Photoelectron Spectroscopy In Air Research Articles

An Alternating Donor–Acceptor Conjugated Polymer Based on Benzodithiophene and [3,4-c]pyrrole-4,6-dione: Synthesis, Characterization, and Application in Photovoltaic Devices

The synthesis is described of a new alternating donor–acceptor semiconducting polymer based on an N-octylthieno[3,4-c]pyrrole-4,6-dione building block together with a newly designed 2,3-bis(2-ethylhexyl)thiophenylethynyl substituted benzodithiophene (BDT). The introduction of electron-rich thiophene units to BDT raises the highest occupied molecular orbital (HOMO) level of the conjugated polymer and the concomitant reduction of the bandgap enhances the harvesting of solar radiation. This modification also introduces less sterically demanding triple bonds, thereby potentially enabling more favourable molecular interactions and an extra dimension of conjugation perpendicular to the main polymer chain. The optoelectronic properties of this new conjugated polymer were evaluated using UV-visible absorption and fluorescence spectroscopy, photoelectron spectroscopy in air, photo-induced charge extraction by linearly increasing voltage (Photo-CELIV), and density functional theory calculations. The polymer absorbs broadly in the wavelength range 300–700 nm in solution and the solid state. The estimated HOMO and LUMO levels of −5.4 and −3.6 eV, respectively, correspond to a bandgap of 1.8 eV. Photovoltaic devices fabricated using the polymer as the active layer displayed power conversion efficiencies (PCEs) of up to 1 %. Photo-CELIV results provide evidence that rapid recombination and poor charge mobility are likely contributing factors to the relatively low PCE values observed.

Widely Controllable Electronic Energy Structure of ZnSe–AgInSe2 Solid Solution Nanocrystals for Quantum-Dot-Sensitized Solar Cells

I–III–VI2-semiconductor-based nanocrystals of ZnSe–AgInSe2 solid solution ((AgIn)xZn2(1-x)Se2, ZAISe) with average sizes of 3.5–6.2 nm were successfully synthesized through thermal reaction of corresponding metal acetates and selenourea in a hot oleylamine solution. The optical property of ZAISe solid solution nanocrystals was tunable in a broad wavelength region from visible to near-infrared light by changing the composition of solid solution, where the energy gap of ZAISe nanocrystals was enlarged from 1.44 to 3.00 eV with an increase in the fraction of ZnSe in ZAISe, that is, with a decrease in x from 1.0 to 0. Both levels of conduction band and valence band edges, determined by photoelectron spectroscopy in air, were monotonously shifted to higher levels with an increase in the fraction of ZnSe. Quantum-dot-sensitized solar cells were fabricated with porous TiO2 film electrodes immobilized with ZAISe nanocrystals using 3-mercaptopropionic acid as a cross-linking agent. The light conversion efficiency ...

Cu2ZnGeS4 Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys

The synthesis of an air and moisture stable germanium complex and its use in the synthesis of ternary and quaternary copper containing nanocrystals (NCs) is described. Through the use of 1H-/13C nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermogravimetric analysis, and powder X-ray diffraction, the speciation and chemistry of this precursor is elucidated. This germanium source is employed in the gram scale, noninjection synthesis of Cu2ZnGeS4 (CZGeS) and Cu2GeS3 (CGeS) NCs using a binary sulfide precursor approach. To demonstrate the versatility of such NCs for fabricating thin films suitable for high-efficiency optoelectronic devices, they are blended with Cu2ZnSnS4 (CZTS) NCs and selenized to form homogeneously alloyed Cu2ZnSnxGe1–xSySe4–y (CZTGeSSe) thin films. The structural, optical, and electronic properties of such thin films are studied using X-ray diffraction, scanning electron microscopy, UV−vis−NIR spectroscopy, and photoelectron spectroscopy in air. These measurements demonstrate collectively that incorporating Ge into micrometer-sized, tetragonal Cu2ZnSnSxSe4–x (CZTSSe) provides a facile manner in which the conduction band energy can be readily tuned. The strategy developed herein provides a pathway to controlled levels of Ge incorporation in a single step process, thus avoiding the need for intra-alloyed Cu2ZnSnxGe1–xS4 nanocrystals.

Evaluation of Electronic States of Polymer Surface Using Photoelectron Spectroscopy in Air

SUMMARYWe investigated shallow surface states relevant to electrification between polymer film and metal ring by photoelectron spectroscopy in air. It was found that the charge transferred from the metal ring to the polyethylene terephthalate (PET) film depended on the work function of the metal ring and the distribution of the density of states in the PET film. It was also found that electrification between the polymer and an aluminum ring was related to the chemical structure, i.e., pendant groups in the polymer.

A Study of Diphenylfumaronitrile and Furan‐Substituted Diketopyrrolopyrrole Alternating Copolymer and Its Thin‐Film Transistors

A fused aromatic furan‐substituted diketopyrrolopyrrole and novel diphenylfumaronitrile conjugated building blocks are used for the synthesis of an alternating copolymer (DPFN‐DPPF) via Suzuki polycondensation. In this paper, the first attempt to use the diphenylfumaro­nitrile building block for the synthesis of conjugated polymer is described. The number‐average and weight‐average ­molecular weights calculated for DPFN‐DPPF are 20 661 and 66 346 g mol−1, respectively. The optical bandgap calculated for DPFN‐DPPF is 1.53 eV whereas the highest occupied molecular orbital (HOMO) value calculated by photoelectron spectroscopy in air (PESA) is 5.50 eV. The calculated HOMO value is lower, which is suitable for stable organic electronic devices. DPFN‐DPPF polymer is used as an active layer in bottom‐contact bottom‐gate organic thin‐film transistor devices and the thin film exhibits a hole mobility of 0.20 cm2 V−1 s−1 in air. image

Synthesis of diketopyrrolopyrrole based copolymers via the direct arylation method for p-channel and ambipolar OFETs

In this paper, we have synthesized two novel diketopyrrolopyrrole (DPP) based donor–acceptor (D–A) copolymers poly{3,6-dithiophene-2-yl-2,5-di(2-octyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-1,5-bis(dodecyloxy)naphthalene} (PDPPT-NAP) and poly{3,6-dithiophene-2-yl-2,5-di(2-butyldecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2-dodecyl-2H-benzo[d][1,2,3]triazole} (PDPPT-BTRZ) via direct arylation organometallic coupling. Both copolymers contain a common electron withdrawing DPP building block which is combined with electron donating alkoxy naphthalene and electron withdrawing alkyl-triazole comonomers. The number average molecular weight (M(n)) determined by gel permeation chromatography (GPC) for polymer PDPPT-NAP is around 23400 g mol(−1) whereas for polymer PDPPT-BTRZ it is 18600 g mol(−1). The solid state absorption spectra of these copolymers show a wide range of absorption from 400 nm to 1000 nm with optical band gaps calculated from absorption cut off values in the range of 1.45–1.30 eV. The HOMO values determined for PDPPT-NAP and PDPPT-BTRZ copolymers from photoelectron spectroscopy in air (PESA) data are 5.15 eV and 5.25 eV respectively. These polymers exhibit promising p-channel and ambipolar behaviour when used as an active layer in organic thin-film transistor (OTFT) devices. The highest hole mobility measured for polymer PDPPT-NAP is around 0.0046 cm(2) V(−1) s(−1) whereas the best ambipolar performance was calculated for PDPPT-BTRZ with a hole and electron mobility of 0.01 cm(2) V(−1) s(−1) and 0.006 cm(2) V(−1) s(−1) .

Intraphase Microstructure–Understanding the Impact on Organic Solar Cell Performance

AbstractA comprehensive study of the effect of intraphase microstructure on organic photovoltaic (OPV) device performance is undertaken. Utilizing a bilayer device architecture, a small molecule donor (TIPS‐DBC) is deposited by both spin‐coating and by thermal evaporation in vacuum. The devices are then completed by thermal evaporation of C60, an exciton blocking layer and the cathode. This bilayer approach enables a direct comparison of device performance for donor layers in which the same material exhibits subtle differences in microstructure. The electrical performance is shown to differ considerably for the two devices. The bulk and interfacial properties of the donor layers are compared by examination with photoelectron spectroscopy in air (PESA), optical absorption spectroscopy, charge extraction of photo‐generated charge carriers by linearly increasing voltage (photo‐CELIV), time‐resolved photoluminescence measurements, X‐ray reflectometry (XR), and analysis of dark current behavior. The observed differences in device performance are shown to be influenced by changes to energy levels and charge transport properties resulting from differences in the microstructure of the donor layers. Importantly, this work demonstrates that in addition to the donor/acceptor microstructure, the intraphase microstructure can influence critical parameters and can therefore have a significant impact on OPV performance.

Isoindigo dye incorporated copolymers with naphthalene and anthracene: promising materials for stable organic field effect transistors

In this paper, we report the design and synthesis of isoindigo based low band gap polymer semiconductors, poly{N,N′-(2-octyldodecyl)-isoindigo-alt-naphthalene} (PISD-NAP) and poly{N,N′-(2-octyldodecyl)-isoindigo-alt-anthracene} (PISD-ANT). A series of donor–acceptor (D–A) copolymers can be prepared where donor and acceptor conjugated blocks can be attached alternately using organometallic coupling. In these polymers, an isoindigo dye acceptor moiety has been attached alternately with naphthalene and anthracene donor comonomer blocks by Suzuki coupling. PISD-NAP and PISD-ANT exhibit excellent solution processibility and good film-forming properties. Gel permeation chromatography exhibits a higher molecular mass with lower polydispersity. UV-vis-NIR absorption of these polymers exhibits a wide absorption band ranging from 300 nm to 800 nm, indicating the low band gap nature of the polymers. Optical band gaps calculated from the solid state absorption cutoff value for PISD-NAP and PISD-ANT are around 1.80 eV and 1.75 eV, respectively. Highest occupied molecular orbital (HOMO) values calculated respectively for PISD-NAP and PISD-ANT thin films on glass substrate by photoelectron spectroscopy in air (PESA) are 5.66 eV and 5.53 eV, indicative of the good stability of these materials in organic electronic device applications. These polymers exhibit p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices in ambient conditions. The highest hole mobility of 0.013 cm2 V−1 s−1 is achieved in top contact and bottom-gate OTFT devices for PISD-ANT, whereas polymer PISD-NAP exhibited a hole mobility of 0.004 cm2 V−1 s−1. When these polymer semiconductors were used as a donor and PC71BM as an acceptor in OPV devices, the highest power conversion efficiency (PCE) of 1.13% is obtained for the PISD-ANT polymer.

Sub-Microscopic Phenomena of Metallic Corrosion Studied by a Combined Photoelectron Spectroscopy in Air (PESA) and Scanning Kelvin Probe Force Microscopy (SKPFM) Approach

Sub-Microscopic Phenomena of Metallic Corrosion Studied by a Combined Photoelectron Spectroscopy in Air (PESA) and Scanning Kelvin Probe Force Microscopy (SKPFM) Approach

A Systematic Approach to the Design Optimization of Light‐Absorbing Indenofluorene Polymers for Organic Photovoltaics

AbstractThe synthesis and optimization of new photovoltaic donor polymers is a time‐consuming process. Computer‐based molecular simulations can narrow the scope of materials choice to the most promising ones, by identifying materials with desirable energy levels and absorption energies. In this paper, such a retrospective analysis is presented of a series of fused aromatic push–pull copolymers. It is demonstrated that molecular calculations do indeed provide good estimates of the absorption energies measured by UV–vis spectroscopy and of the ionization potentials measured by photoelectron spectroscopy in air. Comparing measured photovoltaic performance of the polymer series to the trend in efficiencies predicted by computation confirms the validity of this approach.

Furan substituted diketopyrrolopyrrole and thienylenevinylene based low band gap copolymer for high mobility organic thin film transistors

A novel solution processable donor–acceptor (D–A) based low band gap polymer semiconductor poly{3,6-difuran-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene} (PDPPF-TVT), was designed and synthesized by a Pd-catalyzed Stille coupling route. An electron deficient furan based diketopyrrolopyrrole (DPP) block and electron rich thienylenevinylene (TVT) donor moiety were attached alternately in the polymer backbone. The polymer exhibited good solubility, film forming ability and thermal stability. The polymer exhibits wide absorption bands from 400 nm to 950 nm (UV-vis-NIR region) with absorption maximum centered at 782 nm in thin film. The optical band gap (Eoptg) calculated from the polymer film absorption onset is around 1.37 eV. The π-energy band level (ionization potential) calculated by photoelectron spectroscopy in air (PESA) for PDPPF-TVT is around 5.22 eV. AFM and TEM analyses of the polymer reveal nodular terrace morphology with optimized crystallinity after 200 °C thermal annealing. This polymer exhibits p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices. The highest hole mobility of 0.13 cm2 V−1 s−1 is achieved in bottom gate and top-contact OTFT devices with on/off ratios in the range of 106–107. This work reveals that the replacement of thiophene by furan in DPP copolymers exhibits such a high mobility, which makes DPP furan a promising block for making a wide range of promising polymer semiconductors for broad applications in organic electronics.

Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment

Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. In contrast to established methods that are used in solution or vacuum environments, here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply this new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enables us to answer the outstanding question: why previous attempts to make OPV devices using 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor were not successful.

Electronic Properties and Supramolecular Organization of Terminal Bis(alkylethynyl)-Substituted Benzodithiophenes

Benzodithiophene (BDT) was symmetrically bisubstituted in the terminal positions with five different alkynes C≡C-(C(n)H(2n+1)) with n = 4, 6, 8, 10, 12. The materials were characterized as potential materials for field-effect transistor applications. Electrochemical measurements in solution and photophysical measurements in solution and in the solid state, together with UV photoelectron spectroscopy in air and quantum-chemical calculations, elucidate the nature of the frontier orbitals and of the excited states as well as their deactivation pathways. Structural information on the molecular assembly in the solid state, both at room temperature and at elevated temperatures, is obtained by a combination of DSC, polarized optical microscopy, and 2D-WAXS, which point to the crystallinity of the compounds in all phases and reveal π-stacking arrangements independently of the length of the alkyl side chains.

Mechanism of Perfluoropolyether Chains Bonding on Carbon Overcoat Surface by Ultraviolet Irradiation

To elucidate the bonding mechanism of a non-polar perfluoropolyether lubricant (PFPE lube) under ultraviolet (UV) irradiation, the UV wavelength ranges in which photoelectrons emanate were identified using photoelectron spectroscopy in air (PESA), and the bonding behavior of the lube at various UV wavelengths was studied. The results showed that photoelectrons emanated from magnetic disk substrates at wavelengths of 240 nm or less and the lube was bonded to the substrate in the wavelength range where photoelectrons were emanated. For comparison, Si, SiO2/Si, and Si3N4/Si were also used as substrate; the Si substrate which emanated the highest photoelectron intensity showed the highest bonded ratio of the lube. The lube, however, also bonded to the SiO2/Si and Si3N4/Si substrates under UV irradiation at 222 nm, where photoelectrons do not emanate from the substrates. These observations suggest that the bonding mechanism between the PFPE chains and the carbon overcoat (COC) surfaces under UV irradiation was affected by factors other than photoelectrons.

In-situ Measurement of Photoelectron Spectroscopy in Air of Polypyrrole during Electrochemical Undoping

In-situ measurement of photoelectron spectra of polypyrrole during electrochemical undoping/doping cycles has been carried out by using an open-type electrochemical cell. It has been observed that the ionization potential decreases with decreased electrochemical potential. This result seems to be reasonable because the decreased electrochemical potential corresponds to the undoping or recovery of electrons into vacant state of valence band.

Effect of Microstructure and Crystalline Orientation of Pt Single- or Pt/Ru Bilayer-Electrodes on the Work Function and Leakage Current of SrTiO3 Capacitors

SrTiO3 dielectric thin film capacitors using Pt single layer or Pt/Ru bilayer metal electrodes were prepared on various substrates by pulsed laser ablation. The work function and the surface roughness were evaluated for Pt metal electrodes. In addition, leakage currents and Schottky barrier heights were also characterized for the capacitors. The leakage current was suppressed and the Schottky barrier height enhanced by decreasing the surface roughness and increasing the work function of the metal electrodes. The work function of metal electrodes measured by photo-electron spectroscopy in air was increased by approximately 0.16 eV by decreasing surface roughness. Similar to the work function, the Schottky barrier heights were increased by approximately 0.20 eV. These results allow us to control leakage current behavior of thin film capacitors by tuning of the electrode microstructures.

Transparent conductive thin films based on chemically assembled single-walled carbon nanotubes

Transparent conductive thin films were fabricated by the chemical assembly of carboxylated single-walled carbon nanotubes (ca-SWNTs) onto aminosilane-modified glass plates. The ca-SWNTs were deposited with multilayer structures by amidation reaction between the carboxylic acid groups of ca-SWNTs and amino groups of linker molecules with the aid of coupling agents. Unlike the chemically assembled SWNT layers reported thus far, these SWNT assemblies have high-density structures and excellent surface adhesion due to direct chemical bonding to their substrates, which results in high electrical conductivity. The covalent bonds of the SWNT thin film were characterized by X-ray photoelectron spectroscopy, and the structures of SWNTs in the conductive thin film were analyzed by Raman spectroscopy. Imaging by scanning electron microscopy shows that the SWNT thin film has a uniform morphology with high-density multilayer structure over large areas, and UV–vis spectroscopy analysis shows that the transmittance of SWNT thin film in visible light region is excellent and further improved to greater than 90% upon thermal treatment at 250 °C. The electrical resistivities of the ca-SWNT thin film were found to be 5–10 Ω cm, which was improved to 0.2–0.4 Ω cm by the thermal treatment. The work function measured by photoelectron spectroscopy in air was found to be 5.12 eV.

Electronic Structures of Dyes and Phthalocyanines Estimated with “Photo-Electron Spectroscopy in Air (PESA)”

AbstractThe near-edge electronic structures of several dyes and phtalocyanines were investigated using Photo-Electron Spectroscopy in Air (PESA). The ionization potentials (IPs) of N3, N719, Z907, D102, D149 and D205 were derived from observed photoemission threshold energies to be 5.88, 5.47, 5.44, 5.53, 5.44 and 5.49eV, respectively. Density of states (DOS) of Fe-, Ni-, Cu- and H2-phthalocyanines were deduced from differentiation of observed photoemission yields with incident photon energies. DOS thus obtained were well compared with calculated ones estimated with use of the DV-Xα molecular orbital calculation method.

Cesium-incorporated indium-tin-oxide films for use as a cathode with low work function for a transparent organic light-emitting device

Transparent organic light-emitting devices (TOLEDs) were successfully fabricated utilizing a novel transparent conducting cathode with low work function. Cesium-incorporated indium-tin-oxide film was deposited on the organic layers with negligible damage by simultaneous operation of RF magnetron sputtering using an ITO target and vacuum evaporation of metallic cesium. Incorporation of cesium in the ITO film was confirmed by XPS analysis. The work function (4.3 eV) determined by photoelectron spectroscopy in air (PESA) was lower than that of 0.3–0.4-eV without cesium-incorporation and stable under the atmospheric environment. The electron injection efficiency of cesium-incorporated ITO cathode in the present transparent OLED fabricated was comparable to that of the previous double-layered structure comprising of ITO cathode and an organic buffer layer (BCP) doped by evaporation of cesium [T. Uchida, S. Kaneta, M. Ichihara, M. Ohtsuka, T. Otomo, D.R. Marx, Jpn. J. Appl. Phys., 44, No. 9 (2005) L282].