Photoanode performance

In order to study the influence of surface morphology on the photoanode performance, samples differing in surface structure have been selected. Electron micrographs of representative areas of three different n-WSe2 electrodes are shown in Fig. 2. Substantial differences in surface structure ranging from nearly smooth (Fig. 2a) through moderately structured (Fig. 2b) to extremely high structured (Fig. 2c) are noted. 

Parkinson, B.A., Heller, A., and Miller, B., Effects of cations on the performance of the photoanode in the n-gallium arsenide/potassium selenide (K2Se)-potassium diselenide (K2Se2)-potassium hydroxide/carbon semiconductor liquid junction solar cell, /. Electrochem. Soc., 126,954,1979. 

To detail DSSC technologies, Fig. 18.1 illustrates the modus operandi of DSSCs. Initially, light is absorbed by a dye, which is anchored to the surface of either n- or p-type semiconductor mesoporous electrodes. Importantly, the possibility of integrating both types of electrodes into single DSSCs has evoked the potential of developing tandem DSSCs, which feature better overall device performances compared to just n-or p-type based DSSCs [19-26]. Briefly, n-type DSSCs, such as TiOz or ZnO mesoporous films, are deposited on top of indium-tin oxide (ITO) or fluorine-doped tin oxide (FTO) substrates and constitute the photoanodes. Here, charge separation takes place at the dye/electrode interface by means of electron injection from the photoexcited dye into the conduction band (cb) of the semiconductor [27,28]. A different mechanism governs p-type DSSCs, which are mainly based on NiO electrodes on ITO and/or FTO substrates... 

Iron oxide (Fe Oj) and tungsten oxide (WO ) films have been studied and developed as candidate semiconductor materials for the PEC junction (photoanode). High-temperature synthesis methods, as reported for some high-performance metal oxides, have been found incompatible with multijunction device fabricatioa A low-temperature reactive sputtering process has been developed instead. In the parameter space investigated so far, the optoelectronic properties of WO3 films were superior to those of Fe Oj films, which showed high recombination of photogenerated carriers (Miller et al., 2004). 

A photoanode comprised of flame oxidized carbon doped n-Ti02 films have been reported to perform water splitting with high photoconversion efficiencies [65]. While chemically modified n-Ti02 can be prepared by the controlled combustion of Ti metal in a natural gas flame the authors, in investigating this technique [66], have found reproducibility to be a challenge. Various authors [67,68,69] have discussed in considerable depth issues surrounding the stated photoconversion efficiencies of [65]. 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

The Correlation Between Surface Morphology and Solar Cell Performance of WSe2 and MoSe 2 Photoanodes... 

Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents in addition to corresponding positive and negative photovoltages at solvent-evaporated films of a Chl-oxidant mixture and a Chl-reductant mixture, respectively, on platinum electrodes. Various redox species were examined, respectively, as a donor or acceptor added in an aqueous electrolyte (69). In a typical experiment (71), NAD and Fe(CN)g, each dissolved in a neutral electrolyte solution, were employed as an acceptor for a photocathode and a donor for a photoanode, respectively, and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)J at a Chl-anthrahydroquinone-coated anode were performed under either short circuit conditions or potentiostatic conditions. The reduction of NAD at the photocathode was demonstrated as a model for the photosynthetic system I. In their studies, the photoactive species was attributed to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism for photocurrent generation at the Chi photocathode (71). 

Below stepped-illumination experiments are presented for the photo-assisted electrolysis of water using n-type TiC or SnC photoanode/dark Pt cathode systems. An analysis of these results will be performed, focusing on the influence of the anodic halfcell reaction products upon the electronic state of the semiconductor /electrolyte interface. 

Two types of experimental current scans were performed. After introducing the photoanode into solution, successive current vs. potential (i vs. ) scans were performed at slow sweep rates (2mV/S). All of the photoanode materials studied exhibited an initial drift of the i vs. scans to more anodic bias. This occurred for both uninterrupted scans as well as intermittent scanning. The origin of this aging phenomena is not understood. However, it was observed that after approximately thirty minutes the i vs. scans achieved a profile which was reproduced during further cyclic sweeps. This result was taken as a criterion for proper electrode pre-conditioning. 

Our emphasis in this paper will be to describe and analyze current vs. time scans determined after making stepped-changes in the illumination intensity. These scans were performed after proper photoanode pre-conditioning was indicated. 

Transient Response. The current analysis performed above demonstrates that H+ ion adsorption at the photoanode/electrolyte interface decreases the electronic energy barrier for electron transfer from the bulk conduction band to the electrode surface. 

We have performed an experimental study of photo-assisted electrolysis for illuminated n-type TiC>2 photoanode/dark Pt cathode systems. Analysis of these results indicates that the electronic state of the semiconductor/electrolyte interface is influenced by the electrolysis reaction products, in a manner not previously accounted for. 

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