NOVELTY - Monoadduct fullerene derivatives (I) are new.    USE - (I) are useful in a field-effect transistor device, which is useful in a photodetector, and in a photovoltaic device, which is useful in a solar cell and a solar window (all claimed).    ADVANTAGE - (I) have improved device efficiency and enhanced device stability.    DETAILED DESCRIPTION - Monoadduct fullerene derivatives (I) of formula (II) or (III) are new.    Ring Cn = fullerene core or trimetallic nitride endohedral fullerene core (M3N);    M = Ga, Sc, Ho, Tb, Gd, Dy, Tm or Lu;    D, D1, D2 = electron-rich moiety, preferably triphenyl amine, tetraphenyl biphenyl diamine, carbazole, fluorene, dibenzosilole, dibenzothiophene, dibenzothiophene-5,5'-dioxide, naphthalene, anthracene, 2,3,4-trisubstitued thiophene, thiophene oligothiophene, silole, selenophene, thieno(3,2-b)thiophene, selenolo(3,2-b)selenophene, cyclopentadithiophene, silolodithiophene, stannadithiophene, dithienopyrrole, benzo(1,2-b; 4,5-b')dithiophene, benzo(1,2-b; 4,3-b')dithiophene, phenothiazine, indenofluorene, indolocarbazole, 9-phenylcarbazole, 10-phenylacridine or N,N-diphenyl-4-(2-thienyl)-benzenamine; and    X = non-electron-deficient moiety.    INDEPENDENT CLAIMS are also included for:    (1) a field-effect transistor device (300) comprising at least one electron donor component and (I);    (2) a photovoltaic device comprising (I), where the photovoltaic device further comprises a first electrode, first charge-accepting layer disposed on a surface of the first electrode, and an active layer disposed on a surface of the first charge-accepting layer opposite the first electrode, where the active layer comprises at least one electron donor component and (I); a second charge-accepting layer disposed on a surface of the active layer opposite the first charge-accepting layer; and a second electrode disposed on a surface of the second charge-accepting layer opposite the active layer.    DESCRIPTION OF DRAWING(S) - The figure is a cross-sectional view of a bottom contact field-effect transistor device incorporating (I) in the active layer.    Field-effect transistor device (300)    Substrate (310)    Gate electrode (320)    Insulating layer (330)    Semiconductor layer (340)