TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The n-type layer is doped to an extent that electrons tunnel through the doped n-type layer. The n-type layer is doped with low work function fluorides such as lithium fluoride and cesium fluoride, and low work function salts, or cesium carbonate. The p-type layer is doped with ferric chloride, iodide or hydrogen peroxide. Preferred Components: The n-type layer comprises an n-type metal oxide consisting of zinc oxide (ZnO and ZnOx), titanium dioxide and/or titanium oxide (TiOx), where x is not defined. The p-type comprises a p-type metal oxide consisting of molybdenum trioxide, molybdenum oxide (MoOx), vanadium pentoxide, vanadium oxide (VOx), tungsten trioxide, tungsten oxide (WOx), nickel oxide (NiO and NiOx) and/or graphene oxide. Preferred Properties: The electron-extracting electrode is transparent. The p-type layer is closer to the electron-extracting electrode than the n-type layer. The p-type layer is in physical contact with the n-type layer.    TECHNOLOGY FOCUS - METALLURGY - Preferred Components: The n-type layer is doped with a low work function metal such as sodium, lithium and aluminum.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The polymer used in either the first or second bulk hetero-junction polymer semiconductor layers is benzo(1,2-b;4,5-b')dithiophene compound of formula (II). The device is a polymer solar cell device comprising a bulk heterojunction structure. The bulk heterojunction structure comprises ingredient(s) in addition to the conjugated polymer comprising a fullerene structure. The ingredient(s) is chosen from (6,6)-phenyl C61 butyric acid methyl ester, (6,6)-phenyl C71 butyric acid methyl ester or indene-C60 bisadduct. The photovoltaic device further contains an electron transporting layer between the electron extracting electrode and the first bulk polymer semiconductor layer or second bulk hetero-junction polymer semiconductor layer, and a hole transporting layer between the hole extracting electrode and the first bulk hetero-junction polymer semiconductor layer or second bulk hetero-junction polymer semiconductor layer. The conjugated polymer material is a photovoltaic material. Preferred Properties: The first hetero-junction polymer semiconductor layer is closer to the transparent electrode than second bulk hetero-junction polymer semiconductor layer. The polymer used in the first bulk hetero-junction polymer semiconductor layer has a wider bandgap that the polymer used in the second bulk hetero-junction polymer semiconductor layer. The polymer used in the first bulk hetero-junction polymer semiconductor layer has an absorbance maximum of 400-700 nm and the polymer used in the second hetero-junction polymer semiconductor layer has an absorbance maximum of 700-1000 nm. The absorbance maximum of the polymer used in the first bulk hetero-junction polymer semiconductor layer is more than 20 nm shorter than the absorbance maximum of the polymer used in the second bulk hetero-junction polymer semiconductor layer. The wavelength of the trailing edge at 60% of the maximum absorbance of the absorbance spectrum between 300 nm and 1000 nm of the polymer used in the first bulk hetero-junction polymer semiconductor layer is shorter than or equal to the wavelength of the leading edge at 60% of the maximum absorbance the absorbance spectrum between 300 nm and l000 nm of the polymer used in the second bulk hetero-junction polymer semiconductor layer.    TECHNOLOGY FOCUS - POLYMERS - Preferred Method: The p-type layer is doped with poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate). Preferred Definitions:    R1=2-ethyihexyl;    R3=2-butyloctyl;    Ar1,Ar2=16 five membered heterocyclic compounds e.g. of formulae (i-iii), 16 six membered heterocyclic compounds e.g. of formulae (iv-vi), benzo(1,2,5)thiadiazole moiety of formula (vii), benzo(1,2,5)oxadiazole moiety of formula (viii), benzo(1,2,5)selenadiazole moiety of formula (ix), (1,2,5)thiadiazolo(3,4-c)pyridine moiety of formula (x), (1,2,5)oxadiazolo(3,4-c)pyridine moiety of formula (xi), (1,2,5)selenadiazolo(3,4-c)pyridine of formula (xii) or 2-thia-1,3,5,8-tetraaza-cyclopenta(b)naphthalene moiety;and    R=proton, F, CF3, CN, NO2 or 1-18C alkyl.