Ultraviolet Photoelectron Spectroscopy Spectra Research Articles

Observations on Valence‐Band Electronic Structure and Surface States of Bulk Insulators Based on Fast Stabilization Process of Sample Charging in UPS

AbstractThe valence‐band electronic structure is one of the most basic properties of materials and ultraviolet photoelectron spectroscopy (UPS) is the most commonly used technique for its measurement. To obtain credible UPS spectra on insulator surfaces, eliminating the charging effects at the surfaces is essential but is very difficult. Here, a reliable measurement for the valence‐band structure of bulk insulators is shown. It has been observed that, during UPS measurements without any charge compensation, the charging on irradiated insulator surfaces reaches its steady state very quickly and the fluctuation of the charging‐induced surface potential is very small. Therefore, the whole UPS spectrum undergoes just a parallel shift and can be calibrated by comparison with the corresponding X‐ray photoelectron spectroscopy (XPS) spectrum even though XPS often has a poorer resolution than UPS. From the calibrated valence‐band spectra, it is found that noticeable electronic energy levels (i.e., the “surface states”) exist in the bandgap, and based on the density of states (DOS) calculations, the surface states are attributed to the truncation of the bulk periodic structure at the surfaces. This work is anticipated to provide a way to obtain the valence band structure and the surface states of bulk insulators which play crucial roles in many physical processes.

On the combination of ultraviolet photoelectron spectroscopy with optical absorption studies to investigate Cu2O||TiO2 direct Z-scheme junctions with different Cu2O loading

Among the electronic properties, the positions of the electronic band edges and the work function are essential parameters for determining the potential of a photocatalyst and its ability to function in a solar conversion system. A novel type of photocatalysts, called direct Z-schemes, possesses many advantages over conventional heterojunctions, which all benefit the catalytic performance under solar light. As oxidation and reduction reactions are greatly affected by the electrical characteristics of the material, ultraviolet photoelectron spectroscopy (UPS) is a powerful tool to determine and quantify important electronic parameters of previously fabricated TiO2‖Cu2O junctions. TiO2 nanotubes modified with Cu2O nanoparticles exhibit a reduction in the value of the work function (WF = 3.67 ± 0.01 eV) and ionization potential (IP = 6.01 ± 0.04 eV) with respect to the TiO2 substrate (WF = 4.29 ± 0.02 eV and IP = 7.65 ± 0.05 eV). By varying the electrodeposition time, an optimized amount of deposited Cu2O nanoparticles was proven to reduce the WF and IP to facilitate the excitation of electrons, which could be correlated to the improved absorbance in the visible wavelength range. This work proposes a valuable methodology for band diagram tracing from UPS spectra and provides new insights in the relationship between synthesis, electronic properties and visible light absorption of titania based Z-schemes for photocatalytic applications with a combination of surface sensitive techniques and optical absorption studies.

Impact of photoinduced phase segregation in mixed-halide perovskite absorbers on their material and device stability

Mixed-halide perovskites enable bandgap engineering for tandem solar cell and light-emitting diode applications. However, photoinduced halide phase segregation introduces a compositional instability, that is, formation of I-rich and Br-rich phases, which compromises photovoltaic efficiency and stability. While optical and structural studies of the photoinduced phase segregation in mixed-halide perovskites have been reported, its impact on the material stability is missing. Here, a detailed compositional analysis of mixed-halide perovskite films using x-ray and ultraviolet photoelectron spectroscopy (UPS) was carried out to determine how their stability in various environments depends on the halide ratio. A series of perovskite thin films were fabricated with the composition CH3NH3Pb(IxBr1−x)3, where x = 0.00, 0.25, 0.50, 0.75, and 1.00, and analyzed under different conditions, such as exposure to light in ambient and in nitrogen atmosphere, as well as storage in the dark. From the spectroscopy results, complemented with structural and optical properties, it was found that the deletion of halide ions from the surface is facilitated in mixed-halide perovskites in comparison with pure halide perovskites. A higher stability was found for the mixed-halide perovskite containing less than 25% Br, and it decreases with increasing Br content. This study also established the effect of the Br/I ratio on the energy landscape of the materials. The UPS spectra reveal that photoinduced degradation results in a mismatch of the energy levels at the perovskite/transport layer interface, which may limit the collection of charge carriers. These findings correlate well with the photovoltaic device stability under similar degradation conditions.

Charge transfer and electronic relaxation effects in the photoemission of EMIM-DCA ionic liquid vapor

Ultraviolet photoelectron spectroscopy (UPS) measurements of EMIM-DCA vapors have been conducted. Density functional theory (DFT), Møller–Plesset perturbation theory (MP2) and many-body perturbation theory calculations (GW) have been used to interpret the experimentally measured spectra. The vapor of ionic liquids normally consists of isolated ion-pairs and the corresponding photoemission spectra should reflect the electronic binding energies of the molecular orbitals. The UPS spectra have revealed that many photolines are shifted when compared to ab initio calculation results. The hybrid DFT functionals, which performed very good in the description of the UPS spectra of many other ionic liquid vapors have shortcomings in the EMIM-DCA case. The MP2 calculation and the quasiparticle GW approach are shown to offer a better description of the experimental UPS spectra. The charge distribution analysis showed that there seems to be a favorable charge transfer from the cation to the anion upon ionization. The new data provides strong support to the argument that in some cases DFT still has difficulties in the modeling of IL ion-pairs.

Ultraviolet photoelectron spectroscopy: Practical aspects and best practices

Ultraviolet photoelectron spectroscopy (UPS) is an important technique for measuring the energies of the valence states of metallic, semiconducting and adsorbate-covered metal and semiconducting surfaces. Applications include catalysis, organic electronics, optoelectronic devices and photovoltaics. While it is relatively straightforward to obtain a UPS spectrum in a laboratory photoelectron spectrometer equipped with an ultraviolet discharge lamp, obtaining a meaningful spectrum is more difficult and depends on various factors, including proper sample preparation and the elimination of surface charging. The basics of the technique, along with procedures for proper measurement of valence spectra of bare and adsorbate-covered metal and semiconducting surfaces, and conjugated polymer/oligomer films, are described. Best practices for presenting UPS data and measuring work functions from UPS spectra are also discussed.

(Digital Presentation) Study on the Electrical Characteristics and Band Diagrams of Ga2O3 MOS Gate Stacks

β-Ga2O3 is expected as one of the candidate materials for high-efficient and cost-effective power devices for high-voltage applications [1]. For the fabrication of MOSFETs the understanding of band diagrams of MOS structure is inevitably important. However, the previous reports on band alignments of β-Ga2O3 and various dielectrics including Al2O3 and SiO2 are controversial with significant inconsistencies in their values [2]. Since the ideal band diagram referring to the vacuum level is not clarified we cannot tell the Fermi level of β-Ga2O3 and the flat-band voltage (Vfb) shift of MOS capacitors. In this study we fabricated MOS structures and its band diagram was investigated from photoelectron spectroscopy.We employed SiO2 as the gate dielectric. Single crystalline β-Ga2O3 (001) wafers with ~2×1016 cm-3 n-type doped epitaxial layers were cleaned in diluted HF solution. Then SiO2 films were deposited by electron-beam evaporation of Si in O2 ambient of ~1×10-2 Pa, followed by post-deposition annealing (PDA) in 1 atm-O2 at various temperatures. Au was evaporated as gate electrode to form MOS capacitors. Ultraviolet photoelectron spectroscopy (UPS) was conducted to determine the energy level of the valence band edge on the surface of β-Ga2O3 epitaxial layer referring to the vacuum level. The valence band spectrum of ~4nm-thick SiO2/β-Ga2O3 was also investigated by x-ray photoelectron spectroscopy to determine the valence band offset at the interface.From C-V characteristics of MOS capacitors with different PDA conditions, the higher PDA temperature was found to be beneficial to achieve nearly-ideal C-V characteristics. A negligibly-small hysteresis was obtained for the one fabricated with O2-PDA at 1000℃ [3]. The energy profiles of interface state density (Dit) were determined by conductance method for the samples of different PDA conditions. The values of Dit decreased for higher PDA temperature down to ~1010 cm-3 at the energy level of EC−0.2 eV [3]. These values are significantly lower than typically reported ones for ALD-Al2O3 MOS structures on β-Ga2O3 [4]. The additional annealing at low temperature at 400℃ works for the further reduction of hysteresis to less than 1 mV/nm.Next the band diagrams of the MOS structures were investigated. From the UPS spectra we could deduce the energy level of valence band edge referring to the vacuum level, from the difference between minimum (cut-off) and maximum edges of the obtained kinetic energy spectra of photoelectrons. We also evaluated the valence band offset from the deconvolution of the valence band XPS of thin SiO2/β-Ga2O3 stack. From the energy edge difference between the deconvoluted spectra corresponding to SiO2 and Ga2O3, respectively, the valence band offset ~1.1 eV was deduced, which is not conflicting with the estimated valence band edge energy of Ga2O3 and the well-reported valence band edge energy of SiO2. The conduction band edge energy was also estimated assuming the bandgap of β-Ga2O3 (4.7eV) [1]. Considering the Fermi-level of β-Ga2O3 with n-type doping of ~2×1016 cm-3 was approximated to be ~0.1 eV from the conduction band edge, the Fermi-level on the surface of β-Ga2O3 epitaxial layer after 1000℃ O2 annealing was estimated to be 3.6-3.7eV below the vacuum level [5]. Note that this value would be significantly smaller than the one simply estimated from the previously reported electron affinity of Ga2O3 [6].Finally the Vfb shifts were investigated for the MOS capacitors fabricated with O2 annealing at 1000℃ with various thicknesses, referring to the ideal flat-band voltage determined with the Fermi-level of β-Ga2O3 as discussed above and the work function of Au electrode which was also determined with UPS measurement. As a result, we found the most of the Vfb values were around the ideal flat-band voltage irrespective of the film thickness from 20 to 60 nm, which indicates a small density of fixed charges <2×1011 cm-2 [5].In conclusion, we found that SiO2/β-Ga2O3 (001) MOS capacitors showed nearly-ideal electrical characteristics with small fixed charge density by applying O2-PDA at 1000℃. The band diagram for the MOS stacks referring to the vacuum level was clarified by photoelectron spectroscopy, which enables us to understand the β-Ga2O3 MOS interface properties correctly.

(Invited) Surface/Interface Structures of Perovskite Films Studied By Electron Spectroscopies

Interface of perovskite layer play a crucial role in the charge extraction and recombination process of perovskite solar cell. It is, therefore, of great importance to examine the geometrical and electronic structures of surface/interface of perovskite. Unfortunately, standard experimental techniques such as X-ray diffraction (XRD) and scanning tunneling microscopy (STM) are not suitable to examine practical perovskite films for solar cells. XRD observes the lattice periodicity and is not sensitive to the surface structure. Although STM can precisely observe the surface atoms, it cannot be applied to rough surface or thick films, e.g. solution-processed perovskite surface.We have demonstrated that a combination of electron spectroscopies is powerful and versatile tool to examine both geometrical and electronic structures of perovskite surfaces. Figure 1 shows principles of ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). In UPS, the valence electrons are excited by ultraviolet photons (typically with the energy of 21.22 eV) and kinetic energies of photoelectrons are analyzed to measure the valence electronic states as a difference between the photon and electron kinetic energies. The probing depth of UPS is limited by the mean free path of electrons which is usually several layers a few nanometers (several layers) in the energy range of UPS. In MAES, the excitation source is replaced with the metastable helium atom (with the energy of 19.82 eV). Although the process as electron spectroscopy is similar to that of UPS, MAES is extremely surface sensitive because the helium atom can only interact with the outermost atomic orbitals of the sample surface. By comparing the UPS and MAES spectra, we can obtain information about the surface structure as well as the electronic states. In this presentation, we will demonstrate the following two topics:1. Surface termination of CH3NH3PbI3 [1]The properties of the interface of perovskite ABX3 should be strongly affected by the surface termination, i.e. whether the surface is terminated with AX or BX layer. We examined the surface termination of a CH3NH3PbI3 thin film prepared by the standard one-step solution process. The results show that surface of the perovskite film is terminated with the layer consisting of CH3NH3 and I.2. Surface structure of quasi-2D perovskite films, PEA2m(CH3NH3)n-2mPbnI3n [2]A small amount of large organic cations incorporated in perovskite film (quasi-2D) passivates the surface and enhances the stability of the perovskite solar cell against moisture. We examined surface structure of PEA2m(CH3NH3)n-2mPbnI3n, and confirmed that the PEA segregates to the surface and that the phenyl group covers the outermost surface of the quasi-2D perovskite. From the concentration dependence, we also examined the plausible structure of the quasi-2D film.1) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Yuyama, P. Krüger, H. Yoshida, Adv. Mater. 33, 2004981 (2021).2) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Aihara, M. A. Kamarudin, S. R. Sahamir, S. Hayase, T. Yuyama, P. Krüger, H. Yoshida, Appl. Phys. Express 14, 031006 (2021).

Surface Doping in Poly(3,4-ethylenedioxythiophene)-Based Nanoscale Films: Insights for Polymer Electronics

Conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) are widely researched for application in electronic devices. Researcher’s look to exploit the ability of these polymers to conduct electrical charge. To induce conductivity, the polymers are doped with counterions; for PEDOT, this is typically done with poly(styrenesulfonate) or tosylate (Tos). The Tos anions inserted within the PEDOT nanostructure stabilize positive defects (holes) on the polymer’s conjugated backbone, which, in turn, facilitates electrical conduction. In this study, we use X-ray photoelectron spectroscopy to investigate the Tos doping of PEDOT within the outermost region (<15 nm) of electrochemically oxidized or reduced PEDOT:Tos nanoscale films. Computation of the predicted density of states from density functional theory studies is also conducted to aid in interpreting the ultraviolet photoelectron spectroscopy spectra. We observe that the doping of PEDOT:Tos is more complex than first thought, likely involving the nonionic triblock copolymer used during PEDOT’s oxidative polymerization. This hypothesis is corroborated by time-of-flight secondary-ion mass spectrometry measurements on the outer 2 nm of the oxidized and reduced PEDOT:Tos. The observation of complex and heightened doping near the surface opens opportunities for the deliberate surface engineering of PEDOT:Tos nanofilms in polymer electronic applications such as electrochemical transistors and electrical connections.

Dual Triplet Sensitization Strategy for Efficient and Stable Triplet–Triplet Annihilation Upconversion Perovskite Solar Cells

Dual Triplet Sensitization Strategy for Efficient and Stable Triplet–Triplet Annihilation Upconversion Perovskite Solar Cells

Non-monochromatic He I and He II UPS spectra of polycrystalline silver

The surface composition of clean polycrystalline Silver (Ag) was analyzed by ultraviolet photoelectron spectroscopy (UPS) at optimal instrument operation parameters for both nonmonochromatic He I and He II emission. UPS valence band spectra were obtained using a differentially pumped He gas-discharge lamp (He I: 21.2 eV; He II: 40.8 eV). Fermi levels (Ef) in UPS spectra arising from each source line are observed—where the energy difference between Ef arising from He I or He II is equivalent to the difference in source energy of He I and He II primary lines (19.6 eV), useful for calibration of UPS systems or spectra analysis.

Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed α-Fe2O3(0001) Surface.

Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on α-Fe2O3(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage. However, upon dosing at high temperature (>200 °C), both photoemission techniques have shown distinctive features that strongly suggest electron transfer from adsorbed phenol to Fe2O3 surface atoms and consequent formation of a surface radical. Consistent with the experiment, DFT calculations show that phenoxyl adsorption on the iron oxide surface at RT leads to a minor charge transfer to the adsorbed molecule. The experimental findings at high temperatures agree well with the EPFRs' proposed formation mechanism and can guide future experimental and computational studies.

Tantalum-Doped TiO2 Prepared by Atomic Layer Deposition and Its Application in Perovskite Solar Cells.

Tantalum (Ta)-doped titanium oxide (TiO2) thin films are grown by plasma enhanced atomic layer deposition (PEALD), and used as both an electron transport layer and hole blocking compact layer of perovskite solar cells. The metal precursors of tantalum ethoxide and titanium isopropoxide are simultaneously injected into the deposition chamber. The Ta content is controlled by the temperature of the metal precursors. The experimental results show that the Ta incorporation introduces oxygen vacancies defects, accompanied by the reduced crystallinity and optical band gap. The PEALD Ta-doped films show a resistivity three orders of magnitude lower than undoped TiO2, even at a low Ta content (0.8–0.95 at.%). The ultraviolet photoelectron spectroscopy spectra reveal that Ta incorporation leads to a down shift of valance band and conduction positions, and this is helpful for the applications involving band alignment engineering. Finally, the perovskite solar cell with Ta-doped TiO2 electron transport layer demonstrates significantly improved fill factor and conversion efficiency as compared to that with the undoped TiO2 layer.

A study of dopant incorporation in Te-doped GaAsSb nanowires using a combination of XPS/UPS, and C-AFM/SKPM

We report the first study on doping assessment in Te-doped GaAsSb nanowires (NWs) with variation in Gallium Telluride (GaTe) cell temperature, using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), conductive-atomic force microscopy (C-AFM), and scanning Kelvin probe microscopy (SKPM). The NWs were grown using Ga-assisted molecular beam epitaxy with a GaTe captive source as the dopant cell. Te-incorporation in the NWs was associated with a positive shift in the binding energy of the 3d shells of the core constituent elements in doped NWs in the XPS spectra, a lowering of the work function in doped NWs relative to undoped ones from UPS spectra, a significantly higher photoresponse in C-AFM and an increase in surface potential of doped NWs observed in SKPM relative to undoped ones. The carrier concentration of Te-doped GaAsSb NWs determined from UPS spectra are found to be consistent with the values obtained from simulated I–V characteristics. Thus, these surface analytical tools, XPS/UPS and C-AFM/SKPM, that do not require any sample preparation are found to be powerful characterization techniques to analyze the dopant incorporation and carrier density in homogeneously doped NWs.

Ionic Liquid Vapors in Vacuum: Possibility to Derive Anodic Stabilities from DFT and UPS.

Ultraviolet photoelectron spectroscopy (UPS) investigations of several gas-phase ionic liquid (IL) ion pairs have been conducted. [EMIM][OTF], [PYR14][OTF], [EMIM][DCA], [PYR14][DCA], [PYR14][TCM], [PYR14][FSI], [PYR14][PF6], [S222][TFSI], [P4441][TFSI], and [EMMIM][TFSI] vapor UPS spectra are presented for the first time. The experimental low-binding-energy cutoff value (highest occupied molecular orbital, HOMO energy) of the ionic liquid ion pairs, which is of great interest, has been measured. Many studies use calculated gas-phase electronic properties to estimate the liquid-phase electrochemical stability. Hybrid density functional theory (DFT) calculations have been used to interpret the experimental data. The gas-phase photoelectron spectra in conjunction with the theoretical calculations are able to verify most HOMO energies and assign them to the cation or anion. The hybrid M06 functional is shown to offer a very good description of the ionic liquid electronic structure. In some cases, the excellent agreement between the UPS spectra and the M06 calculation validates the conformer found and constitutes as a first indirect experimental determination of ionic liquid ion-pair structure. Comparisons with recent theoretical studies are made, and implications for electrochemical applications are discussed. The new data provide a much-needed reference for future ab initio calculations and support the argument that modeling of IL cations and anions separately is incorrect.

Inverse photoemission measurements of plutonium metal and oxides

The unoccupied density of states of α-plutonium metal, plutonium sesquioxide (α-Pu2O3) and plutonium dioxide (PuO2) have been measured using inverse photoemission spectroscopy (IPS) in the isochromat mode (9.7 eV). Ultraviolet Photoelectron spectroscopy (UPS) at four different photon energies between 11 and 41 eV are also reported to support the interpretation of the IPS data. The band gap energies of the oxides have been determined from the combination of UPS and IPS spectra; the photoelectric work function was established from the UPS data for the metal and oxides and information from the UPS and IPS spectra have been used to aid interpretation of the core level Pu 4f XPS data. Crown copyright © 2020 Published by Elsevier B.V. All rights reserved.

In-Depth Investigation of the Correlation between Organic Semiconductor Orientation and Energy-Level Alignment Using In Situ Photoelectron Spectroscopy.

Organic semiconductors (OSCs) are of interest for replacing traditional Si-based semiconductors as their flexibility and transparency enable new applications. The properties of OSC materials greatly depend on their orientation and molecular arrangement, which are strongly dependent on the underlying substrate material. Hence, in this study, in situ ultraviolet photoelectron spectroscopy (UPS) is used to elucidate the effect of the substrate on OSC orientation. Two types of OSCs, namely those with shape anisotropy (pentacene, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, and dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene) and those with shape isotropy (N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine, tris(4-carbazoyl-9-ylphenyl)amine, and [6,6]-phenyl C71 butyric acid methyl ester), are deposited on different electrode materials. The differences in the UPS spectra of these materials are observed directly. In general, the orientation of anisotropic OSC molecules significantly depends on the substrate properties, while that of the isotropic ones do not. All the anisotropic OSC molecules grown on poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) electrodes show a greater degree of molecular ordering than those grown on Au and multiwalled carbon nanotube/PEDOT:PSS electrodes. The molecular arrangements within the OSC/electrode structures are reflected in the energy-level shifts in the corresponding UPS spectra and hence in the electronic configurations. The results of this study should aid the design and synthesis of OSC materials with configurations suitable for organic electronic devices.

Elucidating the influence of residual polymer and gas environment on the electronic structure of a graphene layer using in situ APXPS

We use in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and ultraviolet photoelectron spectroscopy (UPS) to develop an effective method for studying changes in the graphene (Gr) electronic structure according to certain circumstance. The amount of polymethyl methacrylate (PMMA) residual polymer (RP), inevitably generated during the Gr transfer process, is significantly reduced from Gr surface by thermal annealing. This processed Gr is then sequentially exposed to specific gas environments (Ar, N2, O2, and CO2), and APXPS or UPS is carried out to investigate the variations in the Gr electronic structure including the work function. When the amount of PMMA RP on Gr is reduced, the position of the main carbon peak shifts by >0.4 eV to a higher binding energy (in XPS spectra), and the secondary electron cutoff moves by about 0.2 eV to a lower binding energy (in UPS spectra). These changes are generally caused by a decrease in the Gr work function. On the other hand, exposure to the gas environments at different temperatures that we investigated did not produce significant changes in the work function and chemical states of Gr. These results confirm that the material in contact with Gr should be considered to achieve the desired Gr performance in electronics.

Plasmon-enhanced perovskite solar cells using ultra-thin LiF spacer isolating AgAl and Au composite nanoparticles from metal electrode

Abstract Plasmon enhanced perovskite solar cells (PSCs) incorporated with AgAl and Au composite nanoparticles (NPs) isolating from AgAl metal electrode with ultra-thin LiF spacer were investigated. PSCs with AgAl-Au composite NPs got 17.02% of the power conversion efficiency (PCE), which is 11.10% higher than PCE (15.32%) of the control PSC with only AgAl electrode. Furthermore, the hysteresis effect of PSCs with LiF modified AgAl and Au composite NPs can be neglected compared to the control cells without NPs and LiF spacer. In the meanwhile, carrier transport and collection probabilities of PSCs with AgAl and Au composite NPs were increased to 96.86% from 89.51% in control cells, indicating the improved electron transport and extraction ability. The enhanced PCEs of PSCs with AgAl(3 nm)/Au(2 nm)/LiF(1 nm) are attributed to the improved light absorption of perovskite layer and the reduced contact barrier at metal electrode, which can be supported with its enhanced photon-to-electron conversion efficiency (△IPCE) and the disappeared negative capacitive behavior. Furthermore, the obviously reduced work function values of AgAl-Au composite NPs and AgAl metal electrode after LiF modification were observed with ultraviolet photoelectron spectroscopy spectra. Our results indicate that thermally evaporating a ultra-thin LiF spacer can well inhibit the energy dissipation of plasmon effects of AgAl-Au composite NPs and reduce the contact barrier between PCBM and AgAl metal electrode, which is an efficient strategy to achieve efficient plasmon-enhanced perovskite solar cells.

Highly-efficient organic light-emitting devices based on poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine:octadecylamine-graphene quantum dots

Abstract Organic light-emitting devices (OLEDs) with a poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD):octadecylamine (ODA)-graphene quantum dots (GQDs) hole transport layer (HTL) were fabricated to enhance their efficiency. Photoluminescence (PL) and PL excitation spectra showed that the optical energy bandgap of the ODA-GQDs was 2.7 eV, and ultraviolet photoelectron spectroscopy spectra demonstrated that the edge of the highest occupied molecular orbital of the ODA-GQDs was 5.3 eV below the Fermi level. While the operating voltage of the OLEDs with a PVK:ODA-GQD (9.77 V, 0.25 wt%) HTL at 10 mA/cm2 was 0.8 V lower than that of the OLEDs with a poly-TPD HTL (10.57 V), their current efficiency was larger by more than 20% than that of the OLEDs with poly-TPD (32.88 cd/A) due to an increase in the number of holes injected from the ITO to the HTL. The number of injected holes was increased due to a reduction of the energy barrier and an increase in the conductivity.

Effects of nitrogen substitution in amorphous carbon films on electronic structure and surface reactivity studied with x-ray and ultra-violet photoelectron spectroscopies

We investigated the effects of incorporating a low percentage of nitrogen on the local and the electronic structures of amorphous carbon (a-C) using X-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy (UPS). Nitrogen-doped amorphous carbon films (a-CNx) with varying nitrogen contents were prepared by a thermal decomposition method using a mixture of CH4 + NH3 under atmosphere. A slight shift of the C 1s core-level spectrum toward the higher binding energy side was detected in a-CNx as a function of nitrogen content, whereas a shift of the Fermi level (EF) cannot be confirmed from the UPS results. This was interpreted as a chemical shift between carbon and nitrogen atoms rather than as a shift of the EF. The C 1s peak shifts can be explained by the presence of two kinds of C–N local structures and the charge transferred bulk C–C components by nitrogen atoms. The two kinds of deconvoluted C 1s components could be well correlated with the two N 1s components. Two localized states were detected below the EF in UPS spectra of a-CNx, which could be assigned to defect bands. These defects played a significant role in the surface reactivity, and were stabilized in a-CNx. The adsorption and reaction of NO were carried out on a-CNx as well as a-C films. It was found that both defect sites and O2– species were responsible on a-C, while O2– species were selectively active for NO adsorption on a-CNx. We concluded that nitrogen doping reduces defect density to stabilize the surface of a-C, while at the same time inducing the selective adsorption capability of NO.