Metal Complexes as Redox Shuttles in Dye-Sensitized Solar Cells Based on Electrodeposited ZnO: Tuning Recombination Kinetics and Conduction Band Energy

Abstract

In this work, a pathway to engineer both interfacial charge recombination kinetics and conduction band energy in dye-sensitized solar cells based on mesoporous electrodeposited ZnO is presented. Especially in solar cells employing metal complex redox shuttles such as Co(bpy)3 [bpy = 2,2’-bipyridine] and Cu(tmby)2 [tmby = 4,4‘,6,6‘-tetramethyl-2,2‘-bipyridine] these factors are crucial in order to obtain efficient devices. Controlling them is achieved by augmenting the liquid redox electrolyte with additives. The most commonly used additive 4-tert-butylpyridine (TBP) induces both an upward shift of the ZnO conduction band energy and retards recombination thus leading to higher device performance. However, the full potential of the cells cannot be exploited by TBP since high concentrations lead to unwanted side reactions. However, adding another additive such as 2,2’-bipyridine or neocuproine can circumvent these problems and opens a full range of options to tune the properties of the ZnO/electrolyte interface. Photoelectrochemical techniques, such as impedance spectroscopy, current-voltage characterization and photocurrent transient measurements are utilized to reveal the underlying mechanisms of the different additives. Using these additives, power-conversion-efficiencies of 3.56% for a Co(bpy)3-based electrolyte and 3.85% for a Cu(tmby)2-based electrolyte are achieved for electrodeposited ZnO sensitized with the organic dye DN216.