Synthesis, Property, and Optoelectronic Device Application of Bis-Lactam-Based Organic Semiconducting Materials

Abstract

Organic semiconductors have attracted great attention in the last decade because of their promising potential as materials for advanced optoelectronic devices. Among organic semiconductors, those based on bis-lactam materials, such as diketopyrrolopyrrole (DPP), isoindigo (II), isoDPP, and thienoisoindigo (TII), have been extensively studied as desirable electron-withdrawing building blocks, because of their unique features, which include (a) a high electron affinity because of the electron-withdrawing effect of the lactam units

(b) a high degree of π-π stacking, deriving from their quasi-planar backbone structure

and (c) the possibility of controlling their solubility by incorporation of suitable alkyl and aryl side chains at the lactam N-atom position. Hence, I studied the bis-lactam-based semiconducting materials as electron-deficient building blocks for high-performance organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In this thesis, I have designed and synthesized three different kinds of novel bis-lactam-based semiconducting materials by modifying their structures, and then investigated their structure-property relationships.

First, a new high-performance small molecular n-channel semiconductor based on diketopyrrolopyrrole (DPP), DPP-T-DCV was designed and synthesized. The frontier molecular orbitals have been engineered by introducing a strong electron-accepting dicyanovinyl group. The well-defined lamellar structure of the DPP-T-DCV crystal displayed a uniform terrace step height corresponding to a molecular monolayer in the solid-state. As a result of the high level of crystallinity derived from the conformational planarity, OFETs made of dense-packed solution-processed single-crystals of DPP-T-DCV exhibited an electron mobility up to 0.96 cm2 V–1 s–1, one of the highest values yet obtained for DPP derivative-based n-channel OFETs. Polycrystalline OFETs also showed decent electron mobility of 0.64 cm2 V–1 s–1 suitable for practical device applications. (Chapter 2)

Second, a novel electron-accepting bis-lactam building block, 3,7-dithiophen-2-yl-1,5-dialkyl-1,5-naphthyridine-2,6-dione (NTDT), and a conjugated polymer P(NTDT-BDT) comprising NTDT as an electron acceptor and benzo[1,2-b:4,5-b']dithiophene (BDT) as an electron donor have been designed and synthesized for producing efficient OPVs. The thermal, electronic, photophysical, electrochemical, and structural properties of NTDT and P(NTDT-BDT) have been studied in detail and compared with those of the widely used bis-lactam acceptor dithienyl-substituted-DPP (DPPT) and its polymer P(DPPT-BDT). Compared to DPPT derivatives, NTDT and P(NTDT-BDT) exhibited remarkably higher absorption coefficients, deeper highest occupied molecular orbital energy levels, and more planar structures. A bulk heterojunction solar cell based on P(NTDT-BDT) exhibited a power conversion efficiency of up to 8.16% with a short circuit current (Jsc) of 18.51 mA cm-2, one of the highest Jsc values ever obtained for BDT-based polymers. It was successfully demonstrated that the novel bis-lactam unit NTDT is a promising building block for use in OPVs. (Chapter 3)

Although the performance of the first successful NTDT-based OPVs, P(NTDT-BDT), was promising, it operated best only for medium-thick active-layer (≈ 140 nm) due to insufficient polymer crystallinity. Therefore, it was considered that the polymer crystallinity should be increased via backbone structure modification in order to produce more efficient (> 9% PCE) thick-active-layer (> 200 nm) OPVs. To this end, new semiconducting polymers, PNTDT-2T, PNTDT-TT, and PNTDT-2F2T, with NTDT as an acceptor, as well as 2,2'-bithiophene (2T), thieno[3,2-b]thiophene (TT) and 3,3'-difluoro-2,2'-bithiophene (2F2T) as donors were designed and synthesized. It was found that PNTDT-2F2T exhibited superior polymer crystallinity and a much higher absorption coefficient than those of PNTDT-2T or PNTDT-TT attributed to the appropriate matching between the highly coplanar acceptor (NTDT) and donor (2F2T) building blocks. A bulk heterojunction solar cell with a thick active layer (>200 nm) of PNTDT-2F2T simply fabricated without using post-fabrication hot processing, demonstrated an outstanding power conversion efficiency of up to 9.63%, with a short circuit current of 18.80 mA cm-2 and a fill factor of 0.70. (Chapter 4)