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Unraveling the Photophysical Properties of Organic Semiconductors

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Organic semiconductors offer numerous advantages compared to their inorganic counterparts, such as low-cost manufacturing, flexibility and lightweight. One important application is their use in new generation solar cells (Figure 1). The active layer of such photovoltaic devices often consists of a conjugated polymer, blended with a fullerene derivative in a solid-state thin film. In order to understand how those solar cells work and to improve their performance, it is important to investigate the lightmatter interactions of the organic materials. We have done so by using a palette of experiments, spanning from ultrafast spectroscopy, via Stark effect methods, terahertz techniques, photocurrent measurements, to nanoscale mobility characterization.

I will briefly present our insights about the excited-state properties of conjugated polymers, with focus on the importance of short-lived delocalization [1,2]. I will then move on to discuss the correlation between the phase-morphology of polymer:fullerene blends and the photogeneration of free charge carriers [3-5]. Indeed, our work has shown that this essential step in solar cell functioning is largely determined by the way in which the polymer and fullerene arrange at the nanoscale. Results will be presented for the pBTTT and PBDTTPD polymers, as obtained by transient absorption, electromodulated differential absorption and terahertz spectroscopy. Finally, I will give an overview of our most recent studies about organic and hybrid semiconductors [6], not just applied to photovoltaics, but also to transistors and biological sensors. [1] N. Banerji, J. Mater. Chem. C. 2013, 1, 3052. [2] M. Scarongella, A. Laktionov, U. Röthlisberger, N. Banerji, J. Mater. Chem. C. 2013, 1, 2308. [3] M. Scarongella, J. De Jonghe-Risse, E. Buchaca-Domingo, M. Causa’, Z. Fei, M. Heeney, J.-E. Moser, N. Stingelin, N. Banerji, J. Amer. Chem. Soc. 2015, 137, 2908. [4] M. Scarongella, A. A. Paraecattil, E. Buchaca-Domingo, J. D. Douglas, S. Beaupre, T. McCarthy- Ward, M. Heeney, J.-E. Moser, M. Leclerc, J.-M. Fréchet, N. Stingelin, N. Banerji, J. Mater. Chem. A. 2014, 2, 6218. [5] A. A. Paraecattil, N. Banerji, J. Amer. Chem. Soc. 2014, 136, 1471. [6] J. Brauer, Y. H. Lee, M. K. Nazeeruddin, N. Banerji, J. Phys. Chem. Lett. 2015, 6, 3675.

This talk is part of the Optoelectronics Group series.

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