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Published asKlahrHowever, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material ( J(V ) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by Atomic Layer Deposition to study the photoelectrochemical properties of this material under water splitting conditions. We employed Impedance Spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat band potential. These findings demonstrate the utility of Impedance Spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement. 3 I ntroductionAs part of the quest to develop better and cleaner energy conversion and storage systems, the direct conversion of sunlight into chemical fuels has become a subject of renewed interest.One attractive example is the use of semiconductors to harness solar photons to split water, thereby producing hydrogen as a chemical fuel. In order to achieve this, a given material must satisfy a number of stringent requirements including visible light absorption, efficient charge carrier separation and transport, facile interfacial charge-transfer kinetics, appropriate positions of the conduction and valence band energy levels with respect to required reaction potentials and good stability in contact with aqueous solutions. 1 While such systems were heavily investigated several decades ago, no material so far has fulfilled all the required conditions. 2,3 Recent advances in nanotechnology and catalysis, however, greatly increase the prospects of developing a combination of materials capable of efficient conversion of sunlight to chemical fuels. 4 Hematite ( -Fe 2 O 3 ) is a very promising material for photoelectrochemical (PEC) water splitting due to its combination of sufficiently broad visible light absorption, up to 590 nm, and excellent stability under caustic operating conditions. 5,6 However, hematite electrodes are adversely affected by a number of factors including a long penetration depth of visible light due to its indirect band gap transition and a very short minority carrier lifetime and mobility; this combination hinders efficient collection of the minority carriers via the required interfacial charge-transfer reactions. Considerable effort has been devoted to improving the actual efficiency by employing nanostructuring strategies, which disconnects the ligh...

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.