Exciton Dynamics in 2D Organic Assemblies for Realization of Next-generation Optoelectronic Devices

Supramolecular assemblies of cyanine dye molecules formed by weak van der Waals forces have gained immense attention for their distinct optical properties (high extinction coefficient, narrow absorption and photoluminescence line-width and small Stokes shift) compared to their monomer counterparts. Strong light-matter interaction in the J-aggregates causes coherent exciton migration and high color purity emission. This motivates to fabricate J-aggregate thin films for different opto-electronic devices such as solar cells, where J-aggregates serve as exciton transport channels, organic light emitting diodes (OLEDs) and polariton lasers. Exciton migration and photoluminescence quantum yield (PLQY) of the J-aggregates in thin films dictate the efficiency of energy harvesting and light emitting devices, respectively. The main focus of this thesis is the development of a new J-aggregate growth route which favors the formation of large, coherent domains of 2D J-aggregate thin films and to investigate the exciton dynamics. Furthermore, non-radiative decay channels suppressing the PLQY of J-aggregates in thin film and in solution were identified. The coherent crystalline domain size in thin films is typically limited to nanoscale length because dye molecules spontaneously self-assemble already in solution. This can be overcome by maintaining equilibrium between monomers and the critical nucleus of a J-aggregate in solution. This approach is shown here as a successful route to achieve large coherent domains on a functionalized substrate. The growth model was further confirmed with small-angle x-ray scattering (SAXS) studies. Using time-resolved photoluminescence (TRPL) spectroscopy, non-radiative decay channels limiting the exciton migration were identified. The results suggest that the increase in domain size and order is directly proportional to the increased radiative decay of the excitons. Furthermore, the PLQY of J-aggregates in water at room temperature is typically ~5%. Upon addition of alkylamine, the quantum yield was drastically improved while maintaining line-width and peak position. Using small-angle neutron scattering (SANS), a two-phase region was identified in the water-dye-alkylamine ternary phase diagram in which high PLQY J-aggregates form. From TRPL studies, an increase in radiative lifetime is in agreement with the increase in PLQY. As an application, ultra-narrowband photodetectors were explored owing to the narrow line-width of J-aggregate absorption. The concept proved advantageous over other strategies for narrowing the response width, with competing figure of merits. Realization of an inkjet printed prototype with higher frequency response compared to spin-coated devices highlights their potential for industrial applications. Excitonic channels using J-aggregate nanowires were formed by complexing J-aggregates with dendronized polymers. SAXS studies revealed a core-shell configuration of the DP-J-aggregate hybrid structure. A correlation between polymer conformation and J-aggregates allowed to assemble J-aggregate nanowires as thin films. Centrosymmetric cyanine dimers are known for their non-luminescent nature. Twisted dimer packing of dye molecules in a polymer matrix are confirmed from circular dichroism spectroscopy studies. This explains a strongly red-shifted photoluminescence with high quantum yield in the thin film.

Nüesch, Frank
Heier, Jakob
Lausanne, EPFL

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 Record created 2019-04-15, last modified 2019-09-17

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