GB2466496A

GB2466496A - Photovoltaic cell based on transition metal oxides of varied band gaps and p/n types

Info

Publication number: GB2466496A
Application number: GB0823441A
Authority: GB
Inventors: Guosheng Shao; Jack Luo
Original assignee: University of Bolton
Current assignee: University of Bolton
Priority date: 2008-12-23
Filing date: 2008-12-23
Publication date: 2010-06-30
Also published as: GB0823441D0

Abstract

The present invention relates to a low-cost and high-efficiency thin film photovoltaic (PV) cell containing at least first and second interfaced or coupled layers. The layers comprise at least first and second photosensitive metal oxide semiconductor materials, wherein the first layer is an n-type photosensitive layer (WGO) and the second layer is a p- type photosensitive layer (NGO), and wherein the first layer has a band gap which is wider than that of the second layer. The p-type NGO semiconductor comprises typically either pure of doped CuO, Cu2O or delaffossite CuFeO2. The N-type NGO comprises either pure or doped materials of any one of TiO2, SnO2, ZnO, NiO, MnO, CoO, FeO, Fe2O3, V203, Mn203 and Ti203. The PV cell employs environmentally friendly metal oxides derived from sustainable natural resources.

Description

Photovoltaic Cell The present invention relates to a low-cost and high-efficiency thin film photovoltaic (PV) cell using environmentally friendly metal oxides derived from sustainable natural resources.

PV cells are operable to generate electrical current in response to incident light, in particular incident solar radiation. They are commonly referred to as solar cells. There are a number of different forms of PV cells, one of which is the thin film PV cell.

Thin film PV cells rely on p-n junctions, either in the form of p-n homojunctions or p-n heterojunctions, as a photosensitive region or component. A p-n junction is an interface formed by combining p-type and n-type semiconductors together in close contact. The homojunctions refer to p-n junctions within a semiconductor material of the same kind, and the heterojunctions to p-n junctions between different semiconductor materials. These p-n junctions, either within a semiconductor material or between different semiconductor materials, function to create photosensitive regions where either emission or detection of photons takes place. Detection of photons occurs when photons are incident in the photosensitive region to create electron-hole pairs, thus causing an electric current to flow. Such a * photovoltaic process is not possible unless the photon energy is higher than the energy * ** * gap, or band gap, of the semiconductor materials.

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* Conventional PV technology is mainly based on silicon material which has the significant setback of high cost and is thus economically unsustainable. Other low-* S cost candidates are often based on materials of limited natural resources (e.g. those which contain a considerable percentage of indium or other group III and/or group V semiconductors), or materials that pose an environmental threat either in service or during processing, or materials having a short service life under solar radiation exposure (e.g. PV cells based on Ti02 and organic sensitising dyes, where the dyes readily decay because of the photo-catalytic functionality of Ti02).

Owing to their potential use in large quantities, sustainable PV technologies need to satisfy the following criteria: they need to be based on materials of enormous natural resources, environmentally friendly, low-cost to process, and durable under prolonged exposure to solar radiation. For effective and efficient PV applications, they also need to be able to absorb a wide range of the solar spectral radiation for energy conversion.

Some metal oxides including M'O2 (e.g. MIV = Ti, Sn), M'2O3 (e.g. M"1 = V, Ti, Cr, Al, Co, Nb, Fe), M"O (e.g. M" Cu, Ni, Mn, Zn), M12O (e.g. Cu20) and the M'M"O2 delaffossite (CuFeO2) type oxides are sustainable in resources and do not pose any significant hazardous threat to the environment. The majority of metal oxide semiconductors, particularly the commonly used wide gap metal oxides are n-type in nature. Only a few p-type oxide semiconductors have been recently discovered.

*: These p-type semiconductors include the copper oxides CuO and Cu20, and the I*

S

M M 02 delaffossite type oxides, where M is a metal with one valence electron (e.g. :4 Cu, Ag) and M"1 is a metal with three valence electrons (e.g. Fe, Al, Cr, Ti, V, Nb).

S *S * * * * * *** *

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Lattice sites of the metal can be doped to enhance electrical properties or to tune the band gap of the materials. The n-type wide gap metal oxides can also be doped into p-type oxides with a significantly reduced band gap. These materials can be used to design novel low-cost and high efficiency PV cells. The key for a sustainable PV technology using these materials lies in the capacity for the utilisation of a large fraction of solar radiation.

The aim of the present invention is to provide a photovoltaic cell that is based on transition metal oxides of varied band gaps and p-/n-types, which are constructed into structurally and electronically compatible sequential layers to allow the absorption of a wide range of spectral solar radiation for energy conversion. It is believed that the present invention provides one or more of these requirements.

According to the invention there is provided a photovoltaic cell containing at least first and second interfaced or coupled layers, which layers comprise at least first and second photosensitive metal oxide semiconductor materials, wherein the first layer is an n-type photosensitive layer (WGO) and the second layer is a p-type photosensitive layer (NGO), and wherein the first layer has a band gap which is wider than that of the second layer.

:::: The band gap for the WGO layer may be up to about 3.3 to 3.5 eV but is * typically less than about 3.0 eV. Possible sizes of the band gap include 1,5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8 or 2.9 eV. A typical band gap size is between about 2.5 and about 3.0 eV. The NGO layer has a band gap which is smaller than that of the WGO layer, enabling good conversion efficiency. Possible sizes of the

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NG0 band gap include 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, or 2.4 eV. A typical size for the band gap of the NGO layer is between about 1.0 and about 1.3 eV. As an example, the WOO band gap may be?: 2.5 eV while the NGO band gap is <2.5 eV.

A three layer construction can also be adopted, wherein one or more further layers of material with an intermediate band gap (100) is included, to form a WOO-IGO-NGO type layered structure. In such a WGO-IGO-NGO layered structure, the WOO layer absorbs the high energy part of solar radiation, and the layers below absorb the lower energy part of solar radiation. For example, the WGO layer, which is typically above the TOO and NGO layers, is used to absorb the solar radiation around the ultra-violet to the blue ranges of solar radiation, the IGO layer is used to absorb the major visible range down to the red edge, and the NGO layer targets the range down to the infrared region. Such a structural design makes it possible to absorb a wide range of solar spectral radiation and reduce intra-band waste of high energy radiation. This helps to overcome the thermodynamic barrier of single junction PV cells, where only a narrow range of solar power is utilised.

By "intermediate band gap" it is meant that the band gap associated with the IGO layer has a size which is between those of the WOO and NGO layers. *.. * * S I. *

The WOO materials include, but are not limited to, any of the stoichiometries S * based on one or more photosensitive metal oxides of the formi.ilae M"O (M" = Zn, Fe, 5555** * Mn, Ni, etc), M"02 (M" = Ti, Sn, etc) and M111203 (M" = Fe, Cr, Al, Nb, V, Ti, Mn etc). The IGO materials include, but are not limited to, the p-type M'M"O2 S..... * S

delaffossite materials, or a material based on the same WOO phase that is doped and/or alloyed to produce a significantly reduced band gap. The NGO material may be based on the p-type CuO or Cu20 materials, or on the delaffossite type materials.

These oxide layers can be doped and/or alloyed to improve the electrical properties or to tailor the band gap, in order to maximise energy harvest.

The interface between the layers can be graded to enhance structural and electronic compatibility. Such a structural design is in line with materials thermodynamics, where the interfaced/coupled regions are thermodynamically stable.

For example, the M'M"02 delaffossite type of IGO layer is a natural product phase through the interfacial reaction of the WOO (e.g. based on M"203, M"02 or M10) and NGO (e.g. based on CuO or Cu20) layers. This makes it reliable to utilise thermal annealing to reduce defects without the risk of violating the structural design. Another advantage is the natural tendency to form good epitaxial relationship between the layers, reducing the complication in band-alignment across layers.

According to one embodiment of the invention, the metal oxides comprise doped or alloyed phases of M10, or M'2O3, M''O2, or M'M"02.

According to a further embodiment of the invention, the p-type NGO semiconductor comprises either substantially pure or doped CuO. * S *

S *.Sa

According to a further embodiment of the invention, the p-type NGO :4 20 semiconductor comprises either substantially pure or doped Cu20.

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** ..SS * S I. * S * 5..

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According to a further embodiment of the invention, the p-type NGO semiconductor comprises either substantially pure or doped delaffossite CuFeO2.

According to a further embodiment of the invention, the p-type IGO semiconductor comprises either substantially pure or doped delaffosite CuFeO2.

By "substantially pure" it is meant that the respective materials contain no less than about 99% of the primary photosensitive metal oxide material.

According to a further embodiment of the invention, a typical WGO-NGO layered structure can comprise of Fe203 (WGO) and CuO (NGO).

The photosensitive layers may be provided between sets of ohmic contacts.

One set of ohmic contacts may have one or more windows provided therein to allow radiation to be incident upon the layers.

The interfaced or coupled layers can be made using any suitable synthesis method apparent to a person skilled in the art, including, but not limited to, such methods as physical vapour deposition (with or without using alloyed andlor doped targets), co-sputtering, reactive sputtering, chemical vapour deposition, molecular beam epitaxy, and ion implantation (including supplementary oxygen implantation to avoid oxygen deficiency due to implantation), and solid-state interfacial reaction. The *..* * * S layers may each be formed using the same synthesis method or may alternatively each * ..S be formed using different synthesis methods as desired or as appropriate. S *

S 55*

* 20 One or more of the layers may be doped andlor alloyed to tune the band gap to a desired value or to tune the gap continuously with a graded compositional and SS** * * associated band gap profiles. Alloying and/or doping techniques may be used to widen the band gap of the second layer or to narrow the band gap of a material of the first layer, as required or as desired. Alloying and/or doping can be conducted either by introducing a single elemental species into targeted lattice sites, or by introducing a combination of more than one alloying and/or doping species into the lattice. Suitable alloying and/or doping elements will depend upon the material forming the layers; which elements are suitable for any given material will be apparent to a skilled person. For example, alloying and/or doping between Ti203 (band gap 2.9 eV) and Fe203 (band gap 2.1 eV) makes it possible to cover the solar spectral radiation from the violet down to the red optical absorption edge. The structure of the phase remains the same due to such alloying and/or doping. Other elements used to dope and/or alloy the Fe203 phase include, but are not limited to, V, Cr, Nb, Co, Mn and Al, with effects similar to Ti. Metal elements with two valence electrons can be used to dope and/or alloy the M'M11102 to obtain improved electrical properties and a tuneable band gap.

According to one embodiment of the invention, the alloying and/or doping elements may include one or more of C, N, S and P which are introduced to one or more oxygen lattice sites.

A two-layer structure can be adopted when the top layer comprises a material * 20 having a continuously tuneable band gap, either due to doping and/or alloying of the *:* M'O, MnO2 or M"203 phase. In such a case, the top layer typically starts with a virgin wide gap material which is gradually alloyed and/or doped to reduce the band gap. Such a layer can be directly coupled with a p-type CuO based layer to form a * *..*** * * heterojunction. Even in such a construction, interfacial reaction during annealing helps to form a nano-layer of delaffossite oxide, and a three-layered structure is useful for improved structural stability/compatibility.

The layers may have a substantially flat interface. Alternatively, the layers may be provided with a more complex -i.e. not flat -interface to increase the interfacial area. This can be achieved by one of the first or second layers having an array of projecting nanowires and the other layer not having the array being deposited over said nanowires. Alternatively, one of the first or second layers may be provided with a network of nanowires and the other layer not having the network being deposited over said nanowires. The array or network of nanowires may be on either the first or the second layer.

According to a further embodiment, the photovoltaic cell may comprise more than one IGO layer, e.g. two, three, four or more IGO layers, between the n-type WGO and p-type NGO layers, such that the band gap gradually reduces from the n-type WGO side to the p-type NGO side.

Also provided in accordance with the invention is a method of manufacturing a photovoltaic cell according to any preceding claim, comprising the steps of: :::: i) providing a first layer of a first photosensitive metal oxide material, *::::* wherein the first layer is an n-type photosensitive layer (WGO); and * 20 ii) interfacing or coupling the first layer with a second layer of a second

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* S.. *6 * S photosensitive metal oxide material, wherein the second layer is a p-m * S * .*

S *...

S S

type photosensitive layer (NGO), and wherein the first layer has a wider band gap than the second layer.

In order that the invention is more clearly understood it is described in further detail below, by way of example only and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a WGO-NGO coupled-layer embodiment of a photovoltaic cell according to the present invention.

Figure 2 is a schematic diagram of a WGO-IGO-NGO three-layer embodiment of a photovoltaic cell according to the present invention.

Figure 3 is a schematic diagram of a multi-layer embodiment of a photovoltaic cell according to the present invention.

Turning first to Figure 1, a schematic diagram of a two-layer design of the PV cell according to the present invention is shown. The cell comprises the n-type WOO and the p-type NGO layers. The layer 1 of WOO material has a wider band gap (e.g. band gap ? 2.5 eV) than the layer 2 of NOO (e.g. band gap <2.5 eV) layer. This allows the WOO layer to absorb the shorter wavelength and the NGO layer to absorb the longer wavelength of the solar radiation. The WOO materials used may include * . but are not limited to materials based on the M'O (e.g. NiO, FeO, CoO, MnO, ZnO), M''O2 (e.g. Sn02, hO2), or M"203 phase (e.g. Ti203, Fe203, Mn203, Cr203, A1203), ** *1 * 20 and the NGO materials include but are not limited to the p-type M'M"02 delaffossite * S materials (e.g. CuFeO2) or based on the p-type CuO or Cu20 materials. For example, .S *..a** * 4' a WGO-NGO can be made of Fe203 (WGO) and CuO (NGO). The layers I and 2 are provided between a pair of ohmic contacts 3 and 4. The contacts 3 and 4 allow the cell to be connected to a circuit. The upper contact 3 is provided with windows to allow radiation to be incident upon the layers.

Figure 2 is a schematic diagram of a three-layer design of photovoltaic cell according to the present invention. The cell comprises a top layer 5 of n-type WGO material, a middle layer 6 is an IGO layer, and a bottom layer 7 is of a p-type NGO material. The band gap of the overlaid layers decreases from top to bottom in the direction of the arrow. The layers 5 to 7 are provided between a pair of ohmic contacts 8 and 9. Radiation is incident upon the WGO layer 5 to absorb short wave-length photons to generate electric charges (e.g. Ti203, band gap about 2.9 eV). Long wave length photons are converted into electric charges by the underlying IGO (e.g. CuFeO2, band gap about 2.0 eV) and NGO (e.g. CuO, band gap about 1.2 eV) layers.

The WGO-IGO-NGO coupling acts to separate the charge carriers, thus generating a photocurrent between the contacts.

All three layers can be thermodynamically compatible. This makes processing involving thermal treatment readily viable so as to reduce or substantially eliminate structural defects and impart enhanced structural compatibility. S.. *

Step change in the band gap at the WGO-IGO pair interface can be avoided **.* using an TOO material based on an energy-band-tuned WOO phase with the band gap **SS.S reducing gradually from that of the WOO layer. This helps make use of a wide range * of solar spectral radiation with little intra-band waste. Similarly, the coupling S..... * S

between the IGO-NGO layers can be improved using a graded compositional profile at the interface.

It will be appreciated that whilst the invention has been described above in respect of a two-layer or a three-layer structure, the invention is additionally applicable to designing efficient PV cells based on multi-layers, as long as the band gap reduces from the n-type top layer to the p-type bottom layer. Such an example is shown in Figure 3, where a number of multi-IGO layers 11 are introduced between the WOO layer 10 and NGO layer 12. In a similar manner to Figures 1 and 2, the layers are positioned between a pair of ohmic contacts 13 and 14, with upper contact 13 being provided with windows to allow radiation to be incident upon the layers.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiments which are described by way of example only. * * * S

S **. S * I...

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*iSSSS * S *S **SS * S *5

SS S 55. *.. * S

Claims

-12 -Claims 1. A photovoltaic cell containing at least first and second interfaced or coupled layers, which layers comprise at least first and second photosensitive metal oxide semiconductor materials, wherein the first layer is an n-type photosensitive layer (WGO) and the second layer is a p-type photosensitive layer (NGO), and wherein the first layer has a band gap which is wider than that of the second layer.
2. A photovoltaic cell according to claim 1 wherein a layer of a photosensitive metal oxide having an intermediate band gap (IGO) is added between the WGO and NGO layers.
3. A photovoltaic cell according to claim 1 or claim 2 wherein the photovoltaic cell contains a plurality of the photosensitive layers of IGO metal oxide materials between the WGO and NGO layers.
4. A photovoltaic cell according to claim 3 wherein the band gap size decreases from the WGO layer to the NGO layer.
5. A photovoltaic cell according to any of claims 1 to 4 wherein the photosensitive layers are provided between sets of ohmic contacts.
6. A photovoltaic cell according to claim 5 wherein one set of ohmic contacts has one or more windows provided therein to allow radiation to be incident * , upon the layers. *S.*
7. A photovoltaic cell according to any preceding claim wherein the layers *.*SSS * are formed using a method selected from physical vapour deposition, co-*S..S* * * * * * * ****** * * -13 -sputtering, reactive sputtering, chemical vapour deposition, molecular beam epitaxy, ion implantation, or solid state reaction.
8. A photovoltaic cell according to any preceding claim wherein the photosensitive layers are each formed using the same synthesis method.
9. A photovoltaic cell according to any of claims 1-7 wherein the layers are each formed using different synthesis methods.
10. A photovoltaic cell according to any preceding claim wherein one or more of the layers are doped and/or alloyed to tune the band gap to a desired value.
11. A photovoltaic cell according to any preceding claim wherein one or more of the layers are doped and/or alloyed to tune the band gap into graded profiles.
12. A photovoltaic cell according to claim 11 wherein alloying andlor doping techniques are used to widen the band gap of the second layer or to narrow the band gap of a material of the first layer.
13. A photovoltaic cell according to claim 11 or claim 12 wherein alloying andlor doping is conducted either by introducing a single element species into one or more targeted lattice sites, or by introducing a combination of more than one alloying and/or doping element species into the lattice sites.
14. A photovoltaic cell according to claim 13 wherein the alloying and/or * doping element species include any one or more selected from Mn, Fe, Nb, Co, Zn, V, Cr, Ni, Cu, Ag, Al, Mg, Ti, and Ga.S

S 5555 * S I. * * * * * *
15. A photovoltaic cell according to any of claims 11-13 wherein alloying andlor doping element species including one or more of C, N, S and P are introduced into one or more non-metal lattice sites.
16. A photovoltaic cell according to any preceding claim wherein the metal oxides are based on the stoichiometries of any one or more metal oxides of the formulae M'O, M111203, MO2, or M1M'1102.
17. A photovoltaic cell according claim 16 wherein the metal oxides comprise doped or alloyed phases of any one or more metal oxides of the formulae M10, or M"203, M"02, or M'M"02.
18. A photovoltaic cell according to any of claims 1 to 17 wherein the p-type NGO semiconductor comprises either substantially pure or doped CuO.
19. A photovoltaic cell according to any of claims I to 17 wherein the p-type NGO semiconductor comprises either substantially pure or doped Cu20.
20. A photovoltaic cell according to any of claims I to 17 wherein the p-type NGO semiconductor comprises either substantially pure or doped delaffossite CuFeO2.
21. A photovoltaic cell according to any preceding claim wherein the n-type WOO comprises either virgin or doped materials of any one or more metal oxide semiconductors selected from Ti02, Sn02, ZnO, NiO, MnO, CoO, FeO, Fe203, V203, Mn203, and Ti203. *.**
22. A photovoltaic cell according to any of claims 2-21 wherein the p-type IGO semiconductor comprises either substantially pure or doped *S..* * delaffosite CuFeO2. S. * . *SS S -15-
23. A photovoltaic cell according to any of claims 2-21 wherein the IGO layer comprises a doped material of any one or more metal oxide semiconductors selected from Ti02, Sn02, ZnO, NiO, MnO, CoO, FeO, Fe203, V203, and Ti203.
24. A photovoltaic cell according to any preceding claim wherein the photosensitive layers have a substantially flat interface.
25. A photovoltaic cell according to any of claims 1-24 wherein the first and second photosensitive layers are provided with a complex interface therebetween to increase the interfacial area.
26. A photovoltaic cell according to claim 25 wherein the complex interface comprises one of the first or second layers having an array of projecting nanowires and the other layer not having the array being deposited over said nanowires.
27. A photovoltaic cell according to claim 25 wherein the complex interface comprises one of the first or second layers having a network of nanowires and the other layer not having the network being deposited over said nanowires.
28. A method of manufacturing a photovoltaic cell according to any preceding claim, comprising the steps of: S... * S * *S Si) providing a first layer of a first photosensitive metal oxide material, wherein the first layer is an n-type photosensitive layer (WGO); and **SS S. ii) interfacing or coupling the first layer with a second layer of a second photosensitive metal oxide material, wherein the second layer is a p-*5SaS Stype photosensitive layer (NGO), and wherein the first layer has a wider band gap than the second layer.
29. A photovoltaic cell substantially as herein described in the description and drawings. S S* *S SIS* .5.55 * S 55.5' * * S. * as.SS -