George Fish1,Frank Nüesch1,2,Jacques-E. Moser1
École Polytechnique Fédérale de Lausanne1,Empa–Swiss Federal Laboratories for Materials Science and Technology2
George Fish1,Frank Nüesch1,2,Jacques-E. Moser1
École Polytechnique Fédérale de Lausanne1,Empa–Swiss Federal Laboratories for Materials Science and Technology2
In organic photovoltaics (OPVs), it is widely accepted that the open circuit voltage (V<sub>OC</sub>) is predominantly determined by the energy level offset between the donor and acceptor material. A consequence of this is that increasing the driving force for charge separation negatively impacts upon the V<sub>OC</sub>.<br/><br/>Recently, symmetry breaking photoinduced charge separation (SB-CS) has become an attractive topic in the field as it would allow for charges to be separated in systems with no energetic offset between the donor and acceptor material, thus increasing the V<sub>OC </sub>and device efficiency.<sup>1</sup><br/><br/>Here, we use ultrafast transient absorption spectroscopy to probe photoinduced charge generation in cyanine and squaraine dyes, both classes of materials with a storied history in the OPV field. The high extinction coefficients of cyanine dyes have led to them being used extensively alongside fullerene acceptors in planar heterojunction devices, as well as being used as model systems to study the dissociation of charge transfer states.<sup>2,3</sup><br/><br/>In this work, we build upon results that demonstrated the possibility of pristine pentamethine cyanine (Cy5) thin films to undergo intrinsic charge generation.<sup>4</sup> We demonstrate that SB-CS takes place in pristine Cy5 thin films and proceeds with a quantum yield of 86%, providing the first direct proof of high-efficiency intrinsic photoinduced charge generation in organic salt semiconductors. The driving force for the SB-CS process was determined to be, in part, due to local electric fields arising from the anions being located outside of H-aggregated cationic chromophore stacks. Furthermore, the counterion of the cyanine dye was varied in order to establish that the degree of aggregation plays a vital role in determining the efficiency of the charge separation process.<sup>5</sup><br/><br/>Comparatively, our ultrafast studies on dicyanomethylene substituted squaraine dyes, which are promising materials for short wave infrared photodetectors<sup>6</sup>, revealed that an intrinsic SB-CS process does not occur. An absence of counterions resulted in there being no local electric fields to provide the driving force for the charge separation to take place.<br/><br/>1 E. Sebastian and M. Hariharan, <i>ACS Energy Lett.</i> 2022, <b>7</b>, 696–711.<br/>2 M. Bates and R. R. Lunt, <i>Sustain. Energy Fuels</i>, 2017, <b>1</b>, 955–968.<br/>3 A. Devizis, J. de Jonghe-Risse, R. Hany, F. Nüesch, S. Jenatsch, V. Gulbinas and J. E. Moser, <i>J. Am. Chem. Soc.</i> 2015, <b>137</b>, 8192–8198.<br/>4 L. Wang, S. Jenatsch, B. Ruhstaller, C. Hinderling, D. Gesevičius, R. Hany and F. Nüesch, <i>Adv. Func. Mater.</i> 2018, <b>28</b>, 1–8.<br/>5 G. C. Fish, J. M. Moreno-Naranjo, A. Billion, D. Kratzert, E. Hack, I. Krossing, F. Nüesch and J. E. Moser, <i>Phys. Chem. Chem. Phys.</i> 2021, <b>23</b>, 23886–23895.<br/>6 K. Strassel, W. H. Hu, S. Osbild, D. Padula, D. Rentsch, S. Yakunin, Y. Shynkarenko, M. Kovalenko, F. Nüesch, R. Hany and M. Bauer, <i>Sci. Technol. Adv. Mater.</i> 2021, <b>22</b>, 194–204.