Physical Chemistry, Short talk
PC-025

Unravelling the Mechanism of Ultrafast Intrinsic Charge Generation in Cyanine Dyes

G. Fish1, J. Moreno Naranjo1, E. Hack2, A. Billion3, I. Krossing3, F. Nüesch4,5, J. E. Moser1*
1Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, École Polytechnique fédérale de Lausanne (EPFL), 2Laboratory for Transport at Nanoscale Interfaces, Swiss Federal Laboratories for Materials Science and Technology, Empa, 3Institut für Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF), Universität Freiburg, 4Laboratory for Funtional Polymers, Swiss Federal Laboratories for Materials Science and Technology, Empa, 5Institute of Materials, École Polytechnique fédérale de Lausanne (EPFL)

Cyanine dyes have a long history in the field of organic photovoltaics and they have several beneficial properties that contribute to their continued use; for example, the absorption profile of the material can be easily tuned, and they possess high extinction coefficients allowing for the active layer of devices to be very thin (up to 20 nm).[1]

Typically, the cyanine dyes are used alongside a fullerene-based acceptor, such as C60, in a planar heterojunction configuration with efficiencies of up to 3.7% being achieved.[2]

To date, ultrafast studies on these systems have tended to focus on the separation of charge transfer excitons, or the role of blend morphology.[3],[4]

However, recent studies have shown that high bulk photoconductivity can be seen in pristine pentamethine cyanine (Cy5) films, without the presence of a donor-acceptor interface.[5]

Here, we employed ultrafast transient absorption spectroscopy to determine the charge transfer mechanism in Cy5 systems, providing the first direct proof of high efficiency intrinsic charge generation in organic salt semiconductors. Furthermore, the impact of counterion size on charge transfer and transport was studied, again using transient absorption spectroscopy alongside other ultrafast techniques. We found that aggregation played a key role in the efficiency of the intrinsic charge generation and the charge transport processes.

[1] B. Fan, F. Araujo De Castro, B. T. Te Chu, J. Heier, D. Opris, R. Hany and F. Nüesch, J. Mater. Chem., 2010, 20, 2952–2955.
[2] E. Berner, T. Jäger, T. Lanz, F. Nüesch, J. N. Tisserant, G. Wicht, H. Zhang and R. Hany, Appl. Phys. Lett., 2013, 102, 98–102.
[3] A. Devižis, J. De Jonghe-Risse, R. Hany, F. Nüesch, S. Jenatsch, V. Gulbinas and J. E. Moser, J. Am. Chem. Soc., 2015, 137, 8192–8198.
[4] J. De Jonghe-Risse, J. Heier, F. Nüesch and J. E. Moser, J. Mater. Chem. A, 2015, 3, 10935–10941.
[5] L. Wang, S. Jenatsch, B. Ruhstaller, C. Hinderling, D. Gesevičius, R. Hany and F. Nüesch, Adv. Funct. Mater., 2018, 28, 1–8.