The performance and recent improvements of polymer-based solar cells can be traced back
to two main factors: the chemical design of new conjugated polymers and the control and
improved understanding of morphology in these solution-processed thin films. When a thin
film is prepared from a blend of a conjugated polymer and the fullerene-based material,
PCBM, demixing determines the final nanostructure, which in turn is influenced by the
polymer-fullerene interactions, the molecules’ tendency to self-organise, and the kinetics of
the film formation. During spincoating, characterized of rapid solvent evaporation, the
kinetics of nucleation and of two-phase separation compete. The resulting film morphology
is important for the performance of photovoltaic devices, because, first of all, excitons need
to diffuse to the donor/acceptor interface to separate into mobile charges, and, secondly,
these mobile charges need to travel to the electrodes. Characterization of the structure,
composition and molecular orientation at these interfaces and in the thin film has been one
major challenge, because of the critical requirements of chemical contrast combined with
lateral or depth resolution. We have used a combination of Atomic Force Microscopy
(AFM), dynamic Secondary Ion Mass Spectrometry (d-SIMS), Near-Edge Absorption Fine
Structure (NEXAFS) spectroscopy, and recently Neutron Reflectometry (NR), to probe the
surface and bulk composition of polymer:fullerene blends. Differences in composition
between surface and sub-surface are observed, and form strong evidence for vertical phase
separation.