Photocurrent Enhancement by Multilayered Porphyrin Sensitizers in a Photoelectrochemical Cell

Multilayer Zn­(II) tetraphenylporphyrin chromophores, assembled using copper-catalyzed azide–alkyne cycloaddition (CuAAC), provide a new sensitization scheme that could be useful in dye-sensitized solar cells (DSSCs). We report on the photoelectrochemical responses of multilayer films of Zn­(II) 5,10,15,20-tetra­(4-ethynylphenyl)­porphyrin (<b>1</b>) assembled on planar ITO substrates operating as a p-type DSSC using three different redox mediators. The traditional I^–/I₃^– redox couple results in the greatest short circuit current densities (<i>J</i>_SC) but very low open circuit potentials (<i>V</i>_OC). The use of cobalt sepulchrate ([Co­(sep)]^2+/3+) and cobalt tris-bipyridine ([Co­(bpy)₃]^2+/3+) as redox mediators generates higher <i>V</i>_OC values, but at the expense of lower photocurrents. These results highlight the inherent differences in the interactions between the redox mediator and Zn­(II) tetraphenylporphyrin multilayer films. Increasing the porphyrin content through multilayer growth proved to be effective in increasing the performance of photoelectrochemical cells with all three redox mediators. Cells using I^–/I₃^– reached maximum performance (power output) at five porphyrin layers, [Co­(bpy)₃]^2+/3+ at five layers, and [Co­(sep)]^2+/3+ at three layers. For all mediators, <i>J</i>_SC increases with the addition of porphyrin layers beyond a monolayer. However, <i>J</i>_SC reaches a maximum value at a point greater than one layer after which it decreases, presumably due to exciton diffusion limitations and the insulating effects of the multilayer film. Similarly, all cells also reach a maximum <i>V</i>_OC beyond one porphyrin layer. We show that porphyrin arrays assembled using newly developed CuAAC layer-by-layer growth may be useful as a multilayer sensitization scheme for use in photoelectrochemical cells