# Spectrally-Selective Energy-Harvesting Solar Windows for Public Infrastructure Applications
## 2 Design Features and Performance Characteristics of Transparent Solar Windows
### Materials and Structures Used within Transparent Energy-Harvesting Glazing Systems
Using scattering, which, in the LSC field, is generally known as a loss  
The main components that define both the visible transparency and the spectrally-selective energy harvesting properties of the glazing system are (i) the high-spectral-selectivity low-emissivity thin-film coating and (ii) the visibly-transparent (when encapsulated using UV-curable epoxy and coverglass) spectrally-selective diffraction grating. The diffraction grating is designed to effectively deflect the incident light in both the UV and the near-IR spectral ranges into multiple orders of transmission. The experimental methods suitable for the fabrication of suitable diffraction gratings are described in detail in Reference [7]. The methods for the production of suitable types of optical thin-film coatings have been discussed in Reference [24]. A unique combination of high-performance luminescent materials, which helps collect a large part of the incident UV energy and also a fraction of the incident near-IR energy from within a rather broad spectral region between about 900-1100 nm has been described in detail within the Supplementary dataset of Reference [21]. Figure 4 (reproduced from Reference [21]) illustrates the functionality of a ZnS:(Ag, Tm)-based luminophore capable of efficiently converting the energy from a broad UV-blue wavelength range into a narrow emissions band centered at 794 nm, which avoids self-absorption losses. Figure 3: Components and structures used for constructing ClearVue PV window modules and their main properties. Graphics are reproduced from Reference [7] for system structure illustration purposes. (**a**) Transmission spectrum of a high spectral selectivity low-e coating (Viracon, Inc. VNE 2463) and graph showing the spectral power density curve for the AM1.5G sunlight reflected off this coating. (**b**) Zero-order (direct) transmission of several transparent spectrally-selective diffraction grating samples measured using a spectrophotometer. (**c,d**) Visual appearance of either the un-encapsulated or the encapsulated diffraction grating. (**e**) Overall schematic of an advanced transparent glazing system for energy harvesting concurrent with high thermal insulation in which any internal air gap(s) are omitted for simplicity. The concentrator samples tested to generate the datasets of Figure 4 were composed of two 1 mm-thick Corning glass plates connected via a UV-cured functionalized epoxy interlayer. Luminophore particles were dispersed uniformly within liquid epoxy at concentrations not exceeding 1 wt %. Strong emission intensities were observed near 794 nm, which is almost saturating the spectrometer detection system under normal-incidence UV excitation and when the sample edge area aperture was aligned with the input aperture of the emissions monochromator. This graphical dataset is reproduced from the Supplementary dataset of Reference [21]. Hybrid-type transparent planar solar concentrators employing the mechanisms of luminescence, scattering, and diffractive deflection of light form the basis of our approach to building practical solar windows suitable for architectural deployment. The apparent synergy between nearly all of the above-named light-collection mechanisms (provided that the sizes and concentrations of all luminophore powder particles were selected carefully) was our significant finding, which governed the design of advanced glazings in multiple laboratory samples and factory-produced window modules.