Electrochemically lithiated V 2 O 5 films: An optically passive ion storage for transparent electrochromic devices
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LiyV2O5 films were produced by reactive dc magnetron sputtering followed by electrochemical posttreatment in LiClO4. X-ray diffraction showed an orthorhombic structure. Spectrophotometric transmittance and reflectance measurements demonstrated that the luminous and solar absorptance increased moderately when y increased from zero to unity. It is argued that LiyV2O5 is useful as an ion storage material operating in conjunction with electrochromic films. I. INTRODUCTION
This paper discusses the structure, optical properties, and electrochemical behavior of L i ^ O s (0 ^ y < 1) thin films produced by reactive magnetron sputtering and subsequent electrochemical treatment. It is argued that this material can serve as an optically passive ion storage in transparent electrochromic devices such as "smart windows".1'2 All-solid-state transparent electrochromic devices comprise five layered materials backed by one glass plate or positioned between two glass plates.1"5 The outermost two layers are transparent electrical conductors; one of them is in contact with an optically active electrochromic layer, and the other is in contact with an ion storage layer. The final layer, located at the center of the five-layer stack, is a transparent ion conductor. Reversible and gradual modulation between a transparent and an absorbing (or reflecting) state is possible when an electric field is set up between the outermost layers so that small mobile ions are moved from the ion storage, via the ion conductor, and inserted into the electrochromic material, or if the process is run in reverse through a change of the field direction. The ion storage, which is the focus of the present study, can be optically passive; i.e., it should remain transparent irrespective of the amount of inserted charge. Such properties can be achieved in LiyV^Os, as we will see below. Alternatively, one can combine the base electrochromic layer with an optically active (electrochromic) ion storage which colors and bleaches in synchronization with the optical change of the base electrochromic layer.6 All-solid-state transparent electrochromic devices can be employed in future "smart windows" which regulate the throughput of radiation in accordance with demands on energy efficiency or human comfort in buildings or cars, in anti-dazzling rear-view mirrors for cars, in (large-area) information displays, etc.2 "'Permanent address: Universidad Nacional de Ingenieria, Facultad de Ciencias, Apartado 1301, Lima, Peru. J. Mater. Res., Vol. 5, No. 6, Jun 1990
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Section II reports on the preparation of V2O5 thin films by reactive dc magnetron sputtering onto In2O3:Sn-coated glass. Section III describes lithiation by electrochemical technology, i.e., the formation of LijA^Os. The Li content can be changed reversibly between magnitudes which correspond to at least one formula unit. X-ray diffraction measurements, discussed in Sec. IV, show that LiyV2O5 has an orthorhombic structure with an interplanar separation which scal
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