Metal-free Phthalocyanine Thin Films Research Articles

The optical properties of a novel metal-free phthalocyanine

In the present work, 4-(4′-dodecyloxycarbonyl) phenoxy phthalonitrile was synthesized and then this phthalonitrile derivative was cyclotetramerized in dodecanol resulting a new metal-free phthalocyanine. The optical properties of this phthalocyanine were investigated. Novel metal-free phthalocyanine thin films were deposited on indium tin oxide-coated glass substrates by spinning method. Surface and microstructural properties of the films were characterized by atomic force microscopy and scanning electron microscopy. Both the transmittance and reflectance spectra of the deposited films were recorded using an NKD analyser. The optical band gap energy, the thickness of thin films, refractive index (n) and extinction coefficient (k) were calculated as 2.7 eV, 300 nm, 1.47 and 0.02, respectively.

Evaluation of charge transfer characteristics of metal-free phthalocyanine thin films by displacement current measurement

We have investigated phthalocyanine thin films using displacement current measurement. Two types of samples were prepared by vacuum evaporation method. One is the sample with a passivation film. The other is the sample without a passivation film. The passivation films were prepared by casting toluene solution containing 3 wt.% of polystyrene. The results obtained by measuring the samples elucidate the relation between carrier behavior and ambience. It is clarified that there are many traps in the sample without the passivation film. On the other hand, in the sample with the passivation film, traps are not observed. Therefore, it is concluded that the passivation film provides the constraint of oxygen doping as well as the preservation of carrier transfer system.

Gas Sensing Mechanism in Chemiresistive Cobalt and Metal-Free Phthalocyanine Thin Films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.

Structural and electrical properties of the α-form of metal-free phthalocyanine (α-H2Pc) semiconducting thin films

Abstract An X-ray structural study of thermally evaporated metal-free phthalocyanine thin films with various film thicknesses was performed. All samples studied had polycrystalline structure and the unit cell was found to be of the α-form. Variation of the deposition rate from 0.5 to 1 nm s −1 had little effect on the structure. The films exhibit preferential orientation at low thickness; however, at higher thickness they become less orientated as additional peaks appear in the spectrum. The increase in the intensity of the first significant low angle peak with increasing thickness is attributed to the increased volume of the crystal probed during the X-ray exposure. The current density–voltage ( J – V ) characteristics of α-H 2 Pc films sandwiched between two aluminum electrodes showed ohmic behavior at low voltages and space–charge-limited conduction (SCLC) at higher voltages. For comparison, similar measurements of the current density as a function of voltage were performed on zinc phthalocyanine, ZnPc, thin films using aluminum electrodes. The J – V characteristics showed ohmic behavior at low voltages followed by SCLC dominated by an exponential trap distribution at higher voltages. Consequently, in both H 2 Pc and ZnPc films, aluminum electrodes act as if they are ohmic contacts. The implied provision of ohmic contacts using aluminum in this case is attributed to the formation of a thin Al 2 O 3 layer during the deposition process.

Electrical and optical studies on metal-free phthalocyanine thin films

Electrical and optical studies on metal-free phthalocyanine thin films

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.

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.

Third-order Nonlinear Optical Properties of Octa-substituted Metal-free Phthalocyanine Thin Films

Third-order nonlinear optical susceptibility, X (3) of symmetrically octa-substituted metal-free phthalocyanine thin films measured by the third-harmonic generation technique are reported. The metal-free phthalocyanine has been found to show a X (3) (-3ω; ω,ω, ω) value as large as 7.73 x 10 -12 esu at 1.80 μm. The figure of merit, X (3) /α was estimated to be 4.17 x 10 -17 esu cm at 1.05 μm and 6.97 x 10 -16 esu cm at 1.65 μm. Both linear and third-order optical properties of liquid-crystalline metal-free phthalocyanines are discussed.

Photovoltaic and Catalytic Activity of Plasma-Polymerized Phthalocyanine Films

Abstract Polymeric metal (Cu, Mg, Zn, and Ni) and metal-free phthalocyanine thin films were prepared from the gas phase by low-temperature plasma polymerization. Electronic and FT-IR spectra indicated that no extensive alteration or destruction of the original ring structure had occurred. The obtained film showed good rectification, photovoltaic, and electro-chromic effects. The film also reduced methyl viologen under visible light, and its catalytic activity was as high as that of monomeric phthalocyanine particles. The formation mechanism and properties of the polymeric phthalocyanine films are discussed briefly.

Photoconduction in Metal-Free Phthalocyanine Films

By using pulsed photoconductivity techniques, transient action spectra have been recorded on vacuum-sublimed a metal-free phthalocyanine thin films in the wavelength range 350-700 nm. The transient photocurrents were determined at fields less than 107 V m-1 by range-limited transport of holes, the μT product of photogenerated holes being c. 2×10-14 m2 V-1. The electron μT product was about 100 times smaller. Carrier generation was a bulk phenomenon. The quantum efficiency was about 1-2% at fields up to 107 V m-1, and it was increased several fold by the presence of a layer of the acceptor pyromellitic dianhydride. Photogeneration in phthalocyanine occurred by an Onsager -type process. When pyromellitic dianhydride was present, an additional process of carrier generation, not of Onsager type, occurred, possibly associated with an exciton reaction with a charge-transfer complex between α metal-free phthalocyanine and pyromellitic dianhydride .

Photovoltage and stability of an n-type silicon semiconductor coated with metal or metal-free phthalocyanine thin films in aqueous redox solutions

An n-type silicon (n-Si) semiconductor coated with an evaporated thin film of metal phthalocyanine (MPc) or metal-free phthalocyanine (H/sub 2/Pc) worked as a fairly stable photoanode in aqueous redox solutions. The photovoltage observed for a photocell, (n-Si/CuPc/Fe/sup 3 +//Fe/sup 2 +/ aqueous solution (pH 4.2) /Pt), was 0.50 V, only slightly less than that for a p-n junction Si photocell (approx. 0.6 V). The action spectrum was similar to that of a bare n-Si electrode, except for a depression caused by photoabsorption by the CuPc film in the red region. The above wet photocell has current-voltage characteristics better than those for a solid photocell, (n-Si/CuPc/Pd).

Singlet–triplet fusion in metal-free phthalocyanine

Fluorescence quenching has been observed in 10−4 to 10−7M solutions of metal-free phthalocyanine in 1-chloronaphthalene at 77°K. This quenching is attributed to singlet–triplet fusion. A triplet lifetime of 200 μsec was measured at 77°K. A dipole–dipole interaction formulation for singlet excitation migration was adapted to the singlet–triplet fusion process. A critical distance of about 200 Å was computed for the system under study. Unlike in singlet excitation migration, this critical distance depends on the experimental illumination conditions. If singlet–triplet fusion plays a role in photogeneration of charge carriers in metal-free phthalocyanine thin films, a photocurrent linear in light intensity, in the absence of recombination and trapping effects, can be understood.