4.2 Tunable Color Filters as Electrochromic Windows
4.2.5 Parametric Analysis
4.2 Tunable Color Filters as Electrochromic Windows
Here,φ3andφ2denote the phase shift upon reflection from the bottom and top Ag layers. The accumulated round trip phase (φ3 +φ2) upon reflection from the bottom and top Ag layers is shown in Fig. 4.11(c). When the accumulated round trip phase becomes zero, maximum transmission (Tmax) is allowed at the resonant wavelength λ0 =550 nm [165].
A comparison among FEM, TMM, and TLM is shown in Fig. 4.11(c). An excel- lent agreement between simulation (FEM) and theory (TMM) is seen for the entire visible spectral regime. TLM allows us to accurately predict the resonant wavelength λ0 =550 nm when the accumulated round trip phase becomes zero. Hence, a perfect match between simulation and theory, indeed, validates our findings. Note that the theoretical derivations shown in this paper can be modified to find the optical response across any thin-film multilayered system for normal as well as the oblique angle of in- cidence. From the next subsection, we will use TMM and FEM to compare theoretical and simulation results.
Additionally, a gradual decrease in the reflection bandwidth is also observed with an increase in top Ag layer thickness. This can be explained using Fabry–Perot cavity the- ory, where an increase in lossy Ag layer thickness increases the quality factor of the reflection spectra and hence, makes the spectral bandwidth narrower [131]. For color filtering application, a narrowband design is considered more suitable for precise color filtering capability, and hence, 40 nm thickness is taken as the optimum value for the top Ag layer thickness to achieve perfect absorption with a very narrow bandwidth of 12 nm.
Next, we vary the thickness of the DAST layer from 40 nm to 120 nm, in steps of 10 nm, as shown in Fig. 4.12(b). We observe a linear trend in the redshift of the resonance wavelength with an increase in DAST layer thickness. The reason can be explained by considering a thicker dielectric cavity, where a longer wavelength mode
Figure 4.12: Contour colour plot for parameter optimization of absorption-mode color filter for varying thickness of (a) top silver (Ag), (b) DAST, and (c) bottom Ag layers; (d) Effect of applied voltage across the DAST layer, and reflectance spectra obtained using (e) gold (Au) / Ag as the top metal, and (f) aluminium (Al) / platinum (Pt) / molybdenum (Mo) / Ag as the bottom metal.
4.2 Tunable Color Filters as Electrochromic Windows
is supported by the medium [166]. We can see that by varying the thickness of DAST layer from 40 nm to 120 nm, the entire visible spectral range can be covered, and hence, any color filter can be designed as per demand. For example, for the design of a green color filter at 550 nm wavelength, 80 nm DAST layer thickness is the optimum value, shown by dotted light green cross line in Fig. 4.12(b).
Thereafter we vary the bottom Ag layer thickness from 10 nm to 100 nm. Similar to the case for top Ag layer, we observe a slight blueshift and a gradually increasing dip in the reflectance spectra at the resonance wavelength, with an increase in the bot- tom Ag layer thickness. The reflectance reaches near-zero value at 60 nm thickness, shown by dotted light green cross line in Fig. 4.12(c), and then the reflectance gradu- ally becomes constant. Hence, for designing an absorption-mode color filter, minimum recommended bottom Ag layer thickness is 60 nm. For completely blocking the trans- mission, we consider bottom Ag layer thickness as 125 nm.
We then study the effect of the applied voltage across the DAST layer, as shown in Fig. 4.12(d). DAST, an EO polymer, having tunability near-visible regime and pos- sessing a large EO coefficient (3.41 nm/V), is desirable to attain maximum tunability with minimum bias voltage. The refractive index of DAST as a function of the applied voltage is given by Eq. (4.3) [165]. By varying the applied potential, over a range of voltage, from−15 V to +15 V, in steps of 3 V, we observe a prominent redshift with a linear trend, as shown in Fig. 4.12(d). We found that by using just±10 V power supply, all types of color filters (blue/green/red) can be realized. When no power is supplied, by default the device acts as a green filter, shown by dotted light green cross line in Fig. 4.12(d).
Finally, we explored different materials to find a suitable choice for the top and the bottom metallic layers. For the case of top metallic layer, we found that only gold (Au) can be used as a possible alternative for Ag. However, the reflectance spectra obtained
using Au is poor compared to Ag, as shown in Fig. 4.12(e). Moreover, Au is much more expensive and gives a very broad bandwidth, hence, not a suitable choice for low-cost and high quality-factor absorption-mode color filter design. For the case of bottom metallic layer, we found many cheaper alternatives to Ag, such as aluminium (Al), platinum (Pt), and molybdenum (Mo). As can be seen in Fig. 4.12(f), Ag outperforms all the metals, in terms of high-quality factor obtained, followed by Mo, Pt, and Al.
Nevertheless, such materials can still be used as a cheaper alternative to Ag to reduce the overall cost of our design, since a relatively thick bottom metallic layer is used for the design of absorption-mode color filter.