Showing papers by "Petko Vitanov published in 2019"

Flexible light valves using polymer-dispersed liquid crystals and TiO2/Ag/TiO2 multilayers.

Abstract: The era of flexible optoelectronics demands development of wearable and bendable structures, foldable touch screens, paper-like displays, and curved and flexible solid-state lighting devices. Here, we demonstrate the fabrication of highly flexible light valves using polymer-dispersed liquid crystal (PDLC) and TiO2/Ag/TiO2 transparent conductive films. TiO2/Ag/TiO2 multilayers were prepared by magnetron sputtering technique on polyethylene terephthalate (PET) substrates at room temperature. By keeping the equivalent TiO2 layers and varying the deposition time of the Ag layer, proper metal nanograins on TiO2 planar plane were formed, providing the best tradeoff between the transmittance, sheet resistance and bending ability. The results are validated by numerical simulations that suggest the best match between the deposition time and individual layer thickness. Based on the performed characteristics of TiO2/Ag/TiO2/PET structures, several flexible light valves are fabricated and characterized. The sheet resistance values of TiO2/Ag/TiO2/PET remain unchanged over 1000 bending cycles. The measured driving voltage and response time values open great potential of TiO2/Ag/TiO2/PET for integration into next-generation ITO-free flexible and stretchable devices.

Tin dioxide thin films deposited by sol – gel technique

Abstract: Wide band gap metal oxide semiconductors are an important material base in optoelectronic industry. Dye sensitized solar cells and especially the organometallic perovskite ones present a new way of application of these materials with specific requirements for their properties. Therefore the reduction of production costs in relation to the efficiency is an important factor for its commercialization. In this work we present sol-gel approach to form dense, uniform and voids free electron selective layers of intrinsic SnO2 for application in nano-sized organometallic photovoltaics. The purpose is to find conditions for deposition at lowest temperatures of compact and well crystallized SnO2 thin films with effective electron blocking ability with perspective to apply on polymer substrates. The process of pyrolysis of the precursors into expected SnO2 was investigated by Differential Scanning Calorimetry. The results were used to have an insight for main reaction steps and to define an adequate annealing regime. The films were characterized by X – ray – photoelectron spectroscopy XPS, diffractometry, UV – Vis spectrophotometry and Scanning Electron Microscopy. Different solvents for the precursor SnCl4.5H2O were investigated in order to minimize their influence onto polymeric substrates. In the row between ethanol, methanol and isopropanol the last ones yielded compact layers without pinholes and cracks. The level of electrical leakage was estimated by cyclic voltammetry measurements of residual current in aqueous redox solution. The quality of the coverage depends on the concentration of the precursor solution but at over 0.5 M SnCl4 the root mean squares (RMS) roughness of the films increases.