E-MRS meeting Spring Meeting Symposium P: Current Trends in Crystalline Organic Semiconductors location:Strassbourg, France date:10/06/03
The charge transport properties of polycrystalline small-molecule organic thin-film transistors are not only determined by the intrinsic bulk crystal properties of the organic semiconductors, but are often even more dependent on their thin-film morphology. To gain a better understanding of organic thin-film vapour-phase growth, the nucleation of various organic semiconductors on dielectric substrates has been interpreted in terms of nearest-neighbour interactions, in analogy with atomistic theory. The parameters required for this nucleation theory are obtained from preliminary molecular mechanical calculations using the MM3 force-field, and compared with experiment to validate their order of magnitude. One important outcome of this study is the existence of a transition from two-dimensional to three-dimensional growth, which is determined by the inter-layer interactions of the molecular crystal. Films that grow three-dimensionally have a tendency to form ill-connected or isolated grains, deteriorating charge transport through the film. Second, the critical supersaturation below which no nucleation occurs depends on the vapour pressure of the material, which in turn is to a large extend determined by the intra-layer interactions. Using both criteria, a materials ability to form well-connected films can be assessed based on its molecular structure. Pentacene, tetracene and perylene are discussed as case-studies and show at least qualitative agreement with experimental data.