A series of highly soluble fullerene derivatives with varying acceptor strengths (i.e., first reduction potentials) was synthesized and used as electron acceptors in plastic solar cells. These fullerene derivatives, methanofullerene [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM), a new azafulleroid, and a ketolactam quasifullerene, show a variation of almost 200 mV in their first reduction potential. The open circuit voltage of the corresponding devices was found to correlate directly with the acceptor strength of the fullerenes, whereas it was rather insensitive to variations of the work function of the negative electrode. These observations are discussed within the concept of Fermi level pinning between fullerenes and metals via surface charges.

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The current–voltage characteristics of methanofullerene [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM)‐based devices are investigated as a function of temperature. The occurrence of space–charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility of μe = 2 × 10–7 m2 V–1 s–1 has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor‐type conjugated polymer poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (OC1C10‐PPV). As a result, the dark current in PCBM/OC1C10‐PPV based devices is completely dominated by electrons. The observed field and temperature‐dependence of the electron mobility of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport‐site separation in PCBM.

Two crystal structures of PCBM, obtained from different crystallisation solvents, are presented; a proposed link with solvent dependence of the efficiency of MDMO-PPV:PCBM solar cells is described.

allowed to proceed for 3 h. The product was moved to a separation funnel and an excess of ethanol was added to remove the surfactants and other reagents.Characterization: FESEM images and EDX data were taken with a JEOL JSM-6700 F microscope with an EDX facility. The TEM images were observed with a JEOL EM-2000 EX II microscope. For the TEM analysis, the samples were deposited on a carbon mesh foil supported on a copper grid. The emission spectrum from the silica nanotubes was obtained with a Shimadzu RF-5301 PC spectrofluorophotometer at room temperature. The excitation wavelength for the emission spectrum was 350 nm, which was the maximum absorption wavelength of silica nanotubes dispersed in ethanol.