F. Stanculescu

Optical Properties of 3,4,9,10-Perylenetetracarboxylic Dianhydride and 8-Hydroxyquinoline Aluminum Salt Films Prepared by Vacuum Deposition

The main purpose of this paper is to investigate the optical properties of PTCDA and Alq3 films, prepared by two steps, vacuum evaporation and deposition processes on platelets of glass, quartz, and indium-tin-oxide (ITO) coated glass. We have emphasised the bands structure of the absorption spectra with peaks situated at 358 nm, 374 nm, 475 nm and 552 nm in PTCDA, respectively 232 nm, 261 nm and 380 nm in Alq3 that confirms the dominant presence of Alq3 meridianal molecular isomer. For PTCDA films deposited on glass coated with ITO, the structure of the weak double peak at low wavelength is partially modified, but the positions of the two important absorption peaks situated at 2.25 eV and 2.61 eV are unchanged. The two different luminescence emission peaks obtained in Alq3 for different excitation wavelengths (λ=360 nm and λ=520 nm) suggest the existence of the facial isomer beside the meridianal one. We have evidenced a significant Stocks shift in the spectra (EPTCDA=0.40 eV; EAlq3=0.9 eV) and a large Frank- Condon shift (0.40-2.3 eV), suggesting important effect associated respectively with the solid state structure and important conformational differences between the ground and excited state.

Melt growth and characterization of pure and doped meta-dinitrobenzene crystals

Abstract Pure and doped crystals of meta-dinitrobenzene have been grown using a Bridgman–Stockbarger method. We have investigated the influence of the dopant (oxine, resorcinol) on the optical properties of the bulk and film samples obtained using crystalline fragments from the melt grown ingots. Transmission variations reflecting the quality of the material are connected with the purity level and the growth conditions.

Preparation and Characterization of Polar Aniline Functionalized Copolymers Thin Films for Optical Non-Linear Applications

This paper presents a study of two monomeric structures containing functional groups with different electronegativities [NH/NH-NH and NO2] and two different polymeric structures prepared by the copolymerisation of maleic anhydride and methylmethacrylate/maleic anhydride and vinyl benzyl chloride, and subsequent functionalization with 2,4 dinitroaniline. Thin films have been prepared by Matrix Assisted Pulsed Laser Evaporation (MAPLE) and UV-VIS, FTIR, Raman and Photoluminescence Spectroscopy have been used to comparatively investigate the properties of the synthesised compounds. We have evidenced the second harmonic emission (SHG), process sustained by the asymmetry of the chemical structure, which is correlated with the crystallization in non-centrosymmeric structures.

Investigations of the Correlation between the Preparation Method and the Properties of Anilinic Derivative Functionalised Polymer Thin Films for Non-Linear Optical Applications

This paper presents a study of two copolymers, maleic anhydride and methylmethacrilate/maleic anhydride and vinyl benzyl chloride, functionalised with polar chromophoric groups such as 2,4 dinitroaniline. Thin films have been prepared by vacuum evaporation and spin coating methods on silicon and glass substrates. UV-VIS, FTIR, XRD and Photoluminescence Spectroscopy have been used to comparatively investigate the effect of the preparation method on the properties of the thin films obtained with functionalized copolymer. SEM has evidenced differences in the morphologies of the layers suggesting a degradation of the polymeric chain during the evaporation process to fragments that conserve unchanged the chromophoric group. This explains the presence of two types of non-linear phenomena, the second harmonic emission and two-photon luminescence emissions for both vacuum evaporated and spin coated films.

Influence of growth conditions on the physical properties of an organic crystalline material for optical application

We present some investigations on the correlation between the crystalline quality controlled by the physical defects induced by the growth conditions and the optical and electrical properties of a benzene disubstituted derivative.

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications

Multilayer or blend heterostructures based on porphyrins and phthalocyanines were obtained on different substrates using VTE and MAPLE methods. Stacked structures based on ZnPc and C60 with NTCDA were prepared by VTE on ITO/glass, their cur‐ rent value being increased by the deposition of the materials in an inverted configu‐ ration or by using ITO/PEDOT:PSS as a substrate. Multilayer structures comprising ZnPc and NTCDA were fabricated by MAPLE on an AZO/glass. Treating the AZO in oxygen plasma, a higher current value was obtained for the deposited heterostructures. The oxygen plasma treatment can increase the work function of the AZO resulting in a decrease of the energetic barrier from AZO/organic interface and finally improving the charge transport. Stacked layers or blend heterostructures having ZnPc, MgPc and TPyP were deposited by MAPLE on ITO/PET. In the case of those containing MgPc and TPyP, an increase in the current value (in dark) was obtained for the blend compared to the stacked layer configuration. For those with ZnPc and TPyP, under illumination, a pho‐ tovoltaic effect was observed for the blend structure. All heterostructures are featured by a large absorption in the visible domain of the solar spectrum and suitable electrical properties for their use in OPV applications.

Laser Prepared Thin Films for Optoelectronic Applications

Laser techniques such as pulsed laser deposition, combinatorial pulsed laser deposition, and matrix-assisted pulsed laser evaporation were used to deposit thin films for optoelectronic applications. High-quality transparent conductor oxide films ITO, AZO, and IZO were deposited on polyethylene terephthalate by PLD, an important experimental parameter being the target-substrate distance. The TCO films present a high transparency (>95%) and a reduced electrical resistivity (5 × 10−4 Ωcm) characteristics very useful for their integration in the flexible electronics. In x Zn1−xO films with a compositional library were obtained by CPLD. These films are featured by a high optical transmission (>95%), the lowest resistivity (8.6 × 10−4 Ωcm) being observed for an indium content of about 44–49 at.%. Organic heterostructures based on arylenevinylene oligomers (P78 and P13) or arylene polymers (AMC16 and AMC22) were obtained by MAPLE. In the case of ITO/P78/Alq3/Al heterostructures, a higher current value is obtained when the film thickness increases. Also, a photovoltaic effect was observed for heterostructures based on AMC16 or AMC22 deposited on ITO covered by a thin layer of PEDOT:PSS. Due to their optical and electrical properties, such organic heterostructures can be interesting for the organic photovoltaic cells (OPV) applications.

Organic Semiconductors for Non-Linear Optical Applications

The organic molecules represent a promising research field because they show special properties which are determined by the conjugated system of π electrons and the groups substituted to the aromatic nucleus offering perspectives for a large area of applications, including the non-linear optics. We have investigated two types of molecular crystalline material prepared from aromatic derivatives, meta-dinitrobenzene (m-DNB) and benzil (Bz), and three types of organic thin films prepared from arylenevinylene (triphenylamine, carbazole) compounds, maleimidic derivatives and anilinic derivatives functionalised copoly‐ mers using different methods (matrix-assisted pulsed laser evaporation, vacuum evaporation, spin coating). The effect of the experimental conditions on the morpho‐ logical and structural particularities of the films has been emphasised. New results are bought about the systems formed from an aromatic derivative (m-DNB and benzil) crystal and inorganic (organic) dopant studied from the point of view of the dopant incorporation considering the stability of the growth interface and its effect on the optical band gap and optical non-linear (ONL) properties of the m-DNB and Bz crystals. The relationship between the morphology and structural order of the maleimidic derivatives monomers in polycarbonate matrix composite layer and the ONL properties is discussed. We analysed the correlation between the molecular structure of organic compound, particularities of the macroscopic order (influenced by the crystal growth and thin-film deposition methods) and optical non-linear (second harmonic generation, two-photon fluorescence) properties.

Organic Semiconductor Based Heterostructures for Optoelectronic Devices

In the last decades organic materials are considered a very important alternative to the inorganic semiconductors in the manufacturing of a large variety of devices because of the great diversity of the organic compounds, their remarkable properties, low production cost, compatibility and complementarity with the very well known silicon technology. Lately the organic semiconductors started to represent a new class of materials very attractive for optoelectronic applications (Huang, 2002; Tang, 1988; Forrest, 2000; Kalinowski, 2003) and electronic applications (Dodabalapur, 1995; Karl, 2000; Katz, 2000 a; Horowitz, 1999; Torsi, 2000; Schon, 2000 a; Inoue, 2005) in particular light emitting devices and lighting sources (Yersin, 2007; Van Slyke, 1996; Wang, 2005; Dini, 2005), solar cells (Lane, 2005; Ameri, 2009; Troshin, 2008; Duggal, 2005), photodetectors (Hofmann, 2005; Troshin, 2008), field effect transistors (Molinari, 2007; Mas-Torrent, 2008) and lasers (Duarte, 2009; Liu, 2009) being considered candidates to replace the inorganic semiconductors. The electroluminescence (EL) of the organic molecules is a well-known phenomenon for more than 50 years, but only in the late ’80th has become utile for practical applications. The successful application of the organic materials luminescence in Light Emitting Devices (OLEDs) requires adequate device structures to overcome the problems associated with the high resistivity of the organic materials and with the difficulty to obtain a good charge injection from the electrodes in the organic layers. These devices have a thin film architecture, which includes both organic/organic and organic/inorganic interfaces. Starting with the first prototype of OLED (Vincett, 1982), lately has been proposed the first efficient OLED based on heterostructures containing thin films from small molecule organic compounds (Tang, 1988) that have shown an attractive efficiency (1% external quantum efficiency, 1.5 lm/W luminous efficiency, higher than 1000 cd/m2 brightness) and driving voltage below 10 V. From this moment, different types of OLEDs have been realised, based on both fluorescence and phosphorescence, that have started to be used in mobile, small dimension displays. Probably, the most spectacular application of the Organic Light Emitting Devices is in the flat panel displays for TV and monitor technology because they are characterized by high brightness and wide viewing angle, which are the most important advantages over the liquid crystals. Significant effort is devoted now to increase the quantum efficiency, lifetime and thermal stability of these devices. Devices with high efficiencies, low working voltages