Thin (~50 nm) film hematite photoanodes doped with different dopants at a concentration of 1 cation% display internal solar to chemical (ISTC) conversion efficiency in the following order: Sn > Nb > Si > Pt > Zr > Ti > Zn > Ni > Mn.
Transparent Fe1-xNixOOH overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn-and Zn-doped hematite (α-Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the co-catalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1-xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1-xNixOOH cocatalysts can play different roles depending on the underlying hematite photoanode. The effect of the cocatalyst is not always limited to changes in the surface properties, but also to an increase in hole current from the bulk to the surface that indicates a possible crosslink between surface and bulk processes. Manuscript Hematite (-Fe2O3) is an attractive material for solar water splitting based on its favorable properties as a photoanode material in photoelectrochemical (PEC) cells. 1 However, the performance of state-of-theart hematite photoanodes 2-5 is still far short of the maximum theoretical efficiency, both in terms of photocurrent and photovoltage. One route for improving photoanode performance is through use of various co-catalysts which reduce the overpotential for water photo-oxidation, thereby leading to a cathodic shift in the applied bias. 6-9 One of the most promising materials for use as a co-catalyst is earth abundant Fe1-xNixOOH. For the remainder of the manuscript, we will refer to Fe1-xNixOOH as "FeNiOx", a commonly used abbreviation. FeNiOx overlayers have shown similar improvements in photoelectrochemical performance as more expensive IrOx based co-catalysts. 10 FeNiOx overlayers can be produced easily by a variety of methods 11-15 and they are stable in alkaline solutions, 12 as the oxyhydroxide phase Fe1-xNixOOH. 16 In addition, it has recently been shown 17 that using a photoelectrochemical deposition method, very thin and transparent FeNiOx overlayers can be deposited to avoid optical (absorption) losses in the photoanode. Significant cathodic shifting of the onset potential for water photo-oxidation is typically observed for FeNiOx coated hematite photoanodes. 18,19 Generally, the changes in performance have been attributed to a reduction in the surface recombination either as a result of surface passivation, 20 hole storage in the overlayer, 21 or p-n junction formation. 22 For ultrathin photoelectrodeposited FeNiOx overlayers, improved catalysis has been suggested as the reason for improvement. 17 I...
Doping with Ti enhances the electron conductivity and photoelectrochemical properties in hematite (α-Fe 2 O 3 ) photoanodes with respect to those of undoped hematite photoanodes. However, the optimal doping level is unknown. This work examined the influence of the Ti doping level on the photoelectrochemical properties of thin-film (∼50-nm) hematite photoanodes. The films were deposited by pulsed laser deposition (PLD) on glass substrates coated with transparent electrodes (fluorinated tin oxide, FTO) from Tidoped Fe 2 O 3 targets with different Ti concentrations: 0 (undoped), 0.25, 0.8, 1, and 7 cation %. The film thicknesses, morphologies, microstructures, and optical properties were nearly the same for all of the photoanodes, thereby enabling systematic comparison of the effect of the doping level without spurious side effects related to morphological variations. The photoelectrochemical performances of all of the Ti-doped photoanodes were considerably higher than that of the undoped photoanode. Among the doped photoanodes, the performance of the heavily doped (7 cation %) photoanode was found to be lower than those of the photoanodes with doping levels of ≤1 cation %. Complementary measurements with a hole scavenger (H 2 O 2 ) and intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that, for the doped photoanodes, both the charge-separation and charge-transfer efficiencies improved with decreasing doping levels and were considerably lower for the heavily doped photoanode than for the lightly doped photoanodes.
H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.
Hematite (α-Fe2O3) is a leading photoanode candidate for photoelectrochemical water splitting. Despite extensive research efforts, the champion hematite photoanodes reported to date have achieved less than half of the maximal...
Strong interference in ultrathin film semiconductor absorbers on metallic back reflectors has been shown to enhance the light harvesting efficiency of solar cell materials. However, metallic back reflectors are not suitable for tandem cell configurations because photons cannot be transmitted through the device. Here, we introduce a method to implement strong interference in ultrathin film top absorbers in a tandem cell configuration through use of distributed Bragg reflectors (DBRs). We showcase this by designing and fabricating a photoelectrochemical-photovoltaic (PEC-PV) stacked tandem cell in a Vshaped configuration where short wavelength photons are reflected back to the photoanode material (hematite, α-Fe2O3), whereas long wavelength photons are transmitted to the bottom silicon PV cell. We employ optical simulations to determine the optimal thicknesses of the DBR layers and the V-shape angle to maximize light absorption in the ultrathin (~10 nm thick) hematite film. The DBR spectral response can be tailored to allow for a more than threefold enhancement in absorbed photons compared to a layer of the same thickness on transparent current collectors. Using a DBR to couple a bottom silicon PV cell with an ultrathin hematite top PEC cell, we demonstrate unassisted solar water splitting and show that DBRs can be designed to enhance strong interference in ultrathin films while enabling stacked tandem cell configuration.
Optimising the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects and photoelectrochemical properties. Here, we use complementary characterisation techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT) and intensity modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat.%) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO) coated glass substrates. The deposition was carried out at 300 °C, followed by annealing at 500 °C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, the non-uniform Ti redistribution seems to be the reason for a reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Tidoped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties.
Potentiodynamic discharge measurements of hematite photoanodes, supported by micro-kinetic modeling, indicate that parallel reaction pathways prevail in water photo-oxidation. Their co-existence may lead to collective phenomena with complex dynamics.
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