WO2015120512A1

WO2015120512A1 - A photovoltaic cell and a method of forming a photovoltaic cell

Info

Publication number: WO2015120512A1
Application number: PCT/AU2015/000085
Authority: WO
Inventors: Fangyang LIU; Xiaojing Hao
Original assignee: NewSouth Innovations Pty Ltd; GUODIAN NEW ENERGY TECHNOLOGY RESEARCH INSTITUTE
Current assignee: NewSouth Innovations Pty Ltd; GUODIAN NEW ENERGY TECHNOLOGY RESEARCH INSTITUTE
Priority date: 2014-02-14
Filing date: 2015-02-13
Publication date: 2015-08-20
Anticipated expiration: 2016-08-14
Also published as: CN107078180A; CN107078180B

Abstract

The present disclosure provides a photovoltaic device comprising a copper-based light-absorbing material and a first material electrically coupled to the light-absorbing material and forming a p-n junction with the light- absorbing material. The device further comprises an intermediate material arranged between the first material and the light-absorbing material. The intermediate material acts to reduce the minority carrier recombination rate at the region between the light-absorbing material and the first material and/or optimise the electronic band alignment in the photovoltaic device.

Description

A PHOTOVOLTAIC CELL AND A METHOD OF FORMING A PHOTOVOLTAIC

CELL

Field of the Invention

Embodiments of the present invention generally relate to a photovoltaic cell and a method of forming a photovoltaic cell, such as a photovoltaic cell comprising a copper- based light-absorbing material.

Background of the Invention

A number of copper-based alloys have suitable properties to be used as light absorbing materials for thin film photovoltaic cells. Copper-based chalcogenides , such as kesterite, have a direct bandgap which can be tuned to match the solar spectrum.

Kesterite is a quaternary compound constituted of copper (Cu) , zinc (Zn) , tin (Sn) and sulphur (S) or selenium

(Se) . Kesterite has the chemical formula Cu₂ nSn(S, Se)₄. Depending on whether the last element is sulphur, selenium or sulphur and selenium, the acronyms are CZTS, CZTSe or CZT(S,Se) are all referred to as Kesterite. Kesterite absorbers with a direct bandgap tunable between -1.0 eV and -1.5 eV and a large absorption coefficient can be formed. These properties are ideal for a thin film

photovoltaic cell absorber. Further, kesterite is

available in abundance on the Earth's crust. Current kesterite photovoltaic cells are realised on soda lime glass substrates coated with a molybdenum (Mo) layer which functions as a back contact. Generally, a kesterite light-absorbing layer is formed by annealing a material containing precursor elements (Cu, Zn, Sn, S, Se) . An re^¬ type cadmium sulphide (CdS) layer is formed on the light- absorbing layer to form a p-n junction and a contacting structure, consisting of zinc oxide and metallic contacts is normally realised on the CdS.

Although it is widely acknowledged that kesterite

photovoltaic cells could potentially outperform other thin film photovoltaic technologies, the current performance of these devices is still well below the market average.

Record efficiencies of kesterite based photovoltaic cells have been reported between 8% and 9% for pure Cu₂ nSnS₄ and Cu₂ nSnSe₄, and 12.7% for Cu₂ZnSn(S Se)4 compared to, for example, 21.7% for Cu(In, Ga) Se₂ (CIGSe) thin film

photovoltaic cells.

One of the causes of the reduced performance of kesterite photovoltaic cells is the recombination of photo-generated carriers in the region around the interface between the kesterite light-absorbing layer and the CdS layer. A high density of recombination sites can be found at the

interface between the kesterite light-absorbing layer and the CdS layer and is generally due to lattice mismatch between the two materials and segregation of impurities. This reduces the open-circuit voltage and the fill factor of the photovoltaic cell affecting its performance. Non- optimal performance is also related to the electronic band configuration within CZTS based devices.

There is a need in the art for kesterite based

photovoltaic cells with a reduced density of recombination sites at the interface between the kesterite light- absorbing layer and the CdS layer and an improved

electronic bands configuration.

Summary of the Invention

In accordance with the first aspect, the present invention provides a photovoltaic device comprising:

a copper-based light-absorbing material;

a first material electrically coupled to the light-absorbing material and forming a p-n junction with the light-absorbing material; and

an intermediate material arranged between the first material and the light-absorbing material, the intermediate material being arranged to reduce the

minority carrier recombination rate at a region between the light-absorbing material and the first material. In an embodiment, a portion of the intermediate material diffuses into the first material during fabrication of the device. The intermediate material may comprise indium or In₂S₃.

In an embodiment, the intermediate material forms an intermediate layer comprising a continuous distribution of clusters. The intermediate layer may have a thickness between 5 nm and 100 nm or preferably between 20 nm and 60 nm.

In an embodiment, the intermediate material forms a plurality of clusters distributed across an interface region of the light-absorbing material and physically separating a portion of the light-absorbing material from the first material. In an embodiment, the intermediate material absorbs at least a portion of photons incoming to the device and in use generates a portion of the current generated by the device . In an embodiment, the device has an external quantum efficiency higher than 70% at a wavelength between 350 nm and 450 nm.

In accordance with the second aspect, the present

invention provides a photovoltaic cell comprising:

a copper-based light-absorbing material;

a first material electrically coupled to the light-absorbing material and forming a p-n junction with the light-absorbing material;

wherein the first material comprises a compound including indium at an interface with the light-absorbing material .

The light-absorbing material and the first material may have a conduction band energy separation smaller or equal to 0.24 eV with conduction band edge of the light- absorbing material being higher than the first material.

In accordance with the third aspect, the present invention provides a photovoltaic cell comprising:

a copper-based light-absorbing material;

a first material having a first portion electrically coupled to a first surface portion of the light-absorbing material and forming a p-n junction with the light-absorbing material; and

an intermediate material arranged between a second portion of the first material and a second surface portion of the light-absorbing material. In an embodiment, the intermediate material is arranged to minimise transport of electrical carriers between the second portion of the first material and the second surface portion of the light-absorbing material. In an embodiment, the intermediate material is selected to decrease the concentration of carrier recombination centres located in the proximity of an interface between the light-absorbing material and the first material.

The intermediate material may have a matching crystalline structure to the crystalline structure of the light- absorbing material. Furthermore, the intermediate material may have a lattice constant substantially identical to the lattice constant of the light-absorbing material.

The intermediate material may be selected amongst zinc sulphide (ZnS) , zinc-oxide (ZnO) , zinc-selenide (ZnSe) , amorphous silicon or aluminium oxide (AI₂O3) .

In addition, the intermediate material may comprise a plurality of particles distributed over the second surface portion of the light-absorbing material. The lateral extension of the particles may be smaller than the

diffusion length of minority carriers in the light- absorbing material and the diffusion length of minority carriers in the first material. The particles may have a semi-sphere shape. In an embodiment, the photovoltaic cell further comprises a second material arranged between the a conductive back contact and the copper-based light-absorbing material, the second material comprising a metallic material which is selected so as to reduce the formation of sulphides and/or selenides in the region between the conductive back contact and the light-absorbing material. The metallic material may comprise silver, gold or a gold-silver alloy.

The copper-based light-absorbing material may comprise a copper-tin-zinc-sulphide-material, a copper-tin-zinc- selenide-material , or a copper-zinc-germanium-tin- chalcogenide-material and the first material may comprise cadmium-sulphide .

In accordance with the fourth aspect, the present

invention provides a method of forming a photovoltaic cell comprising the steps of:

providing a conductive material;

forming a copper-based based light-absorbing material on the conductive material;

depositing an intermediate material on the light- absorbing material;

depositing a first material such that the first material is electrically coupled to the light-absorbing material and forms a p-n junction with the light-absorbing material ;

wherein the intermediate material is arranged to reduce the minority carrier recombination rate at a region between the light-absorbing material and the first

material .

In an embodiment, the method comprises the step of

annealing the intermediate material and the first material in a manner such that a portion of the intermediate material diffuses into the first material.

In accordance with the fifth aspect, the present invention provides a method of forming a photovoltaic cell

comprising the steps of: providing a conductive material;

forming a first material comprising a compound indium at an interface with the light-absorbing material such that the first material is electrically coupled to the light-absorbing material and forms a p-n junction with the light-absorbing material.

In an embodiment, the step of forming a first material comprises

depositing an intermediate material comprising indium on the light-absorbing material;

depositing a cadmium-based material on the intermediate material; and

annealing the intermediate material and the cadmium-based material to form the first material.

In accordance with the sixth aspect, the present invention provides a method of forming a photovoltaic cell

comprising the steps of:

providing a conductive material;

forming a copper-based based light-absorbing material on the conductive material; forming islands of an intermediate material on a second surface portion of the light-absorbing material; and depositing a first material such that the first material has a first portion that is electrically coupled to a first surface portion of the light-absorbing material and forms a p-n junction with the light-absorbing

material . The intermediate material may be arranged to minimise transport of electrical carriers between the second portion of the first material and the second surface portion of the light-absorbing material. Furthermore, the intermediate material may be selected to decrease the concentration of carrier recombination centres located in the proximity of an interface between the light-absorbing material and the first material. The intermediate material may be selected amongst zinc sulphide (ZnS) , zinc-oxide (ZnO) , zinc-selenide (ZnSe) , amorphous silicon or

aluminium oxide (AI₂O3) .

In an embodiment, the step of forming islands of an intermediate material comprises forming a plurality of particles distributed over the second surface portion of the light-absorbing material.

38. The method of claim 37 wherein the particles are formed in a manner such that their final lateral extension is smaller than the diffusion length of minority carriers in the light-absorbing material and the diffusion length of minority carriers in the first material.

39. The method of any one of claims 33 to 38 wherein the step of forming islands of an intermediate material further comprises the step of annealing the structure comprising the conductive material, the light-absorbing material and the intermediate material.

40. The method of claim 39 wherein the annealing is performed at a temperature between 300 ^°C and 600 ^°C.

41. The method of any one of claims 27 to 40, further comprising the steps of:

providing a substrate; depositing the conductive material on the substrate ;

depositing a second material onto the conductive material, the second material comprising a metallic material .

In an embodiment the method further comprises the step of, subsequent the step of depositing the metallic material on the conductive material, annealing at least the substrate, the conductive material and the second material.

In accordance with the seventh aspect, the present

invention provides a method of controlling the conduction band offset between a kesterite layer and a cadmium- sulphide layer comprising the steps of:

depositing an intermediate material comprising indium and sulphur on the kesterite layer, before

depositing the cadmium-sulphide layer; and

annealing the intermediate material and the cadmium-sulphide layer in a manner such that the cadmium- sulphide reacts with the indium and sulphur in the

intermediate material.

In an embodiment, the thickness of the intermediate material is based on a predetermined conduction band offset between a kesterite layer and a cadmium-sulphide layer .

In an embodiment, the annealing temperature is based on a predetermined conduction band offset between a kesterite layer and a cadmium-sulphide layer.

In an embodiment, the annealing duration is based on a predetermined conduction band offset between a kesterite layer and a cadmium-sulphide layer. Advantageous features of embodiments of the present invention are provided by the reduced recombination of photo-generated carriers in the region around the

interface between the light-absorbing material and the CdS material, and in particular at the interface between the light-absorbing material and the CdS material, by

providing an intermediate material which allows optimising the band alignment between light-absorbing material and the CdS material and/or reducing the volume of the device available for carrier recombination.

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 2 show schematic representations of

photovoltaic devices and microscopy images of the devices;

Figures 3 and 4 are flow diagrams outlining methods of forming photovoltaic devices; Figure 5 is an EDS line scan of the structure shown in figure 1 (b) ;

Figure 6 shows an electronic band configuration for the structure of figure 1 (b) ;

Figure 7 shows a plot comparing external quantum

efficiency for different photovoltaic devices;

Detailed Description of Embodiments Embodiments of the present invention relate to

photovoltaic cells comprising a copper-based light

absorbing material, such as a kesterite absorber, a CdS material and metallic contacts. In particular, embodiments of the invention are related to photovoltaic cells

realised on different types of substrates and having an intermediate material between the light-absorbing material and the CdS material. The photovoltaic cells may be deposited on substrates made of glass, stainless steel, flexible polyimide or metallic foil.

In some embodiments, the intermediate material is arranged to minimise the recombination of photo-generated carriers at the interface between the light-absorbing material and the CdS material by electrically passivating a first portion of the surface of the light-absorbing material. In the passivated portion the recombination of photo- generated carriers is minimised. Electrical carriers are extracted from the cell through the p-n junction which is created at a second portion of the surface of the light- absorbing material.

The intermediate material passivates the first portion of the surface of the light-absorbing material minimising recombination of photo-generated carriers at this portion and, at the same time, substantially preventing transport of photo-generated carriers through this portion. As a consequence, the photo-generated carriers move across the second portion of the surface of the light-absorbing material through the p-n junction formed with the CdS material. Generally, this reduces the effective surface area of the p-n junction affecting the value of short- circuit current of the photovoltaic cell. On the other hand, in specific conditions, the reduction of the density of recombination centres, in particular surface

recombination centres at the interface between the light- absorbing material and the CdS material, improves the open-circuit voltage and the fill factor of the device providing improved overall performance.

The relative size of the first to the second portion of the surface of the light-absorbing material may vary depending on several parameters of the photovoltaic cell. In the embodiments where the intermediate material is configured in islands over the light-absorbing material, the relative size of the first to the second portion of the surface of the light-absorbing material is related to the size and the number of islands formed on the light- absorbing material. In these embodiments, a crucial parameter for the design of the size and the number of islands is the diffusion length of minority carriers in the light-absorbing material and the CdS material.

The shape, size and distribution of the islands are in general designed to passivate the surface at the first portion of the light-absorbing material and improve the overall performance of the photovoltaic cell. In some embodiments, the shape and dimension of the islands is also related to the nature of the intermediate material and the fabrication method used to realise the

intermediate material. The intermediate material is generally selected in a manner such that, at least the first portion on the surface of the light-absorbing material is passivated and the presence of recombination centres at the first portion is minimised. Processing steps taken during the formation of the intermediate material may decrease the concentration of recombination centres also at the second portion of the light-absorbing material .

The selection of an appropriate material as intermediate material varies in embodiments of the invention and is related, amongst other cell properties, to the quality and crystal structure of the light-absorbing material. In the embodiments where a high quality kesterite material, with a very low density of defects and recombination centres, is used, the intermediate material may be selected to have crystallographic properties compatible with the kesterite absorber. For example the intermediate material may have a lattice constant and/or a lattice structure similar to the absorber so to avoid the generation of recombination centres at the intermediate material/absorber interface, such as zinc sulphide (ZnS) , zinc-oxide (ZnO) , zinc- selenide (ZnSe) , indium-sulphide (InS) or indium-selenide (InSe) . In the embodiments where an absorber material with a lower quality, higher density of defects and

recombination centres, is used, the intermediate material may be selected to be able to passivate the defects of the absorber. The intermediate material may have

crystallographic properties different from the absorber but efficiently passivate defects in the absorber, while being electrically inactive in the photo-conversion process. For example, the intermediate material may be an amorphous material, such as amorphous silicon or aluminium oxide (AI₂O3) , or a material containing a passivating agent for the copper-based absorber, such as hydrogen. In embodiments of the invention the intermediate material is selected such that it does not participate in the conduction mechanisms of the photo-generated carriers and, at least, does not introduce electrically active

recombination centres. The intermediate material is also configured to prevent transport of carriers between the light-absorbing material and the CdS material at the first surface portion of the light-absorbing material, as discussed above.

In embodiments, an ordered or randomly arranged array of particles of the intermediate material, covering the first portion of the light-absorbing material, may be formed on the light-absorbing material using a single deposition step, such as colloidal deposition or chemical bath deposition. In alternative embodiments the particles may be formed in multiple steps. For example a layer of the intermediate material may be deposited onto the light- absorbing material by, for example, sputtering or

evaporation. The layer may cover a portion or the whole surface of the light-absorbing material. An additional step, for example a thermal annealing or a chemical treatment may be used to reduce the layer of the

intermediate material into an ordered or randomly arranged array of particles. In alternative embodiments a layer of the intermediate material may be deposited on the light- absorbing material through a patterned temporary template, such a metallic mask. By depositing a uniform layer onto the entire surface of the light-absorbing material covered by the metallic mask, an ordered or randomly arranged array of particles of the intermediate material is

obtained upon removal of the metallic mask. In some chalcogenide based solar cells, and more

particularly in some kesterite solar cells, the performance is affected by the non-optimal band alignment between the light-absorbing layer and the CdS layer. This poor alignment affects carrier transport and facilitates carrier recombination. In some embodiment of the invention, the intermediate material is provided in the form of an In₂S₃ continuous layer between the CdS material and the light-absorbing material. The In₂S₃ may create a physical separation layer between the CdS material and the light-absorbing material or, in some instances, may intermix with, at least

partially, into the CdS material during an annealing step. Even when in the form of a continuous layer, the In₂S3 may comprise clusters of different size arranged in a compact layer . The presence of In₂S3 in the region between the absorber and the CdS layer allows optimising the band alignment the solar cell structure. In particular, properties of the In₂S3 layer can be tuned in order to achieve an optimal band alignment around the junction area of the device.

These properties include the initial thickness of the In₂S3 layer and, in the case of an intermixed In₂S3/CdS layer, the annealing conditions of the In₂S3/CdS system.

In order to evaluate the theoretical beneficial effect of using an In₂S3 layer to modify the band alignment, the performance of a kesterite solar cell with different conduction band offsets at the junction have been

calculated. The calculation has been performed for two different surface recombination regimes at the kesterite light-absorbing material surface and no intermediate layer. Table 1 shows the dependence of the performance upon the variation of the conduction band offset at the junction when no interface recombination (fully passivated

surface) .

CBO (eV) Voc ■ (V) JSC FF (%) Eff (%)

(mA/ cm² )

-0.3 1. 058 24. .66 75. .9 19 .81

-0.2 1. 051 24. .60 79. .4 20 .52

-0.1 1. 042 24. .62 79. .1 20 .27

0 1. 033 24. .56 80. .3 20 .44

0.1 1. 027 24. .70 80. .2 20 .34

0.2 0. 969 24. .74 78. .6 18 .84

0.3 0. 871 24. .93 77. .6 16 .84

0.4 0. 771 25. .18 75. .6 14 .69

0.5 0. 768 25. .32 35. .7 6. 13

Table 1

Table 2 shows the dependence of the performance upon the variation of the conduction band offset at the junction with interface recombination (R_v = 10⁷ cm/s) .

CBO (eV) Voc (V) JSC FF (%) Eff (%)

(mA/ cm² )

-0.3 0.638 22 76 76 6 11 .12

-0.2 0.738 22 71 77 8 13 .03

-0.1 0.837 22 73 78 8 14 .99

0 0.934 22 78 79 4 16 .89

0.1 0.996 22 83 78 9 17 .95

0.2 0.964 22 84 76 6 16 .87

0.3 0.869 22 85 75 7 15 .03

0.4 0.77 22 86 74 1 13 .03

0.5 0.66 22 57 39 0 5. 95

Table 2 Table 1 shows that, without substantial interface

recombination, pronounced conduction band offsets can be tolerable. The efficiency of the solar cell in fact is contained within reasonable values over a large interval of offsets (first column) . Table 2 shows that, when interface recombination is present, as in the case of CZTS solar cells, the conduction band offset affects the solar cell performance significantly, in particular the Voc. The optimal conduction band offset for an interface

recombination velocity of R_v = 10⁷ cm/ S was found to be in the range of 0 to 0.2 eV. These simulation results demonstrate the potential in terms of performance

improvement of an In₂S3 intermediate layer applied to the CZTS based solar cells, as provided by embodiments of the present invention.

In some embodiments, the In₂S3 layer plays an active role in the generation of charge carriers in the solar cells, providing a substantial improvement in the EQE of the devices . The intermediate material is formed on the copper-based light-absorbing material after the copper-based compound is formed. Alternatively, the intermediate material is deposited on the precursors of the copper-based material and is annealed with the precursors. The intermediate material is preferably formed using processing techniques compatible with thin film

photovoltaic cells technologies such as sputtering, evaporation, chemical bath deposition, colloidal

deposition, chemical-vapour deposition, atomic layer deposition, successive ionic layer adsorption and reaction method, anodised oxidation, aerosol-assisted chemical- vapour deposition or spray-pyrolysis-like process.

Referring now to figure 1, there is shown a schematic representation of a photovoltaic cell device 100 in accordance with an embodiment of the present invention. The photovoltaic cell consists of a soda lime glass substrate 102 covered with a molybdenum (Mo) layer 104. In this embodiment, the Mo layer 104 is realised by

sputtering a Mo target in a multi-target sputtering machine. However, the Mo layer 104 could be realised using other PVD or CVD techniques, such as e-beam evaporation. The kesterite based light-absorbing layer 108 is formed on the Mo layer 104. The formation of the kesterite layer 108 involves a high temperature annealing step. An In₂S3 layer 109 is formed on the light-absorbing layer 108 by a chemical bath deposition. The solution in bath is prepared by adding thiocetamide (CH₃CSNH₂) , indium chlorite (InCl₃), and ethylic acid into water.

An n-type CdS buffer layer 110 is deposited onto the In₂S3 layer 109. The n-type CdS layer 110 allows forming a p-n junction with the p-type kesterite layer 108. In this embodiment, the CdS layer 110 is deposited by CBD.

The CBD of the CdS layer 110 is realized in stirring aqueous solution using CdS, cadmium nitrite or cadmium chloride as zinc source, thiourea as sulphur source and ammonia as complex agent at temperature of 80 ^°C.

The front contacting structure of the photovoltaic cell 100 is realised with an intrinsic zinc oxide (IZO) layer 112 and an aluminium oxide (AI₂O3) doped zinc oxide (AZO) layer 114. These layers are generally formed by sputtering or ALD. Finally an electrical Al or Al/Ni bi-layer

contacting structure 116 is deposited on the top surface of the photovoltaic cell 100. The Al structure 116 is usually deposited by thermal evaporation with the use of a shadow mask, but could be deposited by other PVD or CVD techniques .

Figure 1 (b) shows a transmission electron microscopy (TEM) image 150 of a section of a device realised in accordance with schematic 100. The TEM image 150 clearly shows the glass substrate 152 covered by molybdenum layer 154. The kesterite light-absorbing material forms a layer 158 with an irregular morphology. A thin In₂S3 layer 159 is visible between absorber layer 158 and CdS layer 160. Layers 159, 160 and the front contacting structure 162, 164 follow the initial morphology of layer 158. In figure 1(b) it can be observed that the In₂S3 layer 159 has a cluster-like morphology and is composed by a compact distribution of clusters .

Referring now to figure 2, there is shown a schematic representation of a photovoltaic cell device 200 in accordance with an alternative embodiment of the present invention. The photovoltaic cell has a similar structure to device 100 except for the alternative nature of

intermediate layer 209. In device 200 an ordered array of particles of ZnS 209 is formed on a first portion of the light-absorbing layer 108 by a CBD step in a stirring aqueous solution using zinc sulphate or zinc chloride as zinc source, thiourea as sulphur source, ammonia as complex agent and methanol as surfactant at temperature of 70^°C.

The ZnS particles 209 have a semi-sphere shape with a diameter of about 50 nm and cover a portion of about 10% of the surface of the light-absorbing material. This provides a reduction of recombination centres of 90%. The n-type CdS layer 110 is deposited onto the light- absorbing layer 108 and the array of particles of the intermediate material 209.

A schematic representation of a section of the CdS layer

110 and the particles of intermediate material 209 is shown in the detail of Figure 2. Figure 2 (b) , there is shown an atomic force microscope image of an internal layer of a kesterite photovoltaic cell in accordance with structure 200. The image shows randomly arranged ZnS particles 250 of the intermediate material on the surface of the kesterite light-absorbing layer 108. The size of the particles is between 100 nm and 200 nm. The ZnS particles are prepared by a CBD technique. The deposition is carried out in a 100 r/min stirring aqueous solution using 0.1 mol/L zinc sulphate as zinc source, 0.2 mol/L thiourea as sulphur source, 5 mol/L ammonia as complex agent and 1 vol% methanol as

surfactant. The deposition temperature and time is 70 ^°C and 30 min, respectively.

The CdS fabricated by CBD method is cubic. The d-spacing of the CdS for the (111) plane is 3.36 A, which is well matched, for example, with CIGS (112) absorbers, whilst quite mismatched with the CZTS main (112) plane (3.13 A) .

The large lattice mismatch may lead to significant

interface states and thereby to a significant interface recombination. The ZnS particles have a similar crystal structure to the CZTS and promote passivation of the CZTS surface and thereby less interface states.

Referring now to figure 3 there is shown a flow diagram 300 outlining a method of forming a photovoltaic cell in accordance with embodiments. The first step 302 consists in providing a substrate to deposit the layers of the photovoltaic cell upon. This substrate is generally, a soda lime glass substrate. However, other types of

substrates can be used. A conductive layer is then

deposited 310 on the soda lime glass, generally a Mo layer. In some embodiments, the substrate covered with the Mo layer is annealed in an annealing furnace to improve the properties of the Mo layer. This step is optional and is not shown in figure 2. Subsequently, a copper-based light-absorbing material is formed 330 on the Mo layer. In this embodiment the copper-based light-absorbing material is a kesterite material and is formed in two sequential steps : depositing a series of layers containing precursor materials for the kesterite layer (for example mixture of ZnS,Cu2S and SnS2) ; and annealing the entire structure at 575°C in a S rich atmosphere for 30 min in a dual zone tube furnace, with the S zone heating to 300°C and N₂ flowing at 20 seem.

Subsequently, a layer of an intermediate material is formed 332 on a surface of the kesterite material. The intermediate material can be formed, for example, by PVD, CVD or CBD. A CdS layer is successively formed 336 by using a CBD step. In some embodiments, after the CdS layer is deposited, the structure is annealed to promote

intermixing of the intermediate material and the CdS layer .

For example, in the case of an In₂S3 intermediate material, an annealing step is performed to allow intermixing of the In₂S3 layer and CdS layer. This promotes formation of an interface compound and improves conduction band alignment in the device. The annealing is generally performed with temperature ramping rates higher than 30 °C/min. The annealing time is shorter than 10 min and the temperature is generally lower than 400 ^°C in order to promote the inter-diffusion between layers.

The structure is then placed in a multi-target sputtering deposition machine to deposit 340 a layer comprising a conductive material to form a front contact. This front contact is formed by depositing in sequence: an IZO layer with a thickness of about 50 nm; and an AZO layer with a thickness of about 300 nm at about 50 ^°C. Finally, Al contacts are thermally evaporated through a shadow mask to create an Al front electrode.

Referring now to figure 4, there is shown a flow diagram 400 outlining a method of forming a photovoltaic cell in accordance with another embodiment of the present

invention. The method 400 of figure 4 shares the initial and final steps 302, 310, 336, 340 of the method 300 of figure 3. However, in the method 400 of figure 4 at least a layer containing the intermediate material is deposited 440 on the layers containing precursor materials for the kesterite layer right after these layers are deposited 330 and before the kesterite compound is formed by annealing the precursors. The kesterite compound is formed and the layer containing the intermediate material is reduced to an ordered or randomly arranged array of particles by annealing 450. The deposition 336 of the first layer containing CdS and the deposition 340 of a conductive material to form front contacts follows as in method 300.

In some cases, two additional steps may be performed after the step of depositing 310 a layer comprising a conductive material to form a back contact and before depositing the kesterite precursors. These two steps consist in

depositing a further layer onto the conductive material used to form a back contact and treating this further layer to improve its structural properties. One or both of these two optional steps may be carried out during

manufacturing. The further layer deposited onto the conductive material reduces the formation of sulphides and/or selenides at the interface between the kesterite light-absorbing layer and the conductive material at the back contact which are detrimental for the performance of the photovoltaic cell. The further layer may be a metallic layer, such as gold or silver, a semiconductor layer, such as titanium boride, or a dielectric layer, such as

molybdenum oxide.

Referring now to figure 5, there is shown an EDS line scan 500 of structure 150. Line scan 500 is divided in four sections representing the ZnO layer 162, CdS layer 160, In₂S3 layer 159 and kesterite layer 158. Traces 502, 504, 506, 508, 510, 512 and 512 respectively indicate the distribution of Cd, In, Cu, Zn, Sn, S, 0. Scan 500 shows that some elements diffuse throughout the device during fabrication. In particular, trace 504 appears to decay quickly at the interface with layer 159. However, a marginal diffusion of In in the CZTS layer 158 is

observed. Trace 504 maintains a higher number of counts through layer 160 and 162. This indicates the diffusion of In into the CdS layer 160. In the same way, trace 502 shows a certain concentration of Cd in In₂S₃ layer 159. The kind of inter-diffusion indicated by traces 502 and 504 acts to blur the interface and facilitates the conjoining of the buffers and absorber layers modifying the band alignment between the light-absorber 158 and CdS layer 160. A chemical reaction could potentially occur between the CdS and In₂S₃ to induce formation of a

compound, for example an alloy containing In and Cd.

One of the causes of the reduced performance of kesterite based solar cells is the non-optimised conduction band alignment between these layers. Research has shown that the conduction band edge of CdS is about 0.3 eV lower than the conduction band edge of CZTS forming a conduction band cliff. Pure In₂S₃ forms a conduction band spike of about 0.41 eV with CZTS. By introducing In₂S₃ layer 159 in the device a compound comprising In and Cd can be formed at the interface with the kesterite layer 158. The value of the conduction band offset around the junction can be modulated to minimise carrier recombination.

Figure 6 shows a schematic band diagram 600 showing a possible electronic band configuration of a structure comprising In₂S3 layer 159. The value of the conduction band offset 602 between 604 and 606 can be modified by varying, for example, the initial relative thicknesses of layers 159 and 160. Offset 602 can be reduced from 0.5 eV to close to 0 eV with an optimum value about 0.11 eV. In diagram 600 the CZTS layer 158 has a bandgap 608 of 1.5 eV while the bandgap of the buffer layer 610 can generally vary between 2.1 eV and 2.4 eV. A shallow spike of about 0.1 eV is formed between conduction band 604 and conduction band 606. Generally a layer 159 with a thickness between 5 nm and 100 nm, or preferably 20 nm and 60 nm, is used for optimal results, with a total thickness of layer 159 and layer 160 below 100 nm.

Referring now to figure 7, there is shown a plot 700 comparing the external quantum efficiency (EQE) for four different photovoltaic devices. Curve 702 shows EQE for a prior art kesterite solar cell based on a CZTS light absorbing layer and a CdS buffer layer. The EQE for curve 702 peaks at about 550 nm. Curve 704 shows EQE for a solar cell device based on a kesterite absorber and a pure In₂S₃ layer, without any CdS layer. Curve 704 shows a clear improvement of the short wavelength response of the solar cell device. However a poor response is obtained at longer wavelengths using this type of device.

Curve 706 shows the EQE for a solar cell with a structure as in figure 1 (a) manufactured in accordance with method 300. The comparison between curve 706 and 702 clearly shows that the addition of the In₂S3 layer substantially improves the short wavelength response of the solar cell device although causing a minor decrease of the EQE for longer wavelengths.

The overall result of the increased ^xblue' response and the improved conduction band alignment discussed above is an increase in the performance of the solar cell device.

Buffer Voc(mV) Jsc (mA/cm2 ) FF(%) Efficiency ( % )

CdS 660 15.9 53.7 5.52

In₂S₃ 716 12.1 30.2 2.62

In₂S₃/CdS 714 17.6 52.6 6.61

Table 3 Table 3 shows a comparison between the performance of a prior art kesterite solar cell based on a CZTS light absorbing layer and a CdS buffer layer. The presence of the In₂S3 layer improves the cell Voc. However, a pure In₂S3 buffer layer causes a significant drop in the Jsc and fill factor, as also demonstrated by the EQE in figure 7, trace 704. The combined In₂S3/CdS buffer layer provides an increased performance with improved Voc, Jsc and FF.

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 device comprising:

a copper-based light-absorbing material;

minority carrier recombination rate at a region between the light-absorbing material and the first material.

2. The device of claim 1 wherein a portion of the

intermediate material diffuses into the first material during fabrication of the device.

3. The device of claim 1 or claim 2 wherein the

intermediate material comprises indium.

4. The device of claim 3 wherein the intermediate material comprises In₂S3.

5. The device of any one of claims 1 to 4 wherein the intermediate material forms an intermediate layer

comprising a continuous distribution of clusters.

6. The device of claim 5 wherein the intermediate layer has a thickness between 5 nm and 100 nm or preferably between 20 nm and 60 nm.

7. The device of any one of claims 1 to 4 wherein the intermediate material forms a plurality of clusters distributed across an interface region of the light- absorbing material and physically separating a portion of the light-absorbing material from the first material.

8. The device of any one of the preceding claims wherein the intermediate material is photoconductive .

9. The device or any one of the preceding claims wherein the intermediate material absorbs at least a portion of photons incoming to the device and in use generates a portion of the current generated by the device.

10. The device of any one of the preceding claims wherein the device has external quantum efficiency higher than 70% at a wavelength between 350 nm and 450 nm.

11. A photovoltaic cell comprising:

a copper-based light-absorbing material;

12. The photovoltaic cell of any one of claims 2 to 11 wherein the light-absorbing material and the first

material have a conduction band energy separation smaller or equal to 0.24 eV.

13. The photovoltaic cell of claim 12 wherein the light- absorbing material has a conduction band edge energy higher than the first material.

14. A photovoltaic cell comprising:

a copper-based light-absorbing material;

an intermediate material arranged between a second portion of the first material and a second surface portion of the light-absorbing material.

15. The photovoltaic cell of claim 14 wherein the

intermediate material is arranged to minimise transport of electrical carriers between the second portion of the first material and the second surface portion of the light-absorbing material.

16. The photovoltaic cell claim 14 or claim 15 wherein the intermediate material is selected to decrease the

concentration of carrier recombination centres located in the proximity of an interface between the light-absorbing material and the first material.

17. The photovoltaic cell of any one of claims 14 to 16 wherein the intermediate material has a matching

crystalline structure to the crystalline structure of the light-absorbing material.

18. The photovoltaic cell of any one of claims 14 to 17 wherein the intermediate material has a lattice constant substantially identical to the lattice constant of the light-absorbing material.

19. The photovoltaic cell of any one of claims 14 to 16 wherein the intermediate material is selected amongst zinc sulphide (ZnS) , zinc-oxide (ZnO) , zinc-selenide (ZnSe) , amorphous silicon or aluminium oxide (AI₂O3) .

20. The photovoltaic cell of any one of claims 14 to 19 wherein the intermediate material comprises a plurality of particles distributed over the second surface portion of the light-absorbing material.

21. The photovoltaic cell of claim 20 wherein the lateral extension of the particles is smaller than the diffusion length of minority carriers in the light-absorbing

material and the diffusion length of minority carriers in the first material.

22. The photovoltaic cell of claim 20 or claim 21 wherein the particles have a semi-sphere shape.

23. The photovoltaic cell of any one of the preceding claims further comprising a second material arranged between the a conductive back contact and the copper-based light-absorbing material, the second material comprising a metallic material;

wherein the metallic material is selected so as to reduce the formation of sulphides and/or selenides in the region between the conductive back contact and the light-absorbing material.

24. The photovoltaic cell of claim 23 wherein the metallic material comprises silver, gold or a gold-silver alloy.

25. The photovoltaic cell of any one of the preceding claims wherein the copper-based light-absorbing material comprises a copper-tin-zinc-sulphide-material, a copper- tin-zinc-selenide-material, or a copper-zinc-germanium- tin-chalcogenide-material .

26. The photovoltaic cell of any one of the preceding claims wherein the first material comprises cadmium- sulphide .

27. A method of forming a photovoltaic cell comprising the steps of:

providing a conductive material;

depositing an intermediate material on the light- absorbing material;

material .

28. The method of claim 27 further comprising the step of annealing the intermediate material and the first material in a manner such that a portion of the intermediate material diffuses into the first material.

29. The method of claims 27 or 28 wherein the intermediate material comprises indium or In₂S₃.

30. The method of any one of claims 27 to 29 wherein the intermediate material is deposited in a manner to form a continuous distribution of clusters or in a manner to form a plurality of clusters distributed across a surface of the light-absorbing material and physically separating a portion of the surface from the first material.

31. A method of forming a photovoltaic cell comprising the steps of:

providing a conductive material;

32. The method of claim 31 wherein the step of forming a first material comprises

depositing a cadmium-based material on the intermediate material; and

33. A method of forming a photovoltaic cell comprising the steps of:

providing a conductive material;

material .

34. The method of claim 33 wherein the intermediate material is arranged to minimise transport of electrical carriers between the second portion of the first material and the second surface portion of the light-absorbing material .

35. The method of claim 33 or claim 34 wherein the

intermediate material is selected to decrease the

36. The method of any one of claims 33 to 35 wherein the intermediate material is selected amongst zinc sulphide (ZnS) , zinc-oxide (ZnO) , zinc-selenide (ZnSe) , amorphous silicon or aluminium oxide (AI₂O3) .

37. The method of any one of claims 33 to 36 wherein the step of forming islands of an intermediate material comprises forming a plurality of particles distributed over the second surface portion of the light-absorbing material .

41. The method of any one of claims 27 to 40, further comprising the steps of:

providing a substrate;

depositing the conductive material on the

substrate ;

42. The method of claim 41, further comprising the step of, subsequent the step of depositing the metallic

material on the conductive material, annealing at least the substrate, the conductive material and the second material .

43. The method of claim 27 to 42, wherein the light- absorbing material comprises a copper-tin-zinc-sulphide material, a copper-tin-zinc-selenide material, or a copper-zinc-germanium-tin-chalcogenide-material and the step of forming the light-absorbing material comprises:

depositing a plurality of precursor materials comprising copper, zinc and tin; and

annealing the plurality of precursor materials in the presence of sulphur or selenium.

44. The method of any one of claims 27 to 43 wherein the first material comprises cadmium-sulphide.

45. A method of controlling the conduction band offset between a kesterite layer and a cadmium-sulphide layer comprising the steps of:

depositing the cadmium-sulphide layer; and

annealing the intermediate material and the cadmium-sulphide layer in a manner such that the cadmium- sulphide reacts with the indium and sulphur in the intermediate material.

46. The method of claim 45 wherein the thickness of the intermediate material is based on a predetermined

conduction band offset between a kesterite layer and a cadmium-sulphide layer.

47. The method of claim 45 or claim 46 wherein the annealing temperature is based on a predetermined

conduction band offset between a kesterite layer and a cadmium-sulphide layer.

48. The method of any one of claims 45 to 47 wherein the annealing duration is based on a predetermined conduction band offset between a kesterite layer and a cadmium- sulphide layer.