Donor Energy Levels Research Articles

Thiazole as a weak electron-donor unit to lower the frontier orbital energy levels of donor–acceptor alternating conjugated materials

We demonstrate that the substitution of the thiophene by the thiazole heterocyclic compound as a weak electron-donor unit, in donor-acceptor alternating conjugated materials, allows a simultaneous downshift of both HOMO and LUMO levels while keeping the energy bandgap almost unchanged.

Synthesis and Photovoltaic Properties of Thieno[3,4-c ]pyrrole-4,6-dione-based donor-acceptor Copolymers

Three donor–acceptor (D–A) 1,3-di(thien-2-yl)thieno [3,4-c]pyrrole-4,6-dione-based copolymers, poly{9,9-dioctylfluorene-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, poly{N-(1-octylnonyl)carbazole-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, and poly {4,8-bis(2-ethylhexyloxyl) benzo[1,2-b:3,4-b′]dithiophene-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c] pyrrole-4,6-dione} were synthesized by Suzuki or Stille coupling reaction. By changing the donor segment, the bandgaps and energy levels of these copolymers could be finely tuned. Cyclic voltammetric study shows that the highest occupied molecular orbital (HOMO) energy levels of the three copolymers are deep-lying, which implies that these copolymers have good stability in the air and the relatively low HOMO energy level assures a higher open-circuit potential when they are used in photovoltaic cells. Bulk-heterojunction photovoltaic cells were fabricated with these polymers as the donors and PC71BM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.22, 0.74, and 3.11% with large open-circuit potential of 1.01, 0.99, and 0.90 V under one sun of AM 1.5 solar simulator illumination (100 mW/cm2). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

The structural and material properties of Cu(In,Ga)Se2 thin films

AbstractWe report on the structural and material properties of Cu‐poor CuIn1‐xGax Se2 (CIGS) thin films with different gallium contents grown using the co‐evaporation technique. Temperature‐dependent photoluminescence (PL) and high‐resolution X‐ray diffraction measurements were performed. The PL emission peaks observed around 1.0‐1.2 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. In addition, the X‐ray diffraction spectra of the samples are simulated using a theoretical model. From the analysis, it is found that the sample with higher gallium content exhibits smaller grain size and the microstructure size uniformity is reduced. The theoretical calculation result is consistent with the experimental results derived from the PL spectra. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Influence of the HOMO Energy Level of the Organic Electron Donor on the Effectiveness of the Anode Buffer Layer in Photovoltaic Solar Cells

Efficiency of organic photovoltaic cells, based on electron donor/electron acceptor junctions (ED/EA) could be strongly improved when the transparent conductive anode, is covered with a thin anode buffer layer (ABL) such as Au or MoO3. To study the influence of the HOMO (Highest Occupied Molecular Orbital) value of the organic material on the effectiveness of the anode buffer layer (ABL), different ED have been probed: pentacene, copper phthalocyanine (CuPc), tetraphenyldibenzoperiflanthene (DBP) and, poly(benzo[c]thiophene-2-oxide) with 5 eV, 5.2 eV, 5.5 eV and 5.52 eV as HOMO value respectively. The effects of a metal (Au) or an oxide (MoO3) ABL are reported. The results indicate that the improvement of the cells performance depends on the value of the ABL work function. A good matching between the work function of the anode and the highest occupied molecular orbital of the organic electron donor is the most important factor limiting the hole transfer efficiency. It is shown that the MoO3 oxide has a wider field of application as ABL than the Au metal.

Role of point defects on the enhancement of room temperature ferromagnetism in ZnO nanorods

Vertically aligned undoped ZnO nanorod arrays (NRAs) have been fabricated on silicon (111) substrates by radio frequency magnetron sputtering. The structural studies illustrate a hexagonal wurtzite structure of the ZnO NRAs with compressive stress. Raman analysis of the E2high phonon mode corroborates the partial relaxation of stress in NRAs by the post growth treatment under oxygen and vacuum atmospheres. The anomalous Raman modes have been attributed to the local vibrations and it corresponds to the silent modes of wurtzite ZnO. The appearance of forbidden modes illustrates the breakdown of the Raman selection rules. The role of point defects on the ferromagnetic behaviour of NRAs was analyzed by optical transitions and correlated with the magnetic properties. Post growth treatment of NRAs under oxygen and vacuum atmospheres significantly suppresses the point defects owing to the enhancement of the crystalline quality. The temperature dependent zero-field cooled and field cooled magnetizations reveal the coexistence of antiferromagnetism and ferromagnetism below 7 K. However, the ferromagnetism is dominant and stable between 7 K and room temperature. The decrease of ferromagnetism in NRAs is directly associated with the compensation of point defects such as zinc and oxygen vacancies as substantiated by the radiative transition between shallow donor and acceptor energy levels. These results confirm that point defects play an important role in enhancing the room temperature ferromagnetism in ZnO NRAs.

Spectra of the energy levels of multicharged nanoclusters of manganese atoms in silicon

In this work, we study the spectra of the energy levels of multicharged nanoclusters of manganese atoms in silicon. It is found that the formation of multicharged nanoclusters significantly changes the structure of the energy states of manganese atoms in silicon and leads to the formation of donor energy levels in the range of E =E V + (0.16–0.5) eV. In these materials, the photocurrent in the range of hν = 0.16–0.6 eV continuously and abruptly increases and has very high values; that is, it exhibits a high impurity photosensitivity. It is revealed that the photosensitivity of these samples in the range of hν = 0.16–0.8 eV increases with the increasing electric field according to the law ∼ E 3.8–4. It is determined that the photoconductivity and photosensitivity of the material can be varied over a wide range via controlling the charge state of the nanoclusters.

Universal energy-level alignment of molecules on metal oxides

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Laser dressed donor impurities in free-standing GaAs films under an electric field

The laser-field dependence of the shallow donor states in a free-standing thin GaAs film under an external static field is studied within the effective mass approximation. The laser dressing effects are considered for the confinement potential of the well as well as for the impurity Coulomb interaction distorted by the dielectric mismatch at interfaces. We found that (i) the increase of the laser intensity dramatically modifies the electron potential energy, which establishes the quantum confinement; (ii) the ground state subband energy is significantly enhanced by the electrostatic self-energy arising from the interaction between the electron and its images; (iii) the impurity binding is much larger than those of the dielectrically homogenous case and it becomes stronger sensitive to the laser intensity variation; (iv) under an electric field parallel to the growth direction, the inversion symmetry with respect to the quantum well center is broken and a red/blue-shift of the binding energy, depending on the impurity position along the field direction, occurs. Therefore, the shallow donor energy levels in the free-standing thin films can be tuned in a wide range by proper tailoring of the structure parameters (well size, impurity position) as well as by varying the external applied fields.

Numerical Analysis of Buffer-Trap Effects on Gate Lag in AlGaN/GaN High Electron Mobility Transistors

Two-dimensional analysis of turn-on characteristics in AlGaN/GaN high electron mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. Effects of buffer traps on gate lag are studied. It is shown that relatively large gate lag arises due to buffer traps, and it is correlated to large source access resistance in AlGaN/GaN HEMTs. The dependences of buffer-related gate lag on the gate length and buffer-trap parameters such as a deep-acceptor density and a deep donor's energy level are also studied. It is shown that the gate lag becomes smaller for a longer gate length because electron injection into the buffer layer occurs at the drain edge of the gate and normalized trapping effects become smaller. It is also shown that the gate lag becomes more significant when the deep-acceptor density in the buffer layer becomes higher, because the trapping effects become larger. In addition, the gate-lag rate is shown to be not so dependent on the deep-donor's energy level. Effects of surface states on gate lag are also described briefly.

Numerical Analysis of Buffer-Trap Effects on Gate Lag in AlGaN/GaN High Electron Mobility Transistors

Two-dimensional analysis of turn-on characteristics in AlGaN/GaN high electron mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. Effects of buffer traps on gate lag are studied. It is shown that relatively large gate lag arises due to buffer traps, and it is correlated to large source access resistance in AlGaN/GaN HEMTs. The dependences of buffer-related gate lag on the gate length and buffer-trap parameters such as a deep-acceptor density and a deep donor's energy level are also studied. It is shown that the gate lag becomes smaller for a longer gate length because electron injection into the buffer layer occurs at the drain edge of the gate and normalized trapping effects become smaller. It is also shown that the gate lag becomes more significant when the deep-acceptor density in the buffer layer becomes higher, because the trapping effects become larger. In addition, the gate-lag rate is shown to be not so dependent on the deep-donor's energy level. Effects of surface states on gate lag are also described briefly.

First Principles Study on the Electronic Properties of Zn64Sb64−xTex Solid Solution (x = 0, 2, 3, 4)

The electronic properties of Te doped-ZnSb systems are investigated by first-principles calculations. We focus on the Zn64Sb64−xTex systems (x = 0, 2, 3, 4), which respond to the 0, 1.56at%, 2.34at% and 3.12at% of Te doping concentration. We confirm that the amount of Te doping will change the conductivity type of ZnSb. In the cases of x = 2 and 3, we find that the Te element in ZnSb introduces some bands originating from Te s and p orbits and a donor energy level in the bottom of the conduction band, which induce the n-type conductivity of ZnSb. From these findings for the electronic structure and the conductivity mechanism, we predict that Te doping amounts such as 1.56at% and 2.34at% can be considered as suitable candidates for use as donor dopant.

Identification of the deep-level defects in AlN single crystals: EPR and TL studies

X-band electron paramagnetic resonance studies were performed on three different types of AlN single crystals. These studies revealed the presence of two different types of shallow donors, oxygen located in the N site of the AlN lattice and carbon or silicon located in the Al site. These findings are supported by the observation of the light-induced EPR signals with almost similar slightly anisotropic g factors typical for shallow donor in AlN, but strongly different anisotropic EPR line-width ΔB. Distribution of the wave-function density of the unpaired donor electron for both shallow donors is discussed. Moreover, findings related to the deep-donor of the V N centers are presented. The V N center EPR spectra intensity increased drastically after the x-ray irradiation of the sample. The annealing results in recombination thermoluminescence and the deep donor (V N) energy level was estimated to be about 0.75 eV.

Physics-based numerical simulation and device characterizations of AlGaN/GaN HEMTs with temperature effects

The research presents AlGaN/GaN HEMTs device characterizations at different temperatures using physics-based numerical simulation. Industry standard simulation tool Silvaco ATLAS is used to characterize the various electronic properties of the device. An extensive theoretical overview is done to achieve the most comprehensive values for GaN and AlGaN properties, as discussed in the paper. This research is mainly focused on simulation of temperature dependent device performances as well as on some other material properties that are not well defined in ATLAS. Energy bandgap, density of states, saturation velocities, surface traps, polarization effect, carrier lifetime and mobility, permittivity, effective Richardson's constant, and donor and acceptor energy levels are considered as critical parameters for predicting temperature effect in ALGaN/GaN HEMT. Various aspects of device performance are analyzed at high temperature along with the different bias configurations.

Photocatalytic Water Splitting into O2 Over Vanadium Doped Nano Rutile TiO2

In this paper, V5+ doped nano rutile TiO2 doped with different concentration V5+ were prepared by low temperature hydrolysis using Tetrabutyl titanate(C16H36O4Ti) as raw material. Powders were characterized by XRD, PL, DRS, BET. The effects of the doping concentration of V5+ to the photocatalytic activity of nano rutile TiO2 was investigated using Fe3+ as electron acceptor and at pH = 2.0 under 365 nm UV irradiation. The relationship between the DRS, PL spectra and the photaocatalytic activity was also discussed. Results show that with appropriate concentration, V5+ dopes into nano rutile TiO2 lattice without causing change in nano rutile TiO2 crystal structure. Therefore, surface oxygen vacancies and the donor energy level near the bottom of the conduction band lead to easier departure of photoinduced electrons from holes to achieve stronger photocatalytic activity. The highest photocatalytic oxygen evolution and PL Spectra intensity were achieved with the O2 evolution speed of 160 umol·L−1·h−1 when the V5+ concentration was 1.0 mol%, which demonstrated certain relationship between photoluminescence performance affected by V5+ concentration and the photocatalytic activity. The photocatalytic oxidation activity also corresponds to the concentration of Fe3+.

Ferroelectric and Semiconducting Properties of Sn2P2S6 Crystals with Intrinsic Vacancies

Thermoelectric properties of Sn2P2S6 (SPS) ferroelectric-semiconductor crystals confirm their hole-type conductivity at room temperature which inverses to the electron-type conductivity at heating to 500 K. Their dielectric susceptibility temperature anomaly near second order phase transition at 337 K and dielectric losses in paraelectric phase both strongly depend on the deviation from the stoichiometric content. Calculated in LDA approach donor energy level of the sulfur vacancies and acceptor level of the tin vacancies are placed near the top of valence band. Such position in the energy gap of donor states mostly determines the semiconductive properties of SPS crystals, and they is related to strong hybridization between sp2 hybrid orbitals of phosphorus atoms and p orbitals of sulfur atoms inside anions (P2S5)4− that contain sulfur vacancy.

MOCVD法制备Cu掺杂ZnO薄膜

Un-doped and Cu-doped ZnO thin films were grown on c-Al2O3 substrate by metalorganic chemical vapor deposition(MOCVD) system.The way of changing the temperature of Cu(tmhd)2 source bottle was used to change the saturated vapor pressure of Cu(tmhd)2,in the meantime to change the quantity of Cu(tmhd)2 carried into the the reaction chamber.Then the content of Cu in ZnO films was tested to study the effects of Cu doping on the luminescent properties of ZnO,and explore the process technique and condition of controlling Cu valence in ZnO films.X-ray diffraction(XRD) show un-doped ZnO and ZnO∶Cu samples have good c-axis preferred orientation growth.The Cu content of these samples were measured by X-ray photoelectron spectroscopy(XPS).The content of Cu in ZnO films were not increased when the Cu(tmhd)2 source temperature was increased,which indicated that increasing the Cu(tmhd)2 source temperature did not lead to any increase of the Cu-doping level,and the solubility of Cu ions in ZnO is quite low.Low-temperature photoluminescence(PL) measurements show a strong and broad blue-violet(BV) light emission peak ranging from 2.8 to 3.3 eV,which could be assigned to donor-acceptor pair(DAP) recombination(between unknown shallow donor level and Cu-related acceptor level).Temperature dependent PL show a red-shift of the BV emission with increasing the temperature from 10 to 180 K,and a new weak shoulder peak appearing at BV emission's high-energy side at 80 K,which could be assigned to eA0.Moreover,the decrease of Cu content in ZnO∶Cu film resulted in quenching the intensity of BV emission red-shift of BV emission's peak position.This phenomenon is interesting and rarely reported in the previous works.However,we still need further study to clarify that if it is the electron transition between Zni-related donor energy level and valence band that causes the BV emission.

Electrical properties of PbxSn1‐xS thin films prepared by hot wall deposition method

AbstractThe PbxSn1‐xS (x = 0 – 0.25) thin films were prepared on glass substrates by hot wall vacuum deposition. The films were polycrystalline monophase in nature and had orthorhombic crystal structure. The thickness of the films was about 2‐3 μm. The temperature dependences of the conductivity were measured in the temperature range from 150 to 420 K. The films revealed p‐type of conductivity. The Seebeck coefficient and conductivity values of the films was in the range of α = 6 – 360 μV/K and σ = 4.8×10‐5 – 1.5×10‐2 Ω‐1·cm‐1, respectively, at room temperature depending on concentration of the lead in the films. The lead atoms created the substitution defects PbSn in the crystal lattice of the PbxSn1‐xS. These defects formed the donor energy levels in the band gap. The activation energy of the films increased in the range ΔEa = 0.121 – 0.283 eV with increasing of the lead concentration. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles study of hydrogen interaction with carbon-doped GaN nanotube

We have performed the first-principles calculations onto the adsorption of hydrogen on the surface of the pristine or carbon-doped (8,0) zigzag gallium nitride nanotube (GaNNT). The results show that, for a hydrogen atom adsorbed on the top site of gallium (H–Ga), an acceptor energy level is present, while for a hydrogen atom on the top site of nitrogen (H–N), a donor energy level is present. There are the impurity bands inside the nanotube band gap for a carbon atom substitution for a gallium atom or a nitrogen atom (C Ga or C N). The impurity bands disappear upon the adsorption of a hydrogen atom on the carbon-doped GaNNT. The adsorption of a hydrogen atom on the carbon atom of the carbon-doped GaNNT almost compensates the doping effect of carbon atom in carbon-doped GaNNT. Compared with pristine (8,0) GaNNT, carbon-doping enhances the binding energy of a hydrogen atom adsorbed on the similar site. The energetically and dynamically favorable site is top site of the carbon atom in carbon-doped GaNNT; this enhancement in hydrogen adsorption is more significant for the H–C N + GaNNT than it is for the H–C Ga + GaNNT with a similar configuration.

Optical polarized properties of the F 2 center in YAlO 3 crystal

The electronic structures, dielectric functions and absorption coefficient of perfect YAlO 3 (YAP) crystal and YAP crystal containing F 2 center (YAP:F 2) have been studied using the CASTEP code with the lattice structure optimized. The calculated electronic structures indicate that the F 2 center in YAP bring donor energy levels in the forbidden band compared with that of perfect YAP. The optical properties of perfect YAP and YAP:F 2 show anisotropy and their optical symmetry coincides with the lattice structure geometry. The calculated polarized absorption spectra of YAP:F 2 exhibit new absorption bands in the region from 190 to 310 nm, which are in agreement with the experimental results.

Research on the direct doping effect of silicon on cubic boron nitride ceramics by UV–VIS diffuse reflectance

Cubic boron nitride (cBN) micro-powders mixed with 1 wt% silicon were sintered at 1450 °C under a pressure of 5.0 GPa. The grain boundaries and silicon distribution in Si-cBN ceramics were studied by scanning electronic microscope (SEM) and energy-dispersive spectrometer (EDS). Optical properties of the ceramics were investigated by UV–VIS diffuse reflectance and photoluminescence spectra at room temperature. Some important parameters of studied ceramics such as absorption coefficient and defect levels were identified from reflection spectra by intercept method. The experimental results indicated the direct n-doped effect of silicon on cubic boron nitride ceramics. With a direct forbidden transition characteristic, the donor energy level of Si in forbidden zone of cBN ceramics was found to be 2.82 eV. The phonon energy related to the direct forbidden transition was 0.235 eV.