Jongbok Lee

Extended Ladder‐Type Benzo[k]tetraphene‐Derived Oligomers

Well-defined, fused-ring aromatic oligomers represent promising candidates for the fundamental understanding and application of advanced carbon-rich materials, though bottom-up synthesis and structure-property correlation of these compounds remain challenging. In this work, an efficient synthetic route was employed to construct extended benzo[k]tetraphene-derived oligomers with up to 13 fused rings. The molecular and electronic structures of these compounds were clearly elucidated. Precise correlation of molecular sizes and crystallization dynamics was established, thus demonstrating the pivotal balance between intermolecular interaction and molecular mobility for optimized processing of highly ordered solids of these extended conjugated molecules.

Donor–acceptor conjugated ladder polymer via aromatization-driven thermodynamic annulation

Composed of alternating electron-rich and electron-deficient repeating units in a fused rigid backbone, donor–acceptor conjugated ladder polymers represent a class of promising material candidates possessing a host of intriguing properties. These polymers are challenging to synthesize because of the typically low reaction efficiency of electron-deficient acceptor units and the possibility of defect formation during the postpolymerization annulation. Herein we report the synthesis of a well-defined donor–acceptor ladder polymer, which overcame these challenges by utilizing aromatization-driven thermodynamic ring-closing olefin metathesis. The good solubility of the polymer allowed for the comprehensive investigation of its optical properties, intramolecular charge transfer, complex formation with dopant, and thin film processing. These results provide a foundation for future applications of donor–acceptor ladder polymers in electronic and optoelectronic devices.

Synthesis of low band gap polymers based on pyrrolo[3,2-d: 4,5-d′]bisthiazole (PBTz) and thienylenevinylene (TV) for organic thin-film transistors (OTFTs)

New low band gap copolymers P1–P4, based on thienylenevinylene (TV) and pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz) units composed of different alkyl side chains, such as 2-octyldodecyl (OD), n-hexadecyl (HD), 2-ethylhexyl (EH), and 9-heptadecyl (HD) groups, respectively, have been synthesized and characterized. Electrochemical and optical studies of the copolymers indicated low energy band gaps in the range of 1.40–1.47 eV. Moreover, theoretical calculation with density functional theory (DFT) and time-dependent DFT calculations demonstrated that the energy band gaps, HOMO energy levels and maximum absorption values in the copolymers were in good agreement with the experimental results. The decomposition temperature of all copolymers was measured to be above 340 °C by thermogravimetric analysis (TGA), which indicates high thermal stability. Thermally annealed OTFT devices based on P1–P4 thin films demonstrated a range of hole mobilities; thus, the P2 based OTFT device exhibited the highest hole mobility of 0.062 cm2 V−1 s−1.

n-Type Electron-Accepting Materials for Organic Solar Cells (OSC)

Electron-accepting materials play a key role in the photovoltaic process in bulk heterojunction (BHJ) solar cells. In order to achieve high-power conversion efficiency (PCE), it is of paramount importance to endow the acceptors with optimized properties, such as balanced energy level, efficient exciton dissociation, ideal phase separation and morphology, complementary absorption spectrum compared to the donor, sufficient electron mobility, feasible synthesis, low cost, and good solution processability. Fullerene derivatives have been the predominant type of acceptors since the first BHJ solar cell was ever reported. Non-fullerene type of acceptors, however, started to emerge recently as a potential substitute to the expensive fullerene derivatives. This chapter reviews the general design principles of acceptors, followed by discussions and comparisons on the structure-property relationships of both fullerene and non-fullerene derivatives. Eventually, it ends with a critical outlook on the future development of next generation acceptor materials.