Mass Transport of Polypyridyl Cobalt Complexes in Dye-Sensitized Solar Cells with Mesoporous TiO₂ Photoanodes

Abstract

The diffusion of Co(DTB) 3 n+ (where DTB ) 4,4′-di-tert-butyl-2,2′-bipyridine) in dye-sensitized solar cells (DSSCs) was investigated. Current vs time experiments reveal that DSSCs with cobalt complex-based mediators display a qualitatively different response to the onset of illumination than do DSSCs with I -/I 3 -, exhibiting a sharp decline in the initial current over ∼1 s or less. The results of these and other experiments conducted on DSSC sandwich cells are consistent with insufficient mass transport of Co(DTB) 3 3+ to the cathode, resulting in solution polarization and limited photocurrents. Rotating disk electrode voltammetry on Co(DTB) 3 n+ and I 3in acetonitrile was performed to compare the rates of diffusion within bulk solution and typical mesoporous titania films. After accounting for viscosity differences, it was found that the effective diffusion coefficient of Co(DTB) 3 n+ through typical titania films in DSSCs is 0.5-3 × 10 -7 cm 2 s -1 , about 1 order of magnitude slower than I 3 -. This difference may be attributed to the greater size and slower bulk diffusion of Co(DTB) 3 n+ , greater viscosity of cobalt complex solutions, and a possible electrostatic surface interaction of Co(DTB) 3 n+ within the TiO 2 film.

A Trisheteroleptic Cyclometalated RuII Sensitizer that Enables High Power Output in a Dye‐Sensitized Solar Cell

“…Note that the high h and IPCE values (e.g., 50 % over 400-600 nm) for 3 were obtained without the use of blocking layers. [30,31] We observe a much larger Warburg feature in our EIS measurements for EL2 than for EL1 ( Figure S5), which we ascribe to mass-transport limitations; [32] thus, investigations are underway to optimize cells of 3 with cobalt electrolytes using thinner TiO 2 films.…”

Trifunctional TiO2 Nanoparticles with Exposed {001} Facets as Additives in Cobalt‐Based Porphyrin‐Sensitized Solar Cells

“…Therefore, despite their theoretical superiority, DSCs equipped with porphyrin sensitizers and Co(polypyridyl) redox couples largely fail to outperform those with ruthenium-based sensitizers and the I − /I 3 − system. Their inferior performance is primarily attributable to the severe molecular aggregation in porphyrin sensitizers induced by π-π stacking of the heterocyclic body on TiO 2 NP surfaces, charge recombination between the electron in the conduction band of the TiO 2 and porphyrin radical cation [ 10 ] as well as the sluggish mass transport of cobalt mediators in conventional mesoporous TiO 2 fi lms, [ 11 ] which lead to lower short-circuit currents ( J SC ) and consequently to low PCE.…”

Improving the efficiency of dye-sensitized solar cells by photoanode surface modifications

“…Use of a LiBr/Br2 mixture as redox in a series of electrolytes led to rather low efficiencies [17]; however, the Br − /Br3 − redox system was shown to give better results using eosin as a sensitizer [18]. As for transition metal systems, the diffusion constant of a cobalt-based redox couple Co(DTB3 2+/3+ ) was found to be at least one order of magnitude lower than that for the triiodide ions inside the pores of dye-sensitized TiO2 [19]. Finding a good way to block recombination reactions or alternatively to identify new one-electron redox systems with inherent slow recombination appears to be the current challenge [8].…”