Electroluminescence in ion gel gated organic polymer semiconductor transistors

This thesis reports the light emission in ion gel gated, thin film organic semiconductor transistors and investigates the light emission mechanism behind these devices. We report that ion gel gated organic polymer semiconductor transistors emit light when the drain source voltage is swept slightly beyond the energy gap of the polymer divided by the elementary charge (Vds > Eg/e). In particular, the light emission in poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) polymer semiconductor, with 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/ poly(styrene-block-ethylene oxide-block-styrene (EMIM TFSI/ SOS) ion gel as dielectric material is reported. The current-voltage characteristics corresponding to the light emission, where the systematic increase of the drain current, correlated with light emission is reported. In low voltage regime, (Vds < Eg/e), well saturated transistor characteristics are observed. By charge modulation spectroscopy (CMS) study we show that there is a prominent electrochemical doping occurring with gate voltages. Further, owing to the movement of ions with voltages, irrespective of the location of electrodes, we show that the ion gel, bilayer planar devices emit light in Vds > Eg/e regime (without any gate voltages), at room temperature. Based on the location of the recombination zone in the proximity of electron injecting electrode and CMS results showing prominent di ffusion of negative ions into the polymer layer, we conclude that the light emitting mechanism is akin to light emitting electrochemical cells (LECs). Even in the the transistor regime, where Vds << Eg/e, with the signatures of increasing drain current for fixed Vg and Vds values, we show that the transistor can not be of purely electrostatic operation alone. We study the fluorescence quenching of an operating bilayer device under a constant bias over a period of time and compare the results with the electroluminescence of the device and show that the formation of the p-n junction within the polymer layer due to the penetrated ions from the gel dielectric into the polymer semiconductor layer on the application of the voltage is the cause behind the light emission. We show that diffusivity of the cation (EMIM) is very low compared to the anion (TFSI). This is consistent with the fact that the recombination zone is near the electron injecting electrode in these devices. We have developed a theoretical model for the ions movement within the semiconductor polymer matrix governed by both diffusion and drift independently, for the bilayer, polymer ion gel planar, light emitting electrochemical cells. We have further developed a 2- dimensional numerical model based on the theoretical model and have compared the results of the numerical model with the results of a fluorescence probing of the bilayer device with time, at constant potential across the bilayer LEC and report that the drift coefficient of 1x10^-13 cm^2/V.s and a diffusion coefficient of 1 x 10^-15cm2/V.s for TFSI ions in F8BT matrix.Gates Cambridge Trust and Cambridge Overseas Trus

Vitrification and Glass Transition of Water: Insights from Spin Probe ESR

Three long standing problems related to the physics of water viz, the possibility of vitrifying bulk water by rapid quenching, its glass transition, and the supposed impossibility of obtaining supercooled water between 150 and 233 K, the so-called 'no man's land'of its phase diagram, are studied using the highly sensitive technique of spin probe ESR. Our results suggest that water can indeed be vitrified by rapid quenching, it undergoes a glass transition at \~ 135 K, and the relaxation behavior studied using this method between 165 K and 233 K closely follows the predictions of the Adam-Gibbs model.Comment: 13 pages, 3 figures; results on slow cooled water added; four figures compressed in to thre

Spin probe ESR studies of dynamics of single walled carbon nanotubes

The highly sensitive technique of spin-probe Electron Spin Resonance (ESR) has been used to study dynamics of carbon nanotubes. The ESR signals were recorded for the nitroxide free radical TEMPO in carbon nanotubes from 5 to 300 K. The onset of the fast dynamics of the probe molecule was indicated by appearance of a narrow triplet at 230 K. The ESR measurements were also done on TEMPO in methanol for the comparative studies in the same temperature range, and in the latter observations, no change in spectra was seen around 230 K. The results indicate the occurrence of a change in the dynamics of carbon nanotubes around this temperature

Vitrification and glass transition of water:insights from spin probe ESR

Three long standing problems related to the physics of water viz, the possibility of vitrifying bulk water by rapid quenching, its glass transition, and the supposed impossibility of obtaining supercooled water between 150 and 233 K, the so-called “No Man’s Land of its phase diagram, are studied using the highly sensitive technique of spin probe ESR. Our results suggest that water can indeed be vitrified by rapid quenching, it undergoes a glass transition at ~ 135 K, and the relaxation behavior studied using this method between 165 K and 233 K closely follows the predictions of the Adam-Gibbs model

Molecular Probe Dynamics Reveals Suppression of Ice-Like Regions in Strongly Confined Supercooled Water

The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice [Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448-11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed

Molecular probe dynamics reveals suppression of ice-like regions in strongly confined supercooled water

The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448-11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed

Spin probe ESR studies of PEG<SUB>x</SUB>LiCIO<SUB>4</SUB> polymer electrolyte systems

The technique of Electron Spin Resonance (ESR) is shown to be useful in the study of dynamics of solid polymer electrolytes (SPE). Through the ESR of the nitroxide radical (2,2,6,6-tetramethyl-1-piperidine-1-oxyl; TEMPO) dispersed in the SPE PEG<SUB>46</SUB>LiClO<SUB>4</SUB> temperature dependence of correlation time is found. The glass transition temperature T<SUB>g</SUB> is estimated to be -51 &#176;C from the measurement of T<SUB>50G,</SUB> the temperature at which the extrema separation 2A<SUB>ZZ</SUB> becomes 50G and is found to be close to that measured using DSC (-51.7&#176; C). T<SUB>g</SUB> for pure PEG-2000, which could not be measured from DSC because of its high crystallinity, is determined to be -72&#176;C by spin probe ESR

Rotational correlation time of TEMPOL in SC and QRW water.

<p>Part of the data are in the “no man's land” (). The two <i>parallel</i> dashed lines with slope kJ/mol are the Arrhenius best-fit of the correlation times of TEMPOL in equilibrium water, SC water (blue) and in the low-mobility S fraction of the QRW water (red). The inset plots the data including the sub- region. Note: i) the change of regime at K close to , ii) the absence of any abrupt change at and, in SC water, at both and .</p

Two idealized ice/water mixtures with different polycrystallinity at .

<p>The scale of the pictures is the same. The two panels refer to the QRW (a) and the SC protocols (b), see text for details. The width of the liquid veins in the two mixtures is very similar and of the order of dozens of nanometers, whereas the size of the ice grains is times larger in SC mixtures. Augmenting of the polycrystallinity increases the water fraction and reduces its confinement due to the additional paths and intersections. According to ref.<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044382#pone.0044382-Banerjee1" target="_blank">[38]</a> and the present study, ice-like patches (blue) with slow (S) mobility are included in the QRW liquid fraction. The patches are suppressed in the SC mixtures, leaving only the less ordered liquid fraction (light blue) with fast (F) mobility. The shape of the patches is unknown.</p