Direct Observation of Interfacial Morphology in Poly(3-hexylthiophene) Transistors: Relationship between Grain Boundary and Field-Effect Mobility

We investigated the effects of microstructural (crystallization and molecular orientation) and morphological alternation (grain boundary) of poly(3-hexylthiophene) (P3HT) films on the field-effect mobility (μ) before (as-spun P3HT) and after (melt-crystallized P3HT) melting of P3HT films. Although grazing incidence X-ray scattering shows that melt-crystallized P3HT has a more highly ordered edge-on structure than as-spun P3HT, the melt-crystallized P3HT reveals μ = 0.003 cm 2 V −1 s −1; this is an order of magnitude lower than that of as-spun P3HT (μ = 0.01 cm 2 V −1 s −1 ). In addition, the interfacial morphologies of the bottom surfaces of P3HT films, which are attached to the gate dielectric, were investigated using a film transfer technique. The melt-crystallized P3HT at this interface consists of well-developed nanowire crystallites with well-defined grain boundaries that act as trap states, as verified by analysis of the temperature-dependence of μ. The remarkable reduction of μ in low-molecular-weight P3HT film (8 kg/mol) that results from melt-crystallization is due to the increased number of well-defined grain boundaries.