PTCDA Thin Films Research Articles

High-energy e-Beam-induced effects in Au/n-Si diodes with pre-irradiated PTCDA interfacial layer

In the presented study, the performance improvement of Au/PTCDA/n-Si diodes was demonstrated with the help of pre-irradiation of PTCDA powders. The crystallographic analysis was carried out by X-ray diffraction method after the pre-irradiation of PTCDA powders. After the vacuum deposition of pre-irradiated (PI) and unirradiated (UI) PTCDA thin films, the Au/PTCDA/n-Si diodes were fabricated. The I–V, C–V, and G/ω–V characteristics were analyzed for Au/PTCDA/n-Si diodes with UI PTCDA as an interfacial layer (D1-pristine) and PI PTCDA with 30 kGy as an interfacial layer (D2). Radiation-induced effects on the produced diodes were investigated with parameters of the barrier height (ϕBo), series resistance (Rs), and the density of interface states (NSS) and compared to the parameters of the pristine diode. It was observed that the electrical characteristics of Au/PTCDA/n-Si diodes, for D1 and D2, were highly influenced by the irradiation. Thus, the device performance could be improved with the pre-irradiation process. By modifying the HF–LF capacitance method to the UI–PI capacitance method, we successfully calculated the radiation-induced NSS values without using C–V measurement at two different frequencies.

Photovoltaic effect on the microelectronic properties of perylene/p-Si heterojunction devices

This paper presents the fabrication and study of light dependent electrical properties of heterojunction device based on small molecular semiconductor 3, 4, 9, 10-perylene tetracarboxylic anhydride (PTCDA). The Ag/PTCDA/P3HT/p-Si heterojunction device is prepared by spin coating a 20 nm layer of poly-3,hexylthiophene (P3HT) on p-Si followed by 80 nm thick film of PTCDA via thermal deposition. Current–voltage (I–V) properties of the device are measured at room temperature in dark condition ~ 20 lx (lx) and different illumination conditions to probe photovoltaic effects on the microelectronic parameters of the device. In dark, the Ag/PTCDA/P3HT/p-Si device exhibited rectifying behavior in the forward bias with a current rectification ratio (RR) of 1990 at ± 3.5 V that confirmed the formation of depletion region. Key microelectronic parameters of the device such as ideality factor (n), barrier height (φb), series resistance (Rs) and shunt resistance (Rsh) are extracted from the I–V characteristics and studied as a function of illumination (2–2700 lx). Charge conduction mechanism and mobility via interface of the device is also investigated from logI–logV curves and conventional I–V characteristics, respectively. The microelectronic properties of the device are correlated with optical properties (absorption spectrum) and morphology of the PTCDA thin films carried out by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transformed infrared (FTIR) spectrum and energy dispersive x-rays (EDX) is performed to validate composition and elemental analysis of PTCDA films.

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Eliminating thermal effects in z-scan measurements of thin PTCDA films.

We investigate the two-photon absorption (TPA) and nonlinear refraction of a micrometer thick 3,4,9,10-perylentetracarboxyl-dianhydride (PTCDA) film using z-scans with tightly focused 100 fs laser pulses. The PTCDA film was grown by organic molecular beam deposition on a Pyrex substrate. To study the influence of sample heating, the pulse repetition rate was varied between 4 MHz and 50 kHz with an acousto-optic pulse selector. We find that thermal effects diminish for pulse repetition times longer than 5 and 0.75 µs when using a 10x or 20x microscope lens, respectively, resulting in a TPA coefficient of 6 cm/GW and a nonlinear refractive index of 1.2 x 10⁻¹³ cm²/W at a wavelength of 820 nm.

Maple prepared organic heterostructures for photovoltaic applications

In this study, we present the deposition of ZnPc, Alq3, and PTCDA thin films using Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. We also report the realisation of multilayer structures, made by the successive application of MAPLE. The films have been characterized by spectroscopic (UV–VIS and Photoluminescence) and microscopic (SEM and AFM) methods, and the effect of different deposition conditions such as fluence, number of pulses, and target concentration on the properties has been analysed. This paper also presents some investigations on the electrical conduction in sandwich type structures ITO or Si/organic layer/Au or Cu and ITO/double organic layer/Cu, emphasising the dominant effect of the height of the energetic barriers at the inorganic/organic and organic/organic interfaces.

Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene

In situ low-temperature scanning tunneling microscopy is used to study the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on epitaxial graphene (EG) on 6H-SiC(0001), as well as on HOPG for comparison. PTCDA adopts a layer-by-layer growth mode, with its molecular plane lying flat on both surfaces. The PTCDA films grow continuously over the EG step edges, but not on HOPG. STS performed on single-layer PTCDA on monolayer EG shows a wide band gap larger than 3.3 eV, consistent with pristine PTCDA films. Synchrotron-based high-resolution photoemission spectroscopy reveals weak charge transfer between PTCDA and EG. This suggests weak electronic coupling between PTCDA and the underlying EG layer.

Optical stability of small-molecule thin-films determined by Photothermal Deflection Spectroscopy

AbstractIn this paper the optical absorption properties of n-type C60and PTCDA, and p-type CuPc small molecule semiconductors are investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). The results show the usual absorption bands related to HOMO-LUMO transitions in the high absorption region of the transmission spectra. PDS measurements also evidences exponential absorption shoulders with different characteristic energies. In addition, broad bands in the low absorption level are observed for C60and PTCDA thin-films. These bands have been attributed to contamination due to air exposure. In order to get deeper understanding of the degradation mechanisms single and co-evaporated thin-films have been characterized by PDS. The dependence of the optical coefficient on exposure to light and air have been studied and correlated to the structural properties of the films (as measured by X-Ray Diffraction Spectroscopy). The results show that CuPc and PTCDA are quite stable against light and air exposure, while C60shows important changes in its absorption coefficient. The bulk heterojunctions show stability in agreement with what observed for single layers, since the absorption coefficient of CuPc:PTCDA is almost not altered after the degradation treatments, while CuPc:C60shows changes for low energy values.

Exciton emission in PTCDA thin films under uniaxial pressure

We study the strain dependent photoluminescence (PL) of a $90\text{\ensuremath{-}}\mathrm{nm}$-thick polycrystalline 3,4,9,10-perylene tetracarboxylic dianhydride film on Si(001) between 20 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Uniaxial pressure up to $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ is applied along the molecular stacking direction using a specially designed pressure cell. With increasing pressure, we find a quenching of the integrated PL intensity, which is mainly attributed to the creation of defects. At low temperature, the charge transfer exciton emission (CT2) gains intensity relative to the Frenkel exciton emission. Furthermore, the CT2 transition reveals a shift to lower energies by up to $24\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. At room temperature, the PL is dominated by the excimer transition, which also shows a redshift of $\ensuremath{\sim}24\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at the highest uniaxial pressure. The relative increase of the CT2 transition at low temperature and the redshift of the emission bands are attributed to an increased exciton trapping probability and an enhanced binding energy with reduced distance between stacked molecules. This explanation is supported by a comparison with total energy calculations for pairs of adjacent molecules. Moreover, the calculated pressure-induced energy shifts of the CT2 and excimer transitions are in good agreement with the experimentally observed values.

Molecular orientation of 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride thin films at organic heterojunction interfaces

In situ low-temperature scanning tunneling microscopy and near-edge x-ray absorption spectroscopy measurements have been used to investigate the molecular orientation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films at the interface of organic heterojunctions of PTCDA on copper(II) phthalocyanine (CuPc). On the CuPc monolayer on highly oriented pyrolitic graphite, PTCDA molecules form a well-ordered in-plane herringbone structure with their molecular plane parallel to the substrate surface. The formation of multiple in-plane hydrogen bonding between neighboring PTCDA molecules is responsible for the flat-lying PTCDA on CuPc monolayer, and gives rise to the lying-down orientation of PTCDA thin films on both standing-up and lying-down CuPc thin films, as well as on Au(111) passivated by a self-assembled monolayer of octane-1-thiol.

Quasi-One-Dimensional K-O Chain in PTCDA Thin Films: Evidence from First-Principles Calculations

Using density functional theory calculations we have found that K atoms in a PTCDA (3,4:9,10-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic oxygen of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 Angstrom) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PTCDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eV in the K-doped system, confirming that the interaction occurs between different molecular planes.

Growth and characterization of thin PTCDA films on 3C-SiC(0 0 1)c(2 × 2)

The growth of thin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films on a 3C-SiC(0 0 1)c(2 × 2) substrate has been studied by means of photoelectron spectroscopy (PES) and atomic force microscopy (AFM). In the first monolayer the molecules interact with the substrate mainly through the O atoms in the end groups of the molecule. The O atoms have a higher binding energy in the first molecular layer compared to the following layers. No chemical shifts are observed in the Si 2p spectra or in the C 1s spectra from the perylene core of the molecules. From the VB spectra and LEED pattern we conclude that the substrate remains in the c(2 × 2) reconstruction after PTCDA deposition. For thicker films a Stranski–Krastanov film growth was observed with flat lying molecules relative to the substrate.

Ultrafast relaxation and exciton–exciton annihilation in PTCDA thin films at high excitation densities

Femtosecond pump–probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S 1 manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (>10 19 cm −3), the major de-excitation mechanism for the relaxed excitons is exciton–exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Förster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Förster radii for room and liquid Helium temperature.

Substrate Dependent Anisotropic Diffusion Of Indium Atoms On Ptcda Thin Films Studied by Peem

Anisotropic surface diffusion of In atoms was observed by photoelectron emission microscopy (PEEM) when a square-shaped microstructure of In metal was deposited onto thin films of perylene-3,4,9,10-tetracarboxyric acid-dianhydride (PTCDA) grown on cleaved MoS 2 and GeS surfaces. For In microstructure on PTCDA/MoS 2 system, the PEEM images showed a triangle pattern, although the deposited shape of the In microstructure was square, indicating that In atoms diffuse to three directions on PTCDA/MoS 2. Such triangle pattern was not observed on PTCDA/GeS but an oval structure was observed, revealing that In atoms diffuse to two directions on PTCDA/GeS with different diffusion constant. The two directions correspond well with surface crystal axes of GeS. Since the unit cells of PTCDA adsorbed on these two crystal surfaces are nearly the same, it is suggested that different anisotropic diffusion of In in these systems is related to strong In-PTCDA interaction which may be affected by the surface structure of underlying substrate.

IR and SFM study of PTCDA thin films on different substrates

FT-IR spectroscopy and SFM were used to investigate the growth of thin films of the organic semiconductor 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) deposited by vacuum sublimation onto various substrates, i.e. Ag(111) layers on mica, KBr(100), mica, oxidized Si, and TiO 2 nanoparticles on Si. Layer thicknesses of PTCDA varied from 10 to 1500 nm. The anhydride vibrations of PTCDA differ for the used substrates, which can be connected to the orientation of the molecules relative to the substrate surface and the film morphology as detected in the SFM pictures.

Preparation and characterization of single-phase potassium-doped PTCDA thin films

Single-phase films with K/3,4:9,10-perylenetetracarboxylic dianhydride stoichiometry have been synthesized using the method of fractional sublimation in vacuum. This demonstrates that this method can be successfully applied to alkali metal-doped organic molecular crystals which is an important prerequisite for a number of spectroscopic studies of such systems where an in situ sample preparation is required.

Raman monitoring of In and Ag growth on PTCDA and DiMe-PTCDI thin films

The formation of In and Ag interfaces with 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and N, N′-dimethyl-3,4,9,10-perylene tetracarboxylic diimide (DiMe-PTCDI) was investigated by in situ Raman spectroscopy in order to determine the interaction between the metals and the organic molecules. A significant enhancement of internal vibrational modes is observed in all cases, clearly indicating the presence of surface enhanced Raman scattering. The molecules having direct contact with the metal are involved in a weak ground state dynamical charge transfer resulting in a breakdown of selection rules. However, there is no indication for the formation of new chemical bonds. In the case of In deposition on PTCDA, this contradicts previous models of covalent bond formation between In and O atoms in PTCDA. The enhancement factors of the Raman signals can be employed to extract information on indiffusion and morphology of the metals. Both metals exhibit rough morphologies of different degrees and depending on the organic substrate. In addition, In shows the strongest tendency of indiffusion when deposited on PTCDA. This is corroborated by the rapid disappearance of intermolecular, i.e. phonon-like modes in the frequency range below 120 cm −1.

Growth and vibrational properties of PTCDA thin films on InSb(1 1 1)A

X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the growth of ordered thin films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on InSb(1 1 1)A. XPS and HREELS spectra of the PTCDA covered surface both show significant differences in the sub-monolayer regime when compared to multilayer PTCDA films. At sub-monolayer coverages the XPS measurements indicate a significant shift in the binding energies of the main C 1s and O 1s peaks when compared to multilayer PTCDA films. HREELS results show that several vibrational modes of the molecule undergo significant changes in intensity and frequency as the PTCDA coverage is increased. Furthermore, the sub-monolayer spectra indicate the presence of softened carboxylic stretching modes. The results are consistent with some charge transfer at the PTCDA/InSb interface during the initial stages of film growth, however there is no spectroscopic evidence for any molecular fragmentation.

Thermal stability and partial dewetting of crystalline organic thin films: 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111)

The thermal stability and dewetting effects of crystalline organic thin films on inorganic substrates have been investigated for a model system for organic epitaxy, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111). The thin films deposited under a variety of growth conditions have been annealed stepwise and studied by in situ x-ray diffraction and noncontact atomic force microscopy. It has been found that comparatively smooth films deposited at temperatures Tg≲350 K are metastable, while films deposited at Tg≳350 K are stable against annealing on a time scale of several hours. The thermal expansion of thin PTCDA films and bulk samples has been determined.

Morphology, structure and photophysics of thin films of perylene-3,4,9,10-tetracarboxylic dianhydride

Abstract The influence of the growth conditions on the properties of PTCDA thin films deposited by organic molecular beam deposition on glass substrates is presented. Powder X-ray diffraction studies show that only the α polymorph is formed, regardless of the growth conditions. By contrast, the film morphology depends specifically upon the deposition parameters, the observed crystal size increasing with growth temperature or after post-growth annealing. The spectroscopic properties (electronic absorption, fluorescence excitation and emission) are also found to be influenced by heat treatment and these are correlated with the changing crystal size in the PTCDA films. A qualitative model is presented in order to account for the photophysical properties of the films.

Time-resolved photoluminescence characterisation of thin PTCDA films on Si(100)

3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) thin films on Si(100) substrate are investigated by time-resolved photoluminescence (PL). A sub-band structure together with a bi-exponential decay of the PL spectra have been observed which are attributed to excimer and monomer recombination processes. A comparison with single PTCDA crystals shows that the excimer transition is related to the crystallinity of the PTCDA films. The dependence of the photoluminescence intensity and decay times on the film thickness and on the substrate temperature give further information about the crystallinity of PTCDA films.