Photoelectrochemical Processes at n‑GaAs(100)/Aqueous HCl Electrolyte Interface: A Synchrotron Photoemission Spectroscopy Study of Emersed Electrodes

High-resolution synchrotron photoemission spectroscopy has been applied to detail the electrochemical and photoelectrochemical corrosion reactions at the liquid junction n‑GaAs(100)/1 M aqueous HCl solution. Under anodic polarization of 1.8 eV, the main process initiated by the presence of holes in the Ga–As bonding states of the valence band is the formation of soluble gallium chloride complexes and insoluble elemental arsenic on the surface. In addition, arsenic hydroxide forms, which reacts further to soluble HAsO₂. In toto, the As/Ga atomic ratio increases, which is accompanied by an increase of the work function. The anodic decomposition reaction is enhanced by illumination as more holes reach the n-semiconductor/electrolyte junction. Under cathodic polarization of 1.5 eV, only minor changes are observed in Ga and As core-level spectra, giving no indication of corrosion, but specific adsorption of hydrated HCl molecules and/or Cl^– ions considerably modifies valence band spectra

Synchrotron Photoemission Spectroscopy Study of p‑GaInP₂(100) Electrodes Emersed from Aqueous HCl Solution under Cathodic Conditions

(Photo)­electrochemical processes occurring under cathodic polarization at the p-GaInP₂(100)/1 M HCl_aq solution interface were investigated in detail by high-resolution surface sensitive synchrotron-radiation photo­emission spectroscopy. It was found that on application of the cathodic bias in the dark to the p-GaInP₂(100)/1 M HCl_aq solution interface the electrochemical processes are started at a bias of about −1.0 V vs reversible hydrogen electrode (RHE), where cathodic current passing through the semiconductor/electrolyte interface starts to rise. Under higher cathodic bias applied in the dark, hydroxyl groups and metallic gallium are accumulated at the surface, which is accompanied by a decrease in work function of the semiconductor. Accumulation of hydroxyl groups can be related only to splitting of water molecules at the semiconductor/electrolyte interface, since the aqueous HCl solution contains no hydroxyl groups intrinsically. Accumulation of hydroxyl groups and metallic gallium is accelerated under visible light illumination, which indicates participation of photogenerated electrons in the surface electrochemical reactions. The formation of the metallic gallium without simultaneous metallic indium formation testifies that the In–P bonds of the GaInP₂ compound are more stable against cathodic corrosion than the Ga–P bonds

Hybrid Perovskite/Perovskite Heterojunction Solar Cells

Recently developed organic–inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices

Band Alignment Engineering at Cu₂O/ZnO Heterointerfaces

Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices, yet the search for material combinations with suitable band alignments remains a challenge for numerous applications. In this work, we demonstrate how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system. The energy band alignment at the heterointerface between Cu₂O and ZnO was studied using photoelectron spectroscopy with stepwise deposition of ZnO onto Cu₂O and vice versa. A large variation of energy band alignment depending on the deposition conditions of the substrate and the film is observed, with valence band offsets in the range Δ<i>E</i>_VB = 1.45–2.7 eV. The variation of band alignment is accompanied by the occurrence or absence of band bending in either material. It can therefore be ascribed to a pinning of the Fermi level in ZnO and Cu₂O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu₂O. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as Δ<i>E</i>_VB ≈ 1.5 eV, being favorable for solar cell applications

Dopant Diffusion in Sequentially Doped Poly(3-hexylthiophene) Studied by Infrared and Photoelectron Spectroscopy

The diffusivity of dopants in semiconducting polymers is of high interest as it enables methods of sequential doping but also affects device stability. In this study, we investigate the diffusion of a bulky sequentially deposited p-dopant in poly­(3-hexylthiophene) (P3HT) thin films using nondestructive <i>in situ</i> infrared (IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant diffusion into the polymer film at varying coverage by differentially evaluating electron transfer in the bulk and at the surface. Thereby it is possible to determine dopant coverages at which both electron transfer and incorporation of dopants are saturated. By use of PES, neutral and charged dopants can be distinguished, revealing that charged dopants are less mobile in the diffusion process than neutral molecules. We further compare the diffusivity in semicrystalline and fully amorphous P3HT. We find that at high coverage semicrystalline P3HT seems to yield a higher capacity for dopants than fully amorphous P3HT. A temperature-dependent measurement of sequential doping shows directly that the incorporation of dopants is thermally activated and requires temperatures close to room temperature

Nanostructured SnO₂–ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO₂–ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO₂ particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO₂ particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption analyses, transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectroscopy. The SnO₂–ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO₂ nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV–visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO₂–ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO₂, that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO₂–ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO₂–ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO₂ and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO₂–ZnO photocatalyst because of the energy difference between the conduction band edges of SnO₂ and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO₂–ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications

Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes

We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A^–1, whereas maximum luminance values are as high as 8000 and 3000 cd m^–2 for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime

Influence of Fermi Level Alignment with Tin Oxide on the Hysteresis of Perovskite Solar Cells

We tune the Fermi level alignment between the SnO_<i>x</i> electron transport layer (ETL) and Cs_0.05(FA_0.83MA_0.17)_0.95Pb­(I_0.83Br_0.17)₃ and highlight that this parameter is interlinked with current–voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL–perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs

Preparation of RuO₂/TiO₂ Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations

Nanoporous RuO₂/TiO₂ heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO₂ nanoparticles in a ruthenium­(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO₂ amount (0.5–5 wt %), the as-prepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO₂ nanocrystallites modified with RuO₂ according to N₂ sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO₂/TiO₂ heterojunction. At first, a model experiment involving stepwise deposition of RuO₂ on the TiO₂ film and an <i>in situ</i> XPS measurement showed a shift of Ti 2p_3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO₂/TiO₂ heterojunction. The band bending for the heterostructure RuO₂/TiO₂ nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO₂/TiO₂ nanocatalyst led to twice higher activities than pure anatase TiO₂ and reference commercial TiO₂ P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO₂/TiO₂ heterojunction. Finally, these mesoporous RuO₂–TiO₂ heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications

New Insights into the Photocatalytic Properties of RuO₂/TiO₂ Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition

Photocatalytic activities of mesoporous RuO₂/TiO₂ heterojunction nanocomposites for organic dye decomposition and H₂ production by methanol photoreforming have been studied as a function of the RuO₂ loading in the 1–10 wt % range. An optimum RuO₂ loading was evidenced for both kinds of reaction, the corresponding nanocomposites showing much higher activities than pure TiO₂ and commercial reference P25. Thus, 1 wt % RuO₂/TiO₂ photocatalyst led to the highest rates for the degradation of cationic (methylene blue) and anionic (methyl orange) dyes under UV light illumination. To get a better understanding of the mechanisms involved, a comprehensive investigation on the photogenerated charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of O^–, Ti³⁺, and O₂^– trapping centers, was performed. Along with the key role of superoxide paramagnetic species in the photodecomposition of organic dyes, ESR measurements revealed a higher amount of trapped holes in the case of the 1 wt % RuO₂/TiO₂ photocatalyst that allowed rationalizing the trends observed. On the other hand, a maximum average hydrogen production rate of 618 μmol h^–1 was reached with 5 wt % RuO₂/TiO₂ photocatalyst to be compared with 29 μmol h^–1 found without RuO₂. Favorable band bending at the RuO₂/TiO₂ interface and the key role of photogenerated holes have been proposed to explain the highest activity of the RuO₂/TiO₂ photocatalysts for hydrogen production. These findings open new avenues for further design of RuO₂/TiO₂ nanostructures with a fine-tuning of the RuO₂ nanoparticle distribution in order to reach optimized vectorial charge distribution and enhanced photocatalytic hydrogen production rates