Linking morphology to electronic properties in small-molecular organic semiconductors

Abstract

In organic electronics, morphology inevitably affects important device properties. The understanding of these relationships and ultimately control of morphology in small-molecular organic materials is necessary for their further development. Understanding the influence of morphology on device properties is critical not only to improving device efficiency, but also to extending usable device lifetimes by careful design to reduce material degradation. Difficulty in imaging these organic materials due to their sensitivity to damage from ion, electron and X-Ray irradiation, as well as their lack of elemental contrast between molecular species has so far limited the study of their morphology. Atom probe tomography (APT) is used as a tool to provide chemically-sensitive three-dimensional tomographs of organic materials which can be used to study morphological phenomena in these systems. In this work, several results are presented which demonstrate the importance of morphology on device properties in organic semiconductor materials. First, the presence of a chemical product confined to the donor-acceptor interface in tetracene/C60 organic photovoltaics (OPVs) is identified using APT and Fourier-transform infrared spectroscopy (FTIR), and it is shown to increase device open-circuit voltage with increasing concentration. Next, APT and high-angle annular dark-field scanning electron microscopy (HAADF-STEM) are used to characterize organic light-emitting diode (OLED) emissive layer (EML) films, showing that the emissive dopant aggregates in codeposited host/dopant films, influencing several important device properties, specifically the hole mobility and rates of triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA). Two strategies are then used to kinetically limit aggregation of the dopant and as a consequence, change electronic properties: reduction of the substrate temperature during film growth, and the addition of a wide-gap co-host molecule.