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 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.

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.