TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The layer of an n-type semiconductor material comprising SnO2 having at least one surface-coating of the surface coating material is formed by sintering of a layer of fine SnO2 particles followed by surface coating of the sintered layer with the surface coating material. The porous layer of an n-type semiconductor material comprising SnO2 having at least one surface-coating of the surface coating material is formed by surface coating fine SnO2 particles with the surface coating material, followed by sintering of a layer of the coated SnO2 particles. The layer is a compact n-type semiconductor material comprising SnO2 coated with the surface coating material. Preferred Components: The surface coating material comprises at least one single metal oxide, compound metal oxide, doped metal oxide, carbonate, sulfide, selenide, teluride, nitrides and/or multicompound semiconductor (preferably magnesium oxide (MgO)). The SnO2 is coated with at least two materials and where the outermost coating closest to the p-type material is a surface-coating of the surface coating material. The SnO2 is additionally coated with at least one intermediate coating between the core SnO2 material and the surface coating of the surface coating material, where the intermediate coating comprises a material with a higher band-gap and/or a conduction band closer to the vacuum level than SnO2 and/or a material with a band gap similar to that of SnO2. The SnO2 is additionally coated with at least one intermediate coating between the core SnO2 material and the surface coating of the surface coating material, where the intermediate coating comprises a material with a lower band-gap and/or a conduction band further from the vacuum level than SnO2 and/or a material with a band gap similar to that of SnO2. The SnO2 is essentially pure SnO2 or is doped throughout with at least one dopant material of greater valency than Sn (greater than 4, n-type doping) and/or is doped with at least one dopant material of lower valency than Sn (less than 4 p-type doping, or reducing degree of the natural n-type state due to defects). The SnO2 is doped with at least one element selected from fluorine (F), antimony (Sb), nitrogen (N), germanium (Ge), silicon (Si), carbon (C) and/or aluminum (Al).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Heterojunction: The heterojunction is sensitized to light by additionally comprising at least one sensitizer, preferably at the junction of the p-type and n-type materials. The solid state p-n heterojunction has at least one intermediate coating of a material with a band gap similar to that of SnO2, such as 3 plus minus 1.5 eV; and has at least one intermediate coating of a material selected from a single metal oxide, compound metal oxide, doped metal oxide, carbonate, sulfide, selenide, teluride, nitrides and/or multicompound semiconductor. The heterojunction is an organic solid state p-n heterojunction. Preferred Components: The sensitizer is an organic dye, a metal-complexed dye and/or quantum-dot photosensitizers. The dye is at least one selected from ruthenium complex dye, metal-phalocianine complex dye, metal-porphryin complex dye, squarine dye, thiophene based dye, fluorine based dye, polymer dye and/or quantum dot sensitizer. 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 selected from 5 compounds as given in the specification e.g. compounds of formulae (I)-(III). The organic hole-transporter is a compound of formula (XVII). The n-type material is porous, preferably having a surface area of 1-400 m2g-1 and preferably in the form of a electrically continuous layer, especially of thickness 0.5-20 mu m. The n-type material is planar and the heterojunction forms a planar junction. The surface coating material has a band gap of 4.6-8 eV, and/or a conduction band edge of less than or equal to -4.8 eV negative relative to vacuum level.N=nitrogen atom;A=mono-, or polycyclic system comprising at least one pair of a conjugated double bond (-C=C-C=C-), the cyclic system optionally comprising at least one heteroatom, and optionally being substituted, whereby in a compound comprising several structures A;A1 and A2=A;v=number of cyclic systems A linked by a single bond to the nitrogen atom and is 1-3;Rw=optional residue selected from 1-30C hydrocarbon residue optionally substituted and optionally comprising at least one heteroatom;w=0-2;v+w= not greater than 3;R1a=residue capable, optionally together with other R1a present on the same structure (I-III), of decreasing the melting point of an organic compound and is selected from a linear, branched or cyclic alkyl or a residue comprising at least one oxygen atom, where the alkyl and/or the oxygen comprising residue is optionally halogenated;x=number of residues R1a linked to an A and is selected from 0 to a maximum possible number of substituents of a respective A, from the number x of other residues R1a linked to another A optionally present;two or more R1a=oxygen-containing ring;R1p=optional residue enabling a polymerization reaction with compounds comprising structure (I-III) used as monomers, and/or a cross-linking between different compounds (I-III);z=the number of residues R1p linked to an A and is 0-2, independently from the number z of other residues R1p linked to another A optionally present;((R1)a\p)x\z and ((R2)a\p)x\z=R1a or R1p; andR= 1-6C alkyl or 1-6C-O-alkyl.R1p can be linked to an N-atom, to an A and/or to a substituent R1p of other structures (I-III), resulting in repeated, cross-linked and/or polymerized groups of (I-III). Provided that per structure (I-III) there is at least one R1a being an oxygen containing residue. EXAMPLE - No suitable example given.