  Surface-pressure isotherms, X-ray reflectivity and X-ray near-total
reflection fluorescence were used to study the properties of
1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C$_{61}$ (PCBM) that was pre-mixed
with cesium carbonate and spread as a film at the air-water interface. The
pre-mixed PCBM with cesium carbonate demonstrated a strikingly strong effect on
the organization of the film. Whereas films formed from pure PCBM solution were
rough due to strong inter-molecular interactions, the films formed from the
mixture were much smoother. This indicates that the cesium carbonate moderates
the inter-molecular interactions among PCBM molecules, hinting that the cesium
diffusion observed in inverted organic photovoltaics and the likely ensuing
ionic Cs-PCBM interaction decreases aggregation tendency of PCBM. This implies
that the use of cesium salts affects the morphology of the organic layer and
consequently improves the efficiency of these devices.

Materials Science