New Synthesis of P-Type Semiconductor Mixed Metal Sulfides from Metal-Based Ionic Liquids
Authors: Balischewski Christian , Taubert Andreas , Behrens Karsten , Zehbe Kerstin , Günter Christina , Mies Stefan 
Published: December 2020

In light of 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 current work is aiming for the establishment of ionic liquid precursors (ILPs) for the synthesis of complex metal chalcogenides. In organic photovoltaic systems these can then be used, e.g., as the active layers due to them acting as additional p-type semiconductors. 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 general approach is using 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 complex 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, thermal, optical and electrochemical properties were analysed. All compounds are ILs with melting points between 69 and 93 °C. The successful synthesis of mixed metal ILs with distinct compositions was confirmed using ICP OES, with single crystal X-ray diffraction showing consistent asymmetric units. Furthermore, X-ray diffraction shows that all ILs are isostructural, thus confirming that both the anion and the cation are stable and reliable building blocks for MILs. A possible application regarding the direct use of these ILs in electrochemical devices was further analysed. The electronic conductivity at room temperature is between 10-2 and 10-8 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 1.47 ∙ 10-1 at 70 ºC. As a result, future electrochemical applications are possible, especially in a moisture-sensitive environment at room temperature. Moreover, corrosive behaviour and the correlation between conductivity and composition were investigated for the first time, e.g. showing a positive influence with the addition of manganese into the compound, possibly acting as an electron scavenger. Additionally, cyclic voltammetry shows promising electrochemical stability windows between 2.5 and 3.0 V.5 Furthermore, a big 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., , DOI:10.1063/1.4991622.
2 A. Abouserie, G. El-Nagar, B. Heyne, R. Sarhan, Y. Kim, C. Pries, E. Ribacki and C. Günter, Hierarchically structured Copper Sulfide Microflowers with Excellent Amperometric Hydrogen Peroxide Detection Performance, 2019.
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, 2017, 5640–5649.
5 C. Balischewski, K. Behrens, K. Zehbe, C. Günter and A. Taubert, In preparation.