Real-Time Changes in the Optical Spectrum of Organic Semiconducting Films and Their Thickness Regimes during Growth

We present real-time in situ studies of optical spectra during thin film growth of several prototype organic semiconductors (pentacene, perfluoropentacene, and diindenoperylene) on SiO(2). These data provide insight into surface and interface effects that are of fundamental importance and of relevance for applications in organic electronics. With respect to the bulk, the different molecular environment and structural changes within the first few monolayers can give rise to significant optical changes. Similar to interface-driven phenomena in, e. g., magnetism, spectral changes as a function of thickness d are a very general effect, decaying as 1/d in the simplest approximation. We observe energy shifts of 50-100 meV, rather small changes of the exciton-phonon coupling, and new transitions in specific systems, which should be considered as general features of the growth of organics.