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Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting

DOI:10.1038/s41563-021-00955-y
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Daniel Grave at Ben-Gurion University of the Negev
Daniel Grave
David S. Ellis
David S. Ellis
David S. Ellis
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Yifat Piekner
Yifat Piekner
Yifat Piekner
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Moritz Kölbach at Oxford PV
Moritz Kölbach
  • Oxford PV
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Abstract and Figures

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 TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining ξ(λ). Although the photogeneration yield spectrum is a key property for photoabsorbers in photovoltaic and photoelectrochemical cells, its characterization remains challenging. An empirical method to extract this parameter through quantum efficiency measurements of ultrathin films is proposed.
Extraction of the photogeneration yield spectrum from optical and photoelectrochemical EQE measurements of a 7-nm-thick haematite film a, Measured (solid line) and calculated (dashed line) transmittance (T), reflectance (R) and absorptance (FA) spectra in front illumination in air. The calculated absorptance in the haematite layer only (Ahaematite) is also shown. b, The calculated haematite absorptance spectra in water and EQE spectra measured at 1.6 VRHE in front and back illumination (FI and BI, respectively). c, The extracted p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectra for front (black) and back (blue) illumination.
Extraction of the photogeneration yield spectrum from optical and photoelectrochemical EQE measurements of a 7-nm-thick haematite film a, Measured (solid line) and calculated (dashed line) transmittance (T), reflectance (R) and absorptance (FA) spectra in front illumination in air. The calculated absorptance in the haematite layer only (Ahaematite) is also shown. b, The calculated haematite absorptance spectra in water and EQE spectra measured at 1.6 VRHE in front and back illumination (FI and BI, respectively). c, The extracted p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectra for front (black) and back (blue) illumination.
… 
TRMC measurements of a 150-nm-thick haematite film a, Photoconductivity transients normalized by the absorbed photon fluence measured by TRMC at different wavelengths. b, Photoconductivity transients normalized by the peak photoconductivity for each wavelength. c, Peak photoconductivity normalized by incident light intensity but not absorptance (left y axis), overlaid with the absorptance FA (right y axis) spectra. The TRMC data are averaged over 300 measurements. The associated error bars in Fig 2c are calculated based on uncertainty in the measured incident photon fluence.
TRMC measurements of a 150-nm-thick haematite film a, Photoconductivity transients normalized by the absorbed photon fluence measured by TRMC at different wavelengths. b, Photoconductivity transients normalized by the peak photoconductivity for each wavelength. c, Peak photoconductivity normalized by incident light intensity but not absorptance (left y axis), overlaid with the absorptance FA (right y axis) spectra. The TRMC data are averaged over 300 measurements. The associated error bars in Fig 2c are calculated based on uncertainty in the measured incident photon fluence.
… 
Comparison of TRMC and photoelectrochemical EQE analysis Comparison of the p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectrum for an ultrathin 7-nm-thick haematite film photoanode with the ϕλ∑μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi \left( {\it{\lambda }} \right){\sum} \mu$$\end{document} spectrum extracted from TRMC measurements of a 150-nm-thick epitaxial haematite film deposited on sapphire. The ϕλ∑μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi \left( {\it{\lambda }} \right){\sum} \mu$$\end{document} spectrum was scaled to plot the data on the same y scale as p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document}. The data points for the TRMC measurements in Fig 3 represent an average of 300 measurements. The solid symbols correspond to measurements taken within an absorbed fluence range of (6–8) × 10¹³ photons per cm² per pulse. The open symbols correspond to measurements taken outside this fluence range. The error bars from the TRMC measurements are calculated based on uncertainty in the measured incident photon fluence and its effect on the measured photoconductivity. The error bars from the EQE measurements reflect the difference between the calculated p¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p$$\end{document}ξ(λ) spectra for front and back illumination.
Comparison of TRMC and photoelectrochemical EQE analysis Comparison of the p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectrum for an ultrathin 7-nm-thick haematite film photoanode with the ϕλ∑μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi \left( {\it{\lambda }} \right){\sum} \mu$$\end{document} spectrum extracted from TRMC measurements of a 150-nm-thick epitaxial haematite film deposited on sapphire. The ϕλ∑μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi \left( {\it{\lambda }} \right){\sum} \mu$$\end{document} spectrum was scaled to plot the data on the same y scale as p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document}. The data points for the TRMC measurements in Fig 3 represent an average of 300 measurements. The solid symbols correspond to measurements taken within an absorbed fluence range of (6–8) × 10¹³ photons per cm² per pulse. The open symbols correspond to measurements taken outside this fluence range. The error bars from the TRMC measurements are calculated based on uncertainty in the measured incident photon fluence and its effect on the measured photoconductivity. The error bars from the EQE measurements reflect the difference between the calculated p¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p$$\end{document}ξ(λ) spectra for front and back illumination.
… 
Contributing and non-contributing components of the absorption spectra a–d, Absorption coefficient (α, black curve) and average contributing absorption coefficient (αC, orange curve) of a 7-nm-thick haematite film (a), estimate for a 150-nm-thick haematite film (described in the text; b), an 11-nm-thick TiO2 film (c) and an 11-nm-thick BiVO4 film (d). For the ultrathin-film cases in panels a, c and d, the contributing absorption coefficient values were estimated from the p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectra extracted from EQE measurements by setting lower and upper limits (dotted lines), where the lower limit corresponds to p¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p$$\end{document} = 1 and the upper limit corresponds to maximal ξ(λ) = 1. The orange and blue shading represent the contributing and non-contributing portions of the absorption spectrum, respectively, for the average estimate.The insets of a–d show the spectra of the non-contributing component, αNC, corresponding to the upper blue-shaded regions in the main figures, along with the upper and lower limits (dotted lines).
Contributing and non-contributing components of the absorption spectra a–d, Absorption coefficient (α, black curve) and average contributing absorption coefficient (αC, orange curve) of a 7-nm-thick haematite film (a), estimate for a 150-nm-thick haematite film (described in the text; b), an 11-nm-thick TiO2 film (c) and an 11-nm-thick BiVO4 film (d). For the ultrathin-film cases in panels a, c and d, the contributing absorption coefficient values were estimated from the p¯ξλ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p \xi \left( {\it{\lambda }} \right)$$\end{document} spectra extracted from EQE measurements by setting lower and upper limits (dotted lines), where the lower limit corresponds to p¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar p$$\end{document} = 1 and the upper limit corresponds to maximal ξ(λ) = 1. The orange and blue shading represent the contributing and non-contributing portions of the absorption spectrum, respectively, for the average estimate.The insets of a–d show the spectra of the non-contributing component, αNC, corresponding to the upper blue-shaded regions in the main figures, along with the upper and lower limits (dotted lines).
… 
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Articles
https://doi.org/10.1038/s41563-021-00955-y
1Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel. 2Department of Materials Engineering and Ilse
Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Be’er Sheva, Israel. 3The Nancy & Stephen Grand Technion Energy
Program (GTEP), Technion – Israel Institute of Technology, Haifa, Israel. 4Institute for Solar Fuels, Helmholtz‐Zentrum Berlin für Materialien und Energie,
Berlin, Germany. 5These authors contributed equally: Daniel A. Grave, David S. Ellis, Yifat Piekner. ✉e-mail: dgrave@bgu.ac.il; avnerrot@technion.ac.il
At the heart of any semiconductor photoabsorber material in
photovoltaic and photoelectrochemical cells for solar energy
conversion is the ability to absorb light and thereby gener-
ate excess mobile charge carriers (electrons and holes) that give rise
to photocurrent and photovoltage1. Many photoabsorber materials
have intricate and complex electronic and optical properties, to the
effect that light can excite electrons and holes into a variety of dif-
ferent states. Although some of these optically excited states are or
can eventually become mobile charge carriers, others are localized
and cannot contribute to photocurrent. These non-contributing
absorption processes include ligand field (d→d) excitations that can
be optically allowed by distortions from inversion symmetry and
many-body effects2,3. Likewise, organic photovoltaic materials also
have different types of photoexcitations and excitons, which have
a direct impact on the photocurrent4,5. The various types of opti-
cal excitations, contributing and non-contributing ones, can over-
lap in wavelength across the material’s absorption spectrum. The
wavelength-dependent ratio between the contributing and overall
absorption defines the photogeneration yield spectrum, ξ(λ), equiv-
alent to the probability that an absorbed photon of wavelength λ
will ultimately generate a mobile charge carrier. This quantity is
therefore of core importance for the development of photovoltaic
and photoelectrochemical devices. However, ξ(λ) can be difficult to
properly characterize, because most of the pertinent measurements
can only detect the photocurrent action spectrum, Jph(λ), which
comprises different optical and electronic factors. An intermingling
of the inherent material properties (for example, absorption pro-
cesses and cross-sections, electronic transport and recombination
mechanisms in bulk and through surface, optical constants), and
the more macroscopic-level, adjustable properties (for example,
lengths and interfaces) of the device tested, are all factors that com-
bine to produce Jph(λ) for the particular device. It is therefore desir-
able, as a basic measure, to characterize the system by only a few
basic parameters, including ξ(λ), that have straightforward physical
definitions.
Here, we present a method for extracting ξ(λ) empirically, with-
out any a priori assumptions about its shape. The derivation begins
from a spatial collection efficiency model6,7, but using ultrathin
films to remove all the spatial effects, thereby greatly simplifying the
computation and avoiding a priori assumptions. As a case study, we
first applied this method to study haematite (α-Fe2O3) photoanodes
for water photo-oxidation in photoelectrochemical (PEC) cells for
solar water splitting8. We then compared the results with those
obtained from photoconductivity measurements using an entirely
different, non-contact technique, namely time-resolved microwave
photoconductivity (TRMC), which should, in principle, provide
an action spectrum that is also proportional to ξ(λ) if the charge
carrier mobility is wavelength-independent9. Consistent with this
prediction, the shapes of the extracted ξ(λ) and TRMC action spec-
tra were found to be similar. The consistency between these two
diverse techniques, one an entirely direct-current method in a PEC
cell under applied bias, the other a totally non-contact method at
Extraction of mobile charge carrier
photogeneration yield spectrum of ultrathin-film
metal oxide photoanodes for solar water splitting
Daniel A. Grave 1,2,5 ✉ , David S. Ellis1,5, Yifat Piekner 3,5, Moritz Kölbach 4, Hen Dotan1,
Asaf Kay1, Patrick Schnell4, Roel van de Krol 4, Fatwa F. Abdi 4, Dennis Friedrich 4 and
Avner Rothschild 1,3 ✉
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 photocur-
rent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorp-
tion 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 effi-
ciency 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 photo-
anodes 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 TiO2 and BiVO4 photoanodes, demonstrating the broader utility
of the method for determining ξ(λ).
NATURE MATERIALS | VOL 20 | JUNE 2021 | 833–840 | www.nature.com/naturematerials 833
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... 7 Rothschild et al. have demonstrated that more absorbed photons were lost to noncontributing absorption in the bulk hematite than in an ultrathin hematite film. 8 In general, nanoscale thin-film semiconductors with a rough surface could effectively improve the optical absorption efficiency. Hematite (α-Fe 2 O 3 ) is a promising material for solar water splitting. ...
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... While nanowire photoanodes including TiO 2 and ZnO absorb mostly in the ultraviolet (UV) region, other oxides including WO 3 (120), BiVO 4 (121), and Fe 2 O 3 (122) extend their absorption in the visible spectrum (Table 2). However, even those suffer from low photovoltages, which may be due to a high recombination, low charge carrier mobility, or noncontributing excitations (9,123). Therefore, WO 3 only displays an onset potential of 0.5 to 0.7 V versus RHE for oxygen evolution (51,124), whereas the 0.8 to 1.0 V versus RHE onset of hematite (125)(126)(127) is even more positive. ...
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Full-text available
  • Sep 2019
Hematite (α-Fe2O3) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (<100 cm-1). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices.
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Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity
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The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 µs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies. Time‐resolved terahertz microwave and terahertz spectroscopy were used to study the charge carrier dynamics of epitaxial hematite films. A physical basis for the wavelength dependent photoelectrochemical behavior of hematite is provided, demonstrating that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.
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First-Principles Modeling of Polaron Formation in TiO2 Polymorphs
Article
Full-text available
  • Jun 2018
We present a computationally efficient and predictive methodology for modelling the formation and properties of electron and hole polarons in solids. Through a non-empirical and self-consistent optimization of the fraction of Hartree-Fock exchange (alpha) in a hybrid functional, we ensure the generalized Koopmans' condition is satisfied and self-interaction error is minimized. The approach is applied to model polaron formation in all known stable and metastable phases of TiO2 including anatase, rutile, brookite, TiO2(H), TiO2(R) and TiO2(B). Electron polarons are predicted to form in rutile, TiO2(H) and TiO2(R) (with trapping energies ranging from -0.02 eV to -0.35 eV). In rutile the electron localizes on a single Ti ion, whereas in TiO2(H) and TiO2(R) the electron is distributed across two neighboring Ti sites. Hole polarons are predicted to form in anatase, brookite, TiO2(H), TiO2(R) and TiO2(B) (with trapping energies ranging from -0.16 eV to -0.52 eV). In anatase, brookite and TiO2(B) holes localize on a single O ion, whereas in TiO2(H) and TiO2(R) holes can also be distributed across two O sites. We find that the optimized alpha has a degree of transferability across the phases, with alpha=0.115 describing all phases well. We also note the approach yields accurate band gaps, with anatase, rutile and brookite within six percent of experimental values. We conclude our study with a comparison of the alignment of polaron charge transition levels across the different phases. Since the approach we describe is only two to three times more expensive than a standard density functional theory calculation it is ideally suited to model charge trapping at complex defects (such as surfaces and interfaces) in a range of materials relevant for technological applications, but previously inaccessible to predictive modelling.
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Elucidating the Pulsed Laser Deposition Process of BiVO 4 Photoelectrodes for Solar Water Splitting
Article
  • Feb 2020
BiVO4 thin films for the use as photoelectrodes for solar water splitting are prepared by pulsed laser deposition (PLD), a powerful technique to synthesize compact multinary metal oxides films with a high electronic quality. Here, the PLD process of BiVO4 films by ablating a BiVO4 target is systematically elucidated with a special focus on deviations from an ideal stoichiometric target-to-substrate transfer. By correlating the V:Bi ratio of the films with their charge carrier transport properties and PEC performance, AM1.5 sulfite oxidation photocurrents of ~2.4 mA cm⁻² at E = 1.23 V vs. RHE with stoichiometric films are achieved without any deliberate doping or surface modification. In addition, we prepare BiVO4 photoelectrodes for the first time by the alternating ablation of Bi2O3 and V2O5 targets. This approach is found to be an attractive alternative route to control the cation stoichiometry and produces BiVO4 films that generate AM1.5 sulfite oxidation photocurrents of up to 2.6 mA cm⁻² at E = 1.23 V vs. RHE. Our results provide important insights into the PLD process of ternary oxide semiconductors and help to accelerate the synthesis and investigation of new multinary metal oxide photoelectrodes.
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Implementing Strong Interference in Ultrathin Film Top Absorbers for Tandem Solar Cells
Article
  • Nov 2018
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 V-shaped 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.
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Enhanced Carrier Transport and Bandgap Reduction in Sulfur-Modified BiVO4 Photoanodes
Article
  • Nov 2018
Recent progress on bismuth vanadate (BiVO4) has shown it to be among the highest performing metal oxide photoanode materials. However, further improvement, especially in the form of thin film photoelectrodes, is hampered by its poor charge carrier transport and its relatively wide bandgap. Here, sulfur incorporation is used to address these limitations. A maximum bandgap decrease of ~0.3 eV is obtained, which increases the theoretical maximum solar-to-hydrogen efficiency from 9 to 12%. Hard X-ray photoelectron spectroscopy (HAXPES) measurements as well as density functional theory (DFT) calculations show that the main reason for the bandgap decrease is an upward shift of the valence band maximum. Time-resolved microwave conductivity measurements reveal a ~3 times higher charge carrier mobility compared to unmodified BiVO4, resulting in a ~70% increase in the carrier diffusion length. This work demonstrates that sulfur incorporation can be a promising and practical method to improve the performance of wide-bandgap metal oxide photoelectrodes.
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Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency
Article
  • Feb 2018
The operando quantification of surface and bulk losses is key to developing strategies for optimizing photoelectrodes and realizing high efficiency photoelectrochemical solar energy conversion systems. This is particularly true for emerging thin film semiconductors, in which photocarrier diffusion lengths, surface and bulk recombination processes, and charge separation and extraction limitations are poorly understood. Insights into mechanisms of efficiency loss can guide strategies for nanostructuring photoelectrodes, engineering interfaces, and incorporating catalysts. However, few experimental methods are available for direct characterization of dominant loss processes under photoelectrochemical operating conditions. In this work, we provide insight into the function and limitations of an emerging semiconductor photoanode, γ-Cu3V2O8, by quantifying the spatial collection efficiency (SCE), which is defined as the fraction of photogenerated charge carriers at each point below the surface that contributes to the measured current. Analyzing SCE profiles at different operating potentials shows that increasing the applied potential primarily acts to reduce surface recombination rather than to increase the thickness of the space charge region under the semiconductor/electrolyte interface. Comparing SCE profiles obtained with and without a sacrificial reagent allows surface losses from electronically active defect states to be distinguished from performance bottlenecks arising from slow reaction kinetics. Combining these insights promotes a complete understanding of the photoanode performance and its potential as a water splitting photoanode. More generally, application of the SCE extraction method can aid in the discovery and evaluation of new materials for solar water splitting devices by providing mechanistic details underlying photocurrent generation and loss.
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The Spatial Collection Efficiency of Charge Carriers in Photovoltaic and Photoelectrochemical Cells
Article
  • Jan 2018
The spatial collection efficiency portrays the driving forces and loss mechanisms in photovoltaic and photoelectrochemical devices. It is defined as the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent. In stratified planar structures, the spatial collection efficiency can be extracted out of photocurrent action spectra measurements empirically, with few a priori assumptions. Although this method was applied to photovoltaic cells made of well-understood materials, it has never been used to study unconventional materials such as metal-oxide semiconductors that are often employed in photoelectrochemical cells. This perspective shows the opportunities that this method has to offer for investigating new materials and devices with unknown properties. The relative simplicity of the method, and its applicability to operando performance characterization , makes it an important tool for analysis and design of new photovol-taic and photoelectrochemical materials and devices. Introduction In photovoltaic (PV) and photoelectrochemical (PEC) cells, volume absorption of photons generates charge carriers with excess free energy, whose net flux gives rise to electric current, commonly termed the photocurrent. The spatial collection efficiency (SCE) is defined as the fraction of photogenerated charge carriers at a specific position within the cell that contribute to the photocurrent that flows out of the cell. Since the photocurrent can be used to produce electrical power or to drive an electrochemical reaction, empirical extraction of the SCE may shed light on processes that govern the energy conversion efficiency and transduction mechanisms that are important for a wide range of applications.
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Wavelength Dependent Photocurrent of Hematite Photoanodes: Reassessing the Hole Collection Length
Article
  • Nov 2017
The photoelectrochemical behavior of a planar 1 µm thick Ti-doped hematite film deposited on F:SnO2 coated glass was studied with both front and back illumination. Despite low quantum efficiency, photocurrent was observed upon back illumination with low wavelengths, indicating that some photogenerated holes are able to traverse at least 700 nm across the hematite film and effectively oxidize water. This cannot be accounted for using the commonly accepted hole collection length of hematite based on fitting to the Gartner model. Furthermore, under back illumination, 450 nm excitation resulted in increased photocurrent as compared to 530 nm excitation despite most of the light being absorbed further away from the surface. These results demonstrate that the photocurrent is strongly dependent on the optical excitation wavelength, and related to both delocalized holes with long lifetime and localized excitations rather than only being dependent on the proximity of the absorption to the surface.
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