Films Of Metal-free Phthalocyanine Research Articles

Enhancement of the electrical characteristics of metal-free phthalocyanine films using cold isostatic pressing

Spatial gaps between grains and other grains, substrates, or electrodes in organic electronic devices are one of the causes of the reduction in the electrical characteristics. In this study, we demonstrate that cold isostatic pressing (CIP) is an effective method to crush the gaps and enhance the electrical characteristics. CIP of metal-free phthalocyanine (H2PC) films induced a decrease in the film thickness by 34%–40% because of the gap crush. The connection of smaller grains into a larger grain and planarization of the film surface were also observed in the CIP film. The crystal axes of the H2PC crystallites were rearranged from the a-axis to the c-axis of the α-phase crystal structure in a direction perpendicular to the substrate by CIP, indicating favorable hole injection and transport in this direction because of a better overlap of π orbitals. Thermally stimulated current measurements showed that deep hole traps disappeared and the total hole-trap density decreased after CIP. These CIP-induced changes of the film thicknesses, crystal axes and the hole traps lead to a marked increase in the hole mobility of the H2PC films from 2.0 × 10−7 to 4.0 × 10−4 cm2/V s by 2000 times in the perpendicular direction. We believe that these findings are important for unveiling the underlying carrier injection and transport mechanisms of organic films and for enhancing the performance of future organic electronic devices.

Ionization sensitization of doping in co-deposited organic semiconductor films

Sensitization of the dopant ionization in co-deposited films of organic semiconductors was found. The ionization rate of cesium carbonate (Cs2CO3), which acts as a donor dopant in single films of metal-free phthalocyanine (H2Pc) and fullerene (C60), was increased from 10% to 97% in a H2Pc:C60 co-deposited film. A charge separation superlattice model that includes electron transfer from the conduction band of H2Pc to that of C60, which increases the rate of dopant ionization, is proposed.

Space charge limited hole transport in evaporated thin films of α-H2Pc

Thin films of metal-free phthalocyanine (H2Pc) have been grown on different substrates via physical vapor deposition. The films grown at room temperature have been shown to have α-polymorph, which was converted into β-phase when the films were subjected to thermal treatment at 608 K for 2 h. The phase transition was confirmed via extensive characterization of the treated and untreated films. This paper describes hole transport investigations in α-H2Pc thin films for which the samples were prepared in hole only configuration and current density–voltage (J–V) characteristics were studied at different temperatures for two different thicknesses of α-H2Pc. The investigations show that the hole transport in α-H2Pc is governed by space charge limited conduction with single-level traps in the energy space. The studies enabled us to calculate the hole mobility in H2Pc which was observed to be independent of temperature and electric field.

Phase and molecular orientation in metal-free phthalocyanine films on conducting glass: Characterization of two deposition methods

In this study, metal-free phthalocyanine has been deposited on a conducting glass surface by two methods: by spreading the molecular powder directly on the substrate in air and by vapor sublimation under ultra-high vacuum conditions (evaporation). The films have been characterized by means of core level X-ray Photoemission Spectroscopy, X-ray Absorption Spectroscopy (XAS) and Ultra Violet and Visible absorption spectroscopy (UV-Vis). Our results show that the two deposition methods produce molecular overlayers in different polymorphic phases; the UV-Vis measurements indicate that the film obtained by powder deposition is of x-phase type whereas sublimation leads to an α-polymorph structure. The XAS results show that in the powder deposited film the molecules are mainly oriented parallel to the surface. This is opposite to the case of the vapor deposited film, where the molecules mainly are oriented orthogonal to the surface.

Luminescence of Phthalocyanine Thin Films

The near-IR luminescence in thin films of metal-free phthalocyanine and phthalocyanine complexes is investigated at room temperature. It is shown that the intensity of the luminescence peaks depends on the polymorphic modification and the structure of the complexes, whereas the peak positions remain virtually unchanged.

Determination of the anisotropic dielectric function for metal free phthalocyanine thin films

Thin films of metal free phthalocyanine (H 2Pc) were grown by organic molecular beam deposition (OMBD) in high vacuum (HV) simultaneously on different substrates. The substrates (hydrogen passivated Si(111) and NaCl) were kept at room temperature during the deposition. From the absorption spectra in the visible and ultraviolet region the films are found to consist predominantly of the α crystalline phase. All the films are uniaxial and strongly optically anisotropic. The dielectric function for H 2Pc films was obtained in the spectral range of 0.7–4.5 eV from the simulation of ellipsometry spectra. The orientation of the molecules with respect to the substrate is derived from the difference between the in-plane and out-of-plane components of the dielectric function and the results of infrared (IR) measurements.

Unoccupied electronic states in phthalocyanine thin films studied by inverse photoemission spectroscopy

Inverse photoemission (IPE) spectra of thin films of metal-free phthalocyanine (H 2Pc), 3d-metal phthalocyanines (MPcs: M=Mn, Fe, Co, Ni, Cu and Zn) and lithium phthalocyanine (LiPc) have been measured to directly observe the electronic structures of their unoccupied states. In particular, IPE spectra of MPcs obtained systematically for 3d metals have been studied with the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states focused on. The difference spectra between MPcs (M=Mn, Fe, Co, Ni and Cu) and ZnPc as the reference are compared with the reported results from X-ray absorption spectroscopy as well as extended Hückel and density functional calculations to show that they reflect the density of unoccupied states derived from the central metals with permitting tentative assignments of their features.

Photovoltaic Properties of Ultramicrostructure-Controlled Organic Co-Deposited Films

Co-deposited films of metal-free phthalocyanine and perylene pigments evaporated on a cooled substrate were found to generate a considerably large photocurrent. These films have an ultramicrostructure consisting of nanocrystals of perylene pigment covered with amorphous phthalocyanine. The efficient photocurrent generation can be explained by the large area of contact between the two pigments and the formation of individual routes for both photogenerated electrons and holes. A power conversion efficiency of 1% was observed in a three-layered cell incorporating a co-deposited interlayer fabricated under low-temperature conditions.

Trapping of Charge Carriers in Organic Molecular Materials: Phthalocyanine Thin Films Revisited

The technique of temperature-modulated space-charge-limited currents was employed to study the energetic distribution of local states in thin films of metal-free phthalocyanine. Four well-defined local levels were detected within the energy range 0.5-0.3 eV, accompanied by an onset to a manifold of shallower states. The morphology of the films, depending on the deposition rate and changing from polycrystalline to nearly amorphous, affects the densities of local states but not their energies.

AC conductivity and dielectric measurements of metal-free phthalocyanine thin films dispersed in polycarbonate

The dielectric constant and the dielectric loss of thin films of metal-free phthalocyanine dispersed in polycarbonate using ohmic gold electrodes are investigated in the frequency range 20–10 5 Hz and within the temperature range 300–388 K. The frequency dependence of the impedance spectra plotted in the complex plane shows semicircles. The Cole–Cole diagrams have been used to determine the molecular relaxation time, τ, The temperature dependence of τ is expressed by thermally activated process. The AC conductivity σ AC (ω) is found to vary as ω s with the index s⩽1, indicating a dominant hopping process at low temperatures. From the temperature dependence of AC conductivity, free carrier conduction with mean activation energy of 0.33 eV is observed at higher temperatures. Capacitance and loss tangent are found to decrease with increasing frequency and increase with increasing temperature. Such characteristics are found to be in good qualitative agreement with existing equivalent circuit model assuming ohmic contacts.

Thin film properties and surface morphology of metal free phthalocyanine films grown by organic molecular beam deposition

The structure, optical properties and surface morphology of thin films of metal free phthalocyanine (H2Pc) deposited in an ultra-high vacuum environment by organic molecular beam deposition have been studied using a variety of ex-situ techniques. The growth conditions have a strong influence on the properties of the films. H2Pc undergoes a phase transition (α→β) at a deposition temperature of ∽330°C, or upon post annealing a film grown at room temperature. Both the structure and optical properties of the films change and powder X-ray diffraction, electronic absorption spectroscopy, Raman and photoluminescence spectroscopies are used to characterise the differences between the two phases. Atomic force microscopy and Nomarski interference microscopy show that the lower temperature α-phase is characterised by a smooth morphology with spherical islands that show no apparent long-range order. By contrast, the β-phase has a much greater root mean square roughness and long thin needle-like crystals are observed on the surface of the films. The morphology of the β-phase depends on the method of preparation and there are two distinct types, β1 and β2. The crystallites show a preferential orientation and alignment with respect to each other for growth at room temperature followed by annealing (β1), but are randomly oriented for films grown at elevated substrate temperatures (β2).

Space-charge-limited conductivity in evaporated α-form metal-free phthalocyanine thin films

The electrical properties of thermally evaporated thin films of metal-free phthalocyanine (H 2Pc), having gold electrodes, have been investigated. Capacitance measurements as a function of film thickness at a standard frequency of 1 kHz showed the usual geometric dependence, with a relative permittivity value of ε r = 2.9. Room temperature dc current density-voltage ( J-V) measurements showed a linear ohmic dependence at low voltages, followed by a power-law dependence, of exponent 4, at higher voltage levels. These results confirm that gold acts as an ohmic contact to H 2Pc, and indicate space-charge-limited conductivity (SCLC) at higher voltages. Measurements of current density as a function of inverse temperature at constant applied voltage yielded a hole mobility value μ = 6.25 × 10 −7 m 2 V −1 s −1. In conjunction with the room temperature J-V data this gave a value of p 0 = 8 × 10 18 m −3 for the hole concentration. The SCLC at higher voltages was dominated by an exponential distribution of traps, P( E) = P 0 exp(− E/ KT t , where P 0 = 1.29 × 10 46 J −1 m −3, T t = 900 K and total trap concentration N t = 1.6 × 10 26 m −3. The results suggested desorption of oxygen, regrowth of the phthalocyanine and the appearance of crystal defects during heat treatment.

Silicon-based surface plasmon resonance combined with surface-enhanced Raman scattering for chemical sensing

A Si-based surface plasmon resonance (SPR) technique has been successfully applied to NO2 sensing at ppm level, with estimated detectability at less than 100 ppb. Surface-enhanced Raman scattering (SERS) has been used in this scheme as an inherent additional data acquisition channel capable of providing spectroscopic selectivity and amplified sensitivity. The behavior of both the SERS spectrum and the SPR-induced photosignal produced by Au-on-Si grating structures coated with thin 18-crown-6 metal-free phthalocyanine films was simultaneously recorded for exposure of the films to 10 ppm of NO2 in air and its reversal in clean air. Both responses have been found to be reversible. The combination of Si-based SPR and SERS looks promising for thin-film and surface explorations, both in fundamental and sensor applications.

Determination of two-dimensional structures of ultrathin films of H 2 and Cu phthalocyanine on MoS 2 by angle-resolved ultraviolet photoemission and low energy electron diffraction

The angle-resolved ultraviolet photoelectron spectra of ultrathin films of metal-free phthalocyanine (H 2Pc) and copper phthalocyanine (CuPc) on MoS 2 single crystal surfaces were measured using synchrotron radiation. The take-off and azimuthal angle dependencies of the photoelectron intensity were analyzed using independent-atomic-center approximation combined with molecular orbital calculation. Further, the low energy electron diffraction was observed for the same ultrathin films. The results indicate that H 2Pc and CuPc molecules form a square lattice with respect to the three equivalent crystal axes of the MoS 2 surface, and at the lattice point lie flat on the substrate with a molecular azimuthal rotation of about 7°

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.

Structure and photoelectrical behaviour of vacuum-evaporated metal-free phthalocyanine films

Vacuum-evaporated metal-free α-phthalocyanine films sandwiched between two gold electrodes have been prepared and their photoelectrical properties studied in vacuo (5 X 10 −4 Pa). The density of the films depends on their thickness: the thicker the film, the lower the mean density of the material. The geometrical non-uniformity along the direction perpendicular to the substrate plane results in the formation of a spatial trap distribution which influences the built-in field and the magnitude of the open circuit voltage induced by sample irradiation.

Use of LINbO 3 for Design of Device-type Catalysts with Activity Controllable Functions

A poled ferroelectric LiNbO 3 single crystal with high spontaneous polarization was employed as a catalyst support, and the static and dynamic effects of the polar substrate surfaces on the adsorptive and catalytic properties of thin film catalysts deposited were shown. For the static effect, it was demonstrated that the surface conductivity of copper and metal-free phthalocyanine films increased by the adsorption of NO gas, to a larger extent, when the substrate was the negative polar surface, compared to the positive one. The role of the polar substrates is explained In terms of a band bending model. For the dynamic effect, the Rayleigh surface acoustic wave generated on the LiNbO 3 substrate was shown to enhance the catalytic activity for ethanol oxidation on Cu and Ag thin film catalysts deposited on the propagation path. It is proposed that the catalyst activation is mainly due to an electric Held produced. The use of poled LiNbO 3 is suggested to be effective for the development of a device-type catalyst with activity controllable functions.

X-ray photoelectron spectroscopy and dark DC conductivity of argon-ion implanted organic films over a wide temperature range

Films of metal-free phthalocyanine (MFPC) and 1,8-dihydrodibenzo(b,i)-(1,4,8,11)tetra-aza-(14)-annulene (TAA) were studied. The compounds were deposited by vacuum sublimation on to: (i) quartz substrates for UV-vis spectroscopy; (ii) Teflon disks for reflection measurements; (iii) stainless steel stubs for X-ray photoelectron spectroscopy (XPS); and (iv) interdigitated platinum electrodes on alumina for electrical conductivity measurements. The transmission UV, visible and IR spectra of the compounds were unchanged following bombardment by 10 kV argon ions, indicating that any damage caused was confined to only a thin surface layer. The x-ray photoelectron spectra of both of the compounds were substantially modified following bombardment by 10 keV argon ions. In both cases the hydrocarbon-type carbon levels increased, the intensities of the nitrogen peaks were decreased, and a new peak due to implanted argon was observed, indicating severe damage to the surface of the films. The electrical conductivities of all samples exhibited increases of up to six orders of magnitude following argon-ion exposure. On the basis of the XPS results and the form of the conductivity against temperature curves, it was concluded that the damaged surface was composed substantially of amorphous carbon.

Variable angle spectroscopic ellipsometry studies of oriented phthalocyanine films

Thin films of metal-free phthalocyanine (H2Pc), vapor deposited onto sputtered copper substrates, are characterized using a fully automated Woollam variable angle spectroscopic ellipsometer. Data from over a dozen H2Pc films in the thickness range of 1000 to 1140 Å were acquired over a 4000–8000 Å wavelength range in 100 Å steps, and 45°–79° incidence angle range in 2° steps. Two classes of H2Pc films were formed, uniaxially oriented with the molecular stacking b axis in either a standing or surface-parallel configuration, by carrying out the deposition at two different substrate temperatures and deposition rates. Reflection absorption infrared spectroscopy was used to verify the different orientations. It is observed that the three dimensional plots of the ellipsometric parameters Δ and Ψ versus wavelength and incidence angle have different characteristic shapes for the two classes of oriented films, implying anisotropic dielectric functions. Using a multilayer optical modeling code capable of treating fully anisotropic films, we attempt to determine the in-plane and out-of-plane optical constants for each of these ‘‘orthogonally’’ oriented film types. Significant differences in the optical constant spectral solutions are found for the two classes when treated with isotropic film models, but convergent solutions of the direct anisotropic regression analyses have not been found. However, due to the high degree of uniaxial ordering in the two film classes, it is possible to extract approximate solutions to the parallel and perpendicular optical constants taken with respect to the crystalline b axis.

Dynamics of third-order nonlinear optical processes in Langmuir–Blodgett and evaporated films of phthalocyanines

The results of degenerate four-wave mixing studies performed on evaporated films of metal-free phthalocyanine and Langmuir–Blodgett films of a silicon phthalocyanine using subpicosecond 602 nm pulses are reported. The third-order nonlinearity of all the samples is high permitting the observation of the degenerate four-wave mixing signal from even a monolayer. In both cases, the third-order nonlinearity has a resonant character and the observed time-resolved phase conjugate signals provide information on the dynamics of excitons. Simultaneously monitoring first- and second-order diffractions from the transient gratings formed in our samples allowed us to obtain more-detailed information on the factors influencing the temporal behavior of the nonlinear response. It is concluded that the dynamics of excitons determining the nonlinearity is mainly governed by the presence of bimolecular exciton–exciton interaction. Saturation behavior is observed at high light intensities.