X-ray and neutron reflectivity and electronic properties of PCBM-poly(bromo)styrene blends and bilayers with poly(3-hexylthiophene)

Stuart B. Kirschner (Johns Hopkins University)

We used neutron reflectivity to complement X-ray reflectivity characterization of PCBM-based layers formed on poly(3-hexylthiophene) (P3HT). Single-layer analyses were used to provide reliable scattering length density values for bilayer fitting. Atomic force microscopy analyses showed trends similar to the reflectivity experiments when observing upper surfaces. Styrene polymers added to PCBM in small concentrations (ca. 10%) led to processing advantages while retaining substantial electron mobility, about 0.001cm^2 V^-1 s^-1. The further introduction of a relatively heavy bromo substituent on the styrene rings greatly increased the film smoothness, as revealed by increases of the oscillation amplitudes in the reflectivity. In addition, the bromine heavy atom increased the X-ray reflectivity scattering length density of the upper layer. Finally, we present data consistent with PCBM becoming partially mixed with the P3HT as the PCBM is spin coated from a solution with poly(bromostyrene) to form an overlying film, consistent with predictions based on published phase diagrams of the P3HT-PCBM system.

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