Fullerene Thin Films Research Articles

Increased performance of A-DA’D-A type acceptor based organic solar cell using a thermally coevaporated cathode-modifying blend layer

The blend thin films of bathocuproine and fullerene (BCP:C60) have been thermally coevaporated to modify the active layer/cathode interfaces in organic solar cells. The BCP forms a much smaller-barrier contact with cathode than the C60. Because the surface zone of active layer is polymer donor-rich and small-molecular acceptor-deficient, C60 molecules make insufficiently large contact area with electron acceptor, leading to inefficient electron extraction. However, BCP molecules penetrate the surface of active layer, due to the smaller molecular size of BCP than that of C60, and likely form sufficiently large contact area with electron acceptor thereby to enable efficient electron extraction. Compared to conventional 10 nm BCP, 10 nm BCP:C60 (1:1 in mass ratio) enables higher efficiency based on the active layer using A-DA’D-A type acceptor, mostly because C60 constituent dissociates some excitons photogenerated in the polymer donor of surface zone and thereby increases short-circuit current density. The current research provides novel insights into the development of cathode-modifying layers for organic solar cells.

Weak ferromagnetism in thin fullerene films as a consequence of 600 keV carbon ion irradiation

Weak ferromagnetism in thin fullerene films as a consequence of 600 keV carbon ion irradiation

The effect of substrate deposition temperature on the electrical and optical properties of C60 thin films

We report the effects of the substrate deposition temperature on the photovoltaic performance of fullerene (C60) thin films. These thin films were deposited via thermal evaporation at substrate temperatures from 20 to 250 °C. The optical and electrical properties of the films were dependent on the substrate deposition temperature, and improvements in film optical properties were noted for certain temperatures. Both the optical constants n and k and the dielectric constants show theoretical and experimental decreases with the substrate temperature due to the creation of crystalline structures in the thin films. Despite differences between the theoretical and experimental optical constants, the nature of the decrease is similar. The film porosity calculated from the optical constants increased more than 50% for greater substrate temperatures. The conductivity of the films also increased with the substrate temperature and the activation energy ranged from 0.25 to 0.31 eV. We demonstrate an increased energy gap with temperature using a Tauc-plot, and the photopic and solar transmittances improved with the increased substrate temperature during deposition.

Tunable room temperature ferromagnetism in fullerene thin film induced by 1 MeV proton microbeam irradiation

We report tunable ferromagnetic properties of thin films of fullerene upon 1 MeV proton microbeam ion irradiation, by varying the ion fluence. Focused microbeam scanning of 1 MeV proton ions have been performed on the fullerene thin films. Consequently, a stable and significant ferromagnetic ordering at room temperature has been observed in the fullerene thin films. The proton microbeam irradiation induces a maximum magnetic ordering in fullerene with optimum dosage and subsequently higher fluence irradiations yield, diminishing effect upon the observed ferromagnetic ordering due to the higher degree of damages. The X-ray diffraction and Raman analysis confirm the damage due to proton microbeam irradiation. The reduction in the saturation magnetic moment induced, is very sharp as a result of exposure to the larger ion fluence. The approximate distance between defects has been simulated computationally, and its relation with observed ferromagnetism has been established. The irradiation has been performed at moderate ion flux (low microbeam current) to avoid the possible annealing effects of the ion irradiation.

Evaluation of cold plasma effect to achieve fullerene and zinc oxide-fullerene hydrophobic thin films

Evaluation of cold plasma effect to achieve fullerene and zinc oxide-fullerene hydrophobic thin films

Structural, surface morphology and optical properties of annealing treated Copper Phthalocyanine doped Fullerene (CuPc: C60) thin films

The doping process with materials related to carbon has become a newly emerged approach for achieving an improvement in different physical properties for the obtained doped films. Thin films of CuPc: C60 with doping ratio of (100:1) were spin-coated onto pre-cleaned glass substrates at room temperature. The prepared films were annealed at different temperatures of (373, 423 and 473) K. The structural studies, using a specific diffractometry of annealed and as deposited samples showed a polymorphism structure and dominated by CuPc with preferential orientation of the plane (100) of (2θ = 7) except at temperature of 423K which indicated a small peak around (2θ = 31) for C60. AFM remarks showed the existence of a compact packing film with a smooth surface. The FTIR spectra emphasized the bonding between CuPc and C60. UV-Vis Absorption spectroscopy confirmed that the CuPc was dominating the optical properties with a small shift in Q band towards higher wavelengths.

Photoinduced Resist-free Imprinting (PRI) in fullerene thin films as revealed by Grazing Incidence Small-angle X-ray scattering

In this article we explore the potential of synchrotron Grazing Incidence Small Angle X-ray Scattering (GISAXS) to evaluate the extension of a Photoinduced Resist-free Imprinting (PRI) effect for grating-like Laser Induced Periodic Surface Structures (LIPSS) on spin-coated thin films of the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Valuable information about the nature of both the original fullerene LIPSS and of the remaining resist-free nanostructure in terms of geometry, size distribution, lattice periodicity and degree of order can be achieved by simulations of the GISAXS patterns. One of the most interesting features provided by the GISAXS analysis is the estimation of the paracrystalline distortion factor, which can be useful to evaluate the level of order in the nanostructure. Data revealed that LIPSS in PC71BM films can be described as one-dimensional paracrystalline lattices with levels of order in accordance with those typically observed in LIPSS of other materials. The PRI effect leaves a qualitatively similar nanostructure but about two times more disordered than the original LIPSS one. Our results show that GISAXS is a powerful tool to characterize Resist-free Imprinting procedures of potential relevance in nanotechnology.

Atomically Smooth Graphene‐Based Hybrid Template for the Epitaxial Growth of Organic Semiconductor Crystals

AbstractAtomically thin 2D materials are good templates to grow organic semiconductor thin films with desirable features. However, the 2D materials typically exhibit surface roughness and spatial charge inhomogeneity due to nonuniform doping, which can affect the uniform assembly of organic thin films on the 2D materials. A hybrid template is presented for preparation of highly crystalline small‐molecule organic semiconductor thin film that is fabricated by transferring graphene onto a highly ordered self‐assembled monolayer. This hybrid graphene template has low surface roughness and spatially uniform doping, and it yields highly crystalline fullerene thin films with grain sizes >300 nm, which is the largest reported grain size for C60 thin films on 2D materials. A graphene/fullerene/pentacene phototransistor fabricated directly on the hybrid template has five times higher photoresponsivity than a phototransistor fabricated on a conventional graphene template supported by a SiO2 wafer.

Влияние масс активных слоев гетероструктур фуллерена C60-4-метилфенилгидразона N-изоамилизатина на их выпрямляющие характеристики

An organocarbon heterostructure consisting of thin films of fullerene and 4-methylphenylhydrazone N-isoamylisatin was obtained from a solution by irrigation. The procedure for obtaining the samples, the microscopy, and synthesis of the initial powder material have been described. The results of an alternate X-ray phase analysis of the substances used has been presented. The analysis of changes in the electrical and optical characteristics of the active layers, depending on the mass of the solid phase has been carried out. The relationship between a sequential increase in film thickness and an increase in the absorption coefficient of infrared electromagnetic radiation has been revealed. Having determined the optimal thicknesses of the active layers of the heterostructure, based on fullerene C60 and 4-methylphenylhydrazone N-isoamylisatin, we achieved an increase in photoconductivity by two orders of magnitude. The obtained light current – voltage characteristics of such thin-film structures are rectilinear in nature with a direct load of up to 10 V.

Ultrafast Charge-Transfer Exciton Dynamics in C60 Thin Films.

The high flexibility of organic molecules offers great potential for designing the optical properties of optically active materials for the next generation of optoelectronic and photonic applications. However, despite successful implementations of molecular materials in today’s display and photovoltaic technology, many fundamental aspects of the light-to-charge conversion in molecular materials have still to be uncovered. Here, we focus on the ultrafast dynamics of optically excited excitons in C60 thin films depending on the molecular coverage and the light polarization of the optical excitation. Using time- and momentum-resolved photoemission with femtosecond extreme ultraviolet (fs-XUV) radiation, we follow the exciton dynamics in the excited states while simultaneously monitoring the signatures of the excitonic charge character in the renormalization of the molecular valence band structure. Optical excitation with visible light results in the instantaneous formation of charge-transfer (CT) excitons, which transform stepwise into Frenkel-like excitons at lower energies. The number and energetic position of the CT and Frenkel-like excitons within this cascade process are independent of the molecular coverage and the light polarization of the optical excitation. In contrast, the depopulation times of the CT and Frenkel-like excitons depend on the molecular coverage, while the excitation efficiency of CT excitons is determined by the light polarization. Our comprehensive study reveals the crucial role of CT excitons for the excited-state dynamics of homomolecular fullerene materials and thin films.

Contact formation of C60 to thin films of formamidinium tin iodide

Abstract

Magnetic Depth Profiling of the Co/C60 Interface Through Soft X-Ray Resonant Magnetic Reflectivity

We have probed the structural and magnetic properties of a ferromagnetic/organic interface constituted by a polycrystalline Co layer deposited on a fullerene thin film through resonant soft X-ray reflectivity measurements. The fitting analysis of the reflectivity indicates the formation of a sharp interface with limited intermixing and a null remanent magnetization in a ~1 nm thick region of the Co film at the interface with C60. This information contributes to elucidate the role of organic–inorganic interfaces in the charge and spin transport inside organic spintronic devices.

Morphological, Optical and Electrical Characterization of the Interfaces in Fullerene-Porphyrin Thin Films.

The role of the interfaces on the optoelectronical properties of porphyrin-fullerene composites has been studied by means of ultraviolet-visible (UV-Vis) and Raman spectroscopy, atomic force microscopy (AFM) and electric conductivity measurements. A simple method of synthesis of donor- acceptor complexes has been performed by subsequent deposition of C60 fullerene and tetraphenylporphyrin (H₂TPP) thin films, using physical vapor deposition (PVD) on a (100) silicon substrate. UV-Vis spectra showed that the interaction of π-orbitals leads to a more ordering for the dipole moments arrangement and the π-orbitals overlapping between C60 and H₂TPP molecules. Besides, Raman spectra presented intensity changes at 960 and 1000 cm-1, both related to the vibration of the pyrrole ring and the rocking of the H on the C atoms within the macrocycle. Therefore, it can be expected that the interface C60-H₂TPP should have a main role in the electric response of the multilayer films. The measurements of surface conductivity indicated that interface has specific contribution, and the value of surface conductivity is enhanced by charge delocalization mechanisms occur by π-π stacking interactions. It was found that the transversal conductivity of 3-layer films was enhanced by a factor of 4 in comparison to 2-layer film, due to charge transfer mechanisms occur in the junctions that could extend the diffusion length of the charge carriers. Finally, the interface generated between C60 and H₂TPP films, without any linking molecule, enhance charge transport mechanism through the films.

Highly charged fullerene anions electrochemically stabilized by anionic polymers

The dissolution of redox-active molecules on electrodes hinders the durability of the corresponding redox reactions. In this study, we propose the overcoating of anionic polymers as a possible solution to the dissolution problem. Fullerene was chosen as a model redox-active molecule due to its ability to accept multiple electrons. The barrier function of two anionic polymers was evaluated by monitoring the peak currents for the redox reactions of fullerenes using cyclic voltammetry. Nafion exhibited a better barrier effect on the dissolution of fullerene species and stabilized up to four and five redox reactions of C60 and C70, respectively. On the other hand, although sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene showed a more negative cathodic limit in the electrochemical potential window, the dissolution of fullerene species occurred as fast as for uncovered fullerene thin films.

Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon

The C60/glass, C60/Si, C60/porous Si/Si as well as porous Si/Si thin film structures for comparison were prepared and studied by photoluminescence spectroscopy at room temperature. Along with a broad-band low-energy emission at 1.774 eV in the photon energy range of the nominaly forbidden optical transitions, the C60 films on glass substrate exhibit high-energy emissions at 2.115 eV and 2. 342 eV in the range of allowed dipole transititions. On the other hand, C60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C60/porous Si/Si is 200 times higher than that in C60/Si and at least 20 times exceeds the one for the emission observed on porous Si/Si. Such a dramatic increase in emission intensity is indicative of the relaxation of the selection rules for nominally forbidden optical transitions and suggests strong interaction of C60 molecules with porous silicon walls. Molecular signatures of the C60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (Ag-mode energy) between the irradiative levels, observed on 1.774 eV emission band in C60/porous Si/Si. The origin of the 2.115 and 2.342 eV emission bands is analyzed within a standard band insulator approach, together with the obtained ellipsometric data. The first corresponds to the band-to-band transitions at band gap. The appearance of the second can be related to the splitting of the valence band in the X point of the Brillouin zone of C60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV.

Development of Fullerene Thin-Film Assemblies and Fullerene-Diamine Adducts towards Practical Nanocarbon-Based Electronic Materials

Abstract Fullerenes are attractive spherical aromatic molecules with good electron acceptor capabilities and good utility as an n-type organic semiconductor. By using a fullerene-amine addition reaction, it was possible to fabricate ultrathin-film assemblies of fullerene on the surface of substrates, which were confirmed by photoelectric conversion applications. Addition reactions between fullerenes and primary aliphatic diamines can also occur to form insoluble adduct particles consisting of fullerenes and diamines. In one example, C60-ethylenediamine adduct particles can be solubilized by addition of alkylacid chloride to residual amino groups of the adducts. Spin-coated or dip-coated thin-films of C60-ethylenediamine adducts from their solutions are useful as n-type organic semiconductors which was confirmed with solar cell application. In this account, the history of the fabrication and application of fullerene thin-film assemblies and fullerene-diamine adducts using the fullerene-amine addition reaction is introduced and summarized from the early studies to more recent developments.

Substrate Temperature Influence on Optical Properties of C60 Thin Films Within the Visible Range

Fullerene thin films of about 200 nm thicknesses have been deposited by thermal evaporation method on soda lime glass at substrate temperature 303 and 403K under pressure about 10-5 mbar. This study concentrated on the influence of substrate temperature on the optical properties of C60 thin films within the visible range. Optical characterization has been carried out at room temperature using the absorption spectra, at normal incidence, in range (200-900) nm.
 The absorption and extinction coefficients of the samples have been evaluated according to the variation in the UV- Visible spectrum. Increasing substrate temperature causes decreasing in optical band gap energy, for direct allowed transitions, and slightly changing in refractive index. This incident was due to the reducing of interatomic intervals, which may be correlating a decrease in the amplitude of atomic vibrations around their equilibrium sites.

Evolution of SPR in 120 MeV silver ion irradiated Cu (18%) C60 nanocomposites thin films

Copper metal embedded fullerene C60 thin films are deposited on various substrates via resistive heating co evaporation technique. Rutherford back scattering simulated spectrum confirms the thickness and composition of thin film to be ~ 32 nm and ~ 18 at%, respectively. The deposited thin films are irradiated by 120 MeV Ag ion beam for different fluences within the range, from 1 × 1012 to 3 × 1013 ions/cm2. UV–vis absorption spectroscopy study reveals the appearance of surface plasmon resonance (SPR) band in pristine thin film at ~ 622 nm; this phenomenon is ascribed to the presence of copper nanoparticles in C60 matrix. The SPR band intensity increases with rising fluences, which is suggestive of the growth of Cu nanoparticles. Growth of Cu nanoparticles is further confirmed by transmission electron microscopic (TEM) and X-ray diffraction (XRD) study. Whereas the conversion of C60 into amorphous carbon (a-C) is confirmed by Raman spectroscopy, XRD results of pristine Cu–C60 thin film record the presence of Cu2O within the nanocomposite thin film. However, both X-ray photoelectron spectroscopy (XPS) and XRD studies on irradiated thin film samples authenticate the growth of Cu metal nanoparticles with concurrent removal of oxygen.

Desorption of Fullerene Dimers upon Heating Non-IPR Fullerene Films on HOPG

Low energy ion beam deposition of mass-selected fullerene ions was used to generate thin films of non-IPR fullerenes, C2n (2n = 48–58 and 52–68) on graphite under UHV. These were probed by mass-resolved thermal desorption spectroscopy. We observe the emission of dimers in addition to dominant monomer desorption for 2n = 58–68, while for 2n = 48–56 no dimer desorption was detected. Dimer signals were typically at <1% level compared to the monomers. The desorption temperature ranges of these dimer species were higher and much narrower than for the corresponding monomers. Building also on theoretical considerations, we came to the conclusion that coalescence reactions between the fullerenes must take place below 1000 K and that fullerenes with more than three pairs of adjacent pentagons cross-link too strongly to allow the desorption of dimers.

Functional nanostructured surfaces induced by laser on fullerene thin films

We report a method to produce nanosecond pulsed laser induced periodic surface structures (LIPSS) on spin-coated [6,6]‑phenyl C71-butyric acid methyl ester (PC71BM) films exhibiting a photoinduced resist-free imprinting (PRI) property. LIPSS in PC71BM are resist free and can be solvent developed, under certain conditions, leaving the laser illuminated zone as an undissolved residue. LIPSS prepared with a wavelength of 532 nm selectively preserve the electrical conductivity on the LIPSS valleys. The heterogeneous conducting pattern consisting in conducting valleys and less conducting hills observed in the LIPSS has been interpreted as due to a reduction in the crystallinity of the hills provoked by the fast heating/cooling process induced by every laser pulse. On the basis of the experiments we propose that irradiated PC71BM films: i) suffer a fast heating/cooling process tending to reduce the overall crystallinity of the LIPSS and therefore decreasing the electrical conductivity of the hills while leaving a remnant conducting film underneath the hills, ii) during laser irradiation the sample suffers a heat treatment leading to the development of a buried phase with enhanced crystallinity, close to the silicon substrate, formed by larger crystals with enhanced electrical conductivity and iii) further solvent treatment reveals the buried phase which remains in the form of partially electrically conducting nanowires. These results point towards the potential use of the described technology for nanofabrication of organic electronic devices.