A Fast-Response Vertical-Alignment In-Plane-Switching-Mode Liquid Crystal Display

Abstract

Fast-response liquid crystal (LC) displays have attracted attention for use as gaming displays with high frame rate and field-sequential displays. This work presents an LC display mode with vertical alignment and horizontal electric field driving, which achieves millisecond-scale response time. The proposed LC display mode may achieve an average grayscale-to-grayscale response time of 1.42 ms using low-rotational-viscosity LC material, optimized device architecture, and overdrive, offering a potential application for gaming displays and color-sequential displays.

1. Introduction

Liquid crystal displays (LCDs) are the preeminent display solutions in the current market, with ubiquitous applications across multifarious sectors. Given the advancements that have been made to improve their contrast ratio, viewing angle, and color accuracy, slow response time is one of the most notable issues for LCD development [1].

For material-based approaches to improve response time, nematic liquid crystals (NLCs) with low rotational viscosity and high dielectric anisotropy could improve response time [2]. However, there is a fundamental trade-off between viscosity, birefringence, and temperature stability [3]. Blue phase liquid crystals (BPLCs) exhibit exceptionally fast response, attributed to their unique isotropic-to-anisotropic electro-optic switching mechanism and alignment-free, self-assembled cubic nanostructures that eliminate slow surface anchoring dynamics [4]. However, BPLCs require high operating voltages due to their Kerr effect-dominated response, and they suffer from noticeable hysteresis in grayscale transitions as well as weak temperature stability [5]. Ferroelectric liquid crystals (FLCs) excel in ultimate switching speed, and their grayscale and stability issues in practical implementation could be solved by the advantages of using controllable anchoring energy, where photoalignment provides FLC samples with uniform alignment and a high contrast ratio [6]. Nevertheless, FLCs also face challenges related to manufacturability, making them difficult to scale up for commercially viable mass production [7].

For device structure and mode innovation, in-plane switching (IPS) mode is renowned for its excellent wide viewing angles and stable color reproduction, yet it typically suffers from relatively low transmittance and a higher operating voltage [8]. The Fringe-Field Switching (FFS) mode, as an evolution of IPS, achieves higher transmittance and better image quality under low-voltage driving by utilizing both horizontal and vertical fringe fields, but it may still face challenges in achieving ultra-fast response times required for high-refresh-rate applications [9]. Meanwhile, the Polymer-Stabilized Vertical Alignment (PS-VA) mode provides superior static contrast ratios and fast response, particularly in terms of the fall time, but the introduction of a polymer network can lead to increased driving voltage and potential long-term reliability concerns [10]. Other structural solutions have also been proposed, such as using alternating (or multi-) rubbing directions to alter the virtual wall pitch length, and hence also alter the response speed and transmission of fast-response LC devices [11]. Some studies have proposed new designs using a three-electrode design while maintaining the fast response speed of these devices [12]. Optically Compensated Bend (OCB) mode is designed in a pre-tilted bend state which allows the LC molecules to switch primarily through a bend deformation and achieves fast response [13]. However, this mode, which is sensitive to cell gap variations, requires a high bias voltage to induce the bend state and temperature-dependent optical compensation films to maintain the contrast ratio, resulting in an increase in the cost [14]. The overdrive technique accelerates switching speed by inputting a time-dependent waveform that initially exceeds the target voltage [15]. Alternative electro-optic modes can be employed to improve the response time, for instance, black insertion [16], blinking backlights [17], double frame rate [18] and motion-compensated inverse filtering [19].

Despite these advancements [20], there remains a persistent demand for an LC mode that can achieve ultra-fast response [21]. To solve these problems, a vertical alignment in-plane switching (V-IPS) mode [22] with low-rotational-viscosity positive LC materials is demonstrated which may achieve a fast response time that is reduced to the 1 ms level, offering potential applications for gaming displays and color-sequential displays.

2. Materials and Methods

The response time of an LC device is fundamentally governed by the electro-optic dynamics of LC molecule reorientation. The interdigitated pixels and common electrodes generate a predominantly horizontal electric field, inducing a torque that rotates the LC directors azimuthally. This specific reorientation mechanism is central to its optical modulation and determines its temporal characteristics. This transient behavior is classically described by a balance of viscous and elastic torques under the influence of an electric field, often approximated by the following simplified equation for the total response time, where γ₁ is the rotational viscosity of the LC material, K is the effective elastic constant, and d is the cell gap. According to the physical model of the response time shown in Equation (1), the intrinsic response time is proportional to γ₁/K and the square of d. In a real situation, the anchoring energy plays an important role in affecting the LC response time. Within a weak anchoring regime, the LC decay time is inversely proportional to the anchoring energy. For instance, if we can find a polyimide that has a somewhat larger anchoring with the LC material, then the decay time can be improved [23]. Strong anchoring facilitates faster relaxation of the LC device and effectively reduces the decay time, while on the other hand, it may necessitate an increased driving voltage.

$${\tau }_{\mathrm{fall}}={\displaystyle \frac{{\gamma }_1{\mathrm{d}}^2}{{\pi }^2\mathrm{K}}}$$

(1)

Due to the relatively small twist elastic constant (K22) of the LC materials, the IPS mode always shows a slower response time compared with the vertical alignment (VA) mode. To improve the response time, the V-IPS mode is proposed, which involves the vertical alignment of positive LC molecules with a horizontal electric field. Owing to its out-of-plane-to-in-plane reorientation mechanism, the V-IPS mode could achieve a fast response time because of the relatively large splay elastic constant, K11, and bend elastic constant, K33. The application of a horizontal electric field could effectively drive the reorientation of positive LC molecules. A schematic cross-section of the V-IPS device architecture is presented in Figure 1.

3. Simulation

The response time and transmittance of our V-IPS device was simulated with the DIMOS.2D (Version 3.0.0, AUTRONIC-MELCHERS) software. The LC cell comprises the following layers in sequence from top to bottom: glass substrate, Si₃N₄ passivation layer, vertical alignment layer, LC layer, vertical alignment layer, ITO electrode layer, and glass substrate. Considering the mass production process capability, optical transmittance, and fast response of the device, the cell gap is set as 3 μm, and the width and spacing of the interdigitated electrodes are also set as 3 μm. Homeotropic boundary conditions with a pretilt angle of 88 degrees were applied at both substrates. A schematic cross-section of the V-IPS device architecture and the transmittance curve in simulation is presented in Figure 2, where the pixel electrodes are highlighted in red, and the common electrodes are highlighted in blue. The material parameters of the LC used in the simulation are specified in

Table 1. DIMOS.2D simulation parameters.

Δn (589 nm, 25 °C)	0.142
ne (589 nm, 25 °C)	1.640
no (589 nm, 25 °C)	1.498
Δε (1 kHz, 25 °C)	4.2
ε∥ (1 kHz, 25 °C)	7.0
ε⊥ (1 kHz, 25 °C)	2.8
K11 (25 °C)	13.6
K22 (25 °C)	8.2
K33 (25 °C)	14.2
γ1 (25 °C)	46

.

The voltage–transmittance (V-T) curves and response times of the V-IPS and conventional IPS modes were compared using LC devices with equivalent structural parameters. To control variables, the material properties detailed in Table 1 were maintained consistently across the V-IPS and IPS mode devices. Figure 3 depicts the normalized V-T curve of IPS and V-IPS cells with the width and spacing of the interdigitated electrodes of 3 μm. The on-state voltage (Von) of the IPS and V-IPS modes is 5.0 V and 6.2 V, respectively. The relatively high Von of the V-IPS mode can be attributed to the vertical alignment of the LC molecules. According to the V-T curve of the IPS and V-IPS modes, the response waveforms of the switch-on and switch-off processes of the two modes are simulated. As shown in Figure 4, the rising times of the V-IPS and IPS modes are 2.9 ms and 4.1 ms, respectively, and the falling times of the V-IPS and IPS modes are 1.7 ms and 7.8 ms, respectively. It can be seen that both the rising and falling times of the V-IPS mode show much improvement compared with those of the IPS mode. The transmittance of the V-IPS mode is similar to that of the IPS mode with the same structural parameters (22.9% and 23.9%).

To investigate the influence of interelectrode spacing on the transmittance and response time of the V-IPS mode, the electrode spacing was set from 5 μm to 3 μm with the fixed electrode width 3 μm, in accordance with current manufacturing constraints, as shown in

Table 2. Impact of electrode spacing variations on the response time and transmittance.

	V-IPS (G = 5 μm)	V-IPS (G = 4 μm)	V-IPS (G = 3 μm)	IPS (G = 3 μm)
Rising time	5.1 ms	3.9 ms	2.9 ms	4.1 ms
Falling time	2.4 ms	2.0 ms	1.7 ms	7.8 ms
Transmittance	23.3%	23.1%	22.9%	23.9%

. The simulation results show that the rising time decreased from 5.1 ms to 2.9 ms, and the falling time decreased from 2.4 ms to 1.7 ms, when the spacing of the interdigitated electrodes was decreased from 5 μm to 3 μm. Meanwhile, the transmittance results under different electrode spacings were similar, only decreasing from 23.3% to 22.9%. The simulation results showed that short electrode spacing improved the response time significantly and almost retained the same transmittance. Therefore, the optimized electrode spacing was set as 3 μm with an electrode width of 3 μm.

To investigate the influence of pretilt angles on the response time and transmittance of the V-IPS mode, we simulated results for varying pretilt angles while keeping the device structure and applied voltage constant, as shown in

Table 3. Impact of pretilt angle variations on the response time and transmittance.

Pretilt Angles	90°	89°	88°	87°	86°	85°
Rising time	3.3 ms	3.0 ms	2.9 ms	2.9 ms	2.9 ms	2.9 ms
Falling time	1.8 ms	1.7 ms	1.7 ms	1.7 ms	1.7 ms	1.7 ms
Transmittance	22.9%	22.9%	22.9%	22.9%	22.8%	22.8%

. The simulation results show that the response time reduced from 3.3 ms to 2.9 ms when the pretilt angle was decreased from 90° to 88°. As the pretilt angle continued to decrease, the response time showed no variation. Meanwhile, the transmittance was similar under different pretilt angles, only decreasing from 22.9% to 22.8%. The simulation results showed that decreasing the pretilt angle improved the response time and almost retained the same transmittance. Therefore, the optimized pretilt angle was set as 88°.

4. Experimental Results and Analysis

According to the simulation results, V-IPS mode cells with a cell gap of 3 μm, electrode width of 3 μm, and electrode spacing of 3 μm were fabricated. LC material, the physical parameters of which are illustrated in Table 1, was inserted into the cells. Figure 5 depicts the measured normalized V-T curve of the V-IPS cells. Figure 6 shows the measured response time of the V-IPS cells and their on/off states; the rising time is about 1.7 ms and the falling time is about 1.8 ms. A comparison with the simulation results reveals that while the fall time is in close agreement, the rise time demonstrates a more favorable outcome than initially anticipated.

With the aim of comparing the viewing angle of the IPS and V-IPS modes, we measured the relative contrast ratio at ±30° and ±60°, using the on-axis contrast as the 100% reference, as shown in

Table 4. Impact of viewing angle of the IPS and V-IPS modes in the horizontal direction.

Viewing Angle	0°	+30°	+60°	−30°	−60°
IPS	100%	≈75%	≈30%	≈75%	≈30%
V-IPS	100%	≈40%	≈10%	≈40%	≈10%

and

Table 5. Impact of viewing angle of the IPS and V-IPS modes in the vertical direction.

Viewing Angle	0°	+30°	+60°	−30°	−60°
IPS	100%	≈70%	≈20%	≈70%	≈20%
V-IPS	100%	≈25%	≈10%	≈25%	≈10%

. The V-IPS mode has a narrower viewing angle than its IPS counterpart, which stems from the constraints imposed by the vertical alignment of the LC molecules.

Grayscale-to-grayscale (GTG) response time quantifies the switching speed between any two intermediate gray levels, which provides a more accurate characterization of the visual presentation quality. It is defined as the mean transition time between the specified initial and final gray levels. A total of 255 grayscale levels were divided into nine segments for GTG response time measurement in order to avoid excessive measurement volume resulting from overly fine partitioning. The corresponding measured voltages of grayscale are presented in

Table 6. Measurement of grayscale and corresponding voltages.

Grayscale	Voltage (V)
0	0
31	1.8
63	2.3
95	2.7
127	3.1
159	3.5
191	4
223	4.6
255	6.2

.

$${\mathrm{t}}_{\mathrm{a}\mathrm{v}\mathrm{g}}={\displaystyle \frac{{\mathrm{t}}_{0-31}+{\mathrm{t}}_{31-0}+{\mathrm{t}}_{0-63}+{\mathrm{t}}_{63-0}+\cdots +{\mathrm{t}}_{223-255}+{\mathrm{t}}_{255-223}}{72}}$$

(2)

The measurement results of GTG response time between different grayscale levels are presented in

Table 7. Measurement of GTG response characteristics of the V-IPS cell.

	Start Voltage (V)
End voltage (V)		0	1.8	2.3	2.7	3.1	3.5	4	4.6	6.2
	0	N/A	1.8 ms	2.8 ms	1.6 ms	1.4 ms	1.7 ms	1.9 ms	1.5 ms	1.8 ms
	1.8	3.6 ms	N/A	2.6 ms	1.4 ms	1.5 ms	1.4 ms	1.8 ms	1.7 ms	1.8 ms
	2.3	3.2 ms	2.7 ms	N/A	2.3 ms	1.9 ms	1.1 ms	1.4 ms	1.6 ms	1.7 ms
	2.7	4.5 ms	4.2 ms	3.8 ms	N/A	2.1 ms	1.9 ms	1.2 ms	1.4 ms	2.0 ms
	3.1	4.2 ms	4 ms	4.2 ms	4 ms	N/A	2.6 ms	1.8 ms	1.3 ms	1.9 ms
	3.5	3.3 ms	3.8 ms	4.1 ms	3.3 ms	2.2 ms	N/A	1.4 ms	1.2 ms	2.3 ms
	4	4.1 ms	4.2 ms	4.2 ms	2.5 ms	2 ms	2.4 ms	N/A	1.8 ms	2 ms
	4.6	3.6 ms	3.1 ms	3.8 ms	2.1 ms	2.1 ms	2.1 ms	2.6 ms	N/A	2.1 ms
	6.2	1.7 ms	1.8 ms	1.9 ms	1.6 ms	1.4 ms	1.9 ms	1.6 ms	1.2 ms	N/A

, with the average GTG response time numerator being the sum of these values. The average GTG response time was 2.37 ms. The average GTG rising time is 2.96 ms, whereas the average GTG falling time is only 1.84 ms. With an increase in the difference between the starting and ending voltages, the rising time increased and then decreased. This phenomenon arises from the distinct effects of voltage on molecular dynamics. In the low-voltage regime, the increase in the steady-state tilt angle outpaces the acceleration of molecular rotation, causing the response time to rise. Conversely, beyond a certain voltage threshold, the rotational acceleration dominates, leading to a net reduction in response time. From the measurement results, we can see that most of the GTG falling time values are less than 2 ms, but the GTG rising time, especially between low gray levels, are relatively slow. Switching on is voltage-driven, but switching off is achieved by a relaxation process. The relatively fast intrinsic response time is a direct consequence of employing the vertically aligned mode together with the LC material whose parameters are listed in Table 1. Therefore, we introduced driving optimization to improve the GTG rising time.

Overdrive is an effective technique for optimizing the rising edge response time of the device, which solves the limitations of materials by manipulating the driving waveform. Based on the varying response time in the rising edge region, the magnitude of the overdrive voltage is a critical factor. If the overdrive voltage is too low, the response time can not be decreased significantly, and if the overdrive voltage is too high, a peaking waveform will appear. To improve the GTG rising time effectively, the corresponding overdrive voltage was optimized for every grayscale, as presented in

Table 8. Measurement of grayscale and corresponding overdrive voltage.

Grayscale	Voltage (V)	Overdrive Voltage (V)
0	0	0
31	1.8	2.5
63	2.3	3
95	2.7	3.5
127	3.1	3.5
159	3.5	4
191	4	4.5
223	4.6	5
255	6.2	6.5

. For each GTG voltage level, three independent measurements were conducted, and the average value was calculated to mitigate the influence of anomalous data points.

Comparing the measurement results in Table 8 and

Table 9. Measurement of GTG response characteristics of V-IPS cell with overdrive.

	Start Voltage (V)
		0	1.8	2.3	2.7	3.1	3.5	4	4.6	6.2
End voltage (V)	0	N/A	1.8 ms	2.8 ms	1.6 ms	1.4 ms	1.7 ms	1.9 ms	1.5 ms	1.8 ms
	1.8	0.9 ms	N/A	2.6 ms	1.4 ms	1.5 ms	1.4 ms	1.8 ms	1.7 ms	1.8 ms
	2.3	0.9 ms	0.8 ms	N/A	2.3 ms	1.9 ms	1.1 ms	1.4 ms	1.6 ms	1.7 ms
	2.7	1 ms	1 ms	0.8 ms	N/A	2.1 ms	1.9 ms	1.2 ms	1.4 ms	2.0 ms
	3.1	1 ms	1.1 ms	0.9 ms	0.6 ms	N/A	2.6 ms	1.8 ms	1.3 ms	1.9 ms
	3.5	1.1 ms	1.2 ms	0.9 ms	0.8 ms	0.7 ms	N/A	1.4 ms	1.2 ms	2.3 ms
	4	1 ms	0.9 ms	1 ms	0.9 ms	0.8 ms	0.7 ms	N/A	1.8 ms	2 ms
	4.6	1.2 ms	1.1 ms	1.1 ms	1.2 ms	0.9 ms	0.8 ms	0.6 ms	N/A	2.1 ms
	6.2	0.9 ms	1.3 ms	0.8 ms	1.2 ms	1.2 ms	1.5 ms	1.6 ms	1.2 ms	N/A

, it can be seen that the average GTG rising time is decreased from 2.96 ms to 0.96 ms, and the average GTG response time is decreased from 2.37 ms to 1.42 ms. The overdrive technique improved the GTG response time significantly. Finally the V-IPS-mode cell with an optimized electrode structure achieved a fast response time, GTG 1.42 ms, using low-rotational-viscosity LC materials and the overdrive technique.

5. Conclusions

A vertical-alignment LC device with in-plane electric field driving is demonstrated to achieve a fast response time. With low-rotational-viscosity LC materials, optimized structure, and overdrive technique, the proposed device achieved an average GTG response time of 1.42 ms while maintaining high transmittance, offering a potential application for gaming displays and color-sequential displays.