Ionic space-charge effects in solid state organic photovoltaics

The effect of mobile ions on the operation of donor-acceptor bilayer solar cells is studied. We demonstrate the large effect ions can have on the energetics of the solar cells, illustrated by (for instance) changing the output voltage of a cell in situ from 0.35 to 0.74 V. More importantly, it is shown ionic species do not obstruct the charge generating properties of the photovoltaic devices and ionic space charge can be used in situ to improve their efficiencies. The results obtained are explained by taking into account energetic changes at the donor-acceptor interface as well as built-in potentials, giving clear guidelines on how ionic species can offer many new and exciting functionalities to organic photovoltaics.

Nanoscale structuring of semiconducting molecular blend films in the presence of mobile counterions

The controlled fabrication of submicrometer phase-separated morphologies of semiconducting organic materials is attracting considerable interest, for example, in emerging thin-film optoelectronic device applications. For thin films of spin-coated blends of PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) and cationic cyanine dyes, we used atomic force microscopy scans to infer the structure formation mechanism: The solutions separate into transient bilayers, which further spinodally destabilize because of long-range molecular interactions. A thin layer ruptures earlier than a thick layer, and the earlier instability determines the morphology. Consequently, the resulting morphology type mainly depends on the ratio of the layer thicknesses, whereas the periodicity of structures is determined by the absolute film thickness. These findings allow control of the feature sizes, and nodular domains with diameters well below 50 nm were produced. Films prepared with dyes possessing a mobile counterion were always unstable. To rationalize the findings, we developed a thermodynamic model showing that electrostatic forces induced by the mobile counterions act as destabilizing pressure.