Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most popular anode buffer coated on indium tin oxide. It is thought to improve the inorganic-organic contact, but little is known about its role in organic-organic contact. This study addresses the latter issue by examining how the PEDOT:PSS layer affects the crystallization process of the neighboring layer composed of p-type organic semiconductors in an organic photovoltaic device. Low landing voltage scanning electron microscopic analysis of crystals and aggregates of two donor compounds, tetrabenzoporphyrin (BP) and poly(3-hexylthiophene) (P3HT), showed that PEDOT:PSS effectively nucleates the crystallization or aggregation of the donor material on its surface to form a uniformly thick film of polycrystalline BP or aggregated P3HT molecules. By contrast, a graphitic surface cannot induce structural order of the donor molecules on it. This result implies that pinning of the donor molecules to the acidic PEDOT:PSS surface promotes the heterogeneous nucleation at the organic-organic interface. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 833-841