III–V semiconductors are promising light absorbers for solar photochemical synthesis, but their surfaces tend to corrode and form defect-rich interphases under photochemical conditions. Here, we report a plasma-enhanced atomic layer deposition (ALD) GaN coating for InP(100), a model high-efficiency absorber, that forms an oxygen-nearly-free, atomically abrupt interface at low temperatures (<350°C). This coating reduces the population of deleterious surface states to below one-thousandth of those at a benchmark TiO2/InP interface. The resulting Pt/GaN/InP photocathode exhibits a barrier height of ~1.2 eV, delivering a ~0.82 V photovoltage, ~97% external quantum efficiency, and over 160 hours of stability in a pH 0 electrolyte. Additionally, the remaining states support ambipolar electron and hole transport through a ~10-nm-thick GaN layer, thereby facilitating photocatalytic H2 evolution and reversible redox-mediator oxidation. Our study demonstrates electronically benign and chemically robust III–V/liquid interfaces for efficient and stable photochemical charge separation and solar energy conversion, enabled by conformal, isotropic, acid-stable GaN coatings.

Photogenerated charge separation across micro- to nanometer scales is essential for photoelectric and photocatalytic conversion. However, identifying microstructures that sustain efficient charge separation and elucidating the underlying mechanisms remain challenging. Here, by combining surface photovoltage microscopy with optical imaging, we show that optically resonant cavity structures generate highly non-uniform light-field distributions that subsequently drive efficient charge separation through asymmetric electron and hole diffusivities. Spatiotemporal imaging of carrier dynamics from femtoseconds to seconds reveals that this charge separation originates from the combined contributions of ultrafast hot-electron diffusion (∼3 ps) and long-lived trap-limited transport (∼5 ms). Leveraging these effects, we demonstrate control over both the magnitude and direction of charge separation via optical structure engineering. These findings deepen the fundamental understanding of diffusion-driven charge separation in semiconductors and establish optical-architecture engineering as a viable approach for manipulating this process, providing a blueprint for advancing solar energy conversion and optoelectronic technologies.

Photocatalytic overall water splitting remains limited by inefficient charge separation and utilization in reactions. Al-doped SrTiO3 exhibiting near-100% apparent quantum efficiency for overall water splitting indicates nearly complete charge separation and surface catalytic efficiency. Although Al doping has been assumed to enhance charge separation and transfer, the exact role of Al is still unclear. Here, using spatiotemporal surface photovoltage imaging, we show that a gradient Al doping in Al-doped SrTiO3 generates a built-in electric field that drives photogenerated holes from the bulk toward surface trap sites in the form of hydroxylated Al-O-Ti, prolonging their lifetime from ~100 ns to 10 ms. Spectroscopic analyses reveal that these hydroxylated Al sites serve as key centers for water adsorption, facilitating water oxidation. These findings underscore the pivotal role of Al in the spatiotemporal alignment of hole transfer and surface catalytic water oxidation, enabling high-efficiency photocatalysis in overall water splitting.

Defect related transition energies and charge transfer along grain boundaries were studied in polycrystalline diamond by using a new type of mirrorless double-prism monochromator based on fused silica and by introducing modulated transient surface photovoltage spectroscopy. Polycrystalline diamond served as a model system containing small crystallites at the seed and large crystallites at the growth sides. Illumination was performed between 0.4 and 7.3 eV for seed and growth side orientations of the sample which allowed for homogeneous excitation of defect related transitions across the sample and for variation of the penetration depth under fundamental absorption in diamond. Photogenerated electrons were separated towards the seed side independently of the defect related transition for excitation below the range of the bandgap of diamond. Under strong absorption, photogenerated electrons were preferentially separated towards the surface. We found that photogenerated electrons are transferred faster than photogenerated holes along grain boundaries and that photogenerated holes are preferentially trapped at defects in crystallites. The enhancement of electron transfer via grain boundaries also explains the observed preferential electron trapping at surface states of diamond.

It has been shown on the example of high pressure high temperature (HPHT) diamond samples that effective absorption (or photogeneration) cross sections of defect states can be obtained by fitting modulated surface photovoltage (SPV) spectra. Spectra of in-phase and phase-shifted by 90 degrees SPV signals were measured without and with bias light at different wavelengths. In the fits, positive and negative features in the SPV spectra have been considered low and independent. Values of absorption energies and broadening parameters of up to 10 defect-related transitions were obtained for two rather different HPHT diamond samples. The influence of the phase of SPV signals and of bias light on the analyzed parameters is briefly discussed. Theresults demonstrate the opportunity of precise analysis of complex SPV spectra over the entire bandgap of diamond. The general applicability of the developed fitting procedure has been shown for precise fitting of absorption spectra below the bandgap of diamond.

Unraveling the interfacial dynamics of photogenerated charges at active sites is fundamental to advancing photocatalysis and related solar energy conversion systems. However, direct observation of the charge behavior within the electric double layer (EDL) under working conditions remains a major challenge. Here, we investigate nanoscale charge dynamics on a single BiVO4 particle model system by directly visualizing light-induced charge distribution within the EDL using a combination of spatially and temporally resolved techniques. Our findings reveal that oxygen vacancies stabilize the coexistence of localized electrons and holes, efficiently driving photocatalytic oxidation and reduction reactions between adjacent atoms. Moreover, defect-induced small polaron formation is shown to suppress ultrafast exciton recombination, extending hole lifetimes to 32 ms from subnanoseconds. Simultaneously, photogenerated electrons are effectively extracted by Fe3+ at the interface, overriding the influence of the net internal electric field and preventing their migration into the bulk. This study elucidates a synergistic mechanism involving reaction-driven electron transport and defect-assisted small-polaron-mediated oxygen evolution. This mechanism challenges the traditional mean-field electrostatic model of interfacial charge transport and highlights the critical role of localized charges for understanding how catalytic reactions reshape charge flow at solid-liquid interfaces. Our findings open avenues for the rational design of defect-engineered photocatalysts and operando-responsive materials in solar-to-chemical energy conversion.

Two-dimensional (2D) aluminum nitride (AlN) represents a promising material with unique properties predicted by density functional theory (DFT), characterized by a honeycomb lattice where Al and N atoms exhibit threefold in-plane coordination. However, the synthesis of free-standing AlN nanosheets has been challenging due to the crystal configurations of the well-known bulk AlN, which presents a hexagonal wurtzite structure with a tetrahedral coordination, preventing its exfoliation to obtain nanosheets. Herein, we propose a facile method involving the preparation of layered-structured aluminum carbonitrides, Al5C3N, followed by exfoliation into AlN nanosheets, offering a potential route for producing 2D AlN. The Al5C3N precursor was chemically etched in hydrofluoric acid (HF), breaking the Al-C bonds and exposing the AlN nanosheets. The development of this synthesis method opens up opportunities towards the preparation of 2D AlN and the investigation of its unique properties for applications in sensors and microelectronics.

Ferroelectric materials, known for their non‐inversion symmetry, show promise as photocatalysts due to their unique asymmetric charge separation, which separates hydrogen and oxygen evolution sites. However, the strong depolarized field induces a relaxed surface structure, which in turn directly leads to slow hole charge transfer dynamics, hindering their efficiency in water splitting. In this study, a fundamental breakthrough in dramatically enhancing the overall water‐splitting activity is presented, through the synergistically regulating of the surface behaviors of photogenerated carriers, resulting in nearly perfect parallel dynamics and balanced amounts. By depositing atomic layers of TiO 2 onto the surface of PbTiO 3 , surface vacancies are effectively passivated, significantly prolonging the hole lifetime from 10 −6 to 10 −3 s. Spatially resolved transient photovoltage spectroscopy showed that improved hole dynamics led to a 180° phase shift between photogenerated electrons and holes, indicating nearly identical extraction dynamics. Notably, hole and electron concentrations increased to equivalent levels. This leads to a nearly 578‐fold increment in the apparent quantum yield, resulting in significantly increased overall water‐splitting rates, with a quantum yield of 5.78% at 365 nm. The strategy is also effective with Al 2 O 3 and SiO 2 , demonstrating its versatility across varied materials, providing a valuable method for creating high‐performance ferroelectric photocatalysts.

Two-dimensional layered perovskite oxides have emerged as promising photocatalysts for solar-driven hydrogen evolution. Although doping has been widely employed to enhance photocatalytic performance, its role in modulating the electronic structure and the local chemical environment of these materials remains poorly understood. Here in this study, we investigate the codoping of Rh and La into exfoliated nanosheets of the Dion-Jacobson perovskite KCa2Nb3O10 to enhance photocatalytic hydrogen evolution reaction (HER) activity. A substantial increase in H2 evolution rate, from 12.3 to 69.0 μmol h-1, was achieved at an optimal doping level of 0.2 wt % Rh and 1.3 wt % La. Comprehensive structural and spectroscopic analyses, including synchrotron techniques and high-resolution microscopy, revealed that Rh3+ substitutes Nb5+ to introduce shallow 4d acceptor states that mediate charge separation, while La3+ substitutes Ca2+, compensates for aliovalent charge imbalance, and modulates local lattice distortions and oxygen vacancy formation. This codoping strategy enhances charge carrier lifetime and separation efficiency through a trap-mediated mechanism. The observed volcano-shaped activity trend highlights a narrow compositional window, where electronic and structural factors are optimally balanced. These findings establish a mechanistic foundation for defect engineering in layered perovskites and offer a pathway for the rational design of photocatalysts.

Oxide semiconductor photocatalysts are widely used for solar energy conversion, and the abundant intrinsic hydroxyl groups as defect sites on their surfaces play a key role in photocatalytic performance. However, the nature of surface hydroxyl-related defect states and their effect on the behavior of photogenerated charges, especially if targeted for charge separation, and whether the electrons and holes facing these hydroxyl sites in the same temporal and spatial ranges compete and conflict are unknown. Understanding these may help us to reasonably control defect-induced charge separation. Here, we perform an energy-, time-, and space-resolved study to reveal the effect of surface hydroxyl variation of BiOCl photocatalyst particles on photogenerated charge dynamics. We reveal that picosecond-level trapping and millisecond-level stability of holes initiated in electron-occupied states induced by hydroxyl sites are the greatest contributor to hole separation but the culprits that hinder electron utilization. Eliminating them can reduce unnecessary recombination and introduce unoccupied states, resulting in electron trapping and stabilization on the surface. Guided by these findings, selectively removing and holding hydroxyl sites on specific crystal planes can achieve the effective spatial separation of electrons and holes and show the associated enhanced reaction performance, especially in photocatalytic reduction. Operando imaging indicates that the surface hydroxyl-related charge-transfer sites align with reaction sites. This study reveals the critical role of surface defect states related to surface hydroxyl variation on charge separation and transport, which helps to understand the pivotal role of surface states in the entire photocatalytic process, also providing a valuable reference for most oxide semiconductor photocatalysts with intrinsic surface hydroxyl groups.

Techniques of surface photovoltage (SPV) spectroscopy allow for the highly sensitive characterization of transitions in ultrawide bandgap materials without the need for contact preparation. Furthermore, SPV techniques can give access to electronic transitions at buried interfaces. Tools for continuous measurements of dc, ac (modulated), and ac (transient) SPV signals with the same perforated electrode and a charge amplifier are developed for spectral ranges from the near infrared up to the deep ultraviolet and for time domains from the ns to s…h to ranges. The analysis of modulated SPV transients is applied. The high empirical potential of SPV spectroscopy is demonstrated by studies of a diamond single crystal, a layer of β‐Ga 2 O 3 grown by pulsed laser deposition, and an AlN layer epitaxially grown on sapphire. Defect‐related transitions are characterized over the entire bandgap for all mentioned materials. Several phonon‐assisted transitions near the bandgap of diamond are well detected. Different defect transitions dominating near the β‐Ga 2 O 3 surface or near the β‐Ga 2 O 3 /sapphire interface are distinguished. Several defect transitions near the bandgap of AlN are observed. Uncertainties for interpretation of transition energies are discussed.

Photoelectrochemical (PEC) cells have emerged as a promising and sustainable route for the production of valuable chemicals and have attracted significant attention over the last few decades. However, photoelectrodes, essential components of PEC devices, are limited by solar-to-hydrogen conversion efficiencies significantly below the theoretical values, as well as by low stability under working conditions. An important hurdle toward achieving high efficiency is the limited number of available tools that can directly probe the interplay between the local morphology and charge carrier transport dynamics within the photoelectrode top layer. This layer has different functions: providing photocatalytic active sites and protecting underlying layers. Therefore, the development of a powerful technique to reveal the influence of morphology on transport dynamics is crucial. In this work, we show how to precisely correlate local morphology with optoelectronic properties via the investigation of photoinduced charge transport processes, by time-dependent Kelvin probe force microscopy (KPFM) measurements. Our approach allows the extraction of time-dependent photovoltage for each pixel of a KPFM scan. We used crystalline-amorphous mixed phase TiO_{2} as a well-established model system to apply and validate our method. Through local correlations between structure-morphology and optoelectronic properties, our results reveal more efficient photoinduced charge separation on the regions corresponding to the crystalline TiO_{2} phase, which exhibit faster transport, and generate a significantly higher surface photovoltage signal (approximately 440 mV) upon UV illumination (super-band-gap illumination). This finding supports the considerable role of microstructure in charge carrier dynamics, and the ability of our analytical method to resolve it. The analysis technique outlined here should find broad applicability, enabling in-depth insight into the effect of local microstructure, and contributing to the design and development of more efficient photoelectrodes by tailoring of the microstructure at the nanoscale.

Charge separation is a critical process for achieving high photocatalytic efficiency, and ferroelectrics hold significant potential for facilitating effective charge separation. However, few studies have demonstrated substantial photocatalytic activity in these materials. In this study, we demonstrate that in ferroelectric PbTiO3, surface Ti vacancy defects near the positively polarized facets impede photocatalytic performance by trapping electrons and inducing their recombination. To tackle this issue, we selectively grew SrTiO3 nanolayers on the polarized facets PbTiO3, effectively mitigating interface Ti defects. This modification establishes a efficient electron transfer pathway at the interface between the positively polarized facets and the cocatalyst, extending the electron lifetime from 50 microseconds to the millisecond scale and significantly increasing electron participation in water-splitting reactions. Consequently, the apparent quantum yield for overall water splitting achieves the highest values reported to date for ferroelectric photocatalytic materials. This work provides an effective strategy for designing advanced ferroelectric photocatalytic systems.

A mirrorless fused silica based double-prism monochromator was developed for continuous measurements over a wide spectral range. Lens positions were adjusted with respect to the spectral dependencies of the focal lengths. A guided optical rail has been implemented between both prisms. The monochromator enables measurements at photon energies between 0.4 and 7.3 eV. Straylight is suppressed by more than eight orders of magnitude, and simulated and measured spectral resolutions are compared. Suitable applications of the monochromator were demonstrated for material characterization by photoelectron emission, optical transmission, photocurrent, and surface photovoltage spectroscopy. The monochromator broadens the opportunities for the detection of defect states, especially by surface photovoltage spectroscopy in ultra-wide bandgap semiconductors.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BA(2)PbI(4)), one of the most prominent 2D HaPs. We combined complementary defect- and exciton-sensitive methods such as photoluminescence (PL) spectroscopy, modulated and time-resolved surface photovoltage (SPV) spectroscopy, constant final state photoelectron yield spectroscopy (CFSYS), and constant light-induced magneto transport (CLIMAT), to demonstrate striking differences between charge separation induced by dissociation of excitons and by excitation of mobile charge carriers from defect states. Our results suggest that the broad sub-band gap emission in BA(2)PbI(4) and other 2D HaPs is caused by radiative recombination via defect states (shallow as well as midgap states) rather than self-trapped excitons. Density functional theory (DFT) results show that common defects can readily occur and produce an energetic profile that agrees well with the experimental results. The DFT results suggest that the formation of iodine interstitials is the initial process leading to degradation, responsible for the emergence of midgap states, and that defect engineering will play a key role in enhancing the optoelectronic properties of 2D HaPs in the future.

Photosystem I (PSI) is a photosynthetic protein which evolved to efficiently transfer electrons through the thylakoid membrane. This remarkable process attracted the attention of the biomolecular electronics community, which aims to study and understand the underlying electronic transport through these proteins by contacting ensembles of PSI with solid-state metallic contacts. This paper extends published work of immobilizing monolayers of PSI with a specific orientation, by using organophosphonate self-assembled molecules with hydrophilic heads on ultra-flat titanium nitride. Electrical measurements carried out with eutectic GaIn top contacts showed current rectification ratios of up to ~200. The previously proposed rectification mechanism, relying on the protein's internal electric dipole, was inquired by measuring shifts in the work function. Our straightforward bottom-up fabrication method may allow for further experimental studies on PSI molecules, such as embedding them in solid-state, transparent top contact schemes for optoelectronic measurements.

Photodegradation and defect passivation of hybrid metal halide perovskites are still challenges toward related highly stable optoelectronic devices such as solar cells. Herein, we show that photodegradation and defect density can be drastically reduced in 2D/3D perovskites by engineering the conditions for carrier delocalization. For this purpose, conjugated, partially conjugated and non-conjugated spacer molecules of similar sizes as well as n values have been systematically changed in 2D/3D perovskites based on archetypical MAPbI3. The decrease of calculated HOMO-LUMO gaps of complexes with spacer molecules (EHL-complex) correlated well with the decrease of the Urbach energy, the decrease of the trap-state density, and the strong reduction of photodegradation of 2D/3D perovskite films, whereas the photodegradation was practically independent of the n value for conjugated spacer molecules. In addition, the short circuit current density (Jsc) and open circuit voltage (Voc) of the prepared solar cells increased with decreasing EHL-complex; hence, maximum initial values of Jsc and Voc larger than those of identically prepared reference solar cells based on 3D MAPbI3 were reached. We also demonstrate that the photodegradation of solar cells decreased monotonously with increasing initial power conversion efficiency. Thus, delocalization by optimum coupling of electron wavefunctions between conjugated spacer molecules and binding moieties at the perovskite enhances defect passivation and strongly reduces photodegradation in 2D/3D perovskites.

α‐SnWO4 is an emerging photoelectrode material for photoelectrochemical water splitting, with several promising properties such as the favorable bandgap of 1.9 eV and suitable positions of the valence and conduction band. However, a major challenge remains: unprotected α‐SnWO4 undergoes surface passivation that blocks further charge transfer, and α‐SnWO4 electrodes that are covered with a protection/catalytic overlayer (e.g., NiOx, CoOx) show limited photovoltage. Earlier studies reveal that interfacial oxidation occurs due to the deposition of the overlayer. This negatively impacts the photovoltage that can be extracted, which is attributed to Fermi‐level pinning at the interface. The exact origin of this Fermi‐level pinning mechanism, however, remains unclear. In the present study, a combination of surface photovoltage analysis and hard X‐ray photoelectron spectroscopy is used to elucidate the electronic structure of the α‐SnWO4/oxide interface. Both techniques offer compelling and consistent evidence for the presence of a defect state that is energetically located within the bandgap energy of α‐SnWO4 and is likely responsible for the Fermi‐level pinning.

Numerous electronic transitions in polycrystalline chemical vapor deposition diamond with characteristic nucleation and growth sides were obtained by highly sensitive surface photovoltage (SPV) spectroscopy in dc (Kelvin probe), ac (modulated) and ac (transient) regimes from near infrared to deep ultraviolet. In the dc regime, defect transitions D1 to D8 were detected at 0.8–0.9, 1, 1.37, 1.78, 2.27, 3.15, 4.2 and 5.42 eV, respectively. Hints for more transitions were found for measurements in the ac (modulated) regime in the range near D3. SPV measurements in the ac (transient) regime showed the importance of disorder for relaxation of SPV signals excited at different photon energies. Phonon assisted transitions were observed at E g− E x− hν LA,LO, E g− E x + hν LA,LO and E g− E x + hν TO. The developed SPV techniques are suitable for applications in research and quality control not only for diamond but also for any other semiconductor with ultra-wide bandgap.

Halide perovskite-based photon upconverters utilize perovskite thin films to sensitize triplet exciton formation in a small- molecule layer, driving triplettriplet annihilation upconversion. Despite having excellent carrier mobility, these systems suffer from inefficient triplet formation at the perovskite/annihilator interface. We studied triplet formation in formamidinium-methylammonium lead iodide/rubrene bilayers using photoluminescence and surface photovoltage methods. By studying systems constructed on glass as well as hole-selective substrates, comprising self-assembled layers of the carbazole derivative 2PACz ([2-( 9H-carbazol-9-yl)ethyl]phosphonic acid) on indium-doped tin oxide, we saw how changes in the carrier dynamics induced by the hole-selective substrate perturbed triplet formation at the perovskite/rubrene interface. We propose that an internal electric field, caused by hole transfer at the perovskite/rubrene interface, strongly affects triplet exciton formation, accelerating exciton-forming electron-hole encounters at the interface but also limiting the hole density in rubrene at high excitation densities. Controlling this field is a promising path to improving triplet formation in perovskite/annihilator upconverters.