A Porous Multilayer Dye-Based Photoelectrochemical Cell That Unexpectedly Runs in Reverse

Abstract

Porous dye multilayers based on cavity-containing porphyrinic molecular squares are capable of sensitizing flat indium−tin oxide (ITO) electrodes to photocurrent production with visible light. In aqueous I3-/I- solutions the sensitized electrodes unexpectedly produce cathodic photocurrents, the opposite of the anodic photocurrents generally observed with conventional dye-senstized solar cells (DSSCs). In constrast to DSSCs, which work by electron injection from a dye excited state into the photoelectrode's conduction band, the mechanism of photocurrent generation is I3- quenching of the porphyrin square excited state to produce an oxidized dye. Quenching is possible, despite the short excited state lifetime, because of ground state donor (chromophore)/acceptor (quencher) complex formation. Following quenching, ground state redox hopping through the multilayer structure delivers the oxidizing equivalent to the ITO electrode. One consequence is the photocurrent production can be systematically increased by adding dye layers and collecting more light (behavior not usually seen with DSSCs). Another is that only small photovoltages can be produced. A third is that the photocurrent production scheme is inoperable when ITO is replaced by titanium dioxide, because the electrons produced are energetically incapable of reaching the TiO2 conduction band. Perhaps most importantly, to the extent that the first part of the scheme (molecular quenching) occurs in conventional cells, the photocurrent efficiencies of these cells will be diminished.