Bismuth vanadate (BiVO_4) is a prototypical oxide photocatalyst that occurs in both tetragonal and monoclinic scheelite phases with markedly different photocatalytic and photoelectrochemical activities. Accurately identifying the monoclinic phase as the ground state and explaining the origin of its symmetry-breaking distortion are unusually challenging from a theoretical perspective, with various levels of theory and associated physical interpretations for this behaviour reported in the literature. Here, we resolve these discrepancies by systematically assessing the role of exact exchange with and without spin-orbit coupling, demonstrating that an accurate treatment of electronic localization is essential to stabilize the monoclinic scheelite structure. Using this framework, we compute the electronic band structure through dense sampling of the Brillouin zone and show that the band edges in monoclinic and tetragonal BiVO_4 lie far from conventional high-symmetry paths, leading to substantial differences in band gaps and carrier effective masses. Choosing the exchange-correlation functional that best reproduces the crystal structure leads to excellent predictions of the band gap once excitonic and thermal effects are taken into account. In addition, we show that the monoclinic distortion is driven by charge transfer between non-equivalent oxygen sites, which breaks the lattice symmetry and is suppressed by self-interaction errors when using semi-local DFT. These results establish a direct connection between the exchange-correlation functional, electronic localization, chemical bonding, and structural stability in BiVO_4, providing a foundation for robust ab initio descriptions of phase stability and optoelectronic properties in such complex oxides.

Abstract Understanding how material and defect characteristics govern photoelectrochemical performance is essential for developing efficient and stable photoelectrodes. Although bulk properties are often emphasized, surfaces and interfaces can equally determine activity and stability under operation. Here, we use depth-sensitive characterization to disentangle surface and bulk properties of Ta 3 N 5 thin films. By preparing photoelectrodes from TaO x , TaN x , and Ta precursors, we systematically vary shallow and deep-level defect concentrations. Structural, compositional, and optoelectronic analyses show that the surfaces consistently exhibit oxygen enrichment, increased structural disorder, and higher deep-level defect densities than the bulk. However, the specific surface structure and its spatial extent depend strongly on precursor chemistry. Ta 3 N 5 derived from TaO x forms an extended, amorphous, oxide-rich surface with fewer deep-level defects, whereas TaN x and Ta-derived Ta 3 N 5 films exhibit thinner, more crystalline surfaces with increased mid-gap defect densities. A brief hydrofluoric acid treatment removes the disordered surface layer, improving crystallinity and hydrophilicity and enhancing photoelectrochemical performance and stability for ferrocyanide oxidation. These results highlight interfacial and defect engineering as routes toward durable, high-efficiency Ta 3 N 5 photoelectrodes.

ABSTRACT Multi‐absorber III–V semiconductors represent the state of the art in high‐efficiency solar‐to‐electricity and solar‐to‐fuel conversion owing to their tunable bandgaps and favorable optoelectronic properties. GaInP is widely used in monolithic tandem devices as a top absorber or charge‐carrier‐selective contact because of its suitable electronic structure. However, GaInP and related III–V materials are prone to photocorrosion under photoelectrochemical (PEC) operation and require protective layers that ensure chemical and electronic passivation. Atomic layer deposition (ALD), particularly plasma‐enhanced ALD (PE‐ALD), enables conformal deposition of films that enhance interfacial stability while maintaining efficient selective charge carrier transport. Here, the band alignment at the GaInP(100)/ interface is examined to determine whether the initial GaInP surface condition–either an atomically well‐defined, phosphorus‐rich surface or a naturally oxidized surface–affects interface formation during mild, low‐power remote oxygen PE‐ALD. Angle‐dependent X‐ray and ultraviolet photoelectron spectroscopy were used to probe the chemical and electronic structure of the buried interfaces. The results show that interface composition and band alignment are largely insensitive to the initial surface condition, with only minor differences in attenuation and interfacial energetics. These findings demonstrate the robustness of mild PE‐ALD for reproducible interface formation and support the design of protected III–V photoelectrodes for PEC applications.

Molybdenum oxide (MoOx) thin films have been extensively investigated for selective hole extraction in solar cells, including as an efficient alternative to the well-studied p-type a-Si:H layers in Si heterojunction cells. Among the various methods for producing MoOx films, atomic layer deposition (ALD) enables the growth of conformal, thin, and electronically tunable materials. However, while several studies have examined ALD-grown MoOx, a comprehensive understanding of its electronic structure, density of states, and the influence of growth conditions on its interface and energetic alignment with c-Si is still evolving. In this study, we investigate and comparatively analyze the properties of MoOx films deposited using different ALD processes, including those based on oxygen plasma-enhanced and ozone-assisted ALD growth. Through analysis of optical absorption spectra, we find that different ALD processes can be used to tune the densities of active defects, including both near-band edge tail states and deep defect states located similar to 1.1 eV below the conduction band edge. Our comparative analysis reveals that ozone-assisted growth leads to increased defect densities compared to oxygen plasma-enhanced growth. In addition, all films are characterized by a prominent sub-gap absorption feature that is consistent with the formation of small polarons. Finally, we used a combination of optical and X-ray spectroscopic methods to evaluate the band alignment between ALD MoOx thin films and c-Si, confirming the presence of a small energetic barrier that can permit selective hole injection across the interface. Overall, ALD enables highly controllable growth of films with variable defect concentrations and film properties that are of key relevance for the development of heterojunction solar cells.

Perovskite-type tantalum-based oxynitride photocatalysts are promising candidates for water splitting due to their suitable band positions and extended light absorption beyond 600 nm. However, their associated photocatalytic activities and quantum yields remain relatively low. Here, we show that a nano-sized single-crystalline BaxSr1-xTaO2N solid-solution perovskite photocatalyst exhibits state-of-the-art activity in separate oxygen and hydrogen evolution half-reactions. The improved performance is attributed to the nanoscale particle sizes, as well as the reduced defect densities achieved by using a mixed precursor comprising TaS2 and Ta3N5. The half-reaction activities can be modulated by applying a post-synthetic high-temperature treatment. Assessments of charge carrier dynamics, in conjunction with a mechanistic kinetic model, reveal that exponential-tail trap states are formed during this post-treatment. Such trap states, present on the photocatalyst surface, facilitate participation of holes during the oxygen evolution reaction. The development of such solid-solution photocatalysts broadens the range of potential materials for solar-driven hydrogen production. In addition, the present findings are expected to enable the selective tuning of bifunctional photocatalysts for either the hydrogen or oxygen evolution reaction.

ABSTRACT Multi‐absorber III–V semiconductors represent the state of the art in high‐efficiency solar‐to‐electricity and solar‐to‐fuel conversion owing to their tunable bandgaps and favorable optoelectronic properties. GaInP is widely used in monolithic tandem devices as a top absorber or charge‐carrier‐selective contact because of its suitable electronic structure. However, GaInP and related III–V materials are prone to photocorrosion under photoelectrochemical (PEC) operation and require protective layers that ensure chemical and electronic passivation. Atomic layer deposition (ALD), particularly plasma‐enhanced ALD (PE‐ALD), enables conformal deposition of TiO2 films that enhance interfacial stability while maintaining efficient selective charge carrier transport. Here, the band alignment at the GaInP(100)/TiO2 interface is examined to determine whether the initial GaInP surface condition–either an atomically well‐defined, phosphorus‐rich surface or a naturally oxidized surface–affects interface formation during mild, low‐power remote oxygen PE‐ALD. Angle‐dependent X‐ray and ultraviolet photoelectron spectroscopy were used to probe the chemical and electronic structure of the buried interfaces. The results show that interface composition and band alignment are largely insensitive to the initial surface condition, with only minor differences in attenuation and interfacial energetics. These findings demonstrate the robustness of mild PE‐ALD for reproducible interface formation and support the design of protected III–V photoelectrodes for PEC applications.

TiO2 is widely applied as a photoanode material for the oxygen evolution reaction (OER), as well as for corrosion protection. However, its stability during photoelectrochemical (PEC) processes is still under debate. Here, we systematically investigate the effect of illumination, electrode potential, and electrolyte pH on the stability of rutile-type TiO2 nanowires during PEC OER. We provide operando insights into the photostability of TiO2, quantifying the photodissolution to identify degradation mechanisms in acidic and alkaline electrolytes. The structural evolution of TiO2 nanowires is correlated with their photodissolution as well as local pH changes during OER. Based on these findings, we propose a model that links the pH-dependent degradation of PEC performance with dissolution processes, thereby establishing a comprehensive understanding of the durabilities of TiO2 photoanodes in different electrolyte environments. Overall, this work identifies and rationalizes the PEC stability boundaries of TiO2, which is key for its practical application in photo/electro-catalysis and as a corrosion protection layer.

Ta_3N_5 is among the most intensively studied photoanode materials for solar-driven water oxidation, yet its performance often remains limited by short carrier lifetimes and defect mediated recombination. Although transient absorption spectroscopy is widely used to probe carrier dynamics in photoelectrodes, spectral assignments are frequently ambiguous due to overlapping contributions. Here, microsecond-to-second transient absorption of Ta_3N_5 thin films is combined with complementary optical spectroscopies to disentangle contributions from lattice heating, electrostatics, and defect states. Photoreflectance reveals three critical points in the Ta_3N_5 band structure, including two anisotropic near-edge transitions at 2.14 eV and 2.27 eV and a higher-lying transition near 2.80 eV, all closely aligned with dominant transient absorption features. A previously unreported photo-induced absorption at 2.80 eV is attributed to pump-induced lattice heating, while potential-dependent measurements reveal that near-edge bleach features arise from pump-induced band flattening and subsequent surface photovoltage relaxation. Fitting transient absorption spectra with independently measured thermal and electrostatic components enables quantification of both thermal and photovoltage dynamics, while the sub-bandgap response provides insight into the redistribution of defect charge states. Thus, this approach to quantifying thermal, electrostatic, and defect-mediated contributions to microsecond-to-second transient absorption provides broadly applicable insights into photoexcitation and relaxation mechanisms in functional semiconductor photoelectrodes.

ABSTRACT is a promising semiconductor for solar‐driven water splitting, but its performance is limited by poor charge transport and inefficient carrier extraction. We present a systematic approach to overcome these limitations by combining Ti compensation doping with engineered TiN back contact interlayers. Time‐resolved terahertz and microwave photoconductivity reveal that Ti incorporation suppresses trapping at mid‐gap defect states and reduces grain boundary barriers, yielding enhanced mobilities, longer lifetimes, and reduced carrier localization. These results elucidate the mechanisms of improved transport in Ti‐doped (Ti:) and the role of compensation doping in suppressing bulk recombination losses. Despite these improved bulk transport characteristics, efficient photoelectrochemical (PEC) function also requires optimized back contacts. To address interfacial losses, we introduce ultrathin (8 nm) TiN interlayers that remain metallic during high‐temperature ammonolysis, act as effective diffusion barriers that protect substrates, and enable efficient majority carrier extraction on both fused silica and Si. Tandem integration of Ti: with TiN interlayers on n‐type Si yields significantly improved PEC performance. Overall, this work establishes a fundamental basis for advancing nitride‐based photoelectrodes through coordinated defect and interface engineering, while enabling cost‐effective fabrication of semi‐transparent Ti: photoanodes for in situ optical studies and nitride‐based tandem solar cells.

Ultrafast small-polaron formation profoundly shapes the electronic and catalytic behaviour of transition metal oxides (TMOs). Despite its significance, spectroscopic investigations of photoexcited polaron hopping in TMOs have been scarcely explored. Here, we present the first optical spectroscopic observation of photoexcited small-polaron hopping across the first-row TMOs, using femtosecond transient absorption spectroscopy. This polaronic feature rises within 500 fs as Drude-type absorption converts to localized, polaronic absorption. Fitting with a small-polaron optical conductivity model yields polaron relaxation energies of 400-650 meV, evidencing substantial energy loss upon self-trapping. Kinetic analysis shows that oxides with open d-shells localize charge most readily: polaron formation activation barriers are low in all TMOs (0-10 meV), whereas hopping barriers remain much higher (200-350 meV). This work establishes key spectroscopic and kinetic insights, highlighting the trade-off between charge localization and mobility, as well as the critical role of polaron formation in TMOs photocatalysts.

Ta 3 N 5 ${\rm Ta}_3{\rm N}_{5}$ is a promising semiconductor for solar-driven water splitting, but its performance is limited by poor charge transport and inefficient carrier extraction. We present a systematic approach to overcome these limitations by combining Ti compensation doping with engineered TiN back contact interlayers. Time-resolved terahertz and microwave photoconductivity reveal that Ti incorporation suppresses trapping at mid-gap defect states and reduces grain boundary barriers, yielding enhanced mobilities, longer lifetimes, and reduced carrier localization. These results elucidate the mechanisms of improved transport in Ti-doped Ta 3 N 5 ${\rm Ta}_3{\rm N}_{5}$ (Ti: Ta 3 N 5 ${\rm Ta}_3{\rm N}_{5}$ ) and the role of compensation doping in suppressing bulk recombination losses. Despite these improved bulk transport characteristics, efficient photoelectrochemical (PEC) function also requires optimized back contacts. To address interfacial losses, we introduce ultrathin ( ∼ $\sim$ 8 nm) TiN interlayers that remain metallic during high-temperature ammonolysis, act as effective diffusion barriers that protect substrates, and enable efficient majority carrier extraction on both fused silica and Si. Tandem integration of Ti: Ta 3 N 5 ${\rm Ta}_3{\rm N}_{5}$ with TiN interlayers on n-type Si yields significantly improved PEC performance. Overall, this work establishes a fundamental basis for advancing nitride-based photoelectrodes through coordinated defect and interface engineering, while enabling cost-effective fabrication of semi-transparent Ti: Ta 3 N 5 ${\rm Ta}_3{\rm N}_{5}$ photoanodes for in situ optical studies and nitride-based tandem solar cells.

The physical properties of bismuth vanadate (BiVO4) make it an appealing semiconductor photoanode for water oxidation in photoelectrochemical cells that aim to produce hydrogen or other solar fuels. However, it has been estimated that its relatively wide bandgap limits achievable photocurrent densities to approximately 7.5 mA/cm2 under 1 sun AM1.5G illumination. Here, we perform high-sensitivity external quantum efficiency measurements and demonstrate that sub-bandgap states within BiVO4 also contribute to photocurrent generation, regardless of the fabrication method or mesoscopic structure. Based on these results and considering Lambertian scattering at the electrolyte/BiVO4 interface, we show that the maximum theoretical current density from BiVO4 can be as high as 12.2 mA/cm2, when assuming complete absorption and conversion of sunlight photons extending to the lowest photon energy for which we experimentally measured photocurrent generation promoted by sub-bandgap states. This finding opens new avenues for design of BiVO4 photoanodes with efficiencies that are much greater than were previously assumed to be possible.

Direct solar water splitting is a promising approach for sustainably producing hydrogen, but significant materials challenges must be overcome to achieve high efficiency and long‐term stability. This work demonstrates a tailored interface treatment combined with multifunctional surface coatings that significantly enhance the efficiency and lifetime of GaAs/GaInP tandem cells capable of unassisted solar water splitting. In particular, it is shown that exposure of the top AlInP window layer to a remote H 2 plasma effectively reduces the interfacial oxide, enhancing charge extraction and maximizing the available photovoltage. Subsequent atomic layer deposition (ALD) of a bilayer coating comprising a TiO 2 corrosion protection layer and Pt nanoparticles enhances the durability of the device, enables efficient electron extraction, and provides high catalytic activity. By tuning the Pt ALD process, a nanoparticulate morphology is achieved, ensuring high catalytic activity at low loading, thus minimizing parasitic light absorption and improving adhesion. The optimized dual‐junction photoelectrode achieves an initial maximum solar‐to‐hydrogen (STH) conversion efficiency of 17.1%, stabilizing at 16.2% for 170 min of continuous operation. Importantly, the tailored interfaces of the device result in a considerable photovoltage surplus, providing a route to systems offering higher STH efficiencies or for integration of Group III‐V semiconductor‐on‐Si tandems.

Catalyst screening for electrochemical nitrogen (N2) reduction to ammonia (NH3) suffers from spurious results caused by various sources of contamination associated with the experimental setup and the environment. As a crucial component of the electrochemical cell, a separator membrane is necessary to prevent the oxidation of as-synthesized NH3, specifically for low-yield aqueous electrochemical nitrogen reduction reactions (EC-N2RR). However, there remains a key need for a robust and contamination-free membrane, including systematic assessments of its electrochemical suitability for this reaction. Here, we report a simple yet effective cleaning process for the recently introduced microporous Celgard membrane. We show that the cleaned Celgard membrane outperforms other commonly used EC-N2RR membranes in alkaline electrolytes, exhibiting superior ion transport compared to a Nafion proton exchange membrane and enhanced chemical inertness and structural stability compared to a Fumasep anion exchange membrane. Beyond guiding membrane selection, this study provides a comprehensive strategy for managing contamination and a framework for designing and operating nearly contamination-free EC-N2RR workstations, enabling reliable catalyst screening for aqueous EC-N2RR.

Zinc nitride (Zn3N2) comprises earth-abundant elements, possesses a small direct bandgap, and is characterized by high electron mobility. While these characteristics make the material a promising compound semiconductor for various optoelectronic applications, including photovoltaics and thin-film transistors, it commonly exhibits unintentional degenerate n-type conductivity. This degenerate character has significantly impeded the development of Zn3N2 for technological applications and is commonly assumed to arise from incorporation of oxygen impurities. However, consistent understanding and control of the role of native and impurity defects on the optoelectronic properties of this otherwise promising semiconductor have not yet emerged. Here, we systematically synthesize epitaxial Zn3N2 thin films with controlled oxygen impurity concentrations of up to 20 at % by plasma-assisted molecular beam epitaxy (PA-MBE). Contrary to expectations, we find that oxygen does not lead to degenerate conductivity but instead serves as a compensating defect, the control of which can be used to achieve nondegenerate semiconducting thin films with free electron concentrations in the range of 1017 cm-3, while retaining high mobilities in excess of 200 cm2 V-1 s-1. This understanding of the beneficial role of oxygen thus provides a route to controllably synthesize nondegenerate O-doped Zn3N2 for optoelectronic applications.

Ternary compounds obtained by alloying wurtzite AlN with transition metals have emerged as promising materials with significantly enhanced piezoelectric characteristics relative to binary AlN. The increased electromechanical coupling in these compounds boosts the performance of high-frequency acoustic devices. So far, progress has largely focused on Al_1-xSc_xN, which is costly and poorly compatible with complementary metal-oxide-semiconductor (CMOS) technologies. Here, we investigate aluminum hafnium nitride (Al_1-xHf_xN) as a scalable and potentially CMOS-compatible alternative to Al_1-xSc_xN. Using reactive co-sputtering on both Si and sapphire substrates, we demonstrate wurtzite Al_1-xHf_xN thin films (x ≤ 0.17) with strong c-axis texture and nearly isotropic lattice expansion upon Hf incorporation. X-ray absorption spectroscopy indicates cross-gap hybridization between N 2p and Hf 5d states, which can enhance the Born effective charge and, thereby, the piezoelectric response. Correspondingly, we observe a nearly two-fold enhancement in the piezoelectric coefficient, d_33, relative to AlN, despite increasing structural disorder in Al_1-xHf_xN. Building on this finding, we demonstrate Al_1-xHf_xN GHz surface acoustic wave (SAW) resonators that exhibit enhanced performance, as well as efficient excitation of bulk acoustic waves with low propagation losses. These results establish Al_1-xHf_xN as a promising platform for next-generation high-frequency electromechanical devices, with prospects for further piezoelectric enhancements through improved epitaxy.

Bismuth vanadate (BiVO_4) is a key photocatalyst for solar fuel applications, yet fundamental questions remain regarding the nature of photogenerated polaronic states and the lattice dynamics that govern its light-to-chemical pathways. Here, we use femtosecond optical pump-X-ray probe measurements to track the photoinduced electronic and structural dynamics in BiVO_4 across multiple length and time scales. Transient X-ray absorption spectroscopy captures sub-picosecond electron localization within VO_4 tetrahedra, consistent with small polaron formation, whereas time-resolved X-ray diffraction reveals a slower, multi-picosecond lattice reorganization into a hidden photoexcited state that is structurally distinct from both the monoclinic ground state and the high-temperature tetragonal phase. Supported by density functional theory, we show that hole-lattice interactions dynamically reduce the ground state monoclinic distortion, stabilizing the hidden state. Our results demonstrate that electron- and hole-lattice coupling jointly shape the excited state landscape, with implications for carrier transport, interfacial energetics, and light-to-chemical energy conversion pathways.

The photoelectrochemical hydrogen peroxide evolution reaction (HPER) has attracted increasing attention as an environmentally friendly approach to generate a commercially and industrially valuable water oxidation product. BiVO4 photoanodes operated in bicarbonate-containing electrolytes have been shown to offer remarkable performance characteristics for HPER, with HCO3 - serving as a reaction mediator. However, the factors affecting the stability of both the semiconductor photoanode and the aqueous electrolyte remain poorly understood. Here, we investigated BiVO4 photoanodes to quantitatively assess the roles of electrolyte composition, bias potential, and illumination on competitive reaction pathways associated with HPER, oxygen evolution reaction, and photocorrosion. Our results confirm that HCO3 - serves as a highly efficient mediator, leading to rapid hole extraction and near complete suppression of interfacial recombination on BiVO4. In addition, these favorable hole transfer kinetics significantly decrease the rate of photocorrosion, leading to dramatically enhanced stability compared to bicarbonate-free electrolytes. While the elevated pH of unbuffered bicarbonate electrolyte leads to gradual chemical attack of BiVO4, the stability is greatly enhanced in near-neutral buffered bicarbonate electrolytes. Finally, we confirm that HCO3 - is regenerated during the photoanodic reaction, though pH swings during operation in an unbuffered electrolyte can lead to electrolyte instabilities. Overall, we find that BiVO4 photoanodes operating in buffered bicarbonate-containing solutions exhibit significantly enhanced stability and can efficiently drive water oxidation reactions, including HPER, thus providing a route to robust production of high value oxidation products.

Semiconducting ternary nitrides are a promising class of materials that have received increasing attention in recent years, but often show high free electron concentrations due to the low defect formation energies of nitrogen vacancies and substitutional oxygen, leading to degenerate n-type doping. To achieve non-degenerate behavior, we now investigate a family of amorphous calcium-zinc nitride (Ca-Zn-N) thin films. By adjusting the metal cation ratios, we demonstrate band gap tunability between 1.4 and 2.0 eV and control over the charge carrier concentration across six orders of magnitude, all while maintaining high mobilities between 5 and 70 cm2 V-1 s-1. The combination of favorable electronic properties, low synthesis temperatures, and earth-abundant elements makes amorphous Ca-Zn-N highly promising for future sustainable electronics. Moreover, the successful synthesis of such materials, as well as their broad optical and electrical tunability, paves the way for a new class of tailored functional materials: amorphous nitride semiconductors - ANSs.