Raman spectroscopic characterization of thin films to be used in Electrochromic Devices

Abstract

Electrochromism is related to the optical properties of some materials when subject to an electric potential. These properties are exploited in special electrochromic devices formed by these materials and others that together provide for their implementation and operation. In recent years, the overall performance of these devices has increased considerably, although further improvements are currently being studied by the scientific community. The aim of this thesis work has been the study of materials for potential use in electrochromic devices, the creation of an electrochromic device capable of modulating solar radiation as well as a part of the heat associated with it and its complete characterization. The instrumental analyses used for the characterization of the various materials and devices were Raman spectroscopy, Cyclic Voltammetry and UV-visible spectroscopy. Raman spectroscopy is a formidable investigation technique because it is non-destructive and it can reveal a lot of structural chemical and physical information from the samples analysed. In recent years it has gained great importance in the analysis of biological samples because of the development of the Raman technique named as SERS (Surface Enhanced Raman Spectroscopy). The results of the thesis are presented in different chapters that deal with the theoretical concepts related to electrochromism (chapters 1 and 2) and those related to the investigation techniques used during the course (chapter 3). The results of the analyses are set out in Chapter 4. Chapter 5 presents the conclusions and future perspectives. In the first part of Chapter 4, Raman analyses have been conducted on different polymeric materials such as polymethylmethacrylates (PMMA) and polyethylene oxide (PEO) as well as natural polymers such as chitosan for potential operational (the former) and environmental benefits (the second) that these can bring in the realization of an electrochromic device. Polyelectrolyte materials can be used as an electrolyte layer in electrochromic device, in this thesis work the SERS effect of polyelectrolyte layers on the Raman spectrum of acetic acid has been observed. Chitosan films doped with zinc complex have been analysed by Raman spectroscopy. The interpretation of the results obtained made it possible to establish that the interaction between the natural polymer and the zinc complex takes place at the level of the N-H and O-H groups. Bipyridine ligands are often used in the formation of metallopolymer molecules with electrochromic properties. In this thesis, the SERS effect of a gold plate on the Raman spectra of bipyridine samples has been studied, showing an interesting improvement of bipyridine Raman signals. Graphite and graphene can improve the conductivity of electrochromic devices by improving their performance. In this thesis work, through the use of Raman spectroscopy, the structural characteristics of functionalized graphite and graphene nanoplates have been shown. The SERS effect of Ag thin layer on the Raman spectra of deposited 6G rhodamine has been demonstrated. The Raman characterization of TiO2 films has been shown. In the second part of Chapter 4, electrochromic devices consisting of an electrochromic solution of viologen and ferrocene to which glycerolate bisphenol A has been added in certain proportions have been developed. The optical properties of the devices have been studied with UV-vis-NIR spectroscopy before and after polymerization induced by exposure to UV light. In particular, the analysis of the transmission properties of the devices in the coloured and transparent states (ON and OFF states, respectively) have been shown. In addition, the electrochemical properties of the devices have been studied by means of cyclic voltammetry measurements that show the differences between the ON and OFF states of the devices. Concluding remarks have been made in the last chapter of the thesis, with research on electrochromic technology continuing with further structural analysis of materials.