CDT 및 BDT 기반 D-A copolymer에서 acceptor 단위체 탄소 원자 일부를 친 전자성 종으로 치환할 때 전하 이동에 미치는 영향

Abstract

Recently, with the increasing surge of the internet of Things, flexible electronics based on electronic circuitry on flexible substrates have attracted a lot of attention for their great potential in many important wearable electronic applications. Polymer thin-film transistors (PTFTs) where synthesized polymer semiconductor is served as an active layer are regarded as a suitable basic switching device for flexible electronics, owing to their inherent mechanical flexibility of polymer films. PTFTs for flexible electronic circuitry can be truly achieved when the development of complimentary polymer semiconductor set, including both P-type and N-type semiconductor is prepared. In this regard, the control of charge-transfer characteristics in the semiconducting polymers with rational molecular design is essential. In particular, recent reports on donor-acceptor (D-A) type conjugated copolymers show that it is possible to obtain high field-effect mobility values up to 30 cm2V-1s-1. Indeed, D-A based semiconducting copolymers are intriguing materials because they exhibit high carrier mobility as well as ambipolar characteristics, in spite of their highly disordered microstructure that is rather counter-institutive considering the conventionally well-known high mobility polymers such as liquid-crystalline or semicrystalline thiophene-based semiconductors.

The experiment consists of two parts. In the first part, benzodithiophene-thiadiazoloquinoxaline (BDT-TAQ) based polymer semiconductors were used as active layers of transistors. We also investigated the effects of charge transport and bandgap energy on the acceptor unit of BDT-TAQ when the trifluoromethyl (CF3) was added. The electrical properties of BDT-TAQ and BDT-TAQ-CF3 were compared. BDT-TAQ showed ambipolar characteristics and BDT-TAQ-CF3 showed only N-type characteristics. The reason for these results is that by adding CF3, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the polymer are totally lowered, and thus the electron transport from the electrode to the LUMO level is facilitated. In the second part, cyclopentadithiophene-alt-benzothiadiazole CDT-BTZ was used as an active layer in transistor fabrication. The effect of substituting a part of BTZ with an electrophilic atom (F, N) on charge transport properties was investigated. Comparing the electrical properties of the CDT based polymer, it was found that the charge transport characteristics are improved in the fluorine or nitrogen substituted polymer. The Arrhenius equation and the Vissenberg-Matters (V-M) model were applied to analyze the charge transport in the CDT based copolymer. Firstly, a polymer backbone is believed to be planarized by the repulsive forces between the electrophilic atoms in the polymer. If the polymer semiconductor is highly planar, the charge can be moved more easily along the polynmer backbone. Second, as the charge moves in the polymer, the trap is filled and the charge moves to the next energy level. In electrophilic atom-substituted polymers, traps are a little closer to the valance band, so it can fill the trap with less energy. As a result, carrier transport to other levels becomes easy.