(Invited) Crystallization and Stability of Flexible P-Type Semiconductors and Their Thin Film Transistor Application
Authors: Lee Sunghwan 
Published: May 2020

In recent years, electronic devices that require superior mechanical flexibility have been extremely sought for potential applications in next-generation displays, sensors, bio-implantable and disposable medical devices. Polymeric materials, of which the flexibility far exceeds that of conventional brittle inorganic counterparts, are widely considered for various flexible devices due to their excellent stretchable, foldable and bendable characteristics.
Oxidative chemical vapor deposition (oCVD) is an emerging technique that provides not only a capability to process mechanically flexible polymeric films but also strategies to address current challenges claimed in typical solution-based polymer processing1-2. The oCVD technique allows for greater uniformity, excellent conformality, enhanced performance (e.g., conductivity) and an ability of large area polymer processing that are limited in conventional solution-processed polymers3-4.
Crystallization and air stability of organic materials have been issues that have to be mitigated to warrant high performance and sustainable device applications. The oCVD technique, in addition to the advantages described above, could also address the issues of crystallization and air stability to enhance the carrier transport and overall conductivity, and to secure sustainability. We recently found that an in-situ oCVD capability of substrate heating during depositions resulted in an substantial increase in ordered structures in conjugated polymers, leading to enhanced crystallization of the resulting oCVD polymers5. Consequently, the enhancement of the carrier transport and conductivity was obtained due to the increase in favorable conduction paths in the crystalline phase. Further, the vapor phase processing of oCVD may be of crucial relevance to achieve air stability since oCVD does not require any monomer solubility in specific solvents and, therefore, is able to polymerize insoluble materials6 that are not available for conventional solution processing techniques such as spin-coating or dip-casting. The reactivity of insoluble monomers is quite limited due to their rigid backbone structures; however, this limited reactivity is the key to achieve air stability since the reactions between insoluble materials and water (H2O)/oxygen, which are the main origins to cause instabilities of organic materials in air, are significantly prohibited.
In this presentation, we report our recent findings of enhanced crystallization and air-stability of oCVD polymers. Particularly, the structure, chemical and electrical properties of two oCVD polymers, poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) and unsubstituted polythiophene, will be compared with focuses on the crystalline state, dope-ability and air-stability. Further, thin film transistors (TFTs) that employ the two oCVD polymers as TFT channel are presented and the device performance of output and transfer characteristics will be compared with focuses on the carrier mobility and on/off switching properties.

Acknowledgement: This work was supported by Purdue University and partly by an NSF grant, Award No. ECCS-193108.

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Figure 1