D Dimov

Polyimide coatings containing azo-chromophores as structural units

Results are presented on the vacuum deposition of polyimide layers containing azochromophore as structural unit. The imidization process of the polymer thin films was studied by FT-IR and optical spectroscopy. After microwave and thermal treatment the, polyimide layers are E-diastereomers only, with a homogeneous smooth surface devoid of cracks. The technique used is suitable for preparing polyimide coatings containing chromophores with tailored properties.

Synthesis, spectroscopic and TD-DFT quantum mechanical study of azo-azomethine dyes. A laser induced trans-cis-trans photoisomerization cycle

This paper describes the synthesis, spectroscopic characterization and quantum mechanical calculations of three azo-azomethine dyes. The dyes were synthesized via condensation reaction between 4-(dimethylamino)benzaldehyde and three different 4-aminobenzene azo dyes. Quantum chemical calculations on the optimized molecular geometry and electron densities of the trans (E) and cis (Z) isomers and their vibrational frequencies have been computed by using DFT/B3LYP density-functional theory with 6-311++G(d,p) basis set in vacuo. The thermodynamic parameters such as total electronic energy E (RB3LYP), enthalpy H298 (sum of electronic and thermal enthalpies), free Gibbs energy G298 (sum of electronic and thermal free Gibbs energies) and dipole moment μ were computed for trans (E) and cis (Z) isomers in order to estimate the ΔEtrans→cis, Δμtrans→cis,ΔHtrans→cis, ΔGtrans→cis and ΔStrans→cis values. After molecular geometry optimization the electronic spectra have been obtained by TD-DFT calculations at same basis set and correlated with the spectra of vapour deposited nanosized films of the dyes. The NBO analysis was performed in order to understand the intramolecular charge transfer and energy of resonance stabilization. Solvatochromism was investigated by UV-VIS spectroscopy in five different organic solvents with increasing polarity. The dynamic photoisomerization experiments have been performed in DMF by pump lasers λ=355nm (mostly E→Z) and λ=491nm (mostly Z→E) in spectral region 300nm - 800nm at equal concentrations and times of illumination in order to investigate the photodynamical trans-cis-trans properties of the CHN and NN chromophore groups of the dyes.

Development of Nanostructured Materials with CBRN Agents Sensing Properties

Azobenzene containing polymers are a class of optical materials extensively investigated in the past decade. Due to their possibility to undergo reversible trans-cis-trans photoisomerization cycles and through their attractive photophysical properties, the azopolymers could be used as storage medium in optical devices, as optical switches and sensors, as responsive surfaces for biomedical applications, in polarization holography, and in photonics. The aim of this paper is to summarize the information available in the scientific literature about the possibility to use these materials not only for optical application but also as sensing platforms for the detection of biological agents listed as category A biological weapons. Furthermore, this review attempts to give the readers insights into the difficulties concerning the detection of biological agents within environmental samples as well as to outline the progress and the weaknesses in the development of sensors based on nanostructured materials to help to combat terrorisms.

Vacuum-deposited diphenyl-diketo-pyrrolopyrrole solar cell structures

Photoelectrical parameters were measured of solar cell ITO|PEDOT:PSS|composite| Al samples. The active composite film was deposited in vacuum by co-evaporation of 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP(TBFu)2) and fullerene (C60). Additional DPP(TBFu)2:C60 composite films were studied by spectroscopy in the ultraviolet and visible region (UV-VIS) and scanning electron microscopy (SEM). It was found that solvent annealing (SVA) of composite DPP(TBFu)2:C60 vacuum-deposited films with tetrahydrofuran vapors improves the solar cell parameters by increasing the efficiency more than tenfold. This could be related to the more homogenized structure of the SVA composite film, as observed by SEM. The increased light absorption, as shown by UV-VIS spectroscopy, around the peak at 350 nm contributed to the better SVA solar cell performance. Photogeneration in the samples follows a monomolecular mechanism.

Improvement of the organic solar cells functional parameters

By changing the concentration ratio (zinc-phthalocyanine-ZnPhTc:Fullerene-C60 from ZnPhTc:C60 = 1:0 through ZnPhTc:C60 = 1:1, to 1 C60: ZnPhTc = 1:0) in the active layer, as well as the architecture of the organic solar cells (OSC), a substanial increase of their short-circuit current is achieved. We suggest that the blurred interlayer boundaries lead to an enhanced motion of the holes and the electrons to the electrodes – ITO and Al respectively. Regardless of the shorter distances between the places of excitons creation, the excitons have enough life-time for diffusion to the suitable places where their decomposition to the charge carriers – holes and electrons, takes place. This is one of the well-known approaches to achieve a higher efficiency of the OSCs. We thus suggest that the possible interactions and processes discussed are obviously essential prerequisites for improving the functional parameters of the OSCs.

Thin dye layers ? vacuum deposition and structure

Vacuum-deposited layers of zinc phthalocyanine and perylene-3,4,9,10-tetracarboxylic dianhydride were obtained with regard to the development of converters of solar radiation into electricity, namely, organic solar cells. It was shown that there were no differences in the composition of the initial dyes and the deposited ones, which is a proof of their thermal stability and a prerequisite for the reproducibility of the composition of the single and mixed layers under study. The results obtained are interpreted in view of the possible applicability of the layers studied for the fabrication of organic solar cells in which the processes of creation, the lifetime and the diffusion paths of the current carriers depend significantly on the film composition and the defects formed at the time of deposition of the active dye layers.

Enhanced hole transport in multilayer organic based devices

Multilayer Organic Based Devices (OBDs) were constructed by subsequent casting of organic films (from polymers, soluble in the same organic solvent). The problem with dissolution of the underlying layer was avoided by using electrophoretic deposition technique. Optimized conditions for electrophoretic deposition (EPD) of thin films with homogeneous and smooth surfaces, as confirmed by SEM, were found. The EPD deposition, carried out at constant current, requires continuous increase of the voltage between the electrodes. In this way the decreased deposition rate caused by the decreased concentration of the material in the suspension and the increased thickness of the film deposited is compensated. The SEM images and the current voltage characteristics recorded, show that the hole transport polyvinylcarbazole (PVK) underlayer survive the treatment with the suspension used for the electrophoretic deposition of the active poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] electroluminescent layer. The introduced PVK hole transport layer increases the device current, as confirmed by the current-voltage measurements. The results obtained demonstrate the possibility of OBDs preparation for electroluminescent and photovoltaic application.

Multilayer organic based structures with enhanced hole transport

Multilayer Organic Based Devices (OBDs) were constructed by subsequent casting of organic films (from polymers, soluble in the same organic solvent). The problem with dissolution of the underlying layer was avoided by using electrophoretic deposition technique. Optimized conditions for electrophoretic deposition (EPD) of thin films with homogeneous and smooth surfaces, as confirmed by SEM, were found. The EPD, carried out at constant current, requires continuous increase of the voltage between the electrodes. In this way the decreased deposition rate caused by the decreased concentration of the material in the suspension and the increased thickness of the film deposited is compensated. The SEM images and the current voltage characteristics recorded, show that the hole transport polyvinylcarbazole (PVK) underlayer survive the treatment with the suspension used for the electrophoretic deposition of the active poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] electroluminescent layer. The PVK hole transport layer increases the device current, as confirmed by the current-voltage measurements. The results obtained demonstrate the possibility of OBDs preparation for electroluminescent and photovoltaic applications.

Ion – beam assisted process in the physical deposition of organic thin layers

A novel method was developed for physical deposition of thin polyimide layers by applying an argon plasma assisted process. The influence was investigated of the plasma on the combined molecular flux of the two thermally evaporated precursors – oxydianiline and pyromellitic dianhydride. The effects observed on the properties of the deposited films are explained with the increased energy of the precursor molecules resulting from the ion-molecular collisions. As could be expected, molecules with higher energy possess higher mobility and thus determine the modification of the films structure and their electrical properties.