A new electrical equivalent circuit (EEC) has been proposed to model antiferroelectric liquid crystal devices. This circuit includes a constant phase element to take into account the ferroelectric part of the dielectric response in these devices. Electrical characterization of samples has been carried out using a specific experimental protocol based on impedance spectroscopy. The parameters of waveforms used in impedance measurements have been optimized. The procedure to obtain the components of the EEC has also been explained. Finally, the EEC has been validated by comparing experimental and simulated impedance results. A reasonable agreement between both of them has been obtained in a wide frequency range for all selection voltages.
Liquid crystals are a growing technology bringing solutions for a number of applications in high performance displays
featuring video-rate, color and high resolution images, and in prototypes of photonic devices. Electrooptic response of
antiferroelectric liquid crystals (AFLC) might be superior to nematic liquid crystals that are been customarily employed
nowadays. AFLC show reduced time response being promising candidates for portable multimedia devices, optical
routing applications, among others.
In this work, temperature and frequency dependence of impedance measurements, in passive devices of commercial
antiferroelectric liquid crystals, has been studied.
Measurements of the temperature dependence of optical transmission have been obtained. 1Hz triangular waveforms
with different amplitude have been applied to the devices to carry out such characterization.
Simultaneous measurements of optical transmission and electrical impedance have been performed. Specific addressing
schemes have been tested in order to obtain the optimum electrooptical performance. Display blanking takes place when
a saturation pulse is applied. Results achieved show that increasing temperature shifts the dynamic range of the analogue
grayscale towards lower voltages. Impedance analysis of these devices upon switching has been performed as well.
Temperature and frequency dependence of the impedance measurements have been characterized. Negative phase
responses show there is a combined capacitive and resistive behavior. As the frequency increases the capacitive effect
grows. Magnitude shows a linear decrease on a log-log frequency scale. As temperature increases, phase profile becomes
slight more complex. New capacitive effects are suggested in a model of the electric response of AFLC cells at low
Liquid crystals are customarily used in several kinds of flat panel displays. Besides usual nematic liquid crystals, smectic tristate antiferroelectric liquid crystals have shown analogue grayscale and full color video rate at high-end devices with passive multiplexing. These devices ultimately are intended to be applied to small size devices on microdisplay applications. When a symmetric driving signal is applied, the electrooptic response of the devices usually consists of two symmetric hysteresis lobes. An asymmetric hysteresis cycle can be developed by using dissimilar aligning layers onto the two glass plates of the cell. This kind of devices can lead to analogue optical multistability, i.e., devices whose optical transmission may be arbitrarily set and maintained reducing or eliminating the bias voltage. In this work, a study of the asymmetric behavior of cells filled with commercial antiferroelectric liquid crystal is presented. Optical hysteresis cycles have been obtained applying a low frequency triangular waveform to the devices. Analogue grayscales have been generated only at one lobe of the hysteresis cycle. Electrical characterization has been carried out measuring the switching current of the cells test. Multiplexed driving waveforms have been applied with and without bias voltage in order to evaluate the stability of the optical transmission for video rate working. Results demonstrating analogue optical multistability on asymmetric antiferroelectric cells have been obtained. Narrow dynamic ranges, compatible with standard electronics for dynamic grayscale in data columns have been found. Preliminary measurements of the frequency dependence of impedance have been obtained on the capacitive device.
Flat panel display technology constitutes the fastest growing segment of the semiconductor industry. Presently, the majority of display related research is focused on the application of electrooptic effects due to the ease and efficiency of molecular reorientation with an applied voltage. One such area of study is the surface stabilized ferroelectric liquid crystal (SSFLC) display, which has many advantages over conventional cathode ray tubes as well as other types of liquid crystal displays. Antiferroelectric liquid crystal (AFLC) displays have unique electrooptical properties such as tristate switching behaviour, fast response, intrinsic analogue grey scale and a wide viewing angle that lead these materials in very attractive candidates for their potential use in high-resolution flat panel displays and microdisplays for computers and TV. To become a competitive display technology, they should work at video frequency and give full colour and a significant number of the grey levels. These features depend on the AFLC material used in the display device as well as the addressing schemes employed.
We present a new programmable driver for addressing passive matrix AFLC displays based on a microcontroller system. This prototype was built with commercial electronic subsystems and it is able to range voltage levels for the row selection of ±40V and has a minimum time resolution of 5 microseconds to shape the frame complete (selection pulse, bias, well and reset slots). The driver can address a 16x160 pixels AFLC display. A grey scale will showed in a preliminary 4x4 pixels AFLC display by using this prototype.