Ionization Energy Level Research Articles

Intrinsic Photoconductivity Spectral Dependence as a Tool for Prediction of Open-Circuit Voltage in Organic Solar Cells

Organic materials are known for their variety of molecules. Methods to predict the parameters of organic photovoltaic (OPV) cells are required to avoid the time- and resource-consuming processes of manufacturing and testing OPVs. Usually, the open-circuit voltage (Uoc) is estimated as the difference between the ionization energy level of the electron donor molecule (Id) and the electron affinity level of the electron acceptor molecule (EAa). Various measurement methods are used to determine the energy level values of pure materials, which, when combined with energy level shifts due to the donor:acceptor interactions, make these estimations less precise. In this work, photoconductivity measurements were applied to the donor:acceptor films. Near threshold energy, the electron can be directly transferred from the donor to the acceptor molecule. The obtained charge transfer energy (ECT) shows the difference between Id and EAa in the film. This difference was compared to the Uoc value of an OPV made of the same donor:acceptor combination. We show that this approach provides less scattered results and a higher correlation coefficient compared to the Uoc estimation using energy level values.

Study on Relative Electrical Strength of SF6 Substitute Gas Based on Density Functional Theory

In order to achieve the goal of “double carbon” as soon as possible, SF₆ substitute gas is more environmentally friendly and applied in electrical equipment. Based on density functional theory (DFT), the ionization energy, polarizability and HOMO-LUMO energy level of the SF₆ substitute gas were calculated. First, some relevant electrical parameters representing the substitute gas were linearly fitted to their experimental values. Secondly, the prediction simulation equation of the relative electric strength of the substitute gas is obtained by multivariate nonlinear fitting, and then the prediction results are screened. Finally, the relationship between the electrical parameters of the surrogate gas molecule and the relationship between the relative molecular mass under the external electric field is further studied. The research results show that the ionization energy, average electron polarizability and HOMO-LUMO energy level of substitute gas molecules have a strong positive correlation with their relative electric strength, and HFOtype substitute gases can be used as SF₆ substitute gases. Under the external electric field, the ionization energy and HOMO-LUMO level of the surrogate gas molecules decrease with the increase of the gas molecular mass, and the average electron polarizability increases with the increase of the gas molecular mass. For the screening of alternative gases, the carbon chain should be selected to be shorter, the relative molecular mass is more significant, and the average molecular weight is higher. The fluoride of the electron polarizability functional group provides a more precise direction for screening SF₆ substitute gases and has a particular guiding significance.

Beryllium sulfur doped with N, Li and Na: Promising p-type transparent semiconductor

The properties for group IA and VA atoms doped BeS are studied by using the first-principles method. Our studies show BeS is a transparent semiconductor. The visible light transmittances could reach 80.0 %, as the thickness increases to 60.0 nm. The holes effective mass along the Г-X and Г-L directions are 0.56 and 1.34 m0 (m0 is electron’s static mass). The defective behaviors for group IA and VA atoms in BeS show that N substituting S, together with the Li and Na substituting Be are shallow acceptors, and the corresponding ionization energy levels ε(0/-), compared to the valence band maximum, are 0.166, 0.177 and 0.25 eV, respectively. Formation energies imply under S-rich condition, NS, LiBe and NaBe defects can be realized experimentally by using the equilibrium growth conditions, while the detrimental hole killers, such as, VS, Bei, Nint, Liint and Naint would be inhibited as samples fabrication. These properties indicate N, Li and Na doped BeS may offer new opportunities in transparent electronics and optoelectronic applications.

Investigation of Double Excitation- Ionization Resonance Profiles of Helium Atom at 10o-80o Angular Range

Bu çalışmada, elektron spektrometresi kullanılarak, Helyum (He) atomunun rezonans düzeylerinin açısal değişimleri ölçülmüştür. 200 eV enerjili elektron demeti, yüksek vakum ortamında He atomları ile çarpıştırılarak koparılan ve saçılan elektronlar dedekte edilmiştir. Elektron spektrometresi deney düzeneği; vakum sistemi, elektron tabancası, hedef gaz He, saçılan ve koparılan elektronları dedekte eden iki elektron enerji analizörü, elektron dedektörleri, sinyal işleme ünitesi ve Faraday Elektron Toplayıcıdan (FET) oluşmaktadır. Bu çalışmada, soygaz element serisinde olan Helyum atomunun kendiliğinden iyonlaşma enerji seviyeleri 10o-80o açı aralığında incelenmiştir. Enerji değerleri 32,5-35 eV olarak alınmıştır. Küçük açılarda rezonans profillerinin simetri durumlarının daha baskın olduğu görülürken, büyük açılara doğru kayıldıkça hem simetrik hem asimetrik yapıda oldukları görülmüştür.

Radiation-induced electron and hole traps in Ge1 − xSnx (x = 0–0.094)

The band structure of germanium changes significantly when alloyed with a few percent concentrations of tin, and while much work has been done to characterize and exploit these changes, the corresponding deep-level defect characteristics are largely unknown. In this paper, we investigate the dominant deep-level defects created by 2 MeV proton irradiation in Ge1 − xSnx (x = 0.0, 0.020, 0.053, 0.069, and 0.094) diodes and determine how the ionization energies of these defects change with tin concentrations. Deep-level transient spectroscopy measurements approximate the ionization energies associated with electron transitions to/from the valence band (hole traps) and conduction band (electron traps) in the intrinsic regions of p-i-n diode test structures. The prominent deep-level hole traps may be associated with divacancies, vacancy–tin complexes, and vacancy–phosphorous complexes (V2, V–Sn, and V–P, respectively), with the presumed V–P hole trap dominating after room temperature annealing. The ionization energy level of this trap (approximated by the apparent activation energy for hole emission) is close to the intrinsic Fermi level in the 0% and 2% Sn devices and decreases as the tin concentration is increased, maintaining an approximately fixed energy spacing below the indirect conduction band edge. The other hole traps follow this same trend, and the dominant electron trap ionization energies remain roughly constant with changes in tin concentrations, indicating they are likewise pinned to the conduction band edge. These results suggest a pattern that may, in many cases, apply more generally to deep-level defects in these alloys, including those present in the “as-grown” materials.

Synthesis and investigation of charge transport properties in adducts of hole transporting carbazole derivatives and push-pull azobenzenes

In order to investigate the viability of a material design for bulk heterojunction (BHJ) organic solar cells, where hole transporting group is bound to the donor moiety, we report the synthesis and charge transport characteristics of 3-(diphenylamino)carbazolyl-functionalized derivatives of 2-(4-((4-(dimethylamino)phenyl)diazenyl)benzylidene)-1H-indene-1,3-dione (DMAAzi) chromophore. Three different bounding configurations were examined in these adducts. Additionally, a trityl-functionalized derivative of DMAAzi was prepared and used for comparison purposes. All of the synthesized materials form thin amorphous films from volatile organic solvents and exhibit glass transition temperatures in the range from 89 °C to 124 °C. The molecular ionization energy and electron affinity energy levels in thin films were measured. Photo-induced time of flight (ToF) method was used in to determine charge carrier drift mobilities. It was found out that the formation of deep charge trap states with local energies at approximately 0.60–0.78 eV takes place and has a considerable negative effect on the hole drift mobility of the investigated compounds.

Nonrelativistic energy levels of helium atoms

The nonrelativistic ionization energy levels of a helium atom are calculated for $S$, $P$, $D$ and $F$ states. The calculations are based on the variational method of "exponential" expansion. The convergence of the calculated energy levels is studied as a function of the number of basis functions $N$. This allows us to claim that the obtained energy values (including the values for the states with a nonzero angular momentum) are accurate up to 28-35 significant digits. Calculations of the nonrelativistic ionization energy of the negative hydrogen ion H$^-$ are also presented.

Fluorinated Thiophene-Based Synthons: Polymerization of 1,4-Dialkoxybenzene and Fluorinated Dithieno-2,1,3-benzothiadiazole by Direct Heteroarylation

The incorporation of fluorine atoms along the conjugated polymer backbone is an effective strategy to tune the electro-optical properties of donor–acceptor based copolymers. We report here an efficient way to synthesize and purify new mono-fluorinated thiophene derivatives for the synthesis of fluorinated dithienobenzothiadiazole (DTBT) comonomers. It was observed that the reactivity and regioselectivity of the direct (hetero)arylation polymerization (DHAP) were modified upon the amount and the positioning of the fluorine atoms on the DTBT moiety. Indeed, the polymerization time went from 66 h (for non-fluorinated DTBT, M1) to only 11 min (for tetra-fluorinated DTBT, M2). In addition to enhanced reactivity, the degree of fluorination of the flanking thiophene of the DTBT moiety also modulates the electro-optical properties, lowering both the bandgap and stabilizing the ionization energy level. Indeed, P1 (with non-fluorinated flanking thiophene) has a bandgap of 1.73 eV and an ionization energy (IE) of 4....

Energy levels of a helium atom

The nonrelativistic ionization energy levels of a helium atom are calculated for S, P, and D states. The calculations are based on the variational method of “exponential” expansion. The convergence of the calculated energy levels is studied as a function of the number of basis functions (N). This allows us to claim that the obtained energy values (including the values for the states with a nonzero angular momentum) are accurate to 20 significant digits.

Study on the synthesis and characterization of 3,3′-bicarbazole and pyridine based luminescent copolymers

A new series of carbazole-pyridine copolymers containing 3,3′-bicarbazolyl moiety in the main chain have been synthesized and characterized. The polymers exhibit good solubility in common organic solvents as well as good thermal stability above 300°C and the glass transition temperature above 150°C indicating their expediency in the device fabrication. The photophysical properties divulge emission in the blue region from 402 to 442nm. All compounds have reversible oxidation and desirable ionization energy and electron affinity level owing to 3,3′-biscarbazole and pyridine moieties. These results signify prospective use of polymers as promising blue luminescent materials in optoelectronics applications.

Direct electron injection into an oxide insulator using a cathode buffer layer.

Injecting charge carriers into the mobile bands of an inorganic oxide insulator (for example, SiO2, HfO2) is a highly complicated task, or even impossible without external energy sources such as photons. This is because oxide insulators exhibit very low electron affinity and high ionization energy levels. Here we show that a ZnO layer acting as a cathode buffer layer permits direct electron injection into the conduction bands of various oxide insulators (for example, SiO2, Ta2O5, HfO2, Al2O3) from a metal cathode. Studies of current–voltage characteristics reveal that the current ohmically passes through the ZnO/oxide-insulator interface. Our findings suggests that the oxide insulators could be used for simply fabricated, transparent and highly stable electronic valves. With this strategy, we demonstrate an electrostatic discharging diode that uses 100-nm SiO2 as an active layer exhibiting an on/off ratio of ∼107, and protects the ZnO thin-film transistors from high electrical stresses.

Computational prediction of lattice defects in multinary compound semiconductors as photovoltaic materials

In the past 60 years development of photovoltaic semiconductors, the number of component elements has increased steadily, i.e., from silicon in the 1950s, to GaAs and CdTe in the 1960s, to CuInSe2 in the 1970s, to Cu(In, Ga) Se2 in the 1980s, to Cu2ZnSnS4 in the 1990s, and to recent Cu2ZnSn(S, Se)4 and CH3NH3PbI3. Whereas the material properties become more flexible as a result of the increased number of elements, and multinary compound semiconductors feature a dramatic increase of possible point defects in the lattice, which can significantly influence the optical and electrical properties and ultimately the photovoltaic performance. It is challenging to characterize the various point defects and defect pairs experimentally. During the last 20 years, first-principles calculations based on density functional theory (DFT) have offered an alternative method of overcoming the difficulties in experimental study, and widely used in predicting the defect properties of semiconductors. Compared with the available experimental methods, the first-principles calculations are fast, direct and exact since all possible defects can be investigated one by one. This advantage is especially crucial in the study of multinary compound semiconductors which have a large number of possible defects. Through calculating the formation energies, concentration and transition (ionization) energy levels of various possible defects, we can study their influences on the device performance and then identify the dominant defects that are critical for the further optimization of the performance. In this paper, we introduce the first-principles calculation model and procedure for studying the point defects in materials. We focus on the hybrid scheme which combines the advantages of both special k-points and -point-only approaches. The shortcomings of the presentcalculation model are discussed, with the possible solutions proposed. And then, we review the recent progress in the study of the point defects in two types of multinary photovoltaic semiconductors, Cu2ZnSn(S,Se)4 and H3NH3PbI3. The result of the increased number of component elements involves various competing secondary phases, limiting the formation of single-phase multinary compound semiconductors. Unlike ternary CuInSe2, the dominant defect that determines the p-type conductivity in Cu2ZnSnS4 is Cu-on-Zn antisite (CuZn) defect rather than the copper vacancy (VCu). However, the ionization level of CuZn is deeper than that of VCu. The self-compensated defect pairs such as [2CuZn+SnZn] are easy to form in Cu2ZnSnS4, which causes band gap fluctuations and limits the Voc of Cu2ZnSnS4 cells. Additionally the formation energies of deep level defects, SnZn and VS, are not sufficiently high in Cu2ZnSnS4, leading to poor lifetime of minority carriers and hence low Voc. In order to enhance the formation of VCu and suppress the formation of CuZn as well as deep level defects, a Cu-poor/Zn-rich growth condition is required. Compared with Cu2ZnSnS4, the concentration of deep level defects is predicted to be low in Cu2ZnSnSe4, therefore, the devices fabricated based on the Se-rich Cu2ZnSn(S,Se)4 alloys exhibit better performances. Unlike Cu2ZnSnS4 cells, the CH3NH3PbI3 cells exhibit rather high Voc and long minority-carrier life time. The unusually benign defect physics of CH3NH3PbI3 is responsible for the remarkable performance of CH3NH3PbI3 cells. First, CH3NH3PbI3 shows that flexible conductivity is dependent on growth condition. This behavior is distinguished from common p-type photovoltaic semiconductor, in which the n-type doping is generally difficult. Second, in CH3NH3PbI3, defects with low formation energies create only shallow levels. Through controlling the carrier concentration (Fermi level) and growth condition, the formation of deep-level defect can be suppressed in CH3NH3PbI3. We conclude that the predicted results from the first-principles calculations are very useful for guiding the experimental study.

Nonrelativistic ionization energy levels of a helium atom

The nonrelativistic ionization energy levels of a helium atom are calculated for S, P, and D states. The calculations are based on the variational method of “exponential” expansion. Variational wave functions of bound states were obtained by solving the Schrodinger equation for the quantum three body problem with Coulomb interaction using a variational approach based on exponential expansion with the parameters of exponents being chosen in a pseudorandom way. The convergence of the calculated energy levels is studied as a function of the number of basis functions (N). This allows us to claim that the obtained energy values (including the values for the states with a nonzero angular momentum) are accurate to 20 significant digits.

Ionization energy levels in C-doped InxGa1−xN alloys

The InxGa1−xN alloys present levels as a result of the intentional (doped) or unintentional (contamination) introduction of C atoms into the host semiconductor. The III-V nitride semiconductors and their alloys usually crystallize in the wurtzite structure although the zinc blende structure has also been grown. We obtained the InxGa1−xN:C ionization energies from first-principles calculations of the two ordered wurtzite and zinc blende structures using different exchange and correlation terms. In accordance with the experimental results, the ionization levels could give rise, on some occasions, to a metallic impurity band.

Covalent Anchoring of Re6Sei8 Cluster Cores Monolayers on Modified n- and p-Type Si(111) Surfaces: Effect of Coverage on Electronic Properties

The electronic properties of redox-active transition metal clusters (Re6Se8) covalently immobilized on modified Si(111) surfaces through linear alkyl spacers have been studied as a function of the cluster coverage (1 × 1013-6 × 1013 cm-2). The latter is controlled by using Si(111)/H surfaces modified by dense mixed alkyl/acid-terminated monolayers with variable fraction of the acid grafting sites from 5 to 100% in solution. Quantitative X-ray photoemission analysis, spectroscopic ellipsometry, and scanning tunnelling microscopy indicate a covalent attachment of a submonolayer to densely packed monolayer of Re6Se8 clusters, while the vibrational Raman signature confirms the cluster integrity within the monolayer. Electrical band gaps as deduced from scanning tunnelling spectroscopy have been obtained for low Re6Se8 cluster coverage. Using ultraviolet photoemission spectroscopy, electronic properties such as ionization potential changes and energy level alignments at organic/inorganic interfaces are studied. We show that the lowest unoccupied molecular orbital of the Re6Se8 cluster is close to the bottom of the Si conduction band. At high cluster coverage, this affects the current-voltage characteristics measured using a weakly interacting top mercury contact onto the organic monolayer/silicon junctions. Indeed, on n-type silicon, the high level current at low bias and the shape of the conductance G(V) curve indicate a Schottky barrier height lowering. On the other hand, the current-voltage characteristics are the same for both acid-terminated and low coverage Re6Se8 cluster junctions at low bias; the high Schottky barrier height limits the current at low bias. When the forward bias increases, the current is tunnelling limited. As expected from the band alignment deduced from photoemission data, the opposite behavior is obtained on p-type silicon.

Acceptor and donor ionization energy levels in O-doped ZnTe

The O-doped ZnTe (ZnTe 1− x O x ) alloys present induce levels through O doping into the host semiconductor gap. ZnTe usually crystallizes in the zinc-blend structure and ZnO in the wurtzite structure under normal conditions. Therefore two possible ZnTe 1− x O x phases may coexist, although in different proportions, depending on experimental growth conditions. We present total energy calculations and analyze some of their electronic properties with respect to: the two ordered wurtzite and zinc-blende structures, the concentration ( x from 0.0078 to 0.5), the localized basis set (from single-zeta to quadruple-zeta with polarization basis sets), and the variation of the ionization levels with x and with the distance O–Zn.

Calculations of Energy Levels Using the Weakest Bound Electron Potential Model Theory

In this paper, we introduce a new method for calculation of energy levels in detail and give our results for several iso-spectrum-level series as examples: [He] 2s2p P1, [He] 2s2p P0, [He] 2s2p P2, and [He] 2s3s S1 series of Be-like sequence; [Ne] 3s3d D3/2 series and [Ne] 3s 3d D5/2 series of Al-like sequences; [Ne] 4p P1/2 series, [Ne] 5d D5/2 series, and [Ne] 6f F7/2 series of Na-like sequences. In the method I(Z) = Tlim(Z) − T (Z, n), where I(Z), Tlim(Z), and T (Z, n) denote ionization potential, series limit, and energy level of a given member, respectively. The expression of non-relativistic part of I(Z) is derived from weakest bound election potential model theory and relativistic effects of I(Z) are included by using a six-order polynomial in Z. Our results are compared with the experimental data and with those obtained by other theoretical method.

Ionization energy levels in Mn-doped InxGa1−xN alloys

The Mn-doped InxGa1−xN alloys are very interesting because of the possibility of controlling the gap and the levels induced by the Mn doping. Most of the experimental and theoretical work has been carried out on the wurtzite structure and with x next to zero. However, two possible phases may coexist, although in different proportions, depending on experimental growth conditions. We present total-energy spin-polarized density-functional calculations and analyze some of their electronic properties interesting for both spintronic and optoelectronic applications. In particular, the ionization levels in the entire x range, as well as in the ordered wurtzite and zinc-blende ferromagnetic structures.

Inner-shell chemical shift of DNA/RNA bases and inheritance from their parent purine and pyrimidine

Inner-shell electronic structures, properties and ionization spectra of DNA/RNA bases are studied with respect to their parent pyrimidine and purine species. Density functional theory B3LYP/aug-cc-pVTZ has been employed to produce the geometries of the bases, whereas LB94/et-pVQZ//B3LYP/aug-cc-pVTZ is used to calculate site-related Hirshfeld charges and core (vertical) ionization energies, as well as inner-shell spectra of C1s, N1s and O1s for DNA/RNA bases and their parent pyrimidine and purine species. The site-dependent variations of properties indicate the changes and inheritance of chemical environment when pyrimidine and purine become substituted. In general, although the changes are site-dependent, they are also ring-dependent. Pyrimidine bases change less significantly with respect to their parent pyrimidine than the purine bases with respect to their parent purine. Pyrimidine bases such as uracil, thymine and cytosine inherit certain properties from their parent pyrimidine, such as the Hirshfeld charge distributions and the order of core ionization energy level etc. No particular sites in the pyrimidine derivatives are engaged with a dramatic chemical shift nor with energy crossings to other sites. For the core shell spectra, the purine bases inherit very little from their parent purine, and guanine exhibits the least similarities to the parent among all the DNA/RNA bases.

Deep-level transient spectroscopy study of theEcenter inn−Siand partially relaxedn−Si0.9Ge0.1alloy layers

We have employed deep-level transient spectroscopy to investigate the electronic properties of defects introduced during high energy He-ion irradiation of epitaxially grown phosphorous-doped $n\text{\ensuremath{-}}\mathrm{Si}$ and partially relaxed $n\text{\ensuremath{-}}{\mathrm{Si}}_{0.9}{\mathrm{Ge}}_{0.1}$. It is found that He-ion irradiation introduces two major defects in Si and ${\mathrm{Si}}_{0.9}{\mathrm{Ge}}_{0.1}$. These have been attributed to a doubly negative charge state of the divacancy $({V}_{2}^{=∕\ensuremath{-}})$ and $V\text{\ensuremath{-}}\mathrm{P}$ pair ($E$ center). The germanium dependence of the activation enthalpy $({E}_{H})$ for both $({V}_{2}^{=∕\ensuremath{-}})$ and $V\text{\ensuremath{-}}\mathrm{P}$ pair is found to be relatively minute with a small decrease and increase in the corresponding ${E}_{H}$ with respect to that of Si, respectively. Comparison was made with earlier reported results in phosphorous-doped fully strained $n\text{\ensuremath{-}}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ and antimony-doped totally relaxed $n\text{\ensuremath{-}}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ layers to directly assess the influence of strain relaxation on radiation-related deep levels in ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$. It is shown that the energy level of the $V\text{\ensuremath{-}}\mathrm{P}$ pair in fully strained and partially relaxed ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ lies closely to that reported for $V\text{\ensuremath{-}}\mathrm{Sb}$ in fully relaxed ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$. This result indicates that the $V\text{\ensuremath{-}}\mathrm{P}$ level is independent of strain, suggesting that such defect is pinned to the conduction band. Moreover, our calculation using full-potential linearized augmented plane wave method shows nonpreferential site occupancy of the phosphorous relative to Ge atoms. This implies a chemical disorder in the vicinity of the $V\text{\ensuremath{-}}\mathrm{P}$ center which leads to a fluctuation of the ionization energy level of the $E$ center. This fluctuation is associated with a distribution of the electron emission rate between the $V\text{\ensuremath{-}}\mathrm{P}$ level and the conduction band edge.