WO2006120237A1

WO2006120237A1 - Portable and disposable solar dosimeter and oxygen sensor

Info

Publication number: WO2006120237A1
Application number: PCT/EP2006/062259
Authority: WO
Inventors: Monica Lira-Cantu; Frederik C. Krebs
Original assignee: Riso National Laboratory; Danmarks Tekniske Universitet; Consejo Superior de Investigaciones Cientificas CSIC
Current assignee: Riso National Laboratory; Danmarks Tekniske Universitet; Consejo Superior de Investigaciones Cientificas CSIC
Priority date: 2005-05-12
Filing date: 2006-05-11
Publication date: 2006-11-16
Anticipated expiration: 2007-11-12
Also published as: GB0509767D0

Abstract

A method of detecting the presence of oxygen using a photovoltaic device, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light harvesting layer is capable of forming electron-hole pairs on excitation using light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with oxygen; and the reaction of the excited light-harvesting layer with oxygen produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; and wherein: the photovoltaic device is exposed to a test atmosphere; the photovoltaic device is illuminated with light of appropriate wavelength; the photovoltaic device is inspected to ascertain whether its appearance has altered, and/or the photovoltaic efficiency of the photovoltaic device is measured; the presence or absence of oxygen in the test atmosphere is inferred from the appearance of the photovoltaic device and/or the measurement of the photovoltaic efficiency.

Description

Portable and Disposable Solar Dosimeter and Oxygen Sensor

This application concerns the provision of methods for determining the exposure of an item to light and to oxygen.

In certain embodiments, the invention provides a method of detecting the presence of oxygen using a photovoltaic device, whose appearance and photovoltaic efficiency is dependent upon both the intensity of incident light and the presence of oxygen.

In certain embodiments, the invention provides a method of measuring cumulative exposure to light using a photovoltaic device, whose appearance and photovoltaic efficiency are dependent upon both the intensity of the incident light and the presence of oxygen.

In certain embodiments, the invention provides an oxygen- excluding enclosure incorporating a photovoltaic device, in order that the presence of oxygen within the enclosure may be detected.

Cumulative solar dosimeters are known in the art. For example, US 6132681 describes a self-adhesive disposable solar dosimeter comprising two active layers: an active compound dispersed within a polymeric matrix, optionally also containing a dye, and an absorbing material having a graded thickness. This construction would be complex and expensive to manufacture in bulk due to the number of active ingredients and the graded thickness required for one layer. US 6060321 discloses an ultraviolet light monitoring device made by screen printing of a photoactive chromogenic substance dispersed within a polymeric matrix on to a self- adhesive backing. The photoactive layer changes colour in response to cumulative UV exposure in the UV-A and UV-B regions; the colour of the photoactive layer is calibrated against a colour comparison chart printed with conventional inks matching the appearance of the sensing layer. WO9318377 describes a UV-B active dosimeter whose appearance changes to reflect cumulative radiation exposure. A photosensitive layer is provided containing a mercury (I) and silver (I) oxalate sol dispersed in a polymeric matrix. The preparation of the required solutions for the active layer uses several steps at different temperatures, making this device expensive to manufacture, and the mercury compounds used are toxic to humans and the environment .

US 5436115 provides a dosimeter that uses a compound capable of forming an acid on exposure to UV light and a compound whose colour changes on exposure to an acid to give a visible indication of cumulative light exposure.

The disadvantages presented by the solar dosimeters known in the art are: the devices are not configurable to monitor cumulative exposure over all of the ranges of wavelengths known to be active in skin ageing and sunburn; the materials and manufacture of the devices are sometimes expensive and unsuitable for bulk production of the devices; the devices may not be stable to elevated temperatures and may require low- temperature storage; the components may be toxic to the user of the device or to the environment.

An aim of the inventors is therefore to provide a method of measurement of cumulative solar exposure that overcomes one or more of the disadvantages outlined above. There are also examples of oxygen sensors in the literature. For example, WO03/000557 describes a device having an oxygen-sensitive indicator blended with a thermoplastic polymer, whose colour or luminescence properties change in an oxygen-dependent manner upon illumination. The oxygen-sensitive indicator may be a polycyclic aromatic hydrocarbon. Oxygen concentration may be measured by observing the luminescence lifetime of the indicator or by measuring the intensity of the luminescence . GB 2368908 describes a device for measuring either or both of the oxygen and water content of a gaseous atmosphere. The device is intended for use as an indicator of the condition of degradable products such as food or photographic materials. After opening of the package, the device indicates the lifetime of the contents from its level of exposure to oxygen and/or water by means of colour changes. A range of materials is required for devices intended to respond to oxygen or water over different lengths of time. Different manufacturing methods are required depending on the material to be used in the device.

The disadvantages of such devices for monitoring oxygen exposure are their complex manufacture, and in the former case, the necessity for an external spectrometer to read the device. Another aim of this invention is therefore to provide a method of sensing oxygen exposure that overcomes one or more of these disadvantages .

It is a further aim of the invention to provide a method of sensing oxygen and/or light exposure using a device comprising flexible components, such that the device may be distorted before or during use and maintain its functionality.

Photovoltaic devices are known in the art having a photoactive polymeric compound and an inorganic semiconductor compound, present either in separate layers or as a mixture, provided between two electrodes, at least one of which is transparent. Such devices may function as solar cells or LEDs.

Incident light having sufficient energy may cause exciton (electron - hole pair) formation in the photoactive polymer. The electrons and holes may travel through the device to respective opposite electrodes causing current flow when the device functions as a solar cell. The inorganic semiconductor may also absorb a photon of light to promote an electron from the valence band to the conduction band, creating charge carriers that also may travel through the device to the appropriate electrodes. A known problem of such photovoltaic devices is bleaching of the photoactive polymer, preventing it from absorbing light and creating charge carriers (electrons and holes) . It has been proposed by the present inventors that the cause of the bleaching is capture by the excited polymer of a superoxide radical anion formed by molecular oxygen trapping a conduction band electron from the excited inorganic semiconductor, by analogy with the mechanism proposed for degradation of dyes in dye-sensitised solar cells containing TiO₂ (Qamar et al . , Dyes Pigm. 2005, 65, 1-9) . It is therefore inevitable that devices of this type will degrade if used in the presence of molecular oxygen.

It has also been found by the inventors that in devices in which the semiconductor compound is a metal oxide, the device requires oxygen in order to function. In the presence of light, oxide vacancies may be created in the metal oxide layer, and the electrons from the excited polymer layer may be trapped by these vacancies, causing reduction of the metal centres. This prevents the electrons reaching the electrode and generating current. However, if oxygen is present in the atmosphere surrounding the device, the oxide vacancies are filled before the metal centres can become reduced, and the device can function as a solar cell.

As a result of these processes, the photovoltaic efficiency of the device is zero in the absence of both oxygen and light, low in the presence of light but no oxygen (decreasing slowly over time) , and is initially high in the presence of both light and oxygen but reduces over time to zero as the polymer is bleached. The appearance of the device also alters depending on the conditions to which it is exposed, with the polymer retaining its initial colour whilst in the presence of neither light nor oxygen, or light but no oxygen, and bleaching over time to become colourless in the presence of both light and oxygen. The inventors have realised that this knowledge may be applied in order to provide a method of monitoring for the presence of oxygen in a test atmosphere that is exposed to light. This knowledge may also be used to provide a method of monitoring cumulative exposure of an item to both light and oxygen .

Accordingly, in a first aspect, the invention provides a method of detecting the presence of oxygen using a photovoltaic device, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light harvesting layer is capable of forming electron-hole pairs on excitation using light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with oxygen; and the reaction of the excited light-harvesting layer with oxygen produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; and wherein: the photovoltaic device is exposed to a test atmosphere; the photovoltaic device is illuminated with light of appropriate wavelength; the photovoltaic device is inspected to ascertain whether its appearance has altered, and/or the photovoltaic efficiency of the photovoltaic device is measured; the presence or absence of oxygen in the test atmosphere is inferred from the appearance of the photovoltaic device and/or the measurement of the photovoltaic efficiency.

The measurement of the photovoltaic efficiency may take the form of determining the voltage across or current flowing between the electrodes of the device when exposed to light. Other methods of determining the photovoltaic efficiency of the device known in the art may also be used. For example, said means for measurement of photovoltaic efficiency might also monitor the rate of change of the current or voltage produced by the illuminated device with respect to time. Suitably, one may infer a degree of cumulative light exposure by integration of the function of the current or voltage produced by the illuminated device during the period of light exposure with respect to time, that is, evaluation of the area under a graph of current or voltage against time as recorded by the monitoring means. Preferably, said means should allow compensation for changing intensity of illuminating light during the period of light exposure. Suitably, said means may incorporate a second photovoltaic device whose photovoltaic efficiency does not decrease in response to cumulative light exposure, and the cumulative light exposure of the devices may be inferred using the comparison between the initial and final photovoltaic efficiencies of both devices. Suitably, said means may comprise a user interface to indicate the cumulative exposure value at a given time. Alternatively, said means may be interrogated by an external device in order for the user to read the cumulative exposure value.

In one set of embodiments, the photovoltaic device is provided within an oxygen-excluding enclosure containing the test atmosphere. Thus, the photovoltaic device may be provided as part of an item of packaging, and the test atmosphere is the atmosphere contained within the packaging. Preferably, the packaging is used to enclose food or drink, or other oxygen- sensitive contents.

Alternatively, the photovoltaic device is provided as part of an apparatus for the handling of substances under oxygen-free conditions, and the test atmosphere is the atmosphere contained within the apparatus. For instance, the apparatus may be a glove box or glove bag, or an item of laboratory glassware such as a Schlenk tube. In a second aspect, the invention provides a method of quantifying cumulative exposure of an item to light using a photovoltaic device, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light-harvesting layer is capable of forming electron-hole pairs on excitation using light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with an available oxygen species; and the reaction of the excited light-harvesting layer with the available oxygen species produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; and wherein : the photovoltaic device is attached to the item; the item is exposed to light; the photovoltaic device is inspected to determine whether its appearance has altered, and/or the photovoltaic efficiency of the photovoltaic device is measured; the degree of light exposure is inferred from the degree of change in appearance of the photovoltaic device and/or the measurement of the photovoltaic efficiency.

In one example of this, the photovoltaic device is placed on the outer clothing or skin surface of a person, and the degree of light exposure measured is used to determine whether that person has exceeded a time or intensity limit for exposure to light, particularly to prevent sunburn. Preferably, the appropriate wavelength range of the light spans all the ranges of solar light known to cause sunburn.

Alternatively, the photovoltaic device is placed on or in the near vicinity of a plant, and the degree of light exposure measured is used to optimise the situation of the plant. All of the features described above in relation to the first aspect of the invention may be used in relation to this second aspect of the invention in so far as they are not incompatible therewith. Additionally, in a third aspect, the invention provides an oxygen-excluding enclosure, e.g. an item of packaging, comprising a photovoltaic device for sensing the presence of oxygen inside the enclosure, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light-harvesting layer is capable of forming electron-hole pairs on excitation with light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with oxygen; the reaction of the excited light-harvesting layer with oxygen produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; at least a region of the enclosure is transparent to light; the photovoltaic device is incorporated into the enclosure such that it is not in contact with an external atmosphere whilst the enclosure remains effective in excluding oxygen; a visible irreversible change in the appearance of the photovoltaic device and/or a reduction in the photovoltaic efficiency of the light-harvesting layer occurs in the presence of oxygen upon illumination of the photovoltaic device with light.

Of course, it is the intention that the enclosure does not initially contain oxygen, but part of the function of the photovoltaic device is to determine whether this condition has actually been met. Brief Description of the Figures

Figure 1 shows a cross-section through the device used in the methods of the invention.

Figure 2 shows graphs of photocurrent versus wavelength for a device containing (a) CeO₂ and (b) TiO₂ as the inorganic oxide .

Figure 3 shows a comparison of graphs of photocurrent versus wavelength for a device containing a combination of CeO₂ and TiO₂ in the inorganic oxide layer when the device is operated under vacuum and in air.

Figure 4 shows a graph of current density versus time for three devices having different concentrations of ZnO.

Figure 5 shows a graph of current density versus time for a device containing ZnO in the inorganic oxide layer used for detection of the presence of oxygen.

Figure 6 shows a graph of current density versus time for a device containing TiO₂ in the inorganic oxide layer, showing its response to an inert atmosphere and subsequently an oxygen- containing atmosphere. Figure 7 shows a graph of current density versus time for a device containing CeO₂ as the inorganic oxide after exposure to light in an ambient atmosphere, and an inset absorption spectrum for the device.

The invention will be described in more detail with reference to the Figures and the Examples.

Referring to Figure 1, a cross-section through a device used in the method of the invention is shown.

The device is constructed of component layers formed on a support layer (2) . The support layer may be made from glass or a polymer, and should be substantially transparent to the light wavelengths of interest for use in the method. The support should be sufficiently rigid that the layers formed upon it are not damaged by being distorted during use.

Formed on one face of the support layer (2) is the second electrode layer (3) . Suitably, the second electrode layer is capable of transmitting at least a part of the light wavelengths of interest, and is preferably substantially transparent to the light wavelengths of interest for use in the methods of the invention. Materials that may be used for the second electrode layer are: indium tin oxide (ITO), gold, silver, aluminium, calcium, platinum, graphite, gold-aluminium bilayer, silver- aluminium bilayer, platinum-aluminium bilayer, graphite- aluminium bilayer, calcium-silver bilayer. The second electrode layer may be formed by methods known in the art for forming electrodes in organic LED devices and dye-sensitised solar cells. For example, the second electrode layer may be formed by vacuum deposition methods. Pode et al. (Applied Physics Letters, 2004, 84, 4614-4616) describes a method of forming a transparent calcium-silver bilayer electrode; Hatton et al . (Journal of Materials Chemistry, 2003, 13, 722-726) describes a method of producing a transparent gold electrode; Neudeck and Kress (Journal of Electroanalytical Chemistry, 1991, 437, 141- 156) describe the formation of laminated gold micro-meshes for use as transparent electrodes.

Upon the second electrode layer is formed the photoactive active layer (s) of the device. In Figure 1, these are depicted as two separate layers, (4) and (5); however, these layers may be provided as a single layer combining the photoactive compounds. A layer (4) comprising at least one inorganic oxide, and a layer (5) comprising a light-harvesting conductive polymer may be formed sequentially on layer (3) . The layers (4) and (5) are formed using a dissolution or dispersal of the respective active compounds in a suitable solvent. For example, the light- harvesting conducting polymer may be dissolved in chlorobenzene, and the inorganic oxide may be dispersed or present as a sol-gel in ethanol. The solutions may then be applied to the device as thin layers by conventional printing techniques such as screen printing, dip coating, spin coating, etc. The thickness of the layers (4) and (5) are each preferably not greater than 200 nm.

Examples of light-harvesting, conducting polymers for use in layer (5) are poly (phenylenevinylene) (PPV), poly (p- phenyleneethynylene) (PPE), and poly (arylene) type polymers. For example, poly [2-methoxy-5- (2' -ethylhexyloxy) -p- phenylenevinylene] (MEH-PPV), poly [2-methoxy-5- (3' , 7' - dimethyloctyl ) p-phenylenevinylene] (MDMO-PPV), poly (3- hexylthiophene) (P3HT) , poly [N-phenylamino-1, 4-phenylene-l, 2- ethylene-1, 4- (2, 5-dioctoxy) -phenylene-1, 2-ethylene-l, 4- phenylene] (PA-PPV), or poly ( 9, 9' -dioctylfluorene-co- bithiophene) (PDFTH) may be used. Preferably, MEH-PPV is used as the polymer in layer (5) . Examples of inorganic oxides that may be used in layer (4) are ZnO, Nb₂O₅, TiO₂, CeO₂, TiO₂-CeO₂, Fe₂O₃, In₂O₃, SnO₂, SnO₂- In₂O₃, WO₃, Ta₂O₃, SrTiO₃, CuO, CdO, and V₂O₅. The inorganic oxide layer may be treated under vacuum before the device is used. The inorganic oxide (s) may be used in the form of nanoparticles . Preferably, the inorganic oxide used is ZnO, Nb₂O₅, TiO₂, CeO₂, or a combination of CeO₂ and TiO₂.

If the device is formed with separate layers (4) and (5), the light-harvesting conducting polymer containing layer is preferably formed in contact with the first electrode, especially when the second electrode is ITO and the first electrode is a metal.

Upon the photoactive layers (4) and (5) is formed the first electrode layer (6) . The first electrode layer may be formed of the same materials as the second electrode layer (3) above, and may be formed using the same methods. It is not necessary for this layer to be transparent.

Optionally, an adhesive layer may be formed on the first electrode layer (6) in order to attach the device to the item whose environment is to be monitored. Optionally, a removable cover film (1) may be provided on the opposite face of the support layer (2) from the photoactive layer (s) (4) and (5) . This cover film is opaque to light, and is removed before use of the device. Alternatively, the device may be provided in a sealed, opaque container or bag, which must be opened and discarded before the device is used.

Optionally, an electrical circuit comprising means for measuring photovoltaic efficiency is formed between the two electrodes (3) and (6) . This circuit is provided if it is intended to monitor the environment of the test item by monitoring the photovoltaic efficiency of the device rather than by inspection of the appearance of the device. The means for measuring the photovoltaic efficiency may be, for example, a multimeter, a voltmeter, an ammeter or a source meter. Alternatively, other means for measuring photovoltaic efficiency known in the art may be used.

Tailoring of the wavelengths of light to which the device responds is possible by using different inorganic oxides. For example, if the device is to be used as a solar dosimeter, it is preferred that the device should reflect the cumulative exposure to wavelengths of light in the regions of the solar spectrum known to produce a response in the skin such as tanning, burning or ageing. These ranges are the UV-B range (280-315 nm) , the UV-A range (315-400 nm) , and the photosynthetic active radiation (PAR) range (400-700 nm) .

Graphs of the photocurrent obtained at different irradiation wavelengths for devices of the type depicted in Figure 1 having (a) CeC>₂ and (b) TiC>₂ as the inorganic oxide in layer (4) are shown in Figure 2. It can be seen that the maximum photocurrent is obtained for CeC>₂-containing devices at 280 nm, 850 nm and 1000 nm, and for TiO₂ the maximum is at 490 nm. Therefore, CeO₂ in layer (4) will be useful for monitoring UV-B exposure, and TiO₂ for monitoring PAR exposure. A combination of CeO₂ and TiO₂ may be used in the inorganic oxide layer; this gives maxima in the photocurrent versus wavelength graph at 280 nm, 450 nm and above 600 nm (see Figure 3) , suitable for monitoring the UV-B and PAR regions.

The device may also be tailored to respond to cumulative light exposure or oxygen exposure over a different length of time. This may be achieved by alteration of the amount of inorganic oxide present in the device. Figure 4 shows a graph of current density versus time for three devices containing no inorganic oxide, a low concentration of inorganic oxide, and a high concentration of inorganic oxide; the inorganic oxide used is ZnO nanoparticles . It can be seen from this graph that the device ceases to work (the current density falls to zero and the polymer is bleached) in around 20 min when using a high concentration of ZnO (60 wt%) , over 12-16 h when using a lower concentration of ZnO (35 wt%) , and is not fully bleached after 16 h when no ZnO is present. Alternatively, it is found that the use of different inorganic oxides allow the time over which the device bleaches to be altered. For example, a device containing TiO₂ takes around 20 times longer to bleach than a device containing the same concentration of ZnO. This allows the method for measuring cumulative sun exposure to be tailored to an appropriate response time for different skin types. It also permits the method for sensing the presence of oxygen to be tailored to be more or less sensitive depending on the situation in which the method is to be used. Example 1 - Preparation of a device having the configuration ITO/TiOz/MEH-PPV/Ag or ITOZNb₂O₃ZMEH-PPVZAg

Indium tin oxide substrates (equivalent to layers (2) and (3) of Figure 1) were purchased from Delta Technologies Ltd. (USA; coated on one surface, Rs = 10 D ± 5 D) , having dimensions 25 x 50 x 1.1 mm, and were etched and prepared following the method described by Krebs et al . (Sol. Eng. Mater. Sol. Cells 2005, 86, 449-516) . After etching, the substrates were washed with acetone followed by ethanol in an ultrasound bath for 10 min each. A sol-gel of TiO₂ was prepared using a modification of the procedure of Hattori et al . (Langmuir 1999, 15, 5422-5425). Thus, Ti(O²Pr)₄ (Aldrich, 5 ml) was mixed with acetylacetone (Riedel-de-Haen, 3.5 ml) added dropwise with stirring. Ethanol (96%, 25.5 ml) was then added to the mixture. The sol-gel TiO₂ formed was orange-yellow in colour, and was stable for several weeks at room temperature. In order to prepare the thin film, the sol-gel prepared above was diluted with ethanol (96%, 0.5 ml per 1 ml sol-gel) , and the diluted solution spin-coated on to the ITO substrate at 3000 rpm for 30 s. The prepared film was then calcined at 450 °C for 2 h with a heating rate of 3 °C/min. Once sintered, the thin TiO₂ film was transparent and presented a light-blue colour, and had an absorbance of 0.1 ± 0.05 absorbance units at ~ 440 nm. A solution of Nb₂O₃ was prepared using a modification of the method described by Ozer et al. (Thin Solid Films 1996, 277, 162-168). Niobium ethoxide (4.5 ml, 99.5%, Aldrich) was mixed with ethanol (99.9%, 3 ml) and acetic acid (0.015 ml), and the mixture stirred for 15 min. An aliquot of this solution (0.5 ml) was diluted by the dropwise addition of ethanol (99.9%, 10 ml) with stirring for 2 h until the turbidity of the solution disappeared. This solution was then filtered with a 45 μm filter. This solution is stable for a maximum of two weeks. The Nb₂θ₅ thin films were prepared by spin coating the diluted solution prepared above on to the ITO substrates at 3000 rpm for 30 s, followed by calcining the thin film at 450 °C for 2 h at a heating rate of 3 °C/min. Once sintered, the Nb₂O₅ thin film was transparent and presented a light purple-blue colour, and had an absorbance of 0.1 ± 0.02 absorbance units at ~ 830nm. After formation of the thin film of inorganic oxide, the MEH-PPV solution was spin-coated on top of the inorganic oxide layer. MEH-PPV was synthesized according to the method of Krebs et al. (Sol. Eng. Mater. Sol. Cells 2005, 86, 499-516), and was applied as a microfiltered (45 μm filter) solution in chlorobenzene at a concentration of 16 mg ml^"1. Spin-coating was carried out at 1500 rpm for 30 s on a substrate which had been heated at 100 °C for 10 min to remove water molecules adhering to the surface, and then cleaned with chloroform by spin coating. The absorbance of the polymer film was controlled between 0.9 and 1 absorbance units at ~ 490 nm. The active area of the device was 3.3 ± 0.2 cm². A silver electrode was thermally evaporated on to the polymer film at a pressure of < 10^~5 mbar through a mask. The device was mounted using a conductive silver epoxy glue for the electrical contacts. Example 2 - Detection of the presence of oxygen in a glovebox

For the detection of oxygen in the atmosphere of an apparatus containing a controlled atmosphere such as a glovebox, it is desirable that the user is alerted to the presence of oxygen as soon as possible after the oxygen has entered the apparatus. The method of the invention is therefore employed using a photovoltaic device containing ZnO as the inorganic oxide. The response of such a device to exposure to oxygen is shown in Figure 5. It can be seen that the device is bleached, providing a visual indication of the presence of oxygen, and the current density falls to zero, providing an electrical measure of the presence of oxygen, within 20 minutes of the introduction of oxygen. Either or both of the detection methods (visual and electrical) may be used in this situation. Alternatively, the increase in photovoltaic efficiency of the device upon exposure to oxygen may be observed to provide a more immediate indication of oxygen exposure.

Example 3 - Detection of the presence of oxygen in a food package For the detection of oxygen in the packaging of perishable goods, such as packaged foods, it is desirable that the detection method should respond to the presence of oxygen over a timescale reflecting the stability of the perishable goods to the presence of oxygen. That is, the polymer layer of the photovoltaic device should become bleached, and/or the current density measured should fall to zero, when or slightly before the perishable goods are no longer in a usable condition. The method will therefore indicate, for example, that the packaged food should no longer be eaten. Figure 6 shows the response of a photovoltaic device containing TiO₂ as the inorganic oxide to an inert atmosphere, as for an unbroken item of packaging, followed by exposure to an oxygen containing atmosphere on opening of the package. It can be seen that the device maintains a constant low current density during exposure to the inert atmosphere; no bleaching of the polymer occurs. On exposure of the device to oxygen, the current density rises sharply and then gradually decreases over a period of around one day to zero; this decrease is accompanied by bleaching of the polymer. When the current density falls to zero, and/or the polymer is bleached (either or both indicators may be used) the contents of the package should be considered unusable.

Example 4 - Long-term UV-B solar dosimeter Figure 7 shows a graph of current density versus time for a photovoltaic device containing CeO₂ as the inorganic oxide, and (inset) the absorption characteristics of the device. After exposure of the device to light in an ambient atmosphere, the current density reaches a maximum and then falls slowly over 130 h in tandem with the bleaching of the polymer to transparency. The device may therefore be applied to the method of monitoring cumulative solar exposure on the invention, the cumulative exposure to the UV-B range being monitored, and might find application for a personal dosimeter for a person working outdoors and wishing to monitor their long-term solar exposure. Alternatively, the solar exposure of an indoor plant could be monitored using such a device in order to optimise growing conditions .

Claims

1. A method of detecting the presence of oxygen using a photovoltaic device, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light harvesting layer is capable of forming electron-hole pairs on excitation using light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with oxygen; and the reaction of the excited light-harvesting layer with oxygen produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; and wherein: the photovoltaic device is exposed to a test atmosphere; the photovoltaic device is illuminated with light of appropriate wavelength; the photovoltaic device is inspected to ascertain whether its appearance has altered, and/or the photovoltaic efficiency of the photovoltaic device is measured; the presence or absence of oxygen in the test atmosphere is inferred from the appearance of the photovoltaic device and/or the measurement of the photovoltaic efficiency.

2. A method according to claim 1, wherein the photovoltaic device is provided as part of an oxygen-excluding enclosure, and the test atmosphere is the atmosphere contained within the enclosure.

3. A method of quantifying cumulative exposure of an item to light using a photovoltaic device, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light-harvesting layer is capable of forming electron-hole pairs on excitation using light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with an available oxygen species; and the reaction of the excited light-harvesting layer with the available oxygen species produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; and wherein: the photovoltaic device is attached to the item; the item is exposed to light; the photovoltaic device is inspected to determine whether its appearance has altered, and/or the photovoltaic efficiency of the photovoltaic device is measured; the degree of light exposure is inferred from the degree of change in appearance of the photovoltaic device and/or the measurement of the photovoltaic efficiency.

4. A method according to claim 3, wherein the appropriate wavelength range of the light spans all the ranges of solar light known to cause sunburn.

5. A method according to any preceding claim, wherein the light-harvesting layer comprises at least one inorganic oxide and a light-harvesting conjugated polymer.

6. A method according to claim 5, wherein the light- harvesting layer comprises a layer of the at least one inorganic oxide in contact with the second electrode layer, and a layer of the light-harvesting conducting polymer in contact with both the first electrode layer and the layer of the at least one inorganic oxide. 7. A method according to claim 5 or claim 6, wherein the light-harvesting polymer is selected from the group consisting of: poly [2-methoxy-5- (2' -ethylhexyloxy) —p— phenylenevinylene] (MEH-PPV), poly [2-methoxy-5- (3' ,

7' - dimethyloctyl) p-phenylenevinylene] (MDMO-PPV), poly (3- hexylthiophene) (P3HT) , poly [N-phenylamino-1, 4- phenylene-1, 2-ethylene-l, 4- (2, 5-dioctoxy) -phenylene-1, 2- ethylene-1, 4-phenylene] (PA-PPV), poly (9, 9'- dioctylfluorene-co-bithiophene) (PDFTH) .

8. A method according to claim 7, wherein the light- harvesting polymer is MEH-PPV.

9. A method according to any previous claim, wherein the at least one inorganic oxide is selected from the group consisting of: ZnO, Nb₂O₅, TiO₂, CeO₂, TiO₂-CeO₂, Fe₂O₃, In₂O₃, SnO₂, SnO₂-In₂O₃, WO₃, Ta₂O₃, SrTiO₃, CuO, CdO, V₂O₅.

10. A method according to claim 9, wherein the at least one inorganic oxide is TiO₂.

11. A method according to claim 10, wherein the at least one inorganic oxide is CeO₂.

12. A method according to claim 10, wherein the at least one inorganic oxide is a combination of TiO₂ and CeO₂.

13. A method according to claim 10, wherein the at least one inorganic oxide is Nb₂O₅.

14. A method according to claim 10, wherein the at least one inorganic oxide is ZnO.

15. A method according to any previous claim, wherein the material of the first electrode is selected from the group consisting of: gold, silver, aluminium, calcium, platinum, graphite, gold-aluminium bilayer, silver- aluminium bilayer, platinum-aluminium bilayer, graphite- aluminium bilayer, calcium-silver bilayer.

16. A method according to claim 15, wherein the first electrode layer is a silver electrode layer.

17. A method according to any previous claim, wherein the second electrode layer is capable of transmitting at least a part of the light of appropriate wavelength.

18. A method according to any previous claim, wherein the material of the second electrode is selected from the group consisting of: indium tin oxide (ITO), gold, silver, aluminium, calcium, platinum, graphite, gold- aluminium bilayer, silver-aluminium bilayer, platinum- aluminium bilayer, graphite-aluminium bilayer, calcium- silver bilayer; and is formed in such a way as to be substantially transparent to the light wavelengths of interest .

19. A method according to claim 18, wherein the second electrode layer is an indium tin oxide (ITO) layer.

20. An oxygen-excluding enclosure comprising a photovoltaic device for sensing the presence of oxygen inside the enclosure, wherein: the photovoltaic device comprises at least a first electrode layer, a light-harvesting layer, and a second electrode layer; the photovoltaic device optionally further comprises a circuit connecting the two electrodes and comprising means for measuring photovoltaic efficiency; the light-harvesting layer is capable of forming electron-hole pairs on excitation with light of appropriate wavelength; the excited light-harvesting layer is capable of reaction with oxygen; the reaction of the excited light-harvesting layer with oxygen produces a visible irreversible change in the appearance of the photovoltaic device and/or reduces the photovoltaic efficiency of the light-harvesting layer; at least a part of the enclosure is transparent to light; the photovoltaic device is incorporated into the enclosure such that it is not in contact with an external atmosphere whilst the enclosure retains its oxygen-excluding property; a visible irreversible change in the appearance of the photovoltaic device and/or a reduction in the photovoltaic efficiency of the light-harvesting layer occurs in the presence of oxygen upon illumination of the photovoltaic device with light.