Browsing by Author "M.N. Kamalasanan"

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Charge transport and microstructure in PFO:MEH-PPV polymer blend thin films
(2010) Manisha Bajpai; Ritu Srivastava; M.N. Kamalasanan; R.S. Tiwari; Suresh Chand
Current density-voltage characteristics of poly(9,9′-dihexyl fluorenyl-2,7-diyl) (PFO) thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (100-290 K) in pure form and with blending (∼0.25-50 wt%) of poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV). It has been found that in the case of pure PFO the charge transport at low fields show an ohmic region which is followed by space charge limited conduction region. Blending of PFO with MEH-PPV at low ratio (up to 2 wt%) increases the current density. As the ratio is increased further, the current density decreases. Analysis show that there is a change in conduction mechanism up to 6 V from SCLC to thermally activated ohmic conduction upon blending indicating the creation of new energy level near the transport states of PFO. Morphology of the polymer blended thin films was obtained by atomic force microscopy (AFM) technique. It has been found that the surface roughness of the investigated films is significantly increased upon blending indicating aggregation as well as phase separation at high blending ratios. The decrease in conductivity at high blending ratio can be related to the change in morphology of the films. © 2010 Elsevier B.V.
Charge transport study of polymer light emitting diodes by impedance spectroscopy
(SPIE, 2012) Manisha Bajpai; Ritu Srivastava; M.N. Kamalasanan; R.S. Tiwari; Suresh Chand
The transport properties of polymer blend have been investigated by using impedance spectroscopy (IS) and current density -voltage (J-V) measurements based on a single-layer structure of ITO/polymer/Al. Impedance spectroscopy measurements showed that the device can be simulated by an equivalent parallel R-C network with a contact resistance Rs in series, the resistance R decreases along with bias voltage. The variation of the whole conductance and capacitance of the device with frequency and bias can be understood by the carrier response and the transit time dependence on the bias in the polymer film. © 2012 SPIE.
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.
Dipolar alignment and consequent enhanced charge transport in poly (9, 9′ di octyl fluorene)-2, 7-ylene ethylnylene
(2011) Manisha Bajpai; Ritu Srivastava; M.N. Kamalasanan; R.S. Tiwari; Suresh Chand
Current density-voltage characteristics of poly (9, 9′ di octyl fluorene)-2,7-ylene ethylnylene thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (290-100 K) in unpolarized and polarized samples. The hole mobility has been found to be enhanced as a result of dipolar alignment by exposure to a dc electric field via cooling at all elevated temperatures. At higher field, current density has been found to be governed by trapped charge limited currents (TCLC) with hole mobility strongly dependent on electric field and their respective charge transport parameters have been obtained for both samples. The density of trap states has been found to be decreased on polarization from 1.1 × 10 18 to 7.6 × 1017 cm-3 and trap energy has correspondingly decreased from 43 to 35 meV. The TCLC model with Poole-Frenkel-type field-dependent mobility has been fitted into the data and found to be in excellent agreement. Temperature dependence of zero field mobility (0) and disorder parameter (σ) also has been estimated. We conclude that the relatively higher hole mobilities may be due to the orientational ordering of polar molecules and displacement of excess charges. © 2011 American Institute of Physics.
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.
Electric field and temperature dependence of hole mobility in electroluminescent PDY 132 polymer thin films
(2010) Manisha Bajpai; Kusum Kumari; Ritu Srivastava; M.N. Kamalasanan; R.S. Tiwari; Suresh Chand
The current density-voltage (J - V) behavior of polymer PDY 132 thin films has been investigated in hole-only device configuration, viz., ITO/poly(ethylene-dioxthiophene):polystyrenesulphonate (PEDOT:PSS)/PDY 132/Au, as a function of polymer (PDY) film thickness (150 nm and 200 nm) and temperature (290-90 K). Hole current density was found to follow two distinct modes of conduction, (i) low electric field region I: ohmic conduction where slope ∼ 1, and (ii) intermediate and high electric field region II: non ohmic conduction where slope ∼ 2. Region I has been attributed to the transport of intrinsic background charge carriers while region II has been found to be governed by space charge limited currents (SCLC) with hole mobility strongly dependent on electric field and temperature. The respective hole transport parameters determined from the SCLC regime, μp 0 is 3.7 × 1 0- 3 m2 / V s, μp (0, T) is 3.7 × 1 0- 8 m2 / V s, and zero field activation energy (Δ0) of 0.48 eV is obtained. © 2010 Elsevier Ltd. All rights reserved.
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.
Modification of metal-organic interface using F4-TCNQ for enhanced hole injection properties in optoelectronic devices
(2012) O. Rana; R. Srivastava; G. Chauhan; M. Zulfequar; M. Husain; P.C. Srivastava; M.N. Kamalasanan
Incorporation of thin layers of tetrafluro-tetracyanoquinodimethane (F 4-TCNQ) has been found to modify the work functions of a number of substrates. Surface potential measurement using Kelvin probe method (KPM) has been used to monitor the change in work function of the modified substrates. The results support the integer charge transfer model by which the Fermi levels of the substrates are aligned with the negative polaron states of F 4-TCNQ. Further, we found that the work function of the substrates increases with increase in F4-TCNQ thickness from 0 to 7nm and then saturates for further increase in thickness. The variation in work function has been attributed to the low surface coverage of F4-TCNQ islands on the substrates. The ITO and Au substrates with increased work functions were used as the electrode for hole only devices of common hole transport materials. The hole injection property has been found to increase with increase in F 4-TCNQ thickness and for ITO surface modified with 7nm F 4-TCNQ layer, ohmic conduction has been achieved for HTLs with HOMO level up to 5.4eV. When these modified substrates were used as hole injecting contacts in organic light emitting diodes (OLEDs), they gave substantially higher electroluminance, power efficiency and lower operating voltages. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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.