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The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite compo-nent to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small-and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h., QC 20230613

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4 . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V2 O5 at grain boundaries of Mo-BiVO4 thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4 . After photo-electrochemical operation, the degraded Mo-BiVO4 films show similar grain center and grain boundary spectra indicating V2 O5 dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4 . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V2 O5 at grain boundaries of Mo-BiVO4 thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4 . After photo-electrochemical operation, the degraded Mo-BiVO4 films show similar grain center and grain boundary spectra indicating V2 O5 dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells., QC 20220406

We elucidate the role of subsurface oxygen on the production of C2 products from CO2 reduction over Cu electrocatalysts using the newly developed grand canonical potential kinetics density functional theory method, which predicts that the rate of C2 production on pure Cu with no O is ∼500 times slower than H2 evolution. In contrast, starting with Cu2O, the rate of C2 production is >5,000 times faster than pure Cu(111) and comparable to H2 production. To validate these predictions experimentally, we combined time-dependent product detection with multiple characterization techniques to show that ethylene production decreases substantially with time and that a sufficiently prolonged reaction time (up to 20 h) leads only to H2 evolution with ethylene production ∼1,000 times slower, in agreement with theory. This result shows that maintaining substantial subsurface oxygen is essential for long-term C2 production with Cu catalysts.

The binary Ta-N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach - by which the carrier concentration is reduced by the oxygen impurity - offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.

We elucidate the role of subsurface oxygen on the production of C2 products from CO2 reduction over Cu electrocatalysts using the newly developed grand canonical potential kinetics density functional theory method, which predicts that the rate of C2 production on pure Cu with no O is ∼500 times slower than H2 evolution. In contrast, starting with Cu2O, the rate of C2 production is >5,000 times faster than pure Cu(111) and comparable to H2 production. To validate these predictions experimentally, we combined time-dependent product detection with multiple characterization techniques to show that ethylene production decreases substantially with time and that a sufficiently prolonged reaction time (up to 20 h) leads only to H2 evolution with ethylene production ∼1,000 times slower, in agreement with theory. This result shows that maintaining substantial subsurface oxygen is essential for long-term C2 production with Cu catalysts.

The binary Ta-N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach - by which the carrier concentration is reduced by the oxygen impurity - offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.

Chemical transformations that occur on photoactive materials, such as photoelectrochemical water splitting, are strongly influenced by the surface properties as well as by the surrounding environment. Herein, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge-carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on bismuth vanadate. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron-trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge-carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding functional characteristics of water splitting photoanodes, devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.

Chemical transformations that occur on photoactive materials, such as photoelectrochemical water splitting, are strongly influenced by the surface properties as well as by the surrounding environment. Herein, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge-carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on bismuth vanadate. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron-trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge-carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding functional characteristics of water splitting photoanodes, devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.

The surface chemistry of 1,6-dibromo-3,8-diiodopyrene (Br2I2Py) is comparatively studied on Au(111) versus Ag(111) surfaces under ultrahigh vacuum conditions by a combination of high-resolution scanning tunneling microscopy (STM) and X-ray spectroscopy. The chemical state of the molecular networks, that is, the dehalogenation and the possible formation of organometallic intermediates, is assessed by X-ray photoelectron spectroscopy. In addition, pyrene tilt angles are quantified by carbon K-edge near edge X-ray absorption fine structure experiments. Upon room-temperature (RT) deposition of Br2I2Py onto Au(111), only partial deiodination was found, and STM revealed the coexistence of ordered arrangements of both intact Br2I2Py molecules and organometallic dimers as well as few larger aggregates. Further annealing to 100 C triggered full deiodination followed by the formation of organometallic chains of otherwise still brominated molecules. By contrast, on Ag(111), iodine is fully and bromine is partly dissociated upon RT deposition of Br2I2Py. The initially disordered organometallic aggregates can be reorganized into more ordered structures by mild annealing at 125 degrees C. Yet, the conversion of the organometallic intermediates into well-defined cross-linked quasi 2D covalent networks was neither possible on Au(111) nor on Ag(111). This is attributed to the large steric hindrance in the covalently linked adsorbed state., Funding Agencies|The excellence cluster Nanosystems Initiative Munich; Max Planck Society

Photoelectrochem. water splitting is a promising route for efficient conversion of solar energy to chem. fuel. However, the development of an efficient photoanode remains the primary materials challenge in the establishment of a scalable technol. for solar water splitting. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious recombination sites. In addn., the functional behavior of photocatalytic systems strongly depends on the presence of structural defects and heterogeneity over different length scales. Indeed, charge trapping at interfaces and/or (photo) corrosion processes can affect catalytic performance and selectivity. Here, we show an approach to the discovery integrated photoanodes based on bismuth vanadate light absorber. Among different photoelectrode materials, BiVO is one of the most actively investigated oxide semiconductors due to its moderate bandgap, favorable conduction band position, and relatively long photocarrier lifetimes. By use of in situ optical spectroscopy and comparisons between the metal oxide coatings and their extrinsic optical and electrocatalytic properties, we present a suite of data-driven discoveries, including the scale-up of compn. regions which form optimal interfaces with BiVO . We use photoconductive at. force microscopy to correlate local surface morphol. with generated photocurrent maps at individual grain facets in BiVO . The photocurrent maps resolve the contributions from individual grains with nanometer spatial resoln. This careful anal. allows us to identify charge transfer and loss mechanism at the nanoscale that ultimately contribute to det. the macroscale properties of the material.

The on-surface synthesis of covalent organic nanosheets driven by reactive metal surfaces leads to strongly adsorbed organic nanostructures, which conceals their intrinsic properties. Hence, reducing the electronic coupling between the organic networks and commonly used metal surfaces is an important step towards characterization of the true material. We demonstrate that post-synthetic exposure to iodine vapor leads to the intercalation of an iodine monolayer between covalent polyphenylene networks and Ag(111) surfaces. The experimentally observed changes from surface-bound to detached nanosheets are reproduced by DFT simulations. These findings suggest that the intercalation of iodine provides a material that shows geometric and electronic properties substantially closer to those of the freestanding network., Funding Agencies|DFG [LA1842/4-1]; Nanosystems Initiative Munich; Helmholtz-Zentrum Berlin

The synthesis of two-dimensional organic nanostructures on metal surfaces is studied under ultra-high vacuum conditions by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy. The surface-supported structures are stabilized by various different intermolecular interactions resulting in the formation of self-assembled structures and covalent networks. For on-surface polymerization of covalent nanostructures different coupling reactions, substrate materials, and reaction parameters are investigated., Die Synthese von zweidimensionalen, organischen Nanostrukturen auf Metalloberflächen wird unter Ultrahochvakuum-Bedingungen mittels Rastertunnelmikroskopie und Röntgenphotoelektronenspektroskopie untersucht. Die oberflächengestützten Strukturen basieren auf verschiedenen intermolekularen Bindungen, wodurch selbstassemblierte Strukturen und kovalent gebundene Netzwerke ausgebildet werden können. Für die oberflächenbasierte Polymerisation von kovalenten Netzwerken werden verschiedene Substrate, Kupplungsreaktionen und Reaktionsparameter untersucht.

The synthesis of two-dimensional organic nanostructures on metal surfaces is studied under ultra-high vacuum conditions by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy. The surface-supported structures are stabilized by various different intermolecular interactions resulting in the formation of self-assembled structures and covalent networks. For on-surface polymerization of covalent nanostructures different coupling reactions, substrate materials, and reaction parameters are investigated., Die Synthese von zweidimensionalen, organischen Nanostrukturen auf Metalloberflächen wird unter Ultrahochvakuum-Bedingungen mittels Rastertunnelmikroskopie und Röntgenphotoelektronenspektroskopie untersucht. Die oberflächengestützten Strukturen basieren auf verschiedenen intermolekularen Bindungen, wodurch selbstassemblierte Strukturen und kovalent gebundene Netzwerke ausgebildet werden können. Für die oberflächenbasierte Polymerisation von kovalenten Netzwerken werden verschiedene Substrate, Kupplungsreaktionen und Reaktionsparameter untersucht.