CuPc Thin Films Research Articles

Electronic structure of copper phthalocyanine monolayer: a first-principles study

The electronic structure of copper phthalocyanine (CuPc) monolayer was investigated by the first-principles all-electron full-potential linearized augmented plane wave (FLAPW) energy band method. The magnetic properties of the CuPc monolayer were investigated with spin-polarized calculation. It was found that the Cu atom has a magnetic moment of 0.56 μ B, but it does not affect strongly the paramagnetic properties of the monolayer. The ground-state electronic structure of the CuPc monolayer found in spin-polarized calculation is indistinguishable from the paramagnetic case in the energy range from −10 to −1.5 eV and above 1.0 eV, with respect to the Fermi level ( E F), but taking into account the magnetic properties of the open-shell Cu atom results with the splitting of bands near E F . The obtained total density of states and the calculated values of work function (4.66 eV) and ionization potential (5.23 eV) of the CuPc monolayer were found to be in a good agreement with the experimental data concerned CuPc thin films.

Bromine and alkoxyl substituted copper phthalocyanine derivative thin films studied with spectroscopic ellipsometry

Abstract Tetrabromo-tetraalkoxyl substituted copper phthalocyanine (TBTACuPc), tetraalkoxyl substituted copper phthalocyanine (TACuPc) and copper phthalocyanine only (CuPc) thin films were prepared on single-crystal silicon. The optical and dielectric properties of these films were studied on a new type of scanning photometric ellipsometer in which the analyzer and polarizer rotate synchronously. The optical constants, dielectric function and absorption coefficients of these films have been obtained in the wavelength range 500–800 nm. It has been found that the substituted groups on the conjugated phthalocyanine macrocycles strongly influence resonance wavelength and abnormal dispersion of the thin films.

Growth of CuPc Thin Films on Structured SiO2/Si(100) Studied by Metastable Electron Emission Microscopy and Photoelectron Emission Microscopy

A combined metastable electron emission microscopy (MEEM) and photoelectron emission microscopy (PEEM) study has been performed for copper phthalocyanine (CuPc) thin films vacuum deposited on Si(100) with a lithographic SiO2 pattern. The MEEM image of SiO2 pattern edges changed considerably upon deposition of a very small amount of CuPc on the SiO2/Si(100) substrate, whereas no such changes were observed in the PEEM images. The different contrast features in MEEM and PEEM can be the result of different surface sensitivities of these microscopy techniques. The MEEM contrast characteristic for the pattern edges after the CuPc deposition may be ascribed to the difference of the molecular orientation of CuPc which cannot be observed by PEEM.

On the correlation between morphology and electronic properties of copper phthalocyanine (CuPc) thin films

The morphology of vacuum deposited copper phthalocyanine (CuPc) thin films surface deposited on Si(111) have been studied using the contact mode Atomic Force Microscope (AFM). The influence of substrate temperature during deposition and of the post-deposition UHV annealing on surface roughness as well as on the average and maximum grain height was determined. The observed changes of surface morphology were in a good correlation with the shift of surface Fermi level position in the band gap after O 2 exposure determined in our recent photoemission studies.

Bottom-contact organic field-effect transistors having low-dielectric layer under source and drain electrodes

An organic thin-film transistor (OTFT) having a low-dielectric polymer layer between gate insulator and source/drain electrodes is investigated. Copper phthalocyanine (CuPc), a well-known organic semiconductor, is used as an active layer to test performance of the device. Compared with bottom-contact devices, leakage current is reduced by roughly one order of magnitude, and on-state current is enhanced by almost one order of magnitude. The performance of the device is almost the same as that of a top-contact device. The low-dielectric polymer may play two roles to improve OTFT performance. One is that this structure influences electric-field distribution between source/drain electrodes and semiconductor and enhances charge injection. The other is that the polymer influences growth behavior of CuPc thin films and enhances physical connection between source/drain electrodes and semiconductor channel. Advantages of the OTFT having bottom-contact structure make it useful for integrated plastic electronic devices.

Heteroepitaxial copper phthalocyanine on Au(0 0 1) studied by high-resolution X-ray photoelectron spectroscopy

We report the formation of heteroepitaxial copper phthalocyanine (CuPc) thin film grown on a reconstructed Au(001) surface and the high-resolution X-ray photoelectron spectroscopy study thereof. The deposition of CuPc onto the “5×20” reconstructed Au(001) surface at room temperature results in a well-ordered CuPc overlayer. The angle-resolved XPS data reveal that adsorbed CuPc is molecular and stoichiometric. The low energy electron diffraction (LEED) further shows that the overlayer is incommensurate to the substrate with its square unit mesh measured approximately 12.7 Å by 12.7 Å along 〈110〉 and 〈11̄0〉 of the substrate. From the combined XPS and LEED data, a model of one flat-lying CuPc molecule per unit cell, which is different from the structures of the well-known α- and β-polymorphs, is proposed.

Peculiar features in the electrical characteristics of CuPc based diodes

We have investigated electrical properties of copper phthalocyanine (CuPc) thin films by means of current–voltage (I–V), capacitance–voltage (C–V) and charge deep-level transient spectroscopy (Q-DLTS) measurements. CuPc films were deposited on indium tin oxide (ITO) via organic molecular beam deposition (OMBD) at room temperature followed by Ag evaporation through a shadow mask, resulting in contacts of different sizes of dots. The measurements were carried out in situ at a pressure below 8×10−10mbar. No noticeable changes of electrical characteristics have been found after storing the devices for a few days under these conditions. Diodes of 330nm CuPc sandwiched between ITO and Ag show very low reverse currents with hysteresis-like characteristics for short delay times, which are the times between two consecutive voltage steps in an I–V measurement. Especially the bias at which current passes through zero is drastically affected by the sweep voltage direction. Almost identical curves in both directions can only be achieved using an extremely long delay time of 100s. It was found that the offset voltages for delay times of 5 and 100s are −0.85 and −0.1V for up-sweep and 0.14 and 0V for down-sweep voltage directions, respectively. This effect is likely due to the presence of deep traps which require a large time constant for charging and discharging in response to a change of the external voltage. The forward current, on the other hand, is virtually not affected, increasing initially over many orders of magnitude in a narrow voltage range and then increasing at a reduced slope. This behaviour is not stable when a high external voltage is applied. Q-DLTS measurements show presence of traps with time constants exceeding 50ms at ambient temperature.

High sensitivity chlorine gas sensors using Cu–phthalocyanine thin films

High sensitivity chlorine (Cl 2) gas detection has been realized by newly developed gas sensors using Cu–phthalocyanine (CuPc) thin films. The CuPc thin film gas sensors exhibited an increase of conductivity with exposure to Cl 2 gas. The sensing properties of the CuPc thin film gas sensors were strongly dependent on the preparation conditions of CuPc thin films. The highest sensitivity for Cl 2 gas was obtained in the gas sensor using CuPc thin film prepared under the optimized preparation conditions: substrate temperature of 170 °C, evaporation temperature of 475 °C and film thickness of 50 nm. It was found that Cl 2 gas detection was realized at a minimum concentration of 0.18 ppm.

Enhancement of Photoelectric Effect in Organic Dye Thin Film Cells by Surface Plasmon Excitation

ABSTRACTPhotoelectric properties have been investigated for the organic dye thin film cells utilizing surface plasmon (SP) excitation. The cells fabricated in this work had a prism/Al thin film/copper phtalocyanine (CuPc) thin film/Ag thin film structure, which was the Kretschmann configuration in the attenuated total reflection (ATR) method. Since the CuPc thin film exhibits p-type conduction, the Schottky and Ohmic contacts can be obtained at the interfaces between the CuPc and Al thin films and between the CuPc and Ag thin films, respectively, and the cells used in this work show photoelectric properties. The ATR and short-circuit photocurrent properties have been simultaneously measured as a function of the incident angles of the laser beams. The experimental and calculated results of the ATR and short-circuit photocurrent properties and the optical absorptions of the organic dye layers were discussed. It was found that the photoelectric effect in the organic dye thin film cells the Kretschmann configuration in the ATR method was enhanced by the SP excitation.

Structural and Transport Properties of Copper Phthalocyanine (CuPc) Thin Films

Thin films of CuPc of various thicknesses have been deposited onto clean glass substrates using thermal evaporation technique at room temperature. The structural studies, using X-ray diffractometry and IR spectral analysis, showed that the films are mainly of β- form with preferential orientation of the plane (100). The dark electrical resistivity and the thermoelectric power measurements were carried out at different temperatures in the range 300-450 K. Room temperature current density-voltage characteristics showed ohmic conduction in the lower voltage range and spacecharge-limited – conductivity (SCLC) in the relatively high voltage. The SCLC are controlled by an exponential distribution of traps above the valence band edge. The temperature dependence of current density allows the calculations of some essential parameters such as the hole mobility, the relative permittivity, the trap concentration, the characteristic temperature and the trap density

Grazing Incidence In-Plane X-Ray Diffraction Study on Oriented Copper Phthalocyanine Thin Films

Uniaxially aligned vacuum deposited films of copper phthalocyanine (CuPc) have been studied by means of angle-dispersive grazing-incidence X-ray diffraction. The lateral structure of the top surface of a 100-nm-thick film was successfully probed. It was revealed that the microcrystals near the surface have a finite in-plane distribution with a full-width at half maximum of 17.5°. In-plane diffraction profiles were transformed into a reciprocal space map that reproduced the electron diffraction pattern of a CuPc thin film reported previously.

Structural Investigation of Copper Phthalocyanine Thin Films Using X-Ray Diffraction, Raman Scattering and Optical Absorption Measurements

Structural properties of copper phthalocyanine (CuPc) powder and its thin films coated on glass plate are investigated using X-ray diffraction, Raman scattering and optical absorption techniques. These experiments are carried out at room temperature on CuPc powder and as-coated, 320 °C and 520 °C annealed, CuPc thin films. From the X-ray diffraction (XRD), we find that the CuPc powder exists in monoclinic phase, as-coated film is in orthorhombic α-phase and the film annealed at 320 °C exists in monoclinic β-phase with different lattice parameters as compared to those of the powder phase. The XRD peaks in 520 °C annealed film do not show the presence of CuPc but indicate the presence of copper oxide (CuO) nanoclusters of approximately 15 nm size. The optical Raman phonon frequencies of CuPc powder, as-coated and 320 °C annealed CuPc films show a slight shift in the intramolecular high frequency modes. Whereas, their low frequency Raman spectra from intermolecular modes are characteristically different indicating different phases. The different characteristic behavior of the optical absorption spectra of the as-coated and 320 °C annealed CuPc films corroborate the structural changes in the annealed film.

Device preparation and characteristics of CuPc transistor

Static and dynamic characteristics of an organic static induction transistor (SIT) fabricated using copper phthalocyanine (CuPc) thin films are reported. Layered structure using CuPc as active layer consists of Al (drain)/CuPc/Al (gate)/CuPc/FTO (source)/glass. Electrical characteristics obtained are excellent at low negative gate voltages.

Femtosecond Z-scan Measurement of the Third Order Nonlinear Optical Coefficient of CuPc Thin Film

The third order nonlinear refractive index and absorption coefficient of copper-phthalocyanine (CuPc) thin film were measured by Z-scan method in ultrafast time regime. By changing pulse separation, we found that the nonlinear absorption behavior is different for the different pulse repetition rate. At 4MHz, the film exhibits RSA (reverse saturable absorption), while at 800kHz and 400kHz it shows SA (saturable absorption). Also the nonlinear refractive index is found to be different at 4MHz and other repetition rates.

Optical Microstructure of CuPc Thin Film Prepared by Thermal Evaporation

The α-type CuPc thin film prepared by thermal evaporation has the b-axes parallel to the substrate plane. We report the comparison of the optical structure and the height image of CuPc layer deposited on Si surface using the Near-field solid immersion lens (NSIL) and a contact AFM. The optical microstructure of CuPc surface was measured by focusing a laser beam(λ=442 nm) in a solid immersion lens(SIL).

Static induction transistor characteristics of organic copper phthalocyanine films

Static and dynamic characteristics of an organic static induction transistor (SIT) fabricated using copper phthalocyanine (CuPc) thin films are reported. Layered structure using CuPc as active layer consists of Au(drain)/CuPc/Al (gate)/Al(discontinuous)/CuPc/Au(source)/glass. Excellent electrical characteristics are obtained at low negative gate voltages.

Photoemission study of the electronic properties of in situ prepared copper phthalocyanine (CuPc) thin films exposed to oxygen and hydrogen

The electronic properties of space charge layer of the copper phthalocyanine (CuPc) thin films in situ prepared by an UHV evaporation method and, subsequently, exposed to molecular oxygen (O 2) and molecular hydrogen (H 2) have been studied using the high resolution photoemission yield spectroscopy (PYS) technique. The influence of interaction of these active gases on the work function ϕ and ionisation energy Φ was determined, together with the changes of effective density of filled electronic states localised in the band gap below the Fermi level and in the upper part of the valence band. In order to direct comparison to the commonly used procedure for UPS and XPS techniques, the changes of macroscopic parameters of the space charge layer were expressed by the surface Fermi level position in the band gap E F– E v. The observed changes of surface Fermi level position upon adsorption/desorption of molecular oxygen O 2 and hydrogen H 2 were attributed to decrease/increase of the induced charge in near surface region.

Photoconductive imaging in a photon scanning tunnelling microscope capable of point-contact current sensing using a conductive fibre probe.

A photoconductive photon scanning tunnelling microscope was developed to investigate the point-contact photoconductive properties of condensed matter. In order to detect the current and the optical signal at a local point on a surface, we coated the edge of a bent type fibre probe with indium tin oxide. Thus it was possible to measure both photocurrent and optical property with subwavelength resolution. The performance of the novel microscope was evaluated by analysing an organic thin film of copper phthalocyanine (CuPc), which is known to be an efficient photoconductive material. Photocurrent and current-voltage characteristics were observed at the local point on the CuPc thin films. Furthermore, photoconductive images were obtained with topography and near-field optical imaging using this system. The photoconductive PSTM shows potential in various areas of future optics and electronics.

Charge-carrier injection into CuPc thin films: a scanning tunneling microscopy study

Injection of charge carriers into the LUMO and HOMO levels of copper phthalocyanine (CuPc) is investigated by scanning tunneling microscopy (STM) and spectroscopy. The threshold for injection of charge carriers emitted by the tip of the STM allows us to determine the so-called single-particle band gap (Egsp) of CuPc polymorphs as well as the band edge alignment with respect to the Fermi level of the Au(111) substrate. We find Egsp values significantly lower than the optical band gap. This indicates the presence of intermolecular interactions, giving rise to in-gap states that can play a predominant role in controlling injection as well as transport properties. The lowest value of Egsp is found for the crystalline phases of CuPc obtained by thermal annealing, whereas the largest band gap and inferior charge-transport properties are found for samples grown on substrates at room temperature.

Electronic properties of the space charge layer of in situ prepared copper phthalocyanine thin films exposed to oxygen

The electronic properties of the space charge layer of copper phthalocyanine (CuPc) prepared under ultrahigh vacuum (UHV) conditions were in situ studied by photoemission yield spectroscopy (PYS). Depending on the oxygen exposure, as well as subsequent UHV annealing the variation of the work function ϕ, ionization energy Φ and the surface Fermi level position in the band gap E F− E v were analyzed, together with the effective density of filled electronic states localized in the band gap below the Fermi level E F . Moreover, the obtained results were compared to the recently determined ex situ studied CuPc thin films.