Raman spectroscopy applied to reveal polycrystalline grain structures and carrier transport properties of organic semiconductor films: Application to pentacene-based organic transistors

We have fabricated high performance polycrystalline pentacene-based thin-film transistors using several dielectrics with different surface properties, including inorganic oxide and polymeric materials. These materials provide excellent samples for the analysis of charge transport properties, particularly the impact of the grain boundary and the molecular structural quality within the grain on the efficient charge transport. The carrier transport in polycrystalline organic films with grain structures is often interpreted using the grain boundary model. Assuming a large amount of charges are trapped at the boundaries, the model neglects the microstructural quality inside a grain. According to joint experimental and theoretical Raman spectra and normal modes analysis on these pentacene films, we present a new observation that the microscopic hopping transport parameters, i.e., intermolecular interactions and reorganization energy, in polycrystalline films govern the carrier transport. An obvious, positive correlation is found between the mobility and the molecular vibrational characteristics, especially the intermolecular vibrational coupling energy, under all varieties of grain size morphology. MicroRaman mapping methodology reveals that the grain size of pentacene films should not be taken for granted in structural quality and efficient charge transport. The microstructural qualities inside the grain play an important role in efficient charge transport.