TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The n-type semiconductor material comprises at least one single metal oxide, compound metal oxide, doped metal oxide, carbonate, sulfide, selenide, telluride, nitrides and/or multi-compound semiconductor, preferably titanium dioxide. The n-type material is porous, preferably having a surface area of 1-1000 m2g-1 and in the form of electrically continuous layer having thickness 0.5-20 mu m. The n-type material is substantially planar and the heterojunction forms a substantially planar junction. The n-type material is oxide of titanium, zinc, tin and/or tungsten, where the n-type material is optionally surface coated. The n-type semiconductor material is essentially pure material or is doped throughout with at least one dopant material of greater valence than the bulk material (n-type doping) and/or is doped with at least one dopant material of lower valence than the bulk (p-type doping), where the n-type material is optionally surface coated.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The peak absorption wavelength of the donor sensitizing agent is shorter than that of any intermediate sensitizing agents and the peak absorption wavelength of the acceptor sensitizing agent is longer than that of any intermediate sensitizing agents. The donor sensitizing agent has a maximum absorbed photon to electron conversion efficiency of less than 40% in an equivalent heterojunction when used as sole sensitizing agent. The solid state p-n heterojunction comprises: organic p-type material in contact with n-type material, where the n-type material is surface-sensitized by two sensitizing agents comprising donor sensitizing agent and acceptor sensitizing agent, and the emission spectrum of the donor sensitizing agent overlaps with the absorption spectrum of the acceptor sensitizing agent; or organic p-type material in contact with n-type material, where the n-type material is surface-sensitized by at least three sensitizing agents comprising donor sensitizing agent, acceptor sensitizing agent and at least one intermediate sensitizing agent, the emission spectrum of the donor sensitizing agent overlaps with the absorption spectrum of the acceptor sensitizing agent and/or with the absorption spectrum of at least one intermediate sensitizing agent, and the emission spectrum of at least one intermediate sensitizing agent overlaps with the absorption spectrum of the acceptor sensitizing agent. The solid state p-n heterojunction is organic solid state p-n heterojunction. The use of at least two sensitizing agents in the solid state p-n heterojunction, generates increased charge transfer in comparison with any of the individual sensitizing agents used as sole sensitizer in equivalent heterojunction. The increased charge transfer occurs at least partially by resonant energy transfer between the donor sensitizer and the acceptor sensitizer, between the donor sensitizer and at least one intermediate sensitizer where present and/or between at least one intermediate sensitizer where present and the acceptor sensitizer.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: At least one of donor, acceptor and any intermediate sensitizing agent is organic dye, metal-complexed dye and/or quantum-dot photosensitizer. The dye is at least one ruthenium complex dye, metal-phthalocyanine complex dye, metal-porphyrin complex dye, squarine dye, thiophene based dye, fluorine based dye, polymer dye and/or quantum dot sensitizer. The solid state p-n heterojunction comprises a donor sensitizing agent and an acceptor sensitizing agent, where the donor and acceptor sensitizing agents correspond to a combination (Donor-Acceptor): indoline dye-(metal-phthalocyanine dye); indoline dye-squaraine dye; indoline dye-(metal-porphyrin sensitizer); indoline dye-lead(II) sulfide nanoparticles; indoline dye-lead selenide nanoparticles; (metal-ruthenium complex dye)-(metal-phthalocyanine dye); (metal-ruthenium complex dye)-squaraine dye; (metal-ruthenium complex dye)-(metal-porphyrin sensitizer); (metal-ruthenium complex dye)-lead(II) sulfide nanoparticles; (metal-ruthenium complex dye)-lead selenide nanoparticles; (metal-porphyrin complex sensitizer)-(metal-phthalocyanine dye); (metal-porphyrin complex sensitizer)-squaraine dye; (metal-porphyrin complex sensitizer)-lead(II) sulfide nanoparticles; (metal-porphyrin complex sensitizer)-lead selenide nanoparticles; polyfluorene polymer dye-(metal-phthalocyanine dye); polyfluorene polymer dye-squaraine dye; polyfluorene polymer dye-(metal-porphyrin sensitizer); polyfluorene polymer dye-lead(II) sulfide nanoparticles; polyfluorene polymer dye-lead selenide nanoparticles; polythiophene polymer-(metal-phthalocyanine dye); polythiophene polymer-squaraine dye; polythiophene polymer-(metal-porphyrin sensitizer); polythiophene polymer-lead(II) sulfide nanoparticles; or polythiophene polymer-lead selenide nanoparticles. The solid state p-n heterojunction comprises a donor sensitizing agent, at least one intermediate sensitizing agent and an acceptor sensitizing agent, where the donor, first intermediate and the acceptor sensitizing agents correspond to a combination (Donor-Intermediate-Acceptor): (cyano-(4-(4-(2,2-diphenyl-vinyl)-phenyl)-1,2,3,3a,4,8b-hexahydro-cyclopenta(b)indol-7-yl)-acetic acid)-((5-(4-(4-(2,2-diphenyl-vinyl)-phenyl)-1,2,3,3a,4,8b-hexahydro-cyclopenta(b)indol-7-yl)-4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid)-phthalocyanine dye of formula (VIII); indoline-indoline-indoline; indoline-indoline-(metal-phthalocyanine); indoline-indoline-squaraine dye; indoline-indoline-(metal-porphyrin); indoline-indoline-(lead(II) sulfide/lead selenide); indoline-ruthenium complex-(lead(II) sulfide/lead selenide); indoline-(metal-porphyrin)-(lead(II) sulfide/lead selenide); indoline-squaraine-(metal-phthalocyanine); indoline-(metal-phthalocyanine)-(lead(II) sulfide/lead selenide); indoline-squaraine-(lead(II) sulfide/lead selenide); ruthenium complex-(metal-phthalocyanine)-(lead(II) sulfide/lead selenide); ruthenium complex-squaraine-(lead(II) sulfide/lead selenide); or (metal-porphyrin)-squaraine-(lead(II) sulfide/lead selenide). The p-type material is an organic hole-transporter, preferably amorphous organic hole transporter. The organic hole-transporter is at least one optionally oligomerized, polymerized and/or cross-linked compound of formula ((R1a)x-A-(R1b)z) (I), ((R1a)x-)A)v(R1b)z-N-(R1c)w) (II), (-(-A((R5)x/z)-N(-A1-((R1)x/z)-A2((R2)x/z)-)n-) (III), (-(-A((R5)x/z)-N(-A1-((R1)x/z)-A2((R2)x/z)-N(-A3-((R3)x/z))-A4((R4)x/z)-)n-) (IV) and/or (-(-A((R5)x/z)-N(-A1-((R1)x/z)-A2((R2)x/z)-A3((R3)x/z)-N(-A4-((R4)x/z))-)n-) (V), preferably substituted N2,N2,N2',N2',N7,N7,N7',N7'-octaphenyl-9,9'-spirobi(fluorene)-2,2',7,7'-tetraamine compounds of formula (VII).A = mono- or polycyclic system comprising at least one pair of a conjugated double bond (-C=C-C=C-) (where the cyclic system optionally comprises at least one heteroatom, and is optionally substituted);A1-A4 = A;v = number of cyclic systems A linked by a single bond to N atom including 1-3;R1c = 1-30C hydrocarbyl (optionally substituted and optionally comprising at least one heteroatom);R1a = residue (optionally together with other R1a) capable of decreasing melting point of organic compound including optionally branched or cyclic alkyl or residue comprising at least one O atom (both are optionally halogenated);x = number of R1a linked to A including 0 to maximum possible number of substituents of a respective A or the number x of other residues R1a linked to another A optionally present; eitherR1b = optional residue enabling polymerization reaction with compounds comprising (I)-(IV) used as monomers and/or cross-linking between different compounds comprising (I)-(V); orR1bN, R1bA, R1bR1b = repeated cross-linked and/or polymerized moieties of (I)-(V);R1-R5 = R1a or R1b;z = number of R1b linked to A including 0, 1 and/or 2, or number z of other residues R1b linked to another A optionally present;M = transition metal ion including cobalt, iron, ruthenium or zinc ion;R1-R3 = tert-butyl;R4 = CO2H;R11 = 1-6C alkyl or 1-6C O-alkyl;n = 1-20; andw = 0-2.Provided that: (v+w) does not exceed 3; and each of (I)-(V) has at least one R1a, which is oxygen containing residue, and two or more R1a optionally form oxygen-containing ring. EXAMPLE - Fluorine doped tin oxide coated glass sheets were etched with zinc powder and hydrochloric acid (4 N) to give the required electrode pattern. The sheets were cleaned and treated under oxygen plasma for 10 minutes to remove any organic residues. The treated sheets were then coated with a compact layer of titanium dioxide (100 nm) by aerosol spray pyrolysis deposition at 450 degrees C using oxygen as the carrier gas and titanium-acetylacetonato dissolved in ethanol at 1:10 (vol/vol.) ratio as the precursor. The sheets were then slowly heated to 500 degrees C over 30 minutes and baked at this temperature for 30 minutes under oxygen flow. After cooling, the sheets were cut into slides of the required size and stored in the dark. The nano-porous films were soaked in aqueous solution of titanium tetrachloride (0.02 M) at 70 degrees C for 1 hour. After rinsing with deionized water and drying in air, the films were baked at 500 degrees C for 45 minutes under oxygen flow with subsequent cooling to 70 degrees C and placed in a dye solution for the specified times. 2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (180 mg/ml) was dissolved in chlorobenzene, at 100 degrees C for 30 minutes. The reaction solution was cooled and tert-butyl pyridine was added. Lithium bis(trifluoromethylsulfonyl)imide salt ionic dopant was pre-dissolved in acetonitrile (170 mg/ml), and then added to the above reaction solution. The dye coated mesoporous films were briefly rinsed in acetonitrile and dried in air for 1 minute. The films were then placed in a thermal evaporator where silver electrodes (150 nm) were deposited through a shadow mask under high vacuum to obtain solar cell.