CuPc Films Research Articles

High temperature study of FT-IR and Raman scattering spectra of vacuum deposited CuPc thin films

In this work we present the polarized and nonpolarized Raman spectra of copper phthalocyanine CuPc layers deposited on (001)Si and borosilicate glass substrates. These spectra are compared with FT-IR spectra of CuPc molecules in KBr pellets and thin layers of CuPc on (001)Si substrates obtained by thermal evaporation method. The observed Raman and FT-IR bands are assigned to definite phonon modes on the basis of their symmetry and comparison with the frequencies predicted by lattice dynamical calculations (LDC). The films of CuPc grown on Si substrate show very good out-of-plane ordering and have X-ray rocking widths of around 0.03°. For layers grown at different temperatures of substrate, we found a change of the molecular out-of-plane tilt angle, as obtained from Raman scattering. Observed feature is accompanied by a change of the out-of-plane lattice parameter.

Nanomechanical properties of ordered phthalocyanine Langmuir–Blodgett layers

The mechanical properties of monolayer and multilayer structures of several symmetrically substituted alkoxy copper phthalocyanine, CuPc(OR)8, Langmuir–Blodgett (LB) films were evaluated. One-dimensional compliance of the monolayers was determined from the slopes of the surface pressure–area isotherms. Multilayers were examined using a SPM and a commercial nanoindentation system. The nanomechanical studies show a distinct relationship between the position and length of alkoxy substituents on the Pc macrocycle and the LB film elasticity and hardness. The phthalocyanine complexes with peripheral octabutoxy and octaoctyloxy substituents form stiff LB monolayers and multilayers. The nonperipheral alkoxy derivetized CuPc films were less stiff. For monolayer films, it appears that the extent of π-π interaction determines the strength of the film. In multilayers, significant additional stability can be imparted through interdigitation of long paraffinic chains which play a significant role in determining the interlayer structure of the LB layers.

Spectroscopic consideration of the surface potential built across phthalocyanine thin films on a metal electrode

The nonlinear optical properties of tert-butyl phthalocyanine copper Langmuir-Blodgett (CuttbPc LB) films and vacuum-evaporated phthalocyanine copper (CuPc) films deposited on a metal surface were investigated by second-harmonic generation (SHG) spectroscopy. At the organic/metal interface, a space charge field is formed due to the presence of excess charge injected from a metal electrode to the organic layer. Since the Pc molecule has D4h symmetry, an inversion center is present and the optical SH process is not allowed under the electric-dipole approximation. However, the space charge field at the interface directly influences the symmetric structure of the electrons in the Pc molecule. We investigated the contributions of the surface potential to the SHG using Pc LB and vacuum-evaporated films deposited on aluminum (Al) and gold (Au) metal electrodes, where a distinctive difference in the spectrum for the Pc films on the Al and Au surfaces was observed. The contribution of the surface potential was revealed based on the resonant conditions of the SH process, taking into account the electric-quadrupole transition and dc-field-induced electric-dipole transition.

Time-of-Flight Measurement of Lateral Carrier Mobility in Organic Thin Films

Lateral carrier transport in a copper phthalocyanine (CuPc) thin film has been investigated by the time-of-flight technique using a micro-excitation system. Drift mobility in the CuPc film has been estimated from the transient photocurrent measured for various electric field strengths. The drift mobility has been compared to the field-effect mobility of a thin-film transistor with CuPc as the channel material. The field-effect mobility was comparable to the drift mobility measured by the TOF technique.

Thin Film Transistors with Oriented Copper Phthalocyanine Crystals Fabricated by Physical Vapor Deposition under DC Electric Field

Crystalline copper phthalocyanine (CuPc) films have been fabricated by physical vapor deposition using a cylindrical furnace. Field effect transistors (FETs) of the crystalline CuPc film are fabricated by optimizing the deposition conditions. Crystal orientation along the electric field is enhanced by an application of a DC electric field during crystal growth. The electric-field-assisted orientation is effective in a submillimeter area. The degree of crystal orientation improves with increasing electric field strength. High current and a remarkable threshold for the drain voltage are observed in the FET characteristics of the oriented crystalline film.

Copper-phthalocyanine ultra thin films grown onto Al(1 0 0) surface investigated by synchrotron radiation

In this work X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are employed to study the CuPc/Al(1 0 0) organic–inorganic interface. The C 1s core level, investigated for three different coverages, shows sharp differences in passing from bulk to monolayer range CuPc film: in particular the shake-up satellite is not visible in the early stages of adsorption, suggesting a strong molecule–substrate interaction. XAS at the Cu L 3 edge was employed to study the molecular orientation with respect to the substrate. In the case of a 40 Å CuPc film the molecular plane was found to be almost perpendicular to the substrate surface.

Interface formation between Cu-phthalocyanine films and CdS and GaAs semiconductor surfaces

Total current electron spectroscopy (TCS) is applied as a key experimental technique to studies of electronic structure of organic semiconductor films interfacing inorganic semiconductors. Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on CdS(0 0 0 −1) and GaAs(1 0 0) substrates. The surface potential, the surface work function and the structure of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) was monitored during the film deposition, using an incident beam of low-energy electrons according to the TCS method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. Electronic work function of the CuPc films changed during the film deposition until it reached a stable value 4.5±0.1 eV at the film thickness 8–10 nm. The deposition of the CuPc under 1.5 nm resulted in formation of the intermediate DOUS structures that were different for the cases of the two substrates used in the study. Fragments of the CuPc molecules were found to constitute mainly the intermediate deposit layers. Along with the increase of the deposit thickness to 8–10 nm, the intermediate DOUS structures were replaced by the DOUS structure typical for the CuPc film, although the S, Ga and As atoms migrated from the substrates were present in the CuPc film. Electron charge transfer layer at the CuPc/GaAs interface extended more than 5 nm from the GaAs surface, while the interaction at the CuPc/CdS interface resulted in band bending in the CdS subsurface region.

Gas sensing characteristics of copper phthalocyanine films: effects of film thickness and sensing temperature

Copper phthalocyanine (CuPc) was vacuum-deposited onto gold patterned glass substrate and used for low temperature operation of NO 2 sensing. The effects of film thickness, as well as the sensing temperature, on the sensitivity and recovery characteristics of the CuPc film were investigated. The results show that both film thickness and sensing temperature have significant effect to the sensing characteristics. At room temperature operation, the recovery characteristic is satisfactory for the 50 nm film, and it becomes poor as increasing film thickness. Besides, the sensitivity shows little dependence on the film thickness. When the sensing temperature is elevated to 100 °C, the recovery ratio of the thicker films can be improved satisfactorily, but the sensitivity of thinner films (50, 100 nm) decrease as a result. However, the sensitivity increases notably with increase of film thickness and approaches the maximum value, which is comparable or even superior to that at 25 °C, at about 200 nm. The present results indicate that the poor recovery characteristic found in the literature can be improved by reducing the film thickness and NO 2 doping period. Besides, to get superior sensing properties, a small film thickness (50 nm) should be controlled for room temperature operation, but a higher thickness (200 nm) is required when the operation is elevated up to 100 °C.

Post-Deposition-Annealing-Induced Alignment of Copper Phthalocyanine Thin Films under UV Irradiation and Their Electrical Properties

Copper phthalocyanine(CuPc), a widely-used semiconducting material, was deposited onto quarts substrates via a vacuum process. SEM and XRD analysis were employed to characterize the structure-controlled CuPc films. As-deposited CuPc thin films were weakly oriented according to the XRD result and were homogeneous. The conductivity of CuPc film was estimated using four-point probe measurement technique and the values of the conductivity were 8.0 × 10−8– 3.5 × 10−7Ω−1cm−1. It was observed that thermal annealing under UV irradiation could make CuPc film well-aligned since UV irradiation could participate to increase the interaction among the CuPc molecules and between the CuPc molecule and the surface of the substrate leading to closer stacking of CuPc molecules. For well-aligned CuPc thin films, the threshold voltage in I-V characteristics of the ITO/CuPc/Al device was decreased by 20%compared to as-deposited CuPc films.

Density functional calculations on the ionization potentials of (CuPc) n ( n=1–6)

We use density functional theory approaches to calculate ionization potentials (IP) of copper phthalocyanine (CuPc) n ( n=1–6). The IP of solid-state CuPc is estimated to be 5.01 eV from the calculated molecular results. Our result is in good agreement with ultraviolet photoelectron spectroscopy and cyclic voltammetry measurements on solid-state films of CuPc.

Low-energy electron mean free path in thin films of copper phthalocyanine

The formation of thin organic films of copper phthalocyanine (CuPc) deposited onto the surface of gold-coated quartz crystal resonator was studied in situ under ultrahigh vacuum conditions by means of total electron-beam-induced current spectroscopy in combination with deposit thickness determination by piezocrystal microbalance technique. Variations in the fine structure of the total current spectra of CuPc layers of var-ious thicknesses in the 0–8 nm interval have been analyzed and the electron mean free path in thin CuPc films was determined as a function of the electron energy. For electron energies of 5.0, 7.2, 14.4, and 18.0 eV above the Fermi level, the mean free path is 6.4, 3.9, 2.6, and 2.3 nm.

Photovoltaic Characteristics of Copper Phthalocyanine-poly(p-phenylene) Co-evaporation Films

Copper phthalocyanine (CuPc) and poly(p-phenylene) (PPP) composite films (CuPc–PPP) were produced by a simultaneous deposition method. Schottky barrier photovoltaic cells consisting of semitransparent aluminum and CuPc–PPP composite films were fabricated. The effects of composition and optimization on the photovoltaic properties of the CuPc–PPP composite films by simultaneous deposition were investigated. The short circuit photocurrent of the CuPc–PPP cells increased in comparison with that of the CuPc film. This behavior was described as follows. One is that an effective surface area of the CuPc is increased by the PPP, which functions as a binder polymer. The other is that the bulk conductivity of the composite film is improved by the PPP, which functions as a carrier transfer material (CTM) for CuPc. It is found that the optimum conditions for simultaneous evaporation are as follows; film thickness: 1200 Å, deposition ratio: 5:1.25 Å/s (CuPc:PPP), temperature: room-temperature. The power conversion efficiency under these conditions was found to be 2.33%.

Static electric field effect in the second harmonic generation from phthalocyanine film/metal electrode

The effect of static electric field on the second harmonic generation (SHG) from copper phthalocyanine (CuPc) was investigated using metal/CuPc/metal structure. An external DC voltage was applied to the sample to induce an electric dipole which allowed second order polarization in the centrosymmetric molecular system (CuPc film). The second harmonic generation (SHG) spectra and its intensity vs. the external bias at fixed fundamental frequency were investigated. Considering the induced electric quadrupole transition and the electric field induced dipole transition, the SHG generated from the CuPc film was attributed to the electric quadrupole effect and the electric field induced SHG (EFISHG). A quadratic relation between the SHG intensity and the external applied bias was observed. It was in accordance with theoretical analysis.

HTSC polaroinc quasiparticle injection devices with an organic copper (II) phthalocyanine injector

We report the nonequilibrium effect of polaronic quasiparticle (QP) injection using an organic injector in a high T/sub c/ three terminal device. The organic, copper (II) phthalocyanine (Cu-Pc), used as the injector, is a photoconductor and a p-type semiconductor. The transport properties of Au/Cu-Pc/Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// (BSCCO) tunnel junctions were investigated in the dark and under the He-Ne laser (/spl lambda/=632.8 nm) radiation. We observed that the injection of polaronic QP from the organic Cu-Pc film into the BSCCO film generated a substantially larger nonequilibrium effect as compared to the normal QP injection current. We could increase the current gain by He-Ne laser excitation of the organic photoconductor injector. The tunneling spectroscopy of a Cu-Pc/BSCCO junction exhibited an enhancement of the zero bias conductance peak under the He-Ne laser excitation. The above phenomena are of importance in developing optically controlled three terminal superconducting devices.

Effects of sensing temperature on the gas sensing properties of copper phthalocyanine and copper tetra- tert-butyl phthalocyanine films

Copper phthalocyanine (CuPc) and copper tetra- tert-butyl phthalocyanine (CuTTBPc) were vacuum deposited onto alumina substrates. The surface morphology and crystalline structures of the films prepared by the two compounds were studied first. These films were then used for the experiments of NO 2 sensing. The effects of gas sensing temperature, as well as the NO 2 concentration, on the sensing properties were studied for both films. The experimental results show that both CuPc and CuTTBPc films exhibit fine-grain morphology and low crystallinity. Due to the tetra- tert-butyl (ttb) substitution on the periphery of phthalocyanine, CuTTBPc film has a larger lattice spacing, higher film resistance, and a higher recovery ratio in the NO 2 sensing experiments. On the contrary, the response rate and sensitivity of a CuPc film are superior to that of a CuTTBPc film. By varying the sensing temperature, it was found that the elevation of sensing temperature can improve the sensing response, recovery ratio, and sensitivity of the sensing films. At high concentrations of NO 2, a longer response time is required and the recovery ratio becomes smaller. In addition, the disadvantage caused by the elevation of NO 2 concentration can also be improved by using an elevated sensing temperature.

Photoconductivity and oxygen adsorption of Cu-phthalocyanine thin films on cadmium sulphide surfaces

Sandwich structures based on 10nm thin Cu-phthalocyanine (CuPc) films vacuum deposited on a 500nm thick cadmium sulphide CdS film surface—deposited onto a glass substrate—demonstrated pronounced photoconductivity at 105Pa O2 pressure and in vacuum. The photoconductivity transient profiles were measured throughout the spectral range from 300 to 900nm and the photoconductivity spectra were built on the basis of these data. Two photoconductivity components that had different relaxation time and spectral distribution were registered in the pressure range from 10−6 to 101Pa. Only the faster photoconductivity component was left after the structure under study was exposed to 105Pa O2. Formation of an interface layer between CdS and CuPc films, having electronic properties different from each of the films, is suggested. The faster photoconductivity component is attributed to incident light absorption in a CdS subsurface region that is possibly modified by the presence of the CuPc molecules. The slower photoconductivity component is attributed to the processes in the interface layer or in the CuPc film.

Small-angle X-ray scattering and neutron reflectivity studies of Langmuir–Blodgett films of copper tetra-tert-butyl-azaporphyrines

Langmuir-Blodgett (LB) films of tetra-tert-butyl-substituted copper azaporphyrines, namely phthalocyanine (CuPc'), tetra-3-nitro-phthalocyanine (CuPc*) and porphyrazine (CuPaz'), are studied by small-angle scattering technique. Two kinds of systems are under investigation: first, the CuPc'/Cd-stearate-d 35 superlattices; second, the LB films of CuPc', CuPc* and CuPaz' doped with benzene-d 6. X-ray diffraction patterns of the superlattices exhibit bilayer structural units indicating existence of not only 1D but also 3D nano-aggregates of CuPc'. The neutron reflectivity study on the films heavily doped with benzene-d 6 indicates that π-deficiency and size of the azaporphyrine nuclei favor interactions between the nano-aggregates.

Surface and interface analysis for copper phthalocyanine (CuPc) and indium-tin-oxide (ITO) using X-ray photoelectron spectroscopy (XPS)

Indium-tin-oxide (ITO) has been widely used as a hole injection electrode for organic light-emitting devices (OLEDs), but the work function of ITO film usually mismatches the highest occupied molecular orbital (HOMO) of the hole transport materials. Copper phthalocyanine (CuPc) has been used as a hole injection buffer to enhance the hole injection from ITO to hole transport layer (HTL). A thin CuPc layer was thermally evaporated onto ITO-coated glass substrate and the surface and interface electron states of the CuPc/ITO close contact were measured and analyzed by X-ray photoelectron spectroscopy (XPS) technology. Results show that, in the interface of CuPc and ITO film, the role of oxygen is very crucial, the oxygen in ITO film may diffuse into the organic semiconductor and cause the alteration of electron states in organic materials.

Fermi level pinning in Cs doped CuPc

We have investigated the electronic structures of pristine and alkali metal doped organic films using photoemission and inverse photoemission spectroscopy (PES and IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) were directly observed simultaneously. We found that the Fermi level position in the organic film was modified by alkali metal doping, and the observed LUMO of the CuPc film was shifted by the Cs doping to less than 0.2eV above the Fermi level. The observation is a direct confirmation of widely used assumption that the LUMO can be inferred from HOMO position in organic films. The result indicates that energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Electronic charge distribution at interfaces between Cu-phthalocyanine films and semiconductor surfaces

Total current electron spectroscopy (TCS) that uses a probing beam of low energy electrons was applied to study electronic charge transfer at interfaces between Cu-phthalocyanine (CuPc) films thermally deposited in situ onto ZnO, oxidized and crystalline silicon substrates. Analysis of the TCS data provided us also with new data on the density of unoccupied electron states (DOUS) of the CuPc films at 0–25 eV above E F. A most significant electronic charge transfer from the CuPc film to SiO 2/n-Si and n-Si(1 0 0) was observed and the polarization layer extended up to 10 nm into the CuPc film bulk. The electronic structure of the CuPc molecules on n-Si(1 0 0) and on ZnO(0 0 0 1) was perturbed within 1–2 nm of the deposit due to interaction with the substrates. Admission of O 2 and NO 2 at 10 −5 Pa and 300 K resulted in a reversible decrease and increase of the film surface potential, respectively. Formation of TC peaks related to O-orbitals on the gas adsorption on the CuPc surface was also registered.