In this Perspective, we introduce intensity modulated photocurrent/voltage spectroscopy (IMPS and IMVS) as powerful tools for the analysis of charge carrier dynamics in photoelectrochemical (PEC) ...cells for solar water splitting, taking hematite (α-Fe2O3) photoanodes as a case study. We complete the picture by including photoelectrochemical impedance spectroscopy (PEIS) and linking the trio of PEIS, IMPS and IMVS, introduced here as photoelectrochemical immittance triplets (PIT), both mathematically and phenomenologically, demonstrating what conclusions can be extracted from these measurements. A novel way of analyzing the results by an empirical approach with minimal presumptions is introduced, using the distribution of relaxation times (DRT) function. The DRT approach is compared to conventional analysis approaches that are based on physical models and therefore come with model presumptions. This work uses a thin film hematite photoanode as a model system, but the approach can be applied to other PEC systems as well.
Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. One of the greatest challenges towards large-scale utilization of this technology is ...reducing the hydrogen production cost. The conventional electrolyser architecture, where hydrogen and oxygen are co-produced in the same cell, gives rise to critical challenges in photoelectrochemical water splitting cells that directly convert solar energy and water to hydrogen. Here we overcome these challenges by separating the hydrogen and oxygen cells. The ion exchange in our cells is mediated by auxiliary electrodes, and the cells are connected to each other only by metal wires, enabling centralized hydrogen production. We demonstrate hydrogen generation in separate cells with solar-to-hydrogen conversion efficiency of 7.5%, which can readily surpass 10% using standard commercial components. A basic cost comparison shows that our approach is competitive with conventional photoelectrochemical systems, enabling safe and potentially affordable solar hydrogen production.
Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others ...result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ(λ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ(λ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite's absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO
and BiVO
photoanodes, demonstrating the broader utility of the method for determining ξ(λ).
Semiconductor photoelectrodes for solar hydrogen production by water photoelectrolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers. Iron oxide (α-Fe(2)O(3)) is one ...of few materials meeting these requirements, but its poor transport properties present challenges for efficient charge-carrier generation, separation, collection and injection. Here we show that these challenges can be addressed by means of resonant light trapping in ultrathin films designed as optical cavities. Interference between forward- and backward-propagating waves enhances the light absorption in quarter-wave or, in some cases, deeper subwavelength films, amplifying the intensity close to the surface wherein photogenerated minority charge carriers (holes) can reach the surface and oxidize water before recombination takes place. Combining this effect with photon retrapping schemes, such as using V-shaped cells, provides efficient light harvesting in ultrathin films of high internal quantum efficiency, overcoming the trade-off between light absorption and charge collection. A water photo-oxidation current density of 4 mA cm(-2) was achieved using a V-shaped cell comprising ~26-nm-thick Ti-doped α-Fe(2)O(3) films on back-reflector substrates coated with silver-gold alloy.
In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active ...field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.
Hematite is a promising photoanode material for photoelectrochemical water splitting. The main challenges toward achieving efficient hematite photoanodes are reviewed. Particular focus is placed on linking the electronic structure to the processes of light absorption, charge transport, and the surface electrochemical reaction. The main losses are identified and discussed, while promising avenues for future research are highlighted.
The oxygen evolution reaction (OER) at the surface of semiconductor photoanodes is critical for photoelectrochemical water splitting. This reaction involves photo-generated holes that oxidize water ...via charge transfer at the photoanode/electrolyte interface. However, a certain fraction of the holes that reach the surface recombine with electrons from the conduction band, giving rise to the surface recombination loss. The charge transfer efficiency, η
, defined as the ratio between the flux of holes that contribute to the water oxidation reaction and the total flux of holes that reach the surface, is an important parameter that helps to distinguish between bulk and surface recombination losses. However, accurate determination of η
by conventional voltammetry measurements is complicated because only the total current is measured and it is difficult to discern between different contributions to the current. Chopped light measurement (CLM) and hole scavenger measurement (HSM) techniques are widely employed to determine η
, but they often lead to errors resulting from instrumental as well as fundamental limitations. Intensity modulated photocurrent spectroscopy (IMPS) is better suited for accurate determination of η
because it provides direct information on both the total photocurrent and the surface recombination current. However, careful analysis of IMPS measurements at different light intensities is required to account for nonlinear effects. This work compares the η
values obtained by these methods using heteroepitaxial thin-film hematite photoanodes as a case study. We show that a wide spread of η
values is obtained by different analysis methods, and even within the same method different values may be obtained depending on instrumental and experimental conditions such as the light source and light intensity. Statistical analysis of the results obtained for our model hematite photoanode show good correlation between different methods for measurements carried out with the same light source, light intensity and potential. However, there is a considerable spread in the results obtained by different methods. For accurate determination of η
, we recommend IMPS measurements in operando with a bias light intensity such that the irradiance is as close as possible to the AM1.5 Global solar spectrum.
A 2‐nm thick Nb2O5 underlayer deposited by atomic layer deposition increases the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection. ...Absorbed photon‐to‐current efficiencies (APCE) as high as 40%, which are one of the highest ever reported with hematite photoanodes, are obtained at 400 nm at +1.43 V vs. RHE.
Numerous studies have shown that the addition of different impurities as dopants in hematite (α-Fe 2 O 3 ) photoanodes improves water photo-oxidation. The improvements observed may have resulted from ...electronic and/or catalytic effects, but also from changes in the layer morphology and microstructure induced by different precursors. The latter could be quite substantial, especially in mesoporous layers produced by chemical routes, making it difficult to make a systematic comparison between different dopants. This work attempts to overcome this difficulty by comparing different dopants in thin films produced by pulsed laser deposition (PLD), a physical deposition method that produces highly reproducible films with no significant variations in the microstructure and morphology. This enables systematic comparison of the effect of different dopants without spurious side effects due to variations in the microstructure and morphology. Thus, we examine the effect of Sn, Nb, Si, Pt, Zr, Ti, Zn, Ni and Mn dopants on the photoelectrochemical properties of thin (∼50 nm) film hematite photoanodes deposited by PLD from Fe 2 O 3 targets doped with ∼1 cation% of the respective dopants onto FTO coated glass substrates. The morphology and microstructure of the films were nearly the same, independent of the different dopants in the films. The Sn-doped hematite photoanode outperformed all the other photoanodes that were examined in this work in both the photocurrent and photovoltage, achieving the highest photocurrent (∼1 mA cm −2 ) and the lowest onset potential (∼1.1 V RHE ). Based on a figure of merit that accounts for the maximum photocurrent × photovoltage product ( i.e. , power) as well as the potential at which the maximum power is achieved, our photoanodes ranked in the following order: Sn > Nb > Si > Pt > Zr > Ti > Zn > Ni > Mn. These observations are not always consistent with other reports on doped hematite photoanodes, suggesting that the photoelectrochemical properties and performance depend not only on the identity of the dopant but also on the dopant concentration, distribution and the morphology and microstructure of the photoanode in which the dopant is incorporated.
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