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

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, xi lamba , 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 xi lamba 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 xi lamba 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 xi lamb

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

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, xi lamba , 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 xi lamba 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 xi lamba 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 xi lamb

Low Complexity Regularization of Linear Inverse Problems

Inverse problems and regularization theory is a central theme in contemporary signal processing, where the goal is to reconstruct an unknown signal from partial indirect, and possibly noisy, measurements of it. A now standard method for recovering the unknown signal is to solve a convex optimization problem that enforces some prior knowledge about its structure. This has proved efficient in many problems routinely encountered in imaging sciences, statistics and machine learning. This chapter delivers a review of recent advances in the field where the regularization prior promotes solutions conforming to some notion of simplicity/low-complexity. These priors encompass as popular examples sparsity and group sparsity (to capture the compressibility of natural signals and images), total variation and analysis sparsity (to promote piecewise regularity), and low-rank (as natural extension of sparsity to matrix-valued data). Our aim is to provide a unified treatment of all these regularizations under a single umbrella, namely the theory of partial smoothness. This framework is very general and accommodates all low-complexity regularizers just mentioned, as well as many others. Partial smoothness turns out to be the canonical way to encode low-dimensional models that can be linear spaces or more general smooth manifolds. This review is intended to serve as a one stop shop toward the understanding of the theoretical properties of the so-regularized solutions. It covers a large spectrum including: (i) recovery guarantees and stability to noise, both in terms of $\ell^2$-stability and model (manifold) identification; (ii) sensitivity analysis to perturbations of the parameters involved (in particular the observations), with applications to unbiased risk estimation ; (iii) convergence properties of the forward-backward proximal splitting scheme, that is particularly well suited to solve the corresponding large-scale regularized optimization problem

In situ characterizations of photoelectrochemical cells for solar fuels and chemicals

Environmental concerns deriving from fossil fuel dependency are driving an energy transition based on sustainable processes to make fuels and chemicals. Solar hydrogen is the pillar of this new green economy, but the technological readiness level of PV electrolysis and direct photoelectrochemical (PEC) electrolysis are still too low to allow broad commercialization. Direct conversion through PEC technology has more potential in the medium\u2013long term but must be first guided by the scientific enhancements to improve device efficiencies. For this purpose, in situ and operando photoelectrochemistry will guide the discovery of new materials and processes to make solar fuels and chemicals in PEC cells

External Quantum Efficiency Spectra of BiVO4 Thin Film Photoanodes under Bias Illumination

External quantum efficiency EQE of bismuth vanadate thin film photoanodes, measured in a pH 7 potassium phosphate buffer solution with sodium sulfite hole scavenger, was observed to substantially decrease when measured under white light bias LB . While the EQE exhibited a fast initial decrease across its full spectral range, a amp; 8764;3.5 eV 350 nm feature under front illumination conditions became disproportionally suppressed after being under LB strongest when it is also incident on the front side of the sample for several tens of minutes, in spite of this wavelength being outside the spectral range encompassed by the LB source. Applied potential does not have a strong effect on the qualitative behavior. From its different decay time, the wavelength specific decrease of the 3.5 eV feature, and its responsible mechanism, is distinct from the initial, spectrally uniform decrease of EQE, which happens at a faster timescale and is similar for all illumination conditions. To more closely examine the suppression of the 3.5 eV feature, we compare calculated depth dependent optical generation profiles and behaviors under different illumination conditions, which imply the involvement of in gap states and long lived states deeper into the conduction or alternatively, valence band. Possible mechanisms are discusse

Electronic excitations of alpha Fe2O3 heteroepitaxial films measured by resonant inelastic x ray scattering at the Fe L edge

Resonant inelastic x ray scattering RIXS spectra of hematite amp; 945; Fe2O3 were measured at the Fe L3 edge for heteroepitaxial thin films which were undoped and doped with 1 Ti, Sn, or Zn, in the energy loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers q, conducted at both low temperature T 14 K and room temperature. While we cannot rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed L edge RIXS intensity originates from mostly nondispersive ligand field excitations. Summing the RIXS spectra over q and comparing the results at T 14 K to those at T 300 K revealed pronounced temperature effects, including an intensity change and energy shift of the amp; 8776;1.4 eV peak, a broadband intensity increase of the 3 4 eV range, and higher energy features. The q summed spectra and their temperature dependencies are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti doped sample s spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discusse