Synthesis, characterization of the pentacene and fabrication of pentacene field-effect transistors

A comprehensive understanding of the organic semiconductor material pentacene is meaningful for organic fieldeffect transistors （OFETs）. Thin films of pentacene are the most mobile molecular films known to date. This paper reported that the pentacene sample was successfully synthesized. The purity of pentacene is up to 95%. The results of a joint experimental investigation based on a combination of infrared absorption spectra, mass spectra （MS）, element analysis, x-ray diffraction （XRD） and atom force microscopy （AFM）. The authors fabricated OFET with the synthesized pentacene. Its field effect mobility is about 1.23 cm^2/（V·s） and on-off ratio is above 10^6.     

Growth of pentacene on Ag(1 1 1) surface: A NEXAFS study

Thin films of pentacene (C₂₂H₁₄) have become widely used in the field of organic electronics. Here films of C₂₂H₁₄ of thickness ranging from submonolayer to multilayer were thermally deposited on Ag(1 1 1) surface. The determination of molecular geometry in pentacene films on Ag(1 1 1) studied by X-ray absorption at different stages of growth up to one monolayer is presented.XAS spectra at the C K-edge were collected as a function of the direction of the electric field at the surface. The different features of the spectra were assigned to resonances related to the various molecular unoccupied states by the comparison with the absorption coefficient of the pentacene gas phase. The transitions involving antibonding π states show a pronounced angular dependence for all the measured coverages, from submonolayer to multilayer. The spectra analysis indicates a nearly planar chemisorption of the first pentacene layer with a tilt angle of 10°.     

The interaction of oxygen and ozone with pentacene

We use ultraviolet photoemission spectroscopy (UPS) to investigate the effect of oxygen and air exposure on pentacene thin film electronic structure. It is found that O₂ and water do not react noticeably with pentacene on the timescale of several hours, whereas a mixture of oxygen atoms, singlet oxygen and ozone readily oxidizes the organic compound. We obtain no evidence for irreversible intercalation of oxygen into pentacene or considerable p-type doping after re-evacuation. Infrared spectroscopy and atomic force microscopy are used to study the oxidation of pentacene thin films. Our data suggest the oxidation of pentacene with reactive oxygen species to yield highly volatile reaction products as evidenced by significant mass-losses of the films.     

Electronic state observation of inner organic thin films beneath electrodes: Fluorescence-yield X-ray absorption spectra of pentacene derivative films

The electronic states of inner organic thin films have been investigated by X-ray absorption spectroscopy (XAS) in a bulk-sensitive fluorescence-yield (FY) mode with theoretical analysis. The thin films of the synthesized pentacene derivative, i.e., 6,13-dihydrodiazapentacence (C₂₀N₂H₁₄), on SiO₂-covered Si substrates were fabricated and their morphology and crystallinity were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis, respectively. The observed N K-edge FY-XAS spectra were different from the surface-sensitive XAS spectra measured in a partial-electron-yield (PEY) mode. The peaks on the FY-XAS spectra were well reproduced by the theoretical calculation of the unoccupied states of an HAPn molecule. In addition, the incident angle dependence of the FY-XAS spectra was consistent with the expected molecular orientation in the thin films. As a result, we successfully obtained the N and C K-edge FY-XAS spectra of the inner HAPn thin films even beneath Au electrodes.     

Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy

We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene OFET on the rubbed PVA layer, however, the optical anisotropy is observed and the anisotropy of the electrical performances directly corresponds to the optical anisotropy of the pentacene thin-film on the rubbed PVA layer.     

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene

We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic light-emitting devices (OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 °C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120 ℃ annealed pentacene film and n-doped electron transport layer (ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m² at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.     

Pulsed laser deposition of c * axis oriented pentacene films

Pulsed laser desorption and film deposition behaviors have been investigated on pentacene as an organic molecule primarily due to its applications in electronics. The laser desorption time-of-flight (LDTOF) mass spectrum exhibited a single parent peak when a pressed pentacene pellet was ablated by an N₂ laser beam of its fluence lower than 100 mJ/cm², indicating that pentacene could be evaporated without an appreciable photodecomposition by the pulsed laser beam. Nd:YAG pulsed laser deposition of pentacene films was performed using such optimization parameters as laser fluences and wavelength (second, third and forth harmonic generations (SHG, THG, FHG)). The analyses with AFM, XRD and UV-Vis spectroscopy revealed the fabrication of c^* axis oriented pentacene films on quartz, silicon, and CaF₂ substrates by the SHG. The SHG films have a surface morphology superior to those of films deposited by THG and FHG. PACS 81.15.Fg; 81.05.Lg; 82.80.Rt     

The effect of oxygen exposure on pentacene electronic structure

We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene single crystals and thin films. It is found that O₂ and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O₂ at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface properties.     

H2O induced structural modification of pentacene crystal

Water is one of the obstacles for prolonged life-time of organic based field-effect-transistors (OFETs). Here, we show how H₂O molecules adsorb and affect pentacene crystal by using photoelectron spectroscopic methods. Diffusion into the crystal is accompanied with accumulation of H₂O onto the crystal. Valence band, core level, and X-ray absorption spectra show that H₂O molecules physisorb on the pentacene surface via oxygen, resulting in the increase of the hole-injection barrier. The diffused molecules results in the deteriorated crystallinity, and they reflect the weakening inter-molecular interactions of pentacene crystal.     

Microscopy of pentacene thin films

Thin films of pentacene were deposited by vacuum sublimation onto amorphous carbon, glass, silicon and mica substrates, then characterized by X-ray and electron diffraction, transmission electron microscopy (TEM) and atomic-force microscopy (AFM). Sub-monolayer films consist of dendritic islands, which change to more compact fractal shapes upon transforming to a multilayer structure (with decrease in surface area) prior to complete coverage of the substrate. Increased crystallite size was obtained by heating the substrate during deposition and by post-deposition annealing. Irradiation with 200-keV electrons was found to destroy the crystallinity of the films after a dose of 0.6?C/cm² (or 2?C/cm² if the specimen is cooled to 90?K during TEM observation). We argue that, despite this moderate radiation sensitivity, TEM under near-optimal conditions can image monolayers of pentacene with sub-nm resolution.     

The nucleation of pentacene thin films

Photoemission Electron Microscopy was used to determine basic factors for nucleation and growth of thin pentacene films. Dependence of both substrate chemistry and deposition rate on the nucleation density was observed. On SiO₂ pentacene shows a high nucleation density and forms small islands consisting of almost vertically oriented molecules. On Si(001) the nucleation density of this thin-film phase is much smaller, but the pentacene film first forms a flat-lying wetting layer. The thin-film phase only forms on top of this wetting layer. Adsorption of a cyclohexene self-assembled monolayer on Si(001) prior to the pentacene growth suppresses the initial pentacene wetting layer but maintains diffusion parameters similar to pentacene on Si(001). The nucleation of pentacene layers on cyclohexene/Si(001) can be described by classical nucleation theory with a critical nucleus size i6. Simple surface modification techniques such as e-beam irradiation of the substrates prior to pentacene adsorption can also have a significant effect on the pentacene nucleation density. PACS 68.37.Nq; 68.43.Fg; 68.47.Fg; 68.55.Ac     

Composition-spread thin films of pentacene and 6,13-pentacenequinone fabricated by using continuous-wave laser molecular beam epitaxy

Synthesis of continuous composition-spread (CCS) thin films is widely recognized in combinatorial material science as a powerful technique for rapidly investigating the properties of new functional materials. However, there are very few reports of CCS organic thin films due to the fact that the thermal evaporation method with Knudsen cells, which is commonly used to fabricate organic thin films in vacuum, does not offer the necessary level of deposition rate control as, for example, does pulsed laser deposition (PLD). We have successfully fabricated organic CCS thin films of pentacene and 6,13-pentacenequinone by continuous-wave laser molecular beam epitaxy (CWL-MBE), which we developed as a new fabrication method for organic thin films. The composition-spread films were characterized systematically by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy, and two-probe conductivity measurements. The present work brings the advantages of high-throughput parallel synthesis and systematic characterization to the field of organic electronics, allowing for quick exploration and rapid optimization of organic functional materials and devices.     

Photoexcited carrier relaxation dynamics in pentacene probed by ultrafast optical spectroscopy: Influence of morphology on relaxation processes

We present a comparative study of ultrafast photoexcited state relaxation in pentacene single crystals and in pure and C₆₀-doped pentacene films using optical pump-probe spectroscopy. The photoinduced absorption spectra in pentacene crystals is consistent with a dominant singlet-triplet fission decay channel for above-gap excitation. This decay channel is suppressed in thin films and even further suppressed by electron trapping in C₆₀-doped films. Thus we show that suppression of triplet state production, which is necessary for free carrier formation and thus photovoltaic and photodiode performance, is controllable via sample morphology.     

Studies on morphology and molecular arrangement of pentacene on different substrates

Pentacene (C₂₂H₁₄) thin films with different thicknesses were fabricated to study the dynamic growth process and morphology of pentacene on different substrates. A discontinuous monomolecular layer was observed when a pentacene thin film is about 0.5 nm thick on native oxide silicon wafer. The terraced islands and dendritic structure gradually formed with increasing pentacene thin film thickness. The height of each layer is about 1.4 nm which corresponds well with the length of the long axis of the pentacene molecule at 1.45 nm. Experimental results show that the pentacene molecule is perpendicular to the silicon wafer surface with a slight tilted angle. However, the pentacene molecular orientation on a polymer pre-covered indium tin oxide coated substrate could not give any indication on the scale of nanometers. The surface roughness of substrates strongly influences pentacene molecular diffusion and the morphology of pentacene thin films.     

Surface morphology characterization of pentacene thin film and its substrate with under-layers by power spectral density using fast Fourier transform algorithms

Surface morphology of pentacene thin films and their substrates with under-layers is characterized by using atomic force microscopy (AFM). The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD plots of pentacene films on glass substrate are successfully approximated by the k-correlation model. The pentacene film growth is interpreted the intermediation of the bulk and surface diffusion by parameter C of k-correlation model. The PSD plots of pentacene film on Au under-layer is approximated by using the linear continuum model (LCM) instead of the combination model of the k-correlation model and Gaussian function. The PSD plots of SiO₂ layer on Au under-layer as a gate insulator on a gate electrode of organic thin film transistors (OTFTs) have three power values of PSD. It is interpreted that the specific three PSD power values are caused by the planarization of the smooth SiO₂ layer to rough Au under-layer.     

The electronic structure of pentacene revisited

Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2).     

Structural and optical properties of layers of pentacene formed by PLD method

Organic films fabrication offers the possibility of producing electronic devices of low weight, mechanical flexibility and low cost. One suitable material for organic film fabrigation which is the subject of the great interest is pentacene, because it is characterized by the large carrier mobility (∼1 cm²/Vs). In this work, the growth of pentacene layers using pulse laser deposition (PLD) on different substrates (glass/ITO, Si) is described and various processing parameters are investigated. Two pulsed YAG:Nd³⁺ laser wavelengths were used for the ablation of the PLD target: the first harmonic at 1064 nm aGn:dNdth3+e second at 532 nm. The structure of the layers formed was examined using SEM and RHEED methods. The results were compared with results of optical spectroscopy studies. It will be shown that layers deposed using second harmonics have a higher quality than those for first harmonic. The other PLD parametersalso have a strong influence on the structure quality of layers.      

High pressure electrical transport behavior in organic semiconductor pentacene

The high pressure electrical transport behavior of pentacene has been investigated by alternating current impedance techniques and direct current resistivity measurement in a diamond anvil cell (DAC). The resistance decreases with increasing pressure below 17.4?GPa, while it increases above 17.4?GPa, which is caused by the transition of pentacene from an ordered state to the disordered state under higher pressure. From the Raman spectra under various pressures, pentacene becomes amorphous above 17.3?GPa, which is consistent with the impedance results. The charge transport operates in the hopping regime with charges jumping between interacting molecules, and the hopping mechanisms are related to the vibration modes. Above 17.4?GPa, the pressure dependence of the relaxation activation energy is 21.7?meV/GPa and pentacene keeps semiconductor characteristics up to 28.3?GPa.     

In-situ measurement of molecular orientation of the pentacene ultrathin films grown on SiO2 substrates

Molecular orientations of pentacene ultrathin films grown on SiO₂ substrates were studied without the influence of the atmosphere by vacuum atomic force microscopy (V-AFM) and near edge X-ray absorption fine structure (NEXAFS). The experimental processes from deposition of pentacene to characterization of films were performed under vacuum condition without exposure to the atmosphere. V-AFM and NEXAFS measurements showed that pentacene molecules tend to grow on SiO₂ surface with their molecular long axes perpendicular to the substrate surfaces (standing-mode) irrespective of preparation procedure of SiO₂ substrate.     

N-doping of pentacene by decamethylcobaltocene

We demonstrate n-type doping of pentacene with the powerful reducing molecule decamethylcobaltocene (CoCp₂^*). Characterization of pentacene films deposited in a background pressure of CoCp₂^* by X-ray photoemission spectroscopy and Rutherford backscattering confirm that the concentration of incorporated donor molecules can be controlled to a level as high as 1%. Ultraviolet photoemission spectroscopy show Fermi level (E _F) shifts toward unoccupied pentacene states, indicative of an increase in the electron concentration. A 1% donor incorporation level brings E _F to 0.6 eV below the pentacene lowest unoccupied molecular orbital. The corresponding electron density of ∼10¹⁸ cm⁻³ is confirmed by capacitance–voltage measurements on a metal–pentacene–oxide–silicon structure. The demonstration of n-doping suggests applications of CoCp₂^* to pentacene contacts or channel regions of pentacene OTFTs.