Diphenylmethanofullerenes:: New and efficient acceptors in bulk-heterojunction solar cells

A novel fullerene derivative, 1,1-bis(4,4'-dodecyloxyphenyl)-(5,6) C-61, diphenylmethanofullerene (DPM-12), has been investigated as a possible electron acceptor in photovolaic devices, in combination with two different conjugated polymers poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (C1C10-PPv) and poly[3-hexyl thiophene-2,5-diyl] (P3HT). High open-circuit voltages, V-OC = 0.92 and 0.65 V, have been measured for OC1C10-PPV:DPM-12- and P3HT:DPM-12-based devices, respectively. In both cases, V-OC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]-phenyl-C-61 butyric acid methyl ester (PCBM). this is somewhat unexpected when taking into account the identical redox potentials of both acceptor materials at room temperature. The temperature-dependent V-OC reveals, however, the same effective bandgap (HOMOPolymer-LUMOFullerene; HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) of 1.15 and 0.09 eV for OC1C10-PPV and P3HT, respectively, independent of the acceptor used. The higher V-OC at room temperature is explained by different ideality factors in the dark-diode characteristics. Under white-light illumination (80 mW cm(-2)), photocurrent densities of 1.3 and 4.7 mA cm(-2) have been obtained in the OC1C10-PPV:DPM-12- and P3HT:DPM-12-based devices, respectively. Temperature-dependent current density versus voltage characteristics reveal a thermally activated (shallow trap recombination limited) photocurrent in the case of OC1C10-PPV:DPM-12, and a nearly temperature-independent current density in P3HT:DPM-12. The latter clearly indicates that charge carriers traverse the active layer without significant recombination, which is due to the higher hole-mobility-lifetime product in P3HT. At the same time, the field-effect electron mobility in pure DPM-12 has been found to be mu(e) = 2 x 10(-4) cm(2) V-1 s(-1), that is, forty-times lower than the one measured in PCBM (mu(e) = 8 x 10(-3) cm(2) V-1 s(-1)).