Mixed Metal Sulfides from Metallated Ionic Liquids
Authors: Balischewski Christian , Taubert Andreas , Zehbe Kerstin , Behrens Karsten , Günter Christina 
Published: May 2020

Driven by the ongoing transition from non-renewable to renewable energy sources, organic and hybrid solar cells have been studied due to their cost-efficiency and the smaller environmental impact. The goal of the current work is the establishment of ionic liquid precursors (ILPs) for the synthesis of complex metal chalcogenides. These can then be used, e.g., as the active layers of organic photovoltaic systems. The major difference to more conventional approaches is that the ILP not only acts as a metal source but also as a morphology directing template and as a stabilizer for the resulting nanoparticles.1,2 The basic idea is to use metal-containing ionic liquids, where the metal is an integral part of the IL anion as the metal source and to generate mixed metal chalcogenides directly from these single source precursors.3,4

In the current proof-of-concept study, thirteen N-butylpyridinium salts, including three monometallic compounds [C4Py]2[MCl4], nine bimetallic compounds [C4Py]2[M1-x
aMx
bCl4] and one trimetallic compound [C4Py]2[M1-y-z
aMy
bMz
cCl4] (M = Co, Cu, Mn; x = 0.25, 0.50 or 0.75 and y = z = 0.33), were synthesized and their structure and thermal and electrochemical properties were analysed. All compounds are ILs with melting points between 69 and 92 °C, Figure 1. The possibility of the direct use of these ILs in electrochemical devices was further analysed. The electronic conductivity at room temperature is between 10-5 and 10-9 S cm-1. This correlates with the optical band-gap measurements indicating rather poor semiconductors with large band gaps. However, at elevated temperature approaching the melting points, the conductivities reach up to 7.27 ∙ 10-4 S/cm at 70 ºC. Therefore future electrochemical applications are possible, especially in a moisture-sensitive environment at room temperature. Furthermore, cyclic voltammetry shows promising electrochemical stability windows between 2.5 and 3.0 V.5 A further advantage of these ILPs is the fact that their properties, such as band gaps can directly be adjusted by proper choice of the metals in the ILs.
Through a reaction of these metal-containing ILs with a sulphur source the respective metal chalcogenide (MC) nanoparticles will form.1 The p-type semiconductor nanoparticles will then be used as a hole transport material in the bulk heterojunction to improve the charge transport as well as providing a broad optical absorption window.
1 Y. Kim, B. Heyne, A. Abouserie, C. Pries, C. Ippen, C. Günter, A. Taubert and A. Wedel, J. Chem. Phys., 2018, DOI:10.1063/1.4991622.
2 A. Abouserie, G. El-Nagar, B. Heyne, R. Sarhan, Y. Kim, C. Pries, E. Ribacki and C. Günter, In preparation.
3 K. Thiel, T. Klamroth, P. Strauch and A. Taubert, Phys. Chem. Chem. Phys., 2011, 13, 13537.
4 A. Abouserie, K. Zehbe, P. Metzner, A. Kelling, C. Günter, U. Schilde, P. Strauch, T. Körzdörfer and A. Taubert, Eur. J. Inorg. Chem., 2017, 48, 5640–5649.
5 C. Balischewski, K. Behrens, K. Zehbe, C. Günter and A. Taubert, In preparation.

Figure 1