The development of nonfullerene acceptor materials applicable to organic photovoltaics (OPVs) has attracted considerable attention for the achievement of a high power conversion efficiency (PCE) in recent years. However, it is still challenging due to the insufficiency of both the variety of effective electron-deficient units and certain guidelines for the design of such materials. This work focusses on naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) as a key electron-deficient unit. Therefore, a new electron-accepting π-conjugated compound (NTz-Np), whose structure is based on the combination of NTz and the fluorene-containing imide-annelated terminal units (Np), is designed and synthesized. The NTz-Np compound exhibits a narrow optical energy gap (1.73 eV), a proper energy level (−3.60 eV) of the lowest unoccupied molecular orbital, and moderate electron mobility (1.6 × 10⁻⁵ cm² V⁻¹ s⁻¹), indicating that NTz-Np has appropriate characteristics as an acceptor against poly(3-hexylthiophene) (P3HT), a representative donor. OPV devices based on NTz-Np under the blend with P3HT show high photovoltaic performance with a PCE of 2.81%, which is the highest class among the P3HT/nonfullerene-based OPVs with the conventional device structure. This result indicates that NTz unit can be categorized as a potential electron-deficient unit for the nonfullerene acceptors.

A series of oligothiophenes that incorporate cyclopenta[c]thiophene-based units bearing spiro-substituted dialkylfluorene was synthesized. Photophysical measurements indicated that there was no interruption in the conjugation along the oligothiophene backbones, irrespective of the number or position of this unit. Electrochemical measurements showed that the thiophene 7-mers and 11-mer exhibit reversible multi-oxidation waves. The formation of cationic species was clearly observed from UV/Vis/NIR measurements. Furthermore, the UV/Vis/NIR spectra at 223 K under one-electron oxidation conditions revealed that the unsubstituted thiophene or bithiophene units remained in the absence of intermolecular π–π interactions, whereas the formation of π-dimeric species was observed for the thiophene 7-mer containing an unsubstituted terthiophene (U₃) unit. Theoretical calculations indicated that the combination of the U₃ unit and the all-trans conformation decreased the intermolecular steric repulsion between the fused cyclopentene ring and its facing thiophene, which may contribute to the formation of the dimeric structure.

A series of electron-deficient π-conjugated systems with 4,9-dihydro-s-indaceno[2,1-d:6,5-d′]dithiazole-4,9-dione-based structures and fluorinated acyl groups as the terminal units have been designed and synthesized for application as organic field-effect transistor (OFET) materials. The thermal, photophysical, and electrochemical properties and OFET performance of the synthesized compounds were investigated. OFET evaluation revealed that all compounds exhibited typical electron-transporting characteristics, and electron mobilities up to 0.26 cm² V⁻¹ s⁻¹ could be achieved. The air stabilities of OFET operation were dependent on the nature of the compounds and were investigated by X-ray diffraction and atomic force microscopy. The terminal units had a great influence not only on the molecular properties, but also on the film-forming properties and OFET performance.

Solution-processable, electronegative, π-conjugated systems containing dicyanomethylene-substituted cyclopenta[b]thiophene were synthesized as potential active materials for air-stable n-type organic field-effect transistors (OFETs). Electrochemical measurements revealed that these compounds exhibited electrochemical stability and that the lowest unoccupied molecular orbital (LUMO) had an energy level less than −4.0 eV. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements were performed, and the value of intradomain electron mobility was determined to be as high as 0.1 cm² V⁻¹ s⁻¹. The OFETs were fabricated by spin-coating thin films of the compounds as an active layer. The electron mobility of the OFETs was 3.5×10⁻³ cm² V⁻¹ s⁻¹ in vacuum. Furthermore, electron mobility of the same order of magnitude and stable characteristics were obtained under air-exposed conditions. X-ray diffraction measurements of the spin-coated thin films revealed the difference of molecular arrangements depending on the inner conjugated units. Atomic force microscopy measurements of crystalline-structured films exhibited the formation of grains. The accomplishment of air-stability was attributed to the combined effect of the low-lying LUMO energy level and the molecular arrangements in the solid state, avoiding both the quenching of electron carriers and the intrusion of oxygen and/or moisture.

A series of electronegative π-conjugated compounds composed of carbonyl-bridged bithiazole and alkyl-substituted dioxocyclopenta[b]thiophene were synthesized as a candidate for solution-processable n-type organic semiconductor materials and characterized on the basis of photophysical and electrochemical properties. Cyclic voltammetry measurements showed that the first half-wave reduction potentials of these compounds are between −0.97 and −1.14 V versus ferrocene/ferrocenium, which corresponds to lowest unoccupied molecular orbital energy levels between −3.83 and −3.66 eV. Thanks to hexyl or dodecyl groups in the molecules, the compounds are sufficiently soluble to realize the fabrication of their thin films through a spin-coating method. As a result, the prepared organic field-effect transistors based on these newly developed compounds exhibited n-channel characteristics not only under vacuum but also in air, and the best field-effect electron mobility observed under vacuum was 0.011 cm² V⁻¹ s⁻¹ with an on/off ratio of 10⁸ and a threshold voltage of 16 V.

An electronegative conjugated compound composed of a newly designed carbonyl-bridged bithiazole unit and trifluoroacetyl terminal groups is synthesized as a candidate for air-stable n-type organic field-effect transistor (OFET) materials. Cyclic voltammetry measurements reveal that carbonyl-bridging contributes both to lowering the lowest unoccupied molecular orbital energy level and to stabilizing the anionic species. X-ray crystallographic analysis of the compound shows a planar molecular geometry and a dense molecular packing, which is advantageous to electron transport. Through these appropriate electrochemical properties and structures for n-type semiconductor materials, OFET devices based on this compound show electron mobilities as high as 0.06 cm² V⁻¹ s⁻¹ with on/off ratios of 10⁶ and threshold voltages of 20 V under vacuum conditions. Furthermore, these devices show the same order of electron mobility under ambient conditions.