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Title: Electroluminescence in ion gel gated organic polymer semiconductor transistors
Author: Bhat, Shrivalli
ISNI:       0000 0004 2708 6246
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2011
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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 thelocation 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 theelectron 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 modelbased on the theoretical model and have compared the results of the numerical model with theresults 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 cm²/V.s and a diffusion coefficient of 1 x 10⁻¹⁵cm²/V.s for TFSI ions in F8BT matrix.
Supervisor: Sirringhaus, Henning Sponsor: Gates Cambridge Trust ; Cambridge Overseas Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Ion gel gated polymer semiconductor transistor ; emitting electrochemical cell ; Light emission