Electrochemical characterization of Prussian blue type nickel hexacyanoferrate redox mediator for potential application as charge relay in dye-sensitized solar cells

Abstract

A polynuclear electronically/ionically (redox) conducting mixed-valent inorganic material such as nickel (II) hexacyanoferrate(II,III), NiHCF, was considered for potential application as a redox mediator (charge relay) in dye-sensitized solar cell (DSSC). The NiHCF redox reactions were found fast and reversible not only when the system was studied as thin film exposed to an aqueous supporting electrolyte but also as bulk material (pasted powder) in solid state, i.e., in the absence of contact with external liquid electrolyte phase. Usefulness of NiHCF material was diagnosed using conventional electroanalytical approaches, solid-state voltammetric methodology, as well as the dynamic electrochemical impedance spectroscopy technique that permitted monitoring of impedance spectra under potentiodynamic conditions. The material was utilized in a mixed-valent state, i.e., as a mixture of K 4 Ni II [Fe II (CN) 6 ] and K 3 Ni II [Fe III (CN) 6 ] in which iron(II) and iron(III) sites were at the 1:1 ratio. Under such conditions, dynamics of electron-hopping between mixedvalent iron sites was maximized. Our DSSC utilized cisdithiocyanoatobis(4,4′-dicarboxylic acid-2,2′-bipyridine) ruthenium(II) dye (N3) adsorbed onto TiO 2 semiconductor and NiHCF as redox mediator. Although performance of our DSSC was not optimized in terms of the NiHCF film thickness and morphology, as well as lower photocurrents in comparison to those characteristic of the iodine/iodide based DSSC were obtained, our system yielded readily fairly high open-circuit photovoltages on the level of 800 mV. An important issue was that the formal potential of NiHCF was more positive relative to the potential of the iodide/triiodide couple while being still more negative than that equivalent to the ground state of the N3 dye. Thus, NiHCF mediator was able to regenerate the dye.

Cobalt Polypyridyl Complexes as Transparent Solution‐Processable Solid‐State Charge Transport Materials

“…[6] Organic solution-processable CTMs also play a vital role in high-performance perovskite solar cells, affording power conversion efficiencies beyond 20%. [12,13] Very recently, copper phenanthroline complexes have been successfully applied as a hole transport material in dye-sensitized solar cells (DSCs), thus pointing to the general prospect of developing transition metal complexes as an emerging class of CTMs. [8,9] Traditionally these materials have been chosen for their superior light absorption and emission properties.…”

Metal Coordination Complexes as Redox Mediators in Regenerative Dye-Sensitized Solar Cells

“…An additional study by Daenake, et al included the no-corrosive ferrocyanide complexes as redox mediators for aqueous media DSSC [181]. The redox potential reported for the complex Fe(CN) 6 4−/3− varies from 0.466 V versus SHE to 0.514 V versus SHE [182][183][184], depending on the electrolyte, and it is likely to reach higher U OC values than those obtained with iodide-based electrolytes. The high molar extinction coefficient of the carbazole dye MK-2 makes it appropriate for thin film sensitization.…”

Transition metal complex redox shuttles for dye-sensitized solar cells

“…115 The same solar conversion efficiencies of 4.3-5.2% and also IPCE results were attained using Fc, Et 2 Fc and EtFc. 117 The current consensus is that water is an undesirable contaminant in DSSC electrolytes as it causes to the formation of iodate, dye desorption, low device efficiencies and device instability. In addition, their results indicated that Et 2 Fc, EtFc and Fc, promote fast dye regeneration by providing the driving forces of 35-46 kJ mol -1 .…”