CuPc Films Research Articles

Unoccupied electronic states and energy level alignment at interfaces between Cu-phthalocyanine films and semiconductor surfaces

Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on silicon, on highly ordered pyrolytic graphite (HOPG) and on ZnO substrates. The surface potential and the density of unoccupied electron states (DOUS) located 1–25 eV above vacuum level were measured during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Analysis of the TCS data allowed us to obtain DOUS features of the CuPc and to study charge transfer during the interface formation. A significant electron transfer from the CuPc film to SiO 2/n-Si and n-Si(1 0 0) was observed and the charge transfer layer in the film extended up to 10 nm. Band bending in the substrates due to the electron transfer was also monitored. A less significant charge transfer was observed at the interfaces between the CuPc film and the ZnO(0 0 0 1), SiO 2/p-Si and HOPG substrates. A model for the extended charge-transfer layer (extended interface dipole) is suggested to explain the observed spectroscopic features of the interfaces.

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.

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.

Investigation on Copper Liberation in the Multilayer CuPc/Li/Al Cathode of Organic Light-Emitting Diodes

An interlayer Li inserting between copper phthalocyanine (CuPc) and Al improves both electric and luminous properties of an organic light-emitting diode (OLED). X-ray photoelectron spectroscopy (XPS) surface analysis technique is employed to investigate chemical binding conditions of CuPc/Li interface. At varying thicknesses of Li deposited on CuPc, we have studied and XPS of CuPc films. Investigation demonstrates that Cu atoms are liberated from CuPc with increasing Li thicknesses. The results reveal that an OLED with an indium tin oxide glass/triphenyldiamine derivative/tris(8-hydroxyquinoline)aluminum/CuPc/Li/Al structure employing a 2 nm thick Li exhibits optimum performance. It can satisfy commercial demands in not only good reproducibility but also a large area of uniformality. © 2003 The Electrochemical Society. All rights reserved.

A study of photoconductive properties of vacuum-sublimated copper-phthalocyanine/zinc sulphide multilayer films

Copper-phthalocyanine (CuPc)/zinc sulphide (ZnS) multilayer films were prepared by vacuum-sublimation. X-ray diffraction, optical absorption spectra and photoconductive properties were measured for the multilayer films. The results showed that the photoconductive properties of the CuPc/ZnS multilayer films were better than that of the CuPc film. The effects of the pair number on the photoconductive properties of the multilayer films were studied. It was found that three-pair CuPc/ZnS multilayer film had the best photoconductive properties with the light sensitivity S=100 lx s and residual potential V R=17 V.

Preparation and photoconductive properties of vacuum-sublimed CuPc/ZnS multilayer films

Organic–inorganic hetero-multilayer films with good photoconductive properties are expected to be useful in a wide variety of devices, and can be prepared by different techniques. A new organic–inorganic composite CuPc/ZnS (where Pc is phthalocyanine) multilayer film was prepared by vacuum sublimation. The influence of substrate temperature, layer stack number and layer thickness on the structural, optical and photoconductive properties of CuPc/ZnS multilayer films was investigated by atomic force microscopy, X-ray diffraction, infrared absorption, ultraviolet–visible spectroscopy and surface potential decay measurements. It was concluded that the 150 °C-deposited three-pair CuPc/ZnS multilayer with a thickness of 0.1 μm for both CuPc and ZnS layers had the best photoconductive property with a light sensitivity of S = 62.5 lx s under white incident light, and had the ideal multilayer structure with good heterostructural compatibility, interfacial coherence and oriented crystallization, which was preferable for potential applications in photoelectric conversion film-related devices compared with single-layer CuPc films. It was speculated that the charge carrier photogeneration occurred in the CuPc layer, and the ZnS layer provided the role of transportation of electrons in plane after charge separation at interfaces in the CuPc/ZnS multilayer film.

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

Structure of phthalocyanine overlayers on the [formula omitted] surfaces of InSb and InAs

The structure of free-base phthalocyanine (H 2Pc) and copper phthalocyanine (CuPc) overlayers deposited on the indium-terminated InSb(1 0 0) and InAs(1 0 0) (4×2)/c(8×2) reconstructed surfaces have been studied by low-energy electron diffraction and van der Waals (vdW) intermolecular interaction energy calculations. The diffraction pattern for H 2Pc grown on InSb(1 0 0) is a superposition of a (3×3) and (√10×√10) R±18.4° structure, whilst no ordered structure is formed when H 2Pc is grown on InAs(1 0 0). In contrast, CuPc films exhibit a (3×3) structure on InSb(1 0 0) and a (√10×√10) R±18.4° structure on InAs(1 0 0). These differences can be rationalised by the vdW intermolecular interaction energy calculations of the quadratic unit cells of H 2Pc and CuPc. The results show that the lattice structure during the initial stages of thin film growth is influenced strongly by the intermolecular interaction energy of their unit cells.

THE THICKNESS DEPENDENT STRUCTURE OF COPPER PHTHALOCYANINE FILMS

We deposited copper phthalocyanine (CuPc) films on (100) silicon wafers by means of a vacuum evaporation method and investigated the dependence of X-ray diffraction pattern on film thickness. We concluded that the planar molecule of CuPc is stacked normal to the substrate within 700 nm from an interface. Above 700 nm, four types of nearly parallel stacking were also observed. These results show the spontaneous formation of the two-layer structure in CuPc films thicker than 700 nm.

Doping in the Mixed Layer to Achieve High Brightness andEfficiency Organic Light Emitting Devices

Doping in the mixed layer was introduced to fabricate high brightnessand high efficiency organic light emitting devices. In these devices, a copperphthalocyanine (CuPc) film acts as the buffer layer,a naphthylphenybiphenyl amine (NPB) film as the hole transport layerand a tris(8-hydroxyquinolinolate)aluminium (Alq3) film as theelectron transport layer. The luminescent layer consists of the mixtureof NPB, Alq3 (to be called the mixed layer), and an emittingdopant 5,6,11,12-petraphenylnaphthacene (rubrene), where theconcentration of NPB declined and the concentration of Alq3 wasincreased gradually in the deposition process. Adopting this dopingmixed layer, the device exhibits the maximum emission of49300 cd/m2 at 35 V and the maximum efficiency of 7.96 cd/A at10.5 V, which have been improved by two times in comparisonwith conventional doped devices. We attribute this improvement tothe effective confinement of carriers in the mixed layer, which leadsto the increase of the recombination efficiency of carriers.

Photoinduced ferroelectric hysteresis curve in organic CuPc photoconductor/inorganic BaTiO3 ferroelectric heterojunction photomemory

Photoinduced switching of the ferroelectric P–E hysteresis curve has been achieved in organic photoconductor (PC) copper phthalocyanine (CuPc)/inorganic ferroelectric (FE) BaTiO3 (BTO) heterojunction photomemory in the visible ray region. Light irradiation could switch the remanent polarization of ferroelectric BTO in this heterojunction up to 3 μC/cm2, almost full remanent polarization of BTO single layer, whereas remanent polarization of that without light irradiation was almost zero. We have also investigated the effect of CuPc thickness on the photoinduced change of ferroelectric P–E hysteresis curve, and found that the insulating dark resistance of the organic photoconductor CuPc film layer plays an important role for high efficiency of PC/FE heterojunction photomemory. The efficiency of this heterojunction photomemory reached up to 500%.

Spin Polarization in Molecular Orbitals of Copper−Phthalocyanine Deposited on a Magnetized Fe(100) Substrate

From a structural analysis using low-energy ion scattering spectroscopy, it is shown that Cu−phthalocyanine (CuPc) molecules adsorb on the Fe(100) surface with their planes parallel to the surface when the film thickness is one monolayer, while the structure of the film becomes disordered with an increase in the thickness. This thickness dependence of the CuPc film structure agrees with that determined from the metastable deexcitation spectroscopy (MDS) spectra, and the origins of the CuPc peaks in the MDS spectra are determined. The spin polarization in the molecular orbitals of CuPc on Fe is detected by spin-polarized MDS (SPMDS). The polarity of the spin polarization in the CuPc molecular orbitals is the same as that of the Fe surface. The adsorption of oxygen on Fe prior to the CuPc deposition is found to induce a remarkable difference in the spin polarization of the CuPc orbitals.

Optical properties of copper phthalocyanine

Optical functions of copper phthalocyanine (CuPc) have been determined using spectroscopic ellipsometry in the spectral range from 1.55 to 4.1 eV. The samples have been prepared by evaporation on glass substrates. CuPc deposited on unheated and heated (100 °C) substrates have been investigated. The optical functions data were determined by point-to-point fit, as well as the conventional oscillator model and modified oscillator model. Atomic force microscopy images revealed considerable surface roughness of the films and hence surface roughness correction in determination of the optical functions has been performed. Good agreement with experimental spectra has been obtained with all three methods for the sample grown on unheated substrate. However, for the CuPc film deposited on heated substrate the conventional oscillator model failed to achieve good agreement with the experimental data. Also, there are indications that the isotropic model may not be applicable to CuPc films grown on heated substrate.

The copper phthalocyanine/Au( [formula omitted]) interface studied using high resolution electron energy-loss spectroscopy

The vibrational and electronic excitations of copper phthalocyanine (CuPc) sublimed as a highly ordered thin films on Au(1 0 0) have been studied using high-resolution electron energy-loss spectroscopy. The vibrational modes of submonolayer, monolayer and multilayer CuPc films show strong differences in intensity for in- and out-of-plane modes which demonstrates the well ordered film structure. In addition, the interaction between CuPc and Au at the interface is very weak which is indicated by an only very small frequency shift of the vibrational modes and no substantial changes as regards the electronic excitations. Consequently, this interface can be regarded free of interface states.

Interface formation between thin Cu-phthalocyanine films and crystalline and oxidized silicon surfaces

Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on n-Si(1 0 0) and SiO 2/ n-Si substrates. Evolution of the surface potential and density of unoccupied electronic states located 0–25 eV above vacuum level were measured during the film deposition using an incident beam of low energy electrons using total current electron spectroscopy (TCS), and a new approach based on TCS analysis was developed and applied to studies of charge transfer at the interfaces. The analysis showed that a negative electric charge was transferred from the film to the substrate. A positively charged layer was registered in the CuPc films, which extends over a range from the substrate level and up to 10 nm. At a higher film thickness, bulk CuPc films were formed that have a workfunction of 4.5±0.1 eV. CuPc molecules decompose on the n-Si(1 0 0) substrate due to interaction with the crystalline surface while the film thickness is less than 3 nm. The features of the interfaces observed during formation of the interfaces are analyzed and a model for the extended charge-transfer layer (extended interface dipole) is applied.

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.

Direct observation of Fermi-level pinning in Cs-doped CuPc film

The electronic structures of pristine and Cs-doped CuPc films are investigated using photoemission spectroscopy and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital can be directly observed by IPES and ultraviolet photoemission spectroscopy simultaneously. We found that the Fermi-level position in organic film can be modified by Cs doping. The observed onset of the LUMO of the CuPc film is shifted by Cs doping to less than 0.2 eV above the Fermi level. The result indicates that the energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Copper phthalocyanine film grown by vacuum deposition under magnetic field

Copper phthalocyanine (CuPc) film with good orientation is always required when CuPc is to be used as an electronic functional layer in different device applications. In the present study, a magnetic field (∼6 mT) was applied during the vacuum deposition of CuPc. The CuPc film grown under such a magnetic field showed better orientation and a new stacking behavior as shown by X-ray diffraction study (XRD) and atomic force microscopy (AFM). In addition, the film grown under magnetic field exhibited different absorption characteristics in the UV-visible range. This demonstrates the potential of using magnetic field to optimize the growth of CuPc for various applications.

Optical Properties of Copper Phthalocyanine(CuPc)Thin Films

Vacuum sublimed thin films of copper phthalocyanine were prepared at room temperature. The optical properties of thin CuPc films have been investigated in the spectral range 200-2100 nm. The refractive index n as well as the absorption index k were calculated before and after annealing. It is obvious that the refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. The analysis of the spectral behavior of the absorption coefficient, α in the absorption region revealed direct transitions. The analysis of α as well as the dielectric constants e1 and e2 reveals several absorption peaks at photon energies given in the text. The fundamental and the onset energy gaps could also be estimated as 2.74+0.02 and 1.64+0.02 eV respectively.

Photo and gas sensitivity of thin Cu-phthalocyanine films studied by spectroscopy of unoccupied electron states

Thin films of Cu-phthalocyanine were thermally deposited in UHV on pyrolytic graphite, n-Si(1 0 0), SiO 2/n-Si, and SiO 2/p-Si substrates. Atomic composition of the films was tested by Auger electron spectroscopy. Evolution of surface potential and density of electron states located 0–25 eV above vacuum level were monitored during the film deposition by means of total current electron spectroscopy (TCS). The resulting spectra of the 2–3 nm thick CuPc films on the different substrates were however identical. Admission of 10 −5 Pa O 2 and NO 2 resulted in a reversible decrease and increase of the film surface potential, respectively. A new peak in the TCS appeared and the spectrum was attenuated on the admission of the gases. These changes were attributed to the interaction between the film and the gas molecules adsorbed. The changes were reversible on the gas evacuation. White light illumination reduced (increased) surface potential of the Cu-phthalocyanine film on SiO 2/n-Si (SiO 2/p-Si) substrate. The gas sensitivity is attributed to the processes at the gas/CuPc film interface, while the photovoltaic properties are attributed to the processes at the CuPc film/substrate interface.