Semiconductor electrodes can catalyze photo-induced redox reactions with light illumination. Photoexcitation produces excited carriers that subsequently transfer to the front and back contacts as determined by the bulk and surface properties of the photoelectrodes. This transfer defines the resultant quasi-Fermi levels of the photo-generated carriers at the photoelectrode surface, which, in turn, impacts the efficiency of surface photoelectrochemical reactions. However, determining such quasi-Fermi levels is not a simple task. In this study, we introduce a method for estimating the quasi-Fermi level of holes using outer-sphere electron transfer reactions. The quasi-Fermi level of holes is estimated by linking the oxidation photocurrent on photoanodes to the separately measured electrode potential on a stable metal electrode. Using this method, the quasi-Fermi level of holes at the surface is monitored in response to variations in applied potential and light intensity. This approach effectively separates the photocurrents of the CdS model electrode between surface redox reaction and photocorrosion, while concurrently quantifying the dynamic quasi-Fermi level at the surface. This work facilitates quantitative understanding of photoelectrochemical reactions on semiconductor electrodes to design green chemical transformation systems.

Photoelectrochemical (PEC) water splitting using tandem cell devices offers a promising method for generating green hydrogen from solar energy. Developing suitable semiconductor materials is crucial for the cost-effectiveness and scalability of this technology. CuFeO2 has gained attention due to its narrow bandgap, positive photocurrent onset potential, and earth-abundant nontoxic composition. However, CuFeO2 photocathode performance is hindered by weak catalytic activity, Fermi level pinning, bulk defect states, and charge carrier localization. In this work, we employ pulsed laser deposition (PLD) to produce high-purity CuFeO2 thin films with controlled morphology and crystal structure. We optimize the parameters for the PLD process to produce compact and uniform films, minimizing diffuse optical scattering. By doing so, we can accurately determine the absorption coefficient, as well as the direct and indirect band gaps of the films. Our study provides important insights into the carrier dynamics in CuFeO2 thin films, including localization in the picosecond and nanosecond time scales. Important electrical properties such as the dielectric constant and the flat-band potential are identified. Surface chemical states are elucidated by detailed X-ray photoelectron spectroscopy analyses. In addition, photoelectrochemical tests show promising potential for these films in water splitting. These findings highlight key properties and improvement opportunities for CuFeO2 films for photoelectrochemical applications.

Ternary chalcogenides have emerged as potential candidates for ultrathin photovoltaics, and NaBiS2 nanocrystals (NCs) have gained appeal because of their months-long phase-stability in air, high absorption coefficients >10(5) cm(-1), and a pseudo-direct bandgap of 1.4 eV. However, previous investigations into NaBiS2 NCs used long-chain organic ligands separating individual NCs during synthesis, which severely limits macroscopic charge-carrier transport. In this work, these long-chain ligands are exchanged for short iodide-based ligands, allowing to understand the macroscopic charge-carrier transport properties of NaBiS2 and evaluate its photovoltaic potential in more depth. It is found that ligand exchange results in simultaneous improvements in intra-NC (microscopic) and inter-NC (macroscopic) mobilities, while charge-carrier localization still takes place, which places a fundamental limit on the transport lengths achievable. Despite this limitation, the high absorption coefficients enable ultrathin (55 nm thick) solar absorbers to be used in photovoltaic devices, which have peak external quantum efficiencies > 50%. In addition, temperature-dependent transient current measurements uncover a small activation energy barrier of 88 meV for ion migration, which accounts for the strongly hysteretic behavior of NaBiS2 photovoltaic devices. This work not only reveals how the charge-carrier transport properties of NaBiS2 NCs over several length and time scales are influenced by ligand engineering, but also unveils the facile ionic transport in this material, which limits the potential of NaBiS2 in photovoltaics. On the other hand, the discovery shows that there are opportunities to use this material in memristors, electrolytes, and other applications requiring ionic conduction.

Elemental selenium is a promising wide-bandgap ($E_\mathrm{G}\approx$ 1.95 eV) photovoltaic material for the next generation of thin-film solar cells. To realize high-efficiency selenium solar cells, it is crucial to optimize the crystallization process of the selenium thin-film photoabsorber. However, the high vapor pressure of selenium restricts the processing conditions to a compromise between the growth of large crystal grains and the formation of pinholes. In this study, we introduce a closed-space annealing (CSA) strategy designed to suppress the sublimation of selenium, enabling thermal annealing processes at higher temperatures and for longer periods of time. As a result, we consistently improve carrier collection and the overall photovoltaic device performance in our selenium solar cells. By characterizing the carrier dynamics in our devices, we conclude that the observed improvements result from a reduction in charge transfer resistance rather than an increase in carrier diffusion length. The CSA strategy is a promising method for controlling surface morphology and roughness without reducing crystal grain sizes, which paves the way for further advancements in the efficiency and reproducibility of selenium thin-film solar cells.

Metal oxides are considered as stable and low‐cost photoelectrode candidates for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies usually suffer from poor transport of photogenerated charge carriers, which has been attributed previously to a variety of effects occurring on different time and length scales. In search for common understanding and for a better photo‐conducting metal oxide photoabsorber, CuFeO 2 , α‐SnWO 4 , BaSnO 3 , FeVO 4 , CuBi 2 O 4 , α‐Fe 2 O 3 , and BiVO 4 are compared. Their kinetics of thermalization, trapping, localization, and recombination are monitored continuously 100 fs–100 µs and mobilities are determined for different probing lengths by combined time‐resolved terahertz and microwave spectroscopy. As common issue, we find small mobilities < 3 cm 2 V ‐1 s ‐1 . Partial carrier localization further slows carrier diffusion beyond localization lengths of 1–6 nm and explains the extraordinarily long conductivity tails, which should not be taken as a sign of long diffusion lengths. For CuFeO 2 , the localization is attributed to electrostatic barriers that enclose the crystallographic domains. The most promising novel material is BaSnO 3 , which exhibits the highest mobility after reducing carrier localization by annealing in H 2 . Such overcoming of carrier localization should be an objective of future efforts to enhance charge transport in metal oxides.

The principal driving force for charge carrier separation in a PEC cell is given by the QFL gradients. Upon illumination the E-field is reduced by the amount of photovoltage delivered. Selective contacts are thus decisive for device performance.

Resonant inelastic x-ray scattering (RIXS) spectra of hematite (alpha-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 approximate to 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 discussed.

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time-resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 µs by the combination of time-resolved terahertz and microwave spectroscopy for BiVO_{4}, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a-Si:H, c-Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered materials.

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current‐voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter‐laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br) 3 halide perovskite thin‐film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance‐free JV‐curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra‐grain charge transport is characterized by terahertz sum mobilities of ≈32 cm 2 V −1 s −1 . Drift‐diffusion simulations indicate that these intra‐grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best‐realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

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 ∼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 discussed.

The III-V semiconductor GaN is a promising material for photoelectrochemical (PEC) cells, however the large bandgap of 3.45 eV is a considerable hindrance for the absorption of visible light. Therefore, the substitution of small amounts of N anions by isovalent Sb is a promising route to lower the bandgap and thus increase the PEC activity under visible light. Herein we report a new chemical vapor deposition (CVD) process utilizing the precursors bis(N,N'-diisopropyl-2-methyl-amidinato)-methyl gallium (III) and triphenyl antimony (TPSb) for the growth of GaSbxN1-x alloys. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show crystalline and homogeneous thin films at deposition temperatures in the range of 500-800 °C. Rutherford backscattering spectrometry (RBS) combined with nuclear reaction analysis (NRA) shows an incorporation of 0.2-0.7 at% antimony into the alloy, which results in a slight bandgap decrease (up to 0.2 eV) accompanied by enhanced sub-bandgap optical response. While the resulting photoanodes are active under visible light, the external quantum efficiencies remained low. Intriguingly, the best performing films exhibits the lowest charge carrier mobility according to time resolved THz spectroscopy (TRTS) and microwave conductivity (TRMC) measurements, which showed mobilities of up to 1.75 cm2 V-1 s-1 and 1.2 × 10-2 cm2 V-1 s-1, for each timescale, respectively.

The widespread application of solar‐water‐splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal‐oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass‐based transparent conductive‐substrates with low thermal‐stability impedes the use of common synthesis routes of high‐quality metal‐oxide thin‐film photoelectrodes. Nevertheless, rapid thermal processing (RTP) enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phase‐purity challenges in complex metal‐oxides, showing the importance of attaining a single‐phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi 2 O 4 , a prime candidate photoabsorber. Pure CuBi 2 O 4 photoelectrodes are synthesized after studying the relationship between the crystal‐structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single‐phase photoelectrodes exhibit higher fill‐factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes. These findings show the impact of combinatorial approaches alongside radiative heating techniques toward discovering highly efficient multinary photoabsorbers.

α‐SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO4/NiOx interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.

Recent study demonstrated that the performance of α-SnWO4/NiOx photoabsorbers prepared by pulsed laser deposition (PLD) is limited by the interfacial properties; [1] understanding this interface is therefore crucial for further improvement. A thorough α-SnWO4/NiOx interface investigation by means of hard X-ray photoelectron spectroscopy (HAXPES) is presented and correlated with photoelectrochemistry measurements. PLD NiOx introduces strong upwards band bending (~500 meV) at the interface. However, photoemission spectra simulation indicates that at the same time a thin SnO2 layer is formed at the interface. The implications of this SnO2 layer to the interface junction properties and the limited photovoltage will be discussed. [1] Kölbach et al. Chem Mater. 30 (2018) 8322-8331

Recently, metal nanoparticle surface coatings have been found to significantly enhance the ultra-violet luminescence intensity from ZnO, providing a viable means to mitigate optical losses and improve LED performance. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal nanoparticles can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in bulk and nanostructured ZnO coated with a 2 nm Al nanoparticle layer is investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show free exciton (FX) emission enhancement factors up to 12x at 80 K, and reveal that the FX couple more efficiently to the LSPs compared to the localized donor-bound excitons. A strong polarization dependence between the LSPs and FX is observed where FX transitions are more strongly enhanced when polarized in the same direction as the electric field of the incident excitation, which is different for laser and electron beam sources. This result indicates that selective enhancement of the excitonic emission peaks in the ZnO coated with Al nanoparticles can be achieved by choosing the appropriate ZnO substrate orientation.

Recently, alpha-SnWO4 attracted attention as a material to be used as a top absorber in a tandem device for photoelectrochemical water splitting due to its nearly optimum band gap of similar to 1.9 eV and an early photocurrent onset potential of similar to 0 V versus RHE. However, the mismatch between the charge carrier diffusion length and light penetration depth. which is typical for metal oxide semiconductors currently hinders the realization of high photoconversion efficiencies. In this work, the pulsed laser deposition process and annealing treatment of alpha-SnWO4 thin films are elucidated to optimize their charge carrier transport properties. A high-temperature treatment is found to enhance the photoconductivity of alpha-SnWO4 by more than 1 order of magnitude, as measured with time-resolved microwave conductivity (TRMC). A complimentary analysis by time-resolved terahertz spectroscopy (TRTS) shows that this improvement can be assigned to an increase of the grain size in the heat-treated films. In addition, TRTS reveals electron-hole charge carrier mobilities of up to 0.13 cm(2) V-1 s(-1) in alpha-SnWO4. This is comparable to values found for BiVO4, which is one of the best performing metal oxide photoanode materials to date. These findings show that there is a significant potential for further improving the properties of alpha-SnWO4 photoanodes.