Papers by Keyword: Organic Electronics

Abstract: Organic semiconductors are suitable for application in biosensors and sensors based on transistors. The influence of soluble group modifications on the performance of diketopyrrolopyrrole-based organic field-effect transistors (OFETs) is studied. The lowest mobility 1·10^-9 cm²/Vs was observed for non-symmetric substitution O,N. Measurable charge carrier mobility was observed due to reduction of the density charge trapping states after application of organosilane self-assembled monolayers (SAMs) on thinner gate-dielectrics (90 nm). We report similar drift mobility 1·10^-7 cm²/Vs for smallest soluble group “butyl” as for biggest group “EthylAdamantyl” in N,N and O,O substitution prepared by spin-coating.

Abstract: The paper deals with the study of optical and electrical properties of inkjet-printed graphene oxide (GO) layers, which can be used e.g. for the preparation of various types of electronic devices. To ensure stable inkjet printing conditions of GO solution, mixture was thoroughly stirred for 1 h at room temperature or sonicated in the bath for 30 min. The thicknesses of prepared layers were determined by spectroscopic ellipsometry and profilometry. An electrical conductivity of GO was increased by the multistep reduction (due to annealing) – the conductivity was changed by these processes about seven orders of magnitude (GO is an isolator and reduced GO is a conductor). For electrical and dielectric measurements, samples with GO and mixture of GO with PEDOT were prepared. All current-voltage characteristics have a diode character. From AC measurements the bulk electrical conductivity and geometric capacity of prepared layers were determined.

Abstract: Engineering, stability and orientation of semiconducting molecules are necessary to achieve the high efficiency of multifunctional organic-based devices. Several conjugated molecules facilitate the use of external magnetic fields to tailor both their molecular orientation and electronic properties while being processed for bio or opto-electronic applications. In this work, molecular thin films of vanadyl phthalocynine (VOPc) layers forming conducting channels in organic field-effect transistors were investigated. Three systems based on 100 nm thick VOPc thin film were grown, one in absence of magnetic field, while the other two with parallel and perpendicular to the substrate plane, respectively. Devices were ex-situ investigated by electrical characterization and confocal scanning Raman spectroscopy (SRS). All molecular layers growth on Au electrodes presented enhancement of the Raman signal.

Abstract: Organic light emitting diodes (OLEDs) have been the focus of intense study since the late 1980s, when the low voltage organic electroluminescence in small organic molecules such as Alq3, and large organic molecules such as polymers (PPV), was reported. Since that time, research has continued to demonstrate the potential of OLEDs as viable systems for displays and eco-friendly lighting applications. OLEDs offer full colour display, reduced manufacturing cost, larger viewing angle, more flexible, lower power consumption, better contrast, slimmer, etc. which help in replacing the other technologies such as LCD. The operation of OLEDs involves injection of charge carriers into organic semiconducting layers, recombination of charge carriers, formation of singlet and triplet excitons, and emission of light during decay of excitons. The maximum internal quantum efficiency of fluorescent OLEDs consisting of the emissive layer of fluorescent organic material is 25% because in this case only the 25% singlet excitons can emit light. The maximum internal quantum efficiency of phosphorescent OLEDs consisting of the emissive layer of fluorescent organic material mixed with phosphorescent material of heavy metal complexes such as platinum complexes, iridium complexes, etc. is nearly 100% because in this case both the 25% singlet excitons and 75% triplet excitons emit light. Recently, a new class of OLEDs based on thermally activated delayed fluorescence (TADF) has been reported, in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates of more than 10⁶ decays per second. These molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels and provides an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.The OLED technology can be used to make screens large enough for laptop, cell phones, desktop computers, televisions, etc. OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, automotive displays, and even head wearable displays. Presently, the OLEDs are opening up completely new design possibilities for lighting in the world of tomorrow whereby the offices and living rooms could be illuminated by lighting panels on the ceiling. The present paper describes the salient features of OLEDs and discusses the applications of OLEDs in displays and solid state lighting devices. Finally, the challenges in the field of OLEDs are explored. Contents of Paper

Abstract: Organic photosensor made of poly [N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) are promising candidates for bio-sensing applications. This paper investigates the optoelectronic characteristics of 4 different structures through simulation, utilizing PCDTBT as the active absorption layer. The scheme 1 is formed by placing the PCDTBT layer on top of a SiO₂ layer, and then interdigitated electrodes made of aluminium are placed onto PCDTBT. As to the scheme 2, the semiconductor layer is placed between an aluminium layer (bottom) and glass (top) layer coated with thick transparent interdigitated electrodes made of indium tin oxide (ITO). Regarding to scheme 3, layers from bottom to top are SiO₂, cathode, PCDTBT and anode. Cathode has the same area as SiO₂ and PCDTBT layers, but anode covers only partial of the semiconductor. Finally, in the scheme 4, the semiconductor layer is also placed over SiO₂ layer but here the anode and cathode are limiting the PCDTBT layer sides, having the same area for both sides. All schemes have same volume of semiconductor. The simulations have been realized in dark conditions and under light intensities 100 mW/cm² in the wavelength range of 400-550 nm. The best results were obtained for scheme 2, organic photoconductor with Metal-Semiconductor-Metal structure. For in this scheme which is under the conditions of 2 V bias, 500 nm wavelength and 100 mW/cm² illumination, the photocurrent, the internal and external quantum efficiency obtained were 8.53 μA, 88% and 45% respectively. As a conclusion, the scheme 2 Glass/PCDTBT/Aluminium with transparent electrodes has reached high performance desirable for bio-sensing.

Abstract: Gold nanoparticles (Au NPs) with mean size of 1 nm were placed on an indium tin oxide (ITO) electrode by ark plasma gun. The hole injection at ITO/organic semiconductor was significantly enhanced by the NPs. Though the hole current was unstable upon the voltage scan, it can be substantially stabilized by the insertion of hyper-branched polystyrene (HPS) thin layer between ITO and Au NPs. The work function of ITO electrode was also increased by NPs. The ITO/HPS/Au NPs structure was applied to the anode of an organic solar cell (OSC) with a bulk heterojunction active layer. The power conversion efficiency was significantly higher than that of OSC without anode buffer layer and almost comparable to that with a representative buffer material.

Abstract: Whereas single crystal silicon limited to 300 mm in diameter dominates the bulk of electronic devices, when electronics is required on a larger scale we have to rely on either amorphous or multi-crystalline materials. In this category, the organic semiconductors have made rapid in-roads. Among their applications, most notable successes are organic light emitting diodes (OLEDs) based displays. But, these OLEDs could also be made to emit in ultraviolet (UV) as shown through polysilanes devices. Two materials that we have investigated emit in UV or near UV. The emission spectrum could be manipulated by modifying the side groups on the main Si chain. Further discussed in the paper are full colour passive matrix displays we have made and development of thin films transistor for moving towards active matrix displays.

Abstract: Non volatile memory devices have been developed using diphenyl bithiophene derivatives (DPBT) as active layer. The devices, developed with a two terminal vertical structure where the spin cast organic layer is sandwiched between two electrodes, behave as bistable conductance switching memory cells; the modification of the electrodes material and of the organic layer composition introduces significant changes in the electrical behaviour, that give some indications on the molecular origin of the electrical bistability. These data are enriched by in-situ spectroscopic experiments.