ANNUAL JOURNAL OF ELECTRONICS, 2014, ISSN 1314-0078 43 Flexible Thick Film Electroluminescent Devices: Influence of the Mechanical Stress on Layers Behavior Mariya Petrova Aleksandrova, Svetozar Krastev Andreev, Ivelina Nikolaeva Ruskova and Georgi Hristov Dobrikov Abstract - The influence of the mechanical bending on flexible screen-printed organic light emitting devices upon their electrical properties has been examined. Our measurements showed that flexible printed coatings can be bent to small radii (5 mm and 2.5 mm) of curvature and still function, despite the increase of the electrical resistance of the electrodes from 45 to 62.1 Ohms/sq. This is ascribed to the mixture of two classes organic compounds (low molecular weight and polymeric) in suitable ratio, which guarantee high adhesion and stability against strain. Device failure is caused mainly by the transparent indium tin oxide (ITO) electrode that is not resistant to continuous bending at radius 5 mm, as well as to repeated bending over 2 000 that lead to cracks occurring. Internal stress induced in organic electroluminescent layers with different thickness was determined to be in the range |1|-|7| MPa, based on the radius of curvature. Current-voltage characteristics clearly show the limit of bending, at which current flow stops, because of electrodes degradation. Keywords – Flexible OLED, Thick Films, Electroluminescent Coatings, Mechanical Stress I. INTRODUCTION Large-scale display production of all modern visualization devices, such as light emitting devices (inorganic LED or organic OLED) [1,2], electrochromic [3], etc. is accomplished by thin film deposition processes. In the same time the screen printing technology has offered highly reproducible and relatively inexpensive way to produce different electronic devices, such as different sensors and even solar cells [4,5]. Thick film technology is useful for deposition of highly temperature stable organic electroluminescent structures, standing work at maximum brightness for hours, which is hard achievable for thin film OLEDs [6]. The screen printing process includes patterned deposition of inks and solders mostly onto planar substrate by pressing the paste through a laser cut metal stencil or polyester screen mesh [7]. The composition for electroluminescent inks includes a light-emitting fine powder or granular material, binding substance, and a solvent. After deposition, temperature treatment is supplied for full solvent evaporation for a certain time depending on the coating thickness. Annealing temperature depends on the materials specific features – for example substance’s thermal degradation or substrate’s degradation point. Existing papers in the literature about screen printed organic inks for display applications are assigned for glass substrates [8,9]. However, recently flexible devices and in particular displays have entered into the modern life and tent to improve all the time in the meaning of performance [10]. Main building coatings should be investigated for the impact of mechanical stress on their electro-optical behavior. There is information concerning this problem, but it is related to flexible thin nanometric films based OLED [11]. By our knowledge there are still no data provided for mechanical behavior of thick film flexible OLEDs. In this paper we used polyethylenetherephtalate (PET) flexible substrate covered by ITO transparent electrode and screen printed poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyle- nevinylene] (MEH-PPV):polystyrene ink and aluminum electrode to investigate stability of the organic electroluminescent device in term of electro-mechanical properties. II. EXPERIMENTAL SECTION Sheets from polyethyleneterephtalate foil (PET), having degradation point of 90 o C, were cut with sizes 4 cm x 4 cm and were cleaned by ethyl alcohol in ultrasonic cleaner. The substrates were cleaned in it for 90 seconds. Indium tin oxide (ITO) transparent electrode films were prepared in RF vacuum sputtering system, from target consists of In 2 O 3 and SnO 2 in a weight proportion of 95:5 mol%. The sputtering power supplied to the target was set to 60 W (target voltage 500 V and plasma current 120 mA) at deposition time of 20 minutes for 200 nm film thickness. The chamber was evacuated to 8.10 -6 Torr, then the oxygen pressure was fixed at 2.10 -4 Torr and finally the total pressure of reactive gas and sputtering inert (argon) gas was maintained at 2.5.10 -2 Torr. In this way the substrate temperature during film growth was lower then the temperature of PET’s mechanical deformation. To decrease the specific and the sheet resistance, the samples were exposed to ultraviolet light (365 nm, 250W) for 10 min. Soluble modification of low molecular weight electroluminescent compound 8-hydroxyquinoline aluminum salt (Alq) was mixed with polyvinilcarbazole and binder polystyrene in a suitable ratio, and then the M. Aleksandrova is with the Department of Microlectronics and Faculty of Electronic Engineering and Technologies, Technical University - Sofia, 8 Kliment Ohridski blvd., 1000 Sofia, Bulgaria, e-mail: m_aleksandrova@tu-sofia.bg S. Andreev is with the Department of Microlectronics, Faculty of Electronic Engineering and Technologies, Technical University - Sofia, 8 Kliment Ohridski blvd., 1000 Sofia, Bulgaria, e-mail: svetozar_a@tu-sofia.bg I. Ruskova is with the Department of Microlectronics, Faculty of Electronic Engineering and Technologies, Technical University - Sofia, 8 Kliment Ohridski blvd., 1000 Sofia, Bulgaria, e-mail: inch@ecad.tu-sofia.bg G. Dobrikov is with the Department of Microlectronics, Faculty of Electronic Engineering and Technologies, Technical University - Sofia, 8 Kliment Ohridski blvd., 1000 Sofia, Bulgaria, e-mail: georgi_hd@tu-sofia.bg