Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells.
Charge transfer (CT) states at a donor-acceptor heterojunction have a key role in the charge photogeneration process of organic solar cells, however, the mechanism by which these states dissociate efficiently into free carriers remains unclear. Here we explore the nature of these states in small molecule-fullerene bulk heterojunction photovoltaics with varying fullerene fraction and find that the CT energy scales with dielectric constant at high fullerene loading but that there is a threshold C60 crystallite size of ~4 nm below which the spatial extent of these states is reduced. Electroabsorption measurements indicate an increase in CT polarizability when C60 crystallite size exceeds this threshold, and that this change is correlated with increased charge separation yield supported by CT photoluminescence transients. These results support a model of charge separation via delocalized CT states independent of excess heterojunction offset driving energy and indicate that local fullerene crystallinity is critical to the charge separation process.
Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
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Bernardo
IMEC, Leuven B-3001, Belgium.
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Cheyns
IMEC, Leuven B-3001, Belgium.
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Verreet
1] Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
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Schaller
1] IMEC, Leuven B-3001, Belgium [2] Department of Electrical Engineering, Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA.
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Rand
Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
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Giebink