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Keywords:

  • Fullerenes;
  • Solar cells, bulk-heterojunction;
  • Solar cells, polymer

Abstract

A novel fullerene derivative, 1,1-bis(4,4′-dodecyloxyphenyl)-(5,6) C61, diphenylmethanofullerene (DPM-12), has been investigated as a possible electron acceptor in photovoltaic devices, in combination with two different conjugated polymers poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-para-phenylene vinylene] (OC1C10-PPV) and poly[3-hexyl thiophene-2,5-diyl] (P3HT). High open-circuit voltages, VOC = 0.92 and 0.65 V, have been measured for OC1C10-PPV:DPM-12- and P3HT:DPM-12-based devices, respectively. In both cases, VOC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]-phenyl-C61 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 VOC reveals, however, the same effective bandgap (HOMOPolymer–LUMOFullerene; HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) of 1.15 and 0.9 eV for OC1C10-PPV and P3HT, respectively, independent of the acceptor used. The higher VOC 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 μe = 2 × 10–4 cm2 V–1 s–1, that is, forty-times lower than the one measured in PCBM (μe = 8 × 10–3 cm2 V–1 s–1).