Diphenylmethanofullerenes: New and Efficient Acceptors in Bulk-Heterojunction Solar Cells

Authors

  • I. Riedel,

    1. Bavarian Centre for Applied Energy Research (ZAE-Bayern), Division Thermal Insulation and Heat Transfer, Am Hubland, D-97074 Würzburg, Germany
    2. Institute of Physics, Energy and Semiconductor Research Laboratory, University of Oldenburg, D-26111 Oldenburg, Germany
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  • E. von Hauff,

    1. Institute of Physics, Energy and Semiconductor Research Laboratory, University of Oldenburg, D-26111 Oldenburg, Germany
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  • J. Parisi,

    1. Institute of Physics, Energy and Semiconductor Research Laboratory, University of Oldenburg, D-26111 Oldenburg, Germany
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  • N. Martín,

    1. Department of Organic Chemistry, Universidad Complutense de Madrid (UCM), ES-28040 Madrid, Spain
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  • F. Giacalone,

    1. Department of Organic Chemistry, Universidad Complutense de Madrid (UCM), ES-28040 Madrid, Spain
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  • V. Dyakonov

    1. Bavarian Centre for Applied Energy Research (ZAE-Bayern), Division Thermal Insulation and Heat Transfer, Am Hubland, D-97074 Würzburg, Germany
    2. Experimental Physics VI (Energy Research), Department of Physics and Astronomy, University of Würzburg, D-97074 Würzburg, Germany
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  • The authors gratefully thank D. Vanderzande and L. Lutsen (LUC Belgium) for providing OC1C10-PPV, J. C. Hummelen for donation of PCBM, and C. Waldauf, P. Schilinsky, and C. J. Brabec (Konarka, Austria) for discussions and simulations of dark J–V characteristics. Many thanks go to M. Pientka, M. Knipper, D. Chirvase, and H. Koch (University of Oldenburg) for their contribution in various discussions and technical assistance. This work was supported by the European Commission, EC (project HPRN-CT-2000-00127), by the Bundesministerium für Bildung und Forschung, BMBF (BMBF projects 01SF0019 and 01SF0026), and by the Spanish Ministry of Sciences and Technology (Ministerio Ciencia y Tecnología, McyT) of Spain project (BQU2002-00855). G. Wittstock (Univ. Oldenburg, Germany) is acknowledged for providing the AFM setup.

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).

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