Simulation of Surface-Directed Phase Separation in a Solution-Processed Polymer/PCBM Blend

The formation of the surface-induced stratified lamellar composition profile experimentally evidenced in spincoated layers of the photovoltaic donor acceptor blend consisting of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole]/phenyl-C61-butyric acid methyl ester (APFO-3/PCBM), as processed from chloroform, is simulated using square gradient theory extended with terms describing the interaction of the blend components with the air and substrate interfaces. The surface energy contributions have been formulated based on an enthalpic nearest-neighbor model which allows integration of common surface tension theory and experimentally accessible surface energies of the fluid phase constituents with a mean field description of a multicomponent blend confined by substrate and air interfaces. Using estimates for the quench depth and transport properties of the blend components as a function of polymer concentration, the time-resolved numerical simulations yield results that compare favorably with experimental observations, both in terms of the number of lamellae as a function of the blend layer thickness and their compositional order. The effect of blend ratio is reproduced as well, the lamellar pattern becoming more pronounced if the amount of PCBM increases relative to APFO-3.