NOVELTY - Substituted lanthanide compounds (I) are new.    USE - (I) are useful in a spectrum converter which is useful in a solar device (all claimed) e.g. photovoltaic cells and photoelectrolytic cells.    ADVANTAGE - (I) have an optimized quantum efficiency of luminescence with respect to the type of ligands coordinated with the metal and are therefore capable of increasing the number of photons effectively convertible to radiations which can be used by a solar device.    DETAILED DESCRIPTION - Substituted lanthanide compounds of formula (Ln (A1)x(B1)y(C1)z (L1L2) (I) or their mixtures are new.    Ln = a metal of the lanthanide series;    (A1), (B1), (C1) = a mono-anionic ligand comprising substituted beta -diketonates of formula ((R1-C(=O)-C(R2)=C(R3)-O)-) (II), substituted carboxylates of formula ((R1-C(=O)-O)-) (III), substituted alcoholates of formula ((R1-O)-) (IV), substituted amides of formula (((R1)(R3)N)-) (V) or substituted carbamates of formula ((R1-NH-C(=O)-O)-) (VI); either    R1 = aryl, heteroaryl, cycloalkyl, heterocyclic (all optionally substituted) or 1-20C alkyl (optionally containing heteroatoms); and    R2 = aryl, heteroaryl, cycloalkyl, heterocyclic (all optionally substituted), 1-20C alkyl (optionally containing heteroatoms), H, fluoro, chloro or bromo; or    R2R1 = a cycle containing 3-12C optionally substituted with aryl, heteroaryl, cycloalkyl, heterocyclic (all optionally substituted) or 1-20C alkyl (optionally containing heteroatoms), where the cycle optionally containing heteroatoms are O, S, N, Si, P or Se; either    L1, L2 = neutral ligands comprising phosphines, phosphine oxides, phosphates, sulfoxides, sulfones, amines, ethers, thioethers, ketones, ketals or carbonates; or    L1L2 = neutral polydentate ligands e.g. phenanthrolines, dipyridines, terpyridines and diquinolines (all optionally substituted with 1-12C alkyl (optionally containing heteroatoms) or aryl);    R3 = aryl, heteroaryl, cycloalkyl, heterocyclic (all optionally substituted) or 1-20C alkyl (optionally containing heteroatoms); and    x, y, z = 0-2 (where only one of x, y and z is 0 and the sum of x, y and z is 2 or 3 ).    INDEPENDENT CLAIMS are also included for preparation of (I).