Regensburg 2013 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 10: Poster: Charge transfer effects in molecular materials (related to symposium SYCT)

CPP 10.2: Poster

Monday, March 11, 2013, 17:30–19:30, Poster C

Field dependent charge carrier generation via charge transfer states: A blending ratio dependent study — •Julia Kern¹, Clemens Grünewald¹, Hannes Kraus¹, Franziska Fuchs¹, Carsten Deibel¹, and Vladimir Dyakonov^1,2 — ¹Experimental Physics VI, Julius Maximilians University of Würzburg, 97074 Würzburg, Germany — ²Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), 97074 Würzburg, Germany

In organic bulk heterojunction solar cells, the mechanism of exciton dissociation at the polymer–fullerene interface is still controversially debated. This work studies the charge carrier generation via charge transfer states by investigating the electric field dependence of the process for a model system consisting of MDMO-PPV and PC₆₁BM. Focusing on the influence of different donor–acceptor blend ratios, the complementary techniques of field dependent photoluminescence quenching and transient absorption are employed to reveal a substantial change in the electric field behavior. Thus, the binding energy of the respective charge transfer excitons (CTE) is reduced from ∼ 320 meV for a 4:1 blend ratio to ∼ 200 meV for a 1:1 blend, decreasing even further for higher PC₆₁BM concentrations. The observed phenomenon may be ascribed to a change in the average dielectric constant or in the CTE delocalization length of the system. The relevance of these influencing factors is unveiled by combined morphological studies comprising atomic force microscopy and optically detected magnetic resonance. This study demonstrates the importance of a high dielectric constant for charge carrier generation in organic photovoltaics.