Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study

Abstract

We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO(2) surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)(3)](3+) species into contact with the TiO(2) surface. This translates into a high probability of intercepting TiO(2)-injected electrons by the oxidized [Co(bpy)(3)](3+) species, lying close to the N719-sensitized TiO(2) surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.

A Spin Crossover Transition in a Mn(II) Chain Compound

“…With this S = 1/2 spin state, the deduced effective g value is ca. 4.8, close to those observed in other isolated HS Co(II) complexes [22,[48][49][50][51][52][53]. …”

“…3.2 cm 3 •K•mol −1 per Co(II) ion, a value within the normal range (2.8-3.4 cm 3 •K•mol −1 ) observed in octahedral monomeric isolated Co(II) complexes [22,[48][49][50][51][52][53] These values are higher than the spin-only contribution of an S = 3/2 ion (1.875 cm 3 •K•mol −1 ) due to the orbital contribution of the 4 T1g ground state of octahedral high spin Co(II) ions. When the sample is cooled, χmT shows a smooth decrease due to the first-order spin orbit coupling arising from the 4 T1g ground state.…”

A new terpyridine cobalt complex redox shuttle for dye-sensitized solar cells

“…This computational approach has been already validated in our previous work. [45] The solution free energy between difference the oxidized and the reduced species was compared with the experimental oxidation potentials. Oxidation potentials measured against the NHE electrode were reported against the vacuum level set at -4.44 eV vs. NHE.…”

“…This approach was shown to provide reorganization energy values in close agreement to experimental estimates. [30,45] 3. Results and discussion…”

“…An interesting conclusion of our previous work concerned the mechanism of regeneration of the oxidized dye by the reduced Co(II) complex, [45] whereby a low-spin/high-spin equilibrium of the Co(II) ions seems to play a role in accessing fast regeneration rates, therefore high short-circuit photocurrent densities. A lower reorganization energy () was calculated for the low-spin regeneration pathway, with bis-tridentate Co(II)/Co(III) complexes (e.g.…”