  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.
