GB2451108A

GB2451108A - Photovoltaic Device

Info

Publication number: GB2451108A
Application number: GB0713969A
Authority: GB
Inventors: Robert Michael Brady; Edward Thaddeus Samulski
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-07-18
Filing date: 2007-07-18
Publication date: 2009-01-21
Also published as: GB0713969D0

Abstract

A photovoltaic device comprising: a light guiding structure to receive and guide incoming light for conversion to electrical energy by total internal reflection, the structure comprising adjacent inner (1) and outer (2) light guiding regions, where the inner region has a higher refractive index than the outer region; and electrical contacts connected to the inner region and the outer region. The outer region contains a photoactive material to generate electrical charge carriers from an evanescent wave arising from total internal reflection of light at an interface between the inner and the outer regions, and the light guiding structure is configured such that said incoming light undergoes multiple total internal reflections at the interface. A method for producing a photovoltaic device as described above and a photovoltaic device comprising a plurality of light guiding structures are also claimed.

Description

Photovoltaic Devices

FiELD OF THE INVENTION

This invention relates to photovoltaic devices, such as solar cells, and to methods of their manufacture, and to related methods for increasing the efficiency of such devices.

BACKGROUND TO THE INVENTION

A photovoltaic cell, such as solar cell, makes use of the photovoltaic effect to convert light energy into electrical energy (Archer, M. D. and Hill, R., editors, "Clean Electricity from Photovoltaics" Imperial College Press, 2001.) The energy in the light radiated from a hot body such as the sun is carried by photons.

Photoactive materials absorb photons and transfer the absorbed energy into photomc excitations in the material, in particular excited electronic states that move a short distance (the diffusion length) through the material before they dissipate their energy and revert back to the unexcited (ground) state. Depending on the material, these excitations may be uncharged (such as excitons -found in organic materials) or they may be created in pairs of opposite electronic charge (such as electrons and holes).

The most important part of a photovoltaic cell is an interface between a layer of photoactive material and a second layer which is made of a material (a "harvesting material") that harvests excitations whose translational motion has brought them into contact with the interface. Uncharged excitations may be converted into charged excitations near the interface.

if the harvesting material is an n-type material (e.g. an electron acceptor) then it preferentially harvests excitations that are negatively charged, resulting in the generation of electric current and voltage across the interface. If instead a p-type harvesting material is used, positively charged excitations are preferentially harvested, again resulting in the generation of electric current and voltage, in this case in the opposite direction.

In the construction of a photovoltaic cell, conducting contacts are added and the circuit is completed by the conduction of electricity outside the cell across a load. The cells are usually connected to one another, encapsulated and mounted on a structure in the form of a carrier or frame, thereby shaping the solar panel.

There is an important limitation on the efficiency of a photovoltaic cell. If the excitations are created too far from the interface with the harvesting material, their energy can be dissipated before their motion brings them into contact with the interface.

In order to overcome this limitation the prior art has focused on the following methods:- (a) Using photoactive materials where the distance typically moved by the excitations is long. This limits the materials that may be used, and typically requires materials of high purity that may be expensive to produce (Nelson, J. "The Physics of Solar Cells", Imperial College Press 2003).

(b) Using an interface which is shaped so that the photoactive material and the harvesting material interpenetrate on short distance scales, so that there is an interface very close to most points where excitations are created (Coakley, K. M. and McGehee, M. D. "Conjugated Polymer Photovoltaic Cells" Chem. Mater. 2004, 16, 4533.); (Rafi Shikier, Marco Chiesa, and Richard H Friend "Photovoltaic Performance and Morphology of Polyfluorene Blends; The Influence of Phase Separation Evolution", Macromolecules 2006, 39, 5393-5399); poster CC7.3 in the program of the Materials research Society Meeting in December 2006, http://www.mrs.org/s mrs/doc.asp?CJJJ=6982&DID=1 78426, "Extending the Spectral Response of Dye-sensitized, Organic and Polymer Solar Cells", Arie Zaban, Elad Koren, Igor Lubomirsky and David Cahen. This requires challenging nanometer-scale structures or delicately controlled nano- phase separation of mixed materials (a so-called "bulk heterojunction") and may be affected by losses due to resistance through the nanometre-scale structure and other losses such as electron hole recombination.

(c) Making the photoactive layer very thin and causing the photons to traverse the layer multiple times ( L. Zeng,a_ Y. Yi, C. Hong, J. Liu, N. Feng, X. Duan,b_ and L. C. Kimerling B. A. Alamariu "Efficiency enhancement in Si solar cells by textured photonic crystal back reflector" App!. Phys. Lett. 2006, 89, 111111; Liu, J., Namboothiry, M. A. G. and Carroll, D.L. "Optical geometries for fiber-based organic photovoltaics" Appi. Phys. Lett. 2007, 90, 133515). This requires precision nanometer scale fabrication and suffers from losses associated with the contacting electrodes.

(d) Confining the electromagnetic field very close to a metallic surface by creating surface plasmons or resonances (Yamagishi, K., Inoue, J. and Yamashita, M. "Surface Plasmon Resonance in Organic Photovoltaic Cells with Silver or Gold Electrodes" Mo!.

Cryst. Liq. Cryst. 2006, 462, 83.; J. K. Mapel, M. Singh, M. A. Baldo, K. Celebi Plasmonic excitation of organic double heterostructure solar cels" Applied Physics Letters 2007, 90, 12102). This presents difficulties associated with poor coupling of the photons to plasmons or resonances and losses in the metallic material.

There is therefore a need for improved techniques for enhancing the efficiency of a photovoltaic device.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided a photovoltaic device, the device comprising: a light guiding structure to receive and guide incoming light for conversion to electrical energy by total internal reflection, the structure comprising adjacent inner and outer light guiding regions with a total internal reflection (TIR) interface between said inner and outer regions, said inner region being exposed to said incoming light and having, at said TIR interface, a refractive index of at least 1.55 for a light wavelength in the range l300nm to 300nm, and said outer region having, at said TIR interface, a lower refractive index than said inner region; wherein said outer region includes a region of width at least lOnm containing photoactive material to generate excitations from an evanescent wave arising from total internal reflection (TW) of said light at said TIR interface; and wherein said photovoltaic device further comprises electrical contacts to said photoactive material to provide said electrical energy from said excitations in said photoactive material.

In embodiments having a thickness of photoactive material of at least lOnm helps to achieve a high efficiency. The light wavelength in the range l300nm to 300nm may be measured in air. As the skilled person will appreciate, not all of the energy of the incident light energy may be reflected at the TIR interface because some of the energy may be absorbed through the evanescent wave into the photoactive material. Preferably the light guiding structure is configured such that the incoming light enters the inner region and subsequently undergoes more than one total internal reflection at the TR interface. Preferably the T interface is non-planar.

In embodiments the TIR interface may be located in a region of graded refractive index.

Then preferably the inner region has a refractive index of at least 1.55 no more than a wavelength away from the TW interface, more particularly at a point at or no more than 200nm away from the TIR interface.

In embodiments the TIIR interface comprises a surface of the photoactive material, and therefore there need be no layer of material between the photoactive material and the inner region.

In other embodiments one or more additional layers of material, for example a transparent or organic conductor and a harvesting material, are present between the photoactive material and the inner region. In such embodiments, light may be refracted through intermediate angles before reaching the TIR interface.

In other embodiments, the surfaces may be roughened, for example in order to achieve greater harvesting of excitations. In such embodiments, the refractive index of the inner region may be high, for example at least two, and the refractive index of the outer region maybe low, for example 1.3. (Here the refractive index of the inner material, not the average refractive index, is preferably greater than two). The roughening may be on a scale shorter than a wavelength of light so that the average refractive index, which determines the path of the light and the TIR interface, is graded smoothly. The precise location and shape of the hR interface may depend on factors such as the wavelength and the angle of incidence of the light inside the inner region.

Thus one or both of the inner and outer regions may have a graded refractive index such that there is a gradual change in refractive index at the TIR interface. In such a case light may travel on a curved path tangential to the TIR interface.

In preferred embodiments the light guiding structure is configured such that for light at a wavelength in the range l300nm to 300nm that is incident at an angle of 80 degrees to the normal to the T interface, the evanescent wave penetrates beyond the TIR interface sufficiently that the energy density of the evanescent wave where it first encounters the photoactive material is on average at least 15%, preferably at least 30% of the energy density of the evanescent wave at the T interface. However preferably the light guiding structure is also configured such that for light at an angle of 80 degrees to the normal an energy density of the evanescent wave decays to l/e of a value it has at a surface of a layer of the photoactive material closest to (ic. towards or adjacent) the TW interface after having penetrated into the photoactive material by no more than 6Onm, preferably no more than SOnm, 4Onm, 3Onm, or 2Onm, or by no more than 1.5 or 1 diffusion lengths of an excitation in the photoactive material. We may define, for example, that if an excitation is created one diffusion length away from a flat harvesting surface then 50% of the excitations reach the harvesting surface (ie before their energy dissipates to a lower/ground state).

The skilled person will appreciate that an embodiment of the device need not, in operation, have light incident at 80 degrees to the normal of the TIR interlace. However in embodiments the device is configured such that if light were incident at 80 degrees to the normal of the TIR interface the aforementioned conditions are satisfied.

Thus in embodiments the light guiding structure is configured so that the evanescent wave decays to l/e of its value at the T interface after having penetrated into the photoactive material by no more than 5Onm, preferably no more than 4Onm, 3Onm, or 2Onm. Additionally, or alternatively the structure may be configured so that the evanescent wave decays after having penetrated no more than approximately a diffusion length of an electronic excitation within the photoactive material, the excitation being an excitation which provides the electrical energy, for example an exciton in an organic material.

Broadly speaking, it is desirable the configure the device such that glancing angles of total internal reflection are achieved, preferably sufficiently glancing (for example greater than 65 degrees to the normal) to limit the average penetration depth (confine the evanescent field) to less than 75nm, preferably less than 5Onm or 3Onm (about 2Onm appears to be the most preferable). However it is also preferably that these angles are not too glancing, in order to make it practicable to construct the device.

The inventors have recognised and understood how a number of different parameters interact. Thus firstly (as described in more detail later) the distance over which the energy of a photon can be confined is reduced in the non-propagating, evanescent wave portion of totally internally reflected (TIR) light by a factor of approximately 1/4w (apart from a geometric factor typically in the range 1-2) relative to the distance over which the photons can be confined using conventional optics. Secondly the inventors have recognised that in a photovoltaic device the structure described above can provide penetration of the evanescent wave to a substantial degree into the photoactive material, thus enabling absorption of the energy in the evanescent wave by the photoactive material and conversion into electrical energy. Thirdly, however, the inventors have recognised that as well as significant penetration of the evanescent wave into the photoactive material there is preferably an additional, potentially conflicting requirement that the penetration depth is short compared with the diffusion length of excitations in the material in order to obtain efficient harvesting of the excitations generated within the photoactive material and delivery to the electrical contacts of the device.

A further factor considered by the inventors relates to the geometry of the structure, and in particular to the number of reflections made by light entering the structure at a particular angle (which, in embodiments that are sufficiently large that diffraction effects are not important, is independent of the physical size of the structure). Thus one condition that may be placed upon the geometry is that light entering at a particular angle, for example, 22.5 degrees to the normal of an optical aperture of the structure, has more than a threshold amount, for example, 40%, of its energy absorbed after an integral number of reflections, for example 5 (in this example there being 13% absorption at each reflection). Moreover, whatever the geometrical structure in some preferred embodiments of the energy absorbed, preferably 40% is absorbed within a diffusion length or 5Onm of an electrical connection.

In some preferred embodiments the inner region of the structure has an average (spatial average) refractive index of at least 2 at a wavelength of operation of the device, for example, at 550nm, or a refractive index of at least 2 at or adjacent the TIR interface.

This significantly facilitates ensuring that at least 50% of the energy of the evanescent wave is inside the photoactive material and within a diffusion length of an interface with a harvesting material.

In embodiments the light guiding structure has a tapered configuration, the inner region of the device narrowing away from a light receiving face or portion of the inner region of the device. Thus in embodiments the device has a flared or generally horn or funnel-shaped structure, in particular to funnel light from a light receiving portion of the device towards a back or end part of the device (which may incorporate a reflector). Such a structure facilitates multiple total internal reflections within the device.

In embodiments of the device connections are made to opposite surfaces of the photoactive material, preferably using electrical conductors which are substantially transparent at a wavelength of operation of the device (at a wavelength chosen in the range l300nm to 300nm) and where one of the electrical conductors is coated with a harvesting material. Thus in embodiments one connection is made to each of the above mentioned outer and inner regions, although depending on the configuration of the device this is not essential.

In some particularly preferred embodiments the inner region comprises or consists of titanium dioxide (which typically has a refractive index of between 2 and 3). The use of titanium dioxide is advantageous because not only is this material substantially transparent, it is also semi-conducting and thus a useful harvester (electron acceptor) and electrical conductor over short distances. Preferably the titanium dioxide is in solid or sintered form; in sintered form the titanium dioxide may comprise, for example, spheres of Ti02 about 2nm to 3Onm in size fused together, and gaps between the spheres that are filled with gas such as air or a space-filling binder such as a polymer. In some preferred embodiments the photoactive material has a refractive index of less than 1.5 and may comprise an organic photoactive material. In this specification conducting includes semi-conducting, that is we consider a semi-conducting material as sufficiently conducting to be a conducting material for the purposes of the device.

In embodiments contacts may be made to the photoactive material and to the titanium dioxide but in other, sometimes preferred embodiments, either or both of the inner and outer regions incorporate one or more electrical conductors or layers of conducting material or harvesting material. Such a harvesting material comprises a material to harvest excitations generated by photons within the photoactive material and to provide electrical energy from the harvested excitations; it may comprise a conductor or semiconductor, in embodiments an organic conductor or semiconductor.

Thus the outer region may incorporate a layer of electrically conducting material adjacent the interface, and for example, an electron acceptor or "blocking layer" such as PEDOT:PSS, poly(ethylenedioxythiophene): poly(styrenesulfonate). In this case it is important that any layers that have a low refractive index are extremely thin, preferably having a thickness of less than 5Onm, 4Onm, 3Onm, 2Onm or lOnm, so that the requirements of a significant proportion of the evanescent wave energy penetrating through these layers and being present in the photoactive material can be met. Thus in embodiments the photoactive material preferably has a surface within 5Onm, 4Onm, 3Onm, 2Onm or 1 Onm or less, of the interface at which total internal reflection occurs.

Additionally the outer region may include a layer of electrically conducting material at the back, that is, on a surface of the photoactive material away from the interface.

Preferably the photoactive material comprises a layer of material with a thickness that is small enough that resistive losses are not important, for example less than 1 micron.

However in embodiments this layer may be thinner, for example less than 200nm. In embodiments the inner region may additionally or alternatively include a layer of conducting material, preferably adjacent to the interface, to provide an electrical connection to this inner region.

In embodiments the inner region includes a layer of electrically conducting material and (preferably) a layer of harvesting material adjacent to the photoactive material to provide a said electrical contact and harvesting interface. Preferably the layer of electrically conducting material and harvesting material has a refractive index of greater than 1.55. In this case although, for example, titanium dioxide may function as both a conductor and a harvesting material preferably separate layers of conducting and harvesting material are employed, for example ITO (indium tin oxide) and titanium dioxide.

In embodiments the light guiding structure has a reflector such as a mirror or grating at one end (more particularly the bottom end, defining the top end as the end receiving the incoming light). In embodiments this reflector may comprise a reflector operating by total internal reflection.

In some preferred embodiments this reflector is configured to reflect light coming directly down the light guiding structure off axis so that on its travel back up the light guiding structure it totally internally reflects from one or more walls and/or takes a longer path through the device. This increases the operational efficiency of the device and/or enables devices to be constructed that are dimensionally less deep. The directed reflection may be implemented by diffraction. The directed reflection may be directed in two directions.

In embodiments the light guiding structure has an optical aperture to collect the incoming light which is substantially rectangular or in the form of a line, preferably a straight line. Thus the light guiding structure considered as a whole may define an optical groove, that is a groove for light, with a long thin optical aperture through which light is accepted and guided down into the groove by total internal reflection at the walls of the groove. In some particularly preferred embodiments the device comprises a plurality of such light guiding structures, preferably regularly arranged, preferably side-by-side. In this manner a light accepting surface is defined. A grooved type structure has the advantage that e.g. for a solar cell application light is accepted over a very wide range of angles about an axis perpendicular to the length of the groove (that is between shining along the groove in one direction, vertical, and shining along the groove in the other direction) and that appropriate alignment of the grooves with the apparent motion of the sun during the day can mean that light penetrates deep into the grooves at all daylight hours. In embodiments the light guiding structure may be tapered or horn-shaped, narrowing away from the optical aperture.

In embodiments of a device with multiple light guiding structures the inner or core regions may be integrally formed from a common block or layer of material, for ease of manufacture. Thus the multiple light guiding structures in embodiments are joined to one another at their optical aperture ends, in embodiments forming a substantially continuous light-receiving surface.

In a further aspect, therefore, the invention provides a method of manufacturing a photovoltaic device, in particular as claimed in any preceding claim, the method comprising: impressing a plurality of grooves into a block or film of precursor material; processing said precursor material to create a block or film of high refractive index material including a plurality of grooves, said high refractive index being at least 1.55 for light of a wavelength between l300nm and 300nm; depositing photoactive material into said grooves to form said device; and providing at least one electrical contact connecting to said photoactive material.

The light wavelength in the range l300nm to 300nni is measured in air. In embodiments of the device the inner region material such as titanium dioxide may provide a second, bulk electrical contact.

In one embodiment of the process the precursor material comprises a sol-gel material.

The grooves may be formed by impressing into the precursor material a pair of long thin electrodes separated between by an insulating layer, the combined structure defining a rod or strip which may be employed for a groove. When the sol-gel material, for example a titanium oxide sol-gel, is heated or sintered it withdraws slightly from around this doublet electrode structure, thus leaving a gap into which a solution or melt of the photoactive material may be deposited. This provides a convenient manufacturing process for embodiments of the device. It will be appreciated that many different groove configurations are possible. In one preferred configuration the grooves taper to a line edge at the bottom (of a physically impressed groove), and hence the doublet electrodes forming the groove have a corresponding shape. In this way because the physical grooves form the back of the photovoltaic device the physical grooves taper towards the front of the device and the optical grooves taper towards the back of the device.

Thus in a still further aspect the invention provides a photovoltaic device, the device comprising: a stack of light guiding structures, each having a longitudinal optical aperture and defining an optical groove down into which light from said aperture is able to propagate, said longitudinal apertures being arranged side-by-side and substantially aligned with one another, and wherein each said light guiding structure comprises a first, inner region and a second outer region of lower refractive index than said inner region such that light is guided down into a said optical groove by total internal reflection at an interface between said regions; and wherein said outer region includes a photoactive material to convert energy in an evanescent wave of said guided light to electrical energy for output.

In embodiments, as previously mentioned, a reflector may be incorporated at the bottom end of the optical groove, preferably configured to reflect light travelling directly ("vertically down") into the optical groove towards a wall of the groove, along the groove, or both towards a wall of and along the groove. In embodiments the reflector may comprise a tilted mirror or grating. In other embodiments the function of the reflection may be performed by a diffractive element.

As previously mentioned, in embodiments the inner region may include a transparent conductor, for example ITO (indium tin oxide) adjacent the interface between the inner and outer regions. In a similar way the outer regions may also include a conductor, for example to provide an electrical connection to a side of the photoactive material opposite to a side closest to the interface. In some preferred embodiments the inner region has a refractive index of at least 1.55, more preferably at least 2 or 2.5 at an operational wavelength (which may comprise a wavelength between 1 300nm and 300nm); in embodiments the photoactive material comprises an organic material. As previously mentioned, the stack of light guiding structures may comprise formations in a single block or layer of material.

In some preferred embodiments of the device, the dimensions of the device are substantially larger than a wavelength of light. Thus in embodiments a physical width of the optical aperture groove is at least 1gm; in embodiments physical depth of a groove it at least lOim. In embodiments a length: width ratio of an optical aperture is at least 10:1.

In use the device may be aligned so that the grooves are substantially parallel to the apparent motion of the sun during the day.

A pair of photovoltaic devices according to an aspect or embodiment of the invention as described above may be stacked one above the other in a tandem cell structure. In such a structure one of the devices may operate most efficiently at a first wavelength and the other at a second, different wavelength. For example the upper device (closest to the incoming light) may comprise a green/blue absorber and the lower device a red absorber.

In a complementary aspect the invention provides a method of converting light energy into electrical energy, the method comprising guiding incoming light by total internal reflection (TIR) through a structure comprising inner and outer regions of respective first and second refractive indexes, said first refractive index being higher than said second refractive index, said light being guided within said inner region; forming an evanescent wave in said outer region by said light guiding; converting energy in said evanescent wave into electrical energy using a photoactive material in said outer region; and wherein said method further comprises: configuring said structure such that, for a light wavelength in the range 1 3Onm to 300nm that illuminates said interface at an angle of 80 degrees to a normal to said interface, said evanescent wave penetrates into said outer region sufficiently for the energy density of said evanescent wave where it encounters the photoactive material to be on average at least 30% of the energy density of said evanescent wave at a point where total internal reflection occurs.

However in variants of the above method the energy density of said evanescent wave where it encounters the photoactive material may be on average only at least 30% of the energy density of the evanescent wave at a point where total internal reflection occurs.

The skilled person will understand that the structure guiding the light may have a range of different geometrical configurations and, in embodiments, the light may be guided (confined) in substantially only one dimension or direction. Thus in embodiments the structure guid ing the light may comprise a substantially planar wave guiding structure.

A skilled person will also understand that some photoactive materials such as P3HT preferentially absorb light in the blue-green part of the spectrum, so that red light will be transmitted through the device. Embodiments of the present invention are thus useful for tandem photovoltaic cells, where one or more further devices are used behind an embodiment of a device according to the present invention, to utilise the wavelengths of light that are transmitted (in this example red light).

Features of the above-described aspects and embodiments of the invention may be combined in any permutation.

BRIEF DESCR[PTION OF THE DRAWINGS These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 illustrates the operation of a simplified photovoltaic device; Figure 2 illustrates a principle of confining evanescent photons using total internal reflection; Figures 3 and 3b show, respectively, an embodiment of a photovoltaic device according to an aspect of the invention, and an enlargement of a reflection zone of figure 3a illustrating schematically the (virtual) evanescent wave photons; Figure 4 illustrates a further embodiment of a photovoltaic device according to an aspect of the invention.

Figures 5a to 5d, show respectively, cross-sectional views through example configurations of electrical connections to the photovoltaic device of figure 3; Figures 6a and 6b illustrate a method of fabricating an embodiment of a photovoltaic device according to an aspect of the invention; and Figure 7 illustrates the properties of light under total internal reflection.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

Figure 1 illustrates one type of photovoltaic device where only one layer is strongly photoactive (horizontal shaded area 2 of thickness T). This layer is in contact with a harvesting material (hashed rectangle 1 at bottom). Incident photons enter the cell from the bottom (arrows a-d -incident photons; a-c create excitons, d passes through cell.

Only photon b creates an exciton within the exciton diffusion distance LD.). The thickness of the photoactive layer T is usually comparable in magnitude to the distance over which light is absorbed LA to minimize light rays which pass through the cell (ineffective photons; ray d on far right). In this case most of the photons interact with the active layer to form excitations (indicated by star symbols). In order for the excitations to contribute to the current, their translational motion through the material must bring them into contact with the interface with the harvesting material. However the excitations only move typically a short distance LD before they lose their energy and decay to the ground state, and therefore only those excitations that are created within approximately LD of the interface (marked with dotted line) contribute to the current. In this figure only the unfilled star (the exciton created by photon b) is sufficiently close to the interface to contribute to the current. The other photons are effectively lost; resulting is low efficiency of the device. In an organic photoactive material typically L0 lOnm to 2Onm (possibly effectively extendable to 5Onm, for example by including bulk heterojunction components including photo-active, conjugated polymers mixed with fullerene-based molecues within the material).

As noted above, only one layer may be strongly photoactive in the relevant part of the solar spectrum. For example, the first layer may be made of an organic polymer such as poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylenevinylenej, also known as MEH-PPV, and the second layer maybe made of titanium dioxide, which acts as an n-type harvesting material. Alternatively, the two layers are made of n-type and p-type semiconductor material respectively, and each layer acts as a harvesting material for excitations that have been created by the absorption of photons in the other layer.

Photons are quantum mechanical entities and are subject to the limitations of the Heisenberg uncertainty principle. They cannot be confined to a region smaller than about a wavelength in size. However, LD is significantly less than a wavelength of light in a number of important materials of interest, for example organic polymers.

When photons in a material of high refractive index illuminate an interface with a material of lower refractive index at a sufficiently glancing angle, then they are reflected at the interface rather than being refracted into the material of lower refractive index), (total internal reflection). When total internal reflection occurs, so-called "evanescent photons" penetrate a short distance into the material of lower refractive index. This is illustrated in Figure 2. Evanescent photons may readily be confined to a region approximately 1/10 of a wavelength in depth using materials currently available. This is because the limitations of the Heisenberg uncertainty principle are modified by a factor of approximately 4ir for evanescent photons; a detailed calculation of the

electromagnetic field in this case is given later.

Broadly, we will describe devices which use the confinement attributes of evanescent photons associated with multiple total internal reflections for the purpose of confining creation of photonic excitations to a region very close to an interface preferably to a distance less than the excitation diffusion length for the purpose of enhancing photovoltaic efficiencies.

Figure 2 illustrates confining evanescent photons using total internal reflection. In this figure, incident light waves 1111 a material of high refractive index n illummate an interface with a material of lower refractive index n2 at an angle to the normal. If the angle is larger than the critical angle O then the photons are reflected as shown along R. Although the photons do not enter the region of low refractive index, there is an electromagnetic field E that penetrates a very small distance d as shown (E decreases exponentially with distance into the low refractive index region and its orientation is polarization dependent). The electromagnetic field is said to be composed of evanescent photons. The distance d can be much smaller than a wavelength of light.

In order to enhance the efficiency of a photovoltaic cell, a material system may cause incoming photons to undergo total internal reflection at or near an interface between a harvesting material and a photoactive material, in such a way that evanescent photons penetrate a short distance into the photoactive material and the reflected photons subsequently undergo further total internal reflections of a similar nature. The confinement of the evanescent photons may confine the creation of excitations to a region close to the interface with the harvesting material. This in turn may make it possible for a greater proportion of the excitations to be harvested before their energy is dissipated, thereby enhancing the efficiency of the photovoltaic cell.

One embodiment of the present invention is shown in Figure 3. The harvesting material is made of a material with high refractive index such as titanium dioxide; the photoactive material is made of a material with lower refractive index such as organic polymer; and the interface is in the form of horn-shaped grooves, which are larger than a wavelength of light in the material. The incoming light illuminates the grooves through the layer of titanium dioxide and undergoes multiple total internal reflections inside the grooves, as shown in figure 3, Electrical contacts (not shown) are made using aluminium, transparent Indium Tin Oxide, or other materials.

In a variation of this embodiment, a photoactive layer may be chosen so that the longer wave-length photons may not be absorbed but pass through the horns. The said photons may subsequently be absorbed by one or more photo voltaic cells of similar or different design wherein the active layers is chose to absorb longer wave-length photons. In this way a tandem cell configuration may be constructed which is able to achieve higher efficiencies.

In another variation of this embodiment, a reflector may be placed on the top of the device so as to reflect or scatter the photons back into the horns. By utilising the reflected photons, this may enable shorter horn lengths to be used whilst absorbing an equivalent number of photons. The reflector may be composed of a metallic mirror, or a shape that uses total internal reflection, or other means such as a scattering surface. It may have a means to direct the photons at preferential angles (such as a difftaction pattern, or a sloped or grooved shape) either along the grooves or perpendicular to them, or both, so to enhance the absorption of reflected photons by directing them in the appropriate direction.

In a variation of this embodiment, it may be combined with other methods which are known to those skilled in the art of solar cell manufacture, such as the use of different materials for the harvesting material and/or photoactive layer, and/or for the use of bulk heteroj unction materials and techniques.

One route to manufacturing the above embodiment would be to emboss the groove profile into a viscous sol-gel of a harvesting material such as titanium dioxide and depositing a conductor (possibly followed by an insulator material) into the bottom of the groove; sintering this composite; and subsequently spin-coating an active layer onto the sintered grooved substrate. An outer electrode could be fabricated by conformally coating the active layer by, for example, vapour deposition of a metal such as aluminium. There are a number of variations including constructing the electrodes in advance and using them to emboss the shape into the viscous sol-gel so that they remain in place, or adding additional layers of ITO/harvesting materials. These will be obvious

to people skilled in the field.

h a variation of this or other embodiments of the present invention, a layer of material with high refractive index (which may or may not be a harvesting or conducting material) is covered with a thin layer of transparent conducting material (e.g., ITO) and/or harvesting material, which is in contact with a photoactive layer with a lower refractive index. The geometry is such that total internal reflection of the incoming light occurs at or near the interface with the photoactive material in such a way that the evanescent photons penetrate a short distance into the photoactive material.

In one implementation of this invention the grooves referred to above may be aligned with the apparent motion of the sun during the day so that the light continues to penetrate deep into the grooves at all daylight times.

In variations of the above embodiments, different materials (e.g.. crystalline silicon, amorphous silicon, cadmium telluride, Cu[lnGa]Se2, and organics or other) may be used for the photoactive layer and/or the harvesting layer that are suitable for creating the conditions for total internal reflection. The skilled person will appreciate that applications of embodiments of the invention are not limited to any particular photoactive materials.

Alternatively the grooves may have different shapes such as being U-shaped or V-shaped, or they may come to a point (or to a separation that is comparable to a wavelength of light) near the bottom of the grooves, or the material of high refractive index may be in the form of fingers or wedges with nearly vertical walls, or slabs of material stacked together as shown in Figure 4. Alternatively, shapes other than grooves may be used, such as cylinders of material of high refractive index (similar to optical fibres) or pyramids which are coated in a similar fashion to the above.

The number of reflections and their angle does not depend on the size of the grooves or other surface shapes, and therefore these may be optimised for other considerations such as electrical resistance or cost of manufacture. However they should typically be larger than half a wavelength of light in the material in order that light penetrates into the structure and creates evanescent photons inside the photoactive material. If any part of the shape approaches this limit in size, then a detailed calculation of the electromagnetic field may be performed by the use of wave analysis (rather than ray-tracing as shown), however this affects the detailed calculation rather than the details of the embodiments and the invention.

Referring to figure 3(a). The photovoltaic cell uses a transparent harvesting material 1 with a high refractive index n1, such as titanium dioxide. The surface of this material is formed into horn-shaped structures of dimensions significantly larger than a wavelength of light. This may be achieved by various methods, such as moulding, embossing or rolling a predetermined shape into a malleable precursor film (e.g., a sol-gel paste) and reacting/sintering. This surface is coated, by spin coating or other methods, with a photoactive material 2 such as poly[2-methoxy-5-(2'-ethylliexyloxy)-1,4-phenylenevinylene], also known as MEH-PPV of lower refractive index n2. Light enters the device from the bottom through the transparent harvesting material and undergoes multiple total internal reflections in the horn-shaped structures (bold arrows a-d). The number of total internal reflections does not depend on the dimensions of the grooves merely their shape provided they are larger than a few wavelengths across. Therefore the dimensions of the grooves may be optimized for other considerations such as amount of material used or electrical properties (such as resistance).

Figure 3(b) is an idealized enlargement of the reflection zones (dotted circles) in Figure 3(a). At each reflection, the electromagnetic field (which describes the evanescent photons) penetrates a short distance into the photoactive layer. Figure 3(b) depicts the intensity of the electromagnetic field decreasing exponentially along the normal to the interface (x-axis). Most of the energy in the evanescent photons is within a distance d of the interface (shaded area). The distance d is typically much shorter than a wavelength of light. By this means the creation of excitations is largely confined to a region very close to the interface with the harvesting matenal and the efficiency of the device is enhanced.

One may estimate the amount of light that reaches the top of the device as follows. At a given photon wavelength, suppose a fraction f of the light is reflected and I-f is absorbed at each reflection. Therefore after n reflections at approximately the same angle, approximately f' of the light remains unabsorbed and reaches the top of the device. The shape of the grooves may be optimized so that f is sufficiently small. As described further in the main body of the text, the light that is transmitted through to the top of the horn may be utilised either by (a) using it to illuminate a further device or devices which are optimised for the appropriate wavelengths (a "tandem cell"), or (b) reflecting, scattering or diffracting it back into the horn for further absorption, for example using a mirror surface.

Figure 4 shows a further, preferred embodiment of the invention. Slides of a material of high refractive index are coated (by spin coating, dipping or other methods) with thin layers of: a transparent conductor (such as ITO i.e. Indium tin Oxide); a harvesting material (such as Ti02) possibly coated with a hole injection barrier (such as poiy-ethylenedioxythiophene):poly(styrenesulfonate) also known as PEDOT:PSS); and a photoactive layer or a bulk heteroj unction composite having components including photo-active, conjugated polymers mixed with fullerene-based molecules (such as poly(3-hexylthiophene):(6,6)-phenyl C61-butyric acid methyl ester, also known as P3HT:PBCM). The slabs are coated with aluminium and stacked side by side. Light illuminates from the top and undergoes multiple total internal reflections inside the material of high refractive index, and at each reflection the evanescent photons penetrate through the light-harvesting layer and into the photoactive layer, so that light of the relevant part of the spectrum is absorbed close to the interface between the harvesting material and the photoactive layer If the optional mirror (shown at the bottom of the device) is not present, then any light that is not absorbed by the photoactive layers will continue through the device. This may be used to make so-called tandem' devices where another device of similar or different design is placed beneath this one that is designed to absorb light in a different part of the spectrum. Using such devices the efficiency may be enhanced further.

Alternatively, a mirror may be introduced at the bottom of the device as shown in the figure. This mirror may be manufactured as shown using a metal such as aluminium, or instead the shape of the device may be arranged so that light is reflected using total internal reflection. The mirror may be angled or corrugated or otherwise shaped, or may have diffraction gratings within it, so that light is reflected at varying angles so as to increase the path length through the device (by for example corrugations in a direction parallel to the plane of the paper) or so that the light reaching the bottom of the device is forced to impinge at an angle on the plates (by for example corrugations in a direction nonnal to the plane of the paper), or both.

Refening to Figure 7 we will now describe properties of light under total internal reflection: Consider two materials, M and N, with refractive indexes m and n. This calculation uses the approximation that the materials are lossless, i.e., m and n are real.

The surface between the materials M and N is governed by the equation y=O A light beam is in the material M. It impinges on the surface y =0 at angle 8 to the normal and is totally internally reflected inside the material M. Symbols A wavelength of incoming light (measured in free space) c speed of light in free space k0 wave-vector of incoming light in free space. k0 2 ir / A (A) angular frequency of incoming light. & = c k0 m refractive index of material M n refractive index of material N 0 angle of incidence to the normal the electrostatic potential associated with the light A the magnetic vector potential associated with the light

A electromagnetic field (4, )

?tir penetration of the evanescent A field inside material N xcits penetration depth for the creation of excitons by the evanescent wave x,y usual directions as per the diagram Analysis The equation for the field inside a material with refractive index z is V2 A = (z2 / c2) d2A/dt2 (1) Inside the material M, the solution for electromagnetic field is A = A0expi(kr -t) where 1k! = m k0. At the surface (y = 0), we have Aro = AOexpi(kXx-c(,t) Where k = mk0sinO (2) Inside material N, the solution to the equation is A Ajexpi(kx+ky-wt) Substituting into equation (1) gives -k2-k2 = -(nw/c)2 Using the relationship w = c I', we obtain:-k2+k2 = n2k02 Substituting equation (2), we obtain k2 = 2 (n2 -m2 sin2 0) (3) If the Right Hand Side is negative then k is imaginary and we can write A = A exp(-JkI y) where we have put the time dependency and x dependency into A and IAc is constant.

This equation describes the decay of the evanescent waves associated with total internal reflection. In this case the evanescent electromagnetic field has a lie decay length Xcicld given by 1 / IkI, that is, 4ieId = l/[ko/( m2sin2O-n2)] A/[2irN( m2sin2O-n2)] The energy in the photons is proportional to the square of the amplitude, !A21 IAI2 exp(-2 kI y), and therefore the lie decay length for the evanescent photons is half of this figure:- >evanescentphotons = A i[4 ir'/( m2 sin2 0-n2)] Typical penetration depths For photons of wavelength 500 nanometres in a vacuum, refractive indexes of 2.1 and 1.3, and an angle of incidence of 70 degrees, the penetration depth of the evanescent photons evanescent photons is approximately 27 nanometres.

For photons of wavelength 500 nanometres in a vacuum, refractive indexes of 3.6 and 1.3, and an angle of incidence of 70 degrees, the penetration depth of the evanescent photons evanescent photons is approximately 13 nanometres.

Figure 5 shows an enlargement of part of the walls of the device shown in figure 3, showing detail including electrical connections.

Referring to the embodiment illustrated in Figure 5a,, a material of high refractive index such as Titanium Dioxide is first coated with a thin layer of Indium Tin Oxide (ITO), which is a transparent conducting material. This layer is shown to the left of figure 5a. In some configurations this layer may be 5Onm in thickness. It may be made by spin-coating or by vacuum deposition. l'his conducting layer forms one of the electrical connections to the device. Secondly, this layer is coated with Titanium dioxide, which acts as an n-type harvesting material. This layer may be deposited by vacuum deposition, or by other methods such as spin-coating of precursor material followed by sintering. Thirdly, the device is coated with a photoactive material such as P3HT, for example by spin-coating. Finally, an aluminium layer is deposited, for example by sputtering. This conducting layer forms the second of the electrical connections to the device.

Figure 5b shows an alternative embodiment in which the material of high refractive index is first coated with a thin layer of Indium Tin Oxide (ITO), which is a transparent conducting material. In some configurations this layer may be 5Onm in thickness. It may be made by spin-coating or by vacuum deposition. This conducting layer forms one of the electrical connections to the device. Secondly, this layer is coated with a thin layer of PDOT:PSS, which acts as a harvesting material. This layer may be deposited for example by spin-coating. This material has a low refractive index and therefore it should be sufficiently thin that the evanescent wave penetrates through it, as illustrated in figure 5c. Thirdly, the device is coated with a thin layer containing photoactive material and a harvesting material in a bulk heterojunction configuration, such as P3HT:PCBM. This may be deposited for example by spin-coating. Finally, an outer contact made of Lithium Fluoride/Aluminium is deposited, for example by sputtering.

This conducting layer forms the second electrical connection to the device.

Figure 5c shows the passage of a photon in the configuration of figure 5b and the penetration of the evanescent wave through the layer of PDOT:PSS. The refractive index of ITO is high and therefore in many configurations the photons do not undergo total internal reflection at the interface with the material of high refractive index (to the left of the diagram), but they are refracted into this layer. The refractive index of PDOT:PSS is much lower than that of ITO and therefore in most configurations total internal reflection occurs at the interface between the ITO and the PDOT:PSS. The diagram shows the evanescent wave penetrating through the PDOT:PSS layer into the layer containing the photoactive material. As can be seen the PDOT:PSS layer is sufficiently thin that enough energy density of the evanescent wave penetrates through it into the photoactive layer.

Figure 5d shows a further alternative embodiment, and illustrates the passage of a photon and the evanescent wave through the device. In the embodiment the layers are made of fluorinated tin oxide (FTO), Titanium Dioxide (Ti02), poly(N-dodecyl-2,5,-bis(2'-thienyl)pyrrole-2,l,3-benzothiadjazole) (PTPTB), which is an electron harvesting material, P3HT, and silver. These layers are deposited similarly to the configurations in figures 5a-5c. The refractive index of the FF0 and hO2 layer is high and therefore in typical configurations the photons do not undergo total internal reflection at the interface with the material of high refractive index (to the left of the enlargement), but they are refracted into these layers. The refractive index of the PTPTB electron-acceptor material is much lower than that of the FTO and Ti02 layers and therefore in most configurations total internal reflection occurs at the interface between the Ti02 layer and the PTPTB as shown in the diagram. The diagram shows the evanescent wave penetrating through the PTPTB layer into the layer containing the photoactive material (P3HT). As can be seen the PTPTB layer is sufficiently thin that enough energy density of the evanescent wave penetrates through it into the photoactive layer sufficiently for adequate efficiency.

One method of manufacturing a device is shown in figure 6.

Firstly a set of formers are made using two pieces of shaped metal and an insulator, as shown in Figure 6a. They will be used to impress a groove into precursor material. The bottom part of the device will act as the negative electrode and also as a reflector to guide light into the grooves of the device. The layer of insulator separates this from the top layer, which will act as the positive electrical connection. Formers like this are used to impress a series of grooves into a material of high refractive index, as shown in figure 6b.

Figure 6b shows a cross-section of the device. The formers are used to impress their shape into a soft material of high refractive index, such as Titanium dioxide precursor.

The precursor is treated (eg by sintering) to convert it into the desired material.

In some configurations, the formers are removed and the material of high refractive index is coated with Indium tin oxide; then the metal cathode and the insulator are reinserted; then a harvesting material such as Titanium Dioxide is deposited and sintered; then the top part of the formers are re-inserted. In other configurations this step is omitted provided that the material of high refractive index itself acts as a harvesting material and has sufficient conductivity for the relevant charge carriers and the dimensions of the grooves are sufficiently small so that resistive losses are controlled.

A photoactive material is inserted into the space between the metal anode and the harvesting material, for example using a vacuum, in order to complete the device.

Photons enter the device through the bottom. In some configurations a mirror surface is inserted at the top of the device to reflect photons back into the device, whereas in other configurations the light that is transmitted to the top of the device is used to power further photovoltaic devices of similar or different design that absorb in wavelengths that are not absorbed in the present device..

Broadly speaking we have described a method to enhance the efficiency of photovoltaic cells by using multiple total internal reflections to confine the creation of photonic excitations to a region very close to an interface with a harvesting material.

In variations of the above embodiments, the shape of one or more interfaces may be modified at length-scales that are small compared to the scales referred to above (for example roughened) so as to harvest excitations more efficiently.

In this document, the evanescent waves have been described in terms of wave optics and the incoming light has been described in terms of ray optics. Where the sizes of the structures are small, for example with apertures a few wavelengths across, it may be convenient to use wave optics also to describe the incoming light. However a skilled person in the field will understand that it is possible to convert one representation into the other for the present purposes.

To take a simple example, light of the lowest mode propagating in the y direction between two reflecting plates at x=O and x=d may be described in wave optical terms as A = sin(kx)exp(iky), where k =4 i / d and k is a constant describing the propagation of light in the y direction. This description maybe converted into a ray optics r epresentation by writing the equation in the following which is mathematically identical:-A = exp(ikx)exp(iky) + exp(-ikx)exp(iky) This form of the equation readily described the incoming waves as two rays of light, each propagating at an angle to the walls of the waveguide.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: 1. A photovoltaic device, the device comprising: a light guiding structure to receive and guide incoming light for conversion to electrical energy by total internal reflection, the structure comprising adjacent inner and outer light guiding regions with a total internal reflection (TIR) interface between said inner and outer regions, said inner region being exposed to said incoming light and having, at said TIR interface, a refractive index of at least 1.55 for a light wavelength in the range l300nni to 300nm, and said outer region having, at said TIR interface, a lower refractive index than said inner region; wherein said outer region includes a region of width at least 1 Onm containing photoactive material to generate excitations from an evanescent wave arising from total internal reflection (TIR) of said light at said TJIR interface; and wherein said photovoltaic device further comprises electrical contacts to said photoactive material to provide said electrical energy from said excitations in said photoactive material.
2. A photovoltaic device as claimed in claim 1 wherein said light guiding structure is configured such that said incoming light enters said inner region and subsequently undergoes more than one total internal reflection at said TIR interface.
3. A photovoltaic device as claimed in claim I or 2 wherein said TIR interface comprises a surface of said photoactive material.
4. A photovoltaic device as claimed in claim 1 or 2 wherein a surface of said photoactive material closest to said T interface is spaced away from said TIR interface; and wherein said light guiding structure is configured such that for light at a said wavelength in the range l300nm to 300nm that is incident from inside said inner region at an angle of 80 degrees to said normal to said TIR interface, said evanescent wave penetrates beyond said TIR interface sufficiently that the energy density of said evanescent wave where it first encounters the photoactive material is on average at least 30% of the energy density of said evanescent wave at said TIR interface.
5. A photovoltaic device as claimed in any one of claims I to 4 wherein said photoactive material comprises a layer of photoactive material in said outer region, and wherein said light guiding structure is configured such that for light at said wavelength in the range l300nm to 300nm at an angle of 80 degrees to said normal, an energy density of said evanescent wave decays to lie of a value it has at a surface of said layer of photoactive material closest to said TIR interface after having penetrated into said photoactive material by no more than 6Onm or by no more than 1.5 diffusion lengths of a said excitation.
6. A photovoltaic device as claimed in any preceding claim wherein said device is configured such that said incoming light undergoes multiple total internal reflections at said interface, and after such multiple total internal reflections, at least 40% of the energy of said incoming light of at least one wavelength in the range between l300nm and 300nm has been absorbed by said photoactive material.
7. A photovoltaic device as claimed in any preceding claim wherein a material of said inner region has, at said interface, a refractive index of at least two at said wavelength.
8. A photovoltaic device as claimed in claim 7 wherein said inner region comprises titanium dioxide.
9. A photovoltaic device as claimed in any preceding claim wherein said photoactive material has a refractive index of less than 1.5 at said wavelength.
10. A photovoltaic device as claimed in any preceding claim wherein said photoactive material comprises an organic photoactive material.
11. A photovoltaic device as claimed in any preceding claim wherein said outer region includes a layer of electrically conducting material and a layer of harvesting material between said photoactive material and said inner region to provide a said electrical contact and harvesting interface, said layer of electrically conducting material and harvesting material having a refractive index of less than 1.55 at said wavelength and a total thickness of less than 7Onm, 6Onm, SOnm, 4Onm, 3Onm, 2Omn, or lOnm.
12. A photovoltaic device as claimed in claim 11 wherein said layer of electrically conducting material and said layer of harvesting material comprise layers of organic material, and wherein said TIR interface comprises a surface of said organic material.
13. A photovoltaic device as claimed in any preceding claim wherein said photoactive material has a surface within 5Onm of said TIR interface.
14. A photovoltaic device as claimed in any preceding claim wherein said outer region includes a layer of electrically conducting material on a surface of said photoactive material away from said interface to provide a said electrical contact, and wherein said photoactive material comprises a layer of photoactive material with a thickness of less than 1 micron or less than 200nm.
15. A photovoltaic device as claimed in any preceding claim wherein said inner region includes a layer of conducting material adjacent to said interface to provide an electrical connection to said inner region, said layer of conducting material having a refractive index of at least 1.55 at said wavelength.
16. A photovoltaic device as claimed in claim 15 wherein said layer of conducting material comprises solid or sintered titanium dioxide.
17. A photovoltaic device as claimed in any preceding claim wherein said light guiding structure has a reflector at one end.
18. A photovoltaic device as claimed in claim 17 wherein said light guiding structure has an axis along a direction in which said incoming light travels and wherein said mirror is configured to reflect a light incident along said axis back from said reflector off-axis.
19. A photovoltaic device as claimed in any preceding claim wherein said light guiding structure has an optical aperture to receive light into said inner region, and wherein said light guiding structure has a tapered configuration, narrowing away from said optical aperture.
20. A photovoltaic device as claimed in any preceding claim wherein said light guiding structure has an optical aperture to receive said incoming light for guiding, and wherein said optical aperture is substantially rectangular.
21. A photovoltaic device as claimed in any preceding claim wherein said light guiding structure has an optical aperture to receive said incoming light for guiding, and wherein said optical aperture comprises a line aperture such that said guiding structure defines an optical groove.
22. A photovoltaic device as claimed in claim 20 or 21 comprising a plurality of said light guiding structures regularly arranged side-by-side such that said optical apertures of said light guiding structures define a light receiving surface.
23. A photovoltaic device as claimed in any preceding comprising a plurality of said light guiding structures and wherein said inner regions of said light guiding structure comprise regions of a common block or film of material.
24. A method of manufacturing a photovoltaic device, in particular as claimed in any preceding claim, the method comprising: impressing a plurality of grooves into a block or film of precursor material; processing said precursor material to create a block or film of high refractive index material including a plurality of grooves, said high refractive index being at least 1.55 for light of a wavelength between l300nm and 300nm; depositing photoactive material into said grooves to form said device; and providing at least one electrical contact connecting to said photoactive material.
25. A photovoltaic device, the device comprising: a stack of light guiding structures, each having a longitudinal optical aperture and defining an optical groove down into which light from said aperture is able to propagate, said longitudinal apertures being arranged side-by- side and substantially aligned with one another, and wherein each said light guiding structure comprises a first, inner region and a second outer region of lower refractive index than said inner region such that light is guided down into a said optical groove by total internal reflection at an interface between said regions; and wherein said outer region includes a photoactive material to convert energy in an evanescent wave of said guided light to electrical energy for output.
26. A photovoltaic device as claimed in claim 25 wherein a said light guiding structure has a reflector at the bottom end of a said optical groove.
27. A photovoltaic device as claimed in claim 26 wherein said reflector is configured to reflect light travelling directly into a said optical groove towards a wall of said groove.
28. A photovoltaic device as claimed in any one of claims 25 to 27 wherein said inner region includes a conductor adjacent the interface between said inner and outer regions to provide an electrical connection to one side of said photovoltaic material.
29. A photovoltaic device as claimed in any one of claims 25 to 28 wherein said outer regions includes a conductor to provide an electrical connection to a side of said photovoltaic material opposite to a side of said photoactive material closest to said interface.
30. A photovoltaic device as claimed in any one of claims 25 to 29 wherein said inner region has a refractive index of at least 1.55 at a wavelength between 1 300nm to 3 OOnm.
31. A photovoltaic device as claimed in any one of claims 25 to 30 wherein said photovoltaic material comprises an organic photoactive material.
32. A photovoltaic device as claimed in any one of claims 25 to 31 wherein said inner regions of said stack of light guiding structures comprise formations in a single block or layer of material.
33. A photovoltaic device as claimed in any one of claims 25 to 32 wherein a physical width of a said optical aperture is at least 1 pm.
34. A photovoltaic device as claimed in any one of claims 20 to 33 wherein a physical depth of a said optical groove is at least 10pm.
35. A method of converting light energy into electrical energy, the method comprising guiding incoming light by total internal reflection (TIR) through a structure comprising inner and outer regions of respective first and second refractive indexes, said first refractive index being higher than said second refractive index, said light being guided within said inner region; forming an evanescent wave in said outer region by said light guiding; converting energy in said evanescent wave into electrical energy using a photoactive material in said outer region; and wherein said method further comprises: configuring said structure such that, for a light wavelength in the range l3Omn to 300nni that illuminates said interface at an angle of 80 degrees to a normal to said interface, said evanescent wave penetrates into said outer region sufficiently for the energy density of said evanescent wave where it encounters the photoactive material to be on average at least 30% of the energy density of said evanescent wave at a point where total internal reflection occurs.
36. A method as claimed in claim 35 wherein for at least one light wavelength in the range l300nm to 300nm illuminating said interface at an angle of 80 degrees to the normal, an energy density of said evanescent wave decays to l/e of a value at said interface after having penetrated into said photoactive material by no more than 5Onm or a diffusion length of an electronic excitation within said photoactive material providing said electrical energy.
37. A method as claimed in claim 35 or 36 wherein said device is configured so that said incoming light undergoes multiple total internal reflections at said interface, and after such multiple total internal reflections, at least 40% of the energy of said incoming light of at least one wavelength in the range between l300nm and 300nm has been absorbed by said photoactive material.