II. Exploiting Ternary Blends to Accurately Control the Coloration of Semi-Transparent, Non-
2.3 Results and Discussion
2.3.4 Optical Calculation
Figure 2. 8. Transmission spectra of color tunable (a) thin and (b) thick active layer films.
Figure 2. 9. Calculated color coordinates of color tunable active layer films for (a) T2-ORH and (b) T2-OEHRH on the 2D CIE (1931) chromaticity diagram and corresponding color representations obtained from CIELAB color space coordinates (L*, a*, b*) for (c) T2-ORH and (d) T2-OEHRH-based color tunable active layer films. Inset graphs show magnified 2D CIE chromaticity diagrams in the region through which the color can be controlled.
In order to accurately characterize the range of colors demonstrated by physical blending, the perceived color of light transmitted through the STOPVs was analyzed using commission internationale de l'éclairage (CIE) 1931 color spaces to create 2D (xy) chromaticity diagrams. The transmitted light is represented by the product of a standard daylight (D65) illuminant spectrum and the transmission
b a
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spectrum of each blend film. The corresponding transmission spectra using optimized conditions for each blend (with film thicknesses in the range of 110−140 nm) are shown in Figure 2. 8a. It should be noted that the high transmittance of the films causes calculated chromaticity values to appear close to white (0.33, 0.33) which would correspond to the chromaticity of a completely transparent film. As shown in Figure 2. 9a, b and Table 2. 4, the PTB7-Th:IEICO-4F blend is located at (0.299, 0.348) towards the green region. The PTB7-Th:T2-ORH and :T2-OEHRH blend system showed reddish- purple and reddish color at (0.391, 0.283) and (0.393, 0.318). As the amount of each of the two ternary acceptors increases in the IEICO-4F system, the color coordinates are shifted continuously into to the blue-purple region, which can be quantified as the following translations in CIE space: (0.297, 0.328)
→ (0.290, 0.295) for T2-ORH and (0.285, 0.316) → (0.303, 0.305) for T2-OEHRH, respectively.
Table 2. 4. Color coordinates (x and y), CIELAB (L*, a* and b*) color space coordinates, and corresponding RGB coordinates of color tunable thin active layer films.
Combination x y L* a* b* RGB Coordinates
IEICO-4F 0.299 0.348 85.0 −16.32 −4.38 (174, 222, 220)
0.90:0.10 0.297 0.328 78.2 −8.57 −10.68 (168, 199, 213)
0.80:0.20 0.290 0.295 65.5 2.27 −20.17 (148, 159, 195)
T2-ORH 0.391 0.283 61.5 42.21 −9.21 (212, 117, 166)
0.95:0.05 0.285 0.316 74.0 −8.83 −15.87 (151, 188, 211)
0.85:0.15 0.303 0.305 70.1 3.17 −16.09 (166, 170, 201)
T2-OEHRH 0.393 0.318 64.5 29.60 1.68 (206, 136, 155)
To quantitively analyze color of each blend combination, the commission internationale de l'éclairage L* a* b* (CIELAB) color space coordinates (L*, a*, b*) were calculated, which include brightness information (L*) in addition to color information;140 these results are summarized in Table 2. 4. In general, negative a* and b* represented greenish and bluish while positive a* and b* represented reddish and yellowish color. The PTB7-Th:IEICO-4F exhibited both negative a* and b* value (−16.32 and
−4.38) but a more negative shift was observed in a*, which indicate a greenish hue. The 0.90:0.10 and 0.95:0.05 mixtures clearly showed more negative values of b* (−10.68 and −15.87) than a* (−8.57 and
−8.83), which represented more bluish colors. On the other hand, both 0.80:0.20 and 0.85:0.15 exhibited positive a* (2.27 and 3.17) and more negative b* value in 0.80:0.20 (−20.17) was observed, which correspond to indigo and purple colors, which comprise both red and blue primary colors. The red, green and blue (RGB) color model in Figure 2. 9c, d was obtained from the CIELAB color space coordinates, a vivid color change (greenish cyan → blue → purple) is observed as the incorporation of the two acceptors into PTB7-Th:IEICO-4F binary system. T2-ORH and T2-OEHRH exhibited clear reddish characteristics with the large positive a* value (42.21 and 29.60), only difference is T2-ORH
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retained more bluish property due to negative b* value (−9.21) than T2-OEHRH (1.68), resulting in a more reddish RGB color in T2-OEHRH.
Figure 2. 10. Calculated color coordinate of color tunable active layer films for (a) T2-ORH and (b) T2-OEHRH on the 2D CIE chromaticity diagram xy (1931). Corresponding color representation obtained from CIELAB color space coordinates (L*, a*, b*) for (c) T2-ORH and (d) T2-OEHRH-based color tunable thick active layer films.
Table 2. 5. Color coordinates (x and y), CIELAB (L*, a* and b*) color space coordinates, and corresponding RGB coordinates of color tunable thick active layer films.
Combination x y L* a* b* RGB Coordinates
IEICO-4F 0.239 0.304 59.2 −21.40 −21.96 (66, 154, 180)
0.90:0.10 0.250 0.286 53.2 −9.69 −23.77 (84, 133, 168)
0.80:0.20 0.249 0.240 37.0 5.41 −28.56 (74, 86, 133)
T2-ORH 0.430 0.252 43.6 53.74 −10.46 (176, 55, 122)
0.95:0.05 0.248 0.288 53.2 −10.99 −23.40 (81, 134, 167)
0.85:0.15 0.282 0.273 49.5 6.55 −22.59 (113, 115, 156)
T2-OEHRH 0.468 0.277 36.6 46.49 0.69 (150, 46, 87)
To demonstrate more intense colors using the same system of materials, transmission spectra of all the blend films were collected using thick films (~350 nm) (Figure 2. 8b) and the same color coordinate calculations were carried out. In Figure 2. 10a, b, as we expected, the color coordinates were located further away from the coordinates of white, indicating more saturated colors compared to the thin film samples. The PTB7-Th:IEICO-4F system is located at (0.239, 0.304) midway between blue and green
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hues. The PTB7-Th:T2-ORH system exhibits a reddish-purple color at (0.430, 0.252), and PTB7- Th:T2-OEHRH system shifts further toward the red region at (0.468, 0.277). An additional color change for both acceptors is observed in ternary blends. As T2-ORH or T2-OEHRH are mixed into the IEICO- 4F binary blend system, the transmitted colors are shifted towards the blue region; (0.250, 0.286) for 0.90:0.10 and (0.248, 0.288) for 0.95:0.05, respectively. The 0.80:0.20 and 0.85:0.15 blend systems both showed purple colors with coordinates at (0.249, 0.240) and (0.282, 0.273), respectively, indicating that the 0.80:0.20 system is closer to true blue in CIE color space than the 0.85:0.15 blend.
Likewise, the CIELAB color coordinates showed similar color differences for thick films compared to thin films. We calculated a* and b* values of (−21.40 and −21.96) for the thick PTB7-Th:IEICO-4F film, which is somewhat different with the thin film result but consistent with a cyan, or blue-green color. The b* values became more than twice as negative (−23.77 and −23.40) than a* (−9.69 and
−10.99) for the thick 0.90:0.10 and 0.95:0.05 films, which indicate a more intense blue color. In the 0.80:0.20 and 0.85:0.15 blend system, positive a* values increased in magnitude (5.41 and 6.55) and negative b* values became more negative (−28.56 and −22.59) in thick films compared to thin films, indicate more vivid and saturated purple colors. RGB color models were also obtained from the CIELAB color coordinates, as summarized in Figure 2. 10c, d, and RGB color coordinates corresponding to each formulation are summarized in Table 2. 5. A dramatic color change (cyan → blue → purple) is observed after mixing with PTB7-Th:IEICO-4F and T2-ORH or T2-OEHRH, respectively. The coordinates of both T2-ORH and T2-OEHRH were estimated to be 53.74 and 46.49 for a*, higher than the thin film, whereas T2-ORH (−10.46) retained more negative b* than T2-OEHRH (0.69), confirming a more reddish RGB color representation for T2-OEHRH. It is obvious that for both thin and thick films, the color differences between not only PTB7-Th:T2-ORH and :T2-OEHRH but also 0.80:0.20 and 0.85:0.15 systems are consistent with the trends observed in CIE color space calculations. The 0.80:0.20 blend results in a more bluish hue while the color becomes reddish for the T2-OEHRH based system, respectively. The results indicate that a continuous range of colors covering green, blue, purple and reddish hues can be achieved by simply adjusting the proportions of donors and acceptors in the ternary active layer.