Influencing optical and charge transport properties by controlling the molecular interactions of merocyanine thin films†

Abstract

In amorphous organic semiconductors, charge transport typically takes place via slow hopping processes, but it is known that molecular aggregation can lead to enhanced exciton and charge transport through coupling of the transition dipole moments. In this work, we investigate the optical, morphological, and electronic properties of thin films of a merocyanine dye, which aggregates easily due to its dipolar character. Firstly, in spin-coated thin films the degree of aggregation can be tuned by thermal annealing, leading to strong spectral shifts alongside with Davydov splitting of >800 meV. At the same time, the mobility increases by approximately three orders of magnitude. We combine variable angle spectroscopic ellipsometry and polarization-resolved transmission spectroscopy with density functional theory to demonstrate that the aggregated molecules are oriented in an upright, standing configuration relative to the substrate surface. This arrangement involves a co-facial orientation of the molecular pi-systems, which is advantageous for lateral charge transport. Secondly, by utilizing highly oriented pyrolytic graphite as an ordered substrate and low-rate vacuum deposition, we are able to template the growth of the merocyanine layer and to substantially improve the in-plane morphological order. By combining atomic force microscopy and photoluminescence microspectroscopy we observe large oriented domains hundreds of μm² in size, emitting linearly polarized light, whereby maintaining the edge-on molecular arrangement. This promises a further significant enhancement of lateral charge carrier mobility.