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Surendra Anantharaman1 2 Thilo Stöferle3 Frank Nüesch1 2 R. F. Mahrt3 Jakob Heier2

1, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Dübendorf, , Switzerland
2, Swiss Federal Laboratories for Materials Science and Technology (Empa), Dübendorf, Zürich, Switzerland
3, IBM Research—Zurich, Säumerstrasse 4,, Rüschlikon, Zürich, Switzerland

The morphology of organic semiconductors influences exciton and charge transport, which are main factors in the performance of organic electronic devices. J-aggregates which are crystalline in nature and show coherent exciton migration experience strong exciton recombination at the grain boundaries. However, improving the crystallinity by tuning their morphologies is limited. Here, we demonstrate that surface-guided molecular assembly helps in realizing different morphologies like monolayer and lamellar crystals with improved crystallinity, which suppresses the non-radiative decay path and improves exciton diffusion.
In this study, we have deposited J-aggregates of a cyanine dye onto a PAMAM (polyamidoamine) dendrimer functionalized surface using a solution-immersion route. The presence of J-aggregates in the solution (water) limits to only island-type growth following the ‘Volmer-Weber’ model as reported so far. The platelets obtained from this growth show a lot of small coherent domains isolated by grain boundaries.
Tuning the size of the J-aggregates in a binary solvent mixture induces strong substrate-molecules interaction following a ‘Stranski-Krastanov’ growth model. The J-aggregates selectively nucleate on the PAMAM surface to form a complete monolayer. Nevertheless, with prolonged exposure, the molecule-molecule interaction dominates the substrate-molecule interaction and quasi-epitaxial vertical crystals were obtained. Furthermore, increasing the dye concentration affects the crystal growth, resulting in lamellar shapes with improved crystallinity. Our finding suggests that having J-aggregates in critical nucleus size is the most important criteria to tune the aggregate morphology on the surfaces.
Optical properties such as absorbance and photoluminescence were characterized in this study. Both platelets and lamellar crystals show similar absorbance at room temperature. Steady-state photoluminescence shows higher fluorescence quantum yield for the lamellar films than platelets. Temperature-dependent photoluminescence studies from 6 K to room temperature also show a similar trend. Using time-resolved photoluminescence spectroscopy, different non-radiative decay pathways were identified. The influence of exciton diffusion on the interplay of energetic disorder and non-radiative decay will be presented. The results suggest that the difference in domain size and order give rise to significantly enhanced radiative decay from lamellar films as compared to platelets or films deposited by spin-coating.

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