41.4 H. Zhang 41.4 Influence of Ion Transport in AFLCDs

• SID 00 DIGEST 1000 41.4: Influence of Ion Transport in AFLCDsH. ZhangAlcatel Bell N.V., Francis Wellesplein 1, B-2018 Antwerp, BelgiumK. D’havé, H. PauwelsDept. of Electronics & Information Systems, Ghent University, St-Pietersnieuwstraat 41, B-9000 Gent, BelgiumD. D. Parghi, G. HeppkeAnisotropic Fluids Research Group, Iwan-N.-Stranski Institute of Physical & Theoretical Chemistry,Sekr. ER11, Technical University of Berlin, Str. des 17 Juni 135, D-10623 Berlin, GermanyAbstractIn this article we present measurement results of greylevel hysteresis in an AFLC mixture in test cells with different alignment layers at various temperatures. Ion content has also been measured in the AFLC mixture in order to study the relationship between ionic contamination and the image sticking phenomenon.1. IntroductionWithin recent years antiferroelectric liquid crystal displays (AFLCDs) have been investigated as an alternative to the conventional nematic liquid crystal displays (NLCDs). AFLCDs offer some advantages, including fast switching speeds, wide viewing angles, double hysteresis (which allows passive matrix addressing) and the possibility for analogue greyscale [1]. Nevertheless, there generally exists ionic contamination in almost all LCDs which influences electrooptical performance. The influence of ion transport in nematic and ferroelectric liquid crystals has been studied extensively [2,3,4], while the influence of ionic contamination in AFLCs has, however, not been considered to be of importance due principally to the symmetric addressing schemes used in devices and consequently has not been studied very much. Nevertheless, it was pointed out recently that as the temperature increases the transit time of ions (the time ions need to move from one side of the cell to the other) will be around the frame period [5,6]. This will lead to the separation of ions and consequently affect the electrooptical performance of an AFLCD. In this paper, in order to investigate the influence of ion transport on AFLC addressing, we present our grey level hysteresis measurement results of an AFLC mixture under a wide temperature range. These measurements have been carried out under various conditions to examine the influence of several factors, such as different types of alignment layers, the frame period and the number of repetition cycles etc. Ionic content in the corresponding test cells has also been measured to ascertain the relationship between the ion mobility, concentration, their temperature behaviour and the detected grey level hysteresis phenomenon .2. ExperimentalThe AFLC mixture used in this paper is pent6’, which is composed of chiral and racemic components. The phase transition temperatures and some physical properties of the mixture are listed in tables 1 and 2 and examples of components present in the mixture are shown in figure 1. The mixture possesses a reasonably wide-temperature SmC A * phase, amoderately high spontaneous polarisation, and a short response time at room temperature.Table 1. Phase transition temperatures of pent6’ (ºC, determined by microscopy and DSC).Recryst. SmC A * SmA* I •<25•72.8 • 94.6 •Table 2. Physical properties of the mixture at room temperature (5µm ITO-coated cell, anti-parallel buffedpolyimide).V th(V ac µm -1) Response time AFO – FO (τ, µs)Tilt angle (θ, °) Max P s (nCcm -2) 13.2 60 28.5 83.2C 12H 2536H 1736H 17C 12H C 10H 2136H 17C 12H 252CH 36H 17(S)(S)(S)Figure 1. Examples of SmC A * and SmC alt -forming materialspresent in the pent6’.3. Results and Discussions 3.1 Ionic ContaminationTransient current measurement has been applied to detect ionic contamination in the mixture [9, 10]. After short-circuiting the cell for a period of time, a series of voltage pulses is applied to the liquid crystal. By measuring the leakage current with a low-noise current-voltage amplifier, several ionic species can be detected. To characterize ionic contamination, both ion mobility µi and concentration n i are calculated [10]. We repeat the measurement at different temperatures (from 30 °C to above 100 °C). This not only improves the reliability of the results but also gives us more information about the ionic contamination in the liquid crystals. Figure 2 shows the measured ion mobility (figure 2a) and concentration (figure 2b) of different ion species as a function of reversed absolute temperature, using polyimide and nylon as alignment materials, respectively. From figure 2a, one sees that atISSN0000-0966X/00/3101-1000-$1.00+.00 © 2000 SIDSID 00 DIGEST • 100130 °C, the mobility of the fastest ion species is about 10-11 V.m/s². Under the holding voltage of about 10 V, as applied in this work, the transit time of these ions is around 25 ms. This is almost comparable to the frame period. Besides, as the temperature rises, the ion mobility increases rather fast and from 40 °C, the ion transit time becomes even smaller than the frame period, which leads to separation of ions at the end of the frame time. Figure 2b shows that the ion concentration is generally of the order of 1020 #/m³. Under this condition, if these ions are completely separated, the electric field generated by these ions is equivalent to an extra voltage of around 1 V. This ionic field will therefore influence the electrooptical performance of the LCDs.1.E-131.E-121.E-111.E-101.E-090.00250.00270.00290.00310.00330.00351/T (K -1)m o b i l i t y (V .m /s ²)(a)1E+191E+201E+211E+220.00250.00270.00290.00310.00330.00351/T (K -1)c o n c e n t r a t i o n (#/m ³)(b)Figure 2. Measured ion mobilities (a), and concentration (b), as a function of reversed absolute temperature in test cells filled with pent6’ and using Nylon6,6 and polyimide alignment materials. ('star' stands for polyimide and 'box' for nylonalignment layers.)3.2 Grey Level HysteresisIn order to study the influence of ion transport on the electrooptical performance of AFLCs and to test the above analysis, grey level hysteresis measurements have been carried out on a test sample with Nylon6,6 alignment layer and filled withpent6'. The test sample has a thickness of 1.6 µm and was placed in a programmable hot stage between crossed polarizers and was oriented in such a way that the optical transmission was symmetric on application of positive and negative voltage signals. The transmission was detected by a photodiode and the results were observed on an oscilloscope and then stored on a computer. A LabVIEW program was written to program the function generator and the oscilloscope so that the entire measurement process could be carried out automatically.Similar to the greylevel hysteresis measurement in an SSFLC, a ‘step up-step-down’ cycle was applied. This cycle began from the dark state: the AFLC was switched, via a few intermediate greylevels, to the bright state. After maintaining the AFLC in the bright state for a number of frames, it was switched gradually back to the dark state, again via various intermediate grey levels. The optical transmission of each state was measured by integrating the transmission throughout the whole frame. The measured transmission on application of identical voltage signals during different processes, i.e. when going from the bright to the dark state and vice versa were compared. In the ideal case, the optical transmission is expected to be always identical since identical voltages are applied. In practice, however, different optical responses were always detected which form a hysteresis. An example of the addressing signal and the optical transmission measured at 40 °C is given in figure 3 and the summarized grey level hysteresis is shown in figure 4. In order to avoid the influence of the assymetric optical response of AFLC to positive and negative voltages, all the transmission results presented here correspond to positive voltages. The experiments show that the results corresponding to negative voltages are similar.0.080.10.120.140.16time (s)Figure 3. Grey level hysteresis measurement results at 40 °C. The applied addressing signal contains a selection pulse of various amplitude, a holding voltage of 9.25 V, a well voltage of 18 V, 100 µs and a reset time of 3 ms. Different grey levels were obtained by varying the amplitude of the selection pulse from 23 V to 27 V. The frame period was 10 ms and each statewas repeated 20 frames .• SID 00 DIGEST1002 In order to characterize this phenomenon quantitatively, the image sticking value is defined asS average Tr Tr Tr Tr bright dark dark bright =−−→→()max minwhere Tr represents the optical transmission. This measurement was repeated with varying frame period from 10 ms to 40 ms and varying number of repetition cycles. The results are given infigure 5.Grey level hysteresis00.511.522.533.52324252627selection voltage (V)T r a n s m i s s i o n (a .u .)Figure 4. Different optical transmissions were detected when the same voltage was applied on going from bright to dark state and vice versa, thus forming a grey level hysteresis.From the experimental results, one can observe that generally a certain grey level becomes brighter or darker after having been kept in that state for a number of frames, which is similar to the situation found in an SSFLC. In SSFLCs, this is explained by charge separation and accumulation at the alignment layer interface under the depolarization field when the crosstalk voltage is applied to the pixel after the line time. In AFLCs, however, as the addressing voltage is always reversed after each frame, the liquid crystal is switched between two optically equivalent but physically different ferroelectric states, in which the depolarization field also reverses. Nevertheless, in contrast to the case in an SSFLC, the averaged driving voltage within each frame is not zero. Instead, a holding voltage of about 10 V is applied during most of the frame period. During the period in which this holding voltage is applied the positive and negative ions become separated. As analyzed previously, from about 40 °C, the transit time of the fast ions is of the order of the frame period. Consequently an ionic field in the opposite direction to that of the applied field is generated. The magnitude of this ionic field can be calculated from the ion concentration measured from the transient leakage current (figure 2b), taking the cell thickness into account. It is found to correspond to an extra voltage of around 1V. This ionic field influences not only the amplitude of the selection pulse, which eventually determines the grey level to be displayed, but also the reset process at the end of each frame, which plays an important role in the reproducibility of the transmission [7]. In comparison with the case in SSFLCs under symmetrical addressing conditions, the image sticking in an AFLC occurs to a much smaller extent. This indicates that AFLCs are less sensitive to ionic influences. Similar to the case of SSFLCs, sometimes, negative image sticking values have been obtained. This is mainly due to the symmetric addressing scheme in which the trapping effect of ions on the surface of the alignment layer becomes non-negligible [8]. Depending on the trapping and release velocity, the ionic field generated by the trapped ions can be either positive or negative, i.e. either in the same or opposite direction of the applied voltage. Increasing the frame period or the number of repetition cycles leads to an increase in image sticking. This is quite logical as ions have more time to separate and to be trapped at the alignment layers.-0.00200.0020.0040.0060.0080.01frame period (ms)i m a g e s t i c k i n g(a)-0.01-0.00500.0050.01number of repetition cyclesi m a g e s t i c k i n g(b)Figure 5. Image sticking at various frame periods (a) and atvarious number of repetition cycles (b).SID 00 DIGEST • 1003This measurement was repeated at different temperatures within the range of the SmC A * phase. The measured image sticking is presented in figure 6. Apparently, image sticking increases as the temperature rises. This is expected as both the ion mobility and the concentration increases with increasing temperature. As a result, the generated ionic field also becomes higher.Nylon 6,6-0.0200.020.040.060.080.10.120.14temperature (°C)i m a g e s t i c k i n gFigure 6. Image sticking measured at different temperaturesusing test cells with Nylon6,6 as alignment layer.The same experiment was repeated on a test cell filled with the same AFLC mixture pent6' but with polyimide as alignment layer. The thickness of the sample is 1.44 µm and the image sticking values at various temperatures are summarized in figure 7.Polyimide-0.0500.050.10.150.20.250.30.35temperature (°C)i m a g e s t i c k i n gFigure 7. Image sticking measured at different temperaturesusing test cells with polyimide as alignment layer.Similar results were obtained. The only exception occurs at 60 °C. It follows that as far as the image sticking alone is concerned, Nylon6,6 alignment layer is preferable to this polyimide.4. ConclusionsThe above grey level image sticking experiments confirm the analysis, from the ion content measurement, that ions areseparated to some extent and even trapped at the alignment layer interface after one frame. This consequently generates an ionic field and as a result, the selection pulse amplitude and the reset process are affected, which gives rise to a shift in optical transmission. This phenomenon increases with increasing temperature. Realizing that symmetric addressing approaches cannot completely avoid ion transport influence in LCDs, for real display applications not only the AFLC mixtures should be further purified but also both addressing technique and cell construction should be optimized in order to minimize this ionic influence.5. AcknowledgementsThis work has been carried out in the framework of the European Network program ‘ORCHIS’.6. 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