Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO 2 electrodes. These capacitance data are complemented by transient optical absorption studies of charge recombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Both measurements are conducted as a function of electrochemical bias applied to the metal oxide film in a threeelectrode photoelectrochemical cell. For both metal oxides, a power law dependence was observed between the half times for charge recombination (t 50% ) and the metal oxide electron density n determined from integration of the capacitance data, t 50% ∝ n -1/R , where R ) 0.27 and 0.30 ( 0.05 for ZnO and TiO 2 , respectively. A numerical model for the recombination dynamics based upon a random walk of electrons between localized sub-bandgap states is found to be in good agreement with experimental observations for both metal oxides. At negative applied potentials, the film capacitance, and therefore electron density, is observed to increase more rapidly with increasingly negative applied potential for the ZnO film compared to the TiO 2 film. This observation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observed for dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamics observed under negative bias may be the origin of the lower open circuit voltages reported previously for dye sensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employing TiO 2 .

The first ruthenium-diiron complex [(mu-pdt)Fe2(CO)5{PPh2(C6H4CCbpy)}Ru(bpy)2]2+ 1 (pdt = propyldithiolate, bpy = 2,2'-bipyridine) is described in which the photoactive ruthenium trisbipyridyl unit is linked to a model of the iron hydrogenase active site by a ligand directly attached to one of the iron centers. Electrochemical and photophysical studies show that the light-induced MLCT excited state of the title complex is localized towards the potential diiron acceptor unit. However, the relatively mild potential required for the reduction of the acetylenic bipyridine together with the easily oxidized diiron portion leads to a reductive quenching of the excited state, instead. This process results in a transiently oxidized diiron unit which may explain the surprisingly high light sensitivity of complex 1.

The effect of the dislocation line density produced by the relaxation of strain in GaAs/In x Ga 1Ϫx As multiquantum wells where xϭ0.155-0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias ͑Ͻ1.5 V͒ dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.

The reaction of a dye cation recombining with an electron in TiO(2), in the presence of Li(+), Ca(2+), and TBA(+) cations, was studied with laser-induced transient absorption measurements. The active cations, Li(+) and Ca(2+), shorten the dye cation lifetime on sensitized TiO(2) but not ZnO electrodes. By combining the absorbance measurements of the dye cation with simultaneous measurements of the current transient, the contribution of the recombination reaction to the current is identified. Furthermore, classical porous electrode theory is used to quantify the behavior of the heterogeneous electrode, and in doing so, the processes contributing to photoinduced current are identified as Helmholtz layer charging, porous electrode charging, recombination reactions, and surface diffusion of the active cations. The rate of charge recombination is proportional to the concentration of initially deposited active cations. The effect of water on the recombination rate and the current is also observed.

ABSTRACT

Nanocrystalline TiO(2) electrodes were studied spectroelectrochemically by observing the simultaneous relaxation of the current and absorbance after applying a voltage step. The absorbance behaved differently in two time regimes: (1) ionic polarization in the oxide electrode, in which charged ions, such as Ti(3+) sites and/or interstitial Ti(4+) sites, move in response to the applied electric field, and (2) the diffusion of Li(+) ions into the TiO(2). These two behaviors were analyzed with equivalent circuit models. Li(+) ions reduce the resistance of the TiO(2) by approximately 90%, increase the capacitance by approximately 350%, and decrease the inductance by approximately 30%. Voltage cycling produces a buildup of intercalated Li(+) ions, lessening the electrode's response to the potential step, and causing it to become a more efficient inductor. The potential distribution in the nanoparticles is described by using a dielectric model in which roughly half the applied potential is dropped across the interface with a Li(+)-ion-containing electrolyte.

Near steady-state photoinduced absorption (PIA) and UV-Vis absorption spectroscopy are used to characterize the pore filling of spiro-MeOTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) into the nanoparticulate TiO2electrode of a solid-state dye-sensitized solar cell (ssDSC). The volumetric ratio of filled to unfilled pore volumes, as well as the optical signature of interacting chemical species, that is, the hole-transfer yield (HTY), are investigated. PIA spectroscopy is used to measure the HTY, relative to the amount of spiro-MeOTAD present, without needing to determine the extinction coefficients of the dye and spiro-MeOTAD cation species. The Beer-Lambert law is used to relate the relative PIA signal to the penetration length of the hole-conductor in the TiO2film. For the sample thickness range of1.4–5 μm investigated here, the optimum characteristic penetration length is determined to be3.1+0.46 μm, which is compared to 1.4 μm for the 200 mgmL−1concentration of spiro-MeOTAD conventionally used. Therefore, doubling the effective penetration of spiro-MeOTAD is necessary to functionalize all the dye molecules in a ssDSC.

No abstract