Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Photoconductivity in thin films of phthalocyanine

The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H₂Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H₂Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.     

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.     

Physical and optical properties of phthalocyanine doped inorganic glasses

Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped glass matrix are reported for the first time. Absorption spectral measurements of H₂Pc, MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV region to obtain the optical band gap (E_g) and the width of the band tail (E_t). Other important optical and physical parameters viz. refractive index (n), molar extinction coefficient (), density (), glass transition temperature (T_g), molecular concentration (N), polaron radius (r_p), intermolecular separation (R), molar refractivity (R_m) are also reported.     

Conduction mechanism in anthracene as a function of temperature

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.     

Impurities in phthalocyanines and structural modifications induced during thih-film deposition

The impurity contents of H₂Pc, CuPc and FePc powders have been determined by neutron activation analysis. It is shown that H₂Pc contains a much higher amount of impurities than expected, especially highly conducting iodine. Their contents are only slightly decreased by annealing in air at 300°C for 2 h. The X-ray diffraction spectrum of H₂Pc indicates that the iodine and calcium containing impurities are partially crystalline. Vacuum evaporation and plasma discharge deposition of H₂Pc yield almost non-crystalline phthalocyanine which is not modified by subsequent annealing at 300°C. These two thin-film deposition techniques also significantly reduce the degree of crystallinity of the impurities. The influence of the impurities and the specific crystalline structures on the electrical properties of phthalocyanines is also briefly discussed.     

Purification and characterization of phthalocyanines

The technical details of a simple train sublimation (carrier gas) system for purification of organic materials are given and the refining procedure is described. A study of the effects of purification on metal free phthalocyanine (H₂Pc), magnesium phthalocyanine (MgPc), copper phthalocyanine (CuPc), vanadyl phthalocyanine (VOPc), zinc phthalocyanine (ZnPc) and lead phthalocyanine (PbPc) was carried out by means of optical absorption, X-ray diffraction, electron paramagnetic resonance, sublimation behaviour and photovoltaic data. It was found that train sublimation improves the quality of the pigments considerably. Spectroscopic results indicate the presence of a second phase in the purified MgPc.     

Study on the characteristics and relative properties of Langmuir–Blodgett films based on substituted bis[phthalocyaninato] rare earth(III) complexes

A new kind of sandwich-like bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] rare earth(III) complexes RE(Pc*)₂ (Pc*=Pc(OC₈H₁₇)₈, RE=Er, Sm, Tb) are used as film-forming materials. Pure RE(Pc*)₂ ultrathin films and RE(Pc*)₂ and octadecanol (OA)/stearic acid (SA) mixed ultrathin films were prepared by the Langmuir–Blodgett (LB) technique. The LB films were characterized by ultraviolet/visible (UV/Vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). The NO₂ gas-sensing properties of Sm(Pc*)₂/OA LB film are studied compared with that of (i-PrO)₄CuPc LB film by XPS spectra and valence band spectra. It is found that the interaction between Sm(Pc*)₂ LB film and NO₂ gas is stronger than that between (i-PrO)₄CuPc LB film and NO₂ gas. Experimental results indicate that bis(phthalocyaninato) rare earth(III) complexes show more remarkable sensitivity than monophthalocyanine, which can be used in high-sensitive gas sensor applications. At the same time, the mechanism of sensitivity of Sm(Pc*)₂/OA mixed LB film to NO₂ gas is also studied in this paper.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Effects of oxidizing gases on semiconductivity and thermal stability of phthalocyanine thin films

The effects of oxidizing gases (O₂ and NO₂) on the p-type semiconductivity of the phthalocyanines H₂Pc, PbPc, ZnPc, CuPc, AlPcCl, AlPc-O-AlPc and (AlPc) _n films treated in nitrogen at 350 ° C have been examined. The conductance increased in the presence of oxidizing gases. Reversible effects of oxidizing gases on the conductance were observed for PbPc, CuPc and ZnPc, all of which are thermally and/or chemically less stable than the other Pcs. For the latter, the conductance in nitrogen increased permanently after exposure to NO₂. The reversibility of the former may be related to the thermal and chemical stability of neutral and/or oxidized phthalocyanines. The conductivity is described byG =G ₀ exp (E/kT) The activation energy,E, decreased in the presence of oxidizing gases. The pre-exponential factor,G ₀, was lowered by the introduction of NO₂ especially for PbPc, CuPc and ZnPc. It seems that the adsorption of NO₂ is limited to the surface of fine crystals and the increase in the conductance caused by exposure to NO₂ is strongly influenced by the crystallinity of phthalocyanine.     

Deep electron trap level in semi-insulating GaAs

The experimental data on the Hall measurements have been used to characterize deep electron trapping levels in Cr doped semi-insulating GaAs crystals. The energy of the level below the conduction band edge has been found to be ∼ 0·8 eV and is thought to be related to Ga vacancies in the host crystal and Cr impurities.     

Space-charge-limited currents and carrier trapping in plasma-polymerized malononitrile films

The dark electrical conductivity of plasma-polymerized malononitrile (PPMa) films in an Ag/PPMa/Ag sandwich structure was investigated over the temperature range 300–525 K at a reduced pressure of 10^-5 Torr. The room temperature current-voltage characteristics indicate space-charge-limited currents. The results for the conductivity as a function of inverse temperature are in accordance with the band model proposed by Barbe and Westgate, and the thermal energy gap between the top of the valence band and the bottom of the conduction band was determined to be 0.86±0.01 eV. At temperatures below about 380±5 K the conduction process is consistent with the presence of an electron trapping level situated 0.34±0.05 eV below the conduction band edge with a density of (5.02±0.05) × 10¹⁶ cm^-3. It is assumed that above about 435±5 K the conduction process is intrinsic. Experiments on the chemisorption of oxygen suggest that electrons are the majority carriers in PPMa films.     

Preparation and characterization of flash-evaporated CuInSe2 thin films

Thin films of CuInSe₂ have been evaporated onto glass substrates by flash evaporation. The as-deposited films are amorphous and annealing in selenium atmosphere produces polycrystalline films. The films were characterized bytem and x-ray diffraction techniques. The optical absorption of the films shows three energy gaps of 1·03, 1·07 and 1·22 eV. The crystal field and spin-orbit splitting are thus found to be 0·04 eV and 0·16 eV respectively. The percentaged-character of the valence band states is ∼35%. The Arrhenius plot of electrical conductivity of films showed impurity ionization ofE _A = 75 meV.     

Surface potential measurement of organic photo-diode consisting of fullerene/copper phthalocyanine double layered device

We investigated the surface potential built across the electrode/fullerene (C₆₀) or copper phthalocyanine (CuPc) interface and C₆₀/CuPc interface as a function of the thickness of the semiconductor film in the dark condition and under illumination. The surface potential of C₆₀ on Au, Al and Mg changes negatively with the increment of film thickness and it saturates at − 0.25, − 1.0 and − 1.5 V within 20 nm. The Fermi level alignment at C₆₀/electrode interface is established within ∼ 20 nm from electrode, and very high electric field exists due to the displacement of negative electronic charges from electrode into C₆₀. On the other hand, the surface potential of CuPc on ITO changes to + 0.1 V, and the work functions of C₆₀ and CuPc were estimated as 5.0 eV and 4.7 eV. C₆₀ film also accepts electrons from CuPc at hetero-junction interface, and the Fermi-level alignment was again obtained at C₆₀/CuPc interface under illumination. The built-in potential of ca. 0.3 V formed at C₆₀/CuPc interface was considered as the origin of the reduction of open-circuit voltage in ITO/CuPc/C₆₀/Au device compared with the optimum value of 0.6 V. On the other hand, the very high electric field formed at C₆₀/Mg contact improved the photovoltaic properties.     

Effect of oxygen on tuning the TiNx metal gate work function on LaLuO3

This paper presents experimental evidence on effective work function tuning due to the presence of oxygen at the TiNx/LaLuO₃ interface. Two complementary techniques, internal photoemission and X-ray photoelectron spectroscopy, show good agreement on the position of the metal gate Fermi level to conduction (2.79 ± 0.25 eV) and valence (2.65 ± 0.08 eV) band edge for TiNx/bulk LaLuO₃ gate stacks. The chemical shifts of Ti2p and N1s core levels and different degree in ionicity of TiNx metal gates correlate with the observed valence band offset shifts. The results have significance for setting the band edge work function and resulting low threshold voltage for ultimately scaled LaLuO₃-based p-metal oxide semiconductor field effect transistor devices.     

Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition

Tin disulfide (SnS₂) hexagonal flakes with diameters in the range of 50−150 nm are synthesized by using SnCl₂.2H₂O and sodium diethyldithiocabamate as source materials via a solvothermal decomposition route. As-prepared SnS₂ hexagonal nanoflakes are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and ultraviolet-visible (UV-vis) spectroscopy. The band gap energy of the SnS₂ nanoflakes is measured to be 2.17 eV, and the conduction band (CB) and valence band (VB) levels of the SnS₂ nanoflakes are calculated to be − 4.34 eV and − 6.55 eV respectively, showing them to be suitable for optional and electronic applications.     

Preparation of AgInS2 chalcopyrite thin films by chemical spray pyrolysis

AgInS₂ thin films were prepared by the spray pyrolysis technique using a water/ethanol solution containing silver acetate, indium chloride and thiourea. We reported our results on the characterization of tetragonal AgInS₂ (chalcopyrite type) films, which were grown from indium deficient spraying solution. The films displayed a n-type conductivity with room temperature resistivities in the range between 10³ and 10⁴ Ω cm. The absorption spectra of sprayed films revealed two direct band-gaps with characteristic energies around 1.87 and 2.01 eV, which are in good agreement with the reported energy values for interband transitions from the split p-like valence band to the s-like conduction band in tetragonal AgInS₂ single crystals.     

Electronic absorption band broadening and surface roughening of phthalocyanine double layers by saturated solvent vapor treatment

Variations in the electronic absorption (EA) and surface morphology of three types of phthalocyanine (Pc) thin film systems, i.e. copper phthalocyanine (CuPc) single layer, zinc phthalocyanine (ZnPc) single layer, and ZnPc on CuPc (CuPc/ZnPc) double layer film, treated with saturated acetone vapor were investigated. For the treated CuPc single layer film, the surface roughness slightly increased and bundles of nanorods were formed, while the EA varied little. In contrast, for the ZnPc single layer film, the relatively high solubility of ZnPc led to a considerable shift in the absorption bands as well as a large increase in the surface roughness and formation of long and wide nano-beams, indicating a part of the ZnPc molecules dissolved in acetone, which altered their molecular stacking. For the CuPc/ZnPc film, the saturated acetone vapor treatment resulted in morphological changes in mainly the upper ZnPc layer due to the significantly low solubility of the underlying CuPc layer. The treatment also broadened the EA band, which involved a combination of unchanged CuPc and changed ZnPc absorption.     

Influence of ambient air exposure on surface chemistry and electronic properties of thin copper phthalocyanine sensing layers

In this paper we present photoemission studies of the influence of 12-hour exposure to the ambient air on the chemical and electronic properties of thin 16-nm copper phthalocyanine (CuPc) sensing layers deposited on n- and p-type silicon Si(111) substrates covered with the native oxide. The surface chemistry and electronic parameters of organic thin film including surface band bending, work function, electron affinity and their variations upon the exposure have been monitored with X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy techniques. We found that after the exposure, the surface chemistry of CuPc remained unaffected, however the work function and surface band bending increased by 0.55 eV and 0.45 eV for the layers on n-Si and by 0.25 eV and 0.30 eV for those on p-Si. Additionally, we detected a slight surface dipole at CuPc on n-Si manifested by a small shift in electron affinity of 0.10 eV. In order to explain these changes we developed a model basing on the interaction of ionic species with the phthalocyanine surface.     

CпEктп ЭЛEКтPOЛюмИнECЦEнЦИИ мOнoКпнCтAЛЛ AgGaS2

The paper deals with the mechanism of electroluminescent emission observed for the first time in AgGaS₂ single crystals. The recombination radiation is due to the transitions from the level located at 0.368 eV below the conduction band T₆ to the corresponding valence bands (peaks P₁ and P₄) and to the transitions of electrons from the conduction band to the local levels at 0.446 eV and 0.486 eV from the top of the valence band T₇ (peaks P₂ and P₃) respectively.