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We propose an ultrafast nematic liquid crystal (LC) device without alignment layers, where both the dark and bright states can be realized by applying an electric field. A vertical electric field is applied to vertically align the LCs for the dark state, whereas an in-plane electric field is applied to homogeneously align the LCs for the bright state. We achieved a total response time of less than 3 ms in the proposed device. This device may contribute, not only to a significant improvement of the switching speed in liquid crystal devices, but also to the simplification of the device fabrication by the omission of the alignment layer coating and the rubbing process.
©2012 Optical Society of America
1. Introduction
As the performance of liquid crystal displays (LCDs) has improved, LCDs have become widely being used in small and large displays for applications such as smart phones, monitors, tablet PCs, TVs, etc. LCDs show high performance in regards to the contrast ratio, viewing angle, and have a low power consumption [1, 2]. However, LCDs still suffer from a slow response time. A rapid response time is needed in order to reduce motion blur, obtain a good field sequential color, and improve the low-temperature operation. In particular, as the market for three-dimensional (3D) displays has been rapidly expanding worldwide, a fast response time is considered to be a very important factor in LCDs because the slow response time of liquid crystals (LCs) is the main cause for the crosstalk problem in 3D displays. Many different technologies have been proposed in attempts to improve the response time. Most of these efforts either use a thin cell gap [3], overdrive schemes [4, 5], optimization of the LC materials [6], or the use of new switching modes [7]. Another approach used to improve the switching speed is to incorporate nematic LCs into the polymer matrices [8, 9]. However, these technologies have only delivered a limited improvement in LC device switching times. Recently, active studies on blue phase LCs with a fast sub-millisecond switching time are in progress [10]. However, display devises using the blue phase LC have yet to be commercialized, because several problems still remain, such as low transmittance, a narrow operating temperature range, and a high driving voltage of over 50 V for example.
In one of our previous works we proposed a homogeneous-aligned 3-terminal (3T) electrode structure used to reduce the response time of a LC cell [11]. By employing the proposed switching method, an accelerated turn-on time of 7.4 ms was observed. Notably, a very fast turn-off time of 600 μs has been observed due to the vertical electric field enforcing that the LCs align vertically. A fast response time was achieved with no decrease of the transmittance. However, the turn-on time still needed to be improved.
In this paper we present a fast-switching 3T nematic LC device that does not require alignment layers. An electric field is applied to the randomly-aligned LCs for both the bright and dark states. A vertical electric field is applied to vertically align the LCs for the dark state, whereas an in-plane electric field is applied to homogeneously align the LCs for the bright state. We achieved a total response time of less than 3 ms in the proposed device.
2. Proposed structure for fast switching
Figure 1 shows the operational principle of the proposed LC device without alignment layers. A LC cell can be fabricated without alignment layers on the top and bottom substrates so that initially the LCs are randomly aligned. In order to realize the proposed LC device, the cell structure shown in Fig. 2 is employed. The top electrode is not patterned, whereas the bottom electrodes consist of the common and grid electrodes. An in-plane field is applied by the voltage difference between the grid and common electrodes, whereas a vertical field is applied by the voltage difference between the top and the common electrodes. To verify the switching characteristics of the proposed device, a unit cell employing the structure shown in Fig. 2 was fabricated. The optical anisotropy of the LC used in the fabrication was 0.29. The cell thickness was maintained at 3 µm by using silica spacers. For comparison, a cell employing two-terminal (2T) electrodes was also fabricated with the same cell parameters used for the proposed device, except for the absence of the top electrode.
Fig. 2 Structure of the proposed LC cell without alignment layers. An in-plane as well as a vertical electric field can be applied by using 3T electrodes.
3. Experiment results and discussion
Figure 3 shows the electro-optical characteristics of a conventional 2T cell with homogeneous alignment layers, a 3T cell with non-rubbed homogeneous alignment layers, and a 3T cell without alignment layers. Figure 3(b) shows the measured optical responses of the three cells. The turn-on time for our purposes is defined as the transient time from 10 to 90% of the maximum transmittance and vice versa for the turn-off time. To measure the response time of the 2T cell, 5.8 V was applied between the grid and the common electrodes for turn-on, and then removed after several seconds. The measured turn-on and turn-off times were 24 and 21 ms, respectively. The 3T cell without alignment layers had an accelerated turn-on time of 1.2 ms at 5 V. Notably, a much faster turn-off time of 500 μs was observed because the vertical field forces the LCs to be aligned vertically. We also measured the response time of the 3T cell with non-rubbed homogeneous alignment layers, which had a turn-on time of 10.3 ms at 6 V. In summary, ultrafast switching of LCs can be obtained by removing the alignment layers, but not by removing the alignment process that is needed to determine the alignment direction of LCs. To analyze the cause of the fast response time, we measured the azimuthal anchoring energy of each cell [12]. Since we cannot measure the anchoring energy of a LC cell in which the LCs are not aligned, we rubbed indium-tin-oxide glass substrates to determine the alignment direction of the LC molecules to measure the anchoring energy of the LC cell without alignment layers. We found that the measured azimuthal anchoring energy 1.0 × 10−6 J/m2 of rubbed LC cell without alignment layers is much lower than 6.9 × 10−5 J/m2 found for a LC cell with alignment layers. We believe that a LC cell without alignment layers has a much faster turn-on time than a LC cell with alignment layers because the former has a lower anchoring energy than the latter
Fig. 3 The electro-optical characteristics of a conventional 2T cell, a 3T cell with non-rubbed homogeneous alignment layers, and a 3T cell without alignment layers, (a) the voltage-transmittance curves, (b) the temporal response.
Although very fast switching was achieved, the transmittance was decreased, as shown in Fig. 3(a). A 3T cell without alignment layers had a maximum transmittance of 14.1% when 3.2 V was applied between the grid and the common electrodes. This was much lower than the maximum transmittance of 29.1% at 5.8 V in a 2T cell with rubbed alignment layers because of the partially tilted LCs in the initial state and the LCs between the grid electrodes. Therefore, for practical applications, we need to improve the maximum transmittance of the 3T cell without alignment layers.
In order to achieve a higher transmittance we introduced an organic-compound and a reactive mesogen (RM) that have an anisotropic molecular shape similar to nematic LCs. The organic-compound used was ‘hexadecyltrimethylammonium bromide (HTAB)'. The organic-compounds mixture used for the LCs had a 0.5 wt% of RM and 0.1 wt% of HTAB. While the mixture was injected into an empty cell, an in-plane field was applied to uniformly align the LCs. The cell was then exposed to a UV light in order to align the LCs in the bulk and surface regions. Figure 4(a) shows the voltage-transmittance curves of an alignment-layer-less 3T cell doped with RM, a non-alignment 3T cell doped with HTAB and RM, and a conventional 2T cell. When only RM was doped into the LCs, the maximum transmittance of the proposed cell was 23.1% at an applied in-plane voltage of 8.5 V. When both HTAB and RM were doped into the LCs, we obtained a maximum transmittance of 28.9% at an applied in-plane voltage of 9 V. It is almost identical to the 29.1% of a conventional 2T cell. Figure 4(b) shows the measured temporal response of a RM-doped 3T cell without alignment layers and an alignment-layer-less 3T cell doped with HTAB and RM. The measured turn-on times of the cells were 2.3 and 2.2 ms, respectively.
Fig. 4 The electro-optical characteristics of a RM-doped 3T cell and a 3T cell doped with HTAB and RM, (a) the voltage-transmittance curves, (b) the temporal response.
In order to check the alignment state, the polarizing microscope images of the fabricated LC cells were observed, as shown in Fig. 5 . The polarizing microscope images of a conventional 2T cell show uniform brightness for each state. However, the 3T cell without alignment layers exhibits texture distortion. The texture distortion in the bright state causes a low transmittance. When RM-mixed LCs or LCs mixed with organic-compounds and RM were used, the texture distortion was reduced. Notably, the texture distortion in the bright state was reduced drastically when both HTAB and RM were doped into the LCs. Therefore, we conclude that HTAB and RM promote LC alignment.
Fig. 5 Polarizing microscopy images of a conventional 2T cell and 3T cells without alignment layers. Each images show the initial, dark, and bright states.
The proposed fast switching LC mode can be applied to obtain a field sequential color (FSC) LCD [13–16]. Since the fast switching characteristics of the LC mode are required to realize a FSC-LCD, the twisted nematic (TN) mode with a low cell-gap [13], the optically compensated bend (OCB) mode [14], the ferroelectric LC (FLC) mode [15], and the blue phase LC mode [10] were employed. However, they all exhibit problems, such as a low yield in the manufacturing process, non-uniform bend transitions, bias and a high driving voltage requirements, difficulties in achieving a high quality alignment without defects, a low transmittance, and a narrow operational temperature range. The fabrication process and the driving scheme for the OCB mode are compatible with conventional thin-film-transistor (TFT) LCD process. However, the OCB mode requires a special driving technique for FSC driving [16], which has impeded its wide use in commercial applications in spite of its various merits. Accordingly, we expect that the proposed device will be provisionally an economic and efficient solution for realizing FSC-LCDs.
We have demonstrated that an ultrafast LC cell can be obtained by removing the alignment layers. The response time of the cell could be further reduced by overdriving the turn-on, as discussed in [4] and [5]. The manufacturing process can be simplified by removing the alignment layer coating and rubbing process.
4. Conclusions
In summary, we have presented an ultrafast 3T LC device without alignment layers. By adding HTAB and RM to LCs, we can obtain a transmittance similar to a conventional 2T cell. The proposed device neither decreases the transmittance nor complicates the fabrication process. The device may contribute not only to a significant improvement of the switching speed but also to the simplification of the fabrication process of liquid crystal devices.
Acknowledgments
This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No.2011-0029198) and by the LCD R&D Center of Samsung Electronics Corporation.
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