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Optimum molecular substitution in organic thin films is established to facilitate electron and ion transport and, thereby, fast reversible electrochromic switching.

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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-conversionefficiencies 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. Dye-sensitized solar cells (DSSCs) are receiving continued interest as photovoltaic devices because they can be fabricated with low preparation expenses, 1,2 with solar-to-electrical power conversion efficiencies (PC Es) up to 14.3% under one sun illumination.3 Thereby they offer options with low energy payback times for a sustainable photovoltaic technology on a very large scale, or, at least for niche applications. In such cells, light is absorbed by dye molecules that are adsorbed to a mesoporous semiconductor. Following light absorption, the dye injects excited electrons into the conduction band of the semiconductor from where they are transported to the front contact. Then the oxidized dye molecules are regenerated by a redox mediator that, on the other hand, is regenerated by the counter electrode at the back contact. 4 Highest PC E is achieved by using nanoparticulate TiO 2 as semiconductor which has to undergo high-temperature treatments of about 400• C to ensure high electron collection efficiency. 5This treatment limits the applicability of DSSCs, since it increases the required energy input and also prevents the use of polymer foils as substrate material. In order to circumvent such high-temperature processes, ZnO has been applied as porous semiconductor because a well-connected network can be directly grown by electrodeposition from aqueous solutions at a typical process temperature of 70However, the PC E of ZnO-based DSSCs is inferior compared to TiO 2 -based cells. One of the reasons is the low quantum efficiency when ZnO is sensitized with dyes that were optimized ...

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