Exploration of the Structure-Function Relationship in Organic Photovoltaic Devices by X-Ray Scattering

Synchrotron wide- and small-angle x-ray scattering are used to study structure formation in photovoltaic polymer blends on the 10-nm length scale.

The structural characterisation of novel semiconducting polymers is of great importance for the further development of new materials for organic electronic applications such as organic photovoltaics (OPVs) and organic field effect transistors (OFETs). Conventionally, blending together a donor (p-type) and an acceptor (n-type) material forms the active layer, the morphology of which is of great importance. It is influenced by inherent polymer phase separation as well as the potential crystallisation of the components. An interpenetrating network on the nanometre scale with a large interface between both material phases is thought to be optimal for OPVs. Our research aims at understanding both the kinetics of phase separation as well as the structure formation of semi-crystalline conjugated materials. These two factors have a combined effect on the morphology of blends, the understanding of which is important as the interplay of these two mechanisms can be influenced through the device manufacture protocol.

Furthermore, we make use of block copolymers (BCPs), which consist of covalently linked donor and acceptor blocks. BCPs are well-known to phase separate into highly ordered nanostructures caused by the immiscibility of the two blocks and their molecular interconnectivity.   This so-called microphase separation results in highly ordered morphologies on the 10-30 nm scale.

Organic solar cells can greatly benefit from nanostructures on the 10-nm length scale, comparable to the exciton diffusion length. We aim at gaining a fundamental understanding on the mutual crystallisation and microphase separation of composite materials which are indispensable for the further development of organic and organic / inorganic hybrid electronic devices. We therefore combine structural studies with electro-optical characterisation with the aim to optimising the performance of solar cells and organic field effect transistors.

The thermodynamic properties of novel and chemically well-defined semi-crystalline polymers, polymer - fullerene and polymer - polymer composites as well as block copolymers and hybrid nanocomposites are investigated in a broad q-range using small- and wide-angle X-ray scattering (SAXS / WAXS) as well as grazing incidence small- and wide-angle X-ray scattering (GISAXS / GIWAXS) on both bulk and thin film nanocomposites. All the studies are pursued at top world synchrotron facilities, including PSI (Switzerland), DESY (Germany), Lawrence Berkeley Laboratory (U.S.A.), Cornell CHESS (U.S.A.), ESRF (France) and Diamond (U.K.). Only in-situ real-time synchrotron SAXS / WAXS and GISAXS / GIWAXS measurements with high time resolution can cover all processes from crystallization to main-chain-side-chain separation to microphase separation and blend phase separation. Furthermore our state-of-the-art X-ray scattering laboratory is a perfect complement to synchrotron measurements for a large variety of nanocomposite materials, granting us with a local access to synchrotron-like static measurements, including SAXS / WAXS, GISAXS / GIWAXS and X-ray reflectivity (XR) and diffraction (XRD).


Adolphe Merkle Institute - Chemin des Verdiers 4 - CH-1700 Fribourg - Phone +41 26 300 9254