UV Photoemission Spectroscopy Research Articles

Advanced Characterization and Optimization of NiOx:Cu‐SAM Hole‐Transporting Bi‐Layer for 23.4% Efficient Monolithic Cu(In,Ga)Se2‐Perovskite Tandem Solar Cells

AbstractThe performance of five hole‐transporting layers (HTLs) is investigated in both single‐junction perovskite and Cu(In, Ga)Se2 (CIGSe)‐perovskite tandem solar cells: nickel oxide (NiOx,), copper‐doped nickel oxide (NiOx:Cu), NiOx+SAM, NiOx:Cu+SAM, and SAM, where SAM is the [2‐(3,‐6Dimethoxy‐9H‐carbazol‐9yl)ethyl]phosphonic acid (MeO‐2PACz) self‐assembled monolayer. The performance of the devices is correlated to the charge‐carrier dynamics at the HTL/perovskite interface and the limiting factors of these HTLs are analyzed by performing time‐resolved and absolute photoluminescence ((Tr)PL), transient surface photovoltage (tr‐SPV), and X‐ray/UV photoemission spectroscopy (XPS/UPS) measurements on indium tin oxide (ITO)/HTL/perovskite and CIGSe/HTL/perovskite stacks. A high quasi‐Fermi level splitting to open‐circuit (QFLS‐Voc) deficit is detected for the NiOx‐based devices, attributed to electron trapping and poor hole extraction at the NiOx‐perovskite interface and a low carrier effective lifetime in the bulk of the perovskite. Simultaneously, doping the NiOx with 2% Cu and passivating its surface with MeO‐2PACz suppresses the electron trapping, enhances the holes extraction, reduces the non‐radiative interfacial recombination, and improves the band alignment. Due to this superior interfacial charge‐carrier dynamics, NiOx:Cu+SAM is found to be the most suitable HTL for the monolithic CIGSe‐perovskite tandem devices, enabling a power‐conversion efficiency (PCE) of 23.4%, Voc of 1.72V, and a fill factor (FF) of 71%, while the remaining four HTLs suffer from prominent Voc and FF losses.

Enhance the Properties of BiI3‐Based Resistive Switching Devices via Mixing Ag and Au Electrodes

AbstractThe correlation between the electrodes and the properties of the BiI3 device is systematically investigated. The X‐ray and UV photoemission spectroscopies are carried out to study the chemical state and electronic structure of the metal/BiI3 interfaces formed by the in‐situ deposition of BiI3 on the metal. This revealed the upward diffusion of Ag and the self‐formation of a conductive filament; the latter is further confirmed via tunneling electron microscopy. By coating the Au substrate with Ag layers of various thicknesses as a buffer layer, the morphology, surface roughness, chemical states, and quantity of the filament in the BiI3 layer can be manipulated. The device with the structure of Au/10 nm Ag/BiI3/Au demonstrated an ultrahigh on/off ratio of 109, good retention of 104 s, and multistate data storage. This study provides not only a fundamental insight into the interaction of BiI3 with metal, which will be helpful to design resistive switching devices and optoelectronics based on BiI3 material, but also a facile method to control the quantity of conductive filament in the BiI3 layer.

Probing the correlation between morphology and optical anisotropy in ZnTPP films grown at different temperatures

Films of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting graphite electrode surfaces from degradation. The effectiveness of the protection depends on both the molecule–substrate and molecule–molecule interactions, which determine the type of film growth and its morphology. For instance, meso-tetraphenyl porphyrin-Zn(II) (ZnTPP) arranges differently depending on the substrate, conditions used for deposition and post-growth treatments. Since many parameters influence film morphology and strategies to reach an efficient protective coverage, here we investigate the role of the substrate temperature in determining morphological variations in thick ZnTPP films. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system on a highly oriented pyrolytic graphite (HOPG) substrate, controlled in temperature by a variable temperature cryostat. The film morphology was characterized by atomic force microscopy (AFM), whereas the type of growth by reflectance anisotropy spectroscopy (RAS), which is highly sensitive to the orientation and type of arrangement of molecules on the substrate; changes in the film valence electronic structure were monitored in situ by UV photoemission spectroscopy (UPS). The comparison between AFM and RAS investigations clearly correlates a ZnTPP film morphology to the characteristic optical signal, both influenced by temperature conditions and diffusivity of molecules on the substrate.

Study of the effect of density of states distribution on carrier injection at organic/electrode interface through high-sensitivity photoemission spectroscopy and injection simulation

Carrier injection, which is a key factor in controlling and improving organic device properties, has been predominantly studied using the injection barrier height derived from HOMO and LUMO positions. The weak density of states (DOS) within the HOMO–LUMO energy gap is also important to understand the practical injection properties. In this study, the DOS of the α-NPD/electrode model interfaces are investigated using high-sensitivity UV photoemission spectroscopy. The nature of hole injection is discussed based on the observed DOS and a simple simulation. The results indicate that the weak DOS close to the Fermi level is critical for carrier injection.

Defense interplay of the zinc-oxide nanoparticles and melatonin in alleviating the arsenic stress in soybean (Glycine max L.)

Present study showed the successful application of the modified hydrothermal method for synthesizing the zinc oxide nanoparticles (ZnO-NPs) efficiently. Well as-synthesized ZnO-NPs are analyzed for various techniques viz., X-ray diffraction (XRD), SEM micrographs, EDAX/Mapping pattern, Raman Spectroscopy Pattern, UV, Photoluminescence (PL) and X-ray photoemission spectroscopy (XPS) analysis. All these measurements showed that ZnO-NPs are highly pure with no internal defects, and can be potentially used in the plant applications. Hence, we further determined the effect of these nanoparticles and melatonin for the modulation of the As tolerance in soybean plants by examining the various growth attributes and metabolic parameters. Our results demonstrated that As-stress inhibited growth (∼34%), photosynthesis-related parameters (∼18–28%) and induced ROS accumulation; however, all these attributes are substantially reversed by the ZnO-NPs and melatonin treatments. Moreover, the As stress induced malondialdehyde (MDA; 71%) and hydrogen peroxide (H2O2; 82%) are partially reversed by the ZnO-NPs and melatonin in the As-stressed plants. This might have resulted due to the ZnO-NPs and melatonin induced activities of the antioxidants plant defense. Overall, the ZnO-NPs and melatonin supplementation separately and in combination positively regulated the As tolerance in soybean; however, the effect of their combined application on the As tolerance was more profound relative to the individual application. These results suggested the synergetic effect of the ZnO-NPs and melatonin on the As tolerance in soybean. However, the in-depth mechanism underlying the defense crosstalk between the ZnO-NPs and melatonin needs to be further explored.

Exploration of Gas-Liquid Interfaces for Liquid Water and Methanol Using Extreme Ultraviolet Laser Photoemission Spectroscopy.

We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas-liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas-liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a1 band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a1 splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na+ penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a1 splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.

In-gap states of an amorphous In–Ga–Zn–O thin film studied via high-sensitivity ultraviolet photoemission spectroscopy using low-energy photons

Low-density electronic states in the energy gap of an amorphous In–Ga–Zn–O film control device performance. Herein, density of states (DOS) distribution from valence band to the in-gap states of 1014 cm−3 eV−1 level was determined using high-sensitivity UV photoemission spectroscopy. Exponential tail states accompanying two energetically-localized states were directly observed as reported previously. The observed slope of the exponential tail state was different from the Urbach energy derived using photothermal deflection spectroscopy, indicating the importance of directly observing the DOS of in-gap states.

(Invited) Estimating Band Edge Energies of Monolayer-Tethered Semiconductor Nanocrystals: Comparison of Photoemission and Spectroelectrochemical Approaches and the Implications for Photocatalysis

This presentation will focus on recent studies to characterize the energetics of semiconductor nanocrystals (NCs - e.g. CdSe, CdS and hetero-structured CdSe@CdS nanorods and tetrapods) using both spectroelectrochemistry and X-ray and UV-photoemission spectroscopies (XPS/UPS), for nanocrystals tethered as monolayers to ITO electrodes (spectroelectrochemistry) or Au (XPS/UPS). Understanding the parameters which control conduction and valence band edge energies in semiconductor nanocrystals is challenging and directly impacts on our ability to design new photocatalysts and photoelectrochemical platforms to produce fuels from sunlight. We have recently shown that UPS and XPS can be used to determine valence band maximum (VBM) energies and vacuum-level corrected ionization potentials (IP), and using the bandgap energies, conduction band energies (CBM) and electron affinities (EA), for monolayers of CdSe NCs tethered to Au, where processing occurred in ambient conditions or in completely inert (O2, H2O free) environments. These studies demonstrated the extreme sensitivity of band edge energies for these nanocrystals to even small changes in surface composition, and if NCs are processed in inert environments, the IP/EA values track the diameter and bandgap energy of the NC quite closely to the trends predicted by the effective mass approximation (EMA). Ambient processed NCs show evidence for loss of the chalcogenide from the surface, with large differences in local vacuum level, and poor tracking of the EMA as NC diameter is varied. More recently we have returned to the characterization of monolayer-tethered, ligand capped CdSe NCs on ITO electrodes, using attenutated-total internal reflectance (ATR) spectroelectrochemical methods, which provide an optical approach to monitoring electron injection into these low surface coverage NCs, allowing us to estimate CBM for isolated NCs in contact with electrolyte solutions. In our most recent studies we find that these estimates of CBM versus NC diameter functionally track the EMA approximation, but with significant shifts in electrochemical potentials (on the vacuum scale) relative to energies estimated via photoemission. In the presence of polar solutions and high concentrations of supporting electrolyte the CBM is shifted by up to 800 meV farther from vacuum than for what is observed via photoemission, emphasizing the critical role played by solvent, surrounding ions, and even the electrode surface in screening the extra charge in the isolated NCs acquired as a result of electrochemical "reduction." For NCs which are precisely synthesized, where processing conditions are optimized, and where size dispersity is minimized, however, it appears that it will eventually be possible to quantify these effects in ways that will be useful in the design of photocatalysts, where VBM/CBM energies control the rates of electron transfer to electron donors/acceptors as part of fuel forming processes.

Design and characterization of a magnetic bottle electron spectrometer for time-resolved extreme UV and X-ray photoemission spectroscopy of liquid microjets

We describe a magnetic bottle time-of-flight electron spectrometer designed for time-resolved photoemission spectroscopy of a liquid microjet using extreme UV and X-ray radiation. The spectrometer can be easily reconfigured depending on experimental requirements and the energy range of interest. To improve the energy resolution at high electron kinetic energy, a retarding potential can be applied either via a stack of electrodes or retarding mesh grids, and a flight-tube extension can be attached to increase the flight time. A gated electron detector was developed to reject intense parasitic signal from light scattered off the surface of the cylindrically shaped liquid microjet. This detector features a two-stage multiplication with a microchannel plate plus a fast-response scintillator followed by an image-intensified photon detector. The performance of the spectrometer was tested at SPring-8 and SACLA, and time-resolved photoelectron spectra were measured for an ultrafast charge transfer to solvent reaction in an aqueous NaI solution with a 200 nm UV pump pulses from a table-top ultrafast laser and the 5.5 keV hard X-ray probe pulses from SACLA.

Polaron-Adsorbate Coupling at the TiO2(110)-Carboxylate Interface.

Understanding how adsorbates influence polaron behavior is of fundamental importance in describing the catalytic properties of TiO2. Carboxylic acids adsorb readily at TiO2 surfaces, yet their influence on polaronic states is unknown. Using UV photoemission spectroscopy (UPS), two-photon photoemission spectroscopy (2PPE), and density functional theory (DFT) we show that dissociative adsorption of formic and acetic acids has profound, yet different, effects on the surface density, crystal field, and photoexcitation of polarons in rutile TiO2(110). We also show that these variations are governed by the contrasting electrostatic properties of the acids, which impacts the extent of polaron–adsorbate coupling. The density of polarons in the surface region increases more in formate-terminated TiO2(110) relative to acetate. Consequently, increased coupling gives rise to new photoexcitation channels via states 3.83 eV above the Fermi level. The onset of this process is 3.45 eV, likely adding to the catalytic photoyield.

Interaction of ultra-thin CoTPP films on Fe(001) with oxygen: Interplay between chemistry, order, and magnetism

This work focuses on the stability against the oxidation of the chemical, structural, and magnetic properties of the system consisting of a layer of Co tetra-phenyl porphyrins grown on the top of Fe(001) – p(1 × 1)O. Such a system is characterized by a very high degree of structural order and the existence of magnetic coupling between the molecules and the substrate, even at room temperature, as we recently reported [Jagadeesh et al., Appl. Phys. Lett. 115, 082404 (2019)]. We highlight, by using x-ray photoemission spectroscopy, the effect of porphyrins in screening the substrate from oxidation. The coupling between the magnetic response of the system and the order of the molecular layer is investigated by means of spin-resolved UV photoemission spectroscopy and low-energy electron diffraction, respectively. As a result, a link is eventually found between this response and the chemical and structural stability of the interface.

Stoichiometry, band alignment, and electronic structure of Eu2O3 thin films studied by direct and inverse photoemission: A reevaluation of the electronic band structure

The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

In-Depth Spectroscopy and New Heights for Organic Solar Cells

In this issue of Joule, Lami et al. describe a method that enables UV photoemission spectroscopy (UPS) in the transverse dimension of polymeric semiconductor layers with nanometer-scale resolution. The approach is based on the use of Argon gas cluster ion beam (GCIB) etching instead of monoatomic ion bean bombardment. The use of GCIB reduces surface damage, enabling in depth UPS. The method is applied to the study of critical electronic levels and photovoltage in organic solar cells.

Application of High-Sensitivity UV photoemission Spectroscopy to Examine the Electronic Structure of Thermophilic Rhodopsin

The electronic structures of materials such as HOMO and LUMO are essential information to understand their functions. Photoemission spectroscopy, which is the most standard technique to examine the electronic structures of various materials, has been not well applied to biomolecules mostly due to sample charging and poor sensitivity. Very recently, we have developed high-sensitivity photoemission spectroscopy (HS-PES) using low photon energy. In this study, HS-PES has been successfully applied to a thermophilic rhodopsin (TR) film. The high sensitivity of our technique enabled to observe the HOMO region of retinal part in TR without any problem due to sample charging. This technique is expected to explore the electronic structures of various proteins.

Understanding the role of organic cations on the electronic structure of lead iodide perovskite from their UV photoemission spectra and their electronic structures calculated by DFT method

Organometal halide perovskites are very attractive as they can be used as efficient light sensitizing materials for solar cells with high power conversion efficiency. In this paper, we report the results of a study on various lead iodide perovskite crystals, which were prepared with different organic cations, and their properties in relation to their electronic structures, particularly their valence bands. For this purpose, we have carried out the XRD and UV Photoemission Spectroscopy (UPS) measurements along with the electronic structure computations. Among the organic cations studied here, it is found that not all organic cations can favorably form a perovskite crystal, though it meets the Goldschmidt tolerance factor. The organic cation, which shows the favourability of the formation of perovskite crystal structure, exhibits a band-edge shifting in the UPS spectrum of its perovskite crystal structure toward to smaller binding energy in comparison to the UPS spectrum of its organic salt. Such band-edge shifting is corresponding to the formation of the valence band in its perovskite crystal structure. The electronic structure calculations using Density Functional Theory (DFT) method, confirmed the formation of an energy gap that is in good agreement with the experimental results. However, the valence band is mainly constructed by the I− orbitals, while the conduction band is mainly constructed by the Pb2+ orbitals, as reported in some literatures. The present study results then suggests that the organic cations play only a minor role in their photovoltaic processes, though they substantially contribute to the valence band formation by transferring their valence electrons to the iodide elements.

Surface resonance of thin films of the Heusler half-metal Co2MnSi probed by soft x-ray angular resolved photoemission spectroscopy

Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of AlOx capped epitaxial thin films of the ferromagnetic half metal Co2MnSi ex-situ by soft X-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX- ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, which does not originate from bulk states, but from a surface resonance. This provides strong evidence for the validity of the previously proposed model based on this resonance, which was applied to explain the huge spin polarization of Co2MnSi observed by angular-integrating UV-photoemission spectroscopy.

Electronic structure of designed [(SnSe)1+δ]m[TiSe2]2 heterostructure thin films with tunable layering sequence

Abstract

Oxidation behavior with quantum dots formation from amorphous GaAs thin films

ABSTRACTWe investigated the oxidation behaviour of an amorphous GaAs thin film deposited onto a micro/nanotextured Si surface by an electron beam. After the deposited film was exposed to air, microcrystallites were formed with octahedral cubic and monoclinic structures of arsenic oxides. Short time exposure after thin film deposition showed the formation of cubic arsenolite while long time exposure showed the formation of monoclinic claudetites as well as cubic arsenolites. These oxide microcrystallites at the GaAs thin film surface disappeared after the sample annealing process. However, the amorphous GaAs thin film included high-density GaAs nanodots. From UV and inverse photoemission spectroscopies, the thin film showed n-type band structure with an energy gap of 2.73 eV. Photoluminescence measurement showed an emission peak at (450–513) nm with the energy of (2.41–2.75) eV corresponding to dot size of (4.1–4.5) nm.

Impact of ambient environment on the electronic structure of CuPc/Au sample

The performances of organic devices are crucially connected with their stability in the ambient environment. The impact of 24 h. Ambient environment exposure to the electronic structures of about 12 nm thick CuPc thin film on clean Au substrate have been studied employing UV photoemission spectroscopy technique. X-ray photoemission spectroscopy (XPS) was used to find out the origin of the change of the electronic structures in the sample with the exposure. The XPS study suggests that the oxidation occurs at the CuPc thin film. Due to the adsorption of oxygen in the CuPc film from the ambient air, charge carriers are formed within the CuPc film. Moreover, the XPS results imply that the CuPc film is sufficiently thinner for diffusing oxygen molecules through it and gets physically absorbed on Au substrate during the ambient exposure. Consequently, the hole injection barrier height of pristine CuPc film, grown on Au substrate, is reduced by about 0.50 eV and work-function of the pristine CuPc sample is enhanced by around 0.25 eV in the exposure. The findings will help to understand the mechanism that governs the degradation of performance of CuPc based devices in ambient environment.

Topologically Enclosed Aluminum Voids as Plasmonic Nanostructures.

Recent advances in the ability to synthesize metallic nanoparticles with tailored geometries have led to a revolution in the field of plasmonics. However, studies of the important complementary system, an inverted nanostructure, have so far been limited to two-dimensional sphere-segment voids or holes. Here we reveal the localized surface plasmon resonances (LSPRs) of nanovoids that are topologically enclosed in three-dimensions: an "anti-nanoparticle". We combine this topology with the favorable plasmonic properties of aluminum to observe strongly localized field enhancements with LSPR energies in the extreme UV range, well beyond those accessible with noble metals or yet achieved with aluminum. We demonstrate the resonance tunability by tailoring the shape and size of the nanovoids, which are truncated octahedra in the 10-20 nm range. This system is pristine: the nanovoid cavity is free from any oxide or supporting substrate that would affect the LSPRs. We exploit this to infer LSPRs of pure, sub-20-nm Al nanoparticles, which have yet to be synthesized. Access to this extreme UV range will allow applications in LSPR-enhanced UV photoemission spectroscopy and photoionization.