Mariya Aleksandrova

Specifics and Challenges to Flexible Organic Light-Emitting Devices

Several recent developments in material science and deposition methods for flexible organic light-emitting devices (OLEDs) are surveyed. The commonly used plastic substrates are compared, according to their mechanical, optical, thermal, and chemical properties. Multilayer electrode structures, used as transparent electrodes, replacing conventional indium tin oxide (ITO) are presented and data about their conductivity, transparency, and bending ability are provided. Attention is paid to some of the most popular industrial processes for flexible OLEDs manufacturing, such as roll-to-roll printing, inkjet printing, and screen printing. Main specifics and challenges, related to the foils reliability, mechanical stability of the transparent electrodes, and deposition and patterning of organic emissive films, are discussed.

Optical characterization of WO3 -VOx thin films for application in electrochromic devices—‘smart windows’

WO3-VOx thin films were deposited by atmospheric pressure chemical vapor deposition (APCVD). A typical electrochromic device is a sandwichlike structure with two conductive glasses and an electrolyte layer. An electrochromic transition metal oxide film is deposited over one of the glass substrates thus forming the working electrode. The counter electrodes in the devices are bare conductive glasses, and the polymeric electrolyte is used to laminate the two glass substrates and to supply the devices with Li ions. The working electrode material is investigated related to its initial transmittance, vibrational properties, and structure.

Impedance Investigation of Polymer Layers for Electroluminescent Devices

AC electrical properties of polymer light-emitting devices with poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and polyvinylcarbazole (PVK) layers are studied. Polymer nanosized films are deposited by the spin-coating method. Frequency-dependent real and imaginary parts of the impedance are measured and fitted using RC equivalent circuits. Important parameters for such types of structures, like capacitance and contact resistance, are evaluated at different frequencies in the range from 100 Hz to 8 MHz and bias voltages between 0 and 4 V. Transit times of charge carriers at different bias are calculated using the capacitance-frequency curves.

Thermo-Stimulation of Charges by Peltier Element for Trap Analysis in Polymer Layers

Activation energy of traps is determined for polymeric samples with spin-coated phenylene-based thin film. For trap energy level detection, a modified method of thermo-activated charge carriers with cooling cycles is applied. The effect of different revolutions of spinning during film deposition on the measured current is investigated. The experiments are carried out for aluminum/poly(9,9-dihexyl-9H-phenylene-2,7-diyl-vinylene)/aluminum single-layer, unipolar devices, prepared at 900, 1500, and 2000 rpm. In all cases, broad trap distributions were found. This behavior is ascribed to bond damage in the polymer chains, which generate trap states. With increase of centrifugal forces, the mean trap energy level shifts to lower energies.

Study of quartz crystal microbalance NO2 sensor coated with sputtered indium tin oxide film

A study of NO2 gas sorption ability of thin indium tin oxide (ITO) deposited on 16 MHz quartz crystal microbalance (QCM) is presented. ITO films are grown by RF sputtering of indium/tin target with weight proportion 95:5 in oxygen environment. The ITO films have been characterized by X-ray photoelectron spectroscopy measurements. The ITO surface composition in atomic % is defined to be: In-40.6%, Sn-4.3% and O-55%. The thickness and refractive index of the films are determined by ellipsometric method. The frequency shift of QCM-ITO is measured at different NO2 concentrations. The QCM-ITO system becomes sensitive at NO2 concentration ≥ 500 ppm. The sorbed mass for each concentration is calculated according the Sauerbrey equation. The results indicated that the 1.09 ng of the gas is sorbed into 150 nm thick ITO film at 500 ppm NO2 concentration. When the NO2 concentration increases 10 times the calculated loaded mass is 5.46 ng. The sorption process of the gas molecules is defined as reversible. The velocity of sorbtion /desorption processes are studied, too. The QCM coated with thin ITO films can be successfully used as gas sensors for detecting NO2 in the air at room temperature.

Influence of the operating temperature on the organic flexible display's performance for mobile applications

Organic light emitting device is produced on flexible polyethileneterephtalate substrate and is investigated at different operating temperatures from 22 to 50 °C. Power consumption of the produced display is ~500 mW/cm2 at room temperature. Important electrical parameters like a turn-on voltage, trap factor and effective mobility are estimated from the measured temperature dependent current-voltage curves. It is established that the turn-on voltage increase from 1.5 V to 3 V with the temperature, while the breakdown voltage drops from 12.5 V to 4.4 V.

Synthesis and electronic spectra of new low-molecular weight compounds with possible application in electroluminescent layers

AbstractTwo new low-molecular weight compounds — (Z)-4-(4-(dimethylamino)benzylidene)-1-(9-ethyl-9H-carbazol-3-yl)-2-phenyl-1H-imidazol-5(4H)-one and 2-(6-hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) — with possible application in organic light-emitting devices were synthesized. Their photophysical properties in solution and in polymer films were investigated. The determined relative fluorescence quantum yields in solution for both compounds were 0.003 and 0.51, while those in poly(methyl methacrylate) films were around 0.10 and 1.0, respectively. For 1H-imidazol-5(4H)-one derivative, single-layer organic displays with one emitting layer were prepared by spin-coating technology. The applied voltage was 40 V (AC) with 1–3 KHz frequency. The emission maximum of the experimental AC display structures was at 600–630 nm. For displays with 2-(6-hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) the applied voltage was 60 V (AC) with 6-9 KHz frequency, but its future success will require more appropriate binding polymers. Based on the obtained experimental results, it is concluded that the investigated compounds could be applied for preparation of color electroluminescent structures.

Study of flexible organic electroluminescent devices with PEDOT:PSS anodes by impedance measurements

Purpose – Purpose This study aims to apply the impedance spectroscopy (IS) for analyzing the electrical behavior and extracting the equivalent circuit of single-layer flexible organic light-emitting diodes (OLEDs) with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) anode. Design/methodology/approach – The preliminary ultraviolet (UV) treatment of the flexible substrate of polyethylene terephthalate (PET) influenced the conductivity of PEDOT:PSS anodes. Findings – The IS showed that the OLED with UV-treated PET/PEDOT:PSS anodes had lower values of the contact resistance and higher value of the interface capacitance. Originality/value – The obtained data were used for modeling of flexible OLEDs with polymeric anodes and calculation of important display parameters such as pixel refresh ratio, signal delays and energy losses due to contact resistances. These parameters were compared for PEDOT:PSS anodes deposited on PET treated and non-treated by UV.

Programmable setup for Peltier element control with fine smooth regulation of the temperature about testing of semiconductor structures

This article presents the design of non-standard equipment for obtaining experimental data about defects in thin film semiconductor structures. The idea of using low-powered Peltier element programmed to cause small temperature changes in semiconductor structures is scanning of the energy bandgap and thermal stimulation of the trapped charge carriers for defects analysis. Block diagram, control algorithm and PCB layout of the testing stand are given. Samples consisting of organic semiconductor and aluminum electrodes are examined for demonstration of the device working capacity. Energy distribution and concentration of the existing defects are determined.

Thin Film Microsensing Elements, Technology and Application in Microsystems for Environment Control

In this paper microsensing elements with thin piezoelectric films are presented. Three different materials from the basic types – ceramic, metal oxide and polymer, in microsystems for environment parameters control are used. Zinc oxide (ZnO) and lead zirconium titanate (PZT) by sputtering technology from hot targets are deposited. The optimal parameters of the process and thickness of the layers are presented. Layers of piezoelectrical polymer poly (vinylidene fluoride) (PVDF) were prepared by spray deposition technique. The microelements (MEMS) with these piezoelectric films are proposed for direct and indirect measuring of parameters as pressure, vibration, gas and liquid sensing, fluid flow, etc.