Oxidovanadium(IV/V) Complexes as New Redox Mediators in Dye-Sensitized Solar Cells: A Combined Experimental and Theoretical Study

Abstract

Corrosiveness is one of the main drawbacks of using the iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs). Alternative redox couples including transition metal complexes have been investigated where surprisingly high efficiencies for the conversion of solar to electrical energy have been achieved. In this paper, we examined the development of a DSSC using an electrolyte based on square pyramidal oxidovanadium(IV/V) complexes. The oxidovanadium(IV) complex (Ph4P)2[V(IV)O(hybeb)] was combined with its oxidized analogue (Ph4P)[V(V)O(hybeb)] {where hybeb(4-) is the tetradentate diamidodiphenolate ligand [1-(2-hydroxybenzamido)-2-(2-pyridinecarboxamido)benzenato}and applied as a redox couple in the electrolyte of DSSCs. The complexes exhibit large electron exchange and transfer rates, which are evident from electron paramagnetic resonance spectroscopy and electrochemistry, rendering the oxidovanadium(IV/V) compounds suitable for redox mediators in DSSCs. The very large self-exchange rate constant offered an insight into the mechanism of the exchange reaction most likely mediated through an outer-sphere exchange mechanism. The [V(IV)O(hybeb)](2-)/[V(V)O(hybeb)](-) redox potential and the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [V(IV)O(hybeb)](2-) were calculated by means of density functional theory electronic structure calculation methods. The complexes were applied as a new redox mediator in DSSCs, while the cell performance was studied in terms of the concentration of the reduced and oxidized form of the complexes. These studies were performed with the commercial Ru-based sensitizer N719 absorbed on a TiO2 semiconducting film in the DSSC. Maximum energy conversion efficiencies of 2% at simulated solar light (AM 1.5; 1000 W m(-2)) with an open circuit voltage of 660 mV, a short-circuit current of 5.2 mA cm(-2), and a fill factor of 0.58 were recorded without the presence of any additives in the electrolyte.

Controlling Nuclearity and Stereochemistry in Vanadyl(V) and Mixed Valent VIV/VV Complexes of Oxido‐Pincer Pyridine‐2,6‐dimethanol Ligands

“…The EPR parameters for the pydim and pydip derivative are similar to those of related mononuclear O=V IV complexes, , , and comparable dinuclear compounds with class I valence‐isolated [OV IV (μ‐O)V V O] units , , , , . Similar valence delocalized (class II or III) behavior as found for [V 2 O 3 (pyphen) 2 ] · – has been previously reported for the radical anionic complexes [(L)OV IV (µ‐O)V V O(LO] · – with tridentate ONO 2– ligands based on hydroxy phenoxy hydrazides,, , , salenes, carboxy‐salicylaldiminates, or azophenolates,, and also the [OV IV (μ‐O oxido )(μ‐O phen )V V O] · 2+ complexes containing pentadentate (N′NONN′) 3– 2,6‐bis(phenoxymethyl‐ N , N ‐dimethylaminomethyl)aminomethyl)‐phenolates,, or complexes [(L)OV IV (µ‐O)V V O(LO] · 3+ complexes containing neutral N′NN′ ligands of the 2,6‐bis[benzimidazol‐2′‐yl]pyridine or ‐amine type, tris(2‐pyridylmethyl)amine, or 1,4,7‐triazacyclononane derivatives …”

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

“…Therefore, the low difference between TiO2 CB edge and the Fermi level of the mediator, results to a low UOC of the cell. [211]. For DSSCs with N719 as dye and Pt as CE, characteristic performance parameters of UOC = 0.66 V, jSC = 5.2 mA•cm −2 , and PCE = 2% under 1 sun Basic additives such as TBP were excluded from the electrolyte formulation since ESI mass spectra and X-ray structures confirmed that these manganese compounds are susceptible to ligand exchange with TBP to form trans-TBP substituted [Mn(acac) 2 TBP] + .…”

Potential Application of Redox Mediators and Metabolic Uncouplers in Environmental Research – A Review

“…Inorganic redox mediator such as iodide/triodide , manganaese(II/III) , and potassium ferricyanide are generally used in electricity generation. Oxidovanadium(IV/V) and cobalt(II)‐bis[2,6‐bis(1'‐butylbenzimidazole‐2‐yl)pyridine] are complex redox mediators . The structures of the mentioned redox mediators are presented in Fig.…”

“…The main drawback of using triiodide in dye‐sensitized solar cells is the property of corrosiveness. Hence, the triiodide was replaced with oxidovanadium complex redox mediators because of their good electron exchange and transfer rate . In quantum dot‐sensitized solar cells with a redox potential of ∼0.51 V, Mn(II/III) complexes produce high voltages of 1 V. Low solubility prevented the redox mediators from sustaining high photocurrent due to mass transport limits.…”