In the field of organic optoelectronics, enhancing the diffusion of excitons has been a longstanding research focus [¹], due to its crucial role in the functionality of (e.g.) organic solar cells and OLEDs. However, directing these randomly diffusing excitons towards a specific target requires precise modification of the nanometer-scale assembly of organic materials. In a recent JACS publication with our colleagues from Bayreuth, we present a fresh approach to inscribe such energy-gradients into P3HT-nanofibers:

Stäter, S. et al. “Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers.” Journal of the American Chemical Society

For sample fabrication, we employ a needle-like nucleation seed that initiates the growth of P3HT-nanofibers in solution, resulting in a configuration resembling shish-kebab superstructures [²]. Our approach produces P3HT-nanofibers that exhibit remarkable length (several µm), parallel alignment, and close proximity.

We use hyperspectral imaging to collect 1680 PL and absorption spectra, comprehensively characterizing a 40x40µm area containing the superstructure. Analyzing the optical properties measured along nanofibers results in the energy landscape shown below:

Figure: Excited-state energy landscape along P3HT nanofibers within a NA/P3HT superstructure. Adopted without changes from Stäter et al., under CC BY-NC-ND 4.0 license.

Note that the location of the superstrucutre is shown schematically on the bottom of the graph: The P3HT-nanofibers (blue) extend around 5µm left and right from the needle-like nucleation seed (orange square, shown as cross-section). Plotted above this, we see how the edges of the exciton band are modified by the presence of the superstructure, leading to an energy-gradient along the P3HT-nanofibers.

We attribute the continuous variation of energy levels along these P3HT-nanofibers to defect-fractionation during nanofiber growth, as described by Oosterbaan et al. and Roehling et al [³, ⁴]. We hope that our approach, which involves employing a specialized nucleation seed to induce fiber growth in solution, will facilitate the design of systems that achieving directed long-range exciton transport.

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