Plasma As a Complimentary Electrochemical Reaction Process for Thin Film Deposition in Preparing Energy Conversion and Storage Devices
Authors: Kuo Yue 
Published: May 2020

Energy storage and conversion devices are often based on electrochemical reactions. For example, batteries are operated on the principle of charge transfer between two electrodes. On the other hand, the clean energy source - hydrogen - can be generated from water using the electrolysis or membrane electrolysis process upon the application of a large voltage through the solution (1,2). Energy storage or conversion can be carried out using solid state devices. For example, thin film dielectric super capacitors can store a large number of charges that are discharged in a very short period of time (3). Solar cells are used to convert the solar light into electric energy (4).
Thin films are important elements in many of the above devices prepared by electrochemical reaction processes. For example, copper films can be deposited by electrodeposition; tantalum oxide or aluminum oxide films are formed from anodization. These reactions have to be done in aqueous solutions through which charges are transferred according to the polarity of the electric field. Films deposited in electrochemical processes are stoichiometric, i.e., following the thermodynamic laws.
Plasma processes are commonly used in preparing thin films electronic or optoelectronic devices. It is done in vacuum. The reaction is initiated from collisions of high energy electrons with feed gas molecules. Complicated particle-particle interactions occur in the plasma phase and the substrate surface. Eventually, stoichiometric or nonstoichiometric thin films of various compositions and structures are formed on the substrate. The plasma deposition process can be carried out on a very large substrate, e.g., the 3 m X 3m glass for the thin film transistor production, at high rate and good uniformity. The substrate temperature can be low. The plasma deposited films have very low impurity content. Since plasma reactions do not follow the thermodynamic rules, films of various chemical or physical properties can be prepared. For many applications, devices of unique characteristics are created by these films.
For many thin film applications, the initial plasma-film (or substrate) reaction is critical to the growth process, which affects final properties of not only the bulk film but also the interface. For mass production applications, the performance, reliability, and yield of the device are influenced by the growth condition. Examples on the plasma thin film growth process and related film properties will be discussed.
It is possible to take the plasma deposition process as a complimentary technology for the electrochemical deposition process. Then, many new energy storage and conversion devices can be fabricated for a wide range of products.
1. https://www.energy.gov/eere/fuelcells/hydrogen-production-electrolysis
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3. K. Yao, et al., IEEE Trans. Ultrason. Ferroelec. Freq. Contl., 58(9), 1968 (2011).
4. K. Kim and Y. Kuo, ECS J. Solid State Sci. Technol., 6(1), Q29 (2017).