GB2468526A

GB2468526A - A thin film photovoltaic device with alkali metal active region

Info

Publication number: GB2468526A
Application number: GB0904299A
Authority: GB
Inventors: Arnaldo Galbiati
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-03-12
Filing date: 2009-03-12
Publication date: 2010-09-15
Anticipated expiration: 2029-03-12
Also published as: GB2468526B; GB0904299D0

Abstract

A thin film photovoltaic device comprising a first positive electrode 24, preferably transparently thin gold or ITO, and a second negative electrode 30, preferably aluminium, again transparently thin. The electrodes are preferably formed on glass substrates 24a, 30a. Between the electrodes is a light absorbing photoactive layer 28 including an alkali metal, most preferably comprising a Potassium film 3nm to 5 nm thick. Also between the photoactive layer and the first electrode is an electron acceptor layer 26 allowing electrons from said photoactive layer to pass by a tunnelling effect to the electrode, the layer preferably between 1 nm and 5 nm. This electron acceptor layer is an insulator or semiconducting, preferably an alkali oxide such as potassium oxide, or an alkali earth oxide or may comprise C60 fullerene.

Description

A Photovoltaic Device

Field of the Invention

The invention relates to a photovoltaic device and a method of manufacturing such devices.

Background of the Invention

One of the major challenges of the energy industry is the development of a technology that is able to directly convert the immense amount of radiant energy from the sun into electrical energy. This can allow electrical devices to be powered in the cleanest and the most sustainable way without the need to disrupt the finely tuned equilibrium of the natural environment. This technology, which is already available, uses the physical phenomenon of liberating electrons from photosensitive materials upon interaction with photons. It is known that the three most important manifestations of this effect are: (1) the generation of an electromotive force between electrodes when either of these electrodes or intervening medium is illuminated, (2) the reduction of the electrical resistance of materials by illumination, and (3) the emission of the electrons from illuminated surfaces. These phenomena are commonly referred as the photovoltaic effect, the photoconductive effect, and the photoelectric (or photoemissive) effect respectively.

A photovoltaic device, such as a solar cell, is widely applied in remote area power systems, such as in orbiting satellites and space probes. Such devices are also commonly found in a variety of consumer products, such as electronic calculators and watches, as sources of electrical power. Essentially, a photovoltaic device converts solar energy into electrical power via the photovoltaic effect. Solar energy, which is a renewable energy source, has been recognised as a safer and cleaner alternative to replace burning fossil fuels in order to meet a rising global energy demand.

Ever since the first photovoltaic silicon cell was developed, there has been a continual effort to make such cells more efficient. A photovoltaic silicon cell, as described in "A new silicon p-n junction photocell for converting solar into electrical power", D.M.

Chapin et al., Journal of Applied Physics, vol. 25, p. 676-667, 1954, is based on silicon and is still dominant in the world market of solar cells. Conventional semiconductor photovoltaic solar cells typically comprise a p-n junction formed in a silicon substrate, and a region depleted of mobile charge carriers is created which has a high internal electric field across it. The p-doped region is generally a strong electron acceptor material, and the n-doped region is a strong electron donor material. When the photovoltaic cell is exposed to light, the semiconductor material will absorb photons to generate pairs of negative and positive charge carriers. Consequently, electrons are freed from their atoms allowing them to flow through the material to produce electricity.

Holes are also created and flow in the direction opposite of the electrons.

However, it is noted that silicon solar cells are expensive, and that the standard silicon solar cell technology has matured to a stage where cost reductions have became one of the most challenging task that a manufacturer can undertake. Indeed, the high production cost of silicon solar cells represents an obstacle to the widespread use of photovoltaic energy conversion. A further disadvantage of silicon solar cells is its indirect bandgap of 11eV and its low absorption coefficient in the visible spectrum.

For instance, a typical silicon monocrystalline solar cell has to be approximately 300 tm in thickness in order to absorb 80% of the visible light.

The potential of using thin films for photo-detector and solar cells application was suggested in Kakafi, Photonics Spectra 27, 1993. Following the publication of this document, there has been many research works focusing on the implementation of thin films for solar cells, particularly focusing on methods of reducing production costs of large solar cell modules. One of the advantages of using thin film solar cells over silicon solar cells is that they are cheaper to produce than conventional silicon solar cells, and consequently reduces the production cost of solar panels. Another advantage of thin film solar cells is that they are light in weight, and therefore making them suitable for many applications such as lightweight vehicles and clothing. However, one of the disadvantages of thin film solar cells is their low efficiency compared with conventional silicon solar cells.

Solar cells can also be produced using materials such as gallium arsenide (GaAs) and indium phosphide (InP) which have direct energy bandgaps of 1.4 eV. Therefore, solar cells manufactured with these materials generally have higher efficiencies than silicon solar cells. However, this comes with a drawback in that such solar cells are more expensive to produce due to the difficulty in growing high quality crystals of these materials.

Although studies in the recent years have indicated that the manufacture of solar cells produces far lesser air pollutants than conventional fossil fuels technology, environmentalist are increasing concerned about the potential negative impact of solar cell technology. This is because manufacture of solar cells involves hazards primarily associated with the raw materials used, such as arsenic (in GaAs photovoltaic devices), cadmium (in CdTe solar cells), and trichlorosilane (in silicon solar cells).

Therefore, there is a need for an improved solar cells structure which is cheaper and more efficient than conventional solar cells, and a method of manufacturing such solar cells at a lower cost.

Summary of the Invention

In a first aspect of the invention, there is provided a photovoltaic device comprising a first electrode and a second electrode, an intermediate layer arranged between said first electrode and said second electrode, the intermediate layer comprising a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode andlor second electrode, and an electron acceptor layer arranged to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein the photoactive layer includes an alkali metal.

Use of a photoactive layer comprising an alkali metal provides an advantage that the alkali metal is able to directly convert visible light into electrical current with a high quantum efficiency. This is a significant advantage over conventional semiconductor silicon solar cells which rely on the absorption of light by the depleted bulk semiconductor silicon.

Examples of alkali metals include potassium, sodium, and caesium. It is noted that these materials are widely available and are non-toxic.

Alternatively, the alkali metal may be included in any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor, or a layer of potassium doped indium-tin-oxide (ITO).

The photoactive layer may have a thickness between mm to 2Onm.

The photoactive layer may include a layer of potassium having a thickness of 3 to 5nm.

The electron acceptor layer may have a thickness between mm to 5nm.

It is known in the art that the "avalanche effect" in a silicon photodiode is present when high voltages (and consequently high electric fields) are applied. However, it is noted in the present invention that by arranging an electron acceptor layer having a thickness between mm to Sum, between the electrodes allows the "avalanche effect" to take place which gives rise to a high electric field being present, without a high external voltage being applied. Consequently, the high electric field accelerates the electrons, freed from the photoactive alkali layer, and provides these electrons with sufficient energy to cause secondary ionisations in the electron acceptor layer.

The electron acceptor may be an insulating material or a semi-conducting material.

The semi-conducting layer may comprise an alkali oxide layer including any of the following: potassium oxide, sodium oxide, and caesium oxide.

Alternatively, the semi-conducting layer may comprise an alkali-earth layer including any of the following: magnesium oxide, calcium oxide, and barium oxide.

The advantage of using alkali oxide material or alkali-earth material for the semi-conducting layer is that these materials allow electrons to be multiplied when the electrons are impinged on these materials, thereby achieving near optimum efficiency.

The person skilled in the art would appreciate that this is clearly advantageous over conventional silicon solar cells in which electrons are not multiplied.

A further alternative for the semi-conducting material includes a layer of C60 fullerene having a thickness of 1 to 5 nm.

The first electrode may have a higher work function than the second electrode.

The first electrode may include a layer of gold formed on a glass substrate, so as to form a positive metal electrode.

The second electrode may include a layer of aluminium formed on a glass substrate, so as to form a negative metal electrode.

The first electrode may have a thickness throughwhich electromagnetic radiation in the visible spectrum can pass, thereby inducing photo-generation of electrons in the photoactive layer.

In a second aspect of the invention, there is provided a method of making a photovoltaic device, the method comprising providing a first electrode and a second electrode, forming an intermediate layer between said first electrode and said second electrode, wherein forming the intermediate layer further comprises forming a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode andlor second electrode, and an electron acceptor layer arranged such as to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein forming the photoactive layer includes providing an alkali metal.

Brief description of the drawings

Further aspects, features and advantages of the invention will become apparent from the following description of specific embodiments thereof; with reference to the accompanying drawings, in which: -Figure 1 illustrates a sectional view of a structure of a generic photovoltaic device according to an embodiment of the invention; Figure 2 illustrates a sectional view of a structure of a photovoltaic device according to another embodiment of the invention; Figure 3 is a flow chart illustrating the steps for manufacturing the photovoltaic device according to the described embodiments of the invention; and Figure 4 is a flow chart illustrating the process steps to form the photovoltaic device according to the described embodiments of the invention.

Detailed Description

Specific embodiments of the present invention will be described in further detail on the basis of the attached diagrams. It will be appreciated that this is by way of example only, and should not be viewed as presenting any limitation on the scope of protection sought.

Figure 1 is a schematic sectional view of a generic photovoltaic device according to one embodiment of the invention. The photovoltaic device 10 comprises a positive electrode 12 and a negative electrode 18, and an electron acceptor layer 14 and a photoactive layer 16 formed between the positive electrode 12 and the negative electrode 18.

In one example, the positive electrode 12 is a transparent Indium Tin Oxide (ITO) material through which incident light can be transmitted. Another example of forming the positive electrode 12 is a thin layer of gold (Au) or platinum (Pt) having a thickness of less than lOnm to allow transitivity of light. Alternatively, the positive electrode 12 can also be formed by coating a layer of Au or Pt on a glass substrate.

The electron acceptor layer 14 can be made of either insulating or semi-conducting material. Examples of suitable semi-conducting materials include alkali oxides, such as potassium oxide, sodium oxide, and caesium oxide, and alkali-earth oxides, such as magnesium oxide, calcium oxide, and barium oxide.

It is further noted that the semi-conducting material may also include an organic semi-conducting material, such as C60 fullerene.

In this example, the thickness of the electron acceptor layer 14 is between mm to 5nm.

However, it will be appreciated by the person skilled in the art that the thickness of the electron acceptor layer is dependent on the material used. The thickness and the material of the electron acceptor layer 14 are selected such as to allow electrons emitted from the photoactive layer 16 to pass through by a tunnelling effect. For example, it is noted in the present invention that the preferred thickness of an electron acceptor layer 14 made of dynode material (such as potassium oxide, magnesium oxide, and so on) is between inni to 5nni.

The photoactive layer 16 comprises an alkali metal material such as potassium. It is noted that one particular advantage of using potassium in the photoactive layer is that photons of energy greater than 2.2eV are able to free an electron from a potassium surface (that is the outermost electron of the potassium atom). Therefore, this allows incident light to be directly transformed into electrical signal, thereby achieving high quantum efficiency.

It is further noted that the photoactive layer 16 can also comprise other alkali metal material including sodium, and caesium. The characteristic of using alkali metal in the photoactive layer is that alkali metal can be ionised directly by visible light which makes it ideal for solar cells applications.

In an alternative embodiment, the photoactive layer 16 can be made of any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor (for example: alkali antimonide), or a layer of potassium doped indium-tin-oxide (ITO).

The preferred thickness of the photoactive layer 16 is approximately between mm to 2Onm. However, it is noted that the thickness of the photoactive layer is dependent on the material used. For example, the preferred thickness for a photoactive layer made of potassium is approximately 3 to 5 nm.

The negative electrode 18 is made of a thin semi-transparent layer of aluminium having thickness of lOnm. In an alternative arrangement, the negative electrode is a glass substrate having a layer of aluminium coated thereon.

It is noted that 1 Onm of aluminium provides "semi-transparency" to light. This is ideal when the photovoltaic device is implemented in applications where incident light is transmitted to the photoactive layer through the positive and negative electrode, as depicted in figure 2. Conversely, in applications where the photovoltaic device is implemented to allow incident light to be transmitted through the positive electrode, the thickness of the negative electrode can be greater lOnm. This will provide good electrical contact and a more stable structure to support the layers above the negative electrode.

The materials of the positive electrode 12 and the negative electrode 18 are preferably selected such that the material of the positive electrode has a higher work function than the material of the negative electrode. Essentially, this is to allow the electrons freed from the photoactive layer 16 to be driven towards the positive electrode 14 in order to generate an electrical current.

Figure 2 illustrates an exemplary embodiment of the invention in which the photovoltaic device 20 comprises a positive electrode having a thin layer of gold 24 coating on a glass substrate 24a, a potassium oxide electron acceptor layer 26 formed on one surface of the positive electrode 24, and a layer of potassium 28 provided as a photoactive layer between the potassium oxide electron acceptor layer 26 and the negative electrode 30. In this example, the negative electrode is a layer of semitransparent aluminium film 30 formed on a glass substrate 30a. This device is herein referred to as the AuIK2O/KJAI photovoltaic device. In this example, the potassium oxide electron acceptor layer is approximately 5nm.

As shown in figure 2, incident light enters the AuIK2OIKIA1 photovoltaic device through the positive electrode 24 and the negative electrode 30, and induces electrons 32, 34 to be released from the potassium (electron donor or photoactive) layer 28 to the potassium oxide electron acceptor layer 26. The electrons 32 are collected at the positive electrode 24 while the negative electrode 30 acts as an electron injecting contact. The in-built potential given by the different work functions of the negative electrode (Al) 30 and the positive electrode (Au) 24 electrode causes photocurrent 36 to be generated in the device 20. It will be appreciated by the skilled reader that the incident light 22, 22a can enter the photovoltaic device through either the positive electrode 24 or the negative electrode 30. The glass substrates 24a, 30a essentially provide protection to the electrodes 24, 30 from being deteriorated due to oxidation.

In contrast to crystalline silicon solar cell technology, the photovoltaic device of the invention can be produced directly on a single large substrate. Therefore, separate processes of handling of ingots, wafers, and cells can be replaced by a direct manufacturing process. This provides the advantage in that it simplifies the materials handing process, and it eliminates the assembly of the circuit of individual cells, which are required in silicon solar cell manufacturing process.

The manufacturing process of the photovoltaic device of the invention includes deposition of four layers, namely: a base electrode layer of low work function, a thin photoactive alkali layer, a thin oxide layer, and a transparent high work function conductor window layer.

Referring to figure 3, the base layer electrode is first deposited in step 40a using Argon sputtering or thermal evaporation. As described in the preceding paragraphs, the base electrode may be a glass substrate coated with aluminium or a substrate of Al material.

An alkali photoactive layer is deposited onto the base layer in step 40b either through argon sputtering, thermal evaporation or using electrodeposition from a non acqueous solution (e.g.: alcohols, acetone, ethilene carbonate-diethyl carbonate, propilene carbonate) of alkali salts (e.g.: potassium chloride, potassium hexafluorophosphate) or in fused salts (e.g. :molten potassium chloride).

In step 40c the thin oxide layer is deposited either through argon sputtering, thermal evaporation or via introduction of oxygen gas in the deposition chamber.

In step 40d the top layer electrode is deposited using argon sputtering or thermal evaporation.

In step 40e a transparent protective window is deposited on top of the top layer electrode using spray coating, argon sputtering or thermal deposition. The protective window material may be glass, plastic or resin based.

Essentially the transparent protective windows provides protection to the top layer electrode from being deteriorated by the external environment. The leads are connected to the device in step 40f. An IV test is then carried out in step 40g.

Hence, compared to conventional silicon solar cells technologies, the photovoltaic device of the invention provides the following advantages in the manufacturing: * lower consumption of direct and indirect materials; * independence from shortages of silicon supplies; * fewer processing steps; and * uses cheap, abundant, and non toxic raw materials.

In one exemplary embodiment of the invention is illustrated in figure 4.

Referring to figure 4, an aluminium substrate (or other metal whose work function is lower than other electrode metal) is provided on which further materials can be deposited (step 60). In step 62, a thin film, potassium photolayer is deposited in a vacuum chamber onto the aluminium substrate using argon sputtering technique.

Alternately, alkali alloys of K, Cs and Na material can be sputtered to form the photoactive layer.

Subsequently, in step 64, a thin film of potassium oxide is deposited onto the potassium layer using argon sputtering technique. Alternately any other alkali oxides or alkali earth oxides material can be sputtered to form the thin oxide layer.

In yet another alternative in step 64, a thin film of C60 fullerene is thermally evaporated onto the potassium layer.

Finally, a gold layer is sputtered on the oxide layer to act as positive electrode (step 66).

Alternatively, any metal whose work function is higher than aluminium may be employed, e.g.: Platinum, Silver. It is further noted that Carbon may also be used.

While the foregoing specific description of an embodiment of the invention has been provided for the benefit of the skilled reader, it will be understood that it should not be read as mandating any restriction on the scope of the invention. The invention should be considered as characterised by the claims appended hereto, as interpreted with reference to, but not bound by, the supporting description.

Claims

CLAIMS: 1. A photovoltaic device comprising a first electrode and a second electrode, an intermediate layer arranged between said first electrode and said second electrode, the intermediate layer comprising a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode and/or second electrode, and an electron acceptor layer arranged to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein the photoactive layer includes alkali metal.
2. A photovoltaic device according to claim 1, wherein the alkali metal includes any one of the following: potassium, sodium, or caesium.
3. A photovoltaic device according to claim 1 or claim 2, wherein the alkali metal is included in any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor, or a layer of potassium doped indium-tin-oxide (ITO).
4. A photovoltaic device according to any one of the preceding claims, wherein the photoactive layer has a thickness between lnm to 20mm 5. A photovoltaic device according to any one of the preceding claims, wherein the photoactive layer includes a layer of potassium having a thickness of 3 to 5 nm.6. A photovoltaic device according to any one of the preceding claims, wherein the electron acceptor layer has a thickness between mm to 5nm.7. A photovoltaic device according to any one of the preceding claims, wherein the electron acceptor layer is an insulating material or a semi-conducting material.8. A photovoltaic device according to claim 7, wherein the semi-conducting material comprises an alkali oxide layer including any of the following: potassium oxide, sodium oxide, and caesium oxide.9. A photovoltaic device according to claim 7, wherein the semi-conducting material comprises an alkali-earth layer including any of the following: magnesium oxide, calcium oxide, and barium oxide.10. A photovoltaic device according to any one of the preceding claims, wherein the first electrode has a higher work function than the second electrode.11. A photovoltaic device according to any one of the preceding claims, wherein the first electrode includes a layer of gold formed on a glass substrate, so as to form a positive metal electrode.12. A photovoltaic device according to any one of the preceding claims, wherein the second electrode includes a layer of aluminium formed on a glass substrate, so as to form a negative metal electrode.13. A method of making a photovoltaic device, the method comprising providing a first electrode and a second electrode, forming an intermediate layer between said first electrode and said second electrode, wherein forming the intermediate layer further comprises forming a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode andlor second electrode, and an electron acceptor layer arranged such as to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein forming the photoactive layer includes providing an alkali metal.14. A method according to claim 13, wherein the alkali metal includes any one of the following: potassium, sodium, or caesium.15. A method according to claim 14 or claim 15, wherein the step of providing an alkali metal includes providing any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor, or a layer of potassium doped indium-tin-oxide (ITO).16. A method according to any one of claims 13 to 15, wherein the photoactive layer is provided with a thickness between mm to 2Onm.17. A method according to any one of claims 13 to 16, wherein the photoactive layer is provided with a layer of potassium having a thickness of 3 to 5 nm.18. A method according to any one of claims 13 to 17, wherein the electron acceptor layer is provided with a thickness between mm to 5nm.19. A method according to any one of claims 13 to 18, wherein the step of forming the electron acceptor layer includes forming an electron acceptor layer using an insulating material or an electron acceptor layer using a semi-conducting material.20. A method according to claim 19, wherein the semi-conducting material comprises an alkali oxide layer including any of the following: potassium oxide, sodium oxide, and caesium oxide.21. A method according to claim 19, wherein the semi-conducting material comprises an alkali-earth layer including any of the following: magnesium oxide, calcium oxide, and barium oxide.22. A method according to any one of claims 13 to 21 includes providing the first electrode with a higher work function than the second electrode.23. A method according to any one of claims 13 to 22, wherein the step of forming the first electrode includes forming a layer of gold on a glass substrate.24. A method according to any one of claims 13 to 23, wherein the step of forming the second electrode includes forming a layer of aluminium on a glass substrate.25. A photovoltaic device substantially as herein described with reference to figures 1 and 2.26. A method substantially as herein described with reference to figures 3 and 4.Amendments to the claims have been filed as follows CLAIMS: 1. A photovoltaic device comprising a first electrode and a second electrode, an intermediate layer arranged between said first electrode and said second electrode, the intermediate layer comprising a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode and/or second electrode, and an electron acceptor layer arranged to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein the photoactive layer includes alkali metal, wherein the photoactive layer has a thickness between 1 nm to 2Onm.2. A photovoltaic device according to claim 1, wherein the alkali metal includes IS..5.: any one of the following: potassium, sodium, or caesium. *.SS * S3. A photovoltaic device according to claim 1 or claim 2, wherein the alkali metal is included in any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor, or a layer of potassium doped indium- . :. tin-oxide (ITO). * . S * SI4. A photovoltaic device according to any one of the preceding claims, wherein the photoactive layer includes a layer of potassium having a thickness of 3 to 5 nm.
5. A photovoltaic device according to any one of the preceding claims, wherein the electron acceptor layer has a thickness between mm to Snm.
6. A photovoltaic device according to any one of the preceding claims, wherein the electron acceptor layer is an insulating material or a semi-conducting material.
7. A photovoltaic device according to claim 6, wherein the semi-conducting material comprises an alkali oxide layer including any of the following: potassium oxide, sodium oxide, and caesium oxide.
8. A photovoltaic device according to claim 6, wherein the semi-conducting material comprises an alkali-earth layer including any of the following: magnesium oxide, calcium oxide, and barium oxide.
9. A photovoltaic device according to any one of the preceding claims, wherein the first electrode has a higher work function than the second electrode.
10. A photovoltaic device according to any one of the preceding claims, wherein the first electrode includes a layer of gold formed on a glass substrate, so as to form a positive metal electrode.
Ii. A photovoltaic device according to any one of the preceding claims, wherein the second electrode includes a layer of aluminium formed on a glass substrate, so *...S.* as to form a negative metal electrode. * *. * . **:.
12. A method of making a photovoltaic device, the method comprising providing a first electrode and a second electrode, forming an intermediate layer between : .: :* said first electrode and said second electrode, wherein forming the intermediate layer further comprises forming a photoactive layer which upon absorption of light allows electrons to be provided to said first electrode and/or second electrode, and an electron acceptor layer arranged such as to allow electrons from said photoactive layer to pass through to said first electrode or second electrode by a tunnelling effect, wherein forming the photoactive layer includes providing an alkali metal, wherein the photoactive layer is provided with a thickness between mm to 2Onm.
13. A method according to claim 12, wherein the alkali metal includes any one of the following: potassium, sodium, or caesium.
14. A method according to claim 13 or claim 14, wherein the step of providing an alkali metal includes providing any one of the following: a layer of alkali metal alloys, a layer of alkali doped compound semiconductor, or a layer of potassium doped indium-tin-oxide (ITO).
15. A method according to any one of claims 12 to 14, wherein the photoactive layer is provided with a layer of potassium having a thickness of 3 to 5 nm.
16. A method according to any one of claims 12 to 15, wherein the electron acceptor layer is provided with a thickness between 1 nm to 5nm.
17. A method according to any one of claims 12 to 16, wherein the step of forming the electron acceptor layer includes forming an electron acceptor layer using an insulating material or an electron acceptor layer using a semi-conducting material.S18. A method according to claim 17, wherein the semi-conducting material *: comprises an alkali oxide layer including any of the following: potassium oxide, * .. sodium oxide, and caesium oxide. * * * *** *S *.*19. A method according to claim 17, wherein the semi-conducting material : comprises an alkali-earth layer including any of the following: magnesium oxide, calcium oxide, and barium oxide.20. A method according to any one of claims 12 to 19 includes providing the first electrode with a higher work function than the second electrode.21. A method according to any one of claims 12 to 20, wherein the step of forming the first electrode includes forming a layer of gold on a glass substrate.22. A method according to any one of claims 12 to 21, wherein the step of forming the second electrode includes forming a layer of aluminium on a glass substrate.23. A photovoltaic device substantially as herein described with reference to figures I and 2.24. A method substantially as herein described with reference to figures 3 and 4.