Donor Energy Levels Research Articles

Toward the Classical Description of Halogen Bonds: A Quantum Based Generalized Empirical Potential for Fluorine, Chlorine, and Bromine.

The electronic effects in supramolecular systems are a great challenge for computational chemistry, and the understanding of ligand-protein interactions driven by halogen bonds can be limited by molecular mechanics point of view. In fact, the variations of the halogen bond acceptors, such as an aromatic ring and electrons lone pairs, restrict the classical approximations even more. Our work enhances the statement that halogen bonds are led mainly by orbital interactions via σ*C-X. Nonetheless, we have pointed a straight relationship between the maximum ESP value on the σ-hole and the LUMO energy levels of the halogen bond donor. In line with this scenario, the current work introduces a new promising empirical potential based on quantum parametrizations able to describe general halogen bonded systems. The new parameters allow force fields to detect variations on the molecular electronic structure of halogenated organic compounds to improve the description of fluorine, chlorine, and bromine in halogen bonds.

Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells.

A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.

A new method to determine the dislocation density in wurtzite n-GaN

We develop a new method to determine the edge and screw dislocation density in wurtzite n-GaN film. The method is to fit the van der Pauw variable temperature Hall-effect measurements with a analytic expression of low-field electron mobility in n-GaN. Our calculations take the comprehensive effect between the dislocation line and the shallow-donor defects as the main cause to depress the carrier mobility. Because of the crystal distortion near the dislocation line, the energy is so high that shallow-donor defects in the GaN crystal can be captured near the dislocation line. In other words, the shallow-donor defects distribute in lines along the dislocation line, but the shallow-donor defects along the screw and edge dislocation line have different energy levels. The shallow-donor defects take energy from lattice and the carrier, which is in relaxation process, then deliver the energy through ionizing. So, it is found that the following assumptions need to be made in order to obtain the model function for the mobility over a wide temperature range: i) there are 6 shallow-donor defect lines around one dislocation line; ii) two donor energy levels belonging to the screw and edge dislocation respectively must be taken into account; iii) the exchange energy between the carrier and the shallow-donor defect is ħωLO, the energy value of polar optical phonon. Under these assumptions, experiments indicate that our calculation function can fit the experimental curve best. The values of dislocation density from our model and others determined by x-ray diffraction or by chemical etching method are in good agreement, and the values of donor energy levels from our model and Rode iterative method to solve the Boltzmann equation are also in good accordance with each other. This method is applicable for the wurtzite n-GaN films grown by various preparation technologies under any condition, which is for the sample with the peak-mobility temperature about or under 200 K, not for the sample with the peak-mobility temperature about or above 300 K, which room-temperature mobility usually is about or less than 100 cm2/(V·s).

Детекторные свойства Cd0,9Zn0,1Te:Al под влиянием гамма-облучения малой дозы

Clarification of the influence of defects on detecting properties of CdZnTe detectors and understanding of the behavior of defects under the influence of aggressive radiation environment are very important to improve detector performance. The objective was to study the charges collection efficiency and the resistivity of Cd0,9Zn0,1Te:Al detectors operating under the influence of low dose γ-radiation. The study was carried out by computer simulation, where initial data were provided by the experiment results of other researchers. The possible reason for the change of measured signatures of defect levels in high resistance Cd0.9Zn0.1Te:Al during gamma irradiation and 1 month later is the change in compensation degree of the material. The changes in the properties of Cd0.9Zn0.1Te:Al detector have been researched depending on the concentration and energy level of the deep donor for different concentrations of deep acceptors, as well as on the degree of alloying with aluminum. The negative factor for registering properties of Cd0.9Zn0.1Te:Al detector is increased concentration of zinc vacancies, which may arise at manufacturing stage and under influence of -irradiation during operation. The degradation of properties of irradiated detector may occur due to the offset dependence of the resistivity on the aluminum dopant concentration N(Al) towards to higher concentrations of Al when the value of doping is not enough large. Only resistivity will be reduced and charge collection efficiency may increase. The increase in resistivity of Cd0.9Zn0.1Te and charges collection efficiency of the detector occur when there is a sufficiently high level of doping the material with aluminum.

The n-type conduction of indium-doped Cu2O thin films fabricated by direct current magnetron co-sputtering

Indium-doped Cu2O thin films were fabricated on K9 glass substrates by direct current magnetron co-sputtering in an atmosphere of Ar and O2. Metallic copper and indium disks were used as the targets. X-ray diffraction showed that the diffraction peaks could only be indexed to simple cubic Cu2O, with no other phases detected. Indium atoms exist as In3+ in Cu2O. Ultraviolet-visible spectroscopy showed that the transmittance of the samples was relatively high and that indium doping increased the optical band gaps. The Hall effect measurement showed that the samples were n-type semiconductors at room temperature. The Seebeck effect test showed that the films were n-type semiconductors near or over room temperature (<400 K), changing to p-type at relatively high temperatures. The conduction by the samples in the temperature range of the n-type was due to thermal band conduction and the donor energy level was estimated to be 620.2–713.8 meV below the conduction band. The theoretical calculation showed that indium doping can raise the Fermi energy level of Cu2O and, therefore, lead to n-type conduction.

Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers.

This Report presents a nitrogen-doping method by chemically forming self-assembled monolayers on silicon. Van der Pauw technique, secondary-ion mass spectroscopy and low temperature Hall effect measurements are employed to characterize the nitrogen dopants. The experimental data show that the diffusion coefficient of nitrogen dopants is 3.66 × 10−15 cm2 s−1, 2 orders magnitude lower than that of phosphorus dopants in silicon. It is found that less than 1% of nitrogen dopants exhibit electrical activity. The analysis of Hall effect data at low temperatures indicates that the donor energy level for nitrogen dopants is located at 189 meV below the conduction band, consistent with the literature value.

Influence of donor–acceptor materials on the photovoltaic parameters of conjugated polymer/fullerene solar cells

Organic solar cells based on poly(3-hexylthiophene-2,5-diyl): [6, 6]-phenyl-C61-butyric acid methyl ester are the most researched solar cells, but still there is a need to achieve higher efficiency and better stability. The active layer blends of poly [N-9′′-heptadeca-nyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo-thiadiazole)]: [6, 6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM) show a promise in this regard. In this work, PCDTBT:PC71BM thin films were prepared with different blend ratios and concentrations, and were characterized for optical and morphological properties. UV-visible spectral studies showed that the absorption decreases as the acceptor concentration in the blend increases. Atomic Force Microscopy studies showed that surface roughness increases with the increase in acceptor concentration. Optimization of the active layer was carried out by using different blend ratios and concentration to achieve better performance, and it was observed that the highest efficiency was obtained at the optimized blend ratio of 1:4 and a concentration of 12 mg/ml. Further, the variation observed in the open circuit voltage with blend ratio has been explained by the disorder induced tail states in the energy levels of donor and acceptor materials.

Development of a donor polymer using a B ← N unit for suitable LUMO/HOMO energy levels and improved photovoltaic performance

We report a novel approach to tune the LUMO/HOMO energy levels of polymer donors by replacing a C–C unit with a B ← N unit. The polymer containing a B ← N unit exhibits lower LUMO/HOMO levels and a narrower bandgap, leading to an improved photovoltaic performance.

Hyperfine Stark effect of shallow donors in silicon

We present a complete theoretical treatment of Stark effects in doped silicon, whose predictions are supported by experimental measurements. A multi-valley effective mass theory, dealing non-perturbatively with valley-orbit interactions induced by a donor-dependent central cell potential, allows us to obtain a very reliable picture of the donor wave function within a relatively simple framework. Variational optimization of the 1s donor binding energies calculated with a new trial wave function, in a pseudopotential with two fitting parameters, allows an accurate match of the experimentally determined donor energy levels, while the correct limiting behavior for the electronic density, both close to and far from each impurity nucleus, is captured by fitting the measured contact hyperfine coupling between the donor nuclear and electron spin. We go on to include an external uniform electric field in order to model Stark physics: With no extra ad hoc parameters, variational minimization of the complete donor ground energy allows a quantitative description of the field-induced reduction of electronic density at each impurity nucleus. Detailed comparisons with experimental values for the shifts of the contact hyperfine coupling reveal very close agreement for all the donors measured (P, As, Sb and Bi). Finally, we estimate field ionization thresholds for the donor ground states, thus setting upper limits to the gate manipulation times for single qubit operations in Kane-like architectures: the Si:Bi system is shown to allow for A gates as fast as around 10 MHz.

Finite-difference calculation of donor energy levels in a spherical quantum dot subject to a magnetic field

The ground and excited states of a donor impurity at the center of a spherical quantum dot subject to a magnetic field are calculated within the effective-mass approximation. The barriers are infinitely high and the differential equation is solved by combining the finite-difference method with the Richardson extrapolation. The binding and transition energies are more accurate than the available variational values, and excellent agreement is found with the hydrogen atom. The transition energies for a medium-size quantum dot are given.

Efficient and stable polymer solar cells with annealing-free solution-processible NiO nanoparticles as anode buffer layers

Annealing-free solution-processible nickel oxide (NiO) nanoparticles are prepared via sonochemical synthesis. In contrast to the common sol–gel route to prepare a NiOx film that had to undergo high-temperature post-annealing to be converted into NiO nanoparticles, herein, colloidal crystalline NiO nanoparticles with diameters of 4–8 nm are prepared with the metal salt at low temperature via ultrasonic irradiation. The colloidal crystalline NiO nanoparticles were directly used as the anode buffer layer in polymer solar cells (PSCs) without post-thermal annealing, which was compatible with flexible substrates and low-cost high-throughput fabrication. After O2-plasma treatment to the as-deposited buffer layer of NiO nanoparticles, the ITO/NiO contact work function was increased from −4.8 to −5.3 eV, providing superior contact with the highest occupied molecular orbital energy levels of polymer donors. Compared to PCDTBT:PC70BM PSCs with a PEDOT:PSS buffer layer, the power conversion efficiency of PSCs with an O2-plasma treated anode buffer layer of NiO nanoparticles was increased from 5.84% to 6.30% and the device stability was also enhanced.

Large positive magnetoresistance effects in the dilute magnetic semiconductor (Zn,Mn)Se in the regime of electron hopping

Magnetoresistance in dilute magnetic semiconductors is studied in the hopping transport regime. Measurements performed on Cl-doped Zn1–xMnxSe with x < 8% are compared with simulation results obtained by a hopping transport model. The energy levels of the Cl donors are affected by the magnetization of Mn atoms in their vicinity via the s-d exchange interaction. Compositional disorder, in particular, the random distribution of magnetic atoms, leads to a magnetic-field induced broadening of the donor energy distribution. As the energy distribution broadens, the electron transport is hindered and a large positive contribution to the magnetoresistance arises. This broadening of the donor energy distribution is largely sufficient to account for the experimentally observed magnetoresistance effects in n-type (Zn,Mn)Se with donor concentrations below the metal–insulator transition.

Naphthalene Diimide Copolymers with Oligo(p-phenylenevinylene) and Benzobisoxazole for Balanced Ambipolar Charge Transport

A series of alternating and random donor (D)–acceptor (A) copolymers based on naphthalene diimide (NDI) as the acceptor and oligo(p-phenylenevinylene) (OPV) or benzobisoxazole (BBO) as the strong and weak donor, respectively, were designed and synthesized by Suzuki coupling and Horner–Wadsworth–Emmons polymerization. The effect of the varying donor strength of OPV and BBO on the photophysical, electrochemical, and semiconducting properties of the polymers was investigated. Absorption and emission spectra recorded for dilute chloroform solution and thin film showed increased intramolecular charge transfer for NDI-alt-OPV polymer compared to NDI-alt-BBO polymer. Cyclic voltammetry studies along with DFT (density functional theory) studies at the B3LYP/6-31g* level gave insight into the energy level (HOMO/LUMO) and molecular orientation of donor and acceptor along the polymer backbone. NDI-alt-OPV polymer exhibited rigid coplanar structure with extended π-conjugation which induced backbone planarity and crystallinity to the polymer. The inherent poor solubility of the NDI-alt-BBO prevented further device characterization of this polymer. Random copolymer having maximum 30% incorporation of BBO comonomer in NDI-r-OPV/BBO was found to be soluble for further characterization. Compared to NDI-alt-OPV, lowering of both energy levels LUMO (∼0.2 eV) and HOMO (∼0.5 eV) was observed for both NDI-alt-BBO and the NDI-r-OPV/BBO. Bottom gate–top contact organic field effect transistors (OFETs) of NDI-alt-OPV exhibited balanced ambipolar charge transport with average electron and hole mobility of 3.09 × 10–3 cm2 V–1 s–1 and 2.1 × 10–3 cm2 V–1 s–1, respectively, whereas the random copolymer incorporating both OPV and BBO units NDI-r-OPV/BBO showed dominant n-type charge transport with moderate 4 × 10–4 cm2 V–1 s–1 average electron mobility. The present work thus highlights the structure–property relationship and the electronic tunability required in this class of NDI-based polymers to produce ambipolar transistors.

Electro-Optic Modulation Induced Enhancement in Photocatalytic Activity of N-Doped TiO2 Thin Films

In the presence of ultraviolet light, TiO2 photocatalyst finds a lot of applications in degradation of organic and inorganic particles present in environment as well as in water. TiO2 nanoparticles can not be utilized effectively in presence of solar light since solar radiation has very little contribution in UV region. The photoresponse of TiO2 can be shifted to the visible region by nitrogen doping. In the nitrogen-doped TiO2 lattice, the oxygen vacancies are formed below the conduction band in the form of donor energy level. These vacancies increase with increasing nitrogen concentration in TiO2, which results in an increase in the trapping rate. This leads to enhancement of photocatalytic activity of nitrogen-doped TiO2 film. Nitrogen doping results in the creation of defects in nitrogen-doped TiO2, which in other way leads to a modification of the work function of thin films. The observed changes in work function of these films, measured using scanning Kelvin probe microscopy, indicates the shifting of Fermi level with the introduction of defects. The study of work function as well as surface properties of thin films helps in better understanding of the photo activity of nitrogen-doped TiO2 film.

CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> Thin Films Prepared by Co-Evaporation Technique

The optical properties of Cu-poor CuIn1-xGaxSe2 thin films with different gallium contents grown by co-evaporated technique were studied. Measurements of photoluminescence and photoreflectance were performed on the samples. The photoluminescence and photoreflectance emission peaks observed around 1.1 eV are attributed to donor-acceptor pair luminescence. These donor-acceptor pair emissions are considered to originate from relatively shallow acceptor and donor energy levels. With increasing gallium content, the emission peaks shift towards higher levels of photon energy, and the linewidths of the luminescence spectra for the samples become wider, which we attributes to the greater statistical disorder between indium and gallium. Moreover, the conversion efficiency of the CuIn1-xGaxSe2-based solar cells is obtained. The measured results coincide with the inference given by the photoluminescence spectra.

Improvement in the open-circuit voltage of an organic photovoltaic device through selection of a suitable and low-lying highest occupied molecular orbital for the electron donor layer

Abstract

Bridgehead Imine Substituted Cyclopentadithiophene Derivatives: An Effective Strategy for Band Gap Control in Donor–Acceptor Polymers

Bridgehead imine substituted cyclopentadithiophene structural units offer the opportunity to modify the electronic properties, in particular, the HOMO–LUMO energy levels of donor–acceptor polymers with unprecedented precision. Utilizing a common synthetic pathway, copolymers with high average molecular weights, a variety of functionality, and properties suitable for solar cell incorporation can be generated. The fabrication of organic photovoltaic devices with these new materials is demonstrated.

Investigations on the growth and characterization of vertically aligned zinc oxide nanowires by radio frequency magnetronsputtering

Undoped vertically aligned ZnO nanowires have been grown on silicon (111) substrates by the rf magnetron sputtering technique without metal catalyst. The diameter, length and density distributions of the nanowires have been analyzed with respect to the different growth durations. The tapering of the nanowires is observed for the growth duration of 120min owing to the insufficient adatoms on the growth front. In the X-ray diffraction pattern, the dominant (002) peak with narrow full width at half maximum (FWHM) of ZnO nanowires indicates the c-axis orientation and high crystalline nature with hexagonal wurtzite crystal structure. The narrow FWHM of E2low and E2high phonon modes (1.4 and 9.1cm−1) provide an additional evidence for the high crystalline and optical properties of the nanowires. The low temperature photoluminescence spectra are dominated by the green emission at∼2.28eV induced by the electron transitions between shallow donor and acceptor energy levels.

Determination of the transport parameters of nanocrystalline CdSe:Cu thin films

Thin films of copper (Cu)-doped CdSe are prepared by the physical vapor deposition technique. X-ray diffraction analysis shows that the films crystallize in hexagonal structure with (100) as the preferred orientation. Scanning electron microscope studies reveal that the grains are spherical in shape and uniformly distributed all over the surface of the substrates. The transport properties of Cu-doped CdSe thin films have been investigated by the time-of-flight measurement, steady-state photocarrier grating and steady-state photoconductivity techniques. The temperature dependence of dark conductivity and photoconductivity data reflects the existence of two independent donor energy levels. The bimolecular recombination nature of the thin films has been found from the photocurrent-illumination dependence at different applied voltages. The minority carrier diffusion length (Ldiff) is obtained both from the best fit of the experimental photocurrent ratio β versus grating period (Λ) and from the ‘Balberg plot’. A straight line has been obtained from the best fit, which confirms that the measured length is the ambipolar diffusion length.

Influence of the cyanine counter anions on a bi-layer solar cell performance

ABSTRACTWe present normal and inverted solution processed bi-layer solar cells using cationic cyanine dyes as the electron donor and a fullerene as the electron acceptor. The cells exhibit high open circuit voltages up to 1 volt showing the optimal alignment of donor and acceptor energy levels. We demonstrate the large effect that cyanine dye counter ions can have on the energetics of the solar cells and how the S-shaped current density vs. voltage (J-V) curves can be avoided.