Femtosekunden Pump-Probe Setup
Final Report Abstract
When a solution of semiconducting polymers such as the light-emitting MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) is cooled down to cryogenic temperatures, the polymer chains start to aggregate into disordered and ordered domains. This is analogous to the aggregation process that occurs in thin films commonly used for organic semiconductor devices. By using the femtosecond pump-probe system, we could study how this aggregation impacts on the dynamics of the excited states in the sample. The pump-probe spectra show a different reduced absorption (ground state bleach) from either, disordered or ordered domains, as well as stimulated luminescence. By analysing the spectra and their evolution in time, we found that energy transfer takes place within less than 1 ps from disordered, unaggregated chain segments to highly planarized, aggregated chain segments. The short time scale of the energy transfer indicates intimate intermixing of the planarized and disordered polymeric chromophores, e.g. in a core-shell structure. When the femtosecond pump-probe spectra are correlated with coherent 2D spectroscopy, the time regimes dominated by localized and by mobile excitations can be differentiated. These spectroscopic results shed light on key morphological effects frequently encountered in organic semiconductors that are difficult to access by structural approaches. The efficiency of emiconducting polymer devices such as solar cells and light-emitting diodes strongly depend on the intermixing of disordered and ordered phases, so that spectroscopically obtained information is highly valuable.
Publications
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Ultrafast Energy Transfer between Disordered and Highly Planarized Chains of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) ACS Macro Lett 4 (2015) 412-416
Thomas Unger, Fabian Panzer, Cristina Consani, Federico Koch, Tobias Brixner, Heinz Bässler and Anna Köhler
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Reversible Laser Induced Amplified Spontaneous Emission from Coexisting Tetragonal and Orthorhombic Phases in Hybrid Lead Halide Perovskites. Adv. Optical Mater. 2016 (2. Mar 2016)
Fabian Panzer, Sebastian Baderschneider, Tanaji P. Gujar, Thomas Unger, Sergey Bagnich, Marius Jakoby, Heinz Bässler, Sven Hüttner, Jürgen Köhler, Ralf Moos, Mukundan Thelakkat, Richard Hildner and Anna Köhler
