# Dual Thermal-/Electrical-Responsive Luminescent 'Smart' Window
## 3 Results
Figure 2: Schematic (**top row**) and photographs (**bottom row**) of liquid crystal and dye alignments in the initial planar state (**middle column**) and its changes upon the application of an electrical field (**right column**) or increased temperature (**left column**). The relative absorption characteristics are indicated by the rainbow-colored arrows. The external power efficiency (\(\eta_{ext}\), energy emitted at the edges of the device as a function of the energy incident on the device) and the internal optical efficiency (\(\eta_{int}\), photons emitted at the edges of the device per photons absorbed by the device) [28] of the systems at the various temperatures and applied potentials were calculated using  
\[\eta_{ext}=\frac{\text{energy emitted}}{\text{incident energy}}=\frac{\int_{\text{emission}}P_{\text{out}}(\lambda_{\text{em}}) \text{d}\lambda_{\text{em}}}{\int_{\text{sol}}P_{\text{in}}\ \text{d}\lambda_{sol}}*100\% \tag{2}\]  
and  
\[\eta_{int}=\frac{\text{photons emitted}}{\text{photons absorbed}}=\frac{\int_{ \text{emission}}P_{\text{out}}(\lambda_{\text{em}})\frac{\lambda_{\text{em}}} {hc}\text{d}\lambda_{\text{em}}}{\int_{\text{absorption}}P_{\text{in}}(\lambda_{ \text{abs}})\frac{\lambda_{\text{abs}}}{hc}A(\lambda)\ \text{d}\lambda_{\text{abs}}}*100\% \tag{3}\]  
where \(P_{out}(\lambda_{em})\) is the integrated emission from all four edges over the wavelength range (in Watt). Both one parallel and one perpendicular edge were measured and multiplied by 2 to approximate the total output. \(P_{in}\) is the incident light power from the solar simulator (in Watt), \(A(\lambda)\) is the absorption of the sample (in %), \(\lambda\) is the wavelength range over which the summation is made, \(h\) is Planck's constant, and \(c\) the speed of light. The absorption of Dye 1 was integrated over the wavelength range 350-500 nm and the emission over 450-700 nm, while for Dye 2 these values were 450-650 nm and 550-800 nm, respectively. The incoming solar simulator light was integrated over the wavelength range of 350-800 nm. The external power efficiency decreases at higher applied voltages (see Figure 4a,b) as the incident light remains constant, but the emitted light decreases due to the reduced absorption of light. The internal optical efficiency increases at higher applied voltages (see Figure 4c,d) as, despite the decrease in absorption, the light that is absorbed is emitted much more effectively towards the edges of the system due to the favorable dye alignment [23]. In many cases, it might be desirable for the'smart' window to be in the most transparent state, and only upon increasing temperature or application of voltage should it become more absorbing. Such a device was fabricated by replacing the planar with a homeotropic alignment layer and switching from a fully ITO-coated glass substrate to an IDE-coated substrate. In this latter device, the thermal response is unchanged, while the LC and dye are switched from a homeotropic (transparent) alignment to a planar (absorbing) alignment by application of a voltage, as depicted in Figure 5. The reorientation from homeotropic to planar was confirmed by POM (Figure S2), where the initial dark images become bright upon application of a voltage until the isotropic state is reached and the image stays dark. The transmissions at the absorption peaks of the dyes of this modified homeotropic device are shown in Figure 6. As mentioned earlier, the effect of heating are very similar to the planar device: that is, at higher temperatures the transmission decreases and the voltage required to switch the system decreases. The response to changes in potential is the opposite, however: at higher voltages, the absorption increases. The relative difference in transmission upon application of an electrical field is smaller compared to the planar ITO-coated system, most likely as the electrical field between the electrodes is less homogeneous, resulting in less uniform LC alignments.