Poly(3,4-ethylenedioxythiophene) and Viologen Based Materials for Electrochromic Devices

Electroactive materials which undergo reversible optical changes through electrochemical redox reactions by the application of an electric field are called electrochromic (EC) materials. Among various electrochromes, organic electrochromes such as poly(3,4-ethylenedioxythiophene) (PEDOT) and viologens can modulate visible light and offer high contrast ratios and fast switching times. In the present thesis, PEDOT/fulleropyrrolidine films were prepared for the first time and the optical response of the film is completely altered by the presence of the fullerene derivative; a coloration efficiency of 386 cm2 C-1 was acheived. Electrohromic devicves (ECDs) with a PEDOT film as cathode and a Prussian blue (PB) film as anode were constructured and the superior EC performance of the an in-situ polymerized ionic liquid based gel electrolyte compared to a conventional gel electrolyte was demonstrated. The optimized in-situ polymerized gel was then used in a heptyl viologen based ECD and the switching of a large area device (8 cm 6 cm) for antiglare electrochromic rearview (ECRA) mirrors was demonstrated. PEDOT films were then transposed over plastic substrates and their ability to serve as a transparent conductor and an electrochromic electrode was shown. Similarly viologens offer different colors depending on the N-substituent. Although a variety of viologens were synthesized in the past, a reversible and high write-erase efficiency continues to pose a formidable challenge. To address this concern, new derivatives of viologens were synthesized by the use of substituents like butylbenzimidazole and ethylindole, their ECDs were constructed and characterized. Further, a hybrid viologen was also prepared by using a phosphonitrillic trimer (P3N3Cl6) as a central core and the potential of this viologen for durable, high contrast ECDs was shown. These viologen based ECDs showed very high contrast ratios, high coloration efficiencies, short response times and the devices can endure more than 2000 cycles without undergoing degradation. These devices and materials are very much useful for applications such as flexible displays, energy saving switchable windows in buildings and automobiles and ECRA mirrors

A novel 1,1′-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4, 4′-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1′-bis-[4-(5,6-dimethyl-1H- benzimidazole-1-yl)-butyl]-4,4′-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4′-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200 °C, and a stable voltage window of ∼4 V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605 nm), a remarkably high coloration efficiency of 725 cm2 C-1 at 605 nm and switching times of 2-3 s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000 cycles, thus ratifying its suitability for electrochromic window/display application

Electrochromic device response controlled by an in situ polymerized ionic liquid based gel electrolyte

Polymer electrolytes were synthesized by two different approaches and applied to electrochromic devices based on electrodeposited tungsten oxide (WO3) or poly(3,4-ethylenedioxythiophene) (PEDOT) films as the cathode, and a Prussian blue (PB) film as the anode. The first method involved the entrapping of an ionic liquid in a polymer host (poly(methylmethacrylate) or PMMA) and the second approach relied on the in situ thermal polymerization of methylmethacrylate (MMA) in the hydrophobic ionic liquid, yielding a solidified transparent gel. The effect of in situ solid polymer electrolyte formation on device performance characteristics was realized in terms of a larger coloration efficiency of 119 cm2 C21 (l = 550 nm) achieved for the WO3–PB (MMA) device, as compared to a value of 54 cm2 C21 obtained for the WO3–PB (PMMA) device. Similar enhancements in electrochromic coloring efficiency, reflectance contrast, and faster switching kinetics were obtained for the PEDOT–PB (MMA) device. The strategy of introducing an electrolyte to the electrochromic device in a liquid state and then subjecting the same to gradual polymerization allows greater accessibility of the electrolyte ions to the active sites on the electrochromic electrodes and superior interfacial contact. As a consequence, larger optical contrast and faster kinetics are achieved in the MMA based devices. While PEDOT films were amorphous, PB films were semi-crystalline but only in the case of WO3; the hexagonal structure of WO3, equipped with three/four/six-coordinated voids was found to affect bleaching kinetics favorably. The performance of PMMA based electrolyte is limited by high resistance at the electrode–electrolyte interface, and a smaller number of ions available for oxidation and reduction. Large area (y10 cm 6 4 cm) devices were also fabricated using this simple wet chemistry method and their ability to color uniformly without any pinholes was demonstrated

A new organo-inorganic hybrid of poly(cyclotriphosphazene-4,4′- bipyridinium)chloride with a large electrochromic contrast

A new organo-inorganic hybrid electrochromic material poly(cyclotriphosphazene-4,4′-bipyridinium)chloride salt was synthesized wherein each phosphorus atom in the triphosphazene core is linked by diquaternized 4,4′-bipyridyls (PPBP). The electrochrome was characterized by 31P nuclear magnetic resonance, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS confirmed the formation of P-N+ and P-N linkages in the PPBP and the conversion of covalently bound Cl to ionic Cl- species. The transparent PPBP material undergoes three reversible one electron-transfer reactions to yield a radical cation insoluble film which first acquires a purple hue and then turns deep blue. Electrochromic devices were constructed with the PPBP hybrid dissolved in a highly conductive, thermally stable and electrochemically inert ionic liquid based gel electrolyte and a Prussian blue (PB) layer as the anode. The PPBP-PB device showed an extremely large coloration efficiency of 504 cm2 C-1, an exceptionally high transmission modulation of 70.5% at 590 nm, one among the highest reported contrasts in organic electrochromics, a large reflectance contrast of 59.2% at 545 nm and fast switching kinetics. The device was durable as it retained 96.2% of its original transmission after 1000 color-bleach cycles, thus exemplifying its use for both reflective and transmissive electrochromic applications. Electrochemical impedance studies revealed charge transfer to be a less resistive process during reduction relative to oxidation which cumulatively ensues in shorter coloration times. Our studies demonstrate the yet untapped potential of the inorganic (PNCl2)3 in steering the synthesis of new organo-inorganic hybrid materials, capable of undergoing facile redox phenomena, manifesting in unequalled functional properties and thereby offering opportunities to use this trimer for preparing a whole gamut of new high performance electroactive compounds

Fast, Direct, Low-Cost Route to Scalable, Conductive, and Multipurpose Poly(3,4-ethylenedixoythiophene)-Coated Plastic Electrodes

Poly(3,4-ethylenedioxythiophene) (PEDOT) films are deposited, using an electroless method, onto flexible plastic poly(ethylene terephthalate) (PET) substrates of approximately 20×6cm2. The sheet resistance of a PEDOT-PET film is approximately 600Ω per square, and the nanoscale conductivity is 0.103Scm-1. A plastic electrochromic PEDOT-Prussian blue device is constructed. The device undergoes a color change of pale blue to deep violet-blue reversibly over 1000cycles, thus demonstrating its use as a light-modulating smart window. The PEDOT-PET film is also used in a quantum dot solar cell, and the resulting photoelectrochemical performance and work function indicate that it is also promising for photovoltaic cells. The high homogeneity of the PEDOT deposit on PET, the optimal balance between conductivity and optical transparency, and the demonstration of its use in an electro-optical device and a solar cell, offer the opportunity to use this electrode material in a variety of low-cost optoelectronic devices. Captured on film: The synthesis of low-cost, scalable, electrically conducting, uniform, and optically transparent coatings of poly(3,4-ethylenedioxythiophene) on poly(ethylene terephthalate) plastic is presented. This may have a tremendous impact on the development of cost-effective electro-optical and photovoltaic devices

Enhanced electrochromic write–erase efficiency of a device with a novel viologen: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate

A novel cathodically coloring viologen electrochrome: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate (IEV), comprising of a 4,4′-bipyridyl core, sandwiched between two indole moieties, was synthesized using 3-(2-bromoethyl)-indole. An electrochromic device (ECD) was assembled using an electrolyte containing an imidazolium imide ionic liquid and characterized by a large electrical conductivity, thermal stability upto 150 °C, and an electrochemical potential stability range of ∼3.6 V. The IEV viologen was dissolved in the electrolyte and Prussian blue was used as the anodic electrochrome. The indole moieties of the IEV2+ salt owing to their electron donating tendency can act like bleaching agents and bleach the viologen faster (IEV+. → IEV2+) and this hypothesis was used for the improved write-erase efficiency of the device. The device switched between colorless and dark violet-blue hues under applied potentials of ±1.5 V. A large transmission modulation (52%, λ = 605 nm), a high electrochromic coloring efficiency of 533 cm2 C-1 at 605 nm and switching times of ∼2 s and good stability during 2000 cycles were reported herein. The electrochemical activity of the ECD improved when it was maintained at an elevated temperature of 70 °C, with no sign of thermal degradation. Furthermore, we also present the ability of this new viologen to function as an excellent redox mediator as we achieved an 86% enhancement in the power conversion efficiency of a solution processed solar cell, by its' addition in the electrolyte. Our studies demonstrate this new viologen to be a highly versatile electroactive material which can be useful for both electrochromics and photovoltaics