Designing semiconductor performance
Giles Lloyd, Head of Materials Business
January 11, 2021
In previous blogs we’ve discussed the two main types of organic semiconductor materials (OSC) used in organic thin film transistors (OTFTs), namely polymeric materials and small molecule materials. FlexiOM™ materials are polymeric organic semiconductors and dielectrics with world-leading performance in key parameters such as mobility, stability and processability.
Whilst polymeric materials are now the standout class of materials for organic semiconductors, this hasn’t always been the case. The story of the best OSC materials has zig-zagged between both types over the years both through evolution of the requirements placed on the materials as they move closer to mass production but also, crucially, through innovation and development of the chemistry and chemical design of the molecules.
Development of complementary organic dielectrics has taken a less complex path but is crucial to the processing and implementation of optimal organic semiconductors. This will be discussed in a future blog.
Polymer and small molecule structures differ in the length of the molecule. An organic molecule is loosely defined as a group of carbon atoms bonded together in a specific arrangement, sometimes with additional elements such as sulphur or nitrogen. Polymers consist of a “core” molecule that is repeated many times. These long chains can lie together similar to a bowl of spaghetti! Their natural elasticity and flexibility are attractive features in many physical applications. In contrast, small molecule materials are short, defined molecules which may include a small number of repetitions of a core chemical structure. They have a tendency to “clump” together in regions of ordered groups.
These two types of morphology have significant impact on how an electronic charge moves through the semiconducting material, a parameter known as the “mobility” of the electronic charge.
Organic molecules that conduct electronic charge are known as conjugated organic molecules. Early development focussed on improving the mobility of these materials with the first molecules having performance values tens of thousands of times lower than amorphous silicon. In the early 1990’s, simple conjugated polymer materials emerged with performance about 100 times less than amorphous silicon and academic and industrial research expanded significantly.
For an electrical current to flow through an organic material, an electronic charge needs to “exist” on a molecule or specific part of a larger molecule. This charge will then move in the presence of an electric field to another molecule. The proximity of the next molecule is therefore a key determining factor in the movement of that charge. Small molecule materials that packed densely together at the molecular level were seen as the highest performing materials for many years. This concept was developed further with vacuum deposition techniques to form thin and highly controlled films. These deposition techniques are commonly used to deposit metals and dielectrics in LCD manufacturing. The leading organic material used in early OTFT work, pentacene, consisting simply of five conjugated rings of six carbon atoms, fused side by side, dominated research on performance which focussed on optimising the vacuum deposition process.
Whilst headline performance slowly increased with enhanced control of the vacuum process, key commercial questions remained. Amorphous silicon used a proven and scaled process whilst pentacene, with marginally higher mobility values, had a similar but non-scaled deposition technique which potentially increased cost. The novel value position wasn’t immediately obvious.
Vacuum-deposited, small molecule OSCs have been successful in OLED applications, due to different market drivers, which demonstrates technical viability of the organic material class.
Being synthetically designed, small molecules evolved through innovative ideas to create soluble versions. Simply put, additional molecules would be bonded onto the side of the semiconducting molecule to create space between the core molecules. Organic solvents could then be used to disperse and dissolve them. Once dissolved, simple coating and drying techniques enable a much more cost effective and commercially attractive process.
Solubilising small molecules demonstrated the elegance of synthetic chemistry to engineer key parameters but the electronic performance was still dominated by the core molecule. Specifically, the molecule’s capacity to carry a charge and the proximity of the next molecule. During the last 10-15 years, no advancements have been made in small molecule design.
Polymer materials intrinsically addressed these two “problems” by the nature of their structure and design. Proximity of an available electronic state on a molecule could be simply improved by chemically bonding molecules together. Electronic charge can move along the chain of the polymer. Jumping to a different chain nearby is more difficult but unlike small molecule materials, polymers have both options for electronic charge transport.
Optimisation of the capacity to carry an electronic charge is more challenging. Approximately ten years ago, the concept of “donor-acceptor” polymers was developed. This used two different conjugated molecules with opposing, or in some cases complementary, electronic properties to be polymerised together in an alternating pattern. This “alternating co-polymer” design enabled fine control of the electronic properties of the material. Critical parameters such as the semiconductor energy bandgap, a characteristic that dictates how good a semiconductor is, could be engineered with this technique.
The complete organic semiconductor
The invention of “donor-acceptor” polymers enabled the design and optimisation of the electronic properties. This, combined with traditional polymer chemistry to control solubility and hence, thin film processing of the material enabled powerful incremental design and optimisation techniques. Over the last 7-8 years generational performance increases have been realised in this class of semiconducting polymers. FlexiOM materials represent the pinnacle of organic semiconductor research and development from the last 20 years. This includes development of the class of materials from low performing polymers to small molecules to soluble small molecules and finally to a highly optimised and engineered “donor-acceptor” polymer system. The electronic performance of FlexiOM materials now significantly exceeds amorphous silicon as well as competing organic semiconductor technologies. FlexiOM’s designed-for solution processing using common organic solvents, a concept not feasible with competing small molecule materials, enables low temperature production methods compatible with low-cost plastic substrates.
Low-cost flexible displays and electronics are fully enabled by FlexiOM materials. Product enquiries and further information can be obtained from email@example.com.
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