31 research outputs found
Tuning the emitting color of organic light-emitting diodes through photochemically induced transformations: Towards single-layer, patterned, full-color displays and white-lighting applications
No full textPhotochemically induced emission tuning for the definition of pixels emitting the three primary colors, red, green, blue (RGB), in a single conducting polymeric layer is investigated. The approach proposed is based on an acid-induced emission shift of the (1-[4-(dimethylamino)phenyl]-6- phenylhexatriene) (DMA-DPH) green emitter and acid-induced quenching of the red fluorescent emitter (4-dimethylamino-4′-nitrostilbene) (DANS). The two emitters are dispersed in the wide bandgap conducting polymer poly(9-vinylcarbazole) (PVK), along with a photoacid generator (PAG). In the unexposed film areas, red emission is observed because of efficient energy transfer from PVK and DMA-DPH to DANS. Exposure of selected areas of the film at different doses results in quenching of the red emitter's fluorescence and the formation of green, blue, or even other color-emitting pixels, depending on the exposure dose and the relative concentrations of the different compounds in the film. Organic light-emitting diodes having the PVK polymer containing the appropriate amounts of DMA-DPH, DANS, and PAG as the emitting layer are fabricated and electroluminescence spectra are recorded. The time stability of induced emission spectrum changes and the color stability during device operation are also examined, and the first encouraging results are obtained.</p
Energy transfer processes among emitters dispersed in a single polymer layer for colour tuning in OLEDs
No full textThe energy transfer processes taking place in a single polymeric layer that enable the definition of the three primary colours (red, green and blue) in selected areas via photochemically induced emission tuning are discussed. The polymers used as hosts are two wide band gap polymers, PVK and a polyfluorenyl derivative. In the polymer matrix are dispersed the green emitter, 1-(4'-dimethyl-aminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH), the red emitter, 4-dimethylamino-4'-nitrostilbene (DANS) and a photoacid generator (PAG). Upon irradiation, protons are released from the PAG and they react gradually with the two emitters, causing the blue shift of the green emitter fluorescence and the extinction of the red emitter fluorescence. Depending on the protonation extent, the relative concentrations of the emitters and the exposure dose the energy transfer processes occurring inside the matrix result in definition of different colour emitting areas. The understanding of the energy transfer processes with photoluminescence experiments is a necessary first step in order to rationalize the selection of suitable components enabling the definition of the three primary colours in OLEDs.</p
Theoretical investigation on the effect of protonation on the absorption and emission spectra of two amine-group-bearing, red push'pull emitters, 4-dimethylamino-4′-nitrostilbene and 4-(dicyanomethylene)-2-methyl-6- p -(dimethylamino) styryl-4H-pyran, by DFT and TDDFT Calculations
No full textA theoretical investigation on the electronic structure of 4-dimethylamino-4′-nitrostilbene (DANS), 4-(dicyanomethylene)-2-methyl-6- p-(dimethylamino) styryl-4H-pyran (DCM), and their protonated forms is presented in an effort to rationalize recent experimental results on the tuning of the emitted color of organic light-emitting diodes through photochemically induced protonation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out on the neutral and protonated forms of DANS and DCM, employing both the B3LYP and the CAM-B3LYP functionals. It was found that the CAM-B3LYP functional leads to better agreement than the B3LYP of the calculated with the experimental absorption λmax for DANS, whereas B3LYP is more appropriate than CAM-B3LYP for DCM. The results of the calculations aid in a rationalization of the observed differences of the spectra of DANS and DCM upon protonation, and in particular those differences that make DANS a more attractive system for absorbance and emission tuning.</p
Incorporating triphenyl sulfonium salts in polyfluorene PLEDs: An all-organic approach to improved charge injection
No full textAll-organic sulfonium salts are introduced as a class of ionic compounds that show high compatibility with conjugated polymers and may form blends with attractive luminescent properties leading to significant improvement in single-layer polymer light emitting diodes' (PLEDs') performance. We demonstrate that triphenylsulfonium (TPS) triflate:polyfluorene-co-benzothiadiazole (F8BT)-blend based PLEDs show a lower turn-on voltage, an increased luminous efficiency and higher peak luminance values. These results are being rationalized in terms of anionic accumulation and space charge formation at the anode side, which facilitates hole injection, leading to more balanced injection and subsequently to a higher recombination rate. Moreover, we find that the salt anion size plays a critical role in the device operating characteristics. The judicious choice of both the salt and the emitting polymer by considering relative energy level alignment, salt electrochemical stability and acquired thermodynamic stability of blend morphology is important for the achievement of high performance PLEDs without requiring elaborate device architectures.</p
Effect of triphenylsulfonium triflate addition in wide band-gap polymer light-emitting diodes: improved charge injection, transport and electroplex-induced emission tuning
No full textThe presence of mobile anions in the emitting layer of polymer-based OLEDs has been proven to influence substantially the injection characteristics of the diode. In this work we report on the improvement of both injection and transport of charge carriers in blue emitting poly[2-(6-cyano-6-methyl-heptyloxy)-1,4- phenylene] (CN-PPP) based OLEDs upon insertion of the all-organic triphenylsulfonium (TPS) triflate salt in the emitting layer. On one hand, the anion displacement influences the energetics at the polymer/anode interface facilitating hole injection, whereas, on the other hand, the triphenylsulfonium cations act as electron transporting sites. The OLEDs exhibit significantly reduced turn-on voltage to half their initial value and increased luminance at low operating voltage. Moreover, the large energetic mismatch of the polymer and the triphenylsulfonium salt as well as the polarity induced by the ions result in simultaneous dual emission originating from the polymer exciton and from an electroplex, which is proposed to be formed at the triphenylsulfonium salt/polymer interfaces in the bulk. These results show that triphenylsulfonium salts represent an attractive class of materials that can be blended with conjugated polymers and can modify their electrical and/or emissive characteristics.</p
Influence of the anion on the optoelectronic characteristics of triphenylsulfonium salts modified polymer light emitting devices
No full textTriphenylsulfonium salts addition in the emitting layer of polymer light emitting diodes (PLEDs) has been shown to be beneficial for charge injection and transport due to both ionic effects and π-conjugation in the phenyl rings of the cation. In some cases the emission profile can be also modified through an electroplex formation. Herein we investigate the effect of four TPS-salts with different counter anions on the overall PLED performance upon blending each salt with the conjugated polymer poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (CN-PPP). In particular, three perfluoroalkanesulfonate organic anions of increasing size (triflate, nonaflate, PFOS) and a perfluorinated inorganic anion (SbF6) are compared. It is shown that the anion size affects primarily the turn-on and operational voltage, whereas its chemical nature is crucial for achieving high luminance values. The counteranion exerts also a direct impact on the dispersion properties of the salt in the polymer matrix, and thus, the film morphology, which in turn influences the emission colour and efficiency of an electroplex that is proposed to be formed at the sulfonium salt/polymer interfaces in the bulk. This study highlights the importance of properly selecting the counterions of the salts added in the emitting layer of PLEDs, which, in addition to their various functionalities, significantly influence device performance.</p
Solution processed multi-color organic light emitting diodes for application in telecommunications
No full textIn this work we present an all solution processing scheme for the fabrication of the three primary colors, (R-G-B), emitting organic light-emitting diodes (OLEDs) via efficient color tuning of a blue organic semiconducting (OSC) thin film, in particular the poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF), in which different color fluorescent emitters are dispersed to define the final emitting color and thus to simplify the different color device fabrication. The transmission speed of the fabricated OLEDs was also examined for possible application in interactive telecommunications. To increase the response speed of the different color devices we altered both the device geometry and the electron injection efficiency. To this end we increased the emissive layer thickness and decreased the device emissive area and we also performed engineering of the cathode interfaces through the incorporation of solution processed porphyrin interlayers in order to lower the electron injection barrier height. The final devices exhibited improved operational characteristics and, consequently, modulation speeds.</p
Flexible organic light emitting diodes (OLEDs) based on a blue emitting polyfluorene
No full textFlexible OLEDs were demonstrated using a highly efficient blue electroluminescent polyfluorene derivative. The flexible devices were fabricated on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates with a sheet resistance of 35 Ω per sq. The emitting layer was poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF). A significant improvement of the luminance and device efficiency was achieved by confining the exciton formation zone within PF by two wide band-gap materials, namely PVK as a hole transport layer (HTL) and an inorganic oxide layer (IOL) as an electron transport and hole blocking layer. In order to achieve full-color LEDs based on a common host material, we probed the use of suitable dye emitters dispersed in PF at appropriate concentrations. The selection of the emitters is based on their capability to be effective energy transfer acceptors from the blue emitting PF. In particular, energy transfer was demonstrated from blue to green for PF-doped with the green dye emitter 1-[4-(dimethylamino)phenyl]-6- phenylhexa-1,3,5,-triene (DMA-DPH), and from blue to red for PF-doped with the red dye emitter (4-dimethylamino-4′-nitrostilbene) (DANS). This demonstration paves the way for developing highly efficient blue, green and red flexible OLEDs based on a common blue emitting PF host.</p
Investigation of the impact of extracting and exchanging health information by using internet and social networks
No full textIntroduction: Social networks (1) have been embedded in our daily life for a long time. They constitute a powerful tool used nowadays for both searching and exchanging information on different issues by using Internet searching engines (Google, Bing, etc.) and Social Networks (Facebook, Twitter etc.). In this paper, are presented the results of a research based on the frequency and the type of the usage of the Internet and the Social Networks by the general public and the health professionals. Objectives: The objectives of the research were focused on the investigation of the frequency of seeking and meticulously searching for health information in the social media by both individuals and health practitioners. The exchanging of information is a procedure that involves the issues of reliability and quality of information. Methods: In this research, by using advanced statistical techniques an effort is made to investigate the participant's profile in using social networks for searching and exchanging information on health issues. Results: Based on the answers 93 % of the people, use the Internet to find information on health-subjects. Considering principal component analysis, the most important health subjects were nutrition (0.719 %), respiratory issues (0.79 %), cardiological issues (0.777%), psychological issues (0.667%) and total (73.8%). Conclusions: The research results, based on different statistical techniques revealed that the 61.2% of the males and 56.4% of the females intended to use the social networks for searching medical information. Based on the principal components analysis, the most important sources that the participants mentioned, were the use of the Internet and social networks for exchanging information on health issues. These sources proved to be of paramount importance to the participants of the study. The same holds for nursing, medical and administrative staff in hospitals. © 2016 John Pistolis, Stelios Zimeras, Kostas Chardalias, Zoe Roupa, George Fildisis, and Marianna Diomidous
Effect of substituents on the spectroscopic properties of all-trans-1,6-diphenyl-1,3,5-hexatriene
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