Mesoporous photovoltaic devices

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

Photovoltaic cells based on the sensitization of mesoporous titanium dioxide by Ru(II) complex dyes in conjunction with the I.3 /U redox couple as a mediator have proved very efficient at exploiting this principle. In such systems, the ionic mediator travels back and forth by diffusion from the working electrode to the counterelectrode, to shuttle to the sensitizer the electrons that have gone through the electrical circuit [18, 21, 84]. Recently, solid-state devices have been described where the liquid electrolyte present in the pores of the nanocrystalline oxide film is replaced by a large-bandgap p-type semiconductor acting as a hole-transport medium [85 88]. 

The back electron transfer reaction between conduction-band electrons and 13 (Eq. (51)) is the ultimate fate of photoinjected carriers. This reaction can be directly followed by measuring the dark current of the photovoltaic cell. The latter should be kept at a minimal level as it determines the photovoltage and, hence, the overall conversion efficiency of the device [93-95]. On mesoporous Ti02 electrodes sensitized by cw-Ru (dcbpy)2(NCS)2, the rate of the back reaction of injected electrons with 13 was measured from intensity-modulated experiments and was observed to... 

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