Growth and structure dynamics of pentacene thin films with applications to OTFTs and OPVs

The organic semiconductor pentacene has stood out as a model system to study the dependence of the electrical properties as a function of structure and morphology due to the field-effect mobility being among the highest reported so far. Most studies looking for a correlation between morphology and charge transport of thin film transistors and photovoltaic cells have been conducted ex-sites after film growth, however, film growth is a dynamic process where the kinetic rates of various processes determine the final film morphology. Real-time synchrotron x-ray scattering experiments have been utilized to monitor both the morphological and the structural dynamics of pentacene thin films. The morphology dynamics were monitored in the anti-Bragg configuration and the data was fit to a distributed growth model. It was found that the interlayer transport was found to depend on the substrate independent of temperature within the range from 25° to 60° C. For thicker films grown on SiO2, an Ehrlich-Schwoebel barrier of the order of 70 meV dominated downward transfer of pentacene. For films grown on a hydrophobic substrate, significant desorption of pentacene molecules from the substrate at elevated temperatures forced the growth mode towards 3D. Previous studies have established that pentacene films are polymorphic with the "thin-film" phase dominating until some critical thickness where the bulk phase then takes over. We show that the bulk phase actually nucleates as early as the substrate, and continues to nucleate as film growth progresses, "shadowing" the evolution of the "thin-film" phase crystals.