Advanced Materials for Smart and Functional Windows

Dear Colleagues,

There is a great interest in variable or switchable transparency large-area panels, for a variety of reasons, including switchable transparency windows for selected privacy, as well as adjusting solar load, climate-adaptive building shells, including variable reflectivity roofing and more exotic applications, such as variable camouflage. In general, this includes intelligent Smart Glass, which will automatically varies transparency in response to temperature or other parameters, and active Smart Glass, which will vary or switch transparency in response to an electrical control signal. As such, active Smart Glass can be considered a form of optical modulator, or device that varies optical transmission vs. voltage. The most commercial forms of optical modulator are Lithium Niobate electrooptic modulators and Liquid Crystal Displays. The former varies optical transmission via an electrooptic change of refractive index, has picosecond response time, and is used to encode communication data on optical fiber. The latter varies optical transmission via a movement of liquid crystal molecules, has a microsecond response time, and is used in displays. One can see that the form of optical physics used is adapted to the application.

For smart windows or other types of such large panel devices, the most common technology used is Electrochromic. Electrochromic Smart Glass consists of layers of material, for which when voltage causes ions to move from one layer to another, optical transmission varies. The Electrochromic effect illustrates a further distinction of smart glass technologies: it causes a variation from transparency to absorption, rather than transparency to reflection. As such the Electrochromic effect is ideal for applications such as shading, as in varying the reflection of automobile rear-view mirrors. For applications such as adjusting solar transmission through building windows, it should be considered that Electrochromic shading will cause absorption and heating of the windows.

The next common technologies for smart windows are Suspended Particle Devices³ and Polymer Dispersed Liquid Crystal. Both of these technologies have suspended particles or liquid crystal droplets, respectively, which are randomly oriented in the zero-voltage state. For the former, the random orientation results in forward-scattering and absorption in the particles, like Electrochromic, the non-transparency state is absorptive. When voltage is applied, the particles align, lowering the scattering and raising the transparency. For the latter, the droplets result in scattering, and so illustrate a further distinction of optical effect, reflective scattering in the non-transparent state. When a voltage is applied, the droplets align and closely match the refractive index of the matrix, raising transmission. For both of these technologies, there is current in the transparent state, resulting in power consumption on the order of 5–20 W/m².

The aim of this Special Issue is not to disparage any particular technology in favor of another, as clearly, each has its benefits and drawbacks. Rather, it is to explore the technologies that may result in the desired optical effect, and thus develop a range of technologies with different costs and performance. Existing commercial technologies are complex, and thus their widespread application may be limited. Lower complexity should result in lower cost and increase adoption, and thus should be explored even if engineering problems are pushed into ancillary systems. In considering different effects that may result in optical modulation or switching, it should be appreciated that something must move. The simplest example is a shutter, and a reasonable example here are double-pane windows with motorized built-in blinds. In electrochromic or particle technologies, ions, molecules, or particles move. In optofluidics, a fluid moves. This call for articles encourages workers in the field to think both inside and outside the box, finding ways to decrease costs and increase performance for both standard and exploratory smart window technologies.

Prof. Keith Goossen
Guest Editor

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