Side-Chain Dipolar Character Controls Threshold Voltage in Solution-Processed Organic Single-Crystal Transistors

Single-crystal organic semiconductors (OSCs) hold immense potential for high-performance thin-film transistors (TFTs) due to their high charge-carrier mobilities (µ) stemming from defect-free and long-range-order structure. While µ is a key factor of TFT performance and can be tackled by molecular design of OSCs, threshold voltage (Vth) significantly impacts operational voltage and power consumption in integrated circuits and is typically controlled by device engineering. A traditional Vth tuning method involves interfacial modifications of gate-dielectric surface by self-assembled monolayers (SAMs) with different dipolar characters. Yet, the SAM is less compatible with printed electronics using polymeric gate dielectrics. Therefore, controlling Vth through OSC material design is of interest. This work demonstrates that Vth in sin-gle-crystal OSCs can be effectively modulated by engineering the dipolar character of side-chain substituents which are widely employed in solution-processable OSCs. To validate this concept, we focused on 3,4,9,10-benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) derivatives because they have formed compara-ble layered brickwork packing structures mostly irrespective of the side-chain structures. Stemming from the bench-marked, phenethyl-substituted BQQDI (PhC2−BQQDI), here, three PhC2−BQQDI analogues with mono-fluorination at the ortho, meta or para position of the phenyl ring were synthesized. These compounds were designed to maintain isostruc-tural crystal packing while varying the dipolar character of the side chains. In solution-processed single-crystal TFTs, obvious Vth shifts depending on the fluorine substitution position were found. By crystal structure and computational analyses, the Vth shift was correlated with the dipolar character of the side chains at the OSC−gate dielectric interface. This side-chain-engineered dipolar effect is not only useful for Vth control in TFTs but also proposes new avenues for de-signing high-performance OSCs and various thin-film devices with tailored interfacial properties.