Optimum molecular substitution in organic thin films is established to facilitate electron and ion transport and, thereby, fast reversible electrochromic switching.
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 ...
Thin, porous films of WO3 were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB50-b-PEO45), or a combination of PEG and PIB50-b-PEO45 as structure-directing agents. The influence of the polymers on the composition and porosity of WO3 was investigated by microwave plasma atomic emission spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, and gas sorption analysis. The electrochromic performance of the WO3 thin films was characterized with LiClO4 in propylene carbonate as electrolyte. To analyze the intercalation of the Li+ ions, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy were performed on films in a pristine or reduced state. The use of PEG led to networks of micropores allowing fast reversible electrochromic switching with a high modulation of the optical transmittance and a high coloration efficiency. The use of PIB50-b-PEO45 provided isolated spherical mesopores leading to an electrochromic performance similar to compact WO3, only. Optimum characteristics were obtained in films which had been prepared in the presence of both, PEG and PIB50-b-PEO45, since WO3 films with mesopores were obtained that were interconnected by a microporous network and showed a clear progress in electrochromic switching beyond compact or microporous WO3.
The optimization of the porosity and pore radius of ZnO for the application in dye-sensitized solar cells is essential to enhance the cell performance. Porous ZnO films were fabricated by electrodeposition in the presence of eosin Y or in combination with eosin B which was used as a structure directing agent (SDA) for the first time. The influence of the deposition time and the SDA concentration on film porosity and pore radius was characterized by a combined electrochemical and atomic absorption spectroscopy analysis, and confirmed by gas sorption, optical spectroscopy and scanning electron microscopy. The combination of eosin Y with eosin B as SDA results in porous ZnO films with larger pore radius compared to the films prepared with eosin Y only. The larger pores showed a significantly decreased diffusion impedance leading to an increased photocurrent less hindered by mass transport. Successful application of these new ZnO films in dye-sensitized solar cells confirmed improved pathways for large complex ions as redox shuttles through the ZnO pore structure.
Thin films of phthalocyanines can be considered established materials as organic semiconductors and as electrocatalytically active electrodes. As a consequence of their redox chemistry, however, they also have been proposed as electrochromic layers. For this purpose, they have to perform as organic mixed ionic and electronic conductors (OMIEC). Phthalocyanines with electron-withdrawing substituents have shown reversible reduction and re-oxidation. Fluorination has proven particularly useful since chemically reversible reactions can be achieved, even in contact to aqueous electrolytes [1]. The degree of fluorination determines the intermolecular coupling and, hence, the movement of electrons and ions in thin films. For films of perfluorinated copper phthalocyanine (F16PcCu) facile transport of electrons was established, but the rate of the electrochromic reaction was limited by ion transport in the films. For a derivative in which all 8 peripheral F atoms were replaced by perfluoroisopropyl groups (F64PcCu), more facile ion transport was observed, but the electron mobility was strongly suppressed and then limited the rate of electrochromic switching [2]. In this work, a new type of phthalocyanine with only 4 peripheral F atoms replaced by perfluoroisopropyl groups (F40PcCu) is studied. Notably, the distribution of the bulky, perfluoroalkyl group is on adjacent 2 phthalocyanine quadrants, leaving two quadrants able to pi-stack, a feature not present in F64PcCu. Physical vapor deposition yielded homogeneous thin films of 1 nm - 50 nm average film thickness. The dependence of the intermolecular coupling on the film thickness was analyzed by in situ UV/Vis spectroscopy. The films showed constantly small spectral splitting from the monolayer regime throughout the studied range of thicknesses, well into the range of bulk structure. A packing of molecules that leads to weak electronic coupling of adjacent phthalocyanine cores was thereby indicated. The electrochromic characteristics were investigated by electrochemical and spectroelectrochemical measurements with an aqueous solution of KCl as electrolyte. The optical absorption spectra revealed reversible changes of the films upon reduction with intercalation of the K+ counter ions and re-oxidation with extraction of the counter ions. As typical for phthalocyanines, absorption in the Soret- and Q-band was strongly attenuated in the reduced state and an additional absorption band was established between the two, around 500 nm. Therefore, the films showed intense changes of color upon reduction and re-oxidation. The films provided a well-balanced, equally fast transport of electrons and ions with effective diffusion coefficients in the range of 10-10 cm2 s-1. Fast and stable electrochromic switching to 90 % of the final state was obtained within about 0.3 s after applying a potential difference of about 1.5 V upon either reduction or re-oxidation. Reversible and stable switching of the films was achieved over at least 200 cycles. We consider F40PcCu to have an optimized substitution pattern to provide equally fast transport of electrons and ions in the films and, therefore, to provide very attractive switching characteristics which might be of interest as electrochromic layers in smart windows and smart mirrors. 1. S. Nagel, M. Lener, C. Keil, R. Gerdes, Ł. Łapok, S. M. Gorun and D. Schlettwein, "Electrochromic Switching of Evaporated Thin Films of Bulky, Electronic Deficient Metallo-Phthalocyanines", J. Phys. Chem. C, 2011, 115, 8759–8767. 2. J. Weissbecker, A. Loas, S. M. Gorun and D. Schlettwein, "Switching of the Rate-limiting Step in the Electrochromic Reduction of Fluorinated Phthalocyanine Thin Films by Decreased Intermolecular Coupling", Electrochimica Acta, 2015, 157, 232–244.
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