Using Ultraviolet Photoelectron Spectroscopy Research Articles

Impact of surface treatments on the electron affinity of nitrogen-doped ultrananocrystalline diamond

In recent years, various forms of nanocrystalline diamond (NCD) have emerged as an attractive group of diamond/graphite mixed-phase materials for a range of applications from electron emission sources to electrodes for neural interfacing. To tailor their properties for different uses, NCD surfaces can be terminated with various chemical functionalities, in particular hydrogen and oxygen, which shift the band edge positions and electron affinity values. While the band edge positions of chemically terminated single crystal diamond are well understood, the same is not true for nanocrystalline diamond, which has uncontrolled crystallographic surfaces with a variety of chemical states as well as graphitic grain boundary regions. In this work, the relative band edge positions of as-grown, hydrogen terminated, and oxygen terminated nitrogen-doped ultrananocrystalline diamond (N-UNCD) are determined using ultraviolet photoelectron spectroscopy (UPS), while the band bending is investigated using photoelectrochemical measurements. In contrast to the widely reported negative electrode affinity of hydrogen terminated single crystal diamond, our work demonstrates that hydrogen terminated N-UNCD exhibits a positive electron affinity owing to the increased surface and bulk defect densities. These findings elucidate the marked differences in electrochemical properties of hydrogen and oxygen terminated N-UNCD, such as the dramatic changes in electrochemical capacitance.

Tuning Mo Incorporation in Ni-Fe-Mo Solid Solutions: Towards an Improved Hydrogen Adsorption Energy

The production of hydrogen by water electrolysis has emerged as a promising pathway to achieve a 100% renewable and sustainable hydrogen economy. Up to now, alkaline water electrolysis has been highlighted as a truly alternative strategy for hydrogen production due to the usage of non-expensive electrocatalysts. Ni-based electrocatalysts already have a long century history because of high conductivity, stability, and earth abundancy. However, their strong hydrogen interaction energy slows the Hydrogen Evolution Reaction (HER) activity. Tuning the morphology of the catalyst by increasing the active surface area (e.g., production of nanosheets, nanowires, nanoparticles, etc.) and enhancing the intrinsic activity (e.g., doping, alloying, etc.) can improve the catalytic activity. Binary and ternary alloys based on nickel are a common route to boost their intrinsic activity and tune the morphology. Ternary alloys like CuAlNi, NiMoFe and NiMoW have been previously proposed, although the role of each element is still not fully investigated. Among the alloys, Ni-Fe mixtures have shown better catalytic activity towards HER, and the Mo incorporation has resulted in a reduced onset potential due to favorable hydrogen-metal interaction and development of active sites. The strategy for NiFeMo synthesis is mainly by hydrothermal method or electrodeposition which are limited by low material loading and production volume. Therefore, searching for a technique which is capable of producing trimetallic alloys with a scalable amount is of high interest for the development of hydrogen production. Here, we studied the preparation of FeNi3Mox via a fast and straightforward solution precursor plasma spraying (SPPS) method. The liquid precursors containing metal salts of Ni, Fe, and Mo are used to produce trimetallic solid solutions. The non-stoichiometric nanostructured electrocatalysts are deposited onto stainless-steel mesh (SS-mesh), Ni foam, and carbon paper for the HER in alkaline. We found that tuning Mo content (~9 At%) in the solution precursor conducts to an improved overpotential of 112 mV (iR-corrected) at -10 mA cm-2 and a Tafel slope of 109 mV dec-1 in agreement with a Volmer-Heyrovsky HER route. Energy dispersive X-ray spectroscopy (XPS) has confirmed the reduction in the superficial Ni and Fe oxides with Mo insertion which is directly affecting the electron transport. In addition, to investigate the effect of Mo in more detail, we evaluated the work function using ultraviolet photoelectron spectroscopy (UPS) which shows that Mo incorporation changes linearly the electrocatalyst’s work function and it is highly correlated to the hydrogen binding energy. Consequently, the change in the work function, resulting in a weaker hydrogen adsorption energy which leads to an improved hydrogen production. Theoretical catalytic activity maps from Density Functional Theory (DFT) studies have revealed the enhanced HER activity is the responsibility of the active sites near the superficial Mo atoms. Figure 1

Self-driven electron enrichment of ultrafine PdAu nanoparticles for electrochemical CO2 reduction: High applicability of work function as an activity descriptor

Highly coupled metal/dopant-incorporated carbon dyads provide a possibility to modulate the electron density of metallic materials by forming a rectifying interface, thus showing an enhanced activity in electrochemical CO2 reduction reaction (ECRR). However, understanding the promotion effects of dopants for ECRR is limited to the prediction by theoretical interpretation and case-by-case studies. Herein, we report the direct experimental evidence that the work function, regulated by single structural factor-dopant contents, is significantly correlated to the ECRR reaction activity and kinetics. We prepared a series of PdAu/NxC electrocatalysts composed of ultrafine (∼5.7 nm) PdAu bimetallic nanoparticles and tailorable N-doped carbon supports. The wide range of the amount of N dopants allowed the modification of the band gap of the carbon easily. Using ultraviolet photoelectron spectroscopy (UPS) measurements, we demonstrate that the reactivity and kinetics trends of the PdAu/NxC in the ECRR can be intrinsically correlated with the work function of the catalysts. PdAu/N7.50C electrocatalyst with the highest N contents displays a 100% CO2-to-CO conversion and high conversion efficiency over a wide potential window, superior over other reported PdAu catalysts. This work provides a novel way to boost ECRR performance by deliberately lowering the work function of the metal/carbon electrocatalysts through the enhancement by dopants.

Designing of graphene oxide-induced p-NiO/n-MoO3 heterostructures for improved photocatalytic activity through plasmon-enhanced S-scheme mechanism and its biological and electrochemical performance.

An environmentally benign, economically advantageous microwave hydrothermal approach is used in synthesis of desirably tailored graphene oxide-induced p-NiO/n-MoO3 (GNM) heterostructures. Various analytical techniques such XPS, XRD, UPS, EIS, and Mott-Schottky were conducted to comprehend complete morphology and functioning of the novel ternary heterostructure photocatalysts. Also, SEM and HR-TEM images were presented for better interpretation. The strategic plasmonic step scheme (S-scheme) charge migration approach was used to describe the effective charge recombination process. Hydroxyl and oxide active species were corollaries of the reactive radical-scavenging experiments and electron spin resonance. The work function has been confirmed using ultraviolet photoelectron spectroscopy (UPS), which assessed an electron transfer between NiO and MoO3, yielding work function values of 6.32eV and 5.26eV, respectively. The cell apoptosis of the HeLa cell line approves the material's biocompatibility. Cyclic voltammetry and electrochemical impedance spectroscopy reveal electrochemical performance of GNM heterostructures. We anticipate our results would pave the way for current and future applications. In order to ensure eco-restoration such photocatalyst which are eco and cost friendly are synthesized. Assessment of pollutant risks presents the impact of them on human and terrestrial and aquatic animals. Sustainability of material is acknowledge as they use solar light for photocatalysis and dye degradation, and hence can be green material. One such material for the treatment of wastewater, dye-infused waters, and industrial water has been tailored, which is capable of dye degradation, heavy metal, and other pollutant removal. Very importantly, the synthesized material is a biocompatible one.

Band alignment and charge transport study of Au nanoparticles decorated over MoS2/MoSe2 hybrid heterostructure for plasmon enhanced photodetection

This paper demonstrates the fabrication of MoS2/MoSe2 hybrid heterostructure and decorating it with Au nanoparticles for enhanced photodetection. The measured responsivity values of the fabricated devices at 505 nm with and without Au nanoparticles were found to be 2.54 A/W and 0.262 A/W suggesting a clear indication of the enhanced photodetection with Au NPs. Detailed band alignment studies of the hybrid heterostructure are performed using Ultra-Violet Photoelectron Spectroscopy (UPS) which also explains the real-time charge transport phenomenon of the fabricated device. Further, simulations and numerical analysis for Au decorated MoS2/MoSe2 are performed to understand the electrical field distribution across Au NPs and the enhancement of absorption due to Mie Scattering. Successful demonstration of plasmon-enhanced photodetection of MoS2/MoSe2 will be essential in the field of 2D plasmonics with potential applications in optical detection in the harsh environment, photovoltaic solar energy conversion, chemical sensing applications, etc.

Modeling of thickness-dependent energy level alignment at organic and inorganic semiconductor interfaces

We identify a universality in the Fermi level change of Van der Waals interacting semiconductor interfaces. We show that the disappearing of quasi-Fermi level pinning at a certain thickness of semiconductor films for both intrinsic (undoped) and extrinsic (doped) semiconductors over a wide range of bulk systems including inorganic, organic, and even organic–inorganic hybridized semiconductors. The Fermi level (EF) position located in the energy bandgap was dominated by not only the substrate work function (Φsub) but also the thickness of semiconductor films, in which the final EF shall be located at the position reflecting the thermal equilibrium of semiconductors themselves. Such universalities originate from the charge transfer between the substrate and semiconductor films after solving one-dimensional Poisson's equation. Our calculation resolves some of the conflicting results from experimental results determined by using ultraviolet photoelectron spectroscopy (UPS) and unifies the general rule on extracting EF positions in energy bandgaps from (i) inorganic semiconductors to organic semiconductors and (ii) intrinsic (undoped) to extrinsic (doped) semiconductors. Our findings shall provide a simple analytical scaling for obtaining the “quantitative energy diagram” in the real devices, thus paving the way for a fundamental understanding of interface physics and designing functional devices.

The role of externally-modulated electrostatic interactions in amplifying charge transport across lysine-doped peptide junctions

Many evolved biomolecular functions such as ion pumping or redox catalysis rely on controlled charge transport through the polypeptide matrix, which can be regulated by shifts in molecular protonation states and dependent supramolecular packing modes in response to environmental cues. However, the exact roles of such dynamic, non-covalent interactions in peptide charge transport have remained elusive. To tackle this challenge, here we report the modulation of charge transport in a series of lysine (Lys)-substituted hepta-glycine (Gly) peptide self-assembled monolayers (SAMs) on template-striped gold (AuTS) bottom electrodes with eutectic gallium-indium (EGaIn) liquid metal top electrodes. We demonstrate systematic modulation of hydrogen bonding and more general electrostatic interactions by shifting the position of the charged Lys-residue and creating different protonation patterns by changing the environmental pH in the AuTS/peptide//GaOx/EGaIn junctions. The effective modulation is evidenced by current density–voltage (J-V) measurements combined with SAM characterization using ultraviolet photoelectron spectroscopy (UPS) and angle-resolved X-ray photoelectron spectroscopy (ARXPS), polarization modulation–infrared reflection-absorption spectroscopy (PM-IRRAS), and molecular dynamics (MD) simulations. Decreasing the hydrogen bonding inside the peptide SAMs and increasing the electrostatic interactions by environmental counterions amplifies the charge transport differently with Lys-position, which means that the sensitive electrical response of peptide SAMs can be tuned by the peptide sequence. Our results provide insights into the relationship between molecular design and in situ modulation of charge transport properties for the development of bionanoelectronics.

Unraveling Ce3+ detection at the surface of ceria nanopowders by UPS analysis.

A sequential analysis using Ultra-violet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS) on ceria nanopowders has been implemented to identify the influence of the X-ray beam on the surface of this oxide. For the first time, UPS analysis evidenced the photoreductive effect of XPS analysis on ceria after an oxidative in situ pretreatment, leading to an overestimation of the Ce3+/Ce4+ ratio obtained by XPS. Based on this spectroscopy methodology, UPS imposes itself as a leading technique for analyzing powders with minimal impact on the authentic chemical state, thus paving the way for identifying the real ratio of Ce4+ and Ce3+ of ceria after oxidative and reductive in situ treatments.

Influence of carbonaceous species entered during arc plasma synthesis on the stoichiometry of LaB6

Electron emission properties of lanthanum hexaboride are dependent on the crystal structure as well as stoichiometric ratio of boron and lanthanum. Present research deals with the influence of carbon diffusion on the stoichiometric LaB6 synthesized using arc plasma gas phase condensation process.Nanocrystalline LaB6 has been synthesized using arc plasma at different plasma powers viz 2–3.4 kW. Further, an inclusion of carbonaceous species from the graphite anode with increasing plasma power and its correlation with stoichiometry has been elucidated. X-ray photo-electron spectroscopic technique has been used for the identification of surface chemistry of the synthesized powders and more effectively to infer the stoichiometry i. e. ratio of B:La in the samples synthesized with increasing plasma in put power. The influence of plasma parameters on the work function of the synthesized nanoparticles has also been elucidated using Ultraviolet Photoelectron Spectroscopy (UPS).

Energy band diagram for SiO2/Si system as evaluated from UPS analysis under vacuum ultraviolet with variable incident photon energy

We have studied the energy band diagram for the Si surface and SiO2/Si system by using ultraviolet photoelectron spectroscopy (UPS) measurements. In the UPS measurements, monochromatized vacuum ultraviolet with variable incident photon energies below 10.50 eV was used in order to increase the detection limit of the depth from the surface and to understand the electronic states not only at the surface but also in the region near the interface of the stacked structure. From the incident photon energy dependence of the UPS spectral width, the energy level of the valence band top of the H-terminated Si surface and the electrical potential change in the SiO2/Si structure has been evaluated. Also, the vacuum work function value of the hetero-epitaxial Ag(111) surface has been investigated to check this measurement technique.

Spectro-microscopy characterization of CoPc-Au(111) interface

Abstract Co-phthalocyanine molecules have wide range of optoelectronics and sensor applications. Motivated by the importance of this material, we studied the adsorption of CoPc thin to thick films on Au(111) surface by using ultraviolet photoelectron spectroscopy (UPS) coupled with angle resolved UPS (AR-UPS) and scanning tunnelling microscopy (STM). Molecular spectral emissions are found to be at 5.13, 7.94, and 10.32 eV below the Fermi level. Observed UPS and AR-UPS measurements show both changes and shifting in valence band features. The work functions of clean Au(111) surface and thick CoPc film are found circa 5.32 eV and 5.25 eV, respectively. Both the work function variation and shifting of molecular features dictate strong adsorbate-substrate interaction. AR-UPS measurements further illustrates that CoPc molecules lie flat on the Au(111) surface. Our STM results disclose that at sub-monolayer (Sub-ML, 0.4–0.8 ML) coverage and room temperature, CoPc molecules tend to adsorb in random fashions; which reveal a strong interaction at the CoPc-Au(111) interface. However, at the ML coverage and elevated temperature (~520K), long range pattern with the oblique structure is observed. Beyond ML coverage, existence of mixed phases consisting of well arranged patterns and molecular clusters have been observed. Overall, our STM results show that depending on the molecular thickness, CoPc molecules form three types of organized oblique, rectangular and square geometries which is consistent with the findings of CuPc and CoPc on Au(111) surface. Our findings may seek potential use in the organic molecular based electronic devices.

Band structure, work function and interfacial diagrams of oxygen-functionalized carbon nano-onions

Carbon nano-onions (CNOs) are an emerging class of carbon nanomaterials with a wide range of applications. Few is known about their electronic structure and the factors that affect it. Here, we study for the first time the effect of two oxidation treatments on the electronic structure of CNOs using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). CNOs were oxidized by physical (Ar/O2 radio frequency plasma) and chemical (HNO3/H2SO4) methods. It was found that the physical oxidative treatment introduced preferentially hydroxyl and carbonyl groups on the outer shells of CNOs and changed the electronic state from metallic to semiconductor with a work function of 4.3 eV and a valence band located 0.2 eV below the Fermi level. In contrast, the chemical oxidation produced CNOs with hydroxyl and carboxylic acid moieties and increased the work function from 4.3 to 4.8 eV with preservation of the metallic structure of CNO. The variations on the electronic structure were explained in terms of the elimination/generation of structural defects on the outer shells of CNO and the electronic effects that resulted from the introduction of different oxygen-containing functional groups on the graphitic layers. The interfacial energy diagrams for pristine and oxidized CNOs were constructed from the UPS data. These diagrams can be used to explain electronic interfacial properties of these materials and the design of novel electrochemical or photovoltaic devices.

Evaluating the Role of Molecular Heredity in the Optical and Electronic Properties of Cross-Conjugated Benzo[1,2-d:4,5-d']bisoxazoles.

A series of eight benzo[1,2-d:4,5-d′]bisoxazole (BBOs) were synthesized using the heredity principle as a design motif, whereby we investigated which characteristics of the linear parents were inherited by their cross-conjugated children. Four linear parents bearing 4-tert-butylbenzene (P) or 1,3-bis(4-tert-butylphenyl)benzene (M) at either the 2,6- or 4,8-position on the BBO and four cross-conjugated children bearing various combinations of the two isoelectronic aryl substituents were evaluated. Due to the bulky nature of the M substituent compared to that of the P substituent, the influence of steric hindrance along the BBO axes was explored theoretically and experimentally. The optical and electronic properties of each molecule were investigated in the solution and solid state using density functional theory (DFT) and time-dependent DFT (TD-DFT) and characterized using ultraviolet photoelectron spectroscopy (UPS), ultraviolet–visible (UV–vis) spectroscopy, and photoluminescence (PL) spectroscopy. The well-correlated theoretical and experimental results showed that the selective tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels was possible through the strategic placement of substituents without impacting the H → L transition energy. Specifically, the theoretical results demonstrated that for the BBO children the HOMO and LUMO energy levels were inherited from the 4,8- and 2,6-parents, respectively. Each molecule was found to exhibit emission maxima ≤451 nm, making them ideal candidates for blue organic light-emitting diode (OLED) materials.

Nitrogen Incorporated Photoactive Brownmillerite Ca2Fe2O5 for Energy and Environmental Applications

Ca2Fe2O5 (CFO) is a potentially viable material for alternate energy applications. Incorporation of nitrogen in Ca2Fe2O5 (CFO-N) lattice modifies the optical and electronic properties to its advantage. Here, the electronic band structures of CFO and CFO-N were probed using Ultraviolet photoelectron spectroscopy (UPS) and UV-Visible spectroscopy. The optical bandgap of CFO reduces from 2.21 eV to 2.07 eV on post N incorporation along with a clear shift in the valence band of CFO indicating the occupation of N 2p levels over O 2p in the valence band. Similar effect is also observed in the bandgap of CFO, which is tailored upto 1.43 eV by N+ ion implantation. The theoretical bandgaps of CFO and CFO-N were also determined by using the Density functional theory (DFT) calculations. The photoactivity of these CFO and CFO-N was explored by organic effluent degradation under sunlight. The feasibility of utilizing CFO and CFO-N samples for energy storage applications were also investigated through specific capacitance measurements. The specific capacitance of CFO is found to increase to 224.67 Fg−1 upon N incorporation. CFO-N is thus found to exhibit superior optical, catalytic as well as supercapacitor properties over CFO expanding the scope of brownmillerites in energy and environmental applications.

The electronic structure of ionic liquids based on the TFSI anion: A gas phase UPS and DFT study

The valence bands of [EMIM][TFSI], [DEME][TFSI] and [PYR1,4][TFSI] gas-phase ion pairs have been investigated using ultraviolet photoelectron spectroscopy (UPS). The photoelectron spectra are interpreted by using several density functional and ab initio calculation methods. Although the experimental vapor phase spectra are similar, the different calculation methods make different predictions about the HOMO molecular state of the ion-pairs of the ionic liquids. The HOMO state of the [DEME][TFSI] ion-pair is due to the TFSI anion, while in [EMIM][TFSI] it is due to the EMIM cation. However, it is difficult to make conclusive assignments for the [PYR1,4][TFSI] ionic liquid. All calculation methods predict the LUMO to be of cationic origin in all the studied ion-pairs.

Effects of gas cluster ion beam sputtering on the molecular orientation of organic semiconductor films: Ultraviolet photoelectron spectroscopy study of [6]phenacene

The alignment of organic layer and metal layer energy levels within organic semiconductor devices is critical for efficient charge injection/collection at the electrodes. The electronic structure at the interface is usually examined using ultraviolet photoelectron spectroscopy (UPS) as the film thickness of the organic layer is built up incrementally (bottom-up method). As such, the morphology and crystallinity of the films that are examined using this technique may be different from the films used in actual devices because these properties are sensitive to the way in which the films are prepared. We have prepared [6]phenacene thin films with standing and lying molecular orientations on naturally oxidized silicon (SiO2) and highly oriented pyrolytic graphite (HOPG) surfaces, respectively. UPS measurements were performed on films that were incrementally decreased in thickness using gas cluster ion beam (GCIB) sputtering (top-down method). The spectral shapes and ionization energies of the films that were formed using the bottom-up and top-down methods were similar. Importantly, the characteristic features of a monolayer on the HOPG substrate were also observed when a thick film (i.e., not built up in layers using the bottom-up method) was thinned down by GCIB sputtering. We have shown that UPS combined with GCIB sputtering is a useful technique for examining the energy level alignment of interfaces within films that are fabricated using conditions similar to those used for real devices.

Investigation of sol-gel and nanoparticle-based NiOx hole transporting layer for high-performance planar perovskite solar cells

We conduct a comprehensive study on and comparison of sol-gel and nanoparticles (NPs)-based nickel oxide hole-transporting layer (HTL) for high-performance planar perovskite solar cells (PSCs). The characteristics and film properties of sol-gel and NPs were systemically investigated using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and photoluminescence (PL), and its effect on device-performance was also examined using J-V characteristics, quantum-efficiency, and the VOC dependence of the light intensity. Through this comparison of two types of HTL and their device-performances, these studies can provide sufficient and robust information for nickel oxide-based PSCs, and furthermore, the overall results and discussions can be useful for high-performance PSCs.

Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open‐Circuit Voltage of Organic Solar Cells

AbstractFor 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C61‐butyl acid methyl ester or [6,6]‐phenyl‐C71‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

Electronic band structure and carrier concentration of formamidinium–cesium mixed cation lead mixed halide hybrid perovskites

The electronic structure of hybrid perovskite compositions of FA0.83 Cs0.17 PbI3−xBrx (x = 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5) is determined using ultraviolet photoelectron spectroscopy (UPS) and UV–Vis–NIR absorption spectroscopy. With the help of UPS, ionization potential and Fermi energy are determined, and using absorption measurements, bandgap values are obtained. It is observed that for FA0.83 Cs0.17 PbI3−xBrx, as the Br content increases, the bandgap increases. The UPS measurements confirm the n-type nature of all compositions. Additionally, the Hall measurements were carried out for the selected compositions and the n-type carrier concentrations were determined.

Understanding the Impact of Film Disorder and Local Surface Potential in Ultraviolet Photoelectron Spectroscopy of PEDOT.

The spectra of conducting polymers obtained using ultraviolet photoelectron spectroscopy (UPS) exhibit a typical broadening of the tail σUPS ≈ 1 eV, which by an order of magnitude exceeds a commonly accepted value of the broadening of the tail of the density of states σDOS ≈ 0.1 eV obtained using transport measurements. In this work, an origin of this anomalous broadening of the tail of the UPS spectra in a doped conducting polymer, PEDOT (poly(3,4-ethylenedioxythiophene)), is discussed. Based on the semiempirical approach and using a realistic morphological model, the density of valence states in PEDOT doped with molecular counterions is computed. It is shown that due to a disordered character of the material with randomly distributed counterions, the localized charge carriers in PEDOT crystallites experience spatially varying electrostatic potential. This leads to spatially varying local vacuum levels and binding energies. Taking this variation into account the UPS spectrum is obtained with the broadening of the tail comparable to the experimentally observed one. The results imply that the observed broadening of the tail of the UPS spectra in PEDOT provides information about a disordered spatially varying potential in the material rather than the broadening of the DOS itself.