고안정성의 유연 유기박막트랜지스터 구현을 위한 졸겔 유무기 하이브리드 절연층 연구

Abstract

Flexible electronics is an innovative technology due to their potential in providing solutions to next-generation display, conformable sensors, wearable healthcare, etc. Organic thin-film transistor (OTFT) is a key device of flexible electronics. In particular, gate dielectric is an important component for OTFT that can determine electrical performances, stability and reliability, considering that they can create a field-effect in combination with the gate electrode, and a conducting channel is formed in a few semiconductor layer near the gate dielectric layer. Inorganic thin-films have been widely used gate dielectrics because they have shown high dielectric constant (κ), dielectric strengths, and good chemical/thermal resistance. These characteristics promote a stable, reliable and low-voltage operation of OTFTs. However, their inherent brittleness, poor adhesion to organic thin-films, and complex/harsh fabrication process have precluded the production of large-area production and flexible applications. Inorganic thin-films can be fabricated by solution-process, but it is attended with cracks or pin-holes on films. In addition, the incompatibility between inorganic gate dielectrics and organic semiconductors can cause poor semiconductor growth and the formation of interface traps, which results in poor charge carrier transport in the OTFTs. On the other hand, organic gate dielectrics represented by polymer are flexible, solution-processable, and compatible with organic semiconductors; however, they have yet to be commercialized due to several issues such as cross-linking, their relatively low κ and dielectric strength levels. Organic-inorganic (O-I) hybrid gate dielectrics are remarkable in that respect. O-I hybrid materials are elaborately mixed structure of organics and inorganics at the nanoscale level. Many studies have reported that O-I hybrid materials can exhibit new material properties completely different from that of original materials. Therefore, it is expected that O-I hybrid gate dielectrics will feature both excellent dielectric properties and flexibility, simultaneously. However, unfortunately, only few studies have reported the details of O-I hybrid gate dielectrics for flexible OTFTs. In this thesis, sol-gel derived O-I hybrid gate dielectrics were synthesized, developed, and then successfully applied to highly stable flexible OTFTs. Their dielectric properties, transistor characteristics, and flexibility also studied in depth. Chapter 1 is a useful section to know when you are reading this thesis. It lays out a basis of flexible electronics, OTFTs, gate dielectrics, and sol-gel process for help readers understand better. Chapter 2 lays out the effects of amphiphilic polymers in the sol-gel derived O-I hybrid gate dielectrics on the flexible OTFTs. A new type of alkoxysilane-functionalized amphiphilic polymer (AFAP) precursor was synthesized for the preparation of O-I hybrid nanoparticles and thin-films (AGPTi). As AFAP precursors have reactive alkoxysilane groups and an amphiphilic polymer backbone, it can be expected that colloidally stable O-I hybrid nanoparticles are formed during hydrolytic condensation polymerization without using a dispersion agent. Simultaneously, the amphiphilic polymer backbone of the AFAP precursor facilitates the formation of a homogeneous film. The introduction of AFAP in the sol-gel matrix induced dramatic changes. AGPTi sol exhibited more enhanced sol stability due to a strong prevention of aggregation between the nanoparticles and effective inducement of nanophase separation than control group (GPTi sol; this sol have no AFAP precursors). The AGPTi film was observed to be made of smaller and more uniform nanoparticles than the that of GPTi film. As a result, AGPTi gate dielectrics gave improved dielectric properties, transistor performances, and flexibility. Chapter 3 lays out the effects of fluorinated amphiphilic polymers in the sol-gel derived O-I hybrid gate dielectrics on the flexible OTFTs. In Chapter 2, I concluded that amphiphilic polymers can effectively prohibit the aggregation of nanoparticles. It had favorable effects on the not only sol but also thin-film. As a follow-up research, a more stable sol-gel matrix was devised by introducing fluorinated amphiphilic polymer precursor, and applied to flexible OTFTs. As a result, the size of nanoparticles in the fluorinated AGPTi (FAGPTi) become smaller than that of non-fluorinated nanoparticles (AGPTi) and it resulted in enhancing not only the dielectric properties but also flexibility. In conclusion, highly stable flexible OTFTs were demonstrated. It was verified that amphiphilic polymers in the O-I hybrid sol-gel matrix play the key roles of enhancing the flexibility as well as the electrical stability. Fluorination strategy also further developed this system.