Berlin 2008 – scientific programme

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SYSA: Symposium Tayloring Organic Interfaces: Molecular Structures and Applications

SYSA 2: Organic Interface Structure and Growth

SYSA 2.7: Talk

Tuesday, February 26, 2008, 12:45–13:00, H 0105

Ambipolar organic semiconductor blendsfor photovoltaic cells — •Andreas Opitz, Markus Bronner, Julia Wagner, Marcel Götzenbrugger, and Wolfgang Brütting — Institute of Physics, University of Augsburg,Germany

Blends of organic semiconducting materials, so-called bulk-heterojunctions, are widely used in ambipolar field-effect transistors and as photoactive films in solar cells. One particularly interesting material system is a mixture of n-conducting fullerene (C₆₀) and p-conducting copper-phthalocyanine (CuPc) prepared by co-evaporation.

The electronic properties of these blends were analysed by X-ray and ultraviolet photoelectron spectroscopy in dependence on the mixing ratio [1]. A common vacuum level is observed suggesting that a phase separation of the two materials in the blends occurs only on the nanometer scale. The HOMO and the core level spectra show no evidence for a charge transfer or a chemical reaction among the materials in the ground state. These results also indicate a reduced energy offset between the HOMO of the donor and the LUMO of the acceptor in bulk heterojunctions in comparison to planar heterojunctions. As a consequence photovoltaic cells reveal higher built-in and open circuit voltage for the bilayered system. Nevertheless, photovoltaic cells using mixtures yield higher photocurrents due to the presence of donor/acceptor interface in the whole active film [2].

[1] A. Opitz et al., Appl. Phys. Lett. 90 (2007) 212112.

[2] W. Brütting et al., Macromol. Symp. (2007), accepted.