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Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows
MOLLIQ-08768; No of Pages 5 Journal of Molecular Liquids xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows Soon-Bum Kwon a,b,⁎, Seong-Jun Lee a, Da-Som Yoon a,b, Hee-Sang Yoo b, Burm-Young Lee b a b
Dept. of Electronic Display Engineering, Hoseo University, Asan, Chungnam 31499, Republic of Korea NDIS corporation, 20 Hoseo-ro, 79beon-gil, Baebang, Asan, Chungnam 31499, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 November 2017 Received in revised form 14 February 2018 Accepted 27 February 2018 Available online xxxx Keywords: Transmittance variable liquid crystal mode Smart window ECB VA Gray off-state
a b s t r a c t For low power consumption smart windows, we developed new ECB and VA mode LC devices with a specific offstate transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The off-state transmittance of the devices could be chosen to be a value suitable for most use time by selecting appropriate retardation values of LC layer and a retarder, which enables considerable reduction of power consumption when applied to devices, for instance, smart windows. The design rules of them were theoretically derived and the electro-optical properties stated above were exposed by corresponding theoretical simulations and experimental measurements. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Active smart windows are capable of controlling the transmission of sunlight, providing many benefits such as energy saving, eye protection, aesthetic value improvement, and privacy protection when used as windows of buildings and vehicles. Various techniques have been developed for the transmittance variable devices; electrochromic, suspended particle device, liquid crystals [1,2]. As for the representative liquid crystal devices, polymer dispersed liquid crystal and guest-host liquid crystal have been developed for the smart windows [3,4]. Conventional liquid crystal modes such as ECB (electrically controlled birefringence), TN (twisted nematic), VA (vertically aligned) and IPS (in-plane switching) have not been studied well for smart windows because the maximum transmittance of them is low due to use of polarizer. These modes have an advantage of providing sufficiently dark state, almost zero transmittance in addition to low driving voltage, around 5 V. Above-mentioned all devices except for electrochromic require some power needed to maintain a certain transmittance between the minimum and maximum transmittances obtainable by applying electric power. Therefore off-state transmittance of them is the minimum or maximum in full transmission variation range. If the specific transmittance in between the minimum and maximum transmittances is required for most use time, commonly happened particularly in ⁎ Corresponding author at: Dept. of Electronic Display Engineering, Hoseo University, Asan, Chungnam 31499, Republic of Korea. E-mail address: [email protected] (S.-B. Kwon).
automobile window application, it is effective for power-saving to show the specific transmittance in voltage off state. For low power consumption smart windows, we developed new ECB and VA mode LC devices with a specific off-state transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The design rules of new modes and electro-optical properties of them are discussed. 2. Design of LC modes with a specific gray off-state 2.1. New ECB mode A geometrical and optical configuration of new ECB mode LC cell is shown in Fig. 1: optic axes depicted in Fig. 1(b) are transmission axes of polarizer and analyzer and slow axis of uniaxial retarder. The voltage dependent transmittance of the cell is given by Eq. (1) [5]. T ðV Þ ¼
1 ΔnðV Þd−Ro sin2 ðπX ðV ÞÞ; X ðV Þ ¼ ; Δnð0Þ ¼ ne −no 2 λ
ð1Þ
where Ro, ne and no are retardation of the retarder, extraordinary and ordinary refractive indices of the LC respectively. In the ECB cell that the initial transmittance in voltage-off state is larger than the minimum transmittance and smaller than the maximum transmittance, a point where V is 0 can be indicated by PS1 in the Fig. 2. That is, X(0) may be greater than 0.5 and less than 1. In this situation, when V begins to increase from 0, X(V) decreases from X(0), so the transmittance gradually increases to reach the maximum transmittance
https://doi.org/10.1016/j.molliq.2018.02.126 0167-7322/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
2
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Fig. 1. Configuration of new ECB cell: (a) cross-sectional view, (b) optic axes.
and then decreases to reach the minimum transmittance. In order to ensure that the ECB cell necessarily reaches the minimum transmittance with a voltage less than maximum applied voltage Vmax, it should appear as indicated by PS2. That is, X(Vmax) must be zero or less than zero. This is because V increases from 0 to reach the maximum value Vmax at the point where the transmittance is minimum, that is, the point where X reaches 0 or exceeds this point. The conditions of X(0) and X(Vmax) can be expressed by the following Eqs. (2) and (3). 1 Δnð0Þd−Ro bX ð0Þ ¼ b1 2 λ X ðV max Þ ¼
ΔnðV max Þd−Ro αΔnð0Þd−Ro ¼ ≤0 λ λ
ð2Þ
ð3Þ
where α is the minimum birefringence ratio of on-state and off-state, usually around 0.2. From the Eqs. (2) and (3), the following Eqs. (4) and (5) are obtained according to the magnitude relation between R0 + λ and R0/α. 1 Ro α λ Ro þ λbðne −no Þd ≤ ; for Ro b 2 1−α α
ð4Þ
1 α λ Ro þ λbðne −no Þd ≤Ro þ λ; for Ro ≥ 2 1−α
ð5Þ
Of course, as can be seen from Fig. 2, X(0) may be larger than 1 and smaller than 1.5, and X(Vmax) may be 0.5 or less. And X(0) may be larger than 1.5 and smaller than 2, and X(Vmax) may be 1 or less. Hence, Eqs. (4) and (5) can be generalized as Eqs. (6) and (7). Ro þ
α mþ1 Ro m m λbðne −no Þd ≤ λ; for Ro b − λ þ 2 1−α 2 α 2α
ð6Þ
Ro þ
α mþ1 mþ2 m λbðne −no Þd ≤Ro þ λ; for Ro ≥ − λ 2 2 1−α 2
ð7Þ
where m is an integer greater than or equal to zero. Eqs. (4) and (5) can be understood as a case where m = 0. By making a ECB cell satisfying the expressions (6) and (7), it is possible to realize a transmittance variable device in which the initial transmittance is larger than the minimum transmittance and smaller than the maximum transmittance and the transmittance can be varied from the minimum transmittance to the maximum transmittance by changing applied voltage. 2.2. New VA mode Fig. 3 shows a geometrical and optical configuration of new VA mode LC cell. Optical configuration is the same as that of ECB cell, so similar equations can be derived. Main difference between two modes comes from a change of optical anisotropy according to applied voltage. Specifically, in the VA mode, Δn(V) has a minimum value (approximately 0) in offstate, increases as V increases, and finally has β(ne-no). That is, the maximum value of the ratio of the optical anisotropy of the LC layer when V is not 0 for ne-no can be defined as β, which is approximately 0.8 when a conventional liquid crystal is used. As Δn(V) increases as V increases, X (V) also increases as V increases. Therefore, X(V) has the minimum value and the maximum value as shown in the following Eq. (8). X ð0Þ ¼ −
Fig. 2. X-T curve of ECB cell written in Eq. (1): PS1 and PS2 indicate initial (V = 0) and final (V = Vmax) points.
Ro βðne −no Þd−Ro ≤X ðV Þ ¼ ¼ X ðV max Þ λ λ
ð8Þ
Fig. 4 is a graph for explaining the condition that the VA cell of Fig. 3 satisfies. Since the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, the point where V is 0 can be indicated by PS1 in the graph of Fig. 4. That is, X (0) may be greater than −0.5 and less than zero. In this situation, when V starts increasing from 0, X(V) increases from X(0), so the transmittance gradually decreases to reach the minimum transmittance and then increases to reach the maximum transmittance. The point for the maximum applied voltage Vmax is indicated by PS2 in the graph of Fig. 4 in order to ensure that the VA cell can reach the maximum transmittance. That is, X(Vmax) must be 0.5 or greater than 0.5. This is because V increases from 0 to reach the maximum value Vmax at the point where
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
3
Fig. 3. Configuration of new VA cell: (a) cross-sectional view, (b) optic axes.
the transmittance is maximum, that is, X reaches 0.5 or exceeds this point. Of course, as can be seen from the graph of Fig. 4, X(0) may be greater than −1 and less than −0.5, and X(Vmax) may be zero or greater than zero. And X(0) may be greater than −1.5 and less than −1, and X (Vmax) may be −0.5 or greater than −0.5. These conditions can be expressed by the following Eqs. (9) and (10). −
mþ1 Ro m bX ð0Þ ¼ − b− 2 2 λ
ð9Þ
X ðV max Þ ¼
βðne −no Þd−Ro m−1 ≥− 2 λ
ð10Þ
where m is an integer greater than or equal to zero. From Eqs. (9) and (10), the following Eqs. (11) and (12) can be obtained. m mþ1 λbRo ≤ λ 2 2 ðne −no Þd≥
ð11Þ
2Ro −ðm−1Þλ 2β
ð12Þ
By satisfying both Eqs. (11) and (12) simultaneously, the initial transmittance in off-state is larger than the minimum transmittance and smaller than the maximum transmittance, and by adjusting the voltage V, it is possible to realize a transmittance variable device in which the transmittance can be varied from the minimum transmittance to the maximum transmittance. Table 1 Design parameters of ECB mode transmittance variable windows.
(ne-no)d (nm) Ro (nm)
#1
#2
#3
#4
#5
440 92.4
500 105.0
560 117.6
620 130.2
680 142.8
Fig. 4. X-T curve of VA cell written in Eq. (1): PS1 and PS2 indicate initial (V = 0) and final (V = Vmax) points.
Fig. 5. Electro-optical properties of ECB cells with the parameters written in Table 1.
Fig. 6. Electro-optical properties of VA cells with the parameters written in Table 2.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
4
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Table 2 Design parameters of VA mode transmittance variable windows.
(ne-no)d (nm) Ro (nm)
#1
#2
#3
#4
649 55
580 110
511 165
443 220
3. Simulation and experimental results To verify the validity of the design rules of transmittance variable windows, we calculated the electro-optical properties for ECB mode and VA mode LC cells with parameters satisfied the design rules by
using a well-known LCD simulator, “Techwiz LCD 1D” (SANAYI System Co.). Fig. 5 is a simulated result showing a change in transmittance according to a voltage in the ECB mode variable transmittance devices of Fig. 1. It shows the electro-optical properties when the value of (neno)d and the value of Ro are set as shown in the following Table 1 while the wavelength λ of the incident light is fixed at 550 nm. As can be seen from Fig. 5, the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, and it can be seen that the transmittance can be changed from the minimum transmittance to the maximum transmittance as V increases.
Fig. 7. V-T curve of an ECB cell and cell pictures for different applied voltages: (a) 0 V, (b) 0.8 V (c) 1.4 V (d) 2.2 V.
Fig. 8. V-T curve of a VA cell and cell pictures for different applied voltages: (a) 0 V, (b) 3.1 V (c) 3.7 V (d) 6.1 V.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Fig. 6 is a simulated result showing a change in transmittance according to a voltage in the VA mode variable transmittance devices of Fig. 3. It shows the electro-optical properties when the value of (neno)d and the value of Ro are set as shown in the following Table 2 while the wavelength λ of the incident light is fixed at 550 nm. As can be seen from Fig. 6, the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, and it can be seen that the transmittance can be changed from the minimum transmittance to the maximum transmittance as V increases. We fabricated an ECB cell satisfying the above-mentioned design rule with the retardations of LC cell and a retarder of 650 nm and 137.5 nm respectively. The voltage-transmittance characteristic of the cell and visual cell images for different applied voltage are shown in Fig. 7. As can be seen from Fig. 7, a specific off-state transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage could be achieved. We fabricated a VA cell satisfying the above-mentioned design rule with the retardations of LC cell and a retarder of 550 nm and 137.5 nm respectively. The voltage-transmittance characteristic of the cell and visual cell images for different applied voltage are shown in Fig. 8. As shown in the Fig. 8, a specific off-state transmittance and an onstate transmittance variable from minimum to maximum values according to applied voltage could be achieved.
5
4. Conclusion We developed new ECB and VA mode LC devices with a specific offstate transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The off-state transmittance of the devices could be chosen to be a value suitable for most use time by selecting appropriate retardation values of LC layer and a retarder, which enables considerable reduction of power consumption when applied to devices, for instance, smart windows. The design rules of them were theoretically derived and the design rules were proved by theoretical simulations and experimental measurements. For more practical usefulness, viewing angle dependent color and transmittance properties would be studied in next work.
References [1] C.M. Lampert, Sol. Energy Mater. Sol. Cells 52 (1998) 207. [2] R. Baetens, B.P. Jelle, A. Gustavsen, Sol. Energy Mater. Sol. Cells 94 (2010) 87. [3] Y.-W. Seo, H.-S. Yoo, Y. Jin, B.-Y. Lee, S. Lu, S.-B. Kwon, Mol. Cryst. Liq. Cryst. 596 (2014) 97. [4] C.v. Oosten, M. Zitto, SID Digest, 2016 376. [5] P. Yeh, C. Gu, Optics of Liquid Crystal Displays, John Wiley & Sons, 1999 215.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows Soon-Bum Kwon a,b,⁎, Seong-Jun Lee a, Da-Som Yoon a,b, Hee-Sang Yoo b, Burm-Young Lee b a b
Dept. of Electronic Display Engineering, Hoseo University, Asan, Chungnam 31499, Republic of Korea NDIS corporation, 20 Hoseo-ro, 79beon-gil, Baebang, Asan, Chungnam 31499, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 November 2017 Received in revised form 14 February 2018 Accepted 27 February 2018 Available online xxxx Keywords: Transmittance variable liquid crystal mode Smart window ECB VA Gray off-state
a b s t r a c t For low power consumption smart windows, we developed new ECB and VA mode LC devices with a specific offstate transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The off-state transmittance of the devices could be chosen to be a value suitable for most use time by selecting appropriate retardation values of LC layer and a retarder, which enables considerable reduction of power consumption when applied to devices, for instance, smart windows. The design rules of them were theoretically derived and the electro-optical properties stated above were exposed by corresponding theoretical simulations and experimental measurements. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Active smart windows are capable of controlling the transmission of sunlight, providing many benefits such as energy saving, eye protection, aesthetic value improvement, and privacy protection when used as windows of buildings and vehicles. Various techniques have been developed for the transmittance variable devices; electrochromic, suspended particle device, liquid crystals [1,2]. As for the representative liquid crystal devices, polymer dispersed liquid crystal and guest-host liquid crystal have been developed for the smart windows [3,4]. Conventional liquid crystal modes such as ECB (electrically controlled birefringence), TN (twisted nematic), VA (vertically aligned) and IPS (in-plane switching) have not been studied well for smart windows because the maximum transmittance of them is low due to use of polarizer. These modes have an advantage of providing sufficiently dark state, almost zero transmittance in addition to low driving voltage, around 5 V. Above-mentioned all devices except for electrochromic require some power needed to maintain a certain transmittance between the minimum and maximum transmittances obtainable by applying electric power. Therefore off-state transmittance of them is the minimum or maximum in full transmission variation range. If the specific transmittance in between the minimum and maximum transmittances is required for most use time, commonly happened particularly in ⁎ Corresponding author at: Dept. of Electronic Display Engineering, Hoseo University, Asan, Chungnam 31499, Republic of Korea. E-mail address: [email protected] (S.-B. Kwon).
automobile window application, it is effective for power-saving to show the specific transmittance in voltage off state. For low power consumption smart windows, we developed new ECB and VA mode LC devices with a specific off-state transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The design rules of new modes and electro-optical properties of them are discussed. 2. Design of LC modes with a specific gray off-state 2.1. New ECB mode A geometrical and optical configuration of new ECB mode LC cell is shown in Fig. 1: optic axes depicted in Fig. 1(b) are transmission axes of polarizer and analyzer and slow axis of uniaxial retarder. The voltage dependent transmittance of the cell is given by Eq. (1) [5]. T ðV Þ ¼
1 ΔnðV Þd−Ro sin2 ðπX ðV ÞÞ; X ðV Þ ¼ ; Δnð0Þ ¼ ne −no 2 λ
ð1Þ
where Ro, ne and no are retardation of the retarder, extraordinary and ordinary refractive indices of the LC respectively. In the ECB cell that the initial transmittance in voltage-off state is larger than the minimum transmittance and smaller than the maximum transmittance, a point where V is 0 can be indicated by PS1 in the Fig. 2. That is, X(0) may be greater than 0.5 and less than 1. In this situation, when V begins to increase from 0, X(V) decreases from X(0), so the transmittance gradually increases to reach the maximum transmittance
https://doi.org/10.1016/j.molliq.2018.02.126 0167-7322/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
2
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Fig. 1. Configuration of new ECB cell: (a) cross-sectional view, (b) optic axes.
and then decreases to reach the minimum transmittance. In order to ensure that the ECB cell necessarily reaches the minimum transmittance with a voltage less than maximum applied voltage Vmax, it should appear as indicated by PS2. That is, X(Vmax) must be zero or less than zero. This is because V increases from 0 to reach the maximum value Vmax at the point where the transmittance is minimum, that is, the point where X reaches 0 or exceeds this point. The conditions of X(0) and X(Vmax) can be expressed by the following Eqs. (2) and (3). 1 Δnð0Þd−Ro bX ð0Þ ¼ b1 2 λ X ðV max Þ ¼
ΔnðV max Þd−Ro αΔnð0Þd−Ro ¼ ≤0 λ λ
ð2Þ
ð3Þ
where α is the minimum birefringence ratio of on-state and off-state, usually around 0.2. From the Eqs. (2) and (3), the following Eqs. (4) and (5) are obtained according to the magnitude relation between R0 + λ and R0/α. 1 Ro α λ Ro þ λbðne −no Þd ≤ ; for Ro b 2 1−α α
ð4Þ
1 α λ Ro þ λbðne −no Þd ≤Ro þ λ; for Ro ≥ 2 1−α
ð5Þ
Of course, as can be seen from Fig. 2, X(0) may be larger than 1 and smaller than 1.5, and X(Vmax) may be 0.5 or less. And X(0) may be larger than 1.5 and smaller than 2, and X(Vmax) may be 1 or less. Hence, Eqs. (4) and (5) can be generalized as Eqs. (6) and (7). Ro þ
α mþ1 Ro m m λbðne −no Þd ≤ λ; for Ro b − λ þ 2 1−α 2 α 2α
ð6Þ
Ro þ
α mþ1 mþ2 m λbðne −no Þd ≤Ro þ λ; for Ro ≥ − λ 2 2 1−α 2
ð7Þ
where m is an integer greater than or equal to zero. Eqs. (4) and (5) can be understood as a case where m = 0. By making a ECB cell satisfying the expressions (6) and (7), it is possible to realize a transmittance variable device in which the initial transmittance is larger than the minimum transmittance and smaller than the maximum transmittance and the transmittance can be varied from the minimum transmittance to the maximum transmittance by changing applied voltage. 2.2. New VA mode Fig. 3 shows a geometrical and optical configuration of new VA mode LC cell. Optical configuration is the same as that of ECB cell, so similar equations can be derived. Main difference between two modes comes from a change of optical anisotropy according to applied voltage. Specifically, in the VA mode, Δn(V) has a minimum value (approximately 0) in offstate, increases as V increases, and finally has β(ne-no). That is, the maximum value of the ratio of the optical anisotropy of the LC layer when V is not 0 for ne-no can be defined as β, which is approximately 0.8 when a conventional liquid crystal is used. As Δn(V) increases as V increases, X (V) also increases as V increases. Therefore, X(V) has the minimum value and the maximum value as shown in the following Eq. (8). X ð0Þ ¼ −
Fig. 2. X-T curve of ECB cell written in Eq. (1): PS1 and PS2 indicate initial (V = 0) and final (V = Vmax) points.
Ro βðne −no Þd−Ro ≤X ðV Þ ¼ ¼ X ðV max Þ λ λ
ð8Þ
Fig. 4 is a graph for explaining the condition that the VA cell of Fig. 3 satisfies. Since the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, the point where V is 0 can be indicated by PS1 in the graph of Fig. 4. That is, X (0) may be greater than −0.5 and less than zero. In this situation, when V starts increasing from 0, X(V) increases from X(0), so the transmittance gradually decreases to reach the minimum transmittance and then increases to reach the maximum transmittance. The point for the maximum applied voltage Vmax is indicated by PS2 in the graph of Fig. 4 in order to ensure that the VA cell can reach the maximum transmittance. That is, X(Vmax) must be 0.5 or greater than 0.5. This is because V increases from 0 to reach the maximum value Vmax at the point where
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
3
Fig. 3. Configuration of new VA cell: (a) cross-sectional view, (b) optic axes.
the transmittance is maximum, that is, X reaches 0.5 or exceeds this point. Of course, as can be seen from the graph of Fig. 4, X(0) may be greater than −1 and less than −0.5, and X(Vmax) may be zero or greater than zero. And X(0) may be greater than −1.5 and less than −1, and X (Vmax) may be −0.5 or greater than −0.5. These conditions can be expressed by the following Eqs. (9) and (10). −
mþ1 Ro m bX ð0Þ ¼ − b− 2 2 λ
ð9Þ
X ðV max Þ ¼
βðne −no Þd−Ro m−1 ≥− 2 λ
ð10Þ
where m is an integer greater than or equal to zero. From Eqs. (9) and (10), the following Eqs. (11) and (12) can be obtained. m mþ1 λbRo ≤ λ 2 2 ðne −no Þd≥
ð11Þ
2Ro −ðm−1Þλ 2β
ð12Þ
By satisfying both Eqs. (11) and (12) simultaneously, the initial transmittance in off-state is larger than the minimum transmittance and smaller than the maximum transmittance, and by adjusting the voltage V, it is possible to realize a transmittance variable device in which the transmittance can be varied from the minimum transmittance to the maximum transmittance. Table 1 Design parameters of ECB mode transmittance variable windows.
(ne-no)d (nm) Ro (nm)
#1
#2
#3
#4
#5
440 92.4
500 105.0
560 117.6
620 130.2
680 142.8
Fig. 4. X-T curve of VA cell written in Eq. (1): PS1 and PS2 indicate initial (V = 0) and final (V = Vmax) points.
Fig. 5. Electro-optical properties of ECB cells with the parameters written in Table 1.
Fig. 6. Electro-optical properties of VA cells with the parameters written in Table 2.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
4
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Table 2 Design parameters of VA mode transmittance variable windows.
(ne-no)d (nm) Ro (nm)
#1
#2
#3
#4
649 55
580 110
511 165
443 220
3. Simulation and experimental results To verify the validity of the design rules of transmittance variable windows, we calculated the electro-optical properties for ECB mode and VA mode LC cells with parameters satisfied the design rules by
using a well-known LCD simulator, “Techwiz LCD 1D” (SANAYI System Co.). Fig. 5 is a simulated result showing a change in transmittance according to a voltage in the ECB mode variable transmittance devices of Fig. 1. It shows the electro-optical properties when the value of (neno)d and the value of Ro are set as shown in the following Table 1 while the wavelength λ of the incident light is fixed at 550 nm. As can be seen from Fig. 5, the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, and it can be seen that the transmittance can be changed from the minimum transmittance to the maximum transmittance as V increases.
Fig. 7. V-T curve of an ECB cell and cell pictures for different applied voltages: (a) 0 V, (b) 0.8 V (c) 1.4 V (d) 2.2 V.
Fig. 8. V-T curve of a VA cell and cell pictures for different applied voltages: (a) 0 V, (b) 3.1 V (c) 3.7 V (d) 6.1 V.
Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126
S.-B. Kwon et al. / Journal of Molecular Liquids xxx (2018) xxx–xxx
Fig. 6 is a simulated result showing a change in transmittance according to a voltage in the VA mode variable transmittance devices of Fig. 3. It shows the electro-optical properties when the value of (neno)d and the value of Ro are set as shown in the following Table 2 while the wavelength λ of the incident light is fixed at 550 nm. As can be seen from Fig. 6, the initial transmittance at V = 0 is larger than the minimum transmittance and smaller than the maximum transmittance, and it can be seen that the transmittance can be changed from the minimum transmittance to the maximum transmittance as V increases. We fabricated an ECB cell satisfying the above-mentioned design rule with the retardations of LC cell and a retarder of 650 nm and 137.5 nm respectively. The voltage-transmittance characteristic of the cell and visual cell images for different applied voltage are shown in Fig. 7. As can be seen from Fig. 7, a specific off-state transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage could be achieved. We fabricated a VA cell satisfying the above-mentioned design rule with the retardations of LC cell and a retarder of 550 nm and 137.5 nm respectively. The voltage-transmittance characteristic of the cell and visual cell images for different applied voltage are shown in Fig. 8. As shown in the Fig. 8, a specific off-state transmittance and an onstate transmittance variable from minimum to maximum values according to applied voltage could be achieved.
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4. Conclusion We developed new ECB and VA mode LC devices with a specific offstate transmittance and an on-state transmittance variable from minimum to maximum values according to applied voltage. The off-state transmittance of the devices could be chosen to be a value suitable for most use time by selecting appropriate retardation values of LC layer and a retarder, which enables considerable reduction of power consumption when applied to devices, for instance, smart windows. The design rules of them were theoretically derived and the design rules were proved by theoretical simulations and experimental measurements. For more practical usefulness, viewing angle dependent color and transmittance properties would be studied in next work.
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Please cite this article as: S.-B. Kwon, et al., Transmittance variable liquid crystal modes with a specific gray off-state for low power consumption smart windows, J. Mol. Liq. (2018), https://doi.org/10.1016/j.molliq.2018.02.126