Browsing by Author "Ritu Srivastava"

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Carbon Quantum Dot as Electron Transporting Layer in Organic Light Emitting Diode
(Wiley-Blackwell, 2019) Md. Bayazeed Alam; Kanchan Yadav; Devyani Shukla; Ritu Srivastava; Jayeeta Lahiri; Avanish. S. Parmar
Carbon quantum dots derived from banana leaves by a simple hydrothermal process has been used as an electron transport layer to fabricate Organic Light Emitting Diodes. To the best of our knowledge, this is the first report of Carbon QD utilized as an electron transport layer inOLED. Devices with multilayered structureITO/PEDOT: PSS/PFO/CQD/LiF/Al are fabricated on indium tin oxide (ITO) coated glass substrates. Current-Voltage (I−V) curves reveal that the turn-on voltages are reduced from 8 V to 6 V as compared with pristine PFO device.Results show CQD as an ETL has enhanced the performance and efficiency of the OLEDs. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Charge carrier generation in a polymer by using n-type doping for the improvement of electrical properties
(VBRI Press, 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari; Suresh Chand
In this paper, the charge carrier generation in polymer blends by chemical doping has studied. In these studies, we employed n-type dopant molecule decamethylcobaltocene (DMC) which exhibit very strong electron donating nature. We have demonstrated that such type of doping favours the formation of charge transfer complex (CTC) and reduce the recombination probability. We have confirmed the CTC formation form the absorption spectroscopy. Further we have used transient photoluminescence spectroscopy to reveal the reduced initial recombination of charge transfer exciton. We interpret our results based on a reduced formation of emissive charge transfer excitons in doped blends, induced by state filling of immobile tail states in the polymer HOMO. © 2016 VBRI Press.
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 doping on the electron transport in polyfluorene
(American Institute of Physics Inc., 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari
In this paper, electron transport of pure and DMC doped polyfluorne (PF) films have been studied at various doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped PF 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 fims were analyzed 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. © 2016 Author(s).
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.
Enhanced Hole transport in polyfluorene polymer by using hole injection layer
(Springer Science and Business Media Deutschland GmbH, 2014) Manisha Bajpai; Ritu Srivastava; R.S. Tiwari; R. Dhar
Hole transport in poly(9,9-dioctylfluorene) (PFO) are measured by performing current density- voltage (J-V) measurements as a function of temperatures. For the J-V measurements, F4TCNQ is used as hole injection layer (HIL). Two different devices of PFO polymer are fabricated F4TCNQ having device configuration ITO/PFO/Au (with interface modification) and ITO/PFO/F4TCNQ/Au (with interface modification) respectively. Since, F4TCNQ is evaporated on the top of PFO film so that the contacts are taken from Au. © Springer International Publishing Switzerland 2014.
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.
High yield synthesis of electrolyte heating assisted electrochemically exfoliated graphene for electromagnetic interference shielding applications
(Royal Society of Chemistry, 2015) Prashant Tripathi; Ch. Ravi Prakash Patel; Abhishek Dixit; Avanish Pratap Singh; Pawan Kumar; M.A. Shaz; Ritu Srivastava; Govind Gupta; S.K. Dhawan; Bipin Kumar Gupta; O.N. Srivastava
Herein, we demonstrate a facile one pot synthesis of graphene nanosheets by electrochemical exfoliation of graphite. In the present study, we report a significant increase in the yield of graphene by electrolyte heating assisted electrochemical exfoliation method. The obtained results of heating assisted electrochemically exfoliated graphene (utilizing H2SO4 + KOH + DW) synthesis clearly exhibit that the yield increases ∼4.5 times i.e. from ∼17% (room temperature) to ∼77% (at 80°C). A plausible mechanism for the enhanced yield based on lattice expansion and vibration of intercalated ions has been put forward and discussed in details. The quality of graphene was examined by Raman, XPS, FTIR, AFM, SEM, TEM/HRTEM and TGA techniques. The Raman as well as morphogenesis results confirm the quality of the graphene nanosheets. We have used this graphene as electromagnetic interference shielding material where a comparatively large quantity of graphene is required. This graphene exhibits enhanced shielding effectiveness (46 dB at 1 mm thickness of stacked graphene sheets in frequency region 12.4 to 18 GHz) as compared to conventional electromagnetic interference shielding materials, which is greater than the recommended limit (∼30 dB) for techno-commercial applications. Thus the present work is suggestive for future studies on enhancement of yield of high quality graphene by proposed method and the use of synthesized graphene in electromagnetic interference shielding and other possible applications. © The Royal Society of Chemistry 2015.
Ion transport in metasilicate based new gel system: x Na2SiO3 + (100-x) NH4BF4
(1999) Ritu Srivastava; D.K. Rai; S. Chandra
A new gel system xNa2SiO3 + (100-x)NH4BF4 has been prepared. The gel could form only in the limited compositional range x = 15-50 mol %. Gelling has been shown to result in the formation of a new structural compound and the dried gel was stable up to ∼ 400°C. Structural and Ion transport studies have been carried out on the prepared gel system. The highest bulk electrical conductivity has been found to be ∼2.45 × 10-6 S cm-1 for x = 45 mol% at 27°C and RH = 49 %. The total ionic transference number of the gel has been measured by polarization method and found to be ∼ 1.
P-Type doping of tetrafluorotetracynoquinodimethane (F4TCNQ) in poly(para-phenylene vinylene) (PPV) derivative "super Yellow" (SY)
(Royal Society of Chemistry, 2014) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari; Suresh Chand
In this paper, we report a case of hole transport in tetrafluorotetracynoquinodimethane doped poly(para-phenylene vinylene) derivative "Super Yellow". The hole mobility of pristine and doped polymer thin films was determined by impedance spectroscopy. The increase in hole mobility upon doping was also verified by current density-voltage measurements. It was found to be increased by two orders of magnitude upon doping. The increase in hole mobility upon p-type doping was explained on the basis of extended Gaussian disorder model by measuring current density-voltage characteristics in the same devices. At low bias, J-V characteristics exhibit clear space charge limited conduction in pristine state but ohmic behaviour when doped. Further, at higher voltages the current density increases non-linearly due to the field-dependent mobility and carrier concentration by filling of tail states of highest occupied molecular orbital of the host material. The room temperature J-V characteristics were well described with the single value of free hole density at low fields. At higher fields it becomes field-dependent followed by a field enhancement factor γ. This journal is © the Partner Organisations 2014.
Porous silica alcogel matrix entrapped with liquid electrolyte: Ionic conductivity and growth of whiskers
(2002) Ritu Srivastava; S. Chandra
Highly porous silica alcogel, with pores entrapped with liquid electrolyte solutions of KH2PO4, has been successfully prepared by hydrolysis and polycondensation of tetraethylorthosilicate (TEOS) giving interesting "solid-liquid electrolyte composites". The entrapped liquid electrolyte solution gives high liquid-like ionic conductivity (∼10-5 S.cm-1). Further, on storage the entrapped solution slowly reaches the surface pore-heads where it evaporates resulting in the growth of long crystalline whiskers. Higher relative humidity (>60%) slows down the evaporation and has been found to be favorable for whisker growth.
Proton-conducting gel electrolyte
(2002) S. Chandra; S.S. Sekhon; Ritu Srivastava; Narinder Arora
Proton conduction in solid state xerogels and polymeric gels are reported. Xerogels, doped with known proton conductors, were prepared by "sol-gel" method starting either from inorganic precursor sodium metasilicate (termed as hydrogel) or organic precursor tetraethyl orthosilicate (termed as silica or SiO2 alcogel). The dopants chosen for the former were NH4BF4, NH4Cl, NH4H2PO4 and N2H6SO4, while for the latter, the dopants used were H3PO4, NH4BF4, NH4H2PO4 and KH2PO4. The SiO2:H3PO4 alcogel gave the highest room temperature conductivity (10-3 S cm-1). Some of the xerogels studied by us were stable even up to 300 °C. Another interesting group of proton-conducting materials discussed in this paper is polymeric gel which was prepared by dispersing PMMA in the liquid electrolyte obtained by dissolving o-, m-, p-hydroxybenzoic acid; o-, m-, p-nitrobenzoic acid and three dicarboxylic acids, viz., oxalic, malonic and succinic acid, in a high-dielectric constant organic solvent. The role of the dissociation constants of the dissolved acids and the interaction of the polymer were discussed. The addition of polymer, inspite of the increasing viscosity, was found to sometimes lead to an increase in the conductivity of liquid electrolyte, which was explained on the basis of a breathing polymer chain model. © 2002 Elsevier Science B.V. All rights reserved.
Role of reduced pi-pi stacking in the charge transport in polyfluorene
(Elsevier Ltd, 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari
We investigated the effect of blending of a bulky copolymer on the hole transport properties of poly (9,9-dihexyl fluorenyl-2,7-diyl)(PFO) thin films (∼120 nm) at different temperatures by obtaining the current density-voltage measurement. The poly (phenylenevinylene) based copolymer ‘‘Super Yellow” (SY) has been used as a blending component. Such polymer blending matrix is useful as it combines the host polymer (PFO) with the guest polymer of bulky side groups where pi-pi stacking is hindered by bulky side groups. This makes polyflorene much interesting and versatile material for lighting application. It has been found that the current density decreases on blending for a given applied bias. We demonstrated that the current conduction mechanism is bulk limited. It has been analyzed by trapped charge limited currents (TCLC) with electric field. We conclude that the relatively low hole mobility in blends as compared to pristine PFO would result due to increase in the pi-pi stacking distance and disorder. © 2016 Elsevier B.V.
Silica alcogel-based proton conducting solid-liquid electrolyte composite
(2002) Ritu Srivastava; S. Chandra
Silica alcogels doped with proton conducting salts (NH4BF4 or NH4H2PO4 or KH2PO4) have been prepared by sol-gel process starting from organic precursor tetraethyl orthosilicate (TEOS). Structural (IR, XRD, DSC) and electrical (transference number and conductivity) studies showed that the solutions of dopants salts remain entrapped in the porous SiO2-gel matrix giving a solid-liquid electrolyte composite material. An implication of the formation of solid-liquid electrolyte composite, apart from liquid-like conductivity, is that the slow drying of entrapped electrolyte solution on storage for 3-4 weeks results in the growth of whiskers of the dopant salt. © 2002 Elsevier Science 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.