Electrochromic materials have the ability to change their optical properties, gradually and reversibly, when an electrical potential is applied across them. Thin films of transition metal oxides impart electrochromic functionality to the emerging smart window technology. Together with transparent contacts and a solid or gel-like electrolyte they constitute the building blocks of electrochromic devices. A typical device consists of five layers between two substrates, or alternatively deposited on a single substrate. In the former configuration, each substrate is coated by a transparent conducting layer and subsequently by an electrochromic layer. The two sides of the device are laminated by an ion conductor, preferably a polymer-based one. The electrochromic layers are chosen to be complementary. One of them is cathodic (i.e. it colors under cation/electron insertion) and the other is anodic (coloring under cation/electron extraction).
Some of the important performance parameters of an electrochromic device are the optical contrast between transparent and dark states, the charge capacity and the switching speed. Each of these has its roots in the fundamental physical and electrochemical properties of the materials. The optical properties depend on the nature of the electronic transitions and whether the electrons are free or localized. It appears that in many electrochromic materials intervalence transitions between localized states is the most important mechanism. Basically, a good description of the optical properties depends on a good understanding of the electronic band structure. The charge capacity of an electrochromic film depends on the magnitude of the electronic density of states in the reversible potential range. It should be noted that the achievable density of states may be less than the theoretically expected one, since intercalated ions may not be able to penetrate all parts of the material. The switching speed depends on the diffusion coefficient of ions in the electrochromic films, as well as in the electrolyte. The sheet resistance of the transparent conductors is also important in this respect.
A number of electrochemical techniques are suitable for studying ionic transport in intercalation materials, such as for example electrochromic films in contact with an electrolyte. Impedance spectroscopy gives the most detailed characterization of the transport processes but measurements are time consuming, and in certain situations more simple techniques such as chronopotentiometry or choronoamperometry may be preferable. Electronic transport may be studied with conventional electrical measurements with the film samdwiched between two metal electrodes.
In this contribution we will focus on the determination of the diffusion coefficient and the electronic density of states (DOS) by electrochemical methods, and how to use this knowledge to describe the optical properties. It is shown that the features observed in the “electrochemical density of states” often show good agreement with the theoretically computed DOS. However, the magnitude of the electrochemical DOS is always lower that the theoretical one. It seems that the whole film is not accessible to the ion intercalation process. We also address the question whether the electronic states are extended or localized and how to determine this experimentally. Theories for optical properties are reviewed based on the intervalence transfer concept. In some, but not all, cases the transitions may be identified as polaronic. We also comment on the existence of electrochromism due to free electrons and its possible limitations. Examples are given mainly from our work on WO3, but also results on SnO2, IrO2 and NiO-based films will be discussed.
2014. 2-3 p.
European Materials Research Society, Spring Meeting, Lille, France, May 26th-30th, 2014