Faculty Publications

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Author: search.filters.author.Ulla, H.

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    Blue light emitting materials for organic light emitting diodes: Experimental and simulation study
    (2012) Ulla, H.; Garudachari, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.
    Novel blue light-emitting materials were designed by the substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy group. The effect of molecular structure on the photophysical, electronic structure properties of the derivatives was explored by UV-visible absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry and quantum chemical calculations. Both UV-visible absorption and emission spectra of derivatives indicate that the emission is in blue region. Electrochemical studies of the molecules revealed that they have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) and energy levels of the highest occupied molecular orbital (HOMO) indicating that the derivatives possess good electron-transporting or hole-blocking properties. To further reveal the electronic structure and the optical properties, the structural and electronic properties of the synthesized derivatives were calculated. These results indicate that molecules may offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue organic light-emitting diodes. © 2012 IEEE.
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    Blue light emitting naphthalimides for organic light emitting diodes
    (2013) Ulla, H.; Garudachar, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.
    The photophysical, electrochemical, surface morphology and thermal properties of two novel blue lightemitting materials were studied. Results indicate that the molecules offer potential as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue organic light-emitting diodes. © 2013 American Institute of Physics.
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    Electrical Characterization of Hybrid Hetero Interface using n-ZnO and p-CuPc
    (Elsevier Ltd, 2015) Raveendra Kiran, M.R.; Ulla, H.; Fernandes, J.M.; Satyanarayan, M.N.; Umesh, G.
    In the present work we report the fabrication, current density-voltage (J-V) characteristics, capacitance-voltage characteristics and impedance measurements of the n-ZnO nanoparticles/ p-CuPc hybrid junction. ZnO nanoparticles were synthesized by sol-gel method. The J-V characteristics clearly show the diode like behaviour. The junction behaviour has been investigated using surface topography (AFM), capacitance and electrical impedance studies. © 2015 Elsevier Ltd.
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    Blue organic light emitting materials: Synthesis and characterization of novel 1,8-naphthalimide derivatives
    (Elsevier B.V., 2014) Ulla, H.; Garudachari, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.
    A series of naphthalimide derivatives were designed and synthesized by substituting electron-donating phenoxy groups at the 4th position of 1,8-naphthalimide. Photophysical, thermal, electrochemical properties of the synthesized derivatives were studied. The photophysical studies revealed that by varying the substituents at the 4th position of the 1,8-naphthalimide backbone, the photoluminescence spectra can be readily tuned in the range 410-423 nm (solution) and 457-468 nm (thin film). The derivatives have high Stokes' shifts and the Commission Internationale de l'Eclairage (CIE) coordinates are positioned in the deep blue region of the chromaticity diagram. Thermal analysis showed that the melting points are in the range 135-270 C with good thermal stability of 260-275 C. Electrochemical studies show the derivatives to have low-lying energy levels revealing that they possess good electron-transporting and hole-blocking properties. The ionization potentials and electron affinity are in the region of 6.30-6.36 eV and 3.31-3.43 eV, respectively, with energy band-gaps in the range 2.93-3.0 eV. The studies reveal that these energy values are relatively higher than the commonly used electron transporting materials. Hence these derivatives are potential candidates not only as electron transporting but also as hole blocking blue emitters for organic light-emitting diode applications. © 2013 Elsevier B.V. All rights reserved.
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    Investigation of hole-injection in ?-NPD using capacitance and impedance spectroscopy techniques with F4TCNQ as hole-injection layer: Initial studies
    (Academic Press, 2014) Fernandes, J.M.; Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.
    The charge accumulation leading to injection at the organic interface in the sequentially doped hole-only device structure is studied using capacitance and impedance based spectroscopic techniques. In this paper, we investigate the role of p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) in the charge transport properties of N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1?-biphenyl)-4,4?-diamine (?-NPD) through sequential deposition. We show that the hole injection into ?-NPD increases with the increase of interlayer (F4TCNQ) thickness by correlating the current density-voltage, capacitance-voltage, capacitance-frequency and impedance measurements. © 2014 Elsevier Ltd. All rights reserved.
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    Blue emitting halogen-phenoxy substituted 1,8-naphthalimides for potential organic light emitting diode applications
    (Elsevier B.V., 2014) Ulla, H.; Raveendra Kiran, M.R.; Garudachari, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.
    In this paper, we report the synthesis and characterization of six 1,8-naphthalimides [4a-4c and 5a-5c] obtained by the substitution of electron donating halogen-phenoxy groups at the C-4 position. The derivatives were characterized using 1H NMR, 13C NMR, mass spectra, FT-IR, single crystal XRD; photophysical, thermal, surface morphological and electrochemical properties were also investigated. The derivatives exhibit deep blue photoluminescence in the range 414-423 nm (in CHCl3) and 457-466 nm (in thin film state) on UV excitation with high Stokes' shifts and good chromaticity. The TGA and DSC analysis showed that the derivatives possess good thermal stability (271-284 °C) and melting points (138-201 °C). The HOMO and LUMO energy levels estimated by cyclic voltammetry are in the range 6.21-6.34 eV and 3.31-3.41 eV respectively corresponding to energy band gaps of 2.98-3.15 eV. These energy values are relatively higher than the commonly used electron transporting materials. The optical and electronic properties of the derivatives were tuned by the introduction of different electron donating halogen-phenoxy groups through C-4 position of the naphthalimide moiety. The emissive and electron-transporting properties of the naphthalimide derivative 4a were studied by fabricating a bi-layer and tri-layer devices. Further a phosphorescent device with 4a as electron transport layer (ETL) exhibited superior performance than the device without any ETL and was comparable with the device using standard Alq3 as ETL. These results indicate that the synthesized naphthalimide derivatives could play an important role in the development of OLEDs. © 2014 Elsevier B.V. All rights reserved.
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    Investigation of hole transport in ?-NPD using impedance spectroscopy with F4TCNQ as hole-injection layer
    (Academic Press, 2015) Fernandes, J.M.; Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.
    The charge carrier transport is studied in N,N?-di(1-naphthyl)-N,N?-diphenyl-(1,1?-biphenyl)-4,4?-diamine (?-NPD) with the incorporation of sequentially doped p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as hole-injection layer in hole-only device structures. The field dependent mobility of the charge carriers is determined using frequency dependent capacitance, conductance and impedance methods by varying the thickness of ?-NPD. The Poole-Frenkel zero-field mobility and the Poole-Frenkel coefficient thus obtained for each device in all the three methods is found to be almost constant. © 2015 Elsevier Ltd. All rights reserved.
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    Investigation of charge transport in Vanadyl-phthalocyanine with molybdenum trioxide as a buffer layer: Impedance spectroscopic analysis
    (Elsevier Ltd, 2015) Raveendra Kiran, M.R.; Ulla, H.; Krishnamanohara; Satyanarayan, M.N.; Umesh, G.
    Charge transport in organic materials is one of the intrinsic properties, which governs the device performance. In this paper, we report the fabrication and electrical characterization of two diodes ITO/VOPc/MoO3/Al and ITO/VOPc/Al. We investigate the electrical conduction of Vanadyl phthalocyanine (VOPc) in both the devices and also the effect of MoO3 as a buffer layer. Improvement of current density through the device is estimated using current density - voltage characteristics and capacitance - voltage characteristics. Space charge limited current (SCLC) conduction with an exponential trap distribution is observed from Impedance measurements. The dominant hopping charge transport is discussed based on ac conductivity measurements and by adopting Correlated barrier hopping (CBH) model. © 2015 Elsevier B.V. All rights reserved.
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    Pyrene-Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials
    (American Chemical Society service@acs.org, 2016) Chidirala, S.; Ulla, H.; Valaboju, A.; Raveendra Kiran, M.R.; Mohanty, M.E.; Satyanarayan, M.N.; Umesh, G.; Bhanuprakash, K.; Rao, V.J.
    Three pyrene-oxadiazole derivatives were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and electron transporters in three different device configurations, involving hole-injection/hole-blocking materials that showed good current and power efficiencies. To understand the underlying mechanisms involved in the application of these molecules as emitters and transporters, a detailed photophysical characterization of molecules 4-6 was carried out. The absorption, steady-state fluorescence, phosphorescence, fluorescence lifetime, and phosphorescence lifetime measurements were carried out. The high quantum yield and efficient reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a good emitter, and the charge delocalization properties leading to excimer formation make them efficient electron transporters. Isoenergetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in the absence of thermal activation. © 2015 American Chemical Society.
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    Tuning the self-assembly and photophysical properties of bi-1,3,4-thiadiazole derivatives through electron donor-acceptor interactions and their application in OLEDs
    (Royal Society of Chemistry, 2017) Yadav, A.K.; Pradhan, B.; Ulla, H.; Nath, S.; De, J.; Pal, S.K.; Satyanarayan, M.N.; Ammathnadu Sudhakar, A.S.
    We report several shape anisotropic molecules that contain two centrally placed 1,3,4-thiadiazole units, which vary from each other with respect to the number and length of the flexible chains at the termini. The number, position and length of the peripheral chains connected to the termini showed an impact on the thermal behavior of these compounds. The compounds with two terminal tails exhibited an enantiotropic smectic C phase, whereas the compounds with four terminal tails turned out to be crystalline. Surprisingly, among the compounds with six terminal tails, only the compound with a longer terminal chain exhibited a columnar phase with oblique symmetry. It is also to be noted that only compounds with six terminal chains exhibited gelation in long chain hydrocarbons. The xerogel of the hexacatenar with six n-decyloxy chains showed an entangled network of nanofibers of several micrometers in length. The aggregation behavior of the hexacatenar in the hydrocarbon solvent is mainly supported by the attractive ?-? interactions of the aromatic cores and the van der Waals interactions offered by the peripheral flexible tails. The emission behavior is dependent on the number of peripheral tails and not on the length. Furthermore, one of the hexacatenars exhibited solvatochromic emissive behavior. This molecular design helps in the development of long molecular nanowires with a central conducting core and insulating peripheral sheath, which will be helpful for the application in organic electronic devices. The application potential of the columnar liquid crystal material was tested by the fabrication of organic light emitting diodes (OLEDs) either as a single emissive material or as a guest material in a host polymer. Higher efficiency and brightness were noticed in the host guest OLED, which exhibited a technologically important bright blue emission. © 2017 The Royal Society of Chemistry.