High performance organic opto-electronic devices : Fabrication and theoretical analysis

Abstract

최근 몇 년 동안 용액공정용 유기반도체는 다양한 응용 가능성과 저가의 특징으로 말미암아 많은 관심을 받아왔다. 용액공정 동안의 자기조립에 의해 결정되는 분자구조와 박막 몰포로지가 그러한 유기 반도체의 최종 응용 소자인 유기박막트랜지스터나 유기 센서, 유기 태양전지의 성능을 결정하기 때문에, 이의 상관관계를 자세히 규명할 필요가 있다. 이에 본 연구에서는 용액공정용 유기반도체를 이용한 다양한 유기전자 소자의 성능을 향상시키기 위해서 다음과 같은 연구를 수행하였다.2장에서 5장 까지는 유기 저분자 반도체 소재의 응용 가능성을 살펴보았다. 우선 저분자 소재의 파이 오비탈의 오버랩 정도가 박막트랜지스터의 전하 이동도에 어떠한 영향을 주는지를 연구한 다음, 이를 바탕으로 최적의 저분자 소재 반도체를 제작, 유기 광센서에 응용해보았다. 또한 안트라센의 경우 디엘스-알더 반응으로부터 자유로울 수 있다는 점에 착안하여 태양전지에 응용해보았다.5장에서는 유기 반도체 분야의 또다른 문제인 산화 안정성에 대해 살펴보았다. 우선 기존의 잘 알려진 반도체 소재와 보호막으로 기능할 수 있는 소재와의 블렌드를 통하여 안정성을 향상시켜보고자 하였고 또한 새로운 결정구조 도입을 통하여 박막트랜지스터와 태양전지의 안정성을 향상시켜보았다.6장에서 7장까지는 무정형 고분자를 이용한 박막트랜지스터, 태양전지의 응용에 대해 연구해보았다. 무정형 고분자의 경우 결정과 결정 사이의 높은 에너지 베리어로부터 자유로울 수 있어 전하 이동도에 유리한 점이 있다는 것을 밝힐 수 있었고 또한 이로 인하여 다양한 유기전자 소자로의 응용이 가능하다는 것을 알 수 있었다.8 장에서는 유기 태양전지의 성능 향상을 위해 자주 보고되고 있는 첨가제의 역할이 단순히 몰폴로지에 기인한 것이 아닌, 분자간의 에너지 장벽 변화 등의 복합적인 효과에서 비롯된다는 것을 밝혔다.

Soluble semiconductors have received special attention as promising organic semiconductors in recent years because of their superior intermolecular interactions and solution-processability, and provide useful benchmarks for organic thin-film transistors (OTFTs) and organic solar cells (OSCs). Because the molecular ordering and film morphologies of such soluble semiconductors determine device performances of resulting devices, controlling self-organization behaviors of soluble semiconductors are strongly required. Herein, this thesis addresses the systematical investigation of self-organization characteristics in soluble semiconductor based thin-films for enhancing device performance of OTFTs and OSCs In Chapter 2, synthetic strategies to achieve high mobility of organic semiconducting materials retaining ？-stacking structure have been proposed. Comparing TIPSAN single crystal OFETs having five different acene derivatives, and applying the concept of molecular overlap ratio along the long / short axis, we could show that the effective ？-stacking area dominantly determines the field-effect mobility of ？-stacked materials rather than the ？-stacking distance. In the case of TIPSAN-Na, a large ？-stacking area and a small ？-stacking distance enabled the highest field-effect mobility. In addition, paying attention to highly photo-sensitive properties of TIPSAN-Na, I could fabricate all organic two terminal transistors by replacing ？field induced p-channel？ with ？photo induced p-channel？. In Chapter 3, we demonstrated that this ？photogenerated p-channel effect？ is closely related with photodetrapping effect by using the theory describing the behavior of a SCLC under light illumination. (We demonstrated that this ？photogenerated p-channel effect？ can be explained by the Helfrich？s theory describing the photodetrapping behavior of a SCLC under light illumination.) I have also reported the use of TIPSAnthracene derivatives as an electron donor material for organic solar cells application. Different to TIPS pentacene derivatives, TIPSAnt derivatives are not susceptible to a Diels-Alder reaction with PCBM and therefore can be fabricated as solar cells with power conversion efficiency of 1.4% as described in Chapter 4.Air-stability is another important issue for the commercialization of organic electronics. Paying attention to the fact that relatively lower HOMO level or dense structure of organic semiconductor can lead to better air-stability, I have used novel semiconductor and analysis of the thin-film structure by in situ grazing-incidence X-ray diffraction, near-edge X-ray absorption fine structure spectroscopy, and atomic force microscopy showed that not only the low HOMO level of PBDN but also the presence of close-packed frustrated structures in the polymer film were responsible for the superior stability of the devices. (Chapter 5)In organic electronic devices, thermal stability is another important issue for commercialization because in many cases, such devices are exposed to high temperature and generally organic materials, especially semiconductors, are very weak to thermal stress. In Chapter 6, I have studied the possibility of amorphous semiconductor for use as active layer of OTFTs. Despite its amorphous nature, our time of flight measurements and OTFT characteristics demonstrate that the polymer is a good hole transport material with high hole mobility, which is comparable to those of crystalline polymers such as P3HT. Ihave also investigated the current density？voltage (J？V) and mobility？voltage (？？V) relationships as a function of temperature to elucidate the origin of the high field-effect mobility of amorphous semiconductor. I conclude that the relatively low trap density, which originates from the grain-boundary-free amorphous nature of the semiconductor, enables this high field-effect mobility. In Chapter 7, I have used an amorphous polymeric semiconductor as an electron donor material in bulk heterojunction cells and tandem cells. By optimizing the morphology of both subcells and controlling the crystallite size and conductivitiy of titanium oxide layer, I could fabricate successful tandem cells. Finally, in Chapter 8, I found out that film-morphological and molecular-energetic analyses should be performed in parallel to study the effect of additives in solution processed bulk-heterojunction solar cells.