Decrease In Electron Affinity Research Articles

Record-High-Performance Hydrogenated In-Ga-Zn-O Flexible Schottky Diodes.

High-performance In-Ga-Zn-O (IGZO) Schottky diodes (SDs) were fabricated using hydrogenated IGZO (IGZO:H) at a maximum process temperature of 150 °C. IGZO:H was prepared by Ar + O2 + H2 sputtering. IGZO:H SDs on a glass substrate exhibited superior electrical properties with a very high rectification ratio of 3.8 × 1010, an extremely large Schottky barrier height of 1.17 eV, and a low ideality factor of 1.07. It was confirmed that the hydrogen incorporated during IGZO:H deposition increased the band gap energy from 3.02 eV (IGZO) to 3.29 eV (IGZO:H). Thus, it was considered that the increase in band gap energy (decrease in electron affinity) of IGZO:H contributed to the increase in the Schottky barrier height of the SDs. Angle-resolved hard X-ray photoelectron spectroscopy analysis revealed that oxygen vacancies in IGZO:H were much fewer than those in IGZO, especially in the region near the film surface. Moreover, it was found that the density of near-conduction band minimum states in IGZO:H was lower than that in IGZO. Therefore, IGZO:H played a key role in improving the Schottky interface quality, namely, the increase of Schottky barrier height, decrease of oxygen vacancies, and reduction of near-conduction band minimum states. Finally, we fabricated a flexible IGZO:H SD on a poly(ethylene naphthalate) substrate, and it exhibited record electrical properties with a rectification ratio of 1.7 × 109, Schottky barrier height of 1.12 eV, and ideality factor of 1.10. To the best of our knowledge, both the IGZO:H SDs formed on glass and poly(ethylene naphthalate) substrates achieved the best performance among the IGZO SDs reported to date. The proposed method successfully demonstrated great potential for future flexible electronic applications.

Monocyclopropanated fullerene derivatives with decreased electron affinity as promising electron acceptor materials for organic solar cells

Monocyclopropanated fullerene derivatives with decreased electron affinity as compared to standard [60]PCBM acceptor were synthesized, characterized, and investigated as promising electron acceptor materials for fullerene-based organic solar cells. This decrease in electron affinity was attributed to the presence of through-space electronic interactions between the lone electron pairs of oxygen atoms and electron-deficient π-system of the fullerene cage. Organic bulk heterojunction solar cells based on the designed materials blended with conjugated polymers P3HT and PCDTBT have demonstrated spectacular increase in open-circuit voltage by >100 mV and improved light power conversion efficiency as compared to the reference devices comprising [60]PCBM. The application of new fullerene-based acceptor materials in combination with the promising low band gap conjugated copolymer may deliver competitive device performances.

PROPERTIES OF EXCITED MOLECULES IN PHOTOLYSIS OF 2,6-DIPHENYL-1,4-BENZOQUINONE WITH AMINES

To date, it is considered established that quinones with lower energy state, under the action of light tear away a hydrogen atom from a hydrocarbon or an electron donor from inorganic anion-radicals, which have a high reduction potential. However, even for the simplest quinones (1,4-benzoquinone, 1,4-naphthoquinones, 9,10-anthraquinone and their derivatives) there is no consensus in the science literature about the nature of the initial event in photoreaction with compounds which are potential donors of hydrogen atom and electron. The first step in many photochemical reactions is the formation of complexes between donors and acceptors of electrons in the excited state (exiplexes). Photoreactive quinones as elementary acts include the transfer of electron (or) hydrogen atom. The mechanism depends on the presence and strength of donor-acceptor complexes (DAC) of the quinones with the reagents. Studies of triplet exiplexes allow you to set the details of the elementary reaction acts. Only short-lived intermediate product was registered upon photoexcitation of the studied quinone Q in low-polarity solvents. The kinetic of decay of the first order with rate constant of about 2∙106 s-1 in toluene and dibutylphthalate and the introduction of oxygen leads to a decrease in the lifetime of the product in the triplet state. With the introduction of solutions of amines quenching of triplet state (QT) with rate constant close to diffusion was observed. Rate constants of quenching by dissolving in benzene and by dissolving in dibutyl phthalate were determined. It is established that formation of intermediate products is carried out from triplet state (QT). Excited complexes with charge transfer in acetonitrile were not observed. It is concluded that with decrease in electron affinity of the acceptors, when the connection of the molecules in the complex becomes weaker, the lifetime of TE increases significantly.

Synthesis, Self-Assembly, and Solar Cell Performance of N-Annulated Perylene Diimide Non-Fullerene Acceptors

The synthesis, characterization, and photovoltaic performance of a series of N-annulated PDI materials is presented. Four novel N-annulated PDI compounds are reported, each of which can be synthesized in gram scale without the need for purification using column chromatography. N-Annulation of the PDI chromophore results in a decrease in electron affinity and lowering of the ionization potential, and renders the chromophore insoluble in organic solvents. Installation of an alkyl group improves the solubility. Single crystal X-ray analysis reveals a bowing of the aromatic backbone and compression of phenyl rings adjacent to the N atom. A brominated N-annulated PDI derivate represents a valuable synthon for creating novel multi-PDI chromophore materials. To demonstrate the utility of the new synthon for making electron transporting materials, a dimerization strategy was employed to create a dimeric PDI material. The PDI dimer has excellent solubility and film forming ability along with energetically deep HOM...

Electrical and optical analysis of InxSy:Na thin-films with varied sodium concentration as buffer layer in Cu(In,Ga)(S,Se)2 solar cells

Replacing the toxic CdS buffer material in thin-film solar cells based on Cu(In,Ga)(Se,S)2 (CIGSSe) has been a crucial issue for a long time. A promising alternative buffer material is InxSy, which stands out due to its tunable characteristics, e.g. the band gap or the electron affinity by adding third elements. The specific influence of additional elements has been subject of many publications, however, with inconsistent results e.g. concerning the nature of the band gap or the conductivity of the material. The experimental access to the optical and electrical parameters of InxSy:Na thin-films is a necessity to fully understand the formation of the hetero-junction in the solar cell. For this work, CIGSSe solar cells with a varied sodium concentration and indium to sulfur ratio in the InxSy:Na buffer layer and comparable single InxSy:Na layers on sodium-free glass substrates were investigated. The solar cells were characterized by means of current-voltage measurements (IV), whereas spectroscopic ellipsometry (SE) and conductivity measurements were performed on bare InxSy:Na layers. The IV measurements indicate a formation of a transport barrier with increasing sodium content, which is in agreement with a decrease in electron affinity reported in the literature. The SE analysis shows band gap values of Eg=2.0eV for sodium-free thin-film InxSy:Na and Eg=2.45eV for the layers with the highest investigated sodium content. The absorption coefficient shows a clear indirect nature of the band gap transition. The conductivity under illumination is in the range of σ=1·10−5 (1/Ωcm) for all sodium containing layers and shows a strong decrease in the dark. However, a decrease of indium to sulfur ratios lead to a strong increase in conductivity from 1·10−6 (1/Ωcm) to σ=1·10−2 (1/Ωcm).

The possibly important role played by Ga2O3 during the activation of GaN photocathode

Three different chemical solutions are used to remove the possible contamination on GaN surface, while Ga2O3 is still found at the surface. After thermal annealing at 710 °C in the ultrahigh vacuum (UHV) chamber and activated with Cs/O, all the GaN samples are successfully activated to the effective negative electron affinity (NEA) photocathodes. Among all samples, the GaN sample with the highest content of Ga2O3 after chemical cleaning obtains the highest quantum efficiency. By analyzing the property of Ga2O3, the surface processing results, and electron affinity variations during Cs and Cs/O2 deposition on GaN of other groups, it is suggested that before the adsorption of Cs, Ga2O3 is not completely removed from GaN surface in our samples, which will combine with Cs and lead to a large decrease in electron affinity. Furthermore, the effective NEA is formed for GaN photocathode, along with the surface downward band bending. Based on this assumption, a new dipole model Ga2O3-Cs is suggested, and the experimental effects are explained and discussed.

Effect of Ar+ ion etching treatment on the surface work function of Hg3In2Te6 wafer

Synchrotron radiation photoemission spectroscopy (SRPES) was employed to investigate the changes of surface work function of Hg3In2Te6 wafer under different Ar+ ion etching parameters. Hg concentration on the Hg3In2Te6 surface decreased with increasing the etching time. Meanwhile, the calculated work function and electron affinity of Hg3In2Te6 wafer also decreased with increasing the etching time. It was found that the decrease of work function of the Hg3In2Te6 wafer was mainly caused by the decrease of electron affinity which was closely related to the increase of electron concentration due to the escape of Hg2+ on the surface of Hg3In2Te6 wafer.

Influence of the oxidation level on the electronic, morphological and charge transport properties of novel dithienothiophene S-oxide and S,S-dioxide inner core oligomers

Novel inner-core dithienothiophene S-oxide and S,S-dioxide oligomers were designed and synthesized. The controlled oxidation at the central sulfur of the dithienothiophene core allows the combined tailoring of the HOMO–LUMO energies, thermal properties, and thin film morphology. Electrochemical measurements indicate that a marked decrease in electron affinity is obtained by addition of the first oxygen at the central sulfur, while addition of the second oxygen induces only minor effects on both the oxidation and reduction potentials. The newly synthesized materials exhibit high solubility in common organic solvents together with good film forming properties when deposited on SiO2 by solution-based techniques. Similar hole field effect transistor (FET) mobility for solution-deposited and thermally-evaporated films together with FET performances independent of film morphology and molecular orientation call for non-conventional charge transport mechanisms.

Characterization of boron carbon nitride film modified by excimer laser annealing

Boron carbon nitride (BCN) shows promise as a field emitter material because of its mechanical hardness, chemical inertness, and low electron affinity. This study investigated the modification of a BCN film with an amorphous area using KrF excimer laser (wavelength: 248 nm, photon energy: 5.0 eV) annealing without substrate heating. This achieved significant variation in characteristics, such as an increase in bandgap energy and decrease in electron affinity. Laser annealing reduced electron affinity from 0.7 to 0.3 eV. The results indicate that the modification of the BCN film by KrF excimer laser annealing achieves characteristics similar to hexagonal BN (h-BN) film without losing the desirable properties of the BCN film, such as physical stability.

Basics of oxygen and SnO 2 interaction; work function change and conductivity measurements

The interaction of oxygen and nanocrystalline SnO 2 has been investigated by means of simultaneous work function change and resistance measurements. Various oxygen concentrations between 50 ppm and 6000 ppm have been dosed at two different temperatures. At 400 °C, only ionic species have been identified. However, at 200 °C dipolar species play the major role reflected by a massive decrease in electron affinity.

Diatomics AB (A = Be, Mg; B = O, S) and oligomers thereof: A theoretical study

A quantum chemical study of title systems and some related species (CaO, SrO, BaO and BaPo) was performed by B3LYP, H.F., MP2, TD DFT, and SAC-CI procedures. Structural features of isomers, vibrational frequencies, population analysis, ionization potential and in particular, electronic spectra. Electronic spectra of several monomers exhibit in accordance with experiments, the first electronic transitions in the near IR region (SAC-CI and TD DFT techniques). The passage to oligomers is accompanied by a very significant hypsochromic shift of the longest wavelength bands, increase of ionization potential and decrease of electron affinity.

Functionalizing the GaN(0 0 0 1)-(1×1) surface II. Chemisorption of 3-pyrroline

Chemisorption of 3-pyrroline (C 4H 6NH) on the GaN(0 0 0 1)-(1×1) surface has been studied using mainly X-ray-excited Auger electron, electron energy loss and ultraviolet photoemission spectroscopies (XAES, ELS and UPS, respectively). The XAES data show adsorption near room temperature with a total sticking probability of ∼0.05 and a saturation coverage of ∼0.34 molecules per surface lattice site. The ELS data show removal of the characteristic surface-state band centered at ∼3.4 eV and the appearance of a π→π * loss at 7.2 eV due to CC bonds. The UPS data, which have been analyzed with the aid of density functional theory molecular orbital calculations, indicate adsorption with the 3-pyrroline ring remaining intact. The results are consistent with bonding via the molecular N atom in a fourfold coordinated configuration bridging two surface Ga atoms, as proposed earlier for ammonia and aniline. However, in this case adsorption by charge donation from the molecular N non-bonding lone-pair orbital to a single surface Ga dangling orbital cannot be excluded on the basis of UPS alone. Adsorption causes a decrease in electron affinity ( δχ≈−0.92 eV) due to a surface dipole layer, and measurement of δχ and of changes in band bending have been used to construct a partial energy-level diagram for the adsorbate-covered surface. Some results (ELS and UPS) are also given, for comparison, for the chemisorption of pyridine.

Functionalizing the GaN( [formula omitted])-(1×1) surface I. The chemisorption of aniline

Chemisorption of aniline (C 6H 5NH 2) on the GaN(0 0 0 1)-(1×1) Ga-polar surface has been studied using mainly X-ray-excited Auger electron, electron energy loss and ultraviolet photoemission spectroscopies (XAES, ELS and UPS, respectively). The XAES data show adsorption near room temperature with a total sticking probability of ∼0.05 and a saturation coverage of ∼0.28 phenyl rings per surface lattice site. The ELS data show removal of the characteristic surface-state band centered at ∼3.4 eV and the appearance of a π→ π ∗ loss at 6.5 eV due to CC bonds. In contrast, benzene (C 6H 6) does not chemisorb under these conditions. The UPS data, which have been analyzed with the aid of density functional theory molecular orbital calculations, indicate that adsorption occurs with a phenyl–NH group forming a Ga–N–Ga bridge. Adsorption causes a decrease in electron affinity ( δχ≈−0.55 eV) due to a surface dipole layer, and measurement of δχ and of changes in band bending have been used to construct a partial energy level diagram for the aniline-covered surface.

Electron affinity of AlxGa1−xN(0001) surfaces

The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.

Effect of Surface Hydrogen Coverage on Field Emission Properties of Diamond Films Investigated by High-Resolution Electron Energy Loss Spectroscopy

The influence of surface hydrogen coverage on the electron field emission of diamond films was investigated by high-resolution electron energy loss spectroscopy. It was found that hydrogen plasma treatment increased the surface hydrogen coverage while annealing caused hydrogen desorption and induced surface reconstruction. Field electron emission measurements manifested that increase of surface hydrogen coverage could improve the field emission properties, due to the decrease of electron affinity of the diamond surface by hydrogen adsorption.

H2O adsorption on the layered chalcogenide semiconductors WSe2, InSe and GaSe

The interaction of n- and p-type WSe2 and n-InSe and p-GaSe van der Waals planes (0001) with adsorbed H2O is investigated by photoelectron spectroscopy (UPS, XPS) in comparison to the semiconductor/electrolyte interface. H2O is molecularly adsorbed on all compounds without any detected surface reaction. It forms three-dimensional clusters on the (0001) planes. As expected for electron donors, H2O induces strong band bending on p-WSe2 (0.8 eV) and a decrease of electron affinity by 0.65 eV; on n-WSe2 band bending is considerably smaller (0.2 eV), but the decrease of electron affinity is similar (0.7 eV). In correspondence we observe stronger band bending (0.3 eV) on p-GaSe compared to n-InSe (0.2 eV). The electron affinities are decreased by 0.6 to 0.7 eV, respectively. The considerably stronger band bending observed for p-WSe2 is related to the stronger electronic coupling of H2O molecular orbitals to the metal d-based band edges which are absent in InSe or GaSe.

Surface methoxylation as the key factor for the good performance of n-Si/methanol photoelectrochemical cells

We have tried to elucidate the mechanism for the beneficial effect of using the solvent methanol in n-Si-based photoelectrochemical cells. Silicon surfaces were treated by exposure to methanol vapour, then tested in indifferent acetonitrile electrolyte using impedance and electromodulated infrared spectroscopic measurements, and also in a vacuum using Kelvin probe measurements. Similar experiments were also done for silicon surfaces treated with hexamethyldisilazane vapour. Treated surfaces exhibited a decrease in electron affinity and a corresponding negative shift of the flatband potential in acetonitrile electrolyte (−0.4 and −0.25 V for methanol and hexamethyl disilazane treatments, respectively). Stability of the surfaces with respect to oxidation was markedly improved. Finally, it was shown from the electromodulated infrared vibrational spectra that methoxy and trimethylsiloxy groups are attached to the surface upon methanol and hexamethyldisilazane treatment, respectively. The shifts in electron affinity and flatband potential can be simply understood as electric dipole effects. Implications regarding other semiconductor/electrolyte systems are suggested.