Browsing by Author "Manisha Bajpai"

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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.
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).
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.
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.
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.