Browsing by Author "Gayatri Chauhan"

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Dependence of charge carrier mobility of 4,4′,4″-tris(N-3- methylphenyl-N-phenylamino)triphenylamine on doping concentration of tetrafluoro-tetracyano-quinodimethane
(Elsevier B.V., 2012) Gayatri Chauhan; Ritu Srivastava; Arunandan Kumar; Omwati Rana; P.C. Srivastava; M.N. Kamalasanan
Electrical transport of pure and tetrafluoro-tetracyano-quinodimethane doped 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) films have been studied at various temperatures and doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped m-MTDATA were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped films were analysed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility. © 2011 Elsevier B.V. All rights reserved.
Effect of hole transport layer doping on the organic light emitting diode performance
(2008) Gayatri Chauhan; Ritu Srivastava; P.C. Srivastava; M.N. Kamalasanan
Effects of doping on the hole conductivity of α-NPD has been analyzed by fabricating the hole-only device and organic light-emitting devices using pure and doped α-NPD. Doping increases the current densities by 2 orders of magnitude. OLED with 0.4 wt % F4-TCNQ doped α-NPD shows the maximum luminescence and efficiency.
Fabrication of white organic light-emitting diodes by co-doping of emissive layer
(2009) Ritu Srivastava; Gayatri Chauhan; Kanchan Saxena; S.S. Bawa; P.C. Srivastava; M.N. Kamalasanan
Efficient white light emission by mixing of red emission from (Ir-BTPA) [bis(2-(2′-benzothienyl) pyridinato-N,C3) (acetyl-acetonate) iridium(III) and greenish blue from (FIrPic) [bis(2-(4,6-difluorophenyl) pyridinato-N,C2) iridium(III)] has been studied. Ir-BTPA and FIrPic are co-doped into a 4, 4′ bis 9 carbozyl (biphenyl) (CPB) host. The device emission colour is controlled by varying dopant concentration. Photoluminescence (PL), electroluminescence (EL) and colour coordinates have been studied as a function of applied voltage. The EL spectra of the devices with the co-doped emissive layer show three emission peaks at 469, 500 and 611 nm. Commission Internationale de l'Eclairage (CIE) coordinates of the devices are 0.27, 0.32 at 18 V and are well within the white region.
Frequency dependent electrical transport properties of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamine)triphenylamine by impedance spectroscopy
(2010) Gayatri Chauhan; Ritu Srivastava; Priyanka Tyagi; Amit Kumar; P.C. Srivastava; M.N. Kamalasanan
The frequency dependent ac conduction mechanism in 4,4′,4″-tris(N-3-methylphenyl-N-phenylamine)triphenylamine (m-MTDATA) has been studied as a function of applied bias and temperature. The Cole-Cole plot shows a slightly depressed semicircle indicating Debye type relaxation. This result has been explained by an equivalent circuit of the device designed as a two parallel resistor and capacitance network in series with contact resistance. The ac conduction studies under dc bias for hole only devices shows an increase in device conductivity with the increase in bias. The variation of bulk resistance with applied bias indicates Space Charge Limited Conduction (SCLC) mechanism for hole conduction. The hole mobility of the material has also been evaluated from SCLC as 8.859 × 10-6 cm2/V s. The temperature dependent impedance studies show two activation energies indicating two different phase of the material with a phase transition at 235 K. © 2010 Elsevier B.V. All rights reserved.
Thermally activated field assisted carrier generation and transport in N, N′ -di-[(1-naphthalenyl)- N, N′ -diphenyl]-(1, 1′ biphenyl)- 4, 4′ -diamine doped with 2,3,5,6-tetrafluoro- 7, 7′,8, 8′ -tetracyanoquinodimethane
(2008) Gayatri Chauhan; Ritu Srivastava; Virendra Kumar Rai; Arunandan Kumar; S.S. Bawa; P.C. Srivastava; M.N. Kamalasanan
Current density-voltage (J-V) characteristics of N, N′ -di-[(1-naphthalenyl)- N, N′ -diphenyl]-(1. 1′ biphenyl)- 4, 4′ -diamine (α-NPD) doped with 2,3,5,6-tetrafluoro- 7, 7′,8, 8′ -tetracyanoquinodimethane have been studied as a function of doping concentration (0-0.8 wt %) and temperature (105-300 K). The current density was found to increase with increase in doping concentration. In the doped samples as field increases above 3.3× 104 V/cm the current abruptly starts increasing at a higher rate, which is ascribed as due to increased free charge carrier generation in the bulk. The enhanced free charge carrier generation is due to field assisted thermal dissociation of donor-acceptor pairs (Poole-Frenkel process) as well as charge injection at the interface. The released carriers increase the charge carrier density which brings the Fermi level near the highest occupied molecular orbital level of the α-NPD and reduces the space charge region near the interface favoring the tunneling of charge carrier across the interface, which is enough to support Ohmic conduction. The carrier generation has been found to be a thermally activated process. At higher fields (i.e., above 1.52× 105 V/cm) the nonlinear J-V characteristics have been explained as due to field dependent mobility of holes. © 2008 American Institute of Physics.