A number of cobalt complexes of substituted polypyridine ligands were synthesized and investigated as possible alternatives to the volatile and corrosive iodide/triiodide redox couple commonly used as an electron-transfer mediator in dye-sensitized solar cells (DSSCs). The extinction coefficients in the visible spectrum are on the order of 10(2) M(-1) cm(-1) for the majority of these complexes, diminishing competition with the light-harvesting dye. Cyclic voltammetric studies revealed a dramatic surface dependence of the heterogeneous electron-transfer rate, which is surprisingly different for gold, carbon, and platinum electrodes. DSSCs were assembled using a mediator that consisted of a mixture of Co(II) and Co(III) complexes in a 10:1 ratio. DSSCs containing these mediators were used to characterize incident photon-to-current conversion efficiency and photoelectrochemical responses. The best performing of these mediators were identified and subjected to further study. As suggested by electrochemical results, gold and carbon are superior cathode materials to platinum, and no evidence of corrosion on any cathode material was observed. Addition of lithium salts to the mediator solution resulted in a dramatic improvement in cell performance. The observed Li(+) effect is explained in terms of the recombination of injected electrons in the photoanode with the oxidized mediator. The best mediator, based on tris(4,4'-di-tert-butyl-2,2'-dipyridyl)cobalt(II/III) perchlorate, resulted in DSSCs exhibiting efficiencies within 80% of that of a comparable iodide/triiodide-mediated DSSC. Due to the commercial availability of the ligand and the simplicity with which the complex can be made, this new mediator represents a nonvolatile, noncorrosive, and practical alternative as an efficient electron-transfer mediator in DSSCs.

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.

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We realize p- and n-type doping of the organic semiconductor zinc-phthalocyanine using a novel strong organic donor. This allows us to demonstrate the first stable and reproducible organic p-n homojunctions. The diodes show very high built-in potentials, attractive, e.g., for organic solar cells. However, the diode characteristics cannot be described by the standard Shockley theory of the p-n junction since the ideality factor strongly increases with decreasing temperature. We show that this behavior can be explained by deviations from the Einstein relation for disordered materials.

The electrochemical and photoelectrochemical study of noncorrosive electron-transfer mediator mixtures for DSC is reported. These mixtures, characterized by the presence of kinetically fast organic or metal-organic redox couples in conjunction with a Co(II)/(III) relay, exhibited maximum photon-to-current conversion efficiencies (IPCE %) exceeding 80% when employed in the presence of a suitably designed MLCT sensitizer, rivaling with the performances of the classical I-/I3- electrolyte.