NOVELTY - Fullerene derivatives (I) are new.    USE - (I) are useful in a photoactive layer of a solar cell, preferably an organic solar cell. The composition is useful for producing a thin film such as a thin film of a photovoltaic cell, preferably an organic photovoltaic cell (all claimed).    ADVANTAGE - (I) provide considerable improvement in the open circuit voltage and in the energy conversion efficiency of organic solar cells compared to the reference systems, possess good compatibility with different electron donor components e.g. conjugated polymers and oligomers thus resulting in the optimal composite morphology, and have appropriate physical properties e.g. sufficient solubility in organic solvent required for solution processing, sufficient thermal stability and photostability to enable long lifetimes of organic solar cells. The light weight thin film has good mechanical flexibility thus allowing one to adapt them to any curved surfaces.    DETAILED DESCRIPTION - Fullerene derivatives of formula (I) are new.    C2n = carbon cage of 60C or 70C fullerene bearing the pendant cyclopropane-type addend;    X = 2-20C-alkyl, 2-20C-fluorinated alkyl, 2-20C-alkylether, 2-20C-alkoxy, 2-20C-fluoroalkoxy, 2-20C-alkylthio group, 2-20C-fluoroalkylthio group, 5-20C-alkylenearyl, 5-20C-alkyleneheteroaryl (all optionally substituted), H or halo;    Y1 = 1-20C-alkyl, 1-20C-fluorinated alkyl, 1-20C-alkylether, 1-20C-fluorinated alkylether, 5-20C-alkylenearyl, 5-20C-alkyleneheteroaryl, phenyl (all optionally substituted), phenyl (optionally substituted at positions 2 and 4, at positions 2 and 5, or at positions 2, 3 and 4), 2-thienyl group (substituted at position 5), (CH2)1-20COOR group, or H;    R = 1-20C-alkyl, 1-20C-fluorinated alkyl, 1-20C-alkylether or 1-20C-fluorinated alkylether (all optionally substituted); and    R1, R2 = 3-20C-alkyl, 3-20C-fluorinated alkyl, 2-20C-alkylether, 2-20C-fluorinated alkylether, 5-20C-alkylenearyl, 5-20C-alkyleneheteroaryl or (CH2)1-20COOR1.    INDEPENDENT CLAIMS are included for:    (1) a composition comprising 0.1-99.9999% of at least one (I); and    (2) a photovoltaic cell comprising at least two electrodes, preferably a cell comprising two electrodes, and a functional layer sandwiched in between, where the functional layer contains 1-99% of (I).