Fabrication and characterization of electrochromic electrodes and devices

Abstract

The modern era is an era of technology which affects almost all aspects of life covered under the sun. Simplest physical manifestation of the term “technology” takes place in the form of a user friendly device. Designing and developing a smart device, capable of performing at its best, depends on the level of scientific understanding of the basic principles that makes the actual backbone on which the technology relies. Smart devices, the ones with multifunctional capabilities, have applications in energy storage devices (supercapacitor, battery), sensors, solar cells, photo cells, smart windows, etc. This leads to the progressive development and research in electronics, material science and related fields. An electrochromic device is a perfect example that can be useful for a range of applications including electrochromism, the phenomenon of bias induced optical modulation. Electrochromic devices are based on electrochemical activity of the active material and possess varied optical states as a consequence of their attainable redox behaviour due to application of bias. They use various organic, inorganic materials and their possible combinations in a designed paradigm to make an improved and power efficient device. Looking at various perspectives of electrochromic devices, my research work, summarized in this thesis, primarily focuses on developing and understanding of electrochemical and synthesis processes required for fabricating efficient electrochromic device and understanding its operational mechanism. Various organic materials for making improved electrochromic devices in a pre-designed device structure have been explored. Their capability to form flexible electrochromic devices have been studied because their inherent nature of solution processability allows their deposition on flexible substrates very easily. Inorganic materials’ morphologies and structures are controlled involving hydrothermal and electrochemical deposition methods by varying their synthesis parameters to obtain optimized electrodes for enhanced device v performance. These inorganic materials have also been used for designing a flexible electrochromic device. To understand the basic mechanism of color switching by the electrochromic device under an appropriate bias, in situ Raman microscopic investigations were carried out along with spectroscopy to see and visualise live formation and annihilation of polarons in an open face poly(3-hexylthiophene), P3HT based device. This helps in understanding the process of dynamic doping in plastic semiconductors that leads to redox induced electronic property variations. The electrochromic performance of P3HT based device has been improved by combining it with, a carbon based derivative, [6,6]-phenyl-C61- butyric acid methyl ester, PCBM. This PCBM has a tendency to store, then hold & release electrons on demand just like graphene. The overall organic electrochromic device, designed in an appropriate geometry, to improve the performance in terms of switching speed, stability etc. Role of each component in the device has been investigated using various electrochemical methods while the quantification of color obtained has been done using in situ absorbtion spectroscopy and its underlying mechanism with Raman spectroscopy. To obtain a tri-chromic device with P3HT, it is combined with ethyl viologen (EV) in bi-layer device structure and three different colors, maroon, transparent and blue, at different bias conditions have been obtained. In situ UV-Vis and Raman spectroscopy have been applied to see color change and possible switching mechanism. Before device fabrication each electrode was tested electrochemically for its electrochromic behaviour and thus helps in solid state device fabrication. After detailed study of organic electrochromic devices, organicinorganic hybrid electrochromic devices have been fabricated. We have designed EV/PB (prussian blue) and P3HT/WO3 based electrochromic devices. The PB has been grown electrochemically on ITO substrate and combined it with EV. The device functions as dual wavelength filter with improved electrochromic performance with coloration efficiency vi of ~ 237 cm2 /C and 226 cm2 /C for 400 nm and 600 nm respectively along with 1400 switching cycles stability between transparent and dark blue colored states have been achieved. Additionally, PB electrodes grown on flexible substrates have been demonstrated for flexible electrochromic applications. Next P3HT/WO3 hybrid device has been fabricated for dual application in electrochromic window as well as IR filter. The device turned transparent from initial magenta color with only 1V bias and keep the area cooler by ~8°C (22%) to maintain the room temperature as imaged using IR camera. The device exhibit superior electrochromic performance with switching time of 50ms, color contrast of ~60% in IR region, coloration efficiency of ~380cm2 /C and stability of more than 1600 switching cycles. Beyond electrochromic applications, WO3 nanostructures, fabricated on carbon cloth, have been studied for its energy storage applications. At the end, electrochemical properties of herbal hibiscus has been studied in detail and based on experimental findings, herbal electrochromic device has been fabricated for the first time in the field of electrochromic devices. This will provide a new pathway in the field of electrochromic devices and possibly in other electronic applications as well.