WO2013030553A1 - Dye- sensitized soalr cell - Google Patents

Abstract

The present invention provides a solid-state p-n heterojunction comprising an n-type material in contact with a p-type material, wherein at least one of said n-type material and said p-type material comprises an organic polymeric charge transporter having a low band gap, and wherein said heterojunction further comprises a light harvesting antenna (LHA) material having a band gap of no less than 1.5 e V and greater than the band gap of said organic polymeric charge transporter. The invention further provides optoelectronic devices comprising at least one such solid state p-n heterojunction, along with methods for the manufacture of such solid-state p-n heterojunctions.

Description

DYE- SENSITIZED SOALR CELL

The present invention relates to a solid-state p-n heterojunction and to its use in optoelectronic devices, in particular in solid-state solar cells (SSCs),

phototransistors and corresponding light sensing devices. Most particularly, the present invention relates to optoelectronic devices having a polymeric hole transporting material and especially polymeric hole transporters which absorb light.

The junction of an n-type semiconductor material (known as an electron

transporter) with a p-type semiconductor material (known as a hole-transporter) is perhaps the most fundamental structure in modern electronics. This so-called "p-n heterojunction" forms the basis of most modern diodes, transistors and related devices including opto-electronic devices such as light emitting diodes (LEDs), photovoltaic cells, phototransistors, and electronic photo-sensors.

A realization of the pressing need to secure sustainable future energy supplies has led to a recent explosion of interest in photovoltaics (PV). Conventional semiconductor based solar cells are reasonably efficient at converting solar to electrical energy. However, it is generally accepted that further major cost reductions are necessary to enable widespread uptake of solar electricity generation, especially on a larger scale. Solid-state solar cells such as polymer solar cells, dye-sensitized solar cells (DSCs), polymer "bulk heterojunction" solar cells (made from a blend of p-type and n-type polymers) and Polymer Oxide Solar Cells (POSCs) offer a promising solution to the need for low-cost, large-area photovoltaics.

Some recent work has focused on creating gel or solid-state electrolytes, or entirely replacing the electrolytes in dye sensitized solar cells with a solid-state molecular hole-transporter, which transports the charge by movement of electrons rather than diffusion of ionic species, as is the case for electrolytes. Solid phase organic hole- transporters are much more appealing for large scale processing and durability due to their lack of corrosive properties and possibility of better tuning of the electronic properties. However, there are a number of obstacles that need to be overcome if organic hole transporting materials can be used efficiently. Polymeric organic hole transporters offer the potential of high efficiency charge transfer but have frequently been considered to be of limited application because it is difficult to cause the polymer to penetrate and fill the porous network of the n-type material (such as mesoporous metal oxide). Heterojunctions of polymers with inorganic semiconductors can alternatively be made planar rather than by using a porous inorganic material but in that case a very high level of absorption is required in a thin layer junction to avoid a large fraction of the instant light being wasted.

Since the amount of energy available in a solar cell is fundamentally limited by the amount of solar energy absorbed, it is desirable that the device absorbs to a high degree over a broad range of frequencies.

Over recent years there has been tremendous advancement in the synthesis of polymer hole-conductors and their implementation in organic photovoltaics. A successful strategy to improve the performance has been to synthesise polymers absorbing further into the near Infrared and successfully integrate them into bulk heterojunction solar cells (comprising a polymeric hole transporter and a polymeric electron transporter). However, the optical bandwidth for light absorption in organic semiconductors is relatively narrow, as compared to conventional inorganic semiconductor materials. This means that many of the newest polymers do not effectively harvest all the light across the visible spectrum. To a certain extent this has been remedied by replacing PC61 BM (the electron acceptor) which is essentially transparent over the visible spectrum, with PC71 BM as the electron acceptor (electron transporter / n-type material), which absorbs light up to 500nm and acts as an effective "hole-donor" in the operational solar cell. Despite this, as the band gap moves further into the IR for new polymers, a new approach is required to enable uniform light harvesting across the spectrum. Furthermore, since this material is an electron transporter, it is not directly suitable for use in heterojunctions having an n-type inorganic material, such as Ti0₂ or ZnO (either planar or porous). Such inorganic/polymer solar cells show considerable promise and so there is considerable need for light absorption and charge generation within the "p-type", hole-transportation and electron donation layer of the heterojunction.

Absorbing light directly in the same materials which are responsible for charge generation and transport, is the principle adopted for polymer solar cells. However, this puts very stringent demands on the electronic and optical properties of these materials, and it is quite apparent that in the whole of the field of organic

photovoltaics there is only one material, PC71 BM, which simultaneously acts as a reasonable absorber and electron acceptor. High performance materials acting as hole transporters/electron donors and also absorbing substantially uniformly well over a broad frequency range are not known.

In view of the above, it would be a considerable advantage to provide solid-state heterojunctions and optoelectronic devices such as solar cells comprising polymeric charge transporters that were able to absorb light over a large proportion of the visible spectrum. It would be an advantage if solar cells could provide enhanced currents and/or enhanced efficiencies

The present inventors have now surprisingly established that by use of an appropriate species as a "light harvesting antenna", the light absorption provided by a polymeric charge transporter can be enhanced. In particular, light absorption in a low band-gap polymeric charge transporter can be enhanced by use of a light absorber having a higher band gap and/or having absorption at higher frequency than the polymer and having the facility to transfer the energy from that light absorption to the polymer charge transporter.

In a first aspect, the present invention therefore provides a solid-state p-n heterojunction comprising an n-type material in contact with a p-type material wherein at least one of said n-type material and said p-type material comprises an organic polymeric charge transporter having a low band gap (of no more than 1 .9 eV) and wherein said heterojunction further comprises a light harvesting antenna having a band gap of no less than 1 .5 eV and greater (e.g. at least 0.05 eV greater) than the band gap of said organic polymeric charge transporter. Said sensitizer should be capable of transferring excitation energy to said organic polymeric charge transporter.

Suitable organic polymeric charge transporters may be at least one electron transporter (n-type material) or may be at least one hole transporter (p-type materials). An n-type and a p-type organic polymeric charge transporter may be used together to form a "bulk heterojunction" by methods well known in the art (see, e.g. Dennler et al., Adv. Mater. 2009, 21 , 1323-1338). Alternatively, at least one p- type organic polymeric charge transporter may be used with an n-type inorganic semiconductor (planar or porous). Similarly, at least one n-type organic polymeric charge transporter may be used with a p-type inorganic semiconductor (planar or porous). Examples of suitable materials will be well known in the art and include those described herein in all compatible embodiments (see, e.g. Hardin et al.

Nature Photonics 3, 2009, 406-41 1 ).

The heterojunctions of the present invention, along with all other embodiments, may optionally include additional "electron acceptor moieties" to serve to enhance the transfer of electrons from the p-type material to the n-type material. These include those described herein and are particularly preferable in embodiments where an n- type inorganic semiconductor or a p-type inorganic semiconductor is utilised.

All references to a heterojunction herein may be taken to refer equally to an optoelectronic device, including referring to a solar cell, phototransistor or to a photo-detector where context allows. Similarly, while solid-state polymer-oxide solar cells are frequently used herein as illustration, it will be appreciated that such heterojunctions may equally be applied to other corresponding optoelectronic devices including all those described in all sections herein.

A particularly suitable application of the heterojunctions of the present invention (as well as applying to all other aspects of the invention) is in optoelectronic devices. In a further aspect, the present invention therefore provides an optoelectronic device comprising at least one solid state p-n heterojunction as indicated in any embodiment of the present invention and/or formable by any indicated method. Most appropriate optoelectronic devices include all those indicated herein, such as photo-detectors, photo-transistors, solid-state polymer-oxide solar cells, solid-state bulk heterojunction solar cells, solid state dye-sensitised solar cells and/or solid state polymer sensitised solar cells.

Thus, in one embodiment, the optoelectronic device of the present invention is a "bulk heterojunction" solar cell comprising an organic polymeric n-type material and an organic polymeric p-type material wherein at least one of said organic polymeric materials has a low band-gap as described herein. ln a further embodiment, the optoelectronic device of the present invention is a "polymer oxide solar cell" comprising an inorganic n-type material (such as those described herein) and an organic polymeric p-type material having a low band-gap as described herein. The inorganic n-type material is typically a metal oxide but may be any suitable inorganic n-type material, such as those described herein. The inorganic n-type material may be in the form of a planar or substantially planar layer or may be in the form of a porous (e.g. mesoporous) layer.

In a further embodiment, the present invention additionally provides the use of a light harvesting antenna (LHA) having a band gap of no less than 1 .5 eV to enhance the light absorption of at least one organic polymeric charge transporter having a low band gap (of no more than 1 .9eV) wherein the band gap of the light harvesting antenna is greater than the band gap of said organic polymeric charge transporter (e.g. at least 0.05 eV, preferably at least 0.1 eV greater). Preferably the LHA is capable of transferring excitation energy to said organic polymeric charge transporter.

A further aspect of the present invention lies in a method for the manufacture of a bulk heterojunction, said method comprising:

a) coating an anode, preferably a transparent anode (e.g. a Fluorine doped Tin Oxide - FTO anode) with a compact layer of an n-type inorganic semiconductor material (such as any of those described herein);

b) optionally forming at least a partial monolayer of functionalised electron acceptor moieties (as described herein) and optionally sensitizing agents which are photoactive over part of the solar spectrum adsorbed on to the surface of the compact n-type layer;

c) optionally treating said compact layer of n-type material with at least one ionic material such as a lithium salt.

d) optionally forming a porous barrier layer of an insulating material on said compact layer of n-type material;

e) contacting said planar layer of n-type material with at least one solution of at least one low band-gap polymeric organic p-type semiconductor material and at least one light harvesting antenna dye (having a band gap as described herein) whereby to form a layer of polymeric organic p-type semiconductor having distributed therein said at least one light harvesting antenna;

f) contacting said planar layer of n-type material with at least one solution of at least one low band-gap polymeric organic p-type semiconductor material and at least one electron accepting organic n-type semiconductor and at least one light harvesting antenna dye (having a band gap as described herein) whereby to form a layer of an organic bulk heterojunction whereby the polymeric organic p-type semiconductor having distributed therein said at least one light harvesting antenna. g) contacting said planar layer of n-type material with at least one solution of at least one low band-gap polymeric organic n-type semiconductor material and at least one hole accepting organic p-type semiconductor and at least one light harvesting antenna dye (having a band gap as described herein) whereby to form a layer of an organic bulk heterojunction whereby the organic n-type semiconductor having distributed therein said at least one light harvesting antenna.

h) optionally treating said layer produced in "e", "f" or "g" by coating on top with a solution of ionic material, such as Li-TFSI.

i) forming a cathode, preferably a metal cathode (e.g. a silver or gold cathode) in contact with said p-type semiconductor material.

A further aspect of the present invention lies in a method for the manufacture of a solid-state p-n heterojunction comprising an organic p-type material in contact with a layer of inorganic n-type material, said method characterised by; a) coating an anode, preferably a transparent anode (e.g. a Fluorine doped Tin Oxide - FTO anode) with a compact layer of an n-type inorganic semiconductor material (such as any of those described herein);

b) forming a planar or porous (preferably mesoporous) layer of an n-type semiconductor material (such as any of those described herein) on said compact layer,

c) optionally forming at least a partial monolayer of functionalised electron acceptor moieties (as described herein) and optionally sensitizing agents on the surface of the porous n-type layer;

d) optionally treating said compact layer and/or said porous layer of n-type material with at least one ionic material such as a lithium salt. e) optionally forming a porous barrier layer of an insulating material on said porous layer of n-type material;

f) contacting said planar or porous layer of n-type material with at least one solution of at least one low band-gap polymeric organic p-type semiconductor material and at least one light harvesting antenna (having a band gap as described herein) whereby to form a layer of polymeric organic p-type semiconductor having distributed therein said at least one light harvesting antenna;

g) optionally treating said layer produced in "f" by coating on top with a solution of ionic material, such as Li-TFSI;

h) forming a cathode, preferably a metal cathode (e.g. a silver or gold cathode) in contact with said p-type semiconductor material.

Without being bound by theory, it is believed that the use of an ionic material, such as a lithium salt at step d) enhances the generation of an electrically continuous layer of p-type material over the pore-surface within the n-type material. It is therefore preferable that step d) is included. It is more preferable that the ionic material in step d) be a lithium salt and still more preferable that this be Li-TFSI or an analogue or derivative thereof (including any indicated in any section of this application). Devices formed or formable from any of the heterojunctions of the invention or by any of the methods, uses or process of the invention evidently also form additional aspects of the invention in themselves.

In a particularly preferred embodiment optional step d) comprises treating said compact layer and/or said porous layer of n-type material with a mixture of at least two ionic materials. Suitable ionic materials preferably include metal salts

(especially lithium salts such as lithium bis(trifluoromethylsulfonyl)imide lithium salt) in combination with ionic liquids (such as, 1 -Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide).

In all aspects and embodiments of the present invention, the term "ionic liquid" is used to indicate an ionic species with a melting point sufficiently low for convenient processing in the formation of the heterejunctions and devices of the invention. Many suitable low melting-point salts are known and in one embodiment, salts having a melting point of 100 °C or lower are preferable. Salts having a melting point of below 50 °C or even below room temperature may be preferably used. Some suitable ionic liquids, including 1 -Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide have a melting point below 0°C. Highly preferable ionic liquids include those selected from 1 -Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 -Allyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 ,3-Dihydroxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 ,3-Dimethoxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide and mixtures thereof.

The functioning of a polymer oxide or bulk heterojunction solar cell relies initially on the collection of solar light energy in the form of capture of solar photons by the polymer (and/or by the additional light harvesting antenna dye in the present invention). The effect of the light absorption is to raise an electron into a higher energy level in the polymer and/or LHA. This excited electron will eventually decay back to its ground state, but in a solar cell, the junction of the n-type and p-type materials in close proximity to the excited material provides an alternative (faster) route for the electron to leave its excited state, viz. by "injection" of an election into the n-type semiconductor material (or corresponding injection of a "hole" into the p- type material). This injection can be direct or via an intermediate material but in all cases results in charge generation, whereby the n-type semiconductor has gained a net negative charge and the p-type material has gained a net positive charge. Where a dye-sensitizer or electron injecting material is present, this may initially take the positive charge but it will rapidly be passed to the polymer charge carrier. This serves to "regenerate" the dye-sensitizer or portion of the polymer close to the heterojunction by passing the positive charge ("hole") on through the p-type semiconductor material of the junction (the "hole transporter"). The photoinduced excitation on the polymer may also initially donate the excited electron to the dye- sensitizer, or surface adsorbed electron-acceptor, as an intermediate step to the electron being injected into the n-type material (e.g. metal oxide). Where such an optional dye-sensitizer is present, this may be separate from the light harvesting antenna which forms a key component of the various aspects of the present invention. In particular, the key features of the one or more LHAs present include the ability to absorb light at wavelengths shorter than at least one peak in the absorption spectrum of the polymer and to transfer the energy captured in this way to the polymer.

The LHAs referred to in the present invention should transfer energy to the at least one polymer charge transporter. This transfer will typically be by non-radiative coupling, such as FRET. There is no necessity that the LHAs be capable of direct transfer of charge at the heterojunction. In one embodiment, the LHAs are not distributed primarily at the junction of the n-type and p-type materials but are distributed substantially uniformly through at least one of the polymer charge transporters. Thus in one embodiment, no more than 10% (preferably no more than 5%) of the charge generated by the heterojunction is generated by injection of charged species due to the LHA at the junction of the p-type and n-type materials.

In contrast to the LHA, where any optional dye sensitizer is present, this will typically be concentrated at the junction of the n-type and p-type material, such as being deposited as a surface layer on the n-type material before the p-type material is introduced. Thus, where a dye sensitizer is present, at least 5%, preferably at least 10% and more preferably at least 15% of the generated charge will result from injection of charge by the dye sensitizer at the junction of the n-type and p-type materials.

In a solid state polymer oxide or bulk heterojunction device, the hole transporter is in direct contact with the n-type material and/or sensitizer material, while in the more common electrolytic dye-sensitised photocells, a redox couple (typically iodide/triiodide) serves to regenerate a dye and transports the "hole species" (triiodide) to the counter electrode. Once the electron is passed into the n-type material, it must then be transported away, with its charge contributing to the current generated by the solar cell.

While the above is a simplified summary of the ideal working of a polymer oxide or bulk heterojunction solar cell, there are certain processes which occur in any practical device in competition with these desired steps and which serve to decrease the conversion of sunlight into useful electrical energy. Decay of the light- absorbing species (which may be the polymer or dye-sensitizer) back to its ground state was indicated above, but in addition to this, there is the natural tendency of two separated charges of opposite sign to re-combine. This can occur by return of the electron into a lower energy level of the excited species, or by recombination of the electron directly from the n-type material to quench the hole in the p-type material. In an electrolytic DSC, there is additionally the opportunity for the separated electron to leave the surface of the n-type material and directly reduce the iodide/iodine redox couple. Evidently, each of these competing pathways results in the loss of potentially useful current and thus a reduction in cell energy- conversion efficiency. It is therefore essential that each of the desired steps occurs at a rate which is considerably faster than the competing undesirable processes to avoid wasting potentially useful energy. It is also important that there is not too much of a disparity in the speeds of the various steps since a fast step followed by a slow step can lead to a build-up of a short-lived intermediate species which may then follow an energy-wasteful path. Thus it is particularly critical that the polymer hole-transporter is capable of effectively carrying charge away from the site of generation.

A schematic diagram indicating a typical structure of the solid-state polymer oxide solar cell is given in attached Figure 1 a and a diagram indicating some of the key steps in electrical power generation from a polymer oxide solar cell is given in attached Figure 1 b .

Polymer-oxide solar cells, composed of planar or mesoporous metal oxide electrodes contacted with (light absorbing) semiconducting polymers and bulk heterojunctions between two charge-carrying polymers or small molecules have the potential to deliver high power conversion efficiencies while being compatible with low cost large area chemical processing. However, until recently solar-to-electrical power conversion efficiencies have remained below 1 %. For efficient solar cell operation, a suitable fraction of sun light needs to be absorbed in the photoactive layer, excitons formed in the dye and/or polymer need to be ionised at the heterojunction, and the photo-generated charges need to be separated and carried out of the solar cell to their respective electrodes. The latter requires effective percolation for both charge carriers, electrons and holes, in the n-type and p-type materials respectively. The two main issues thought to be responsible for the relatively poor operation of polymer-oxide solar cells have been an inability to effectively infiltrate semiconducting polymers into random porous networks and ineffective charge transport within the polymer phase due to non-crystalline, un- orientated polymer chains.

The number of photos absorbed provides a fundamental limit on the amount of current which can be generated in a solar cell or similar optoelectronic device. Thus, although polymer-containing optoelectronic devices such as solar cells have been improved by the development of low-band-gap charge transporting polymers, the lack of absorption of these materials in the visible region limits their efficiency and especially their photocurrent.

A further factor limiting the efficiency of a photocell or similar optoelectronic device is the efficiency of injection of electrons from the p-type material to the n-type material to generate the charge separation and thus photocurrent. In one optional embodiment of the present invention an electron acceptor moiety may be used to increase injection from the p-type to the n-type materials. These are particularly appropriate where the n-type material is an inorganic material, such as in a polymer-oxide solar cell.

Typical electron acceptor moieties as indicated above will comprise at least one 2- dimensional or 3-dimensional network of carbon atoms, optionally including up to 10% of heteroatoms, functionalised by at least one organic group having affinity for the n-type material surface. Such moieties will typically have molar masses between 100 and 10000 g/mol. Fullerenes are preferred.

Where an electron acceptor moiety is used, this will typically be in the form of an at least partial monolayer at the interface of the n-type and p-type materials.

A key aspect of the present invention is the use of a light harvesting antenna dye to provide a broad spectral absorption of incident light when used in combination with the recently developed low-band-gap polymer charge transporters. There are two key features which such aLHA dye should embody; firstly it should provide absorption in a spectral region at least partially complimentary to that provided by the low-band-gap polymer, and secondly it should be capable of transfer of excitation energy from the sensitizer to the polymer and thus ultimately provide an enhanced photocurrent. With regard to the absorption frequencies of the sensitizer, those having a peak in their absorption spectrum at shorter than around 600 nm wavelength will be highly suitable. Alternatively, since this suitable absorption range will generally correspond to a band-gap greater than that of the polymer, a LHA dye of band gap of at least 1.5 eV (e.g. between 1.5 and 4 eV), preferably at least 2.0 eV. such as at least 2.1 eV will be highly suitable. Since the excitation energy will preferably be transferable onto the polymer charge carrier, it will be desirable that the band gap of the sensitizer be greater than that of at least one of the charge carrying polymers.

With regard to the transfer of energy from the sensitizer to the polymer charge carrier, a typical mechanism would be described by Forster Resonant Energy Transfer (FRET). FRET is a nonradiative energy transfer process facilitated through dipole-dipole coupling of an energy donor-acceptor pair through an electric-dipole field. Forster showed that the energy transfer rate (½-) from a donor (D) to an acceptor (A) is related to the overlap of the emission of the donor and the absorption of the acceptor, the alignment of the dipoles of the donor and acceptor and the distance between hem as [2],

where ² is a measure of the orientation of the donor and acceptor dipoles, QD is the fluorescence quantum yield of the donor,

is the natural lifetime of the donor excited state, NA is Avogadro's number, n is the refractive index of the medium within which the donor and acceptor are embedded, rDA is the distance between donor and acceptor, FD k) is the normalised fluorescence intensity of the donor, and εΑ is the acceptor absorbivity. For a specific system, this simplifies to, k - ¹

ET - 6~ ' (2) ' ^{■ '} ·

^{K T}D ^rDA where R₀ is the D_A distance at which the energy transfer rate from the donor equals the natural decay rate of the donor, termed the Forster radius. This relationship between the Forster resonant energy transfer (FRET) rate and distance between the donor and acceptor is valid for a point-to-point interaction in a dilute environment. The extent of this coupling can be quantified with the Forster radius Ro, which describes the donor-acceptor separation at which this process is 50% probable.

In order to allow FRET to be an effective method of energy transfer between the sensitizer and the polymeric charge carrier it will therefore be necessary for the emission of the LHA dye and the absorption of the charge carrier to overlap. Such an emission is evidently a feature of the nature of the LHA dye and represents the spectrum of emission which would take place (e.g. due to fluorescence) if the excitation energy of the sensitizer were not being transferred by FRET. Thus, where an absorption spectrum is the spectral absorption of a species in the absence of energy transfer and an emission spectrum is the spectral emission from a species in the absence of energy transfer, then to achieve effective FRET, there should be at least partial overlap between the emission spectrum of the LHA dye and the absorption spectrum of the semiconducting polymer.

In view of the above, the LHA dyes suitable for use in all aspects of the present invention will preferably be chosen by reference to the polymeric charge transport material being employed. Thus that the emission spectrum of the sensitizer preferably overlaps at least partially with the absorption spectrum of the polymeric charge carrier (or at least one polymeric charge carrier where more than one may be present). LHA dye will thus preferably have an absorption spectrum with at least one peak at shorter wavelength than the maximum absorption of the polymeric charge transport material. LHA dyes will also preferably have at least one region of their emission spectrum which overlaps with at least one region of the absorption spectrum of the polymeric charge transporter. Evidently, more than one LHA may be chosen and in such a case, each LHA will have an emission spectrum overlapping with the absorption spectrum of the charge transporter and/or at least one other LHA, and at least one LHA will have an emission spectrum overlapping with the absorption spectrum of the charge transporter. Thus a cascade of FRET energy transfer can be set up allowing as much as possible of the absorbed energy to transfer onto the polymeric charge transporter and thus be converted into photocurrent. The most commonly used light sensitising materials in electrolytic DSCs are organic or metal-complexed dyes. These have been widely reported in the art and the skilled worker will be aware of many existing dye-sensitizers, all of which are suitable in all appropriate aspects of the invention and consequently are reviewed here only briefly.

With regard to the distribution of the LHA dye material, this will, in one preferred embodiment, be substantially uniform within the at least one polymeric charge carrier (transporter). Correspondingly, it will be preferable that the concentration of LHA dye within 0.5 nm of the junction between the n-type material and the p-type material (all being as defined in any aspect herein) will be no more than 10% greater than the overall concentration of the LHA dye within the bulk of the polymeric charge carrier material(s).

A common category of organic dye sensitizers are indolene based dyes, of which D102, D131 and D149 (shown below) are particular examples.

The general structure of indolene dyes is that of Formula si below:

Formula si

wherein R1 and R2 are independently optionally substituted alkyl, alkenyl, alkoxy, heterocyclic and/or aromatic groups, preferably with molecular weight less than around 360 amu. Most preferably, R1 will comprise aralkyl, alkoxy, alkoxy aryl and/ or aralkenyl groups (especially groups of formula C_xH_yO_z where x, y and z are each 0 or a positive integer, x+z is between 1 and 16 and y is between 1 and 2x +1 ) including any of those indicated below for R1 , and R2 will comprise optionally substituted carbocyclic, heterocyclic (especially S and/or N-containing heterocyclic) cycloalkyl, cycloalkenyl and/or aromatic groups, particularly those including a carboxylic acid group. All of the groups indicated below for R2 are highly suitable examples. One preferred embodiment of R2 adheres to the formula C_xH_yO_zN_vS_w where x, y, z, v and w are each 0 or a positive integer, x+z+w+v is between 1 and 22 and y is between 1 and 2x+v+1 . Most preferably, z≥2 and in particular, it is preferable that R2 comprises a carboxylic acid group. These R1 and R2 groups and especially those indicated below may be used in any combination, but highly preferred combinations include those indicated below:

Dye Name R1

Indolene dyes are discussed, for example, in Horiuchi et al. J Am. Chem. Soc. 126 12218-12219 (2004), which is hereby incorporated by reference.

A further common category of sensitizers are ruthenium metal complexes, particularly those having two bipyridyl coordinating moieties. These are typically of formula sll below

wherein each R1 group is independently a straight or branched chain alkyl or oligo alkoxy chain such as C_nH₂n₊i where n is 1 to 20, preferably 5 to 15, most preferably 9, 10 or 1 1 , or such as C-(-XCnH2n-)m- CpH₂p₊i , where n is 1 , 2, 3 or 4, preferably 2, m is 0 to 10, preferably 2, 3 or 4, p is an integer from 1 to 15, preferably 1 to 10, most preferably 1 or 7, and each X is independently O, S or NH, preferably O; and wherein each R2 group is independently a carboxylic acid or alkyl carboxylic acid, or the salt of any such acid (e.g. the sodium, potassium salt etc) such as a

C_nH_2nCOOY group, where n is 0, 1 , 2 or 3, preferably 0 and Y is H or a suitable metal such as Na, K, or Li, preferably Na; and wherein each R3 group is single or double bonded to the attached N (preferably double bonded) and is of formula CHa- Z or C=Z, where a is 0, 1 or 2 as appropriate, Z is a hetero atom or group such as S, O, SH or OH, or is an alkyl group (e.g. methylene, ethylene etc) bonded to any such a hetero atom or group as appropriate; R3 is preferably =C=S. A preferred ruthenium sensitizer is of the above formula sll, wherein each R1 is nonyl, each R2 is a carboxylic acid or sodium salt thereof and each R3 is double- bonded to the attached N and of formula =C=S. R1 moieties of formula sll may also be of formula sill below:

Formula sill

Ruthenium dyes are discussed in many published documents including, for example, Kuang et al. Nano Letters 6 769-773 (2006), Snaith et al. Angew. Chem. Int. Ed. 44 6413-6417 (2005), Wang et al. Nature Materials 2, 402-498 (2003), Kuang et al. Inorganica Chemica Acta 361 699-706 (2008), and Snaith et al. J Phys, Chem. Lett. 112 7562-7566 (2008), the disclosures of which are hereby incorporated herein by reference, as are the disclosures of all material cited herein.

Other sensitizers which will be known to those of skill in the art include Metal- Phalocianine complexes such as zinc phalocianine PCH001 , the synthesis and structure of which is described by Reddy et al. (Angew. Chem. Int. Ed. 46 373-376 (2007)), the complete disclosure of which (particularly with reference to Scheme 1 ), is hereby incorporated by reference.

Some typical examples of metal phthalocyanine dyes suitable for use in the present invention include those having a structure as shown in formula sIV below:

Formula sIV

Wherein M is a metal ion, such as a transition metal ion, and may be an ion of Co, Fe, Ru, Zn or a mixture thereof. Zinc ions are preferred. Each of R1 to R4, which may be the same or different is preferably straight or branched chain alkyl, alkoxy, carboxylic acid or ester groups such as C_nH₂n₊i where n is 1 to 15, preferably 2 to 10, most preferably 3, 4 or 5, with butyl, such as tertiary butyl, groups being particularly preferred, or such as OX or C0₂X wherein X is H or a straight or branched chain alkyl group of those just described. In one preferred option, each of R1 to R3 is an alkyl group as described and R4 is a carboxylic acid C0₂H or ester C0₂X, where X is for example methyl, ethyl, iso- or n-propyl or tert-, iso-, sec-or n- butyl. For example, dye TT1 takes the structure of formula sIV, wherein R1 to R3 are t-butyl and R4 is C0₂H.

Further examples of suitable categories of dyes include Metal-Porphyrin

complexes, Squaraine dyes, Thiophene based dyes, fluorine based dyes, molecular dyes and polymer dyes. Examples of Squaraine dyes may be found, for example in Burke et al., Chem. Commun. 2007, 234, and examples of polyfluorene and polythiothene polymers in McNeill et al., Appl. Phys. Lett. 2007, 90, both of which are incorporated herein by reference. Metal porphyrin complexes include, for example, those of formula sV and related structures, where each of M and R1 to R4 can be any appropriate group, such as those specified above for the related phthalocyanine dyes:

Formula sV

Squaraine dyes form a preferred category of dye for use in the present invention. The above Burke citation provides information on Squaraine dyes, but briefly, these may be, for example, of the following formula sVI

Formula sVI

Wherein any of R1 to R8 may independently be a straight or branched chain alkyl group or any of R1 to R5 may independently be a straight or branched chain alkyloxy group such as C_nH₂n₊i or C_nH₂n₊iO respectively where n is 1 to 20, preferably 1 to 12, more preferably 1 to 9. Preferably each R1 to R5 will be H, C_nH₂n₊i or C_nH₂n₊iO wherein n is 1 to 8 preferably 1 to 3 more preferably 1 or two. Most preferably R1 is H and each R5 is methyl. Preferably each R6 to R8 group is H or C_nH_2n+i wherein n is 1 to 20, such as 1 to 12. For R6, n with preferably be 1 to 5 more preferably 1 to 3 and most preferably ethyl. For R7 n will preferably be 4 to 12, more preferably 6 to 10, most preferably 8, and for R8, preferred groups are H, methyl or ethyl, preferably H. One preferred squaraine dye referred to herein is SQ02, which is of formula sVI wherein R1 and R8 are H, each of R2 to R5 is methyl, R6 is ethyl, and R7 is octyl (e.g. n-octyl).

A further example category of valuable sensitizers are polythiophene

(e.g.dithiophene)-based dyes, which may take the structure indicated below as formula sVII

Formula sVII

Wherein x is an integer between 0 and 10, preferably 1 , 2, 3, 4 or 5, more preferably 1 , and wherein any of R1 to R10 may independently be hydrogen, a straight or branched chain alkyl group or any of R1 to R9 may independently be a straight or branched chain alkyloxy group such as C_nH_2n+i or C_nH_2n+iO respectively where n is 1 to 20, preferably 1 to 12, more preferably 1 to 5. It is preferred that each if R1 to R10 will independently be a hydrogen or C_nH_2n+i group where n is 1 to 5, preferably methyl, ethyl, n- or iso-propyl or n-, iso-, sec- or t-butyl. Most preferably, each of R2 to R4 will be methyl or ethyl and each of R1 and R6 to R10 will be hydrogen. The group R1 1 may be any small organic group (e.g. molecular weight less than 100) but will preferably be unsaturated and may be conjugated to the extended pi-system of the dithiophene groups. Preferred R1 1 groups include alkenyl or alkynyl groups (such as C_nH_2n-i _and C_nH_2n_₃ groups respectively, e.g. where n is 2 to 10, preferably 2 to 7), cyclic, including aromatic groups, such as substituted or unsubstituted phenyl, piridyl, pyrimidyly, pyrrolyl or imidazyl groups, and unsaturated hetero-groups such as oxo, nitrile and cyano groups. A most preferred R1 1 group is cyano. One preferred dithiophene based dye is 2-cyanoacrylic acid-4- (bis-dimethylfluorene aniline)dithiophene, known as JK2.

Other types of light absorbing antenna species may also be utilised, such as inorganic films or nanoparticles. Where present, in one embodiment, only a single dye sensitizer will be employed in the p-n heterojunctions herein described (and thus all compatible aspects of the invention), and this may serve to increase absorption in regions of the spectrum where the absorption of the polymer material is relatively low. In an alternative embodiment, two or more dye sensitizers may nevertheless be used. For example, all aspects of the present invention are suitable for use with co-sensitisation using a plurality of (e.g. at least 2, such as 2, 3, 4 or 5) different sensitizing agents, including dye sensitizing agents. If two or more dye sensitizers are used, these may be chosen such that their respective emission and absorption spectra overlap. In this case, resonance energy transfer (RET) results in a cascade of transfers by which an electron excitation steps down from one dye to another of lower energy, from which it is then injected into the n- type material. Where two or more dye sensitizers are used, these will preferably have complimentary absorption characteristics. Some complimentary parings of polymers with dyes include, for example, the near-infra red absorbing low band gap polymers referred to herein in combination with indoline or ruthenuim-based sensitizers to absorb the bulk of the visible radiation.

A particularly suitable group of light absorbing antenna species for use in the present invention has been recently developed and is formed around a

spirobifluorene core of formula S1 :

SI Wherein R is an aromatic group conjugate to the fluorene core such as at least one benzothiadiazole and/or at least one thiophene group each optionally substituted with one or more alkyl groups, such as C₄ to C₈ straight or branched chain alkyl groups. Conjugated thiophenyl-benzothiadiazolyl-thiophenyl groups having at least one C₄ to C₈ alkyl substituent form particularly preferred R groups, where the most preferred R group is of fromula S2:

S2

Wherein each R₂ group is independently a C₄ to C₈ alkyl group, such as an n-butyl, n- pentyl, n-hexyl, n-heptyl or n-octyl group, or any branched equivalent, such as the equivalent sec-, tert-, or iso-alykyl groups, n-hexyl groups are most preferred.

A highly preferred light harvesting antenna for use in the present invention is

2,2',7,7'-tetrakis(3-hexyl-5-(7-(4-hexylthiophen- 2-yl)benzo[c][1 ,2,5]thiadiazol-4- yl)thiophen-2-yl)- 9,9'-spirobifluorene) (spiroTBT) having formula S3:

S3 It is notable that in one preferred embodiment of the present invention the light harvesting antenna moiety does not serve to directly generate charge at the heterojunction as is the previously known function of sensitizing agent in solar cells such as dye sensitized solar cells. Rather, in the present invention, the light harvesting antenna preferably serves solely or primarily as an "antenna" for the polymeric charge transporter, allowing a wider spectrum of light to be absorbed effectively.

In one particularly preferred embodiment of the present invention, a low-band-gap p-type material such as those described herein (e.g. PCPDTBT) may be used in combination with a light harvesting antenna of formula S1 (e.g. spiro TBT).

In a highly preferred embodiment, the combination of a low-band-gap p-type material such as those described herein (e.g. PCPDTBT) may be used in combination with a LHA of formula S1 (e.g. spiro TBT) may be used in generating bulk heterojunction solar cells in combination with a polymeric electron transporter (such as [6,6]-phenyl C₆i butyric acid methyl ester (PC61 BM) and/or [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) ).

In a further highly preferred embodiment, the combination of a low-band-gap p-type material such as those described herein (e.g. PCPDTBT) may be used in combination with a LHA of formula S1 (e.g. spiro TBT) may be used in generating planar polymer-oxide solar cells in combination with at least one n-type metal oxide (such as a planar ZnO layer n-type material).

In a further highly preferred embodiment, the combination of a low-band-gap p-type material such as those described herein (e.g. PCPDTBT) may be used in combination with a LHA of formula S1 (e.g. spiro TBT) may be used in generating mesoporous polymer-oxide solar cells in combination with at least one mesoporous layer of n-type metal oxide (such as a mesoporous Ti0₂ layer n-type material).

As referred to in the present context, a p-type polymer is a material which exhibits good hole-transport characteristics and functions as a hole-transporter in the operating heterojunction (especially solar cell). Its function is to 1 . Accept energy absorbed by the LHA moieties. 2. Transfer an electron from photoexcitations directly on the p-type polymer or absorbed from the LHA to the n-type material resulting in generated charge. 3. Transport the holes remaining on the polymer to the cathode and into the external circuit.

Conversely, an n-type polymer is a material exhibiting good electron-transport characteristics and which can function as an electron transporting material in a heterojunction (such as a solar cell). This may have corresponding functions in that it may 1 . Accept energy absorbed by the LHA moieties. 2. Transfer a hole from photoexcitations directly on the n-type polymer (or molecules) or absorbed from the LHA to the p-type material resulting in generated charge, or accept electrons from the photo-excited p-type material, again generating charge. 3. Transport the electrons on the n-type polymer (or molecules) to the anode and into the external circuit.

In several preferred aspects of the present invention, a polymerised material is used as the p-type material of the heterojunction or device. There are a number of polymeric p-type materials which have been used previously in reported polymer oxide solar cells and any of these may be used. Preferably, the polymeric p-type material is an organic polymer selected from poly fluorenes, poly carbazolenes, poly thiophenes, poly selophenes, polythiadiazoles, poly thienopyrazines, poly p- phenylene vinylenes, poly thieneylene vinylenes, poly(thienylenevinylenes) and mixtures, copolymers and derivatives thereof. Highly suitable polymers include those disclosed in Kroon Et Al. (Polymer Reviews 48, 531 -582 (2008), which is hereby incorporated by reference. In particular, the polymers set out in Table 1 and compounds 1 to 10 (thiophene and thioselenophene polymers), Table 2 (donor- acceptor polymers), Table 3 and Figs 12 to 14 (Poly thenylene vinylene polymers), Table 4 (Thienylene vinylene copolymers), Table 5 and Fig 15 (fluorene polymers) and in Table 6 and Figs 16-18 (carbazolene polymers) are highly suitable.

Suitable conducting polymers preferably have a band gap of 2 eV or less, particularly preferably 1 .9 eV or less. Particularly effective examples of conducting polymers include poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(3,7- dimethyloctyloxy)]-1 ,4-phenylenevinylene) (MDMO-PPV), poly(2,5-bis(3- alkylthiophen-2-yl)thieno[3,2-b]thiophene), and poly[(9,9-dioctylfluorenyl-2,7-diyl)- alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (APFO-3). Low band gap polymers are particularly preferred in the present invention.

Examples of low band gap polymers include poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1 -b;3,4-b0] dithiophene)-alt-4,7-(2,1 ,3-benzothiadia-zole)]

(PCPDTBT), APFO-Green5.

The alternating co-polymers described and disclosed in Zhang et al. (Advanced Materials 18, 2169-2173 (2006)) are highly appropriate and are hereby incorporated by reference. In particular, the structure of APFO-Green5 is given in Figure 1 and its synthesis in the experimental section thereof.

Further information on PCPDTBT may be found from Lee et al. J. AM. CHEM. SOC. 2008, 130, 3619-3623, Chen et al., Macromolecules, Vol. 43, No. 2, 2010 -D and Muhlbacher et al. Adv. Mater. 2006, 18, 2884. These are again

incorporated herein by reference.

By "band-gap" as used herein in the context of organic semiconductors is intended the energy of the transition from the lowest energy singlet ground state (S₀) to first excited state (Si), "low band gap" in this context thus refers to having a low energy for this transition and "high band gap" refers to a high energy transition. Typical transition energies for "low band gap" organic semiconductors may be, for example, less than 2.0 eV (e.g. 1 .45 eV to 2.0 eV), preferably less than 1 .9 eV, more preferably 1 .8 eV or lower. 1 .7 eV or lower is highly preferred (e.g. less than 1 .6 eV or less than 1 .5 eV). A corresponding definition of the band-gap applies for sensitizer molecules although these will preferably have a higher band gap as described herein.

Where present, the optional electron acceptor moieties which may be present in one embodiment for the present invention and are believed to allow the transfer of energy absorbed by the p-type polymer into useful electrical energy by promoting charge transfer to the n-type material. Such functionalised electron acceptor moieties typically comprise at least one 2-dimensional or 3-dimensional network of carbon atoms, optionally including up to 10% of heteroatoms, functionalised by at least one organic group having affinity for the n-type material surface.

Fullerenes are particularly suitable as the 2-dimensional or 3-dimensional network of carbon atoms referred to herein and may comprise heteroatoms as indicated herein. Fullerenes as indicated herein may be in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also known as buckyballs, cylindrical ones are called carbon nanotubes or buckytubes. All such fullerenes are suitable in the present invention. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings. Single graphene sheets and small numbers of stacked graphene sheets (e.g. 2-5) are also suitable in the present invention. Molar masses of 200 to 10000 g/mol would be typical for the electron accepting moieties, preferably 300 to 3000 and more preferably 500 to 1200.

The electron accepting moieties of the present invention are "functionalised" in that they are bound to at least one organic group that is not part of the fullerene-type 2- dimensional or 3-dimensional atom network. Such functional groups will preferably serve to provide an affinity for the n-type material and thus may vary depending upon the n-type material used. Typical groups include polar groups such as carboxylic acids, phosphonic acids, cyano acrylic acids, thioacids, esters, amides, hydroxyl, hydroxamate, thiol and amine groups.

In several preferred aspects, the n-type semiconductor material for use in the solid state heterojunctions (e.g. polymer oxide solar cells) relating to the present invention may be any of those which are well known in the art. Oxides of Ti, Zn, Sn, and mixtures thereof are among those suitable. Ti0₂ and Sn0₂ are common examples, as is ZnO, . The n-type material is used in the form of a layer and will typically be either planar or mesoporous, the former providing the thinnest and most easily fabricated solar cells and the latter providing a relatively thick layer of around 0.05 - 100 μηι over which electron transfer may take place.

In one optional but preferred embodiment, a thin surface coating of a high band-gap / high band gap edge (insulating) material, may be deposited on the surface of a lower band gap n-type semiconductor such as Sn0₂. This can greatly reduce the fast recombination from the n-type electrode, which is a much more severe issue in solid state DSCs than in the more widely investigated electrolyte utilising cells. Such a surface coating may be applied before the oxide particles (e.g. Sn0₂) are sintered into a film or after sintering. The n-type material of the solid state heterojunctions relating to all appropriate aspects of the present invention (e.g. polymer oxide solar cells) is generally a metal compound such as a metal oxide, compound metal oxide, doped metal oxide, selenide, teluride, and/or multicompound semiconductor, any of which may be coated as described above. Suitable materials include single metal oxides such as Al₂0₃, ZrO, ZnO, Ti0₂, Sn0₂, Ta₂0₅, Nb₂0₅, W0₃, W₂0₅, ln₂0₃, Ga₂0₃, Nd₂0₃, Sm₂0₃, La₂0₃, Sc₂0₃, Y₂0₃, NiO, Mo0₃, PbO, CdO and/or MnO; compound metal oxides such as Zn_xTi_yO_z, ZrTi0₄, ZrW₂0₈, SiAI0₃,₅, Si₂AI0₅,₅, SiTi0₄ and/or AITi0_5; doped metal oxides such as any of the single or compound metal oxides indicated above doped with at least one of Al, F, Ge, S, N, In, Mg, Si, C, Pb and/or Sb;

carbides such as Cs₂C₅; sulphides such as PbS, CdS, CuS; SnS, SbS selenides such as PbSe, CdSe; SnSe, SbSe telurides such as CdTe; nitrides such as TiN; and/or multicompound semiconductors such as CIGaS₂.

It is common practice in the art to generate p-n heterojunctions, especially for optical applications, from a mesoporous layer of the n-type material so that light can interact with the junction at a greater surface than could be provided by a flat junction. In the present case, this mesoporous layer may be conveniently generated by sintering of appropriate semiconductor particles using methods well known in the art and described, for example in Green et al. (J. Phys. Chem. B 109 12525-12533 (2005)) and Kay et al. (Chem. Mater. 17 2930-2835 (2002)), which are both hereby incorporated by reference. With respect to the surface coatings, where present, these may be applied before the particles are sintered into a film, after sintering, or two or more layers may be applied at different stages.

Typical particle diameters for the semiconductors will be dependent upon the application of the device, but might typically be in the range of 5 to 1000nm, preferably 10 to 100 nm, more preferably still 10 to 30 nm, such as around 20 nm. Surface areas of 1 -1000 m²g^"1 are preferable in the finished film, more preferably 30-200 m²g^"1 , such as 40 - 100 m²g^"1. The film will preferably be electrically continuous (or at least substantially so) in order to allow the injected charge to be conducted out of the device. The thickness of the film will be dependent upon factors such as the photon-capture efficiency of the photo-sensitizer, but may be in the range 0.05 - 100 μηι, preferably 0.5 to 20 μηι, more preferably 0.5 -10 μηι, e.g. 1 to 5 μηι. Typical thicknesses include 0.05 to 30 μηι, preferably 1 to 5 μηι, more preferably 2 to 3 μηι. In one alternative embodiment, the film is planar or substantially planar rather than highly porous, and for example has a surface area of 1 to 20 m²g^"1 preferably 1 to 10 m²g^"1. Such a substantially planar film may also or alternatively have a thickness of 0.005 to 5 μηι, preferably 0.025 to 0.2 μηι, and more preferably 0.05 to 0.1 μηι.

Where the n-type material is surface coated, materials which are suitable as the coating material (the "surface coating material") may have a conduction band edge closer to or further from the vacuum level (vacuum energy) than that of the principal n-type semiconductor material, depending upon how the property of the material is to be tuned. They may have a conduction band edge relative to vacuum level of at around -4.8 eV, or higher (less negative) for example -4.8 or -4.7 to -1 eV, such as - 4.7 to -2.5 eV, or -4.5 to -3 eV

Suitable coating materials, where present, include single metal oxides such as MgO, Al₂0₃, ZrO, ZnO, Hf0₂, Ti0₂, Ta₂0₅, Nb₂0₅, W0₃, W₂0₅, ln₂0₃, Ga₂0₃, Nd₂0₃, Sm₂0₃, La₂0₃, Sc₂0₃, Y₂0₃, NiO, Mo0₃, PbO, CdO and/or MnO; compound metal oxides such as Zn_xTi_yO_z, ZrTi0₄, ZrW₂0₈, SiAI0₃,₅, Si₂AI0₅,₅, SiTi0₄ and/or AITi0_5; doped metal oxides such as any of the single or compound metal oxides indicated above doped with at least one of Al, F, Ge, S, N, In, Mg, Si, C, Pb and/or Sb;

carbonates such as Cs₂C₅; sulphides such as PbS, CdS, CuS; selenides such as PbSe, CdSe; telurides such as CdTe; nitrides such as TiN; and/or multicompound semiconductors such as CIGaS₂. Some suitable materials are discussed in Gratzel (Nature 414 338-344 (2001 )). The most preferred surface coating material is MgO.

Where present, the coating on the n-type material will typically be formed by the deposition of a thin coating of material on the surface of the n-type semiconductor film or the particles which are to generate such a film. In most cases, however, the material will be fired or sintered prior to use, and this may result in the complete or partial integration of the surface coating material into the bulk semiconductor. Thus although the surface coating may be a fully discrete layer at the surface of the semiconductor film, the coating may equally be a surface region in which the semiconductor is merged, integrated, or co-dispersed with the coating material.

Since any coating on the n-type material may not be a fully discrete layer of material, it is difficult to indicate the exact thickness of an appropriate layer. The appropriate thickness will in any case be evident to the skilled worker from routine testing, since a sufficiently thick layer will retard electron-hole recombination without undue loss of charge injection into the n-type material. Coatings from a monolayer to a few nm in thickness are appropriate in most cases (e.g. 0.1 to 100 nm, preferably 1 to 5 nm).

The bulk or "core" of the n-type material in all embodiments of the present invention may be essentially pure semiconductor material, e.g. having only unavoidable impurities, or may alternatively be doped in order to optimise the function of the p-n- heterojunction device, for example by increasing or reducing the conductivity of the n-type semiconductor material or by matching the conduction band in the n-type semiconductor material to the excited state of the chosen sensitizer.

Thus the n-type semiconductor and oxides such as Ti0₂, ZnO, Sn0₂ and W0₃ referred to herein (where context allows) may be essentially pure semiconductor (e.g. having only unavoidable impurities). Alternatively they may be doped throughout with at least one dopant material of greater valency than the bulk (to provide n-type doping) and/or may be doped with at least one dopant material of lower valency than the bulk (to give p-type doping), n-type doping will tend to increase the n-type character of the semiconductor material while p-type doping will tend to reduce the degree of the natural n-type state (e.g. due to defects).

Such doping may be made with any suitable element including F, Sb, N, Ge, Si, C, In, InO and/or Al. Suitable dopants and doping levels will be evident to those of skill in the art. Doping levels may range from 0.01 to 49% such as 0.5 to 20%, preferably in the range of 5 to 15%. All percentages indicated herein are by weight where context allows, unless indicated otherwise.

In addition to the n-type, p-type and optional sensitizer components, an optional by preferable ionic material such as a lithium salt may also be included in all aspects of the present invention. In one embodiment therefore, this ionic additive will be present. In a more preferable embodiment, this ionic additive will be present and will comprise a lithium salt or compound. Particularly preferable ionic additives are lithium salts such as lithium perchlorate or ionic liquids, such as 1 -Ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 -Allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 ,3-Dihydroxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 ,3-Dimethoxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide. Mixtures of such materials are also highly suitable.

The invention is illustrated further in the following non-limiting examples and in the attached Figures, in which:

Figure 1 a - is a schematic diagram of a polymer oxide solar cell formed with a mesoporous Ti0₂ n-type semiconductor material infiltrated with a semiconducting polymer.

Figure 1 b - shows a schematic energy level diagram and processes occurring in a polymer oxide solar cell during operation, hv indicates light absorption, following light absorption, exciton migration occurs until the exciton undergoes natural decay, or reached the heterojunction with the n-type metal oxide. At this junction electron transfer can occur from the exciton to generate free charge carriers (electrons in the metal oxide and holes in the hole transporter. Ag referes to the silver cathode. Terminology: CB = conduction band; HOMO = highest occupied molecular orbital, LUMO=lowest unoccupied molecular orbital; FTO = Fluorine doped tin oxide.

Figure 2a - shows absorption spectra of a thin film of PCPDTBT, Spiro-TBT and of the PCPDTBT:spiro-TBT blend, exhibiting good spectral coverage of all the visible and near IR spectral region.

Figure 2b - shows molecular structures of PCPDTBT (top panel) and of spiro- TBT (bottom panel).

Figure 2c - shows a schematic energy level diagram for a ZnO/spiro-TBT:

PCPDTBT solar cell highlighting charge generation and energy transfer pathways. Figure 3 - shows pictures from left to right of neat spiroTBT, neat PCPDTBT and the blended spiroTBT:PCPDTBT at a 1 :0.4 blend ratio.

Figure 4 - shows the absorption spectra of a thin film of SpiroTBT spin casted from a 100mg/ml_ solution in CB, and of PCPDTBT. The PL spectrum of spiroTBT is also shown.

Figure 5a - shows photoluminescence spectra (after excitation at 540nm and

700nm) of the neat spiroTBT, the neat PCPDTBT and the

PCPDTBT:spiroTBT system (1 :0.4).

Figure 5b - shows excitation spectra for the neat PCPDTBT and for the

PCPDTBT:spiro-TBT blend.

Figure 6 - is a schematic picture of the flat layer devices, or a bulk

heterojunction device, where the active layer is composed of either neat polymer(PCPDTBT), polymer and LHA dye (spiroTBT) blend (flat layer device of this invention), or polymer and LHA dye and electron acceptor (PC60BM) blend (bulk heterojunction device of this invention).

Figure 7a - shows External Quantum Efficiency measurements for

ZnO/PCPDTBT, ZnO/spiroTBT:PCPDTBT, and

ZnO/spiroTBT:PCPDTBT:PCBM devices.

Figure 7b - shows photocurrent-voltage traces of ZnO/PCPDTBT,

ZnO/spiroTBT:PCPDTBT and ZnO/spiroTBT:PCPDTBT:PCBM devices.

Figure 8 - shows transient absorption measurements on the ZnO/PCPDTBT,

ZnO/spiro-TBT and ZnO/PCPDTBT:spiro.TBT blend. Example 1 - Solar cell fabrication 1 .1 Materials

A low band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1 -b;3,4- bO] dithiophene)-alt-4,7-(2,1 ,3-benzothiadiazole), (PCPDTBT), was used as the hole transporter in solid-state hybrid solar cells. This material has been the first candidate of a new class of copolymers for organic photovoltaics utilizing a cyclopentadithiophene unit as the donor block in the polymer chain. It shows improved charge-transport properties, mobility values as high as 2 ^χ 10^"2 cm²V^"1s^"1 and good processability (see D. Muhlbacher Adv. Mater. 18: 2884-2889, 2006 and J. Peet Nat Mater 6: 498-500, 2007). The ideal energy gap of PCPDTBT of Eg = 1 .46 eV and the improved light harvesting in the near-infrared region make it a good candidate for solar photovoltaic applications. The combination of these properties allowed efficiencies up to 5.5% when blended with PC₇₁ BM in standard bulk heterojunction solar cells, with short-circuit current values up to 16 mAcm^"2 and a high EQE approaching 50% over the spectral range from 400 to 800 nm (D.

Muhlbacher and J. Peet supra).

The absorption spectrum of the PCPDTBT is shown by the lower line in Figure 2, it peaks around 700nm, with a minor peak around 400nm. Its molecular structure is presented in the inset.

Example 1 - Devices

Planer devices:

Fluorine doped tin oxide (FTO) coated glass sheets were etched with zinc powder and HCI (2 Molar) to obtain the required electrode pattern. The sheets were then washed with soap (Hellmanex at 2% in water), de-ionized water, acetone, methanol and finally treated under an oxygen plasma for 10 minutes to remove the last traces of organic residues. The FTO sheets were subsequently coated with a compact layer of ZnO (100 nm) by aerosol spray pyrolysis deposition at 400 ^QC on a Zn powder solution diluted in Methanol, using air as the carrier gas. PCPDTBT was dissolved in chlorobenzene at 30mg/ml_ concentration, heated at 70 ^QC for 1 hour and stirred overnight and spin coated on the substrate at 1000 rpm for 45 seconds. Optionally, Spiro-TBT was added to the PCPDTBT solution at a range of concentrations. The films were then placed in a thermal evaporator where 150 nm thick silver electrodes were deposited through a shadow mask under high vacuum (10-6 mbar), to give rectangular cells with an active area of ~ 0.12cm2.

Bulk heterojunctions

Fluorine doped tin oxide (FTO) coated glass sheets were etched with zinc powder and HCI (2 Molar) to obtain the required electrode pattern. The sheets were then washed with soap (Hellmanex at 2% in water), de-ionized water, acetone, methanol and finally treated under an oxygen plasma for 10 minutes to remove the last traces of organic residues. The FTO sheets were subsequently coated with a compact layer of ZnO (100 nm) by aerosol spray pyrolysis deposition at 400 ^QC on a Zn powder solution diluted in Methanol, using air as the carrier gas. To create the bulk heterojunction, an electron donating semiconducting polymer is blended with an electron acceptor and alight harvesting antenna dye. The semiconducting polymer,for instance poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1 -b;3,4-b0] dithiophene)-alt-4,7-(2,1 ,3-benzothiadia-zole)] ( PCPDTBT), is dissolved in chlorobenzene at a concentration of 30mg/ml. PC60BM is dissolved in

chlorobenzene at a concentration of 40mg/ml. the light harvesting antenna dye, for instance spiroTBT, is dissolved in chlorobenzene at a concentration of 40mg/ml. The three solutions are then mixed at volume ratios of 4:4.2

PC60BM:PCPTBT:SpiroTBT. The ZnO coated FTO substrated are cooled,and coated with the polymer-molecular blend solution via spin-coating at 100rpm for 60 seconds. The coated substrates are then placed in a thermal evaporator to deposit 150nm thick Ag electrodes under high vacuum, which completed the device.

Mesoporous hybrid solar cells

Fluorine doped tin oxide (FTO) coated glass sheets (15 Ω/D Pilkington) were etched with zinc powder and HCI (2 Molar) to obtain the required electrode pattern. The sheets were then washed with soap (Hellmanex at 2% in water), de-ionized water, acetone, methanol and finally treated under an oxygen plasma for 10 minutes to remove the last traces of organic residues. The FTO sheets were subsequently coated with a compact layer of Ti0₂ (100 nm) by aerosol spray pyrolysis deposition at 450^QC, using air as the carrier gas. The standard Dyesol Ti0₂ paste was previously diluted down 1 :3.5 in anhydrous ethanol and stirred and ultrasonicated until completely mixed. The paste was then doctor-bladed by hand using scotch tape and a pipette on the Ti0₂ compact layer coated FTO sheets to get a Ti0₂ average thickness of 2μηι. The sheets were then slowly heated to 550^QC (ramped over 1 ½ hours) and baked at this temperature for 30 minutes in air. After cooling, slides were cut down to size and soaked in a 15 mM of TiCI₄ in water bath and oven-baked for 1 hour at 70^QC. After rinsing in water, ethanol and drying in air, they were subsequently baked once more at 550^QC for 45 minutes in air, then cooled down.

Optionally, the films were cooled to 70^QC and introduced into a dye solution for 1 hour. The indolene dye used was D131 in a 1 :1 volume ratio of tert-butanol and acetonitrile at 0.3 mM concentration. The dyed films were briefly rinsed in acetonitrile and dried in air for 1 minute.

Optionally, the Ti0₂ substrates, optionally dye sensitized, were immersed into a 5 ml_ solution of an electron acceptor self-assembling molecule to form a self assembledmonolayer (SAM) on the Ti0₂ surface. C₆₀-SAMs are preferably used. And left in solution for a set period of time (from 1 min up to 240 min). The samples were then removed from the solution and thoroughly rinsed with THF:CB to remove any excess unbound molecules and dried under a nitrogen stream. Samples were annealed at 140^QC for 5 minutes to improve the uniformity of the chemical bonding of the SAM to the Ti0₂ surface.

The synthesis of C60-substituted carboxylic acid is reported elsewhere (see S. K. Hau et al. supra). Functionalized C60-SAMs with carboxylic acid end-group can be formed onto the surface of dye-sensitized Ti0₂ by a solution immersion technique. First, a 1 mM solution containing the C₆o-SAM molecules in a 1 :1 (v:v) cosolvent in tetrahydrofuran:chlorobenzene (THF:CB) is prepared. The solutions are filtered through a 0.2 μηι PTFE filter prior to immersion of the samples into the solution.

Optionally, a solution of lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) at 0.01 to 0.12 M concentration in acetonitrile was prepared to treat the dye and C₆₀- SAM sensitized Ti0₂ surface prior to PCPDTBT spin-coating. A small quantity of Li- TFSI solution (25 μΙ) was dispensed onto each substrate and left to wet the films for 20 sees before spin-coating at 1000 rpm for 60 sees in air.

PCPDTBT was dissolved in chlorobenzene at 30mg/ml_ concentration, heated at 70 ^QC for 1 hour and stirred overnight and spin coated on the substrate at 1000 rpm for 45 seconds. Optionally, SpiroTBT was added to the PCPDTBT solution at a range of concentrations.

The films were then placed in a thermal evaporator where 150 nm thick silver electrodes were deposited through a shadow mask under high vacuum (10^~6 mbar), to give rectangular cells with an active area of -0.12cm².

Device testing

During testing under simulated sun light, the active areas of the devices were defined by single aperture metal optical masks with a round aperture of 2.5 mm giving an area of 0.0491 cm². We note that all light was excluded from entering the sides of the devices by having them in a "light-tight" sample holder, and the only light entering the solar cell substrate was through the single mask aperture.

Example 2 - Analysis of absorption spectra

The absorption spectrum of spiro-TBT, shown in Figure 3, fits perfectly into the minimum of PCPDTBT absorption, and has a molar extinction coefficient of 38 000 cm^"1 M^"1 at 510 nm. Additionally, the photoluminescence peaking at around 640 nm, is entirely encompassed by the strongly absorbing region of PCPDTBT, setting this system up to be ideally designed for resonant energy transfer.

The optical properties of spiro-TBT and PCPDTBT are near ideal for a FRET energy donor-acceptor pairs with their cooperative absorption entirely spanning the spectrum from the Uv to the nearlR. The energy landscape of the device is depicted in Figure 3b. This schematic also outlines the possible pathways to photovoltaic action after excitation of spiro-TBT. On the basis of the relative offset of the HOMO and LUMO levels, charge generation from light absorbed in the spiro-TBT is possible via hole transfer from spiro-TBT to PCPDTBT and electron transfer to the acceptor, here depicted as PCBM, ZnO or Ti0₂. This electron transfer from spiro- TBT to the acceptor may occur through one of two routes: either direct electron transfer or a cascaded mediated by PCPDTBT. Alternatively, resonant energy transfer from the spiro-TBT excitation to PCPDTBT and subsequent electron transfer to the acceptor may lead to a photoresponse through the standard polymer solar cell mechanism, where the spiro-TBT is purely acting an antennae to harvest the solar energy and channel it into the charge generating system. In Figure 4 the absorption of a PCPDTBT:spiro-TBT (1 :0.4) blend is shown, and in Figure 5 photographs of films of neat spiro-TBT, neat PCPDTBT and the spiro- TBT:PCPDTBT (1 :0.4) blend are shown, very convincingly demonstrating the panchromatic absorption in the "black" blend.

Example 3 - FRET - Photoluminescence and Pump Probe measurements

To verify that FRET occurs in this system between these two molecules, we examined the fluorescence spectra of films of neat spiro-TBT, neat PCPDTBT, and PCPDTBT:spiro-TBT blend. Figure 6 shows the relative PL of all samples for an excitation wavelength of 540 nm. It should be noted that the detection limit of the apparatus at 850nm is just past the emission onset of PCPDTBT so we can only detect the rise in the emission from PCPDTBT and not the peak. The spiro-TBT shows a strong emission peaking at 650nm. This emission is entirely quenched when blended with PCPDTBT and the emission of the blend closely follows that of the neat PCPDTBT film. The excitation spectra of the neat PCPDTBT and the blend films, probing the emission at 850nm is also shown in Figure 6. The signal is noisy due to the relatively low emission at 850nm, however, it is clear that for the neat polymer film there is a dip in the PLE spectrum at 500nm, corresponding to the trough in the absorption spectrum, and for the blend film the PLE spectra follows the absorption spectra remarkably well with the peak in the excitation spectra at 500nm corresponding to the peak of the spiro-TBT absorption. The comparable intensity of the PLE peaks at 500nm (spiro-TBT) and 700nm (PCPDTBT), and the close match to the absorption spectra, indicates a near to unity energy transfer efficiency in this system. Example 4 - Device Testing

In order to verify to role of the spiro-TBT as second absorber in polymer based solar cells we have constructed three types of solar cells, hybrid devices based on planar junctions between ZnO and PCPDTBT, hybrid solar cells based on mesoporous Ti0₂ infiltrated with PCPDTBT and bulk heterojunction solar cells employing PC60BM as the electron acceptor and PCPDTBT as the donor.

Schematic illustrations of the device structures are shown in Figure 7.

The solar cell characteristics of the devices with EQEs are shown in Figure 8, evaluated at AM 1 .5G solar conditions. Figure 8a shows the external quantum efficiencies (EQE) versus wavelength of fully processed devices (FTO conductive glass/compact ZnO spiro- TBT/PCPDTBT/PCBM/Ag electrode, as schematically illustrated in Figure 7). We note the emergence of a broad response exceeding 4% in the 450-550 nm visible region of the spectrum with blending the spiroTBT, corresponding to the absorption band of spiroTBT. Note the lack of this band for the pristine PCPDTBT, used as a reference (black empty squares).

We observe a huge increase in short-circuit current due to the enhanced collection of visible photons. With respect to the pristine PCPDTBT, giving a

Jsc~0.17mA/cm2, we obtain an increase of almost 60% due the light harvesting in the spiroTBT.

CONCLUSIONS

We have demonstrated a unique approach toward improving the optical absorption and the efficiencies of polymer based devices by the integration of secondary absorber which has the ability to participate in both energy transfer and charge transfer. The spectral compatibility of spiroTBT and PCPDTBT is near optimal and we have verified very efficient energy transfer through a survey of the

photoluminescence properties of the FRET pair including emission and excitation profiles and pump probe decay dynamics. Incorporation of the optimal amount of spiroTBT into PCPDTBT solar cells results in a significant increase in the photocurrent (60%) when the spiroTBT is added to the PCPDTBT based flat solar cells and more then 20% increase in power conversion efficiency due to the collection of high energy photons via FRET and cascaded electron transfer. As demonstrated, the multipath enhancement offered in this device architecture results in an increased and extended photoresponse with respect to the individual materials employed and with further engineering of suitable donor-acceptor pairs and optimization of charge separation in conjugated molecular blends has the potential to become a continuing avenue toward enhancing polymer based solar cell efficiencies.

Example 5 - Further device testing - pump-probe spectroscopy Methods

Pump-probe spectroscopy: In a typical pump-probe experiment, the system under study is photoexcited by a short pump pulse and its subsequent dynamical evolution is detected by measuring the transmission changes ΔΤ of a delayed probe pulse as a function of pump-probe delay τ and probe wavelength λ. The signal is given by the differential transmission ΔΤ/Τ [=(T_pumpon-T_pumpoff)/T_pumpoff]. The pump probe set-up is driven by 1 kHz repetition rate pulse train at λ= 780 nm centre wavelength with 150 fs duration coming from a regeneratively amplified

modelocked Tksapphire laser (Clark-MXR Model CPA-1 ). A fraction of this beam is used to pump a non-collinear optical parametric amplifier (NOPA) capable of delivering ultra-broadband pulses in the visible (500 - 700 nm). Details of the NOPA used can be found elsewhere [5]. In the present work we used the NOPA in narrowband configuration to obtained tunable visible pulses with spectral width of 20 nm and time-duration of 180 fs without dispersion compensation. Another small fraction of the Ti: sapphire amplified output is independently focused into a 1 -mm- thick sapphire plate to generate a stable single-filament white-light supercontinuum which serves as a probe pulse. A short-pass filter with 760-nm cutoff wavelength is used to filter out the residual 800 nm pump light thus limiting our probing window to the 450-760 nm region. The pump and probe beams are spatially and temporally overlapped on the sample, controlling the time delay by motorized slit. The minimum detectable signal is ΔΤ/Τ-10^"4. The system has a -150 fs temporal resolution. Details of the experimental set-up can be found elsewhere (e.g. Cerullo, G., Luer, L, & Polli, D., Time-resolved methods in biophysics. 4. Broadband pump- probe spectroscopy system with sub-20 fs temporal resolution for the study of energy transfer processes in photosynthesis. Photochem. Photobiol. Sci. 6, 133- 144 (2007); Lanzani , G. et al., Photophysics of conjugated polymers: the contribution of ultrafast spectroscopy. Phys. Stat. Sol. (a) 201 , 1 1 16-1 131 (2004).). The pump beam density energy used in the experiment is kept deliberately low (10- 50 nJ energy, 300μηι beam size). All the measurements were taken with the samples in a vacuum chamber, to prevent any influence from oxygen or sample degradation. The pump-probe measurements were taken on PCPDTBT and spiroTBT film spin cast on the ZnO substrates, fabricated following device procedure.

Figure 8a shows the pump probe spectrum of the neat PCPDTBT after excitation at 780 nm, corresponding to the absorption peak of the PCPDTBT. The primary photoexcitation dynamics of the PCPDTBT in the visible probe spectral region show two main contributions, both coming from singlets states. The instantaneously photogenerated positive band around 600-800 nm is attributed to photobleaching (PB) and stimulated emission (SE), while the negative band at shorter wavelength (around 450nm) is due to the singlet exciton photo induced absorption (PA), decaying in few hundred ps. Figure 8b represents the primary photoexcitation dynamics of the neat spiroTBT films. It shows a photobleaching (PB) signal at shorter wavelengths, a small charge band feature around 560nm and a SE band at 630nm. The PA at longer wavelength is assigned to singlet exciton absorption, decaying in a few hundred ps. Figure 8c shows the pump probe spectrum of the PCPDTBT:spiroTBT blend. Though the spiro-TBT is being predominantly photoexcited (pump at 500 nm), the spectra appears much more similar to the PCPDTBT spectra. Notably, the photobleaching signal at 700nm, grows in over the first 500fs to 1 ps, in contrast to the neat PCPDTBT film where the photobleaching is already at a maximum within the earliest time window. This is consistent with ultrafast energy transfer from the photoexcited spiroTBT to the PCPDTBT. The exciton decay lifetime (¾) of a solid film of SpiroTBT, as probed in Figure 3b is in the order of 100ps. From the dynamics represented in Figure 8d, probing at 740nm, we extract the rise time of the PB signal of the PCPDTBT with a time constant x=240fs, which we assign to be due to energy transfer after excitation of the spiroTBT. Taking the resonant energy transfer lifetime (T_RET) to be 240fs, we can estimate the efficiency for energy transfer (η_Γ£ί) to be 99.8 % using

The data presented above appears to be consistent with ultrafast and a very efficient energy transfer in this system. Legend to Figures

Figure 1. a) A schematic illustration of a cross section of a polymer oxide solar cell using Ti0₂ as the n-type metal oxide and a semiconducting polymer as the hole transporter and light absorber, b) An illustrative energy level diagram for a polymer oxide solar cell.

Figure 2 Absorption spectra and chemical structure of the material used, sketch of the energy levels, a, Absorption spectra of a thin film of PCPDTBT (dashed line), Spiro-TBT (solid line) and of the PCPDTBT:spiro-TBT blend (circles), exhibiting good spectral coverage of all the visible and near I spectral region, b, molecular structures of PCPDTBT (top panel) and of spiro-TBT (bottom panel), c, Schematic energy level diagram for a ZnO/spiro-TBT: PCPDTBT solar cell highlighting charge generation and energy transfer pathways.

Figure 3 Pictures from left to right of the neat spiroTBT, neat PCPDTBT and the blended spiroTBT: PCPDTBT at a 1:0.4 blend ratio.

Figure 4 The absorption spectra of a thin film of SpiroTBT spin casted from a 100mg/mL solution in CB (light solid line) and of PCPDTBT (heavy solid line). PL spectrum of spiroTBT as dashed black line.

Figure 5 Emission spectra of the neat components and the blend, a,

Photoluminescence spectra (after excitation at 540nm and 700nm) of the neat spiroTBT (squares), the neat PCPDTBT (diamonds) and the PCPDTBT .spiroTBT system (1 :0.4) (circles), b, Excitation spectra for the neat PCPDTBT (diamonds) and for the PCPDTBT:spiro-TBT blend (circles).

Figure 6 Schematic Picture of the flat layer devices, or a bulk heterojunction device, where the active layer is composed of either neat polymer(PCPDTBT), polymer and LHA dye (spiroTBT) blend (flat layer device of this invention), or polymer and LHA dye and electron acceptor (PC60BM) blend (bulk heterojunction device of this invention). Figure 7. Comparison of Spectral response and Photocurrent-voltage traces for ZnO/PCPDTBT, ZnO/spiroTBT:PCPDTBT flat layer devices and

ZnO/spiroTBT:PCPDTBT:PCBM flat layer bulk heterojunction device. For all devices silver was used as the cathode, a. External Quantum Efficiency measure for ZnO/PCPDTBT (solid line), ZnO/spiroTBT:PCPDTBT (circles) and

ZnO/spiroTBT:PCPDTBT:PCBM (squares) devices, b, Photocurrent-voltage traces of ZnO/PCPDTBT (solid line), ZnO/spiroTBT.PCPDTBT (circles) and

ZnO/spiroTBT:PCPDTBT:PCBM (squares) devices measured under AM 1.5G simulated sun light of lOOmWcm^"2. In the inset of b the table showing the short circuit current for the presented devices.

Figure 8 Transient absorption measurements on the ZnO/PCPDTBT,

ZnO/spiro-TBT and ZnO/PCPDTBT:spiro.TBT blend. Time evolution of the pump-probe spectrum of a, ZnO/PCPDTBT cell, b, ZnO/spiro-TBT film, c,

ZnO/PCPDTBT:spiro-TBT blend. The pump wavelength has been tuned to 780nm to excite the neat PCPDTBT and to 500nm (resonant with the spiro:TBT main peak) for the eat spiro-TBT and PCPDTBT: spiro-TBT devices, d, pump-probe dynamics at 740nm probe wavelength of the neat spiroTBT (squares) and of the PCPDTBT:spiro-TBT blend (circles). The solid line represents the fit for the rising component of the blend dynamic, from which a time constant of 240fs is extracted.

Claims

Claims:

1 ) A solid-state p-n heterojunction comprising an n-type material in contact with a p- type material wherein at least one of said n-type material and said p-type material comprises an organic polymeric charge transporter having a low band gap and wherein said heterojunction further comprises a light harvesting antenna (LHA) material having a band gap of no less than 1.5 eV and greater than the band gap of said organic polymeric charge transporter.

2) A solid-state p-n heterojunction as claimed in claim 1 wherein said light harvesting annena is capable of transferring excitation energy to said organic polymeric charge transporter.

3) A solid state p-n heterojunction as claimed in any of claims 1 or 2 wherein the light harvesting antenna is distributed substantially uniformly throughout the organic polymeric charge transporter.

4) A solid-state p-n heterojunction as claimed in any of claims 1 to 3 wherein said polymeric charge transporter has a low band gap of no more than 1.9 eV, preferably no more than 1.7 eV.

5) A solid state p-n heterojunction as claimed in any preceding claim wherein said LHA agent comprises at least one dye selected from a spirobifluorene centred dye, a spiro centred dye, a ruthenium complex dye, a metal-phalocianine complex dye, a metal-porphryin complex dye, a squarine dye, a thiophene based dye, a fluorene based dye, a polymer dye, and mixtures thereof.

6) A solid state p-n heterojunction as claimed in any preceding claim wherein said sensitizing agent comprises at least one LHA agent of formula S1 :

wherein R is an aromatic group conjugate to the spirobifluorene core such as at least one benzothiadiazole and/or at least one thiophene group each optionally substituted with one or more alkyl groups, such as C₄ to C₈ straight or branched chain alkyl groups.

7) A solid state p-n heterojunction as claimed in any preceding claim wherein said sensitizing agent comprises at least one sensitizing agent of formula S1 as defined in claim 5 and each R group is independently a conjugated thiophenyl- benzothiadiazolyl-thiophenyl group having at least one C₄ to C₈ alkyl substituent, each R group is preferably of formula S2:

Wherein each R₂ group is independently a C₄ to C₈ alkyl group, such as an n-butyl, n-pentyl, n-hexyl, n-heptyl or n-octyl group, or any branched equivalent, such as the equivalent sec-, tert-, or iso-alykyl groups, n-hexyl groups are most preferred. 8) A solid state p-n heterojunction as claimed in any preceding claim wherein said sensitizing agent comprises 2,2',7,7'-tetrakis(3-hexyl-5-(7-(4-hexylthiophen- 2- yl)benzo[c][1 ,2,5]thiadiazol-4-yl)thiophen-2-yl)- 9,9'-spirobifluorene) (spiro-TBT).

9) A solid-state p-n heterojunction as claimed in any preceding claim wherein said heterojunction comprises at least one ionic additive.

10) A solid-state p-n heterojunction as claimed in claim 9 wherein said ionic additive is a lithium salt such as lithium bis(trifluoromethylsulphonyl)imide salt (Li-TFSI) or Li perchlorate.

11 ) A solid-state p-n heterojunction as claimed in claim 9 or claim 10 wherein said ionic additive is a mixture of a metal salt and an ionic liquid.

12) A solid-state p-n heterojunction as claimed in any of claims 9 to 11 wherein said ionic additive comprises or consists of an ionic liquid, selected from 1 -Ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -Butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 -Allyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 ,3-Dihydroxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide, 1 ,3-Dimethoxy-2-methylimidazolium

bis(trifluoromethylsulfonyl)imide and mixtures thereof.

13) A solid-state p-n heterojunction as claimed in any preceding claim wherein said p-type material is an organic polymer selected from poly thiophenes, poly p- phenylene vinylenes, mixtures, derivatives and copolymers thereof, most preferably PCPDTBT.

14) A solid-state p-n heterojunction as claimed in any preceding claim wherein said n-type semiconductor material comprises at least one single metal oxide, compound metal oxide, doped metal oxide, carbonate, sulphide, selenide, teluride, nitrides and/or multicompound semiconductor, most preferably Ti0₂, Sn0₂ or ZnO.

15) A solid-state p-n heterojunction as claimed in claim 14 wherein said n-type semiconductor material is in the form of a planar or substantially planar layer. 16) A solid state p-n heterojunction as claimed in any preceding claim wherein said n-type material has a surface area of 1-1000 m²g^"1 and preferably in the form of an electrically continuous layer.

17) A solid state p-n heterojunction as claimed in any preceding claim wherein said n-type material has a thickness of 0.05 to 30 μΐη, preferably 1 to 5 μίπ, more preferably 2 to 3 μΐη.

18) A solid state p-n heterojunction as claimed in any preceding claim wherein said n-type material is a polymeric charge transporter.

19) A solid state p-n heterojunction as claimed in any preceding claim wherein said n-type material is a polymeric or molecular electron transporter such as a substituted fullerene.

20) A solid state p-n heterojunction as claimed in any preceding claim wherein said n-type material is a fullerene butyric acid ester such as [6,6]-phenyl C₆i butyric acid methyl ester (PC61BM) and/or [6,6]-phenyl C₇₁ butyric acid methyl ester (PC71 B ).

21 ) A solid state p-n heterojunction as claimed in any preceding claim in the form of a polymeric bulk heterojunction.

22) An optoelectronic device comprising at least one solid state p-n

heterojunction as claimed in preceding claim.

23) An optoelectronic device as claimed in claim 22 wherein said device is a solar cell, phototransistor or photo-detector, preferably a solid state dye sensitised solar cell and/or solid state polymer sensitised solar cell.

24) Use of a light harvesting antenna dye having a band gap of no less than 1.5 eV to enhance the light absorption of at least one organic polymeric charge transporter having a low band gap (of no more than 1.9eV) wherein the band gap of the sensitizer is greater than the band gap of said organic polymeric charge transporter.

25) The use as claimed in claim 24 wherein the sensitizer is capable of transferring excitation energy to said organic polymeric charge transporter.

26) The use as claimed in claim 24 or claim 25 in a heterojunction as defined in any of claims 1 to 23.

27) A method for the manufacture of a solid-state p-n heterojunction comprising an organic p-type material in contact with a layer of inorganic n-type material, said method characterised by; a) coating an anode, preferably a transparent anode (e.g. a Fluorine doped Tin Oxide - FTO anode) with a compact layer of an n-type inorganic semiconductor material (such as any of those described herein);

b) forming a planar or porous (preferably mesoporous) layer of an n-type semiconductor material (such as any of those described herein) on said compact layer,

c) optionally forming at least a partial monolayer of functionalised electron acceptor moieties (as described herein) and optionally sensitizing agents on the surface of the porous n-type layer;

d) optionally treating said compact layer and/or said porous layer of n-type material with at least one ionic material such as a lithium salt.

e) optionally forming a porous barrier layer of an insulating material on said porous layer of n-type material;

f) contacting said planar or porous layer of n-type material with at least one solution of at least one low band-gap polymeric organic p-type semiconductor material and at least one sensitizer (having a band gap as described herein) whereby to form a layer of polymeric organic p-type semiconductor having distributed therein said at least one sensitizer;

g) optionally treating said layer produced in "f by coating on top with a solution of ionic material, such as Li-TFSI.

h) forming a cathode, preferably a metal cathode (e.g. a silver or gold cathode) in contact with said p-type semiconductor material. 28) An optoelectronic device such as a photovoltaic cell, phototransistor or light sensing device comprising at least one solid-state p-n heterojunction formed or formable by the method of claim 27.