From Amorphous to Polycrystalline Rubrene—Charge Transport in Organic Semiconductors Paralleled with Silicon

Abstract

While progress has been made in the design of new organic semiconductors (OSCs) with improved transport properties (i.e., high mobilities), our understanding of the mechanisms involved is still limited and hinders further development, due in large part to the inherent difficulty when comparing OSCs with different molecular structure and morphologies. For silicon, enhanced understanding finally came once comparisons could be made with variations in morphology from single crystals to fully amorphous films. In this work, we achieve a similar feat in one single organic molecular system. In particular, rubrene is employed as the OSC as it spans transport mechanisms from thermally activated hopping in its amorphous form to band-like transport in highly ordered crystals. Various transport characterizations including variable temperature conductivity, advanced Hall effect and magnetoresistance measurements are performed on rubrene films with varying levels of order (polycrystalline vs amorphous), crystal phase (orthorhombic vs triclinic) and morphologies (platelet-like vs spherulitic grains). We find that conductivity can be tuned over four orders of magnitude when changing the level of order in the film from fully amorphous to polycrystalline with a few high-quality grains. Our results show that transport in polycrystalline orthorhombic films is limited by grain boundaries, as observed in polycrystalline silicon. The use of advanced Hall measurement, for the first time performed on OSC thin films without the use of “gating” (gate voltage applied through addition of a dielectric and gate electrode), provides access to the intrinsic properties of the semiconductor. Despite the very high resistivity of amorphous and triclinic rubrene, we are able to probe a Hall signal, pointing to the existence of a marginal density of delocalized carriers. Overall, our Hall and magnetoresistance measurements suggest a gradual transition from predominantly hopping transport to predominantly band-like transport as order is increased and crystal phase optimized. In summary, through this work we provide a comprehensive understanding of the interplay between order, molecular packing, morphology and charge transport in OSCs, akin to research on silicon decades ago. Our results point toward the application of a unified transport model with varying contributions of delocalized and localized carriers. More importantly, our study highlights that order alone is insufficient whereas intermolecular coupling is paramount for optimal transport, providing guidelines for the design of new molecules.