Evidence for Kinetically Limited Thickness Dependent Phase Separation in Organic Thin Film Blends

We present depth-resolved grazing incidence x-ray diffraction, grazing incidence small angle scattering and x-ray reflectivity studies on the structure of mixed C-60 and diindinoperylene (DIP) films as a function of the mixing ratio. We observe enhanced out-of-plane order and smoothing of the mixed films compared to pure films upon coevaporation of DIP: C-60 thin films (in different mixing ratio) which otherwise phase separate. The mixing ratio of molecules can be tuned to alter the in-plane crystallite size as well as the interisland distances of the mixing molecules. Real-time in situ grazing incidence x-ray diffraction measurements show the kinetics and thickness dependence of phase separation, which appears to proceed only after a certain thickness. The crystallite grain size of the individual phase separated components is significantly larger at the top of the film than at the bottom with implications for the understanding of devices.