Band gap study on zinc-doped cadmium sulphide

Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.     

Photoconductivity of iodine-doped single crystals of phthalocyanine

Single crystals of metal-free phthalocyanine (H₂Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H₂Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H₂Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.     

Application of ZnO single crystals for light-induced water splitting under UV irradiation

This paper presents experimental results of implementation of ZnO single crystals as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation through the process of water splitting. Both, as-grown and O₂-annealed ZnO single crystals were investigated for this purpose. A 351 nm argon laser line was used as the light source. The XRD investigations showed that used ZnO crystals are of good crystalline quality. It was found that the as-grown ZnO single crystals possess higher conversion efficiencies comparing to the O₂-annealed one. The photocurrent density was found to increase significantly with the increase of external bias applied and excitation light intensity. Time dependent photocurrent density characteristics showed that the decay of photocurrent density was not observed within the measurement time. The differences in behaviour of the as-grown and the annealed in O₂ ZnO single crystals are discussed in terms of crystals intrinsic defects.     

Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries

The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.     

Electronic defects in heterogeneous CuInS2

The difference in photoactivity between CuInS₂ crystals grown by chemical vapour transport (CVT) and new heterogeneous material containing phases of Cu_2–x S and In₂S₃ in the mol % range is analysed. Photoluminescence, time resolved microwave conductivity (TRMC) experiments and photocurrent spectroscopy demonstrate a strongly reduced defect density in the heterogeneous samples. EBIC analyses show the terraces occurring from vapour-liquid-solid (VLS) growth of the heterogeneous material to act as recombination sites.     

Mixing antisolvents induced modulation in the morphology of crystalline C60

We present systematic studies of the synthesis of fullerene (C60) crystals with diverse morphologies by liquid-liquid interfacial precipitation (LLIP) method based on different alcohols as antisolvents and aromatic benzene as a solvent. C60 crystals are grown at liquid-liquid interface of mixed isopropyl alcohol (IPA) and tertiary butyl alcohol (TBA) with C60 solution in benzene. The role of mixing IPA and TBA on the morphology of C60 crystal is investigated. One dimensional (1 D) C60 nanowhiskers and polygon-shaped 2D sheets have been grown with individual IPA/benzene and TBA/benzene system, respectively. However, C60 crystals of different morphology (1D, 2D or both), and the self-assembly of nano-sized C60 into micron-sized crystals could be obtained upon mixing IPA and TBA and it is the mixing ratio which determines the morphology. Raman scattering and power X-ray diffraction measurements have shown that these materials are crystalline with cubic and hexagonal structures.     

Normal-incidence reflectance of crystalline K(6)C(60), Rb(6)C(60), and Cs(6)C(60)

The structure factors and normal-incidence reflectances of the alkali-metal-doped fulleride crystals M(6)C(60) (where M = K, Rb, or Cs) were calculated, and the peak reflectances are in the 2-20% range for incident wavelengths of 9-17 ?, which indicates that C(60) may be a promising transmissive (spacer) material for normal-incidence mirrors that have relatively high normal-incidence reflectance in the x-ray region.     

Ohmic contacts to p-type Hg0.3Cd0.7Te by metalorganic chemical vapour deposition of HgTe

Ohmic contacts were obtained to p-type Hg_0.3Cd_0.7Te crystals by metalorganic chemical vapour deposition (MOCVD) of HgTe as an interface material between the crystals and the contacting metal. Deposition at a reduced temperature of 350C did not lead to an obvious change in the material performance. Electronic-transport data are given. Two acceptor levels, at 4.8 and 60 meV above the valence-band edge, were found.     

Charge photogeneration and recombination in poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene composite films studied by photocurrent response

The photocurrent spectral responses of poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene (C60) composites are measured as a function of C60 concentration. At low concentration, the relationship between the external quantum efficiency (EQE) and absorption spectra is exhibited as the strengthened antibatic effect, and the EQE of the composite devices declines with increasing concentration of C60. At higher concentration, however, the maximum EQE gradually coincides with the absorption peak (symbatic response) and the EQE of composite devices begins to increase with increasing C60 concentration. It is proposed that at low concentration, dopant C60 increases the self-absorption rate of composite films, and charge trapping by C60 molecules causes the loss of efficiency. At high C60 concentration, the large-scale aggregations in composite films build pathways for charge carrier transport to respective electrodes, inhibiting the self-absorption effect and charge recombination on C60.     

Photocurrent enhancement of d.c. sputtered copper oxide thin films

Copper oxide (CuO) thin films with photocurrent as high as 25 μA/cm² were deposited on conductive glass substrates using d.c. reactive sputtering. This was the highest reported photocurrent for sputteredp- type copper oxide measured in the electrolyte KI. The photocurrent drastically increased up to 25 (μA/cm² as the sputtering pressure and the substrate temperature were increased up to 8.5 mbar and 192°C, respectively. All the synthesized films contained single phase of CuO in this range of pressure and substrate temperature. Variation of the photocurrent, photovoltage, structure and absorbance with deposition conditions were studied in detail.     

Vapor growth and photoconductivity of single-crystal nickel-phthalocyanine nanorods

Nickel-phthalocyanines (NiPc) with planar aromatic structures are ideal building blocks for organic nanostructures. They can self-assemble into stacks through π-π interaction, exhibit high thermal and chemical stabilities, and possess outstanding electrical and optical properties. Herein, single-crystal NiPc nanorods were synthesized by a facile vapor transfer deposition method. Their nanostructures and compositions were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) in detail. The deposited NiPc nanorods were found to be the β-phase single crystals. Moreover, the NiPc nanorod-based devices were fabricated and exhibited high photocurrent upon white-light illumination. This indicates that the NiPc nanorods can be considered as a candidate material for fabricating photoelectric devices.     

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene)(MEH-PPV) /Nitrogen Containing Derivatives of Fullerene Composites: Optical Characterization and Application in Flexible Polymer Solar Cells

New soluble organofullerenes were synthesized by the reaction of organic amines and azides with the [60]fullerene. The comparative investigations of the IR- and optical absorption spectra of blends MEH-PPV/fullerene derivative in solutions and in films showed no ground-state interaction between the components. Photoluminescence (PL) experiments, and photocurrent-voltage measurements were performed on model photovoltaic (PV) cells. We found that PL of MEH-PPV is completely quenched by a small admixture of fullerene derivative which assumes a high efficiency of charge separation in the composite material. The photocurrent in the PV device containing fullerene derivative is two orders of magnitude higher than that in pure MEH-PPV. An attempt to observe the magnetic field spin effect (MFSE) on the photocurrent in MEH-PPV/fullerene composites was made.     

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene)(MEH-PPV)/Nitrogen containing derivatives of fullerene composites: Optical characterization and application in flexible polymer solar cells

New soluble organofullerenes were synthesised by the reaction of organic amines and azides with the [60]fullerene. The comparative investigations of the IR- and optical absorption spectra of blends MEH-PPV/fullerene derivative in solutions and in films showed no ground-state interaction between the components. Photoluminescence (PL) experiments, and photocurrent-voltage measurements were performed on model photovoltaic (PV) cells. We found that PL of MEH-PPV is completely quenched by a small admixture of fullerene derivative which assumes a high efficiency of charge separation in the composite material. The photocurrent in the PV device containing fullerene derivative is two orders of magnitude higher than that in pure MEH-PPV. An attempt to observe the magnetic field spin effect (MFSE) on the photocurrent in MEH-PPV/fullerene composites was made.     

Counterion effects in cyanine heterojunction photovoltaic devices

We investigated cyanine heterojunction photovoltaic devices using carbocyanine dyes as donors and buckminsterfullerene (C60) as acceptor. In particular, we focused on the influence of cyanine counterions on the photovoltaic device characteristics. It was found that counterions can be displaced in the applied electric field and give rise to important hystereses in the current-voltage characteristics, which are related to charge injection processes at electrode and organic heterointerfaces. Mobile counterions have also a drastic effect on the photocurrent spectrum and are responsible for an important C60 contribution at the organic heterojunction between cyanine and C60. If the counterion is covalently linked to the cyanine dye, the C60 contribution in the blue spectral domain can not be observed.     

Lithium Intercalation into TlInSe<Subscript>2</Subscript> Crystals

p-Type TlInSe₂ crystals (TlSe chain structure) are electrochemically intercalated with lithium. Intercalation is found to raise the equilibrium conductivity of the crystals by 2.5 to 3 orders of magnitude at a charge passed through the cell as small as 0.025 C. After intercalation, however, no significant changes in equilibrium conductivity or photocurrent occur. The intercalation process is accompanied by the accumulation of lithium and selenium on the cathode face of the sample and selenium on the lateral faces. The experimental data are interpreted in terms of lithium and hole diffusion. The drift mobility and diffusion coefficient of Li⁺ ions in the chain structure of TlInSe₂ are estimated.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 910–914.Original Russian Text Copyright © 2005 by Alekseev.     

Photoelectrochemical behaviour of SnO2 thin-film electrodes prepared by ultrasonic spray pyrolysis

Photoelectrochemical behaviour of the SnO₂ thin film prepared by ultrasonic spray pyrolysis was studied as a function of deposition time, annealing time and phosphorus content. The photocurrent increased up to 10 min (1.16 m) with deposition time and then decreased. When the SnO₂ thin film was annealed for 10 min at 400°C in air, the maximum value of photocurrent was obtained. When phosphorus was added to the SnO₂, photocurrent was decreased continuously with increasing phosphorus content.     

Polymorphism and Crystal Growth of Phenylbutazone in Semisolid Preparations. Part One: Characterisation of Isolated Crystals from Commercial Creams of Phenylbutazone

It has been observed that a commercial product of phenylbutazone cream (o/w type) shows gritty appearance. Large Crystals up to 1000 μm in diameter have been isolated from unexpired batches. New batches from the same product show crystal size up to 150 μm in diameter. Investigation of the physical characters of the isolated crystals have been done in comparison with a pure phenylbutazone sample (99.5%). UV-Spectrophotometric analysis of the isolated crystals indicates phenylbutazone content of 90%. However, TLC examination and UV, IR and NMR Spectra examinations indicate the absence of decomposition products. Oily material has been separated from the isolated crystals by phosphate buffer solution (pH. 6.90).

DSC examinations of the isolated crystals show two endothermic peaks at 76°C and 100.1°C and one exothermic peak at 81°C. The endothermic peak at 76°C is interpreted as a result of the interaction between phenylbutazone and the oily material.

The examination of the isolated crystals before and after isothermal transformation and during melting by both hot stage microscope, ESM, (IR) and DSC indicates that the presence of phenylbutazone in the crystals is not a mechanical entrapment. It suggests that phenylbutazone interacts with the fatty base on the molecular level and both crystallise out from the cream.     

A study of optical absorption in 9,10-anthraquinone crystals

Single crystals of 9,10-anthraquinone having dimensions 5 mm × 5 mm × 1.5 mm were grown from the melt using the Bridgeman technique. The starting material was purified by sublimation and zone-refining techniques in specially, designed crystal-growth tubes. The crystals were transparent and yellow in colour, and could be cleaved easily along ab-, bc-planes and poorly along the ac-planes of the crystal. Thin layer chromatography of the final crystals was performed, and no impurity was detected in these crystals by this technique. The cleaved crystals were found to have a surface photocurrent of 10^–11 A, which increased non-linearly with the increasing voltage at room temperature. This is attributed to the formation of triplets in the bulk of the crystal which migrate and dissociate at the surface of the crystal. The optical absorption spectrum in benzene and ether solutions were found to exhibit similar behaviour in the visible range, i.e. 320–450 nm. From the optical absorbance data, the optical band gap was found to be 3.4 eV.     

An organic D-π-A dye for record efficiency solid-state sensitized heterojunction solar cells

The high molar absorption coefficient organic D-π-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm(-2)) in a solid-state dye-sensitized solar cell using 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.     

Field stability of piezoelectric shear properties in PIN-PMN-PT crystals under large drive field

The coercive fields (E(C)) of Pb(In?.?Nb?.?)O?-Pb(Mg(1/?)Nb(2/?)O?-PbTiO? (PIN-PMN-PT) ternary single crystals were found to be 5 kV/cm, double the value of binary Pb(Mg(1/?)Nb(2/?)O?-PbTiO? (PMNT) crystals, further increased to 6 to 9 kV/cm using Mn modifications. In addition to an increased EC, the acceptor modification resulted in the developed internal bias (E(int)), on the order of ~1 kV/cm. The piezoelectric shear properties of unmodified and Mn-modified PIN-PMN-PT crystals with various domain configurations were investigated. The shear piezoelectric coefficients and electromechanical coupling factors for different domain configurations were found to be >2000 pC/N and >0.85, respectively, with slightly reduced properties observed in Mn-modified tetragonal crystals. Fatigue/cycling tests performed on shearmode samples as a function of ac drive field level demonstrated that the allowable ac field levels (the maximum applied ac field before the occurrence of depolarization) were only ~2 kV/cm for unmodified crystals, less than half of their coercive field. Allowable ac drive levels were on the order of 4 to 6 kV/cm for Mn-modified crystals with rhombohedral/orthorhombic phase, further increased to 5 to 8 kV/cm in tetragonal crystals, because of their higher coercive fields. It is of particular interest that the allowable ac drive field level for Mn-modified crystals was found to be ≥ 60% of their coercive fields, because of the developed E(int), induced by the acceptor-oxygen vacancy defect dipoles.