CuPc Films Research Articles

Effects of Gold Nanorods on Photocurrents from Copper Phthalocyanine-Gold Nanorod Composite Films

The fabrication of copper phthalocyanine (CuPc)–gold nanorod (AuNR) composite films on indium-tin-oxide electrodes was performed by electrostatic layer-by-layer adsorption technique. The photocurrents in CuPc–AuNR composite films were larger than those in CuPc films as a reference. The enhancements of the photocurrents in CuPc–AuNR composite films due to AuNR increased with increasing of wavelength up to near-infrared region. The enhancements are attributable to localized surface plasmon resonance (LSPR) due to AuNR. The enhancements at near-infrared wavelength due to the longitudinal LSPR were larger than those at visible wavelength due to the transverse LSPR.

Effect of dispersion of gold nanoparticles on the properties and alignment of liquid crystalline copper phthalocyanine films

The inclusion of hexadecylamine capped gold nanoparticles (AuNPs) into the ordered matrix of hexagonal columnar mesophase of copper phthalocyanine (CuPc) has been investigated. The effects of AuNPs on the liquid crystalline behaviour, optical and electrical properties of CuPc films is investigated by different physical methods. The molecules orientation in the films of CuPc and its composites with AuNPs, deposited on the surface of one substrate and between two electrodes, is studied by polarized Raman spectroscopy and polarized microscopy. It has been shown that thin films of the composites confined between ITO and Al electrodes are characterized by homeotropic alignment while no homeotropic alignment was observed in films deposited on ITO substrate with air constituting the upper interface. The I(V) dependences for the films confined between ITO and Al electrodes were measured. Appreciable enhancement of the DC conductivity is observed in the case of composites containing gold nanoparticles.

Ambience-related adsorbates on CuPc surface—Photoemission and thermal desorption spectroscopy studies for control of organic electronics degradation processes

In present work, using the angle-resolved X-ray photoelectron spectroscopy, the chemical structure of the 32nm-thick copper phthalocyanine (CuPc) films surface was revealed upon long-term ambient air exposure. The existence of ambience-related components was detected in the C1s, N1s and O1s spectra and their depth distributions within the CuPc layer were analyzed. Combined ultraviolet photoelectron and photoemission yield spectroscopies provided information concerning redistribution of electron density in surface region which was reflected in surface band bending increase by 0.91eV, shift in electron affinity of 0.26eV and work function variation of 1.17eV. The kind and amount of desorbed molecules or atoms from phthalocyanine’s surface were investigated by the thermal desorption spectroscopy. Quantified activation energies for desorption of C, N2, O2 and H2O species with values in the range of 120–166kJ/mol indicated chemical adsorption process (of rather covalent than ionic nature) of the ambience related adsorbates on organic surface.

Surface-type nonvolatile electric memory elements based on organic-on-organic CuPc-H2Pc heterojunction**Project supported by the GIK Institute of Engineering Science and Technology, Pakistan and Physical Technical Institute of Academy of Sciences of Tajikistan.

A novel surface-type nonvolatile electric memory elements based on organic semiconductors CuPc and H2Pc are fabricated by vacuum deposition of the CuPc and H2Pc films on preliminary deposited metallic (Ag and Cu) electrodes. The gap between Ag and Cu electrodes is 30–40 μm. For the current–voltage (I–V) characteristics the memory effect, switching effect, and negative differential resistance regions are observed. The switching mechanism is attributed to the electric-field-induced charge transfer. As a result the device switches from a low to a high-conductivity state and then back to a low conductivity state if the opposite polarity voltage is applied. The ratio of resistance at the high resistance state to that at the low resistance state is equal to 120–150. Under the switching condition, the electric current increases ∼ 80–100 times. A comparison between the forward and reverse I–V characteristics shows the presence of rectifying behavior.

Carrier Transport Mechanism of Copper Phthalocyanine Based Photodiode for Solar Cell Applications

Copper phthalocyanine (CuPc)/n-silicon junction was fabricated using thermal evaporator method. X-ray analysis of the CuPc film confirms the b-phase with preferred orientation along (100) direction. The crystallite size of the CuPc film was estimated using XRD data and observed to be about 12.6 nm. The current-voltage characteristics of Au/CuPc/n-Si/Au device was studied in dark and under illumination. The device showed diode characteristics. The diode parameters such as ideality factor, barrier height and series resistance were determined using different techniques such as conventional forward bias I-V characteristics, Cheung method and Norde’s function. A good agreement between the diode parameters calculated form these methods was observed. The analysis of the diode characteristics confirmed that the transport mechanism of the Au/CuPc/n-Si/Au diode at the higher electric fields was governed by the space-charge-limited currents. The photoconducting behavior of the diode suggests that it can be used as a photosensor in optoelectronic applications.

Tuning carrier mobility without spin transport degrading in copper-phthalocyanine

We demonstrate more than one order of magnitude of carrier mobility tuning for the copper-phthalocyanine (CuPc) without spin transport degrading in organic spin valve devices. Depending on the preparation conditions, organic spin valves with the CuPc film mobility of 5.78 × 10−3 and 1.11 × 10−4 cm2/V s are obtained for polycrystalline and amorphous CuPc, respectively. Strikingly, the spin diffusion lengths are almost the same regardless of their mobilities that are ∼50 times different, which is in sharp contrast with previous prediction. These findings directly support that the spin relaxation in CuPc is dominated by the spin-orbit coupling.

Study of metal specific interaction, F-LUMO and VL shift to understand interface of CuPc thin films and noble metal surfaces

The performances of organic electronic devices are significantly associated with their energy level alignment at organic semiconductor/metal–electrode interfaces. The electronic character of an organic semiconducting molecular over-layer on a metal surface can vary from semiconducting to metallic, depending on the nature of the molecular orbitals with respect to the Fermi level of the electrode. The general tendency of extrapolating established models for single crystal substrates to ‘real’ device substrates is highly misleading. Hence, the importance of metal specific interaction, former lowest unoccupied molecular orbital (F-LUMO) and vacuum level (VL) shift have been investigated as a function of thickness of the deposited films by means of photoelectron spectroscopy (XPS and UPS) to understand the interface between CuPc and Cu, Ag, Pt and Au foils sequentially. The XPS data provides the signature of affectability of pyrrole sites of CuPc molecules for partial charge transfer from Cu and Ag substrates while a negligible effect on pyrrole cites resulted for Pt and Au substrates. Furthermore, the appearance of F-LUMO, a hybridized state close to the Fermi level gives confirmatory information about partial charge transfer. Contrary to the general belief that vacuum level shift caused by charge transfer can partially or totally cancel that for push back effect, our observation indicates that the partial charge transfer does not play significant role in the shift of vacuum level. The entire thickness dependent electronic energy level alignment of CuPc films on all noble metal substrates is explained in terms of a combined effect of partial charge transfer and photoemission final state relaxation energy. A systematic variation of HOMO-FWHM was observed with CuPc thickness due to continuous change in molecular orientation.

Effects of Heat Annealing on the Gas Sensing Properties of Spin Coated Unsubstituted Copper Phthalocyanine Films

Unsubstituted copper phthalocyanine (CuPc) thin films on glass substrate were prepared using spin coating technique. The effects of heat annealing of CuPc films on the NO2 gas sensing properties were studied. The results showed that the films annealed at 150°C have highest sensitivity, highest response rate and lower response time. Sensitivity and response rate were decreased and response time was increased after annealing with temperature 200°C and above. The duration over which the films were annealed changes the gas sensing properties. Highest sensitivity, highest response rate, lower response time and highest recovery rate were observed for 2 hour (150°C) annealed films. Longer annealing time causes the decrease of sensitivity, response rate and recovery rate and increase of response time however improves recovery ratio. The heat treatment limited to the temperature of 150°C for 2 hours duration improves the sensing properties of the films. Hence heat annealed unsubstituted CuPc spin coated films can be used as nitrogen dioxide gas sensor.

Effect of thermal annealing on the structural and optical properties of spin coated copper phthalocyanine thin films

Low cost sol–gel spin coating was used to deposit copper phthalocyanine (CuPc) thin films on both fused quartz and glass substrate. The prepared films were studied before and after thermal annealing at 350 °C for 1 h in air. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) were used to study the structural properties. From the structural characterization results, the films transformed from the metastable α-phase to the stable β-phase. Refractive index, absorption coefficient, and lattice dielectric constant were evaluated before and after annealing for the first time for spin coated CuPc thin films using spectrophotometric measurements in the spectral range 200–2500 nm. The values of the direct optical band gap of the as deposited film at 1.52 eV and 2.85 eV were redshifted to 1.4 eV and 2.42 eV for the annealed film. This shift is significant for near infrared photonics. The third order non-linear susceptibility was presented at lower photon energy for the CuPc films showing higher value for the annealed film.

Influence of Substrate Temperature on the Optoelectronic Properties of CuPc Thin Film

The copper phthalocyanine thin films were deposited on quartz and Si/SiO2 substrates at different substrate temperatures. The morphologies, structures and optical properties of CuPc films were characterized by scanning electron microscopy, X-ray diffra ction and UV-Visible spectroscopy. The polymorph of alpha–CuPc was observed in films deposited at temperature ranging from 20°C to 145°C. The transmission and absorption spectra of CuPc films deposited at different temperatures are presented.

Microspot two-photon photoemission spectroscopy for CuPc film on HOPG

Microspot two-photon photoemission (micro-2PPE) spectroscopy has been applied to measure the lateral distribution of unoccupied levels on copper phthalocyanine (CuPc) film on HOPG. In addition to the LUMO-derived level and the image potential state (IPS) on the film, we identified the modified IPS which is stabilized by the hole localized in a molecule. We show that modified IPS is observed only on bilayer area, reflecting the localization of the hole in a molecule. The modified IPS is absent on monolayer area, because the hole strongly interacts with substrate.

Different Photoresponses for Positive and Negative Biases of CuPc/C60 Heterojunction Nanostructures

Photoresponse characteristic from efficient exciton dissociated heterojunction based on copper phthalocyanine (CuPc) and fullerene (C60) layers was observed the different spectrum responses under positive and negative biases. The nanostructures of CuPc and C60 thin films were fabricated between transparent indium tin oxide (ITO) and aluminum (Al) electrodes. The 100 nm thick of CuPc and C60 layers were deposited on patterned ITO glass substrates by thermal evaporation with quartz thickness monitor. Photoresponses of the fabricated devices were investigated by current measuring as a function of wavelength in range of 400 to 700 nm. Measured current in Al/C60/CuPc/ITO structure when applied negative voltage to ITO electrode is higher than that of positive voltage case. Under monochromatic light, the photoresponse characteristic of Al/C60/CuPc/ITO structure under negative bias shows dominate response current peak at around 450 nm and double peaks in range of 500-700 nm originated from C60 and CuPc layers, respectively. These two response characteristics can be described by the combination of responses from Al/C60/ITO and Al/CuPc/ITO structures. The response current characteristics of Al/C60/ITO and Al/CuPc/ITO structures also agree with the optical absorptions of C60 and CuPc layers, respectively. By applying positive bias to Al/C60/CuPc/ITO structure, the photoresponse characteristic has only one peak at about 450 nm that is similar to the response in Al/C60/ITO structure only. This indicates that under positive bias, the photocurrent only from C60 layer can be observed.

The importance of gap states for energy level alignment at hybrid interfaces

Energy level alignment and electronic structure at organic semiconductor interfaces must be controlled to ensure efficient carrier harvesting or injection in next-generation organic optoelectronic technologies. In this context, hybrid organic/inorganic semiconductor interfaces exhibit particularly rich physics. Here, we show that states in the band gap of the inorganic layered van der Waals dichalcogenide SnS2 play an important role in determining energy level alignment at the hybrid interface with copper phthalocyanine (CuPc). By taking advantage of the closely related CuPc film growth on SnS2 and the well-studied interface of CuPc/HOPG, we are able to trace spectroscopic differences to the fundamentally different electronic interactions across the two interfaces. We provide a detailed picture of the role of gap states at the hybrid interface and shed light on the electronic properties of inorganic semiconductors in general and metal dichalcogenides in particular.

Spin coated unsubstituted copper phthalocyanine thin films for nitrogen dioxide sensors

Copper phthalocyanine ( CuPc ) is synthesized chemically and used for making CuPc thin films using spin coating technique. Films were prepared from trifluroacetic acid (TFA) and chlorobenzene mixed solution on the glass substrate. Spin coated films of unsubstituted CuPc films were heat annealed at 150°C for 2 h duration and were used to study NO 2 gas sensing characteristics. α-phase of CuPc is noted by UV-visible absorption spectra. IR spectra of undoped CuPc films and doped CuPc films with NO 2 revealed that, doping of nitrogen dioxide modifies and deletes some of the bands. The effect of NO 2 at various concentrations from 50 ppm to 500 ppm in atmospheric air at room temperature on the electrical conductivity of CuPc films was studied. Sensitivity, response time and repeatability of the CuPc sensor were discussed in this paper.

(TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8]/CuPc two-component bilayer heterojunction-based organic transistors with high ambipolar performance.

Organic thin film transistor (OTFT) devices fabricated by the solution-based QLS technique from a mixed (phthalocyaninato)(porphyrinato) europium complex (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] exhibit air-stable ambipolar performance with mobilities of 6.0 × 10(-5) cm(2) V(-1) s(-)1 for holes and 1.4 × 10(-4) cm(2) V(-1) s(-1) for electrons, respectively. In good contrast, the two-component bilayer heterojunction thin film devices constructed by directly growing (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] on vacuum deposited (VCD) CuPc film using solution based QLS method were revealed to show unprecedented ambipolar performance with carrier mobilities of 0.16 cm(2) V(-1) s(-1) for holes and 0.30 cm(2) V(-1) s(-1) for electrons. In addition to the intrinsic role of p-type organic semiconductor, the VCD CuPc film on the substrate also acts as a good template that induces significant improvement over the molecular ordering of triple-decker compound in the film. In particular, it results in the change in the aggregation mode of (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] from J-type in the single-layer film to H-type in the bilayer film according to the UV-vis, XRD, and AFM observations.

Critical CuI buffer layer surface density for organic molecular crystal orientation change

We have determined the critical surface density of the CuI buffer layer inserted to change the preferred orientation of copper phthalocyanine (CuPc) crystals grown on the buffer layer. X-ray reflectivity measurements were performed to obtain the density profiles of the buffer layers and out-of-plane and 2D grazing-incidence X-ray diffraction measurements were performed to determine the preferred orientations of the molecular crystals. Remarkably, it was found that the preferred orientation of the CuPc film is completely changed from edge-on (1 0 0) to face-on (1 1 −2) by a CuI buffer layer with a very low surface density, so low that a large proportion of the substrate surface is bare.

Structural and Optical Properties of Vacuum Coated and Annealed Copper Phthalocyanine (CUPC) Thin Films

Heat treatment is able to cause the structural transformation of phthalocyanine thin film during the post-deposition heat treatment. The annealing effects of the process on the film characteristics of vacuum deposited copper phthalocyanine thin films were studied. However, the pre-annealing leads to a larger-grain and a more compact structure. Structure transformation was inhibited by post-deposition annealing at 300oC. Thin films of CuPc were deposited using vacuum evaporation technique method onto glass substrates. The films were annealed at 4 different temperatures (323, 333, 343 and 353K) in air at different intervals of time. Optical studies were done and the results are discussed. The optical band gap for the film as obtained from the graph for all the thin films are tabulated and analyzed. X ray diffraction studies on the deposited film showed crystallinity with a major peak for CuPc.

Interface properties of FeMgOCu-phthalocyanine metal-insulator-organic semiconductor structures

Hybrid interface structures consisting of organic copper-phthalocyanine (CuPc) and ferromagnetic metal Fe(001) with and without a MgO(001) cover were investigated by using surface sensitive techniques of X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. A systematic study of the energy level alignment at the interfaces was carried out. For the hybrid interfaces considered here, our results indicate that the insertion of an artificially-grown ultra-thin oxide layer MgO(001) can prevent Femi level pinning and induce a rather large interface dipole, thereby resulting in remarkable CuPc Fermi level shifts when the thickness of the CuPc film is less than 3 nm. This study provides a better understanding of spin filtering in MgO-based organic spin devices and a new way to alter the interface electronic structure of metalorganic semiconductor hybrid systems.

Excitation of optical modes supported by strong absorption in organic thin films at attenuated total reflection geometry

Reflection dips observed at attenuated total reflection (ATR) geometry for metal thin films are interpreted from excitations of surface plasmon polaritons. We have presented that the absorption induced by enhanced electric fields in a metal film is an origin of this phenomenon (Wakamatsu et al (2007 J. Opt. Soc. Am. B 24 2307)). This viewpoint provides that ATR dips can be generated not only in metals but in other materials. Here, the absorption spectra of copper phthalocyanine (CuPc) thin films were observed at ATR geometry without a metal layer. Only from p-polarized ATR measurements was a strong absorption dip observed in the short wavelength region of the CuPc absorption bands. Kramers–Kronig analyses for the transmittance spectra of a CuPc thin film revealed that the refractive index of the film lowers around the spectral position of the p-polarized ATR dip. Transfer matrix analyses indicated that the lowered refractive index of CuPc induces the enhancement of evanescent fields in the film. These results demonstrate that the ATR dip generation is attributable to the absorption derived from enhanced evanescent fields in the CuPc film.

Ultrafast all-optical universal logic gates with graphene and graphene-oxide metal porphyrin composites

We present a detailed theoretical analysis of reverse saturable absorption (RSA) in graphene with picosecond laser pulses at 532 nm, and in graphene-oxide (GO)-metal porphyrin composites with femtosecond (fs) laser pulses at 800 nm. Increase in pulse intensity leads to increased contrast in graphene due to strong two-photon absorption (TPA) and larger excited-state absorption coefficients in comparison to GO and GO-metal porphyrin composite materials. The RSA characteristics are sensitive to pump pulse intensity and pulse width, TPA coefficient and concentration and have been theoretically optimized to design all-optical NOT and the universal NOR and NAND logic gates. Femtosecond operation at relatively lower pump intensities (GW/cm$$^{2})$$2) compared to Cu-Pc doped PMMA thin films (TW/cm$$^{2})$$2) and GO thin films (GW/cm$$^{2})$$2) demonstrates the applicability of GO-metal porphyrin composites for ultrafast all-optical information processing.