Fundamentals of controllable photochromic technology

Abstract

Recently, a new technology termed the controllable photochromic has emerged within the field of chromogenic technology. This technology represents a novel approach to the control of solar heat gain and adjustable transmission by utilizing a combination of chromogenic and radiation sensitive technologies. By harnessing solar energy for coloration, these systems represent a low-energy approach to controllable transmission glazings when compared to other chromogenic technologies available today. The zero external power required to reduce the glazing transmission and to maintain that state will provide opportunities for applications where today's chromogenic technologies are not well suited. One emphasis of this work was the rationalization of a general framework through which present and future controllable photochromic systems can be discussed. This framework discusses the roles and properties of the system components, necessary and desired for the function of the system, including a radiation sensitive electrode, a chromogenic electrode, and an electrolyte. The framework was also examined through a detailed case study of a system involving an anatase TiO₂ radiation sensitive electrode and a WO₃ chromogenic electrode. While this study involved a particular implementation of the technology, it exposed a great many attributes of this system including kinetics, wavelength sensitivity, influence of construction parameters, sensitivity to non-uniform incident radiation, and control over the photochromic response. While the general framework is useful for discussion of the principles involved in device operation, the detailed mechanisms occurring in a particular implementation will generally be more complex. Through careful study of the example system described in this thesis, the primary mechanisms occurring in the device were identified, and a model is proposed which is consistent with the observed findings. The source of degradation occurring due to prolonged cycling in the devices was studied through investigation of changes occurring in individual device components. While only one implementation of the technology was studied in detail in this investigation, three configurations were discussed in terms of the general framework. These systems exhibit an impressive array of desirable properties; however, it is clear that there is still much to be done to bring the technology to full fruition.