Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.166
Publication date: 18th July 2019
The efficient transport of energy in the form of spin-singlet excitons lies at the heart of natural light harvesting for photosynthesis and optoelectronic devices based on synthetic organic semiconductors. For optoelectronic applications, energy transport over long length scales would be highly desirable, but most organic semiconductors exhibit singlet exciton diffusion lengths between 5 – 50 nm [1], with only a handful of systems being suggested to exceed this [2]. This is because the primary mechanism for exciton transport is Förster resonance energy transfer (FRET), which involves site-to-site hopping within a disordered energy landscape, leading to short diffusion lengths. While there have been reports of higher diffusion lengths in highly ordered one-dimensional organic nanowires, these materials have proved challenging to integrate into optoelectronic devices. Hence, what is required is a new methodology that allows for long range energy transport within thin films of organic semiconductors, which form the basis of current optoelectronic devices. Here, we show that long-range and ultrafast transport of energy can be achieved at room temperature in a range of chemically diverse organic semiconductor thin films through strong light-matter coupling to form exciton-polaritons. These effects occur despite the absence of an external cavity, metallic or plasmonic structures. We directly visualize energy transport via femtosecond transient absorption microscopy with sub-10 fs temporal and sub-10 nm spatial precision and find energy transport lengths of up to ~270 nm at effective velocities of up to ~5 ×106 m s-1. Evidence of strong light-matter coupling in the films is provided via peak splittings in the reflectivity spectra, measurement of the polariton dispersion and emission from collective polariton states. The formation of these exciton-polaritons states in organic semiconductor thin-films is a general phenomenon, independent of underlying materials chemistry, with the principal requirements being a high oscillator strength per unit volume and low disorder. These results and design rules will enable a new generation of organic optoelectronic and light harvesting devices based on robust cavity-free exciton-polaritons [3].
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