinc- germanium-tin-chalcogenide based layer or a silver-copper- zinc-tin-chalcogenide based layer.
In embodiments, the first material comprises a suitable metallic material, such as molybdenum, or transparent conductive oxide.
In accordance with the second aspect, the present
invention provides a method of forming a photovoltaic cell comprising the steps of:
depositing an intermediate material comprising carbon on a substrate comprising a first conductive material; subsequent to depositing the intermediate material
forming a copper based light-absorbing material on the intermediate material; and
depositing a second conductive material such that the second material is electrically coupled to the light- absorbing material;
wherein the intermediate material is arranged to at least partially cover inner walls of voids disposed in the region between the light absorbing material and the first conductive material.
In embodiments, the step of forming a copper based light- absorbing material comprises the steps of: depositing a plurality of precursors comprising copper and sulphur and/or selenium; annealing the precursors to obtain sulphurisation or selenisation; and wherein the voids are created by metals, metal sulphides and/or selenides formed during the annealing of the precursors .
The structure comprising the substrate, the first
conductive material and the intermediate material and the precursors may be annealed at a temperature comprised between 450°C and 650°C for a period of time comprised between 10 seconds and 3 minutes.
To deposit the precursors, the precursors may be dissolved in a solution and the solution may be span onto the structure comprising the substrate, the first conductive material and the intermediate material and then dried.
Alternatively, the precursors may be co-sputtered onto the structure comprising the substrate, the first conductive material and the intermediate material .
The intermediate material may be deposited by evaporating or sputtering a carbon layer onto the first conductive material .
In embodiments, prior to depositing a plurality of precursors, the structure comprising the substrate, the first conductive material and the intermediate material is annealed. The annealing may be performed in a manner such that sodium diffuses from the substrate to the first conductive material.
In accordance with the third aspect, the present invention provides a photovoltaic cell comprising: a substrate; a copper based light-absorbing material; a first conductive material disposed between the substrate and the light-absorbing material, the first conductive material being arranged so as to be
electrically coupled to the light-absorbing material a graphene layer arranged between the first conductive material and the light-absorbing material and arranged so as to be electrically coupled to the light- absorbing material, a second conductive material arranged so as to be electrically coupled to the light-absorbing material.
The graphene layer may reduce the formation of metallic sulphide compounds or metallic selenide compounds in a region between the light absorbing material and the first conductive material during a sulphurisation or
selenisation manufacturing step. Further, the graphene layer may be arranged to reduce formation of void regions between the light-absorbing material and the graphene layer. In addition, a layer of graphene with a thickness between 1 nm and 3 nm may provide sufficient conductivity to not affect the conductivity of the device. This
graphene layer may comprise several two-dimensional graphene planes.
In accordance with the fourth aspect, the present
invention provides a method of forming a photovoltaic cell comprising the steps of:
depositing graphene layer on a substrate comprising a first conductive material; subsequent to depositing the graphene layer forming a copper based light-absorbing material on the graphene layer; and depositing a second conductive material such that the second material is electrically coupled to the light- absorbing material.
In embodiments, the step of forming a copper based light- absorbing material comprises the steps of: depositing a plurality o precursors comprising copper and sulphur and/or selenium annealing the precursors to obtain sulphurisation or selenisation .
In embodiments, the step of annealing the precursors comprises performing an annealing of the structure
comprising the substrate and the graphene layer and the precursors at a temperature comprised between 450 °C and 650 °C for a period of time comprised between 10 seconds and 3 minutes. The method further comprises the step of, prior to depositing a plurality of precursors, annealing the structure comprising the substrate and the graphene layer .
In accordance with the fifth aspect, the present invention provides a photovoltaic device comprising: a photon receiving surface; a first photon absorbing layer comprising a material having a first bandgap; a second photon absorbing layer comprising a copper based material having a second bandgap; and an intermediate graphene layer disposed between the first photon absorbing layer and the second photon absorbing layer; the graphene layer being arranged to electrically couple the first photon absorbing layer and the second photon absorbing layer; wherein the first photon absorbing layer is positioned at least partially below the second photon absorbing layer and the photovoltaic device is arranged such that received photons are absorbed by the first or the second photon absorbing layer. In embodiments, the first photon absorbing layer is a silicon layer.
In accordance with the sixth aspect, the present invention provides a method of forming a photovoltaic device comprising the steps of:
forming a first photovoltaic cell comprising a first photon absorbing layer having a material with a first bandgap; depositing an intermediate graphene layer on a surface of the first photovoltaic cell; forming a second photon absorbing layer comprising a copper based material having a second bandgap; and wherein the graphene layer is arranged to electrically couple the first photon absorbing layer and the second photon absorbing layer; the first photon absorbing layer being positioned at least partially below the second photon absorbing layer and the photovoltaic device being arranged such that received photons are absorbed by the first or the second photon absorbing layer .
Studies on solution-based synthesized kesterite solar cells have found carbon content in the devices due to carbon included in the initial solution, mostly from the solvent. This carbon has been found to be detrimental for the device's performance. For example, researchers at IBM decided to opt for toxic hydrozin as a solvent to avoid the carbon problem. Researchers at Dupont, despite not being able to prove a detrimental impact of carbon on their devices, reached the conclusion that the carbon contained in the device reduces the total amount of absorbing material available, negatively affecting the performance of the device in an indirect way.
The research conducted by the Applicants surprisingly shows that by, deliberately adding a thin carbon layer or a graphene layer in a state of the art kesterite device, or fabricating the devices on a thicker carbon template, so avoiding the Mo, the performance of the devices can be enhanced .
Surprisingly, carbon layers with a certain range of thicknesses advantageously allowed to electrically connect voids in the vicinity back contact area when a MoS2/Mo contact is used, reducing the overall series resistance.
Advantageous embodiments of the invention allow mitigating the problems related to the back contact of CZTS solar cells by inserting an intermediate a carbon layer or a graphene layer between CZTS and the Mo layer or by having a thicker full carbon back contact layer (without Mo) .
Brief Description of the Drawings Features and advantages of the present invention will become apparent from the following description of
embodiments thereof, by way of example only, with
reference to the accompanying drawings in which:
Figures 1 and 8 show schematic representation of solar cell devices in accordance with embodiments;
Figure 2 and 9 show flow diagrams with manufacturing steps used to manufacture the devices of figures 1 and 8 respectively; Figure 3 shows an XRD pattern (a) , Raman spectra (b) and Top-view SEM images of the absorber layer of a prior art device (c) and a device in accordance with embodiments (d) ;
Figures 4 and 5 show cross-sectional TEM images (a) and EDS line scans (b) of devices in accordance with
embodiments ;
Figure 6 shows a cross-sectional TEM image of a device in accordance with embodiments, and EDS mapping images for the compositional distribution in respect to several elements; and
Figures 7 shows illuminated J-V curves (a) and EQE curves (b) comparison between prior art devices and devices realised in accordance with embodiments.
Detailed Description of Embodiments
Embodiments of the present invention relate to
photovoltaic cells comprising a copper based chalcogenide light-absorbing material and a carbon layer at the back contact. In some embodiments the carbon layer is provided as a thin carbon layer or a graphene layer between the CZTS and the Mo material. In alternative embodiments, a thicker carbon layer is provided on the substrate and the device does not have any Mo layer. Embodiments also relate to a tandem device comprising a low bandgap bottom cell and a kesterite based top cell. A graphene layer is used to interconnect the top and the bottom cells . Carbon is an inert material to sulphur and has a good conductivity. In the embodiments where a thin carbon layer is positioned between the CZTS absorber and the Mo back contact, the carbon does not entirely block the formation of Mo sulphides or selenides. However, carbon allows
reconnecting the CZTS absorber and the Mo through voids due to the formation of sulphides and/or selenides.
Referring now to figure 1(a), there is shown a schematic representation of a solar cell device 100 in accordance with an embodiment of the present invention. The solar cell consists of a soda lime glass substrate 102 covered with a Mo layer 104 with a thickness of approximately 1000 nm. An intermediate layer 106 made of amorphous carbon separates the Mo layer 104 and the kesterite based light- absorbing layer 108. In this embodiment, the intermediate layer 106 is about 25 nm thick and is grown using an evaporation system.
The kesterite based light-absorbing layer 108 is formed on the surface of the carbon layer 106. The formation of the kesterite layer 108 involves sputtering of Cu-ZnS-SnS precursors and a sulphurisation step which can be
performed in a rapid thermal annealing furnace in a sulphur containing atmosphere. The sulphurisation step can be performed, for example, using a Rapid Thermal Processor (AS-One 100) at 560 °C for 1 min. Alternatively, the kesterite based light-absorbing layer 108 can be formed using a solution based method. The precursor solution can be prepared by dissolving 10.65 g copper (II) acetate monohydrate (Cu (CH3COO) 2 -H20, 99%) (53.3 mmol), 7.32 g zinc acetate dihydrate (Zn(CH3COO)2 2H20, 99%) (33.4 mmol), 6.02 g tin (II) chloride (SnCl2, 98%) (26.7 mmol, AR) and 16.16 g thiourea (SC(NH2)2, 99%) (210 mmol) into 100 mL 2-methoxyethanol ( ( HOCH2CH2 ) 3N, 98%) and stirring at 50 °C for 1 hour to get a light yellow solution. The precursor solution can be spin coated on the carbon layer 106 at 800 rpm for 10 s and then 3000 rpm for 20 s, and dried at 300 °C for 5 min on a hot plate in air. This coating step has to be repeated multiple times to get a CZTS precursor film with a desired thickness. The prepared film still has to be sulphurized, for example at 540 °C in a sulphur/N2 atmosphere for 40 min with
controlled sulphur partial pressure.
During the sulphurisation step sulphur and/or selenium can react with the Mo of the back contact 104 forming
sulphides (MoS2) or selenides (MoSe2) . These can be highly mobile and affect the structural and electrical properties of the devices, for example by creating voids in the device's structure.
Carbon material from layer 106 diffuses through the back portion of the device and allows to electrically connect voids. This, in turn, reduces the overall series
resistance of the device. After the carbon diffusion, the stability of the CZTS absorber is improved as the CZTS is less prone to reaction with carbon than with Mo.
A 70 nm cadmium sulphide (CdS) buffer layer 110 is deposited between the kesterite layer 108 and a conductive structure which forms the front contact of the solar cell. The CdS layer 110 improves carrier extraction from the kesterite layer 108 and provides electronic band alignment between the kesterite layer 108 and the top contacting layers. In this embodiment, the cadmium sulphide layer 110 is deposited by chemical bath deposition. However, the cadmium sulphide layer 110 could be deposited using other PVD or CVD techniques such as PECVD or ALD.
The front contacting structure of the solar cell is realised with a 50 nm intrinsic zinc oxide (IZO) layer 112 and a DC magnetron sputtered 200 nm ITO window layer 114. Finally Al is thermally evaporated on the ITO layer to form top contact fingers via a shadow mask.
Referring now to figure 1 (b) , there is shown a schematic representation of a solar cell device 150 in accordance with an alternative embodiment. The solar cell consists of a soda lime glass substrate 152 on which a carbon layer 156 is deposited. The thickness of carbon layer 156 is in the micrometers range. The kesterite based light-absorbing layer 158 formed on the surface of the carbon layer 156 and layers 160 to 166 are manufactured and configured in a similar manner as for device 100.
Referring now to figure 2, there is shown a flow diagram 200 outlining a method of forming a kesterite solar cell in accordance with embodiments. The first step 210 consists in depositing an intermediate material comprising carbon on a substrate comprising a first conductive material. In this instance, the substrate is a soda lime glass substrate and the first conductive material is a Mo layer with a thickness around 1000 nm, sputtered using a multi-target sputtering tool. Subsequently, at step 220, a copper based light-absorbing material is formed on the intermediate material. Precursors layers are deposited and sulphurised in accordance with one of the techniques describe above At step 230, a second conductive material is deposited on the absorber layer. A front contact structure is
subsequently formed, in accordance with the description provided above with reference to figure 1. Referring now to figure 3, there are shown an XRD patterns (a) , Raman spectra (b) and top-view SEM images of the absorber layer of a prior art device (c) and a kesterite solar cell with an intermediate amorphous carbon layer (d) . The data in figure 3 allow evaluating the effects of the intermediate carbon layer 106 on the overlying CZTS absorber' s crystallinity, phase constitution and
morphology .
The XRD patterns of figure 3 (a) show that both the absorbers with (302) and without (304) the intermediate carbon layer have a kesterite structure. The patterns show no peaks assigned to secondary phases such as CuSx, SnS and SnS2.
The inset of Figure 3 (a) shows a comparison of the enlarged peaks of the CZTS absorbers (112) with and without carbon layer. No peak broadening or shifting can be identified when carbon layer is introduced, suggesting that the introduction of carbon layer has little impact on the crystallinity of CZTS absorber. However, some ZnS and/or Cu2SnS3 (CTS) impurities may be present in the films as they have similar XRD patterns to CZTS.
The Raman data of figure 3 (b) confirm the phase
constitution of the CZTS films with or without
intermediate carbon layer. Figure 3(b) shows that for the CZTS films with an intermediate carbon layer (306) the major Raman peaks appear at 286 cm-1 and 337 cm-1. These peaks are consistent with those of the CZTS thin film without carbon layer (308) . There is no evidence of secondary phases like ZnS (355 cm-1) or CTS. The data in figure 3(a) and 3(b) confirm that the single phase CZTS thin films were achieved in the cases with and without intermediate carbon layer. Therefore, the insertion of the intermediate carbon layer has no effects on the phase composition and crystallinity of the synthesized CZTS films .
A large grain size is favourable for device performance since the grain boundaries tend to attract both chemical impurities and structural defects, and normally create deep levels that act as recombination centers, reducing the mobility of majority carriers in the CZTS films.
Figure 3(c) and Figure 3(d) show the top-view SEM images of sulphurised CZTS thin films without (c) and with (d) an intermediate carbon layer. The figures show that the surface of the absorbers in the structure with the carbon layer is as compact as in the structure without a carbon layer. The average grain sizes are quiet analogous, around 1 \im for both surfaces.
The chemical composition of the CZTS films is another important factor that affects the performance of CZTS devices. The composition (here using atomic ratios) of the CZTS absorber (deposited by sol-gel method and calculated by EDS) is Cu/(Zn+Sn) ~ 0.76 and Zn/Sn -1.05 for the structures grown without the intermediate carbon layer. The composition is Cu/(Zn+Sn) ~ 0.78 and Zn/Sn ~ 1.04 for the structures grown with an intermediate carbon layer. The data show only a minor difference in the chemical compositions . The similar morphology and chemical composition of the absorber layers with and without an intermediate carbon layer suggest that the insertion of the intermediate carbon layer does not cause side effects, and Cu- poor, Zn-rich and dense CZTS absorbers can be formed.
Referring now to figures 4 and 5, there are shown cross- sectional TEM images (a) and EDS line scan images (b) of solar cell structures fabricated with a 25 nm intermediate carbon layer. The solar cell structures of figure 4 have been fabricated using a sol-gel method. As the sol-gel method involves the use of carbon, similar measurements have been made for solar cells structures fabricated using a sputtering method to confirm the results . The
measurements for the cells manufactured by sputtering are shown in figure 5.
The top TEM images, figures 4 (a) and 5 (a) , show some voids and a 200nm M0S2 interfacial layer between the CZTS absorber layer and the Mo back contact for device with carbon layer. The intermediate carbon layer does not entirely inhibit generation of voids or MoS2 during the sulphurisation process. Figures 4(b) and 5(b) show the vertical element distribution measured by EDS. The arrows in figures 4 (a) and 5 (a) mark the regions detected by the EDS line scans. Lines 402, 404, 502 and 504 show the EDS path on voids areas. The composition profiles for these areas are shown in plots 410, 412, 510 and 512
respectively. Lines 406, 408, 506 and 508 show the EDS path on grain areas. The composition profiles for these areas are shown in plots 414, 416, 514 and 516
respectively . In figures 4 (b) and 5 (b) , the carbon signals have a low intensity in the absorber bulk region, but increase considerably and forms peaks in the void regions. The exact location of carbon in the voids can be inferred by comparing the vertical distribution of carbon with the distribution of other elements. By comparing the
distribution of sulphur in figure 4 (b) , for example, it is noticeable that the concentration of carbon rises from 1.13 \im while that of sulphur plunges from 1.1 \im and reaches the lowest point at 1.25 μιη, which indicates that the void forms from around 1.1 \im and therefore carbon mainly sticks to the inner wall of voids in absorber.
The intermediate carbon layer or residual carbon from the precursor solution can provide a source of carbon for the inner walls of the voids. In plots 412 and 512, the carbon concentration is uniform and shows no major differences between the bulk of the absorber and the void regions. By comparing the vertical carbon distribution at void region for CZTS solar cells with and without the intermediate carbon layer, the difference suggests that the carbon on the inner walls of voids comes from the deposited
intermediate carbon layer, for both the sol-gel cells and the sputtered cells . During the sulphurisation, the carbon aggregates at the inner wall of voids. This evidence shows that the highly conductive carbon of the intermediate layer may promote carrier transport between the CZTS and the MoS2 at the void areas, and thereby reduce the contact resistance .
In the structures without void regions the carbon is distributed homogeneously, as shown in plots 414, 514. terms of TEM topology and EDS line scan, the vertical element distribution of CZTS and the thickness of each layer are similar for both CZTS solar cells with or without intermediate carbon layer, and the only difference is the carbon distribution at the void regions without carbon layer.
Referring now to figure 6, there are shown EDS mapping plots obtained by TEM for a sol-gel cell. These
measurements further confirm the observation of the carbon distribution. Subfigure 601 shows the main TEM image.
Subfigures 602 to 609 show related EDS mappings images of a solar cell device with an intermediate carbon layer. The carbon signal in subfigure 608 shows the aggregation of carbon at the inner walls of voids. In addition, the bigger the size of the voids, the easier is to observe the aggregation of carbon on the walls of the voids.
Referring now to figure 7, there are shown J-V 700 and EQE 750 plots of CZTS devices (manufactured by sol-gel method) with (702, 706) and without (704, 708) intermediate carbon layer between the CZTS absorbers and the Mo covered substrates. An increased conversion efficiency of 5.52% (from 4.47%) is measured in the device with the
intermediate carbon layer. This efficiency enhancement is mostly due to a significant boost in the Jsc of the device (from 13.60 mA/cm2 to 16.96 mA/cm2) . This 25% increase in Jsc is attributed to the aggregation of carbon on the inner walls of voids reconnecting the CZTS and back contact. The Voc and the FF of the curves of plot 700 are substantially identical .
The EQE curves, shown in plot 750 reveal that the
difference in Jsc the EQE enhancement in the whole visible region, which may be explained by the reduced Rs (5.1 Ω/cm2 without carbon and 2.0 Ω/cm2 with carbon) .
Without intermediate carbon layer, the large number of voids at the back contact region limits the free-carrier transportation, leading to higher Rs and thereby lower Jsc. With the introduction of intermediate carbon layer, the carbon aggregation at inner wall of voids connect the CZTS and back contact, which can reduce the contact resistance and thereby reduce Rs, facilitating the photo-generated current flow.
The significant improvement in Jsc was also observed in solar cell devices with a CZTS absorber layer formed using a sputtering technique. The devices with the intermediate carbon layer showed an efficiency of 5.2% (from 4.1%). The enhancement in Jsc was of 17% from 17.5 mA/cm2 to 20.5 mA/cm2 (increases by 17%) . In this case the V0c and FF also increased slightly, from 560 mV to 580 mV and from 0.40 to 0.42, respectively. These results prove that the
intermediate carbon layer is effective for devices manufactured using vacuum based methods and non-vacuum based methods .
Other materials have been investigated in the prior art to improve the electrical performance of the back contact of CZTS solar cells. However, it has been demonstrated that the addition of these materials in the devices may either deteriorate the Rs, reduce the FF or the crystal quality, depending on the type of material. In some cases, for example for Ag, the materials may introduce an unfeasible cost. The carbon layer designed by the Applications has a low cost, high accessibility, non-toxicity and no side effects on the V0c and FF. Referring now to figure 8 (a) , there is shown a schematic representation of a solar cell device 800 in accordance with an embodiment of the present invention. Device 800 has a similar structure to cell 100 of figure 1. The main difference is the nature of intermediate layer 806, which in this embodiment is a graphene layer. This graphene layer is generally composed of a few planar graphene sheets to provide sufficient uniformity and coverage. A continuous layer of graphene substantially reduces the chemical reaction between Mo and sulphur or selenium.
Graphene layer 806 does not diffuse through the back portion of the device to the same extent of carbon layer 106 in device 100. However, layer 806 allows to minimise the formation of sulphides and/or selenides from
decomposition of chalcogenide absorber which, in turn, reduces the density of voids and high resistive secondary phases, and increases the stability of the CZTS absorber. At the same time the graphene layer provides sufficient conductivity for carrier transport and extraction. Figure 8 (b) shows a tandem solar cell device 850 which comprises a top kesterite solar cell such as cell 800. The top cell is interconnected to a silicon bottom cell 854 through a graphene layer 856. Carriers are extracted via metal contact 852. The bottom cell may be a different type of solar cell, such as another thin film solar cell.
The graphene layer allows high light transmittance and avoiding a complex tunnel junction for recombination in the device and provides a fast carrier transport channel. Referring now to figure 9, there is shown a flow diagram 900 outlining a method for forming a tandem solar cell as the one described above with reference to figure 8 (b) .
The first step 902 consists in forming a first
photovoltaic cell, for example a silicon solar cell, comprising a first photon absorbing layer having a material with a first bandgap. Subsequently, step 904 an intermediate graphene layer is deposited on a surface of the first photovoltaic cell. The intermediate graphene layer provides a conductive interconnection layer between the bottom cell (first) cell and a top kesterite based cell. The top cell comprises a second photon absorbing layer comprising a copper based material having a second bandgap, formed at step 906. At step 908, a CdS layer and a contacting structure are formed as discussed with reference to figures 1 and 8.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are,
therefore, to be considered in all respects as
illustrative and not restrictive.

Claims

The Claims Defining the Invention are as Follows:
1. A photovoltaic cell comprising: a substrate; a copper based light-absorbing material; a first conductive material disposed between the substrate and the light-absorbing material, the first conductive material being arranged so as to be
electrically coupled to the light-absorbing material; an intermediate material arranged between the light-absorbing material and the first conductive
material, the intermediate material comprising carbon; a second conductive material arranged so as to be electrically coupled to the light-absorbing material; wherein the intermediate material is arranged to at least partially cover inner walls of voids in the region between the light absorbing material and the first conductive material.
2. The photovoltaic cell of claim 1 wherein the
intermediate material is arranged cover at least 50% of inner walls of voids disposed in a region between the light absorbing material and the first conductive
material .
3. The photovoltaic cell of any one of claims 1 or 2 wherein the intermediate material aggregates on inner walls of voids disposed in a region between the light absorbing material and the first conductive material during a sulphurisation or selenisation manufacturing step of the photovoltaic cell.
4. The photovoltaic cell of any one of claims 1 to 3 wherein the intermediate material is arranged to reduce the electrical resistance between the light absorbing material and the first conductive material.
5. The photovoltaic cell of any one of claims 1 to 4 wherein the intermediate material is arranged to reduce the electrical resistance between the light-absorbing material and metallic sulphide compounds or metallic selenide compounds in the region between the light- absorbing material and the first conductive material.
6. The photovoltaic cell of any one of claims 1 to 5 wherein the intermediate material is further arranged to reduce formation of void regions between the light- absorbing material and the first material.
7. The photovoltaic cell of any one of claims 1 to 6 wherein the intermediate material consists of carbon
8. The photovoltaic cell of any one of claims 1 to 7 wherein the carbon is thin film amorphous carbon layer.
9. The photovoltaic device of any one of claims 1 to 8 wherein the intermediate material has a thickness
comprised between 15 nm and 35 nm.
10. The photovoltaic cell of any one of claims 1 to 9 wherein the intermediate material is arranged in a manner such that the crystallinity of the light-absorbing material is not affected by the intermediate material.
11. The photovoltaic cell of any one of claims 1 to 10 wherein the intermediate material is arranged in a manner such that the chemical composition of the light-absorbing material is not affected by the intermediate material.
12. A method of forming a photovoltaic cell comprising the steps of:
depositing an intermediate material comprising carbon on a substrate comprising a first conductive material; subsequent to depositing the intermediate material
forming a copper based light-absorbing material on the intermediate material; and
depositing a second conductive material such that the second material is electrically coupled to the light- absorbing material;
wherein the intermediate material is arranged to at least partially cover inner walls of voids disposed in the region between the light absorbing material and the first conductive material.
13. The method of claim 12 wherein the step of forming a copper based light-absorbing material comprises the steps of: depositing a plurality of precursors comprising copper and sulphur and/or selenium; annealing the precursors to obtain sulphurisation or selenisation; and wherein the voids are created by metals, metal sulphides and/or selenides formed during the annealing of the precursors .
14. The method of claim 13 wherein the step of annealing the precursors comprises performing an annealing of the structure comprising the substrate, the first conductive material, the intermediate material and the precursors at a temperature comprised between 450 °C and 650 °C for a period of time comprised between 10 seconds and 3 minutes.
15. The method of any one of claims 12 to 14 wherein the method further comprises the step of, prior to depositing a plurality of precursors, annealing the structure
comprising the substrate, the first conductive material and the intermediate material.
16. A photovoltaic cell comprising: a substrate; a copper based light-absorbing material; a first conductive material disposed between the substrate and the light-absorbing material, the first conductive material being arranged so as to be
electrically coupled to the light-absorbing material a graphene layer arranged between the first conductive material and the light-absorbing material and arranged so as to be electrically coupled to the light- absorbing material, a second conductive material arranged so as to be electrically coupled to the light-absorbing material.
17. The photovoltaic cell of claim 16 wherein the graphene layer is arranged to reduce the formation of metallic sulphide compounds or metallic selenide compounds in a region between the light absorbing material and the first conductive material during a sulphurisation or
selenisation manufacturing step of the photovoltaic cell.
18. The photovoltaic cell of claim 16 or claim 17 wherein the graphene layer is further arranged to reduce formation of void regions in a region between the light-absorbing material and the graphene layer.
19. The photovoltaic cell of any one of claims 16 to 18 wherein the graphene layer has a thickness between 1 nm and 3 nm.
20. A method of forming a photovoltaic cell comprising the steps of:
depositing graphene layer on a substrate comprising a first conductive material; subsequent to depositing the graphene layer forming a copper based light-absorbing material on the graphene layer; and depositing a second conductive material such that the second material is electrically coupled to the light- absorbing material.
21. The method of claim 20 wherein the step of forming a copper based light-absorbing material comprises the steps of: depositing a plurality of precursors comprising copper and sulphur and/or selenium; annealing the precursors to obtain sulphurisation or selenisation.
22. The method of claim 21 wherein the step of annealing the precursors comprises performing an annealing of the structure comprising the substrate and the graphene layer and the precursors at a temperature comprised between 450°C and 650°C for a period of time comprised between 10 seconds and 3 minutes .
23. The method of any one of claims 20 to 22 wherein the method further comprises the step of, prior to depositing a plurality of precursors, annealing the structure comprising the substrate and the graphene layer.
24. A photovoltaic device comprising: a photon receiving surface; a first photon absorbing layer comprising a material having a first bandgap; a second photon absorbing layer comprising a copper based material having a second bandgap; and an intermediate graphene layer disposed between the first photon absorbing layer and the second photon absorbing layer; the graphene layer being arranged to electrically couple the first photon absorbing layer and the second photon absorbing layer; wherein the first photon absorbing layer is positioned at least partially below the second photon absorbing layer and the photovoltaic device is arranged such that received photons are absorbed by the first or the second photon absorbing layer.
25. The photovoltaic device of claim 21 wherein the first photon absorbing layer is a silicon layer.
26. A method of forming a photovoltaic device comprising the steps of:
forming a first photovoltaic cell comprising a first photon absorbing layer having a material with a first bandgap; depositing an intermediate graphene layer on a surface of the first photovoltaic cell; forming a second photon absorbing layer comprising a copper based material having a second bandgap; and wherein the graphene layer is arranged to electrically couple the first photon absorbing layer and the second photon absorbing layer; the first photon absorbing layer being positioned at least partially below the second photon absorbing layer and the photovoltaic device being arranged such that received photons are absorbed by the first or the second photon absorbing layer .
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