Copper Phthalocyanine Thin Films Research Articles

Copper phthalocyanine nanoparticles and nanoflowers

Molecular organization of copper phthalocyanine (CuPc) thin films deposited at room temperature (30 °C) on quartz and post annealed gold-coated quartz substrates have been studied. The thin films have been characterized by optical absorption, X-ray diffraction, atomic force microscopy and field emission scanning electron microscopy (FESEM). The FESEM images have shown densely packed nanoparticles and nanoflower like structure for the gold-coated quartz substrates annealed at 500 and 750 °C, respectively. The fractal dimension of the assembly of nanostructures in the films has been estimated from FESEM images. The effect of substrates on the surface morphology, optical properties and the fractal dimension have been discussed.

Frontier Electronic Structures in Fluorinated Copper Phthalocyanine Thin Films Studied Using Ultraviolet and Inverse Photoemission Spectroscopies

Ultraviolet photoemission and inverse photoemission spectroscopy was used to compare the electronic structure of copper phthalocyanine, copper octafluorophthalocyanine, and copper hexadecafluorophthalocyanine thin films. A rigid shift to lower energy was observed for both the occupied and unoccupied electronic states as the number of fluorine atoms around the phthalocyanine ring increased. The spectral features of the fluorine-substituted derivatives were otherwise very similar to copper phthalocyanine, and no change in the transport gap energy was observed. Vacuum level shifts were observed at the interface with polycrystalline gold substrates of sufficient magnitude to consistently pin the substrate Fermi level near the middle of the HOMO-LUMO gap. The calculated barrier heights for electron and hole injection across the interface were therefore equal, and no correlation with fluorine substitution was found.

VUV Induced Doping of Cu-Phthalocyanine Thin Films: A Possibility of n-Type Doping

The new type doping method, VUV-induced doping, has been applied to copper phthalocyanine (CuPc) thin film. The highest occupied molecular orbital (HOMO) peak in the ultraviolet photoelectron spectrum of CuPc film was gradually shifted toward high binding energy side upon the irradiation of He Iα radiation. Furthermore, the radiation induced gap states were observed up to the Fermi level. The shift of the HOMO peak in the low-dose region depends in a semi-logarithmic fashion on the irradiation time, indicating that the intentional n-type doping of CuPc film can be realized by VUV irradiation.

Thickness Dependent Characteristics of a Copper Phthalocyanine Thin-Film Transistor Investigated by in situ FET Measurement System

The influence of active layer thickness on transport characteristics of a copper phthalocyanine thin-film transistor was investigated by using the in situ FET measurement system with the film deposition continued up to several hundred nm in thickness. The drain current and mobility showed maximum values in the early stage of film growth and then decreased with the increasing film thickness. This result suggests that the over-grown layer affects the transport characteristics of the conductive accumulation layer, for example, owing to change of the electric field in the device.

Concept and demonstration of all organic Gratzel solar cell (dye sensitized solar cell)

The authors present the concept of “all organic Gratzel/dye sensitized solar cell” and demonstrate such a device using high Tauc band gap (above 2.5eV) amorphous carbon thin films doped with nitrogen (n type) deposited by microwave assisted surface wave plasma chemical vapor deposition and sensitized with copper-phthalocyanine thin films. Open circuit voltage and short circuit current density obtained are about 0.53V (versus Ag∕AgCl, reference electrode) and 8.52×10−6A∕cm2, respectively. The mechanism of photovoltaic action in such cells may be similar to dye sensitized photoelectrochemical cells using nano-TiO2∕ZnO porous thick films.

Phase transition of copper(II) phthalocyanine thin films characterized by a near-field microwave microprobe

Metal phthalocyanine is one of the promising organic compounds due to the possibility of applications in electrooptical devices, photoconducting agents, photovoltaic cell elements, nonlinear optics, electrocatalysis, and other photoelectronic devices. Several metal substituted phthalocyanines have been widely investigated. Among the metal substituted phthalocyanines, copper (II) phthalocyanine (Cu-Pc) has been found to have superior properties. Cu-Pc is a p-type semiconductor and mobility changes could be achieved by employing controlled substrate heating temperature during deposition. The crystal phase and the grain orientation of Cu-Pc depend on the heat-treatment temperatures and substrate heating conditions [1]. In order to better characterize the electrical properties of theαand the β-phase of Cu-Pc thin films, we take advantage of the nondestructive evaluation capabilities of a near-field microwave microprobe (NFMM) [2]. NFMM techniques with high sensitivity have been developed for the microwaveand millimeter-wave ranges. An important ability of the NFMM is nondestructive and contactless characterization of thin films, in particular, the characterization of electrical properties of organic thin films. Organic multilayer thin films have attracted considerable interest in regard to various display applications. Nondestructive and contactless characterization techniques are very useful for these applications. NSMM technique, which directly measures the physical properties such as surface resistance of organic thin films shows practical promise. To study the phase transition of Cu-Pc thin films, wemeasured the surface resistance using a NFMM by measuring the microwave reflection coefficient S11. The crystal structure of Cu-Pc thin films transformed from the α-phase of the orthorhombic crystal to the thermally stable β-phase of themonoclinic crystal as the substrate heating temperatures increased. The surface resistance depended on the crystal structures of the Cu-Pc thin films. As the phase changed from the α-phase to the β-phase, the surface resistance of the Cu-Pc thin films decreased.

An investigation on air stability of copper phthalocyanine-based organic thin-film transistors and device encapsulation

The device performances of copper phthalocyanine (CuPc)-based organic thin-film transistors (OTFTs) in main components of air were studied. We found that the device stored in O2 humidified by water exhibited the changes of electric characteristics including positive-shifted threshold voltage and lower Ion/Ioff but unchanged mobility, which was similar to the device exposed to room air. These changes are attributed to O2 doping to copper phthalocyanine thin film assisted by water. Furthermore, a cross-linked polyvinyl alcohol film was used as encapsulation layer to prevent the permeation of O2 and water, which resulted in excellent stability even when devices were placed in air for over a year. Therefore, current studies will push the development of OTFTs for practical applications.

Optical absorption studies of copper phthalocyanine thin films

The optical absorption of thermally evaporated copper phthalocyanine (CuPc) was studied in the UV-VIS-NIR region. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthalocyanine molecules, namely, the Soret ( B) and the Q-band . The Q-band shows its characteristic splitting (Davydov splitting) of the main absorption peak in the metal phthalocyanine correlates with the relative tendencies of the metal to out-of-plane bonding. Some of the important spectral characteristics such as the molar extinction coefficient (ε molar), the oscillator strength ( f), the electric dipole strength ( q 2 ) and absorption half-bandwidth (Δ λ) of the principle optical transitions were evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.95±0.03 and 1.55±0.02 eV. The spectra of the infrared absorption allow characterization of vibration modes for the powder, as-deposited material and thin films of CuPc annealed at 423 K for two hours. Discussion of the obtained results and their comparison with the previous published data are also given.

Absorption spectra of organic semiconductors in IR-range measured by constant photocurrent method

Constant photocurrent method has been used to study spectral dependence of the absorption coefficient in copper phthalocyanine thin films formed by the molecules with different peripheral groups. Spectral dependence of the absorption coefficient reveals exponential growth in the energy range 1.4–1.6eV and a series of three bands in the energy below 1.4eV. The maximum observed at 1.12eV is attributed to optical transition from ground state S0 to excited triplet state T1 of a molecule. This transition results in creation of triplet excitons. The correspondence between photoluminescence and absorption spectra measured by constant photocurrent method both with respect to peak positions and relative intensities has been found.

On the Correlation between Morphology and Electronic Properties of Fluorinated Copper Phthalocyanine (F16CuPc) Thin Films

We report on the correlation between morphology and electronic properties of fluorinated copper phthalocyanine (F16CuPc) thin films deposited on SiO2/Si substrates at different substrate temperatures. Highly ordered films with the 200 plane spacing of d200 = 14.3 Å is observed. Increasing the substrate temperature significantly improves the molecular ordering of F16CuPc, and the smallest FWHM was gained at a substrate temperature of 120°C. The mobility is strongly dependent on the substrate temperature. Increasing in grain size at higher substrate temperatures improves the mobility of F16CuPc TFTs, but gaps generated between grains degrade the performance of F16CuPc TFTs at a substrate temperature higher than 110°C. When deposition of F16CuPc is done at a substrate temperature of 100°C, the maximum mobility of 4.25 × 10−3 cm2/Vs. can be obtained.

Carrier injection and transport characteristics of copper phthalocyanine thin films under low to extremely high current densities

We investigated current density-voltage (J-V) characteristics of copper phthalocyanine (CuPc) thin films depending on active device areas. We prepared CuPc thin-film devices with active areas smaller than S=625μm2 using a photolithography technique. The maximum breakdown current density (JMAX) and voltage (VMAX) of the devices markedly increased as the active area was decreased from S=625 to 7.9μm2. In the smallest device, with S=7.9μm2, we obtained not only an extremely high current density of JMAX=128kA∕cm2 at VMAX=9.2V, but also unique J-V characteristics, indicating that the carrier conduction process shifted from the Fowler-Nordheim tunneling injection mechanism to shallow-trap and trap-free space-charge-limited current mechanisms.

Tunneling-Current-Induced Light Emission from Copper Phthalocyanine Thin Films

Molecular fluorescence induced by tunneling electrons from a scanning tunneling microscope was investigated for copper phthalocyanine thin films deposited on the highly ordered pyrolytic graphite and the Au(111) substrates. No light emission was observed from the copper phthalocyanine thin films on the highly ordered pyrolytic graphite substrate, on the other hand, the fluorescence of copper phthalocyanine on the Au(111) substrate was observed with plasmon-mediated light emission related to the Au(111) substrate. These results are due to the differences in the charge injection efficiency and surface plasmon resonance mode of two substrates. The obtained fluorescence did not satisfy the quantum cutoff condition, that is, observed photon energy was higher than the energy of injected electrons. [DOI: 10.1380/ejssnt.2006.559]

Nanoindentation Test for Evaluating Intermolecular Elastic and Plastic Characteristics of Copper Phthalocyanine Thin Films

We present the elastic and plastic parameters and the deformation patterns of a copper phthalocyanine (CuPc) thin film as compared with those of a polyester (PE) film and a metal copper (Cu) bulk. The yield stress of the CuPc film, σY=51.9 MPa, was comparable to that of the PE film and was lower by a factor of 10 than that of the Cu bulk. In contrast, Young's modulus, ES=9.29 GPa, of the CuPc film was intermediate between those of the PE film and Cu bulk, and the shearing strain energy per unit volume, US=8.74×1023 eV/m3, was lower by a factor of 10 than those of these two materials. The load–displacement (P–h) curves of the CuPc film were similar to those of the Cu bulk, in contrast to those of the vsicoelastic PE film, indicating that these deformation patterns showed an elastoplastic behavior and could be expressed by the Maxwell model.

Coherent anti-Stokes Raman scattering on single-walled carbon nanotubes and copper phthalocyanine thin films excited through surface plasmons

Experimental data indicating an anti-Stokes Raman emission, reminiscent of coherent anti-Stokes Raman scattering (CARS), that originates in a wave-mixing process between the incident laser light (ωl) and Stokes Raman light (ωs) generated by a surface enhanced Raman scattering (SERS) mechanism are reported. The variation of anti-Stokes SERS spectra of single-walled carbon nanotubes (SWNTs) and copper phthalocyanine (CuPc) as a function of the film thickness, laser excitation intensity and numerical aperture of the microscopic objective used for collecting the scattered light are presented as experimental arguments.

Electrical properties of hybrid phthalocyanines thin films using gold and lead electrodes

The electrical properties of mixed copper phthalocyanine (CuPc) and nickel phthalocyanine (NiPc) thin films devices with gold and lead electrodes have been investigated. The two phthalocyanines were co-evaporated and deposited on to a pre-cleaned glass substrate by thermal evaporation technique at room temperature. By the co-sublimation technique an integrated phthalocyanine sample was prepared. At low voltages the film showed an ohmic conduction whereas at higher voltage levels the conduction mechanisms were dominated by space charged limited conduction (SCLC). The value of mobility of holes were calculated as μ= 3.1×10-8 m2 V-1 s-1 which is lower for the mixed phthalocyanines whereas the trap concentrations Nt(e) = 8.06 ×1025 m-3 have increased in the SCLC region. Further the reverse conduction mechanisms have also been investigated. From the current limitations in the reverse condition a strong rectifying behaviour was evident.

Phase transition of copper (II) phthalocyanine thin films characterized by a near-field scanning microwave microscope

To study the phase transition of copper (II) phthalocyanine (Cu-Pc) thin films, we measured the surface resistance using a near-field scanning microwave microscope (NSMM) by measuring the microwave reflection coefficient S 11. The crystal structure of Cu-Pc thin films transformed from the α -phase of the orthorhombic crystal to the thermally stable β-phase of the monoclinic crystal as the substrate heating temperatures increased. The surface resistance depended on the crystal structures of the Cu-Pc thin films. As the phase changed from the α -phase to the β-phase, the surface resistance of the Cu-Pc thin films decreased.

Stationary and modulated absorption spectroscopy of copper phthalocyanine (CuPc) layers grown on transparent substrate

Copper phthalocyanine thin films grown on glass and quartz substrates by the thermal evaporation method were investigated using stationary and modulated photoabsorption as well as photoreflectance spectroscopy. The stationary and modulated studies at room temperature were performed in the wavelength range 200–1200 nm and 400–1200 nm, respectively. The absorption coefficient of investigated layers was determined directly from the transmission spectrum and fitted in terms of the mixed Lorentz–Lorenz model, which assumed five oscillator components. Sensitive modulated spectroscopy, which enabled us to emphasize weakly visible transitions in the Q-band spectral region, was also performed to estimate the electronic structure of metallophthalocyanine layers. The complex nature of excited states of the metallophthalocyanine layers gave us abundant modulated spectra. These spectra were fitted using the third derivative line shape given by the Aspnes formula. Modulated photoabsorption and photoreflectance spectra for CuPc thin layers are presented for the first time in this paper.

Scanning Near-Field Optical Microscopic Observation of Surface-Enhanced Raman Scattering Mediated by Metallic Particle-Surface Gap Modes

Samples consisting of a copper phthalocyanine (CuPc) thin film sandwiched between Au particles and a Ag film were prepared and their Raman scattering was investigated with a scanning near-field optical microscope (SNOM). The Raman intensity of CuPc was found to be greatly enhanced by the presence of the Au particles and takes a maximum on top of the particles. It was also found that the enhancement factor depends strongly on the diameter of Au particles and the excitation wavelength. With the aid of reflection-absorption spectra, which exhibited an absorption peak attributable to the gap mode of the particle–surface system, the observed behavior of the enhancement factor could be explained by the resonance of incident and scattered light to the gap mode. The present results clearly demonstrate that the gap between the metallic particle and surface is a hot site of Raman scattering.

Improvement in real time detection and selectivity of phthalocyanine gas sensors dedicated to oxidizing pollutants evaluation

A sensor microsystem prototype, using copper phthalocyanine thin film as sensitive layer, and dedicated to ozone evaluation, was developed. The methodology implemented is based on cyclic sensor recalibrations by thermal cleaning of the sensitive membrane, and on pollutant concentration quantification according to the kinetics of sensor response. Results of laboratory experiments for various NO 2 and O 3 concentrations, in the range of 10–200 ppb, illustrate the selectivity of CuPc sensors towards ozone, obtained by our methodology. We have shown that ozone selectivity is especially improved for short time of exposure (few minutes) and for phthalocyanine layer maintained at low temperature (80 °C). For optimal conditions, our microsystem exhibits a threshold lower than 10 ppb, a resolution lower than 10 ppb, and good reproducibility of measurements. Performances obtained in real urban atmosphere are satisfying to ensure real time evaluation of ozone during several days. Long-term stability and the detection of NO 2 by associating chemical filters to our microsystem will be also discussed.

Charge Recombination in CuPc/PTCDA Thin Films

The recombination kinetics of photogenerated charge carriers in perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) and copper phthalocyanine (CuPc) thin films grown by organic molecular beam deposition have been studied using transient absorption spectroscopy. Optical excitation is observed to generate long-lived polaron states, which exhibit power law recombination dynamics on time scales from microseconds to milliseconds. Studies as a function of excitation density and temperature, and comparison between heterostructures and PTCDA single layers, all indicate that this power law behavior results from trapping of PTCDA- polarons in localized states, with an estimated trap state density of approximately 6 x 10(17) polarons cm(-3). This recombination behavior is found to be remarkably similar to that previously observed for polymer/fullerene blends, suggesting that it may be generic to a range of semiconducting materials.