NOVELTY - Homo-aggregates of rylene tetracarboxylic acid diimides (I), which is partially in the form of J-aggregates, are new.    USE - (I) are useful: as emitter materials, charge transport material or exciton transport material; as an active material in organic photovoltaics and excitonic solar cells; in organic electronics, preferably in organic field-effect transistors; and in optical label and invisible marking of products. (I) are useful as: fluorescent dyes in a fluorescent conversion based display and for chemiluminescence based applications; fluorescent labels for biomolecules; pigments in coloring electrophoretic displays; light conducting synthetic material that combines with a solar cell; and an emitter in organic light-emitting diode (all claimed).    ADVANTAGE - (I) exhibits high fluorescence quantum yield.    DETAILED DESCRIPTION - Homo-aggregates of rylene tetracarboxylic acid diimides of formula (I), which is partially in the form of J-aggregates, are new.    R(n1)-R(n4) = H, aryloxy, arylthio, hetaryloxy or hetarylthio, preferably substituted benzene compounds of formulae (a)-(c), substituted pyridine derivatives of formulae (d)-(f), substituted pyrimidine derivatives of formulae (g)-(i) or 2-methyl-(1,3,5)triazine compound of formulae (j)-(l);    x = 2 or 3 (where in compounds (j)-(l), x is 2);    A = bivalent bonded group comprising -O-, -S-, -O-arylene, -S-arylene, -O-arylene-C(=O)O-, -S-arylene-C(=O)O-, -O-arylene-OC(=O)- or -S-arylene-OC(=O)-, preferably -O-;    Ri = 4-30C alkyl, 4-30C alkoxy or 4-30C alkylthio, where the alkyl chain of alkyl-, alkoxy- or alkylthio substituents is interrupted by a non-adjacent oxygen atom, preferably methoxy group;    asterisk = bond to rylene residues; and    n = 1-4.    INDEPENDENT CLAIMS are included for:    (1) the preparation of (I);    (2) an organic field-effect transistor comprising a substrate with at least one gate structure, a source electrode and a drain electrode, and (I) as n-type semiconductor;    (3) a substrate comprising organic field effect transistors, where at least a portion of the field effect transistors contains (I) as n-type semiconductor; and    (4) a semiconductor device comprising a substrate.