A series of iron polypyridyl redox shuttles were synthesized in the 2+ and 3+ oxidation states and paired with a series of wide optical gap organic dyes with weak aryl ether electron‐donating groups. High voltage dye‐sensitized solar cell (HV‐DSC) devices were obtained through controlling the redox shuttle energetics and dye donor structure. The use of aryl ether donor groups, in place of commonly used aryl amines, allowed for the lowering of the dye ground‐state oxidation potential which enabled challenging to oxidize redox shuttles based on Fe2+ polypyridyl structures to be used in functional devices. By carefully designing a dye series that varies the number of alkyl chains for TiO2 surface protection, the recombination of electrons in TiO2 to the oxidized redox shuttle could be controlled, leading to HV‐DSC devices of up to 1.4 V.

Increasing the rate of productive interfacial electron transfer reactions in dye-sensitized solar cells is critically important toward improving device performances. Preorganized electron transfer systems at a metal oxide interface are an interesting approach toward favoring fast electron transfer reactions. This study focuses on facilitating electron transfer reactions from a redox shuttle to an oxidized dye at a TiO 2 surface via a transient redox shuttle− dye coordination complex. By design, the cobalt redox shuttle is supported by a pentadentate polypyridyl ligand with a remaining labile coordination site on the metal. The organic dye is designed with pyridyl groups on the donor region for coordinating to the open site on the redox shuttle to preorganize the redox shuttle−dye pair via a Lewis acid−Lewis base interaction. DSC devices fabricated with this dye−redox shuttle pair are studied via current−voltage curves, incident photon-to-current conversion efficiencies (IPCEs), photocurrent dynamics, electrochemical impedance spectroscopy, and transient absorption spectroscopy. Results show that dye binding to the redox shuttle increases the rate of dye regeneration, even in the complex electrolyte environment where coordinating species such as tertbutylpyridine and multiple oxidation states of the redox shuttle are present, leading to a dramatically higher performance of the DSC device under fluorescent lighting (13.0% for [Co(PY5Me 2 )(MeCN)] 3+/2+ versus 5.6% PCE for [Co(bpy) 3 ] 3+/2+ ).

A Cu complex featuring a hexadentate ligand was synthesized and evaluated as a redox shuttle in dye-sensitized solar cell (DSC) devices, which exhibited excellent performance under low-light conditions. Cu-based redox shuttles (RSs) have been shown to perform remarkably well under low-light conditions; however, most of the known Cu-based RSs employ bidentate pyridyl ligands and often require bulky flanking groups adjacent to the nitrogen donors of these ligands to prevent distortion and binding of exogenous Lewis bases such as 4-tert-butylpyridine (TBP) that are added to enhance cell performance. Without the bulky substituents, the bidentate ligands are susceptible to ligand exchange with TBP. In this context, we have developed a Cu-based RS with a preorganized multidentate ligand designed to facilitate efficient electron transfer kinetics and high stability via the chelate effect. The Cu system, [Cu­(bpyPY4)]2+/+, reported here is supported by the hexadentate polypyridyl ligand bpyPY4 (6,6′-bis­(1,1-di­(pyridine-2-yl)­ethyl)-2,2′-bipyridine) and examined as a RS in DSCs. From X-ray crystallography and variable-temperature 1H NMR studies, bpyPY4 provides a dynamic coordination environment around the metal center. Cyclic voltammetry and UV–visible and NMR spectroscopy indicate that noncoordinated pyridyl donors block binding of TBP to copper. DSC devices using [Cu­(bpyPY4)]2+/+ as the redox electrolyte gave a power conversion efficiency (PCE) value of 4.9% under 1 sun illumination (100 mW/cm2). Strikingly, the device performance increased to 11.11% when irradiated with 2400 lux (0.5 mW/cm2) via a fluorescent lamp light source and improved further to 15.2% PCE at 13500 lux (2.10 mW/cm2). The Cu redox shuttle is an intriguing candidate for implementation with narrow band gap sensitizers with low oxidation potentials, which are important for high photocurrent DSC devices.

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