S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Synthesis, Properties and Applications of Indolo[3,2-b]indole-Based Organic Semiconductors
인돌로[3,2-b]인돌을 기반으로 한 유기반도체의 합성, 특성 및 응용에 대한 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- indolo[3; 2-b]indole; heteroacene; charge transport; hole-transporting; ambipolar; organic electronics; organic semiconductor; organic field-effect transistor; organic solar cell; perovskite solar cell
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 8. 박수영.
- Organic semiconductors have been attracting significant attention as a potential alternative to the prevalent silicon-based semiconductor by the virtue of chemical versatility, good mechanical property, large area and low cost fabrication, flexibility, and so on. Aiming at the realistic device performance, a wide range of π-conjugated molecules have been developed and investigated. In particular, heteroatom-containing fused aromatic molecules (heteroacenes) have been explored as a structurally and an electronically fascinating molecular platform based on their unique electronic characteristics originating from the incorporated heteroatoms, rigid and extended π-conjugation, excellent oxidative stability, and tight molecular stacking. Among various types of heteroacenes, pyrrole-containing ones have been intensively investigated owing to their low redox potential, strong electron donating nature and feasibility for molecular structure modification. However, pyrrole-fused heteroacenes which ensure the superb device performance is still rather limited for few molecular structures. Among various types of pyrrole-fused heteroacenes, I have focused particular attention on indolo[3,2-b]indole (IDID) unit which comprises two inner pyrrole rings and two outer benzene rings fused all together, as a promising π-conjugated backbone structure for high performance organic semiconductor. IDID allows symmetrical structural derivatization in (N,N-), (2,7-), and (3,8-) positions affording diverse opportunities to control their π-conjugation length and solubility with high flexibility, and their fused two pyrrole rings give rise to strong electron donating nature. Despite such promising perspective of IDID core, its organic semiconductor applications have seldom been explored so far. In this regard, this dissertation describes following two main topics
i) rational molecular design of IDID-based π-conjugated molecule to realize high performance organic semiconductor, ii) chemical structure modification of IDID core unit for the development of advanced and versatile core structure.
Firstly, in order to verify the high potential of the IDID core as an organic semiconductor backbone structure, and also to propose molecular design strategy for the high-performance and versatile processable IDID-based semiconductor, a series of IDID derivatives comprising the core unit of N,N-dihexyl-IDID with different aromatic and aliphatic substituents at 2- and 7-position were designed and synthesized. Among others, 4H4TIDID (with 2- and 7-substituents of 5-hexyl-2,2-bithiophene moiety) with excellent solubility (> 20 weight% in chloroform), showed the highest field-effect hole mobility of 0.97 cm2 V-1 s-1 in vacuum deposited (VD) organic field-effect transistor (OFET) device and 0.18 cm2 V-1 s-1 in spin-coated OFET device, respectively, and also both devices identically showed the two-dimensional (2D) molecular orientation favorable for the high transistor mobility.
Besides apparent advantages of pyrrole-fused structure such as facile solubility control and structural derivatization, IDID core is characterized by stronger electron donating nature than those of others. In this regard, to examine the possible bipolar carrier injection and transport, intramolecular charge transfer (ICT)-type IDID derivatives, which comprise IDID as an electron donor (D) and dicyanovinyl (DCV) as an electron acceptor (A) with A-π-D-π-A type architecture were designed and synthesized. It was found that the compensated ICT interaction between IDID and DCV of 2TIDID-DCV derivatives (with thiophene π-spacer) and their efficient electronic interaction in the three-dimensional lamellar π-stacking structure gave rise to the dramatically reduced energy band-gap as well as excellent film morphology. Consequently, I could successfully demonstrate that 2H2TIDID-DCV exhibited highly balanced ambipolar charge transport with hole and electron mobilities of 0.08 cm2 V-1 s-1 and 0.09 cm2 V-1 s-1, respectively, in VD OFET devices. The spin-coated OFET devices using OD2TIDID-DCV, for which hexyl side chains of 2H2TIDID-DCV were replaced by 2-octyldodecyl, also exhibited ambipolar charge transporting nature (mobility of 9.67 × 10-2 cm2 V-1 s-1 for hole, and 3.43 × 10-3 cm2 V-1 s-1 for electron).
Meanwhile, in the course of my investigation, I observed typical molecular interaction tendency of pyrrole-fused heteroacenes from the IDID derivatives, i.e., slipped-herringbone packing structure and distorted π-plane which are regarded as demerit for the ensuring high mobility. Not only to solve these kinds of drawbacks, but also to provide effective strategy for the crystal engineering, I tried to tune the molecular interaction motif of indolo[3,2-b]indole (IDID) derivatives through incorporating the fluorine (F) atom into the IDID core. It was found that the substitution of F atom at 3- and 8-position of IDID core gave rise to extended and planar π-conjugated backbone structure, strong Coulombic force toward π-π stacking, and dense one-dimensional crystal growing for their derivatives. As a consequence, 4H4TIDIDF for which IDID core of 4H4TIDID was replaced by fluorinated IDID, exhibited excellent p-type field-effect mobility of 1.88 cm2 V-1 s-1 in single crystal (SC) OFET device with packing motif of slipped π-stack.
In addition, I could successfully demonstrate that 4H4TIDIDF exhibited as a high-performance crystalline hole transporting material (HTM) for perovskite solar cells (PSCs). A planar π-conjugated backbone linked with a flexible alkyl chain of 4H4TIDIDF enabled a tight molecular stacked arrangement as well as versatile processing, leading to a higher hole mobility than that of p,p-Spiro-OMeTAD in sandwich-type devices. The photoluminescence quenching in perovskite/4H4TIDIDF interface was also more effective as compared to that at the perovskite/p,p-Spiro-OMeTAD. As a consequence, the PSC device fabricated using 4H4TIDIDF showed a superior performance as compared to p,p-Spiro-OMeTAD, exhibiting a best PCE of 19%. Thus, this remarkable result demonstrated fluorinated IDID core-based materials as a new class of HTM for highly efficient PSCs.
Lastly, to elaborate characteristics of fluorinated IDID core as a donor building block for ICT-type molecules, ICT-type low bandgap small molecules were designed and synthesized by altering electron donors (IDID and fluorinated IDID) and electron acceptors (octylcyanoacetate (OCA) and 3-ethylrhodanine (Rho)), for the solution processed bulk-heterojunction organic solar cells (BHJ-OSCs). It was found that the fluorinated IDID derivatives exhibited lower highest occupied molecular orbital (HOMO) energies than those of non-fluorinated counter molecules (ca. 0.1 eV, each), resulting in the higher open circuit voltage (Voc, by ~ 0.1 eV) for the BHJ-OSC devices. Moreover, strong π-π interaction of fluorinated IDID derivatives gave rise to stable micro-structure with edge-on lamellar packing in their blended films (with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)) as well as in their neat films. Consequently, the BHJ-OSC devices using fluorinated IDID derivatives with 3-ethylrodanine and octylcyanoacetate exhibited enhanced device performance than those of non-fluorinated IDID-based devices with a best PCE of 3.90% for OD4TIDIDF-Rho, and 3.07% for HD4TIDIDF-OCA.