TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Material: (I) comprises cerium dioxide (CeO2) doped with lanthanum (La3+), praseodymium (Pr3+ or Pr4+) or terbium (Tb3+ or Tb4+) cations. (I) comprises crystalline domains with dimensions of 1-100 nm. (I) has a BET surface area of 80-300 m2/g and a particle size of 1-50 nm. (I) is in the form of a film with a thickness of 0.01-100 mum. Preferred Device: The photovoltaic device is a solar cell comprising a liquid or gel electrolyte or conductive polymer between (I) and a counterelectrode. The electrolyte contains a self-regenerating redox system, especially I-/I3-, I-/I2/I3-, Fe2+/Fe3+, or Fe2+/Fe3+ in association with thiocyanate or cyanide ligands. (I) is in the form of a film on a transparent conductive support. The device includes an organic dye to improve the efficiency of the photovoltaic effect. The dye is a thiazine, toluidine blue, methylene blue, azure A, phenosaphrin or a derivative thereof, neutral red, fluorescein or a derivative thereof (e.g. erythrosin, rhodamine, eosin or rose bengal), pyronine, a metal phthalocyanine, a metal porphyrin, a coumarin (e.g. coumarin 6) or a ruthenium polypyridine complex EXAMPLE - A mesostructured material was produced using the process of example 1 in WO20010049606. The material (10 g) was impregnated with a 0.93 M lanthanum nitrate solution (6.2 ml). The resulting paste was air-dried overnight and calcined at 400degreesC for 6 hours. The cycle of impregnation, drying and calcination was repeated twice more, giving a product with a lanthanum/cerium molar ratio of 0.3. This (20 mg) was mixed with 1/10 acetylacetone/water (20 mul) and coated onto a transparent indium tin oxide (ITO) electrode, which was then dried at 25degreesC for 48 hours and calcined at 400degreesC for 6 hours. A solution (0.1 cm3) containing 0.5 M sodium iodide and 0.05 M iodine was deposited on the electrode, covered with an aluminum counterelectrode and sealed with epoxy resin. When exposed to sunlight, the resulting solar cell generated a current of 0.9 mA at 0.55 V.