Georgi Kolev

Spray deposition of PVDF layers with application in MEMS pressure sensors

Layers of piezoelectrical polymer polyviniliden fluoride (PVDF) were prepared by spray deposition technique on silicon, glass substrates and glass substrates with thin aluminium (Al) and cooper (Cu) films. The deposition temperature of the substrates and the concentration of the solution were investigated and their influence on the adhesion to the surface and the films morphology were explored. The pre– and post deposition temperature treatment of the sample was conducted and the changes in the layers behaviour was detected.

Flexible optoelectronic device with polymer based electrode on hybrimer substrate — Impact of the bending on the interfacial processes

In this study, novel flexible organic light-emitting diode structure (f-OLED) hybrimer/polymer/metal/polymer/emissive layer/metal was proposed. We studied the effect of the mechanical bending on its electro-optical behavior. The device contains polymeric based anode PEDOT:PSS/Au/PEDOT:PSS, where the conductive polymer was spray deposited on preliminary UV exposed hybrimer surface. It was found enhancement of the films uniformity and adhesion due to this treatment. Furthermore, due to the favorable mechanical properties of the substrate and polymeric anode, improved f-OLED performance was observed, compared to the available devices. Degradation of less than 15% after 100 bending cycles and less than 32% after 1000 was achieved for the current-brightness-voltage characteristics. The results were explained by the stability of the PEDOT:PSS morpholog. Interfacial processes were analyzed, based on the assumption for interfacial barrier variation at bending, which was confirmed by AFM anode topography and capacitance measurements. The nanoscale scanning showed continuous, crack-free polymer surface with roughness lower than 20nm after bending. We concluded that the combination substrate/anode could be reason for the small bending impact on the interfacial processes.

Piezoelectric energy harvesting device with nanobranched ZnO on Polymer/Metal/Polymer coated flexible substrate

This work reports about the fabrication of piezoelectric energy harvesting with ZnO nanostructure grown on polymeric based electrode. The piezoelectric film exhibits nanobranched morphology, which provide favorable conditions for dipoles orientation and thus the device is characterized with relatively low dielectric losses in the range of 0.4–3.9×10−2 and piezoelectric voltage of 3.8V at 4.8 kg mechanical loading. The minimum weight at which piezoelectric reaction arises is 3 gr, corresponding to generated piezoelectric voltage of 200mV.

Investigation of MEMS Piezoelectric Transformer with PVDF Thin Layer

This paper presents the results of experimental work on thin piezoelectric polyvinylidene fluoride (PVDF) film, used as active layer in piezoelectric transformer. PVDF film was deposited by spray deposition technique on flexible polyethylene terephthalate (PET) substrate and its thickness was measured to be 2 μm. Aluminum (Al) bottom and top contacts were deposited by vacuum thermal evaporation. The transfer function of the transformer was measured at different frequencies in the range 50 Hz – 4 MHz. It was observed that at input frequency of 1 MHz, the transfer function started to decrease, which supposed low-frequency AC/AC transformer. Dielectric losses, which characterize piezoelectric devices’ quality, were less that 0.09 in the whole frequency range. This is proof for the efficient energy conversion and stable operation of the microstructure. The work shows that the PVDF transformer performance is comparable to the existing piezoceramic based transformers, which however suffer of high dielectric losses, signal distortions and relatively low boundary frequency.

Spray deposition of organic electroluminescent coatings for application in flexible light emitting devices

Abstract Organic electroluminescent (EL) films of tris(8-hydroxyquinolinato)aluminum (Alq3) mixed with polystyrene (PS) binder were produced by spray deposition. The influence of the substrate temperature on the layer’s morphology and uniformity was investigated. The deposition conditions were optimized and simple flexible light-emitting devices consisting of indium-tin oxide/Alq3:PS/aluminum were fabricated on polyethylene terephthalate (PET) foil to demonstrate the advantages of the sprayed organic coatings. Same structure was produced by thermal evaporation of Alq3 film as a reference. The influence of the deposition method on the film roughness and contact resistance at the electrode interfaces for both types of structures was estimated. The results were related to the devices’ efficiency. It was found that the samples with sprayed films turn on at 4 V, which is 2 V lower in comparison to the device with thermal evaporated Alq3. The current through the sprayed device is six times higher as well (17 mA vs. 2.8 mA at 6.5 V), which can be ascribed to the lower contact resistance at the EL film/electrode interfaces. This is due to the lower surface roughness of the pulverized layers.

Piezoelectric MEMS stress sensor with thin lead zirconate titanate (PZT) layer

In this study results from fabrication and testing of MEMS stress sensor with piezoelectric sensitive layer of lead zirconate titanate (PZT) are presented. PZT layers are deposited by RF-sputtering in vacuum. The dependence of the voltage, generated by the structure, on the applied stress is investigated for two different electrode configurations - sandwich type and lateral type, with aluminum electrodes. The obtained results show high piezoelectric coefficient of 186 pC for thickness of PZT-layer of 180 nm. The maximal generated voltage is 35 mV for lateral structure in the stress range of 0-300 mgr. The sandwich structure is sensitive above 1,1 gr and the maximal voltage is 200 mV.