Printed Amorphous CoS Thin Films for DSSCs Counter Electrodes and Hydrogen Production

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Mirko Congiu, 330
Publication date: 5th February 2015

Dye sensitized solar cell (DSSC)[1]⁠ is a very interesting technology due the facility of production, the good efficiencies up to 13%[2]⁠ and the low costs. In order to reduce the costs associated to the use of platinum, different materials have been used for the counter electrode (CE). Cobalt sulfide (CoS) is a cheap and reliable alternative to Pt in the CEs. In fact, in several studies, it has shown to be more efficient then Pt in the reduction reaction of the I^-/I₃^- redox couple[3,4]⁠. Furthermore CoS can be used as cathode for the electrolysis of water in order to produce hydrogen to store the solar energy in the form of gaseous fuel⁠[5]. Despite the great advantages and the reduced costs, cobalt sulfide counter electrode is not as commonly used as Pt, and remains excluded from industrial production. The reason for not spreading of this excellent material is the lack of an efficient method to perform the industrial production of CoS thin films. Among the modern techniques, used for the industrial large scale production of DSSCs, the screen printing processes are the most used. In fact we can found a great number of commercial products such as screen-printing pastes for the deposition of the mesoporous TiO2 and also for the the Pt CE. One of the most common technique for the fabrication of CoS CEs is the electrodeposition (ED). This technique allows to obtain excellent results, however it requires the use of highly toxic precursors such as thiourea[3,4,6]⁠. In our last research work we have proposed a new method for the deposition of nanocrystalline CoS transparent layers, based on a single precursor ink (Co(II) bis-diethylcarbamate in CHCl₃)[7]⁠. In this new research we focused on a similar process based on another chemical precursor, cheaper and water-soluble, for the large-scale deposition of amorphous CoS counter electrode. Electrochemical impedance spectroscopy (EIS), Cyclic voltammery (CV) have been used in order to perform the electrochemical characterization of the CEs and also to perform the electrolysis experiments. X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS) and electronic microscopy (SEM), have been used to characterize the CoS on both films and powder samples. The CEs have been used also on complete DSSC devices characterized by current-voltage curves (IV) under simulated sun light.
Figure 1: SEM image of a CoS counter electrode on FTO. A little scratch has been made on the uniform deposited layer in order to show the morphologic differences between the amorphous CoS and the substrate.
[1] B. O’regan, M. Grfitzeli, A low-cost, high-efficiency solar cell based on dye-sensitized, Nature. 353 (1991) 737–740. [2] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, C.F. E., N. Ashari-Astani, et al., Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat Chem. 6 (2014) 242–247. http://dx.doi.org/10.1038/nchem.1861. [3] J.-Y. Lin, J.-H. Liao, T.-C. Wei, Honeycomb-like CoS counter electrodes for transparent dye-sensitized solar cells, Electrochem. Solid-State Lett. 14 (2011) D41–D44. [4] M. Wang, A.M. Anghel, B. Marsan, N.-L. Cevey Ha, N. Pootrakulchote, S.M. Zakeeruddin, et al., CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells, J. Am. Chem. Soc. 131 (2009) 15976–15977. [5] Y. Sun, C. Liu, D.C. Grauer, J. Yano, J.R. Long, P. Yang, et al., Electrodeposited cobalt-sulfide catalyst for electrochemical and photoelectrochemical hydrogen generation from water., J. Am. Chem. Soc. 135 (2013) 17699–702. doi:10.1021/ja4094764. [6] J. Yang, C. Bao, K. Zhu, T. Yu, F. Li, J. Liu, et al., High catalytic activity and stability of nickel sulfide and cobalt sulfide hierarchical nanospheres on the counter electrodes for dye-sensitized solar cells, Chem. Commun. (2014). [7] M. Congiu, L.G.S. Albano, F. Decker, C.F.O. Graeff, Single precursor route to efficient cobalt sulphide counter electrodes for dye sensitized solar cells, Electrochim. Acta. (2015).

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