Solar cells based on dye-sensitized mesoporous films of TiO2 are low- cost alternatives to conventional solid-state devices. Impressive solar-to- electrical energy conversion efficiencies have been achieved with such films when used in conjunction with liquid electrolytes. Practical advantages may be gained by the replacement of the liquid electrolyte with a solid charge- transport material. Inorganic p-type semiconductors and organic materials have been tested in this regard, but in all cases the incident monochromatic photon-to-electron conversion efficiency remained low. Here we describe a dye-sensitized heterojunction of TiO2 with the amorphous organic hole- transport material 2,2',7,7'-tetrakis(N,N-di-p-methoxyhenyl-amine)9,9'- spirobifluorene (OMeTAD; refs. 10 and 11). Photoinduced charge-carrier generation at the heterojunction is very efficient. A solar cell based on OMeTAD converts photons to electric current with a high yield of 33%.