GB2451188A

GB2451188A - Photovoltaic cell

Info

Publication number: GB2451188A
Application number: GB0813223A
Authority: GB
Inventors: Guosheng Shao; Jikui Luo
Original assignee: University of Bolton
Current assignee: University of Bolton
Priority date: 2007-07-18
Filing date: 2008-07-18
Publication date: 2009-01-21
Anticipated expiration: 2028-07-18
Also published as: GB2451188B; GB0813223D0; GB0713943D0

Abstract

An upper layer is an inorganic semiconductor with a relatively wide energy gap (eg TiO2, ZnO, ZnSe, ZnTe) and the lower layer 2 is an inorganic semiconductor with a relatively narrow energy gap (eg anatase TiO2 alloyed with Ga, Mn, Fe, Co, Zn, V, Cr, Cu, Ni, Ag and rare earth elements). The lower layer 2 absorbs irradiance of longer wavelengths to provide the layer 1 with photo-generated charges. In addition, higher energy solar irradiance can be directly absorbed by the upper layer 1 to avoid intra-band transitions. This enhances the performance of the photovoltaic cell with respect to longer wavelengths. Alternative device configurations include a graded bandgap region, nanowire active regions (fig 3) or a micro / nanocrystalline alloy of wide, narrow and intermediate bandgap semiconductor particulates (fig 6). The photovoltaic cell made of such inorganic semiconductor materials is chemically inert so as to provide prolonged service life.

Description

-1-2451188 Photovoltaic Cells The present invention relates to photovoltaic cells and in particular relates to low cost thin film photovoltaic cells using band engineered semiconductors including metal oxides such as Ti02.

Photovoltaic cells are operable to generate electrical current in response to incident light. One particular application of photovoltaic cells is in generating electricity from incident solar radiation. In this context, they are commonly referred to as solar cells. There are a number of different forms of photovoltaic cells, one of which is the thin film photovoltaic cell.

There are two main classes of thin film photovoltaic cells. The conventional thin film photovoltaic cells rely on a pn junction or multi-junctions within a * semiconductor material to create photoactive regions, where either emission or S. : detection of photons takes place. Detection of photons occurs when photons are *.*.*.

* incident in the photoactive 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.

Conventional thin film photovoltaic cells can be formed from a metal oxide semiconductor such as hO2 using pn junctions. The energy gaps for the two major phases of Ti02, rutile and anatase, are about 3.0 eV and 3.2 eV respectively. While thin-film hO2 photovoltaic cells have been considered a low-cost and environmentally friendly replacement for the present conventional photovoltaic technologies, they are confronted with the major roadblock of low energy conversion efficiency, particularly in the context of solar applications, because their intrinsic wide band-gap only permits the conversion of the ultraviolet irradiance (-5% of the solar irradiance) to electrical energy.

The other type of Ti02 photovoltaic cells uses organic sensitising dye instead of pn junctions. Such dye-sensitized photovoltaic cells (DSSC) separate the two functions provided by the semiconductor, e.g. silicon, in a conventional photovoltaic cell. In a conventional cell design, the semiconductor not only acts as the source of photo-generated electric charges (e.g. electrons or holes), but also provides the potential barrier to separate the charges and create a current. In the dye-sensitized photovoltaic cell, the semiconductor is used solely for charge separation, the photo-S...

generated electrons are provided from a separate photosensitive dye. Also, the charge S.....

* separation is not provided solely by the semiconductor, but works in concert with a : third element of the cell, an electrolyte in contact with both. The sensitising dyes are I.....

* 15 of narrower energy gaps to allow absorption of visible solar irradiance to generate electrons, which can be injected into the semiconductor phase to result in an electric current. However, the energy conversion efficiency of such organic-dye-sensitised hO2 photovoltaic cells is low, which can be attributed to (a) significant loss at the large angle phase boundaries between the inorganic Ti02 phase and the organic dye/s, and (b) waste due to intra-band transition of high energy photo-generated electrons.

Besides, the photo catalytic functionality of Ti02 under ultraviolet illumination tends I -3-to decompose the organic dye to result in short working life of such dye-sensitised photovoltaic cells.

It is therefore an object of the present invention to provide a photovoltaic cell that at least partially overcomes or alleviates the aforementioned problems.

According to the present invention there is provided a photovoltaic cell comprising: two coupled layers of photosensitive inorganic semiconductor material wherein the first photoactive layer has a relatively wide energy gap (WEG) and a second photoactive layer has a relatively narrow energy gap (NEG).

In such a NEG-WEG band coupled region, the NEG layer absorbs irradiance of longer wavelengths to provide the coupled WEG layer with photo-generated charges. In addition, higher energy solar irradiance can be directly absorbed by the S.....

* WEG layer to avoid intra-band transition. This enhances the performance of the S. : *a: photovoltaic cell with respect to longer wavelengths. Furthermore, a photovoltaic cell *...

* made of such inorganic semiconductor materials is chemically inert so as to permit a prolonged service life.

The two layers may be provided between contacts or electrodes which may be Ohmic contacts. One set of contacts may have one or more windows provided therein to allow radiation to be incident upon the layers.

Preferably, both the WEG and the NEG layers are formed from inorganic semiconductors of similar crystal structure and similar lattice parameters. Such structural compatibility between the layers promotes an epitaxial orientation relationship between the NEG and WEG materials, so as to avoid the formation of large angle grain boundaries, enhancing injection of electrons across the NEG-WEG interface into the WEG layer. The invention may further make use of strain or quantum confinement to generate interfaced materials of different band gaps.

The NEG-WEG layers can be made using any suitable synthesis method including, but not limited to such methods as physical vapour deposition (with or without using alloyed targets), co-sputtering, chemical vapour deposition, molecular beam epitaxy, and ion implantation (including supplementary oxygen implantation to **.. 10 avoid oxygen deficiency due to implantation). The layers may each be formed using the same synthesis method or may each be formed using different synthesis methods as desired or as appropriate.

S..... * .

One or both of the layers may be doped andlor alloyed to tune the energy gap to a desired value. Preferably, one of the NEG or WEG layers are formed from a substantially virgin stoichiometric semiconductor material and the other layer is formed from an alloy of the same material. In this manner, similarity of crystal structure and of lattice parameters may be achieved between the layers whilst providing the desired contrast in energy gap. Alloying may be used to widen the energy gap of a material with a relatively narrow energy gap or to narrow the energy gap of a material with a relatively wide energy gap as required or as desired.

Alloying can be conducted either by introducing a single elemental species into targeted lattice sites, or by introducing a combination of more than one alloying I -5-species into the lattice. Suitable alloying elements will depend upon the material forming the layers.

The inorganic semiconductor material may be a metal oxide semiconductor material. Suitable metal oxide semiconductor materials include, but are not limited to: hO2 and ZnO. Other potentially suitable inorganic semiconductors include, but not limited to, ZnS, ZnSe, and ZnTe. In the case of Ti02, as it has a relatively wide intrinsic energy gap, alloying is preferably used to narrow the energy gap such that it can provide an NEG layer in order to allow effective absorption of visible or infrared irradiance. Suitable alloying elements for this purpose include, but not limited to, Ga, * ** Mn, Fe, Co, Zn, V, Cr, Ni, Cu, Ag and rare-earth (R.E.) elements. Preferably, such elements are introduced into the metal-metal lattice sites. For example, the energy gap of Ti02 can be narrowed into the infrared regime of solar irradiance, when 10% **S*** * of the Ti lattice sites are replaced by Ga. Alloying Ti02 with Mn, Co, Fe or R.E.

: elements can lead to greater narrowing in energy gap. Interstitial elements such as C, e*** * 15 N, S and P may be introduced to replace the non-metal lattice sites.

The NEG and WEG layers may have a substantially flat interface.

Alternatively, the NEG and WEG layers may be provided with a more complex interface to increase the interface area. This can be achieved by forming a first layer such that it is provided with an array of projecting nanowires and depositing the second layer over said nanowires.

In a further embodiment, the photovoltaic cell may comprise a plurality of coupled or interfaced WEG-NEG layers, mounted in a stack between Ohmic contacts.

The layers may be ordered such that energy gap decreases in deeper layers. Such a structure can make use of a wide range of solar irradiance that spans the NEG and WEG energy gaps, and reduce waste due to intra-band transition such that multiple electrons can be generated from high energy incident photons. In some embodiments, the NEG-WEG layers may have a constant energy gap difference. In other embodiments, the energy gap varies continuously or substantially continuously between the coupled layers. This provides an opportunity for photons of both higher and lower energy states to be fully utilised to produce photovoltaic energy.

** * Typically, such a gap may be adapted to decrease in deeper layers of the cell. u

The layers may further incorporate one or more nano-/micro-crystalline alloy S.....

* composite particles. Such particles may be dispersed throughout the host matrix : material forming said layer e.g. an inorganic semiconductor material such as hO2.

*5* S*o Such particles may comprise the same material as the host matrix but have varying alloy concentration e.g. Ti02 with varying rare-earth element (or other element, such as Ga, Mn, Fe, Co, Zn, V, Cr, Ni, Cu, Ag) concentration. The result is a composite cell with varying NEG/WEG band gaps. The distribution of energy gaps may be continuous or substantially continuous. NEG particles absorb lower energy solar radiation, while WEG particles convert high energy radiation into electrons. In addition to converting high energy radiation into electrons, the host matrix provides effective separation barrier and continuous conduction path for electron transportation to the Ohmic contacts. Such a composite structure can thus improve performance.

The composite structure with embedded nano/micro-particles can be synthesized by mixing and sintering pre-formed nano-/micro-particles during layer formation. Such nano-/micro-particles with varying alloy concentration can be manufactured through nanofabrication technologies such as, but not limited to, electroforming, spraying, physical vapour deposition (with or without using alloyed targets), co-sputtering, chemical vapour deposition, molecular beam epitaxy, and ion implantation (including supplementary oxygen implantation to avoid oxygen S. . 10 deficiency due to implantation). * S.. * S *e.

According to a second aspect of the present invention there is provided a SP 55 photovoltaic cell comprising: a single layer or a plurality of layers of photosensitive inorganic semiconductor material wherein the cell has a relatively wide energy gap *...: (WEG) at first edge and a relatively narrow energy gap (NEG) at a second edge, the magnitude of the energy gap at locations between the first and second edges varying continuously or substantially continuously between the relative wide energy gap and the relatively narrow energy gap.

This provides an opportunity for photons of both higher and lower energy states to be fully utilised to produce photovoltaic energy.

The cell of the second aspect of the present invention may incorporate any or all features of the first aspect of the present invention as desired or as appropriate. * -8-

According to a third aspect of the present invention there is provided a photovoltaic cell comprising: a plurality of nano-/micro-crystalline alloy composite particles having a distribution of alloy concentrations such that there are some particles with a relatively wide energy gap (WEG) and some particles with a relatively narrow energy gap (NEG) and some particies with energy gaps intermediate between the relative wide energy gap and the relatively narrow energy gap.

Microstructures of NEG phase/s absorb lower energy solar radiation, while microstructures of WEG phase/s convert high energy radiation into electrons. Such a * S. S'S... composite structure can thus improve performance. S.. * S S...

"s. 10 The cell of the third aspect of the present invention may incorporate any or all S...

5: s. features of the first and/or second aspects of the present invention as desired or as :. appropriate.

S

S.....

* 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 plot of the electron density of states of Ti09Ga01O2, showing the potential for band narrowing to infrared energies by replacing the Ti lattice sites with Ga; Figure 2 is a schematic diagram of a simple embodiment of a photovoltaic cell according to the present invention; Figure 3 is a schematic diagram of a second embodiment of a photovoltaic cell according to the present invention; Figure 4 is a schematic diagram of a multiple layer pair photovoltaic cell based on the embodiment of figure 2; Figure 5 is a schematic diagram of a multiple layer pair photovoltaic cell based on the embodiment of figure 3; and Figure 6 is a schematic diagram of a composite photovoltaic cell with * .* embedded nano/micro particles which can absorb solar ** * * **** irradiance of various energies. **** * S S...

Turning first to figure 2, a schematic diagram of a simple design of photovoltaic cell according to the present invention is shown. The cell comprises a pair of overlaid thin film layers 1, 2. In this particular design, the upper layer 1 is an inorganic semiconductor with a relatively wide energy gap (WEG layer) and the lower layer 2 is an inorganic semiconductor with a relatively narrow energy gap (NEG layer). The layers 1, 2 are provided between a pair of Ohmic contacts 3, 4.

The contacts 3, 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 1, 2.

Radiation is incident upon the WEG layer 1 which absorbs short wave-length photons to generate electric charges. Long wave length photons will be conversed into electric charges by the underlying NEG layer 2. The WEG-NEG coupling acts to separate the charge carriers and thus a photocurrent is generated between the contacts 3,4.

Both layers 1, 2 are formed from inorganic semiconductor materials having a similar crystal structure and lattice parameter. This structural compatibility between layers i and 2 promotes epitaxial orientation relationship between the NEG and WEG materials, so as to avoid the formation of large angle grain boundaries, enhancing injection of electrons across the NEG-WEG interface into the WEG material.

Additionally, as the inorganic materials are substantially chemically inert, this permits * .* a prolonged service life. S.. * S S..

**** 10 A suitable type of inorganic semiconductor material is a metal oxide such as S...

* : hO2. In the present invention, the layers 1, 2 may be semiconductor oxides such as hO2. In such cases, the intrinsic wide energy gap of pure Ti02 enables pure Ti02 to provide the WEG layer 2 whilst the NEG layer I is provided by a layer of Ti02 alloyed or doped with a suitable element to effect narrowing of the energy gap. The extent of the alloying may be varied to achieve an NEG layer 1 with a desired energy gap in order to allow effective absorption of visible or even infrared irradiance.

Suitable alloying elements include Ga, Mn, Fe, Co, Zn, V. Cr, Ni, Cu, Ag and rare-earth (RE.) elements. Such elements are introduced into the Ti lattice sites to effect energy gap narrowing. For example, the energy gap of Ti02 can be narrowed into the infrared regime of solar irradiance, when 10% of the Ti lattice sites are replaced by Ga. This is illustrated in figure 1. Alloying Ti02 with Mn, Co, Fe or RE. elements can be even more effective in energy gap narrowing. Additionally or alternatively,

--

interstitial C, N, S or P may be used to replace the 0 lattice sites in some embodiments.

The specific interfacial area of the photoactive component can be enhanced by nano engineering of microstructures. An example is shown in figure 3, where the WEG substrate is made into an array of nanowires 5 upon which the NEG material 6 is deposited. The structure can also be engineered into inter-connected networks of NEG and WEG materials.

Multi-layered structures can be utilised to reduce waste due to intra-band *::::* transition, so that multiple electrons can be generated from high energy photons. This 10 is illustrated in figure 4 and 5 and is achieved by arranging multiple layers with decreasing energy gap value in sequence, giving opportunity for photons of different energetic state to turn into electrons. In figure 4, three pairs of NEG-WEG coupled *:" regions 9-11 are utilised. The arrow in Figure 4 indicates the direction in which energy gaps for the layers are reduced. This principle may also be applied to nano- engineered embodiments as is shown in figure 5, wherein two nano-engineered NEG-WEG coupled layer pairs 14, 15 are arranged one over the other.

Step change in energy gap at the NEG-WEG interface can be avoided a plurality of layers having a substantially continuous variation in energy gap value or alternatively bt using as single layer and continuously tuning the energy gap such that it narrows gradually from the WEG layer. This can help make use of a wide range of spectral radiation with little intra-band waste.

Figure 6 shows an alternative embodiment of the present invention comprising a plurality of nano-/micro-sized particles of varying energy gap, as the photo-active component, 18. Such particles may comprise inorganic semiconductor material with have varying alloy concentration e.g. hO2 with varying rare-earth element concentration. A cell may be formed directly from a plurality of such particles or may be formed from a plurality of such cells dispersed throughout a host matrix material of inorganic semiconductor material such as Ti02, as si shown in figure 6. The result is a composite cell with varying NEG/WEG band gaps wherein NEG particles absorb lower energy solar radiation, while WEG particles convert high energy radiation into *::::* 10 electrons. In addition to converting high energy radiation into electrons, the host matrix provides effective separation barrier and Continuous conduction path for * : electron transportation to the Ohmic contacts..

The composite structure with embedded nano/micro-particles can be * S synthesized by mixing and sintering pre-formed nano-/micro-particles during layer formation.

It will be appreciated that whilst the invention has been described above in respect of band tuned hO2 layers, wherein a wide gap semiconductor is alloyed to cause energy gap narrowing, the invention is additionally applicable to designing efficient photovoltaic cells based on wide gap semiconductors other than Ti02. As such it may be applied to other wide gap semiconductors, e.g. ZnO-based system through alloying in either the Zn or the 0 lattice sites, to tune band structures for the NEG-WEG coupling. Additionally or alternatively, a NEG-WEG embodiment can be * -13-realised by band widening of a virgin semiconductor of narrow energy gap. A further possibility would be to make use of strain or quantum confinements to generate interfaced materials of different energy gaps.

The NEG-WEG coupled materials can be made using any suitable synthesis S methods such as physical vapour deposition using alloyed targets or co-sputtering, chemical vapour deposition, molecular beam epitaxy, and ion implantation (including supplementary oxygen implantation to avoid oxygen deficiency due to implantation),

for example. * ** * S S * **

*: :: It is of course to be understood that the invention is not to be restricted to the ** 10 details of the above embodiments which are described by way of example only. *5SS

S 5.*.. * S S. * . * ...

* S....

S

Claims

Claims I. A photovoltaic cell comprising: two coupled layers of photosensitive inorganic semiconductor material wherein the first photoactive layer has a relatively wide energy gap (WEG) and a second photoactive layer has a relatively narrow energy gap (MEG).
2. A photovoltaic cell according to claim I wherein the two layers are provided between contacts.
3. A photovoltaic cell according to claim 2 wherein one set of contacts have one * ..

or more windows provided therein to allow radiation to be incident upon the ***. * . S...

layers.
4. A photovoltaic cell according to any preceding claim wherein both the WEG *...
5: and the NEG layers are formed from inorganic semiconductors of similar : crystal structure and similar lattice parameters.

**5*** * . 5. A photovoltaic cell according to any preceding claim wherein there is a strain or quantum confinement between the layers.
6. A photovoltaic cell according to any preceding claim wherein the layers are formed using any of: physical vapour deposition, co-sputtering, chemical vapour deposition, molecular beam epitaxy, or ion implantation.
7. A photovoltaic cell according to any preceding claim wherein both layers are formed using the same synthesis method.
8. A photovoltaic cell according to any preceding claim wherein the layers are each formed using different synthesis methods
9. A photovoltaic cell according to any preceding claim wherein one or both of the layers are doped and/or alloyed to tune the energy gap to a desired value.
10. A photovoltaic cell according to any preceding claim wherein one of the NEG or WEG layers are formed from a substantially virgin stoichiometric semiconductor material and the other layer is formed from an alloy of the same material.
11. A photovoltaic cell according to claim 10 wherein the alloying is used to widen the energy gap of a material with a relatively narrow energy gap or to narrow * : :* the energy gap of a material with a relatively wide energy gap.

*:::: 10
12. A photovoltaic cell according to claim 10 or claim 11 wherein alloying is conducted either by introducing a single elemental species into targeted lattice S...

sites, or by introducing a combination of more than one alloying species into the lattice. * *.*
13. A photovoltaic cell according to any preceding claim wherein the inorganic semiconductor material is ZnS, ZnSe, or ZnTe
14. A photovoltaic cell according to any one of claims 1 to 12 wherein the inorganic semiconductor material is a metal oxide semiconductor material.
15. A photovoltaic cell according to claim 14 wherein the metal oxide semiconductor material is hO2.
16. A photovoltaic cell according to claim 13 wherein the metal oxide semiconductor material is ZnO.
17. A photovoltaic cell according to any one of claims 13 to 16 wherein alloying is used to narrow the energy gap such that it can provide an NEG layer in order to allow effective absorption of visible or infrared irradiance.
18. A photovoltaic cell according to any one of claims 13 to 17 wherein alloying elements for this purpose include Ga, Mn, Fe, Co, Zn, V, Cr, Ni, Cu, Ag and Rare Earth elements.
19. A photovoltaic cell according to claim 18 wherein such elements are introduced into the metal lattice sites.

* :* : :*
20. A photovoltaic cell according to any one of claims 13 to 19 wherein alloying *::::* 10 elements such as C, N, S and P are introduced to non-metal lattice sites.

*.*
21. A photovoltaic cell according to any preceding claim wherein the NEG and : : WEG layers have a substantially flat interface.

*
22. A photovoltaic cell according to any preceding claim wherein the NEG and WEG layers are provided with a complex interface to increase the interface area.
23. A photovoltaic cell according to claim 22 wherein this is achieved by forming a first layer such that it is provided with an array of projecting nanowires and depositing the second layer over said nanowires.
24. A photovoltajc cell according to any preceding claim wherein the photovoltaic cell may comprise a plurality of NEG-WEG layer pairs, mounted in a stack between ohmic contacts.
25. A photovoltaic cell according to claim 24 wherein the NEG-WEG layer pairs have a constant energy gap difference.
26. A photovoltaic cell according to claim 24 wherein the NEG layer energy gaps decrease in deeper layers of the device.
27. A photovoltaic cell according to claim 25 or claim 26 wherein the energy gap varies continuously or substantially continuously between the coupled layers.
28. A photovoltaic cell according to any preceding claim wherein the layers may further incorporate one or more nano-/micro-crystalline alloy composite particles.
29. A photovoltaic cell according to claim 28 wherein such particles are dispersed throughout the host matrix material forming said layer.

*::::* 10
30. A photovoltaic cell according to claim 29 wherein such particles comprise the same material as the host matrix but have varying alloy concentration resulting in a composite cell with varying NEG/WEG band gaps.

:. *
31. A photovoltaic cell according to claim 30 wherein the distribution of energy gaps is continuous or substantially continuous.
32. A photovoltaic cell according to claim 31 wherein the composite structure with embedded nano/micro-particles is synthesized by mixing and sintering pre-formed nano-/micro-particles during layer formation.
33. A photovoltaic cell according to any one of claims 30 to 32 wherein nano-fmicro-particles with varying alloy concentration are manufactured through nanofabrication technologies such as electroforming, spraying, physical vapour deposition (with or without using alloyed targets), co-sputtering, chemical vapour deposition, molecular beam epitaxy, and ion implantation (including supplementary oxygen implantation to avoid oxygen deficiency due to implantation).
34. A photovoltaic cell comprising: a single layer or a plurality of layers of photosensitive inorganic semiconductor material wherein the cell has a relatively wide energy gap (WEG) at first edge and a relatively narrow energy gap (NEG) at a second edge, the magnitude of the energy gap at locations between the first and second edges varying continuously or substantially continuously between the relative wide energy gap and the relatively narrow energy gap. * .* *.S.

10
35. A photovoltaic cell comprising: a plurality of nano-/micro-crystalline alloy composite particles having a distribution of alloy concentrations such that there are some particles with a relatively wide energy gap (WEG) and some particles * with a relatively narrow energy gap (NEG) and some particles with energy gaps intermediate between the relative wide energy gap and the relatively narrow energy gap.