Copper Phthalocyanine Thin Films Research Articles

Electronic properties of the space charge layer of the copper phthalocyanine thin films

Photoemission Yield Spectroscopy (PYS) has been used in studies of the electronic properties of the space charge layer of UHV annealed copper phthalocyanine (CuPc) thin films. The work functionφ, ionization energyΦ, absolute band bendingeVs and surface electron affinityXs is determined. Moreover, two filled electronic state bands localized in the band gap are observed. Their origin is briefly discussed.

Enhancement of Photoelectric Conversion Efficiency in Copper Phthalocyanine Solar Cell by Surface Plasmon Excitation

In order to enhance the photoconversion efficiency, surface plasmon polaritons (SPP) in an organic solar cell consisting of Al, copper phthalocyanine (CuPc) and Ag thin films were excited by an attenuated total reflection (ATR) method. The dependence of the enhancement factor for the short-circuit current (Isc) on the wavelength of the incident light (λin) was measured in detail in the visible region. It was found that the enhancement factor depends strongly on λin and takes a maximum (a factor of 8) around λin=550 nm. The good agreement between the calculated λin dependence of light absorption in the CuPc layer and the observed λin dependence of the enhancement factor suggests that the enhancement is caused by increased carrier generation in the CuPc layer due to increased light absorption upon SPP excitation.

Some comments on the electronic properties of the surface space charge layer of copper phthalocyanine thin films

A simple analysis, using a theory of the surface space charge layer of semiconductors, of the published values of the work function φ and surface ionization energy Φ s of copper phthalocyanine (CuPc) thin films was performed. Using a well known position of the Fermi level E F within the band gap E g the values of its absolute band bending eV s and surface electron affinity X s were determined. A small negative value of the absolute band bending eV s = −0.17 ∓ 0.15 eV has been interpreted by the existence of the filled electronic surface states localized in the band gap below the Fermi level E F. Such states were predicted theoretically for thin films and the crystalline surface of CuPc, and attributed to surface lattice defects of a high concentration.

A.c. electrical properties of thermally evaporated thin films of copper phthalocyanine

The a.c. electrical properties of copper phthalocyanine (CuPc) thin films have been studied in the frequency range 10 2−2 × 10 4 Hz and in the temperature range 173–360 K. The a.c. conductivity σ(ω) was found to vary as ω s with the index s ⩽ 1, indicating a dominant hopping process at low temperatures and high frequencies. At higher temperatures and lower frequencies, free carrier conduction with mean activation energy of 0.3 eV was observed. Capacitance and loss tangent decreased with frequency and increased with temperature. Such characteristics were found to be in good qualitative agreement with the existing model of Goswami and Goswami. Both the conductance and capacitance were lowered over the entire frequency spectrum after heat treatment at 370 K. This reduction was ascribed to the desorption of oxygen by the heat treatment, as previously observed in d.c. conduction measurements.

Photogeneration mechanism of charged carriers in copper-phthalocyanine thin films

The photogeneration properties of copper-phthalocyanine (CuPc) thin films were investigated by measuring photo-induced displacement currents for metal/CuPc/SiO 2/Si devices. The impurity effect that the quantum efficiency (QE) of photogeneration in the layer including impurities is larger than that in the intrinsic layer was quantitatively estimated from this method. The QE increase has been found to be due to the charge transfer reaction from an electron-hole pair generated by photon absorption to an impurity. The QE difference between two layers is discussed in relation to the formation process of bound pairs of opposite charges.

Integrated circuit microsensor for selectively detecting nitrogen dioxide and diisopropyl methylphosphonate

A novel gas-sensitive microsensor, known as the interdiginatated gate electrode field effect transistor (IGEFET), was realized by selectively decositing a chemically-active, electron-beam evaporated copper phthalocyanine (CuPc) thin film onto an integrated circuit (IC). When isothermally operated at 150 °C, the microsensor can selectively and reversibly detect parts-per-billion concentration levels of nitrogen dioxide (NO 2) and diisopropyl methylphosphonate (DIMP). The selectivity feature of the microsensor was established by operating it with a 5 V peak amplitude, 2 μs duration, 1000 Hz repetition frequency pulse, and then analyzing its time- and frequency-domain responses. The envelopes associated with the normalized-difference Fourier transform magnitude spectra, and their derivatives, reveal features which unambiguously distinguish the NO 2 and DIMP challenge gas responses. Furthermore, the area beneath each response envelope may correspondingly be interpreted as the microsensor's sensitivity for a specific challenge gas concentration. Scanning electron microscopy (SEM) was used to characterize the morphology of the CuPc thin film. Additionally, infrared (IR) spectroscopy was employed to verify the α- and β-phases of the sublimed CuPc thin films and to study the NO 2- and DIMP-CuPc interactions.

Interaction of NO 2 with copper phthalocyanine thin films I: Characterization of the copper phthalocyanine films

Thin films of copper phthalocyanine (CuPc) have been deposited by vacuum evaporation on glass or alumina substrates maintained at different temperatures. The morphology and the crystallinity of as-deposited films and of heat-treated and/or NO 2-treated films have been studied by electron microscopy and X-ray diffraction. The crystallinity of the films increases with substrate temperature during deposition: films deposited at room temperature or below are rather amorphous, whereas films deposited at 80°C or 200°C exhibit a highly uniaxial order of their crystallites; this order is altered by heat treatment or NO 2 doping. Scanning electron microscopy shows that the morphology of the film surface also depends on deposition temperature and that heat treatments induce the growth of whiskers from the surface. The structural changes are also observed through the temperatures of maximum conductivity of the CuPc films doped with NO 2. The sensitivity of CuPc to NO 2 could be applied to gas sensing.

Interaction of NO 2 with copper phthalocyanine thin films II: Application to gas sensing

Thin films of copper phthalocyanine (CuPc) can be doped by oxidizing gases such as NO 2. The film conductivity goes through a maximum for a temperature T m which increases as the film evolves from the α phase to the β phase under the action of NO 2 doping and heat treatment. The temperatures of maximum conductivity were determined for CuPc, PbPc, FePc, and LiPc after doping with NO 2, Cl 2 or O 2. The sensitivity and stability of the responses of CuPc thin films to NO 2 are discussed as well as the application of this material as the sensitive element of gas sensors.

Structural Studies of Thermally Evaporated Thin Films of Copper Phthalocyanine

Thin films of the organic semiconductor copper phthalocyanine (CuPc) are deposited onto glass substrates maintained at room temperature, and their structures investigated using X-ray diffractometry and infrared light absorption. Films of thicknesses approximately 0.1 μm are preferentially oriented in the [001] direction. Further growth of thicker films has the effect of randomizing the orientation of the phthalocyanine crystallites, with structure mainly of the α-form. Prolonged heating at 240 °C produces a structural transformation of much of the material from the α- to the stable β-form, with the films preferentially oriented in the [201] direction. These structural changes are confirmed by infrared optical absorption analysis. Dunnschichten des organischen Halbleiters Kupferphthalozyanin (CuPc) werden bei Raumtemperatur auf Glasunterlagen aufgebracht. Ihre Struktur wird mittels Rontgenstrahlbeugung und IR-Absorption untersucht. Schichten mit Dicken von etwa 0.1 pm sind vorzugsweise in [001]-Richtung orientiert. Weiteres Wachstum dickerer Schichten fuhrt zu einer Zufallsverteilung der CuPc-Kristallitorientierungen, vorwiegend mit α-Struktur. Langeres Erwarmen auf 240 °C bewirkt eine Strukturumwandlung eines grosen Teils des Materials von der α-Phase in die stabile β-Phase, wobei die Schichten vorzugsweise in [201]-Richtung orientiert sind. Die Strukturveranderungen werden durch IR-Absorptionsanalyse bestatigt.

Variable angle spectroscopic ellipsometry studies of oriented phthalocyanine films. II. Copper phthalocyanine

Recent studies obtaining the anisotropic complex dielectric spectra of oriented metal-free phthalocyanine (H2Pc) films with variable angle spectroscopic ellipsometry (VASE) have been extended to similarly oriented copper phthalocyanine (CuPc) thin films. The CuPc films were vacuum vapor deposited onto sputter deposited copper substrates. Two classes of anisotropically oriented films with thicknesses on the order of 1000 Å were produced with the molecular stacking b-axis in either a ‘‘standing’’ or surface-parallel configuration by controlling the substrate temperature. Reflection absorption infrared spectroscopy and x-ray diffraction were used to verify the different orientations. VASE data were acquired over a 4000–8000 Å wavelength range in 100 Å steps, and 45°–79° incidence angle range in 2° steps. Three-dimensional plots of the ellipsometric parameters, Δ and Ψ, versus wavelength and incidence angle have different characteristic shapes for the two classes of oriented films, similar to H2Pc, leading to significant differences in the optical constant spectral solutions obtained with an isotropic film model. Knowing the high degree of uniaxial ordering in the two film classes, approximate solutions to the parallel and perpendicular optical constants taken with respect to the crystalline b axis can be extracted from the isotropic solutions using a two-dimensional Bruggeman effective medium approximation.

Micrometer patterning of phthalocyanine derivatives by selective chemical vapor deposition method

Selective chemical vapor deposition was achieved to produce copper phthalocyanine thin films on the copper microelectrodes on insulating and optically transparent materials. By thermally annealing the fabricated films at 500 °C under vacuum, their quality was greatly improved and the conductivity of the modified copper microelectrodes became high.

In Situ Photoconductive Study of C60 and Phthalocyanine Thin Films

ABSTRACTThe in situ measurement of lateral photoconductivity has been carried out on vacuum vapor deposited C60 and phthalocyanine thin films. From the photo-current increase curve during the deposition and the SEM observation of copper phthalocyanine (CuPc) thin films, it has been revealed that, at an early stage of deposition, they consist of small clusters scattered on substrate. As the thickness Increases, the clusters coalesce to eventually form continuous film. We also show that C60 thin film has considerable photoconductivity. The photo-current spectrum agrees well with the absorption one with the exception that a peak at 600 nm appears only in the former. This wavelength region corresponds to a theoretically predicted forbidden transition. An onset of photocurrent at 760 nm is assigned to the absorption edge of solid C60. The photoconductivity of C60 thin films shows a rather unusual temperature dependence In that a small drop of temperature (e.g. from 25° C to 15° C) causes a dramatic increase In photocurrent by more than two orders of magnitude.

Micrometer patterning of phthalocyanines by selective chemical vapor deposition

Micrometer size patterns of copper phthalocyanine derivatives were fabricated from 1,2,4,5-tetracyanobenzene (TCNB) by a selective chemical vapor deposition method. Copper films were patterned on silicon wafers by photolithography and wet etching techniques, sealed in a glass tube with TCNB, and heated at different temperatures. Controlling the treatment temperature, selective chemical vapor deposition was achieved to produce copper phthalocyanine thin films on the copper patterns. After thermal annealing of the deposited film in vacuum, the films were converted to a polymer of the copper phthalocyanines.

Parabolic dispersion and effective mass of hot electrons in oriented thin films of copper phthalocyanine determined by means of low-energy-electron transmission.

Low-energy-electron transmission (LEET) spectra were measured for thin films of copper phthalocyanine (CuPc) with different crystalline order using electron energies below 15 eV. The LEET features were highly dependent on the crystalline order, indicating that they do not originate from the electronic states localized in the individual molecules, but from those which reflect the crystal structure. For oriented polycrystalline films of \ensuremath{\alpha}-form CuPc with the crystal bc plane parallel to the surface, the energy positions of the LEET minima were found to be in excellent agreement with those of the band gaps obtained by assuming a parabolic conduction band in the direction perpendicular to the crystal bc plane. Further, we estimated the effective mass of hot electron ${\mathit{m}}^{\mathrm{*}}$ to be 2.${2}_{2}$${\mathit{m}}_{0}$, where ${\mathit{m}}_{0}$ is the free-electron mass, and the inner potential ${\mathit{V}}_{0}$ to be -1.${3}_{3}$ eV in the direction perpendicular to the bc plane of \ensuremath{\alpha}-form CuPc. The results show that the specular reflection of the incident electron at the band gap is a main factor for the LEET features and the hot-electron state can be well described by the parabolic dispersion even near the vacuum level.

Selective detection of nitrogen dioxide and diisopropyl methylphosphonate with an interdigitated gate electrode field-effect transistor (IGEFET)

A microelectronic integrated circuit, referred to as the interdigitated gate electrode field-effect transistor (IGEFET), has been coupled with a chemically active electron-beam evaporated copper phthalocyanine (CuPc) thin film to realize a novel gas-sensitive microsensor. The sensor has been utilized to selectively detect parts-per-billion concentration levels of nitrogen dioxide (NO 2) and diisopropyl methylphosphonate (DIMP). The sensor is excited with a voltage pulse, and the time- and frequency-domain responses are measured. The envelopes associated with the normalized difference Fourier transform magnitude frequency spectra reveal features which unambiguously distinguish the NO 2 and DIMP challenge gas responses. The area beneath each response envelope can correspondingly be interpreted as the sensor's sensitivity for a specific challenge gas concentration.

Structure and nonlinear optical properties of copper phthalocyanine thin films

This paper describes a joint study of the structure and nonlinear optical properties of vacuum evaporated thin films of copper phthalocyanine (CuPc for brevity). Film thickness ranges from 50 to 500 nm. The anisotropic paramagnetic resonance of Cu++ ions reveals that the Pc rings lie almost parallel to the substrate plane with however a large angular distribution (30° FWHM). Third harmonic optical generation measurements performed at 1.064 μm and 1.907 μm fundamental wavelengths give respectively an average value of the cubic susceptibility 〈χ(3)(-3ωω,ωω)〉=(4±0.4)·10−12 e.s.u. and (2.1+-0.2) · 10-12 These values, although significantly higher than for a common ionic crystal, are about one order of magnitude lower than in conjugated 1-D systems, which shows that the 2-D π-electron delocalization is less profitable than the 1-D one. Besides third harmonic, we have also observed second harmonic generation. Its polarization dependence is characteristic of a quadratic susceptibility enhanced in one direction, almost perpendicular to the substrate, withd eff comprised between 30 and 60 · 10-9 e.s.u. The possible origins ofd eff are discussed.

Effects of annealing on the trap distribution of cobalt phthalocyanine thin films†

Current density-voltage characteristics have been obtained from evaporated cobalt phthalocyanine thin Film sandwich structures using ohmic gold contacts, both before and after annealing. Results show that at low voltages the conduction process is ohmic, while at high voltages space-charge limited conduction is present. These results are consistent with current density-voltage characteristics obtained with copper phthalocyanine thin films. Annealing resulted in little change in current density within the space-charge limited region but a significant reduction within the ohmic region. Subsequent exposure to air gave an increase in current density tending towards levels obtained with fresh samples. Analysis of the results yields a typical room temperature hole concentration of p 0 = 8 × 1017 m−3 for unannealed samples. An exponential trap distribution P(E) = P 0 exp (−E/kT t) was consistent with the higher-voltage data, where typically P 0 = 8 × 1043J−1 m−3 and T t = 885 K. Variations of parameters with heating conditions are discussed, with particular reference to the desorption of oxygen during annealing and its absorption during exposure to air.

High field electronic conduction processes in copper phthalocyanine thin films using lead and gold electrode combinations

The electrical properties of copper phthalocyanine (CuPc) thin films have been studied using lead and gold electrode combinations. Samples with two gold electrodes showed ohmic conduction and space-charge-limited conductivity, in agreement with previous work. Samples having one electrode of each metal showed three separate regions of space-charge-limited conductivity under forward bias (gold positive), but with considerably lower current density when the non-substrate (top) electrode was lead. Diode behaviour was also observed in samples where the lead electrode was on the substrate (bottom) side of the CuPc film. Under reverse bias the injecting lead electrode was found to act as a blocking contact with barrier height approximately 1 eV. The barrier widths were approximately 40 nm and 100 nm when the lead electrode was on the bottom and top sides respectively of the CuPc films. At higher voltage levels the Poole-Frenkel effect was observed when lead was the top electrode. Sample rectification ratios were also dependent on the electrode positions, having a value of several hundred with lead as the bottom electrode and considerably lower values as the top electrode. In the latter case the rectification ratio increased by a factor of about 10 on annealing and exposure to air. The results are consistent with the establishment of a barrier region when the non-substrate side of the CuPc films is in contact with a lead electrode.

Asymmetric electrical conductivity in oxygen-doped thin films of copper phthalocyanine, using gold and aluminium electrodes

Evaporated copper phthalocyanine (CuPc) thin films in the form of metal-CuPc-metal sandwich structures have been extensively studied, particularly with regard to their electrical conductivity and its variation on exposure to various gases. In the present work, CuPc thin films doped by prolonged exposure to oxygen have been incorporated into asymmetric sandwich structures using ohmic gold and blocking aluminium electrodes. DC conduction processes identified depend both on the polarity and on the electric field strength. Under forward bias (gold electrode positive), conductivity was ascribed to space-charge-limited conductivity controlled by an exponential trap distribution at lower fields and by a single dominant level at higher fields, in accordance with prior work on comparable organic films. Under reverse bias, currents were identified with Schottky emission over a potential barrier of approximate height 1 eV and thickness 200 nm. Additional work on annealed samples showed broadly similar behaviour under forward bias with a reduced Schottky barrier thickness under reverse bias. At higher voltages there is evidence of an electrode-to-bulk limited transition, with conductivity dominated by the Poole-Frenkel effect.

Effect of gravity on copper phthalocyanine thin films I: Enhanced smoothness and local homogeneity of copper phthalocyanine thin films grown in a microgravity environment

Closed-cell physical vapor transport (PVT) has been used for the first time to deposit epitaxially thin films of organic pigments. A series of experiments were conducted both in low earth orbit on the Space Shuttle Orbiter and in the laboratory as ground controls using the 3M-PVTOS (PVT of organic solids) apparatus. The experiments used PVT as a process to deposit oriented organic thin films of phthalocyanine and perylene compound derivatives on epitaxially active substrates sealed within demountable ampoules. In a series of three papers, of which this is the first, the complete characterization of the microscopic physical structure and macroscopic optical properties of the thin films produced using copper phthalocyanine as the organic source material is presented, and detailed comparisons are made between the microgravity-grown and unit-gravity (laboratory) grown samples. In this first paper of the series, the results of comparisons of the microgravity and unit gravity films using visual photography, bright field and differential interference contrast microscopy, scanning ellipsometry, visible reflection spectroscopy and direct interferometric phase contrast microscopy are presented. These techniques together imply that the space-grown films are radially more uniform and homogeneous, are an order of magnitude smoother over the submillimeter to submicron range as indicated by their lower relative surface areas, and possess effective optical refractive index vs. position profiles which are distinctly different from those of the laboratory-grown control films. In the next two papers of this series, reflection IR spectroscopy, X-ray diffraction and scanning electron microscopy are used to show further that the microgravity-grown films are more highly uniaxially oriented, significantly denser and essentially contain only crystalline domains which consist of a previously unknown polymorph now designated as M-CuPc.