# 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  
To efficiently convert the available near-IR energy (which, in spectrally-selective LSCs, is routed towards glass panel edges), it is very important to select the appropriate solar cell chemistry and solar module construction type, which will enable the photovoltaic conversion of this near-IR energy within the broadest possible spectral window with the highest possible efficiency. Of particular importance for solar window applications is the selection of PV modules, which can maintain their efficiency (to the maximum extent possible) in the presence of significant adverse factors such as transverse or longitudinal geometric shading from the incoming sunlight as well as \"spectral shading.\" This condition in which the solar module is not illuminated by the full-spectrum sunlight was designed to operate the selected PV module type. Despite the fact that the short-circuit current (I\({}_{\text{sc}}\)) of any PV cell (or module) depends on the incident optical power available at any given wavelength and the corresponding responsivity, the power-generation behavior (and also the fill factor (FF) and efficiency) of PV modules connected to external electric loads becomes markedly more complicated in the presence of a combined influence of the geometric and spectral types of shading. These conditions are usually encountered in concentrator-type solar window applications where the cell modules are located at or near the glass panel edges due to both the geometry of sunlight incidence and the device construction geometry itself. We found experimentally that the properties and performance of Avancis PowerMax 3.5 CuInSe\({}_{2}\) (CIS) PV modules were the most suitable for solar window applications due to a large number of important technical considerations listed below. * Broad spectral responsivity band of the CIS modules, which is a key factor for the design of visibly-transparent solar windows. Their high efficiency (12.3%) is also a factor. * The possibility of fitting the shape and size of CIS modules to the application requirements by cutting and re-encapsulating the suitably-sized strips out of commercial CIS panels. * A high degree of control over the electric circuit configuration performance in window modules. In addition, the possibility of obtaining a high open-circuit voltage V\({}_{\text{oc}}\) (up to about 50-55 V) without extensive series connections. * Mechanical robustness and reliability--the 2 mm glass substrates are much more robust than mono-Si wafers. * Superior (compared to many other cell types) performance of CIS modules in shading conditions and at elevated cell temperatures. * The efficiency in factory-produced CIS cells has been increasing continuously in recent years. Figure 2 shows the spectral responsivity of Avancis PV modules relative to the crystalline Si cells and their external quantum efficiency (EQE) spectrum. Other spectrally-selective functional materials and glazing system components suitable for preferentially harvesting the non-visible parts of the solar spectrum energy in highly-transparent hybrid-type solar concentrators have been described in significant technical detail in References [7; 21]. For illustration and schematic purposes, Figure 3 outlines the main optical properties of glazing system components (the data of Figure 3 is reproduced from Reference [7]). The term \"hybrid-type solar concentrator\" refers to using simultaneously several types of physical mechanisms (e.g., luminescence, diffractive deflection, total internal reflection, and even scattering as a short-range light-trapping mechanism) to promote the spectrally-selective concentration of light in near-edge regions of glazing-system panels.