A photosensitizer complex of formulae (Ia) MX3Lt or (Ib) MXYLt in which M is a transition metal selected from ruthenium, osmium, iron, rhenium and technetium, preferably ruthenium or osmium; each X is a co-ligand independently selected from NCS?-, Cl-, Br-, I-, CN-, NCO-, H¿2O, NCN2-, pyridine unsubstituted or substituted by at least one group selected from vinyl, primary, secondary or tertiary amine, OH and C¿1-30? alkyl, preferably NCS?-¿; and L¿t? is a tridentate ligand comprising heterocycles such as pyridine, thiophene, imidazole, pyrazole, triazole, carrying at least one carboxylic, phosphoric, hydoxamic acid or a chelating group. A photovoltaic cell comprising an electrically conductive layer deposited on a support to which at least one titanium dioxide layer has been applied and, as a photosensitizer applied to the titanium dioxide layer, a photosensitizer complex of formulae (Ia) or (Ib) as specified above.