Browsing by Author "Ulla, H."

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A Perylene-Triazine-Based Star-Shaped Green Light Emitter for Organic Light Emitting Diodes
(Wiley-VCH Verlag info@wiley-vch.de, 2018) Gupta, R.K.; Ulla, H.; Satyanarayan, M.N.; Ammathnadu Sudhakar, A.A.
A star-shaped molecule with a central electron-deficient triazine ring connected to three electron-rich N-annulated perylene tetraesters was synthesized. Its application as an emissive layer in the fabrication of a sole and host–guest solution processable OLED, exhibiting bright green emission with good electroluminescence efficiency is demonstrated. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Blue emitting 1,8-naphthalimides with electron transport properties for organic light emitting diode applications
(Elsevier B.V., 2017) Ulla, H.; Raveendra Kiran, M.R.; Garudachari, B.; Ahipa, T.N.; Tarafder, K.; Vasudeva Adhikari, A.; Umesh, G.; Satyanarayan, M.N.
In this article, the synthesis, characterization and use of two novel naphthalimides as electron-transporting emitter materials for organic light emitting diode (OLED) applications are reported. The molecules were obtained by substituting electron donating chloro-phenoxy group at the C-4 position. A detailed optical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical calculations (DFT) were performed to get a better understanding of the electronic structures. The synthesized molecules were used as electron transporters and emitters in OLEDs with three different device configurations. The devices with the molecules showed blue emission with efficiencies of 1.89 cdA-1, 0.98 lmW?1, 0.71% at 100 cdm-2. The phosphorescent devices with naphthalimides as electron transport materials displayed better performance in comparison to the device without any electron transporting material and were analogous with the device using standard electron transporting material, Alq3. The results demonstrate that the naphthalimides could play a significant part in the progress of OLEDs. © 2017 Elsevier B.V.
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.
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.
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. Â© 2012 IEEE.
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.
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. Â© 2013 American Institute of Physics.
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.
Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies
(Elsevier B.V., 2023) Vishrutha, K.S.; Ulla, H.; Raveendra Kiran, M.; Badekai Ramachandra, B.R.; Vasudeva Adhikari, A.
Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores (TPA-CyP1-7) having triphenylamine (TPA) as a donor unit and 3-cyanopyridine-2-one (CyP) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 °C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host–guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3-based doped device has achieved a maximum current efficiency (ηc), high power efficiency (ηp), and good external quantum efficiency (ηEQE) of 10.72 cd/A, 7.87 lm/W, and 5.32 %, respectively. © 2022 Elsevier B.V.
Effect of deposition rate on the charge transport in Vanadyl-phthalocyanine thin films
(Elsevier Ltd, 2017) Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.
We report fabrication of Vanadyl phthalocyanine (VOPc) based diodes with different deposition rates (0.1, 1 and 5 Å/s) in hole only device configuration: ITO/MoO3/VOPc/MoO3/Al. The dc and ac electrical conductivity of Vanadyl phthalocyanine based devices is investigated by employing Impedance spectroscopy measurements. The frequency dependence of conductivity indicates that the dominant mechanism for charge transport is the hopping type. Further, the dependence of conductivity on temperature and bias voltage clearly indicates that the hopping mechanism is described by the correlated barrier hopping (CBH) model. The thin layer (3 nm) of MoO3 in our devices is seen to enhance the electrical conductivity. J-V measurements indicate that the current density J as well as the charge carrier mobility are higher for the devices fabricated at a relatively lower deposition rate (0.1 Å/s). Our results suggest that the VOPC films deposited at lower rates are more appropriate for the optoelectronic device applications. © 2016 Elsevier B.V.
Effects of annealing temperature on the resistance switching behaviour of solution-processed ZnO thin films
(Academic Press, 2020) Raveendra Kiran, M.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.
In this study, the resistance switching (RS) behaviour of the fabricated devices with the configuration: ITO/ZnO (x annealing temperature)/Al were investigated. It was observed that the area of a hysteresis loop in the Current-Voltage characteristics was reduced with increase in ZnO annealing temperature. Correspondingly, the on/off ratio of the RS also gets reduced. The hysteresis behaviour was highly consistent and repeatable for the films annealed at 150 °C. The films annealed at 450 °C did not show any RS behaviour. Under the high current condition, a reproducible RS behaviour was observed. This was attributed to the synergetic effects of lowering of the barrier height at electrode/ZnO interface and the increase in the grain size with the annealing temperatures. The RS behaviour is ascribed to the conduction mechanism at the ITO/ZnO interface. © 2020 Elsevier Ltd
Efficient non-doped bluish-green organic light emitting devices based on N1 functionalized star-shaped phenanthroimidazole fluorophores
(Elsevier B.V., 2018) Tagare, J.; Ulla, H.; Satyanarayan, M.N.; Sivakumar, S.
In this paper, two star-shaped fluorescent phenanthroimidazole fluorophores, tris(4-(1-(3-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIMCFTPA) and tris(4-(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIPCFTPA) with a D–?–A structure were designed and synthesized by attaching a hole-transporting triphenylamine and an electron transporting phenanthroimidazole moiety. A detailed photophysical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical investigations (DFT) were performed to get a better understanding of the electronic structures. In particular, PIMCFTPA shows blue shifted emission due to the most twisted conformation and reduced intermolecular interaction as compared with PIPCFTPA. Both the fluorophores exhibit high glass transition temperatures and high thermal stabilities with decomposition temperatures up to 377 °C. The excellent stability renders them promising materials in electroluminescent devices. Non-doped organic light-emitting devices (OLEDs) using these fluorophores as emissive materials display bluish green emissions with high efficiency (6.58 cdA?1, 5.91 lmW?1, 3.62% at 100 cdm?2), low turn-on voltages (2.83 V) (PIMCFTPA) and excellent spectral stability. Our results suggest that the molecular design strategy of integrating TPA with phenthroimdazole play an important role in the device performance. © 2017 Elsevier B.V.
Electrical Characterization of Hybrid Hetero Interface using n-ZnO and p-CuPc
(2015) 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.
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. Â© 2015 Elsevier Ltd.
Fluorescent MoS2 Quantum Dot-DNA Nanocomposite Hydrogels for Organic Light-Emitting Diodes
(American Chemical Society service@acs.org, 2020) Pandey, P.K.; Ulla, H.; Satyanarayan, M.N.; Rawat, K.; Gaur, A.; Gawali, S.; Hassan, P.A.; Bohidar, H.B.
In this study, we report the synthesis of water-soluble MoS2 quantum dots (MoS2, QD) by a hydrothermal one-step method. These QDs were mixed in an aqueous solution of 2 kbp DNA to form fluorescent nanocomposite hydrogels at a very low concentration of the nucleic acid (1.0% (w/v), normal gelation occurs at 2% (w/v)). The melting temperature Tmelt of these gels was 50 ± 2 °C while the hydrogels melt at 40 ± 2 °C, and the low-frequency storage modulus/gel strength G0 was 40 ± 2 Pa (9 ± 2 Pa for hydrogel). This clearly implied that MoS2 acted as a pseudo-cross-linker in the nanocomposite hydrogel formation. The remarkable synergy of interaction between DNA and QDs can be gauged from the fact that the gel strength and melting temperature increased with QD content regardless of the fact that both carried negative charge. Dynamic light scattering studies showed arrested dynamics at the onset of gelation, and the gel transition time or ergodicity breaking time ?EB decreased with the increase in QD concentration. Small-angle X-ray scattering data captured the internal structure of these gels. Thus, we have a unique nanocomposite DNA-based hydrogel that is fluorescent, and in 2-D, this soft matter remarkably exhibits the behavior of an organic light-emitting diode (OLED), which imparts sufficient novelty to this work. © © 2020 American Chemical Society.
Hydrogen bond-driven columnar self-assembly of electroluminescent D-A-D configured cyanopyridones
(Royal Society of Chemistry, 2018) Vinayakumara, D.R.; Ulla, H.; Kumar, S.; Pandith, A.; Satyanarayan, M.N.; Shankar Rao, D.S.S.; Prasad, S.K.; Vasudeva Adhikari, A.
Herein, we report the design and synthesis of a new series of flying bird-shaped liquid crystalline (LC) cyanopyridone derivatives with a D-A-D architecture, CPO-1 to CPO-4. Their mesomorphic, photophysical, electrochemical, and electroluminescence characteristics have been investigated in detail. Here, the H-bonding interactions through a central lactam core were shown to be the key driving force for their self-assembly into columnar mesophases. The key role of H-bonding has been confirmed by using newly synthesized similar shaped compounds, MCP-1 to MCP-3. New CPO mesogens were found to be intense greenish blue light emitters with narrow band-gap energies. Conclusions were drawn based on theoretical studies also. Finally, the application potential of the selected mesogen CPO-2 as an emissive material has been demonstrated for the fabrication of doped and non-doped OLED devices with different device architectures, which displayed encouraging results. In fact, this is the first report on the use of emissive H-bond-assisted columnar liquid crystals in devices. The present results provide a new guideline and a versatile approach to the design of new LC molecules for the fabrication of efficient OLEDs. © 2018 The Royal Society of Chemistry.
Imidazole-Pyrene Hybrid Luminescent Materials for Organic Light-Emitting Diodes: Synthesis, Characterization & Electroluminescent Properties
(Elsevier B.V., 2021) Umasankar, G.; Ulla, H.; Madhu, C.; Gontu, G.R.; Shanigaram, B.; Nanubolu, J.B.; Bhanuprakash, B.; Karunakar, G.V.; Satyanarayan, M.N.; Rao, V.J.
A series of multichromophoric, pyrene-imidazole-phenyl based hybrid luminescent small molecules (PA, PI, PnB, PtB, PoM and PnDM) with different donating groups at the para position of the phenyl attached to N1 position of imidazole moiety were designed, synthesized, and characterized, for use in blue organic light-emitting diodes (OLEDs). The photophysical, thermal and electrochemical properties of the molecules were systematically investigated. All the molecules displayed delayed fluorescence at room temperature with a lifetime ranging from 7.1 to 8.5 µs. The synthesized fragment molecules (tetraphenyl-imidazoles: IA, IoM and InDM) revealed high triplet energies of ~ 2.90eV. Glass transition temperatures determined to range from 77°C to 123°C and decomposition temperatures are found to be above 280°C. The molecules possess appropriate HOMO and LUMO energy levels for effective charge injection. The crystal structure for PI is reported. OLED devices were fabricated based on the pyrene-imidazole-phenyl hybrids as emitters and as dopants with CBP as host. OLED devices with PI doped (5% wt.) with CBP exhibits excellent device performance with a current efficiency of 9.82 cd/A, the power efficiency of 8.32 lm/W and external quantum efficiency of 4.64%. © 2021
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.
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.
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.