CuPc Films Research Articles

Flexible solar cells based on copper phthalocyanine and buckminsterfullerene

Flexible solar cells based on copper phthalocyanine (CuPc) and buckminsterfullerene (C 60) were fabricated on flexible indium-tin-oxide (ITO) coated polyethylene terephthalate (PET) substrates. Substrate temperature was found obviously affecting the molecular orientation of CuPc films and low substrate temperature should be adopted in order to transport charges perpendicular to substrate surface. For planar heterojunction cell PET-ITO/CuPc/C 60/CuPc (10 nm)/Al, the appropriate thickness of CuPc layer and C 60 layer was 40 and 80 nm, respectively, in view of light absorption and power conversion efficiency. For hybrid planar-mixed molecular heterojunction cell PET-ITO/CuPc/CuPc:C 60/C 60/CuPc (10 nm)/Al, 33%(mole) of CuPc in CuPc:C 60 blend layer resulted in well-distributed micro grains, which helped to improve photovoltaic performance. As the thickness of blend layer was increased to 90 nm, 0.85% of power conversion efficiency could be obtained. Bending test showed that no significant change happened in photovoltaic performance as tensile curvature up to 1.2 cm −1 and compressive curvature up to 1.35 cm −1, respectively.

Impact of Ground‐State Charge Transfer and Polarization Energy Change on Energy Band Offsets at Donor/Acceptor Interface in Organic Photovoltaics

AbstractThe fullerene (C60)/copper phthalocyanine (CuPc) interface is one of the widely used donor/acceptor (DA) interfaces for organic photovoltaics (OPVs), and information on the electronic structure at the interface is essential for fully understanding the energetics of excitons and carriers in OPVs. Here, an investigation into the energy levels at the C60/CuPc interface is made using UV photoelectron, X‐ray photoelectron, and inverse photoemission spectroscopies. The vacuum level and core levels rise with C60 deposition on the CuPc film, which indicates that the interfacial dipole is formed with the negative charge on the C60 side. The interfacial dipole can be formed by the electron transfer from CuPc to C60 in the ground state at the interface, which is indicated by the analysis of the UV–vis–NIR absorption spectrum of the CuPc/C60 blended film. On the other hand, the highest occupied and lowest unoccupied molecular orbitals of CuPc and C60 shift in opposite directions at the interface. This is attributed to the changes of the polarization energies of CuPc and C60 at the interface. The formation of the interfacial dipole and the change of the polarization energy result in the anomalous energy band offsets at the C60/CuPc interface, which are entirely different from those in inorganic p–n junctions.

Photoconductivity of thin organic films

Thin organic films were deposited on silicon oxide surfaces with golden interdigitated electrodes (interelectrode gap was 2 μm), and the film resistivities were measured in dark and under white light illumination. The compounds selected for the measurements include molecules widely used in solar cell applications, such as polythiophene ( PHT), fullerene ( C 60 ), pyrelene tetracarboxylic diimide ( PTCDI) and copper phthalocyanine ( CuPc), as well as molecules potentially interesting for photovoltaic applications, e.g. porphyrin–fullerene dyads. The films were deposited using thermal evaporation (e.g. for C 60 and CuPc films), spin coating for PHT, and Langmuir–Schaeffer for the layer-by-layer deposition of porphyrin–fullerene dyads. The most conducting materials in the series are films of PHT and CuPc with resistivities 1.2 × 10 3 Ω m and 3 × 10 4 Ω m, respectively. Under light illumination resistivity of all films decreases, with the strongest light effect observed for PTCDI, for which resistivity decreases by 100 times, from 3.2 × 10 8 Ω m in dark to 3.1 × 10 6 Ω m under the light.

Indium tin oxide on copper phthalocyanine – an ellipsometry study

AbstractSpectroscopic ellipsometry was employed to evaluate the influence of ITO deposition on underlying copper phthalocyanine (CuPc) thin films grown on silicon with native oxide. At first the dielectric functions of ITO and copper phthalocyanine were derived from a multi‐sample analysis of single layer samples. While the CuPc films were isotropic in the substrate plane they showed a strong anisotropy in their optical response perpendicular to the film plane. Obvious higher values were observed below 1.9 eV for the real part, and over the whole spectral range (0.7 – 5 eV) for the imaginary part of the dielectric function perpendicular to the film plane. The dielectric functions were applied to model the double layer samples with ITO on top, resulting in a good match of modelled and measured ellipsometry spectra. This is a clear indication that the ITO deposition does not destroy or strongly modify the optically anisotropic CuPc layer. In a next step the CuPc dielectric function was varied to improve the fit resulting in a slight line shape change. This change might be a hint for possible phase transformation of a small part of the CuPc from α to β phase due to ITO deposition. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In‐plane optical anisotropy of copper‐phthalocyanine films: RAS studies and modelling

AbstractCopper‐phthalocyanine (CuPc) films prepared by organic molecular beam deposition on H‐passivated silicon and Si covered with native oxide were investigated by means of ellipsometry and reflection anisotropy spectroscopy (RAS). In addition to the out‐of‐plane anisotropy found from the evaluation of the ellipsometry data (Aspnes and Studna, Phys. Rev. Lett. 54, 1954 (1985) [1]) an in‐plane anisotropy in the order of 10‐2 was detected by RAS. In a first step model calculations assuming homogeneous in‐plane anisotropy over the whole layer thickness were performed in order to extract the anisotropy of the optical constants n and k.In a second step, more advanced models taking into account a change in the anisotropy with film thickness were constructed in order to describe the in‐plane anisotropy spectral evolution with film thickness. For the films on Si/SiO2 the in‐plane anisotropy is found to increase with increasing thickness. On H‐Si this effect is not observed and the behaviour of CuPc is more complicated to model on this substrate. Thus, the interaction of the molecules with the differently treated substrate surfaces plays a crucial role for the molecular alignment. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic Structure of C60/Phthalocyanine/ITO Interfaces Studied using Soft X-ray Spectroscopies

The interface electronic structure of a bilayer heterojunction of C60 and three different phthalocyanines grown on indium tin oxide (ITO) has been studied using synchrotron radiation-excited photoelectron spectroscopy. The energy difference between the highest occupied molecular orbital level of the phthalocyanine (donor) layer and the lowest unoccupied molecular orbital level of the C60 (acceptor) layer (EHOMOD − ELUMOA) was determined. The EHOMOD − ELUMOA of a heterojunction with boron subphthalocyanine chloride (SubPc) was found to be much larger than those of copper phthalocyanine (CuPc) and chloro-aluminum phthalocyanine (ClAlPc). This observation is discussed in terms of the difference of the ionization energy of each donor material. Additionally, we have studied the molecular orientation of the phthalocyanine films on ITO using angle-dependent X-ray absorption spectroscopy. We found that the SubPc films showed significant disorder compared to the CuPc and ClAlPc films and also found that EHOMOD − E...

The influence of orientations and external electric field on charge carrier mobilities in CuPc and F16CuPc films on highly ordered pyrolytic graphite and octane-1-thiol terminated Au(111) substrates

The lying-down and standing-up CuPc and F(16)CuPc films on HOPG (highly ordered pyrolytic graphite) and C8-SAM/Au(111) (octane-1-thiol terminated Au(111)) substrates are investigated by using a hybrid strategy combing the molecular dynamic (MD) simulations with density functional theory (DFT) calculations, in order to understand the influence of packing orientation on charge carrier mobilities. On the basis of the periodic slab model and consistent-valence force field, MD simulations show the populations of various packing configurations and radial distribution of intermolecular distance in the films at room temperature. It is also demonstrated that the external electric field (parallel or perpendicular to the substrate) perturbs the intermolecular distances in CuPc and F(16)CuPc films, especially for the slipped edge-to-face stackings. DFT calculations are then used to evaluate two key charge-transfer parameters, reorganization energy and transfer integral. An electrostatics embedding model is employed to approximately consider the external electrostatics contributions to reorganization energy. The thermal-averaged mobility is consequently estimated by taking account of both electronic structures of charge-hopping pairs and dynamic fluctuations in film morphologies under various experimental conditions. It is found that CuPc has smaller reorganization energy and larger hole (electron) mobilities than F(16)CuPc. Under the external electric field of 10(4)-10(7) V cm(-1), both hole and electron mobilities of CuPc and F(16)CuPc films would decrease to 1-3 orders of magnitude. CuPc (F(16)CuPc) films show substantial orientation dependence of mobilities on the ratio of standing-up versus lying-down orientations falling in the range of 10-1000.

Interpretation of the C1s XPS Signal in Copper Phthalocyanine for Organic Photovoltaic Device Applications

AbstractCopper phthalocyanine (CuPc) belongs to a class of small molecules offering particularly interesting advantages when employed in organic electronic devices. Because of its advantageous attributes like high thermal stability, inertness when exposed to acids or alkalis, relatively high electron conductivity, color and light fastness it has been employed in polymer photovoltaic devices as a unipolar dopants complementing the buckminsterfullerene (C60) acceptors and as a conductive buffer. Other organic applications include ambipolar OFETs and non-linear optics structures. X-ray photoelectron spectroscopy (XPS) has been commonly employed to monitor the quality of thin CuPc films. Although XPS analyses of CuPc have been done for over forty years there has not yet been agreement regarding interpretation of the major C1s signal, particularly in the case of non-stoichometric CuPc composition. This work presents systematic studies of the C1s signal of thin film deposits, fabricated using commercially available CuPc materials. It was found that composite C1s CuPc signal consists of five components: two related to the principal C positions within the CuPc macrocycle (C-C in 6-membered ring, C-C-N in 5-membered ring), two associated with shake-up transitions accompanying principal C transitions, and one due to mostly aliphatic impurities. Detailed analysis showed that the magnitude of shake-up peaks was approximately equal 10% to 12% of their principal transitions, in agreement with the theoretical calculations. Correspondingly, the C1s signal originating from the non-CuPc impurities quantitatively agreed with the IR attenuated total reflectance (ATR-IR) measurement of the C-H aliphatic vibrations originating from these impurities present within the CuPc layer. The proposed C1s interpretation has been successfully tested for a large number of commercial CuPc materials and provides a guideline for a routine XPS analysis of the CuPc in organic photovoltaic devices.

Property Modification of Copper Phthalocyanine Film by Nitrogen Dioxide Gas

Copper Phthalocyanine (CuPc) thin films have a potential to use as nitrogen dioxide (NO2) gas sensor therefore the study of property modification of CuPc thin film by NO2 gas can lead to a better understanding the sensor operation. CuPc thin films were prepared by thermal vacuum deposition on glass substrate and aluminum interdigitated electrodes. The film characteristics were investigated with and without NO2 gas. These characteristics indicated that the resistance of CuPc film decrease as the NO2 concentration increases. In order to achieve high efficient of sensing properties, the post-deposition film was also annealed. The surface modifications were investigated with different exposed time of NO2 gas. Their properties modifications before and after annealing were investigated on the film morphology and crystalline structure. The surface morphology of the film was examined by atomic force microscopy while the crystalline structure of the film was determined by x-ray diffraction. The electrical measurements of CuPc film exposed to NO2 gas were studied in order to investigate property modification of the CuPc film.

Copper Phthalocyanine on Hydrogenated and Bare Diamond (001)-2 × 1: Influence of Interfacial Interactions on Molecular Orientations

The molecular orientations of copper phthalocyanine (CuPc) organic semiconductor molecules on hydrogenated and bare diamond (001)-2 x 1 surfaces are studied using synchrotron-based photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Angular-dependent NEXAFS reveals that the CuPc molecular assemblies are orientationally ordered and lying down on hydrogenated diamond, whereas they undergo a molecular reorientation on bare diamond from lying down at submonolayer coverage to standing up in multilayers. The molecular film on bare diamond also exhibits an order-disorder-order transition in the molecular orientations. The distinct molecular orientation within the CuPc films on both diamond (001) surfaces is explained in terms of the interplay between intermolecular interactions and molecule-substrate interactions.

Orientation-controlled charge transfer at CuPc/F16CuPc interfaces

Molecular orientation-controlled charge transfer has been observed at the organic-organic heterojunction interfaces of copper-hexadecafluoro-phthalocyanine (F16CuPc) or copper(II) phthalocyanine (CuPc) on both standing-up and lying-down CuPc or F16CuPc thin films. In situ synchrotron-based photoemission spectroscopy reveals that the charge transfer at the standing F16CuPc/CuPc or CuPc/F16CuPc interface is much larger than that at the lying F16CuPc/CuPc or CuPc/F16CuPc interface. This can be explained by the orientation-dependent ionization potentials of well-ordered organic thin films, which place the highest-occupied molecular orbital of the standing CuPc film much closer to the lowest-unoccupied molecular orbital of the standing F16CuPc film, facilitating stronger charge transfer as compared to that at the lying OOH interfaces. Our results suggest the possibility of manipulating interfacial electronic structures of organic heterojunctions by controlling the molecular orientation, in particular for applications in ambipolar organic field transistors and organic photovoltaics.

Polymer reptation for molecular assembly of copper phthalocyanine

An efficient molecular assembly in vacuum and on solid state surface is demonstrated by using copper phthalocyanine (CuPc) as the candidate molecule wherein the assembly is dependent on the coassistance of the reptation of polymer chains and the π-π interactions of CuPc. The films assembled by this method exhibit high electronic performance, e.g., the carrier mobility based on the assembled CuPc films is improved by one order compared to that of the devices based on unassembled CuPc films, indicating their prospective application in organic electronics.

Organic photovoltaic cells with vertically aligned crystalline molecular nanorods

We demonstrate the growth of vertically aligned nanorod arrays of copper phthalocyanine (CuPc) using the oblique angle deposition technique in high vacuum. High density nanorods with diameters down to 20 nm have been achieved with either stationary or rotational substrates. X-ray diffraction reveals the polycrystalline nature of the CuPc nanorods. Photovoltaic cells have been fabricated by spin-coating [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) onto the CuPc nanorod films to form the active region, and scanning electron micrographs show excellent infiltration of PCBM molecules into the gaps between the CuPc nanorods. A maximum power conversion efficiency of η p = (1.8 ± 0.1)% under 1 sun AM 1.5G illumination has been achieved in a device with ∼40 nm long CuPc nanorods deposited on a 20 nm thick flat CuPc film, approximately twice of that of optimized bilayer CuPc/PCBM devices.

Ambient induced degradation and chemically activated recovery in copper phthalocyanine thin film transistors

The electrical degradation (aging) of copper phthalocyanine (CuPc) organic thin film transistors (OTFTs) was investigated. Thick (1000 ML) and ultrathin (4 ML) channel thicknesses were used in bottom contact OTFTs to correlate the electrical effects of aging with film microstructure. Proper TFT saturation behavior was unattainable in thick devices subject to ambient aging; however ultrathin devices were significantly less susceptible and maintained good saturation and subthreshold behavior. Therefore 1000 monolayer (ML) CuPc OTFTs were characterized in ambient air, clean dry air, clean humidified air, and NOx environments to isolate the ambient components that induce aging. Thick channel devices which had been aged in ambient air to the point of losing all saturation behavior could be restored to proper saturation behavior by exposure to clean humidified air. The data are consistent with aging resulting primarily from adsorption of strong oxidants from ambient air within the grain boundaries of the CuPc films.

Dissolution of copper phthalocyanine and fabrication of its nano-structure film

The mono-protonated and di-protonated forms of copper phthalocyanine (CuPc) were obtained by increasing concentrations of trifluoroacetic acid (TFA) solution to a fixed concentration of CuPc solutions. UV-Vis spectrum shows that the Q bands of these two derivatives split and shift to the red, which means successive protonation happened and caused the two derivatives to lose their symmetry. After the protonation step, the solubility of protonated CuPc in organic solvent increased 60 times. The CuPc film was fabricated by the electrophoretic deposition (EPD) method from the protonated CuPc dissolved in nitromethane containing TFA. Scanning electron microscopy (SEM) showed that the deposited CuPc film on the indium tin oxide (ITO) substrate is composed of thread-like nanobelts with diameters between 100 nm and 200 nm. Furthermore, the CuPc film is in α phase with stacking direction (b-axis) parallel to the substrate, which was detected by X-ray diffraction.

Molecular Orientation Dependent Energy Level Alignment at Organic−Organic Heterojunction Interfaces

Molecular orientation dependent energy level alignments at organic−organic heterojunction (OOH) interfaces have been investigated with synchrotron based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements. Model systems of the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on both standing-up and lying-down copper hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) thin films have been used to illustrate the molecular orientation dependent interface properties. The formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPc or CuPc thin films. This is attributed to the intrinsic surface dipoles induced in the standing-up F16CuPc (CuPc) film due to the polar intermolecular C−F (C−H) bonds formed at the interface. In situ NEXAFS measurements reveal that the room-temperature deposition of PT...

Morphology of Vapor Deposited Polyimides Containing Copper Phthalocyanine

AbstractA vapor deposition polymerization technique is applied to grow composite films containing polyimide and copper phthalocyanine (CuPc). Specifically, 4,4′‐oxydianiline and 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride are co‐evaporated in the presence of CuPc. Spectroscopy experiments confirm the formation of polyimide segments and suggest that embedded CuPc molecules have less mobility than pure CuPc films. Electron microscopy and X‐ray diffraction (XRD) studies show that the majority of CuPc initially assumes an amorphous phase. Upon curing, partial crystallization of CuPc occurs, forming small crystals (α phase) that are suspended in a polyimide matrix. These crystals exhibit enhanced thermal stability compared to pure vapor‐deposited CuPc.

Preparation of highly oriented copper phthalocyanine film by molecular templating effects for organic field-effect transistor

Highly oriented copper phthalocyanine (CuPc) film consisting of many crystal domains was prepared by introducing titanyl-phthalocyanine (TiOPc) as molecular template at high substrate temperature. CuPc molecules stand up on the top of TiOPc domains exhibiting herringbone structure and high-order arrangement investigated by atomic force microscopy and X-ray diffraction. Then organic transistors were fabricated that exhibited improved device performances, a 0.12 cm 2/V s of mobility and a 10 5 of on/off ratio. Furthermore, template thickness dependence on device performance was disclosed. These results indicated that molecular template is a very effective method to prepare high-mobility organic semiconductor film.

Influence of thermal treatment on the performance of copper phthalocyanine thin-film transistors

Organic thin-film transistors (OTFTs) with bottom-gate and bottom-contact configuration based on copper phthalocyanines (CuPc) as active layer were fabricated. The performance of CuPc OTFTs was studied before and after thermal treatment on CuPc layer. The values of the threshold voltage before and after thermal treatment are −6.3 and −5.7 V, respectively. The field-effect mobility values in saturation regime of CuPc thin-film transistors before and after thermal treatment are 0.014 cm 2/Vs and 0.0068 cm 2/Vs, respectively. The experimental results indicate that there is a heavy decay on the mobility of CuPc based OTFTs mostly due to the crystalline morphology change induced by the thermal treatment, and absolute value of the threshold voltage after thermal treatment decreases with the decrease of the CuPc film thickness and the roughness.

New Structure Improves Copper Phthalocyanine (CuPc) Order and Decrease the Device Resistance

We fabricated high performance organic field-effect transistors with novel hybrid electrode-semiconductor contact geometry. The HC OFETs contain both the top electrodes and the bottom electrodes in a single transistor simultaneously. The bottom electrodes induce an ordering CuPc film during the active layer depositing. Due to a high order CuPc layers and low total device resistance, as well as little charge effect below the top electrodes, OFETs with the hybrid contact geometry exhibit a higher performance. With convenience of the new structure, the contact resistance between the two traditional contact geometries was studied in one single transistor.