Role of Initial and Final States in Molecular Spectroscopies: Example of Tetraphenyldibenzoperiflanthene (DBP) on Graphite

Electronic devices based on organic semiconductors are a fast-growing field of technology. A detailed understanding of the interplay between optical and electronic properties of organic molecular thin films as well as the physical structure is highly beneficial for the optimization of such devices. This requires investigations by means of different yet complementary spectroscopic techniques, where the comparability between the different data sets can become confusing. Consequently, the core aspect of this work is the consistent unification of spectroscopic data obtained by a diversity of experimental techniques within the complementary pictures of molecular states and energy levels. The confusion is obvious in the example of epitaxial films of tetraphenyldibenzoperiflanthene (DBP) on graphite(0001) surfaces being discussed here. One puzzling issue is the widening of the transport gap by around 0.6 eV during film growth, whereas the optical gap remains virtually constant. Furthermore, two-photon photoemission spectroscopy (2PPE) yields several features related to the same unoccupied orbital, and it is a nontrivial task to correlate them with the results of other spectroscopies. These issues can be resolved when all spectroscopic data are consistently interpreted in the framework of a theoretical model which was recently introduced by us, taking the initial and final states of the underlying probe processes into account. Applying that model, all peak shifts are explained here consistently by means of polarization and charging energies and further physical effects in context with the microscopic structure of the DBP thin films.