Ag(I)-Bi(III)-based semiconductors have gained substantial attention as nontoxic, stable alternatives to lead-halide perovskites for optoelectronics, but are widely limited by carrier localization, which severely restricts diffusion lengths. The most efficient Ag/Bi solar absorber is AgBiS2, but diffusion lengths in nanocrystal films are <50 nm. Carrier localization in this rock-salt (Fm-3m) system is believed to arise from cation disorder, and so we herein investigate the layered cation-ordered analogue. Through beyond-DFT simulations combined with neutron and X-ray powder diffraction, we reveal that off-centring of Ag+ and Bi3+ cations is energetically-favoured in this cation-ordered phase. Despite local distortions in the AgS6 and BiS6 octahedra, band-like transport takes place, which, surprisingly, also occurs in the cation-disordered rock-salt phase when these materials are made as bulk powders. The cubic-phase powders have the same degree of cation disorder as the nanocrystals that have carrier localization, which suggests that extrinsic factors play a determining role. We ascribe the intrinsic band-like transport of both phases of AgBiS2 to its close packing, ensuring high electronic dimensionality. These insights offer pathways for designing solar absorbers avoiding carrier localization limitations, and call for future efforts to enhance the efficiency of AgBiS2 photovoltaics to focus on large-grained thin films, or improved nanocrystal surface passivation.

Plasmon-induced interfacial hole injection at metal/semiconductor heterointerfaces represents a complementary pathway to conventional hot electron transfer, thereby broadening the functional landscape of plasmonic systems for energy conversion and optoelectronic technologies. However, direct experimental visualization of the energy-resolved femtosecond dynamics associated with this process remains elusive, as the intricate interfacial properties significantly complicate the underlying mechanism. Here, we reveal unprecedented spectral signatures of an ultrafast nonthermal hole transfer process at the Au/GaN heterointerface, occurring within 49 fs after plasmon excitation, on a timescale comparable to hot electron transfer. This process exhibits pronounced sensitivity to both excitation energy and light polarization, leading to a substantial reshaping of the low-energy electron distribution near the Fermi level by enhancing the low-energy electrons population and reducing the decay rate. Harnessing this ultrafast nonthermal hole transport results in a 14-fold enhancement in hydrogen evolution performance, demonstrating a promising approach for tailoring interfacial charge dynamics and offering mechanistic insights to guide the rational design of advanced plasmonic materials and device architectures.

Ultrafast charge transfer dynamics are key to photocatalytic efficiency, governing energy relaxation and surface reactivity. However, the temporal evolution of carrier energy landscapes following photoexcitation, particularly at complex metal/semiconductor interfaces, remains poorly understood. Here, we present a surface- and energy-resolved investigation of ultrafast electron dynamics across bare and Pt-modified gallium nitride (GaN) surfaces using time-resolved two-photon photoemission spectroscopy. We show that photogenerated electrons rapidly thermalize to the conduction band minimum and undergo sub-picosecond trapping in nitrogen-vacancy-related surface states. Surface modification with Pt suppresses these trapping channels and introduces an energy-independent ultrafast electron transfer pathway (~50 fs) from GaN into Pt. By disentangling interfacial charge transfer from intrinsic relaxation mechanisms through tailored pump-probe configurations, we demonstrate that Pt facilitates picosecond-scale electron transport from the bulk to the surface by photoinduced dynamic band flattening. Modulating these ultrafast dynamics through interfacial engineering significantly enhances charge separation and photoelectrochemical performance. This study deepens the understanding of interface-dependent relaxation and transfer processes of photocarriers and provides valuable guidance for rational design of advanced photocatalytic systems.

Abstract Spatially inhomogeneous plasmonic heterostructures concentrate light into nanoscale volumes and offer powerful routes for plasmon-mediated solar energy conversion. However, how hot carriers evolve and lose energy under such extreme nanophotonic confinement, particularly during interfacial transfer, remains largely unexplored. Here, we demonstrate that rationally engineered plasmonic nanocavities with intense nanoscopic field localization provide a unique platform to bypass these energy-loss channels by enabling nonthermal charge injection on a sub-40 fs time scale. Using polarization-resolved and time-resolved multiphoton photoemission spectroscopy, we reveal that the intense localized fields arising from plasmon-cavity mode coupling establish an accelerated and direct electron transfer channel across the interface. This behavior is accompanied by a distinctive inversion of the polarization dependence and by photon-energy-independent spectral features, confirming charge injection occurring prior to thermalization. Furthermore, the emergence of three-photon photoemission indicates a fundamental reconfiguration of the hot carrier generation and relaxation landscape resulting from the nanoscopic electric field within the plasmonic nanocavity. These insights establish a microscopic understanding of how interfacial charge transfer contributes to the enhanced water-oxidation activity in plasmonic nanocavity systems, offering a guiding framework for the development of advanced plasmonic photocatalysts and optoelectronic interfaces.

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.

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.

Plasmon-induced charge generation and separation in metal/semiconductor heterostructures offer a promising platform for hot carrier-based energy conversion applications. A key challenge is understanding ultrafast hot carrier transfer at heterogeneous interfaces, as the details of plasmonic enhanced charge transfer dynamics and accompanying energy relaxation remain unclear. Here, by tracking charge transfer processes across spatial, temporal, and energy domains, we reveal ultrafast, nonthermal electron transfer directly from gold nanoparticles to gallium nitride (GaN) without energy losses from electron-electron scattering. This process facilitates efficient charge separation and produces a nonthermal distribution of transferred electron in GaN-contrasting with substantial energy dissipation typically observed during conventional interfacial charge transport. Furthermore, we demonstrate the pivotal role of light polarization in modulating charge generation and energy distribution, which enables dynamic control of electron relaxation and enhances the possibility of nonthermal electrons surmounting the Schottky barrier for successful injection. These insights pave the way for advancing hot-carrier management and achieving coherent control of non-equilibrium charge behavior across multiple dimensions for solar energy conversion and optoelectronic applications.

While employing bulky organic semiconductor cations in layered (2D) hybrid halide perovskites can mitigate dielectric and quantum confinement limitations by enabling charge transfer between organic and inorganic layers, applying this strategy to tin-based perovskites is more challenging due to their more compact structure. Herein, we successfully integrate the strongly pi-delocalized naphthalene diimide (NDI) cations with diammonium-functionalized alkyl chain lengths of six carbons (NDI-dH) with tin(ii) iodide, forming a layered hybrid organic-inorganic perovskite with a type-II nanoheterojunction electronic architecture. Compared to typical Sn-based hybrid layered perovskite films (prepared with phenylethylamine), the herein reported (NDI-dH)SnI4 hybrid perovskite demonstrates significantly improved photogenerated charge carrier lifetime and a notable out-of-plane electron mobility reaching 0.1 cm2 s-1 V-1. Additionally, (NDI-dH)SnI4 exhibits improved stability in ambient air and nitrogen environments, highlighting its potential for use in air-stable optoelectronic devices.

The interaction of water molecules with semiconductor surfaces is relevant to various optoelectronic phenomena and physicochemical processes. Despite advances in fundamental understanding of water‐exposed surfaces, the detailed time‐ and energy‐resolved behavior of excited electrons remains largely unexplored. Here, the effects of water exposure on the near‐surface electron dynamics of phosphorus‐terminated p(2×2)/c(4×2)‐reconstructed indium phosphide (100) (P‐rich InP) are studied experimentally and matched to theoretical calculations. The P‐rich InP surface, consisting of H‐passivated P‐dimers, serves as a model for other P‐containing III‐V semiconductors such as gallium phosphide (GaP) or aluminum indium phosphide (AlInP). Electron dynamics near the surface are probed with femtosecond resolution using time‐resolved two‐photon photoemission (tr‐2PPE), a pump‐probe spectroscopic technique. Pulsed water exposure preserves electronic states and significantly increases lifetimes at the conduction band minimum (CBM). Density‐functional theory (DFT) calculations attribute these findings to suppression of surface vibrational modes in the top P‐layer by water exposure, reducing electronic transition probabilities of near‐band‐gap surface states. The results suggest that many near‐surface state lifetimes reported in ultra‐high vacuum may change significantly upon electrolyte exposure. These states may thus contribute more strongly to surface reactions than traditionally assumed. Demonstrating this effect for the technologically relevant P‐rich InP surface opens new opportunities in this underexplored area of surface electrochemistry.

AlInP (001) is widely utilized as a window layer in optoelectronic devices, including world‐record III‐V multi‐junction solar cells and photoelectrochemical (PEC) cells. The chemical and electronic properties of AlInP (001) depend on its surface reconstruction, which impacts its interaction with electrolytes in PEC applications and passivation layers. This study investigates AlInP (001) surface reconstructions using density functional theory and experimental methods. Phosphorus‐rich (P‐rich) and indium‐rich (In‐rich) AlInP surfaces are prepared with in situ monitoring of the process by reflection anisotropy (RA) spectroscopy and confirmed by low‐energy electron diffraction and photoemission spectroscopy. The experimental RA spectra closely match the theoretical predictions obtained by solving the Bethe–Salpeter equation. It is shown that missing hydrogen on P‐rich surfaces and formation of In–In 1D atomic chains on In‐rich surfaces introduce mid‐gap surface states that pin the Fermi level and induce band bending. Time‐resolved two‐photon photoemission measurements reveal ultrafast near‐surface electron dynamics for both P‐rich and In‐rich surfaces, demonstrating photoexcited electrons reaching the surface conduction band minimum and relaxing to mid‐gap surface states on about hundreds of fs. This work provides the most extensive AlInP surface analysis to date, allowing for more targeted surface and interface engineering, which is crucial for the optimization and design of III‐V heterostructures.

Artificial leaves could be the breakthrough technology to overcome the limitations of storage and mobility through the synthesis of chemical fuels from sunlight, which will be an essential component of a sustainable future energy system. However, the realization of efficient solar‐driven artificial leaf structures requires integrated specialized materials such as semiconductor absorbers, catalysts, interfacial passivation, and contact layers. To date, no competitive system has emerged due to a lack of scientific understanding, knowledge‐based design rules, and scalable engineering strategies. Herein, competitive artificial leaf devices for water splitting, focusing on multiabsorber structures to achieve solar‐to‐hydrogen conversion efficiencies exceeding 15%, are discussed. A key challenge is integrating photovoltaic and electrochemical functionalities in a single device. Additionally, optimal electrocatalysts for intermittent operation at photocurrent densities of 10–20 mA cm −2 must be immobilized on the absorbers with specifically designed interfacial passivation and contact layers, so‐called buried junctions. This minimizes voltage and current losses and prevents corrosive side reactions. Key challenges include understanding elementary steps, identifying suitable materials, and developing synthesis and processing techniques for all integrated components. This is crucial for efficient, robust, and scalable devices. Herein, corresponding research efforts to produce green hydrogen with unassisted solar‐driven (photo‐)electrochemical devices are discussed and reported.

Incorporating organic semiconductor building blocks as spacer cations into layered hybrid perovskites provides an opportunity to develop new materials with novel optoelectronic properties, including nanoheterojunctions that afford spatial separation of electron and hole transport. However, identifying organics with suitable structure and electronic energy levels to selectively absorb visible light has been a challenge in the field. In this work, we introduce a new lead-halide-based Ruddlesden-Popper perovskite structure based on a visible-light-absorbing naphthalene-iminoimide cation (NDI-DAE). Thin films of (NDI-DAE)(2)PbI4 show a quenched photoluminescence and transient absorption dynamics consistent with the formation of a charge transfer state or free charge carriers when either the inorganic or organic layer is photoexcited, suggesting the formation of a type II nanoheterostructure. Time-resolved microwave conductivity analysis supports free charge generation with sum mobilities up to 4 x 10(-4) cm(2) V-1 s(-1). Mixed halide (NDI-DAE)(2)Pb(I-x Br1-x )(4 )films show modified inorganic layer band gaps and a photoluminescent reversed type I nanoheterostructure with high bromide content (e.g., for x = 0). At x = 0.5, transient absorption and microwave conductivity measurements provide strong evidence that selective visible-light absorbance by the NDI-DAE cation generates separated free carriers via hole transfer to the inorganic layer (leaving photogenerated electrons in the organic layer), which represents an important step toward enhancing light harvesting and affording the spatial separation of charge carrier transport in stable layered perovskite-based devices.

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.

Renewable ("green") hydrogen production through direct photoelectrochemical (PEC) water splitting is a potential key contributor to the sustainable energy mix of the future. We investigate the potential of indium phosphide (InP) as a reference material among III-V semiconductors for PEC and photovoltaic (PV) applications. The p(2 x 2)/c(4 x 2)-reconstructed phosphorus-terminated p-doped InP(100) (P-rich p-InP) surface is the focus of our investigation. We employ time-resolved two-photon photoemission (tr-2PPE) spectroscopy to study electronic states near the band gap with an emphasis on normally unoccupied conduction band states that are inaccessible through conventional single-photon emission methods. The study shows the complexity of the p-InP electronic band structure and reveals the presence of at least nine distinct states between the valence band edge and vacuum energy, including a valence band state, a surface defect state pinning the Fermi level, six unoccupied surface resonances within the conduction band, as well as a cluster of states about 1.6 eV above the CBM, identified as a bulk-to-surface transition. Furthermore, we determined the decay constants of five of the conduction band states, enabling us to track electron relaxation through the bulk and surface conduction bands. This comprehensive understanding of the electron dynamics in p-InP(100) lays the foundation for further exploration and surface engineering to enhance the properties and applications of p-InP-based III-V-compounds for, e.g., efficient and cost-effective PEC hydrogen production and highly efficient PV cells.

Abstract The knowledge of minority and majority charge carrier properties enables controlling the performance of solar cells, transistors, detectors, sensors, and LEDs. Here, we developed the constant light induced magneto transport method which resolves electron and hole mobility, lifetime, diffusion coefficient and length, and quasi-Fermi level splitting. We demonstrate the implication of the constant light induced magneto transport for silicon and metal halide perovskite films. We resolve the transport properties of electrons and holes predicting the material’s effectiveness for solar cell application without making the full device. The accessibility of fourteen material parameters paves the way for in-depth exploration of causal mechanisms limiting the efficiency and functionality of material structures. To demonstrate broad applicability, we further characterized twelve materials with drift mobilities spanning from 10–3 to 103 cm2V–1s–1 and lifetimes varying between 10–9 and 10–3 seconds. The universality of our method its potential to advance optoelectronic devices in various technological fields.

The current efficiency records for generating green hydrogen via solar water splitting are held by indium phosphide (InP)-based photo-absorbers, protected by TiO2 layers grown through atomic layer deposition (ALD). InP is also a leading material for photonic integrated circuits and computing, where ultrafast near-surface behavior is key. A previous study described electronic pathways at the phosphorus-rich (P-rich) surface of p-doped InP(100) using time-resolved two-photon photoemission (tr-2PPE) spectroscopy. Here, the intricate electron pathways of the P-rich InP surface modified with ALD-deposited TiO2 are explored. Photoexcited bulk InP electrons migrate through a bulk-to-surface transition cluster of states and surface states and inject into the TiO2 conduction band (CB). Energy levels and occupation dynamics of CB states in P-rich InP and TiO2 adlayers are observed, with discrete states preserved up to 10 nm TiO2 deposition. Thermalization lifetimes of excited electrons > 0.8 eV above the InP conduction band minimum (CBM) are preserved for layer thicknesses up to 2.5 nm. Annealing at 300 degrees C to achieve crystalline TiO2 reconstructions destroys interfacial states, affecting charge transfer. These observations enable innovative engineering of the P-rich InP/TiO2 heterointerface, opening new possibilities for studying hot-carrier extraction, adsorbate effects, surface plasmons, and improving photovoltaic and PEC water-splitting devices.

An electron-accepting spacer di-cation based on naphthalene diimide is incorporated into layered and quasi-layered perovskite structure, and the effect of the resulting type II heterostructure on the optoelectronic structure is investigated.

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.

It is widely accepted that metal oxide‐based photoelectrodes (MOPs) hold great promise for future solar hydrogen generation but are facing awkward challenge arising from their low intrinsic carrier mobility. The highly polarized nature of the predominantly ionic metal‐oxygen bond always leads to the formation of small polarons that are responsible for the localized trapping of photo‐generated carriers. Present study explores the reduction of carriers transport barrier via bulk embedding of ferroelectric nanodomains (FNDs) in MOPs that results in a new performance benchmark for the CuBi 2 O 4 photocathode. By embedding laser‐generated sub‐10 nm BaTiO 3 nanocrystals in the bulk of CuBi 2 O 4 photocathode, numerous FNDs are created that can lead to two times enhancement of the carrier mobility, which is proposed to originate from the overlaying of the internal electric fields and effective electrons transport channel at the heterointerfaces of BaTiO 3 /CuBi 2 O 4 . Such strategy leads to the CuBi 2 O 4 photocathode with the photocurrent density of up to 3.21 mA cm −2 at 0.6 V RHE , as well as a pronounced absorbed photon‐to‐current efficiency up to 80% at 400 nm. The universal feature of present technology is further verified by laser embedding of SrTiO 3 FNDs, providing an effective route for addressing the charge transport limitations in MOPs.