Donor–Acceptor–Donor Modular Small Organic Molecules Based on the Naphthalene Diimide Acceptor Unit for Solution-Processable Photovoltaic Devices

Two novel solution-processable small organic molecules, 4,9-bis(4-(diphenylamino)phenyl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (S6) and 4,9-bis(benzo[b]thiophen-2-yl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8 (2H,7H)-tetraone (S7), have been successfully designed, synthesized, characterized, and applied in solution-processable photovoltaic devices. S6 and S7 contain a common electron-accepting moiety, naphthalene diimide (NDI), with different electron-donating moieties, triphenylamine (S6) and benzothiophene (S7), and are based on a donor–acceptor–donor structure. S7 was isolated as black, rod-shaped crystals. Its triclinic structure was determined by single crystal x-ray diffraction (XRD): space group \(P\bar{1}\) , Z = 2, a = 9.434(5) Å, b = 14.460(7) Å, c = 15.359(8) Å, α = 67.256(9) degrees, β = 80.356(11) degrees, γ = 76.618(10) degrees, at 150 Kelvin (K), R = 0.073. Ultraviolet–visible absorption spectra revealed that use of triphenylamine donor functionality with the NDI acceptor unit resulted in an enhanced intramolecular charge transfer (ICT) transition and reduction of the optical band gap compared with the benzothiophene analogue. Solution-processable inverted bulk heterojunction devices with the structure indium tin oxide/zinc oxide (30 nm)/active layer/molybdenum trioxide (10 nm)/silver (100 nm) were fabricated with S6 and S7 as donors and (6,6)-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM) as acceptor. Power conversion efficiencies of 0.22% for S6/PC₇₀BM and 0.10% for S7/PC₇₀BM were achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm^?2). This paper reports donor–acceptor–donor modular small organic molecules, with NDI as central accepting unit, that have been screened for use in solution-processable inverted photovoltaic devices.     

The Frenkel–Poole Effect in the Ionization of an Acceptor Impurity of Boron in Diamond in a Strong Electric Field

Journal of Communications Technology and Electronics - The conductivity of epitaxial diamond films lightly doped with boron has been studied at strong electric fields up to ~5 × 105 V/ cm. It...     

On the local injection of emitted electrons into micrograins on the surface of A<Superscript>III</Superscript>–B<Superscript>V</Superscript> semiconductors

The characteristics of the injection of electrons into a semiconductor from a microprobe–micrograin nanogap are investigated with a tunneling microscope in the mode of field emission into locally selected surface microcrystals of indium antimonide, indium arsenide, and gallium arsenide. The current mechanisms are established and their parameters are determined by comparing the experimental I–V characteristics and those calculated from formulas of current transport. The effect of limitation of the current into the micrograins of indium antimonide and indium arsenide which manifests itself at injection levels exceeding a certain critical value, e.g., 6 × 10¹⁶ cm^–3 for indium antimonide and 4 × 10¹⁷ cm^–3 for indium arsenide, is discovered. A physical model, i.e., the localization of electrons in the surface area of a micrograin due to their Coulomb interaction, is proposed.     

X-Ray Diffraction Analysis of Features of the Crystal Structure of GaN/Al<Subscript>0.32</Subscript>Ga<Subscript>0.68</Subscript>N HEMT-Heterostructures by the Williamson–Hall Method

The fitting of θ/2θ and ω peaks in X-ray diffraction curves is shown to be most accurate in the case of using an inverse fourth-degree polynomial or probability density function with Student’s distribution (Pearson type VII function). These functions describe well both the highest-intensity central part of the experimental peak and its low-intensity broadened base caused by X-ray diffuse scattering. The mean microdeformation ε and mean vertical domain size D are determined by the Williamson–Hall method for layers of GaN (ε ≈ 0.00006, D ≈ 200 nm) and Al_0.32Ga_0.68N (ε = 0.0032 ± 0.0005, D = 24 ± 7 nm). The D value obtained for the Al_0.32Ga_0.68N layer is most likely to result from the nominal thickness of this layer, which is 11 nm.     

Determination of the composition and strains in Ge<Subscript>x</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>-based nanostructures from Raman spectroscopy data with consideration of the contribution of the heterointerface

The procedure of determining the composition and mechanical strains in Ge_xSi_1?x quantum dots with the use of Raman spectroscopy is substantially refined. The parameter x characterizing the composition is determined by analyzing the intensity of the peaks in the Raman spectra that correspond to the Ge-Ge and Ge-Si bond vibrations, taking into account the Ge-Si bonds at the heterointerface. The strains in the quantum dots are determined by analyzing the position of the Raman peaks, taking into account the data obtained for the composition of the quantum dots.     

Effect of Al Substitution on the Thermoelectric Properties of the Type VIII Clathrate Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript>

Single-crystal samples of the type VIII clathrate Ba₈Ga_16−x Al _x Sn₃₀ (0 ≤ x ≤ 12) were prepared by the Sn-flux method, and the structural and electrical properties were studied from 300 K to 600 K. The lattice parameter increases linearly as x is increased from 0 to 10.5, which is the solubility limit of Al. For all samples, the electrical conductivity σ decreases monotonically as the temperature is increased. σ(T = 300 K) increases from 1.88 × 10⁴ S/m for x = 0 to 3.03 × 10⁴ S/m for x = 2, and then gradually decreases to 2.4 × 10⁴ S/m with further increase of x to 8. The increase of σ for Al-substituted samples is attributed to enhancement of carrier mobility. The Seebeck coefficients of samples with 0 ≤ x ≤ 8 are negative with large values, and the absolute values increase from 240 μV/K to 320 μV/K as the temperature increases from 300 K to 600 K. At 300 K, the effective mass m*/m ₀ is in the range from 0.53 to 0.67, and the samples with x = 6 and x = 8 have a rather low thermal conductivity of 0.72 W/mK and 0.78 W/mK, respectively. ZT reaches 1.2 at 500 K for x = 6.     

Specific features of the temperature dependence of the conduction electron concentration in the narrow-gap and zero-gap states of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Results of studies of the conductivity σ and the Hall coefficient R in the Cd _x Hg_{1 − x} Te crystals with x = 0.1, 0.12, 0.14, and 0.15 are analyzed in the temperature range T = 4.2–300 K and the magnetic field range B = 0.005–2.22 T. Using data on the R(B) in low and high magnetic fields and the data on σ(T), electron and hole concentrations and mobilities are determined. It is shown that the electron concentration n in the studied samples is almost independent of T in the range 4.2–15 K, while as T increases, it increases according to the law n ∝ T ^r (r > 3/2), where r = f(n, T, x). It is found that r varies from 1.7 at x = 0.1 to 3.1 at compositions with x = 0.14 and 0.15. The results for n(T) are compared with theory, taking into account nonparabolicity of the variance law for ⃛(T), and with the theory of impurity states in narrow-gap and zero-gap semiconductors. It is shown that the constancy of n(T) up to ∼15 K and the strong dependence n(T) (r > 3/2) at higher temperatures are caused by the intense ionization of electrons localized at acceptor states.     

Effect of constructional features of the insulating gap of open TiN–SiO<Subscript>2</Subscript>–W and Si–SiO<Subscript>2</Subscript>–W “sandwich” structures on the process of their electroforming

It is experimentally shown that the introduction of an additional dielectrical layer several nanometers thick (more than 2.5 and 1.3 nm for TiO₂ and SiO₂, respectively) into open TiN–SiO₂–W and Si–SiO₂–W “sandwich”-structures on their anode (lower electrode, i.e., TiN and Si, respectively) in series with respect to the conducting medium formed in the insulating gap during electroformation appreciably decreases the probability of an electrical breakdown. This effect is produced by the limitation of the current by the resistance of an additional dielectrical layer at the prebreakdown level. Some experimental dependences of the probability of successfully electroforming on the thickness of an additional dielectrical layer (TiO₂) are obtained for the TiN–SiO₂–W structure.     

Specific features of molecules’ pyrolysis on the epitaxial surface in the case of growth of the Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers from hydrides in vacuum

Specific features of molecules’ pyrolysis in the case of growth of Si_{1 ? x} Ge _x films from hydrides in vacuum are considered in the context of kinetic approximation on the basis of a growth experiment. The type of solution characteristic of the kinetic problem was studied in relation to the values of the system’s parameters for a scheme of decomposition of monohydrides of Si and Ge with a dominant role of SiH₂ and GeH₂ radicals in the pyrolysis process. It is shown for the first time that, in the system under consideration, there can exist at least two types of solutions that differ radically from each other in the rate of incorporations of atoms into the growing layer and in the degree of filling of the surface states with products of molecules’ pyrolysis. The type of solution characteristic of the actual experiment is determined from the condition for the segregation-related accumulation of Ge atoms at the growth surface of the Si_{1 ? x} Ge _x film. Numerical analysis performed in relation to the growth temperature made it possible to estimate the rate of the monohydrides’ decomposition and the values of the factor of incorporation of Si and Ge adatoms into the crystal. The estimate of the characteristic decomposition time for monosilane yields the value of 3–4 s at the growth temperatures of 450–700°C; for monogermane, this value is about 2 s.     

Nature of the diamagnetic maximum in the temperature dependences of the magnetic susceptibility of crystals of (Bi2 ? x Sb x )Te3 alloys (0 < x < 1)

The temperature dependences of the magnetic susceptibility ?? of crystals of (Bi_{2 ? x} Sb _x )Te₃ alloys (0 < x < 1) are studied using a SQUID magnetometer in the temperature range from 2 to 400 K with the parallel and perpendicular orientations of the vector of magnetic field strength H relative to the trigonal axis of the crystal C ₃ (H ?? C ₃ and H ?? C ₃). It is found that the diamagnetic susceptibility of the samples with x = 0.2 (Bi_1.8Sb_0.2Te₃) and x = 0.5 (Bi_1.5Sb_0.5Te₃) increases in the range from 50 K to temperatures preceding the emergence of intrinsic conductivity (250 K). It is found that the diamagnetic maximum manifests itself in the same temperature range, in which an anomalous increase in the Hall coefficient is observed. It is shown that the nature of the diamagnetic maximum is associated with the nonparabolicity of the energy spectrum of light diamagnetic holes, a decrease in whose concentration is accompanied by a decrease in their effective masses, which provides an increase in the diamagnetic susceptibility with increasing temperature. These results are confirmed by the dynamics of the temperature variation in the resonance frequency of plasma oscillations of free charge carriers.     

Degradation of the Photoluminescence of ZnTPP and ZnTPP–C<Subscript>60</Subscript> Thin Films under Gamma Irradiation

Porphyrins and their fullerene complexes are promising materials for organic photovoltaic structures. However, the stability of the properties of organic components under hard radiation is poorly studied. Here, the influence of γ irradiation with medium (about 10⁴ Gy) and large (10⁷ Gy) doses on the photoluminescence of thin structurally perfect films of both pure porphyrin ZnTPP and ZnTPP/C₆₀ composite films in the molar ratio of 1.3: 1 is investigated. It is shown that the intensity of the electronic radiative transition (626 nm) decreases under the effect of γ irradiation, and the dose dependence is threshold. The threshold dose is ~20 kGy for the ZnTPP films. The intensity of the electron-vibrational part of the spectral dependence of the PL (670–690 nm) for both types of samples decreased at initial irradiation doses and decreases less with a further increase in the irradiation dose than for the purely electron transition. The addition of a fullerene in nanocomposite films increases the threshold dose, after which the PL of the films started to degrade, by a factor of ~2.5. Herewith, the spectral components of the PL associated with the manifestation of the radiation transition of the fullerene C₆₀ are more stable under γ irradiation.     

The paraxial model of the electron-optical system of a planar gyrotron with the <Emphasis Type="Italic">Т</Emphasis>-mode emission

The issues of shaping a ribbon beam with elliptical and rounded-rectangular cross sections in an external magnetic field directed at an angle to the cathode are discussed based on the classical paraxial theory.     

Formation of acceptor centers under the action of redox media on the surface of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te films

The effect of redox media on the formation of acceptor centers in the Cd _x Hg_{1 − x} Te films grown by molecular beam epitaxy on the GaAs (301) substrates is studied. When tested for long-term stability, the untreated n-type films do not change their parameters, whereas the treated films exhibit a decrease in the conductivity and the mobility of charge carriers by nearly two orders of magnitude. It is shown that, on the treatments, a source of acceptors is formed at the surface, and the acceptors are most likely mercury vacancies.     

Dependence of the electrical parameters of MBE-grown Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te films on the level of doping with indium

Dependences of the minority-carrier lifetime and electron mobility in Cd _x Hg_{1 ? x} Te films on their indium-doping level are studied. Films with x ≈ 0.22 grown by molecular-beam epitaxy on GaAs substrates were in situ doped with indium across their entire thickness. The temperature dependences of the lifetime were studied in the temperature range 77–300 K. The decrease in the lifetime, observed as the doping level increases, is governed by the mechanism of Auger recombination. As the doping level becomes higher, the mobility decreases in qualitative agreement with theoretical calculations.     

Study of the electrical properties of Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te

On the basis of the temperature and field dependences of the Hall coefficient R _H , it was found that samples with a low electron density are, as a rule, compensated, and the degree of compensation changes upon thermal conversion of the conductivity of the sample to p type. For n-Cd_xHg_1?xTe, the ionization energy of the donor level was found from the temperature dependences of resistivity ρ(T): E _d =24–32 meV. For the same samples, after their thermal conversion to p type, the ionization energies of acceptors, which are related to doubly charged vacancies V _Hg ⁺⁺ , were determined: E _a =32 and 48 meV. In addition, a deep level E _t , related to an unknown amphoteric impurity, was found (E _t ?E _v ≈0.7E _g ).     

Local symmetry of the Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>x</Subscript>Se lattice near the zero-gap state

By applying Mössbauer spectroscopy with isotopes ⁸³Kr and ¹¹⁹Sn to Pb_1?xSn_xSe solid solutions, it is shown that the local symmetry and electronic structure of anionic and cationic lattice sites remain unchanged during the transition to the zero-gap state.     

Specific features of electrical properties of the In<Subscript>x</Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>N alloy

Temperature dependences of the Hall coefficient and resistivity of the In_xGa_{1 ? x} N alloys (0 ≤ x ≤ 1) are investigated. It is found that, at x ≤ 0.4, the temperature dependences of the Hall coefficient and resistivity have the activation-related portion. The activation energy depends linearly on the In content in the alloy. At x ≈0.5, the activation portion vanishes. The main scattering mechanism depends on the temperature, on the defect density in the film (this density is largely determined by the used intermediate GaN layers), and on the alloy composition x.     

320 × 256 avalanche array photodetector on the basis of ternary alloys of the A<Subscript>3</Subscript>B<Subscript>5</Subscript> group with an InGaAs absorbing layer and an InAlAs barrier layer

320 × 256 avalanche array photodetector on the basis ternary alloys of the A₃B₅ group with an InGaAs absorbing (in the band of 0.9–1.7 μm) layer and InAlAs barrier layer is studied. The array of 320 × 256 elements was fabricated in a nBp nanoheterostructure by the mesa technology. The number of imperfect elements, the dependence of the dark current on the bias voltage, and the avalanche gain factor are measured.     

Special features of the excitation spectra and kinetics of photoluminescence of the Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:Er/Si structures with relaxed heterolayers

Luminescent properties of heteroepitaxial Si_{1 − x} Ge _x :Er/Si structures with relaxed heterolayers are studied. The results of combined studies of the excitation spectra and kinetics of photoluminescence (PL) are used to single out the components providing the largest contribution to the PL signal of the Si_{1 − x} Ge _x :Er/Si structures in the wavelength region of 1.54 μm. It is shown that relaxation of elastic stresses in the Si_{1 − x} Ge _x :Er heterolayer affects only slightly the kinetic characteristics of erbium luminescence and manifests itself in insignificant contribution of the defects and defect-impurity complexes to the luminescent response of the Si_{1 − x} Ge _x :Er/Si structures. In the excitation spectra of the erbium PL, special features related to the possibility of the rare-earth impurity excitation at energies lower than the band gap of the Si_{1 − x} Ge _x solid solution are revealed. It is shown that a peak the width of which depends on the band gap of the solid solution and the extent of its relaxation is observed in the excitation spectra of the erbium-related PL in the Si_{1 − x} Ge _x :Er/Si structures in the wavelength region of 1040–1050 nm. The observed specific features are accounted for by involvement of intermediate levels in the band gap of the Si_{1 − x} Ge _x :Er solid solution in the process of excitation of an Er³⁺ ion.     

Temperature dependence of the optical energy gap for the CdS<Subscript>x</Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

A temperature dependence of the optical energy gap E _g (T) for the CdS_xSe_1?x quantum dots synthesized in a borosilicate glass matrix was investigated in the range of 4.2–500 K. It was demonstrated that this dependence reproduced the dependence E _g (T) for bulk crystals and is described by the Varshni formula for \(\bar r > a_B \) over the entire temperature range. Here, \(\bar r\) is the average dot radius, and a_B is the Bohr radius for the exciton in a bulk crystal. With the transition to quantum dots with \(\bar r > a_B \), a decrease in the thermal coefficient of the band gap and a deviation from the Varshni dependence were observed in the temperature range of 4.2–100 K. The specific features observed are explainable by a decrease in the resulting macroscopic potential of the electron-phonon interaction and by modification of the vibration spectrum for dots as their volume decreases.