RECENT SIGNIFICANT PROJECTS
Study of the Piezoelectric Response of Layered Microgenerators on Flexible Substrates
The project aims to acquire new knowledge about the mechanisms of charge generation in portable piezoelectric elements on flexible substrate and the dependence of the piezoelectric response on the situation of piezoelectric areas over the flexible surface, on the type of active piezoelectric layer, technological modes for its deposition and the applied mechanical stress. For this purpose is planned design, fabrication and testing of piezoelectric generator elements with a potential use as alternative energy sources (energy harvesting devices), made of layers with nano- and / or micrometer thickness. Variety of technologies for deposition of layers from composite organic and inorganic materials for the active piezoelectric layers will be used, such as screen printing, vacuum sputtering, spray deposition and atomic layer deposition. Scientific tasks will be related to clarification of the physical nature of processes to increase the converting efficiency of mechanical energy into electrical energy and to find a way for their control and optimization. Possibilities for reducing the influence of mechanical stresses on the stability of the piezoelectric efficiency will be examined. The energy barrier at the interface electrode / transducer layer will be estimated, the polarizing processes and the influence of temperature on the dipoles’ mobility and coefficients of linear expansion of the substrate will be studied. Worldwide novelty, in line with the modern trends for the development of biocompatible and environmentally friendly products, will be fabrication and characterization of a completely polymer piezoelectric structure in which even the metal contacts are replaced with conductive polymer from the class of polythiophenes. The study of such a structure would provide new knowledge about the processes developed at the junction between the polymer electrode and the functional polymer layer.
Acknowledgement: grant No ДН 07/13 funded by Bulgarian National Science Fund, 2016-2019
SEM image of ZnO film grown on PET/PEDOT:PSS/Au/PEDOT:PSS substrate, giving enhanced piezoelectric response.
Image of sample piezoelectric flexible energy harvesting device.
Highly Flexible Transparent Electrodes for Optoelectronic Devices – Fabrication and Testing
This project is related to study of the electrical, optical and mechanical properties of polymer (polyethylenedioxithiophene) doped with polystyrenesulfonate (PEDOT:PSS) thin films and PEDOT:PSS based multilayer structures on polyethylene terephthalate (PET) substrates for transparent electrodes in flexible organic light-emitting devices and solar cells. The easily cracked at repeated bends electrode from indium tin oxide is successfully replaced by transparent polymer film produced from PEDOT:PSS solution. Optimized technological regimes for spin coating of this solution is developed for its deposition on flexible PET substrate and a few tenths of nanometers thin films with controllable and reproducible properties are achieved. It is found that pre-treatment of PET with UV light at specific wavelength and dose of irradiation makes the surface hydrophilic, improves the PEDOT:PSS nanofilm uniformity and its adhesion strength. It was achieved 2.6 times higher adhesion of the films that withstand mechanical load of over 2000 cycles without microcracks revealing. It is found that insertion of metal layer with optimized thickness between PEDOT:PSS layers leads to improvement of the sheet resistance in comparison to single PEDOT:PSS film and reach values close to those of ITO. Moreover, this multilayer electrode showed dramatically improved mechanical stability in comparison to ITO, when subjected to multiple bends. Its sheet resistance variation is 3.8 % after applying 2 000 bending cycles at radius of curvature 6 mm in comparison to 6.7 % for single PEODT:PSS film and 11.4 % for ITO.
Acknowledgement: grant No 141PR0005-03 funded by Scientific and Research Sector of Technical University of Sofia.
Deposition methods for organic electroluminescent thin and thick films on flexible substrates
The main goal of the project is replacing of the conventional vacuum thermal evaporation of the organic compounds by processes, conducted at room temperature. In this way there are no mechanical stress, induced in the polymeric flexible substrate and thermal degradation in the organic bonds of the electroluminescent layer. It is achieved long term stability and exploitation life. The suggested approach is spray deposition of organic solution and screen printing of organic paste. One of the targets is achieving of high adhesion strength of the layers during bending of the flexible substrate and in the same time application of cheep and simple technological processes. For OLED with spray deposited layers the quantum efficiency is also improved, because of the higher layer’s uniformity on nano-scale level. For OLED with screen printed thick micrometers layers, it is achieved higher breakdown voltage, because of the better heat dissipation in comparison with the thermal evaporated thin films. Their mechanical stability should be further improved.
Acknowledgement: grant No DMU 03/5 Fund “Scientific Research”, Bulgarian Ministry of Education and Science.
Samples, representing patterned trnasparent electrode for flexible OLEDs, as well as spray depositoed and screen printed organic light emitting devices.
International conference participant.
Awards for best poster presentation and oral presentation
on international conferences.