불소화 알킬 측쇄 함유량에 의존하는 DPP-BTZ 공중합체 박막내에서의 전하이동 연구

Abstract

Recently, donor–acceptor (D-A) type semiconducting copolymers are receiving a lot of interests because of their high field-effect mobility as well as ambipolar characteristics wherein both p- and n-type charge transport can occur. From a circuit design point of view, the ambipolar semiconducting polymer is preferable for facilitating fabrication of such circuits using complementary-like inverters with high gain. In this thesis, a newly-synthesized novel D-A type semiconducting polymers obtained by copolymerization with the electron-deficient diketopyrrolopyrrole (DPP) monomer and benzotriazole (BTZ) monomer are investigated. In particular, the effects of fluorine in the side-chains on the charge transports via the DPP-BTZ semiconducting copolymer films as a active layer of organic field-effect transistors (OFETs) are studied. For this purpose, we prepared four kinds of DPP-BTZ semiconducting copolymers with different ratios of alkyl side-chain to fluoroalkyl side-chain (i.e. alkyl chains : fluoroalkyl chains = 3:7, 5:5, 7:3 and 10:0, respectively). The band gap energies of the four ratios measured by UV-visible simulation were 1.32 eV and were all the same. However, as the ratio of the fluoroalkyl side-chain increases, the peak of the absorption wavelength changes from the red-shift to the blue-shift, which is considered to decrease the planarity of the polymer backbone. From the two-dimensional grazing incident X-ray diffraction (2D-GIXD) results, it was found that the phases in the DPP-BTZ polymer films are changed from face-on to edge-on, as increasing the ratio of fluoroalkyl side-chain. According to the crystallographic parameter data, as increasing the ratio of fluoroalkyl side-chain, the π- π distance between the molecules was shortened and the polymer backbone torsional angle was increased, in order for fluorine groups in polymer to exist in the most stable state. In contrast, as decreasing the ratio of fluoroalkyl side-chain, the π- π distance was increased, however, the torsional angle of the copolymer backbone was decreased and the coherence length of the copolymer backbone was increased. On the other hand, the measured electrical properties of the DPP-BTZ based OFETs showed the ambipolar characteristics for all ratios of fluoroalkyl side-chain and the reduction of currents at the same bias conditions, as increasing the ratio of fluoroalkyl side-chain. We also performed the temperature-dependent electrical measurement to extract the activation energy of the hopping charge transport in the copolymer films. It was found that the more activated energy was required as increasing the ratio of the fluoroalkyl side-chain. In addition, the density of states (DOS) of each fluoroalkyl chain ratios of the DPP-BTZ copolymers was extracted by using the temperature-dependent results. The number of states per unit volume and energy and the width of the energy distribution become larger for hole and electron accumulated regions, respectively, as increasing the ratio of the fluoroalkyl side-chain. From the above results, it was concluded that the effect of the torsional angle of the DPP-BTZ based D-A type semiconducting copolymer backbone on the charge transporting properties is greater than the distance between the molecules.