As many of us learnt during our school years, electronic systems contain three types or materials: conductors, semiconductors and insulators (also known as dielectrics).
This is also true for displays, which themselves contain transistors. The electronic switch inside each and every pixel of a liquid crystal TV is constructed using all three material types, with the dielectric usually being the most abundant because there are typically multiple types of dielectric in one device. Dielectrics define the boundaries of the conductor, ensuring there are controlled paths for electrical charge to flow around the circuit. They allow different conductors to be layered on top of each other separated by the dielectric with minimal interaction between the conductors, or in certain cases form a critical interface that separates the conductor and semiconductor (transistor).
Electrical Properties
In previous blogs, we have discussed the fundamentally important and highly valuable semiconductor material. That material can be controlled to switch from a state where no electrical charge can flow through it to another state where charge will flow. How do we control it? Through the use of high specification dielectric materials. These dielectrics are able to carry an electric field across its bulk without any electrical charge leaking through it. This creates a ‘field-effect’ within the semiconductor, attracting or repelling charge close to the surface of the semiconductor in contact with the dielectric, causing a switch in the conduction state. This so-called gate dielectric is one example of the key role that dielectrics play in the thin film transistor (TFT) located on each pixel of the display. The FlexiOM™ portfolio includes these high specification Gate Dielectrics for use in conjunction with the FlexiOM organic semiconductors to define the key interface where the magic happens.
Structural Properties
It’s not only the electrical properties of the dielectric that matter; the structural properties are also of critical importance. The physical and structural properties of the overall electronic device are determined mostly by the different dielectrics (including the substrate) because they make up the bulk of the mass. The structural properties of the dielectrics will determine not only the qualities of the final device but also the processability of the material during device production. For example, the ability to create a physical hole through the dielectric layer (via-hole) is important to allow conductors to connect between layers in order to form the circuit and interconnects required. Photo-patterning is commonly used to create these via-holes and therefore certain dielectrics need to be designed for compatibility with that type of process. FlexiOM D320 Series is a very high purity Gate Dielectric material and sits directly on top of the organic semiconductor to create that perfect interface that stable device operation requires. FlexiOM D048X Series is a second high purity Gate Dielectric but designed to enable photo-patterning (to form via-holes and other features) and is a perfect complement to both FlexiOM organic semiconductors and the D320 Series.
Optical Properties
As well as the electrical and structural properties of the dielectric, for many applications, including liquid crystal displays, the optical performance is also a key property, particularly because the dielectric layers tend to form the bulk of the fabricated stack of the electronic circuit. The backlights used in LCD TVs must shine through these dielectrics without loss. Gate Dielectrics are typically very thin, a few hundred nano-meters, and as a general rule, the thinner the optical layer, the less opportunity to absorb light. Other dielectric layers in the stack of materials forming the display circuit are typically much thicker, up to a few micro-meters in thickness. At this scale, optical performance is critical to the light and colour performance of the final display and is a fundamental part of the molecule and formulation design of the dielectrics.
Designing Polymer Dielectrics
When designing new dielectric material films, both the design of the material itself and the formulation of it are important tools to produce high yield, readily processable thin film dielectric products.
As discussed in an earlier blog, polymer materials are long chains of repeat units of a core chemical molecule. The composition of that core molecule can be varied depending on application requirements. By adjusting components on that core molecule, or by alternating the core molecule with different additional components, we can adjust the physical properties of the material. This approach is very effective in dielectrics, particularly when designing for physical parameters, such as photo-patternability.
The properties of the film to be deposited based on this polymer can then be further controlled or modified by the design of the formulation. This is often achieved by using ‘additives’ within the solution of the dielectric material where it is dissolved or dispersed in a solvent. Additives can be specific molecules which give additional functionality such as increased photo-sensitivity or changing the fluid properties for coating and drying of the dielectric (film morphology). Modern formulation products can easily include 15-20 different components, all individually supporting different important parameters. Formulation chemistry is used across a broad range of consumer products from shampoos and shower gels through to liquid crystals inside LCD TVs! This concept is particularly relevant for those FlexiOM dielectrics that are located away from the organic semiconductor interface. Second layer Gate Dielectrics (eg FlexiOM D048X Series), planarization layers (immediately below the electronic circuit) and passivation layers (immediately above the circuit) are examples.
FlexEnable’s portfolio of dielectric materials has been developed using these core design elements to create high performance materials optimised for our applications in displays and liquid crystal optics along with our broader and growing portfolio of speciality organic chemicals for flexible display and liquid crystal optics production. Low-cost flexible displays and electronics are fully enabled by FlexiOM materials. Product enquiries and further information can be obtained from info@flexenable.com.
