Factors which affect and limit the performance of substituted tris(bipyridine)cobalt(II) complexes in dye-sensitized solar cells

Significant efforts were made towards the development of a novel, solid state dye-sensitized solar cell (DSSC) with a thin layer of cobalt redox polymer on the surface of the photoanode. Ultimately, a thin cobalt redox polymer film was successfully grown in a surface-initiated radical polymerization. SEM and XPS experiments suggest that polymerization was initiated at the surface by the dye, but that the active radical was not well-deactivated, resulting in isolated regions of uncontrolled polymer growth. Over time, these islands of polymer merged, forming a continuous film on the macroscopic surface of the titania layer. Unfortunately, the film adversely affected photoanode performance.;A suite of DSSC characterization experiments was developed which could be rapidly performed on a standard two electrode sandwich cell. In combination with a novel technique for mediator replacement without cell disassembly, this suite was utilized to probe the influence of several parameters on DSSC functioning in order to identify limitations of cells mediated by cobalt complexes. The results clearly implicate diffusion of cobalt(III) to the cathode as frequently rate-limiting.;A novel three electrode DSSC experiment, modified from two existing techniques, was developed which incorporated a split photoanode in conjunction with a customized potentiostat circuit to evaluate cathode performance. Interestingly, DSSCs with poorer cathodes exhibited only modest (or no) charge transfer overpotentials on the cathode. More perplexing, mass transfer overpotentials were not observed, even in clearly diffusion-limited cases. Whether these results are due to a deficiency in the experiment or are a consequence of the DSSC system being extremely sensitive to small overpotentials is uncertain.;Although conventional electrochemical models do not predict mass transfer of cobalt(III) to be a problem, actual DSSCs are more complicated than these simple models. In particular, diffusion of cobalt(III) through the mesoporous titania layer of the cell may be hampered by the layer's small pore size and/or tortuosity. Preliminary experiments using a modified rotating disk electrode indicate that this is probably the case. Initial results suggest that the effective diffusion coefficient (uncorrected for porosity) of cobalt bipyridine complexes through mesoporous titania films is no greater than 1 x 10 -7 cm2 s-1.