Failures in the operation of liquid crystal displays are often associated with a change in the ionic conductivity of the liquid crystal material. More specifically, an increase in conductivity can degrade the quality of the image produced by a display by reducing the contrast and introducing artifacts that include image sticking and flicker. Failures can also be induced in the control electronics.
Ionic impurity in a liquid crystal layer can arise for many reasons including residuals from material synthesis, decomposition of materials, contamination introduced during device fabrication, DC induced electrochemical reactions and ionization by ultra violet light or radiation.
The starting point for the research reported in this article was the idea that real-time measurement of electrical conductivity could be used as a means to monitor and, thus, enable control and improvement of the liquid crystal device manufacturing processes. It could also be used to analyze and even predict device failure modes.
To implement this idea, a team of researchers headed by Fedor Podgornov within the Institute of Natural Sciences and Mathematics at the South Ural State University (Chelyabinsk, Russia) has been developing the means to measure the ionic electrical conductivity of a liquid crystal layer in complex device structures that include multiple materials.
A recent article by the team on this topic is entitled “Direct current electric conductivity of ferroelectric liquid crystals-gold nanoparticles dispersion measured with capacitive current technique.” The article was published in the journal Liquid Crystals. A copy of the article is available for purchase on-line and can be found here.
First, a few words of background information paraphrased from the article.
When ions carry the charge, it is difficult to measure the electrical conductivity of the material. The reason is that ions cannot move in wires. It follows that, to perform measurements, it is necessary to use electrodes. By this means, an electrochemical reaction can take place in them in which the ions either receive or give electrons in an exchange between the test material and metal electrodes. If the test material contains several types of ions, it is difficult to correctly measure its electrical conductivity. The situation is even more complicated when the device includes heterostructures. (A heterostructure is a semiconductor structure in which the chemical composition changes with position.) Since devices with heterostructures are composed of various materials, it becomes difficult to measure the conductivity of each of them using conventional methods.
In their effort to develop a means to measure ionic conductivity in complex devices, the researchers decided to investigate approaches based on the use of a bias current rather than a more conventional conduction current. In fact, the researchers report investigating three different approaches to making this type of measurement.
Approaches based on dielectric and conductivity spectroscopies were reported as not successful. What was successful was a capacitive technique. It was applied to measuring the “influence of capped gold nanospheres on the DC electric conductivity of a ferroelectric liquid crystal confined in a cell with blocking electrodes.”
The researchers make particular note of the fact that this approach, despite the description just offered, is actually applicable to wide range of liquid crystal devices.
To verify the utility of the proposed method, the researchers investigated the effects of nanoparticles on the conductivity of the liquid crystal layer in a liquid crystal cell. Definitions used in the model appear in the figure below.
Simplified equivalent electric circuit of a liquid crystal cell. Cpol and Clc note the capacitances of the polymer alignment and liquid crystal layers, Rpol and Rlc are resistances of these layers.
In these experiments, it was found that changes appeared after the introduction of nanoparticles, which adsorb impurity ions. This also led to an increase in the resistance of the liquid crystal layer and a decrease in the electro-optic switching time of the liquid crystal device.
In the conclusion of their article, the researchers state that, based on the results of these experiments, they believe it will be possible to diagnose the nature of a range of degradations mechanisms in liquid crystal displays.
However… the researchers went on to comment that further development of the technique is required before it can be used in practical applications. They explain that it will be necessary to conduct detailed studies to determine the influence of other physical conditions on the accuracy of the results. Conditions specifically mentioned as needing additional consideration were the relaxation of double electric layers and the formation of space charge. Finally, the researchers state that it will also be necessary for future research to focus on exploring the possibility of measuring nonlinear ionic conductivity and identifying the mechanisms responsible for its occurrence. –Arthur Berman
South Ural State University, Fedor Podgornov, [email protected]
