Study on the Performance of a Cobalt Complex as Redox Couple in DSSCs Based on ZnO Thin Films Obtained by Electrochemical Methods and Effect of Passivation of Recombination Paths at the Metal Oxide Surface on the Solar Cell Parameters

In recent years increased attention has been placed on the development of more efficient and lower cost devices for the conversion of solar energy into electricity, which is where the dye-sensitized solar cells (DSSCs) has shown promising results [1]. A DSSC is a photoelectrochemical device consisting of a nanostructured and mesoporous film of a metal oxide semiconductor sensitized to sunlight via dye molecules adsorbed to the surface. The dye absorbs visible light, injects an electron from its excited state into the conduction band of the metal oxide, and the oxidized dye is subsequently reduced by an electron donor in the electrolyte solution, which is then regenerated at the counter electrode. However, the process of recombination of photogenerated electrons with electron acceptor species in the electrolyte solution is one of the limiting factors to consider, in order to improve the performance of these cells.

Great efforts have been made with the purpose of suppressing this loss process by slowing down the kinetics, and in the recent literature several strategies have been reported. For example, the use of organic dyes with high extinction coefficients, electrolyte solutions with redox couples based on cobalt complexes [2], passivation or shielding of recombination paths at the metal oxide surface by coadsorption of certain organic molecules, or deposition of a semiconductor interlayer, and combinations of these methods have been explored [3].

On the other hand, efficient DSSCs with lower production costs involves the optimization of processing steps and components used, such as obtaining the semiconductor thin film, which has been used traditionally TiO₂. ZnO is an attractive alternate material for use in these devices having a band gap similar to that of TiO₂, but with a higher electron mobility. In addition, ZnO can be synthesized and deposited by electrochemical methods such as electrodeposition, with a good control over the morphology of the material and at low temperatures, which makes the use of plastic substrates viable.

The work presented here is focused on the study of DSSCs based on nanostructured and mesoporous ZnO films prepared by electrodeposition. We report on the influence of the electrodeposition parameters on the morphology and crystallinity of the ZnO films: it is shown that at low current density, a crystalline film can be deposited, which would allow to avoid a high temperature sintering process. We have also used pulse plating conditions to prepare ZnO film, and we have applied these films as substrates in the DSSC.

For the fabrication of the solar cells, we have used an organic dye, OD-8, as sensitizer and an electrolyte solution prepared with the redox couple [Co(III/II) tris(2,2'-bipyridine)]. Since this redox couple undergoes a one-electron reduction process, recombination through electron transfer from the metal oxide to the oxidized species in generally fast. However, the redox couple is bulky, and the recombination pathway may be blocked in several ways, using co-adsorbing additives or interfacial blocking layers.  In this work, we explore several options to slow down recombination, and we show the influence on solar cell parameters, such as as the open circuit potential (V_OC), short circuit current (I_SC) and fill factor (FF). Electrochemical impedance spectroscopy (EIS) measurements, and chronopotentiometric experiments at open circuit conditions and under intermittent illumination were performed to support our conclusions.

[1] Prog. Photovolt. : Res. Appl. 2013; 21: 1-11.

[2] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A. K. Chandiran, Md. K. Nazeeruddin, E. W.-

G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, SCIENCE 334(2011) 629-633.

[3] N. R. Neale, N. Kopidakis, J. van de Lagemant, M. Grätzel and A. J. Frank, J. Phys. Chem. B 109(2005) 23183-23189.