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    Low Work Function Lacunary Polyoxometalates as Electron Transport Interlayers for Inverted Polymer Solar Cells of Improved Efficiency and Stability
    (AMER CHEMICAL SOC, 2017) Tountas, Marinos; Topal, Yasemin; Polydorou, Ermioni; Soultati, Anastasia; Verykios, Apostolis; Kaltzoglou, Andreas; Papadopoulos, Theodoros A.
    Effective interface engineering has been shown to play a vital role in facilitating efficient charge-carrier transport, thus boosting the performance of organic photovoltaic devices. Herein, we employ water-soluble lacunary polyoxometalates (POMs) as multifunctional interlayers between the titanium dioxide (TiO2) electron extraction/transport layer and the organic photoactive film to simultaneously enhance the efficiency, lifetime, and photo stability of polymer solar cells (PSCs). A significant reduction in the work function (WF) of TiO2 upon POM utilization was observed, with the magnitude being controlled by the negative charge of the anion and the selection of the addenda atom (W or Mo). By inserting a POM interlayer with similar to 40 nm thickness into the device structure, a significant improvement in the power conversion efficiency was obtained; the optimized POM-modified poly[[4,8-bis [(2-ethylhexyl) oxy] benzo [1,2-b :4,5-b'] dithiophene-2,6-diyl] [3-fluoro-2-[(2-33 ethylhexyl) carb onyl] thieno [3,4-b] thiophenediylp [6,6]-phenyl-C-70 butyric acid methyl ester (PTB7:PC70BM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO2 cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PC70BM or indene-C-60 bisadduct (IC(60)BA), which enhanced their efficiency up to 4.34 and 6.21%, respectively, when using POM interlayers; this represents a 25-33% improvement as compared to the reference cells. Moreover, increased lifetime under ambient air and improved photostability under constant illumination were observed in POM-modified devices. Detailed analysis shows that the improvements in efficiency and stability synergistically stem from the reduced work function of TiO2 upon POM coverage, the improved nanomorphology of the photoactive blend, the reduced interfacial recombination losses, the superior electron transfer, and the more effective exciton dissociation at the photoactive layer/POM/TiO2 interfaces.
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    A multifunctional ınterlayer for solution processed high performance ındium oxide transistors
    (NATURE PUBLISHING GROUP, 2018) Kyndiah, Adrica; Ablat, Abduleziz; Guyot-Reeb, Seymour; Schultz, Thorsten; Zu, Fengshuo; Koch, Norbert; Amsalem, Patrick; Chiodini, Stefano; Alıç, Tuğbahan Yılmaz; Topal, Yasemin; Kuş, Mahmut; Hirsch, Lionel; Fasquel, Sophie; Abbas, Mamatimin
    Multiple functionality of tungsten polyoxometalate (POM) has been achieved applying it as interfacial layer for solution processed high performance In2O3 thin film transistors, which results in overall improvement of device performance. This approach not only reduces off-current of the device by more than two orders of magnitude, but also leads to a threshold voltage reduction, as well as significantly enhances the mobility through facilitated charge injection from the electrode to the active layer. Such a mechanism has been elucidated through morphological and spectroscopic studies.
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    A silanol-functionalized polyoxometalate with excellent electron transfer mediating behavior to ZnO and TiO2 cathode interlayers for highly efficient and extremely stable polymer solar cells
    (ROYAL SOC CHEMISTRY, 2018) Tountas, Marinos; Topal, Yasemin; Verykios, Apostolis; Soultati, Anastasia; Kaltzoglou, Andreas; Papadopoulos, Theodoros A.; Auras, Florian
    Combining high efficiency and long lifetime under ambient conditions still poses a major challenge towards commercialization of polymer solar cells. Here we report a facile strategy that can simultaneously enhance the efficiency and temporal stability of inverted photovoltaic architectures. Inclusion of a silanolfunctionalized organic-inorganic hybrid polyoxometalate derived from a PW9O34 lacunary phosphotungstate anion, namely (nBu(4)N)(3)[PW9O34(tBuSiOH)(3)], significantly increases the effectiveness of the electron collecting interface, which consists of a metal oxide such as titanium dioxide or zinc oxide, and leads to a high efficiency of 6.51% for single-junction structures based on poly(3-hexylthiophene): indene-C60 bisadduct (P3HT: IC(60)BA) blends. The above favourable outcome stems from a large decrease in the work function, an effective surface passivation and a decrease in the surface energy of metal oxides which synergistically result in the outstanding electron transfer mediating capability of the functionalized polyoxometalate. In addition, the insertion of a silanol-functionalized polyoxometalate layer significantly enhances the ambient stability of unencapsulated devices which retain nearly 90% of their original efficiencies (T-90) after 1000 hours.
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    Synthesis and solar-cell applications of novel furanyl-substituted anthracene derivatives
    (ELSEVIER SCIENCE BV, 2017) Kivrak, Arif; Er, Omer Faruk; Kivrak, Hilal; Topal, Yasemin; Kus, Mahmut; Camlisoy, Yesim
    At present, novel furanyl-substituted anthracene derivatives; namely 9,10-di(furan-2-yl)anthracene (DFA), 5,5'-(anthracene-9,10-diyl)bis(furan-2-carbaldehyde) (DAFA) and 2,2'-((5,5'-(anthracene-9,10-diyl)bis(furan-5,2-diyl))bis(methanylylidene))dimalononitrile (DCNFA) were designed and synthesized successfully by employing Stile Cross-Coupling, Vilsmeier-Haack and Knoevenagel condensation reactions, respectively. This methodology provides a practical new route for the synthesis of furanyl-substituted anthracene derivatives bearing strong electron-withdrawing groups. The electrochemical and electro-optical properties of these novel furanyl-substituted anthracene derivatives were also examined with strong acceptor-pi-donor-pi-acceptor interactions. Furthermore, Highest occupied molecular orbital (HOMO), Lowest Unoccupied molecular orbital (LUMO), and band gap (Eg) values were investigated by using spectroscopic methods. Electrochemical and electro-optical properties were calculated and compared to DFA, DAFA and DCNFA. Eg was found as 2.85, 2.71, and 2.33 eV, respectively. Consequently, Organic Solar Cells (OSC) were fabricated to investigate their solar cell performances. The strong electron withdrawing groups did not increase the solar cell performance of furanyl-anthracenes. Surprisingly, DFA was found to exhibit the best OSCs performance (Efficiency = 3.36). As a result, one could note that these novel furanyl-substituted anthracene derivatives are good candidate for the applications of the OSCs. Our results might help in the development of new materials with important electrochemical functions by giving the advantage of designing and further derivatization of new generation small organic molecules for photovoltaic device applications. (C) 2017 Elsevier B.V. All rights reserved.
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    Synthesis of thiophenyl-substituted unsymmetrical anthracene derivatives and investigation of their electrochemical and electrooptical properties
    (ELSEVIER SCIENCE BV, 2017) Kivrak, Arif; Calis, Hatice; Topal, Yasemin; Kivrak, Hilal; Kus, Mahmut
    Novel thiophenyl-substituted anthracene derivatives (D-A)bearing a variety of electron-withdrawing groups were designed and synthesized for organic solar cells (OSCs). Their electrochemical and electro-optical properties were examined with strong donor-acceptor interaction. The electrochemical properties were examined by cyclic voltammetry (CV) measurements. These estimated values from CV measurements are in good agreement with the optical band gaps. HOMO, LUMO, and Eg values of designed organic materials were estimated. Furthermore, these new generation organic materials were fabricated to find their solar cell performances.
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    Water-Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes
    (WILEY-V C H VERLAG GMBH, 2016) Tountas, Marinos; Topal, Yasemin; Kus, Mahmut; Ersöz, Mustafa; Fakis, Mihalis; Argitis, Panagiotis; Vasilopoulou, Maria
    High performance solution-processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)](F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 mu m W-1 and a current efficiency up to 14.0 cd A(-1) which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9-vinylcarbazole) (PVK): 1,3-bis[2(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl] benzene (OXD-7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 mu m W-1 and 30.0 cd A(-1) power and luminous efficiency, respectively, which are among the best performance values observed to date for all-solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10(-2) cm(2) V-1 s(-1).