Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on Solution-Processable Low Band Gap Small Molecules

We synthesized a solution-processable low band gap small molecule, Si1TDPP-EE-COC6, for use as a semiconducting channel material in organic thin film transistors (OTFTs). The Si1TDPP-EE-COC6 is composed of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) and carbonyl units. SiTDPP-EE-COC6-based OTFTs with Au source/drain electrodes were fabricated, and their electrical properties were systematically investigated with increasing thermal annealing temperature. The hole and electron mobilities of as-spun Si1TDPP-EE-COC6 were 3.3 × 10^-4 and 1.7 × 10^-4 cm² V^-1 s^-1, respectively. The carrier mobilities increased significantly upon thermal annealing at 150 °C, yielding a hole mobility of 0.003 cm² V^-1 s^-1 and an electron mobility of 0.002 cm² V^-1 s^-1. The performance enhancement upon thermal annealing was strongly associated with the formation of a layered edge-on structure and a reduction in the π-π intermolecular spacing. Importantly, the use of atomically thin single-layer graphene (SLG) source/drain electrodes that were grown by the chemical vapor deposition (CVD) method further increased the carrier mobilities. The 150 °C annealed Si1TDPP-EE-COC6-based OTFTs with SLG source/drain electrodes exhibited a hole mobility of 0.011 cm² V^-1 s^-1 and an electron mobility of 0.015 cm² V^-1 s^-1. The improved electrical performances of the SLG OTFTs were attributed to the stepless flat surface of the SLG electrodes and the better interfacial contact between the Si1TDPP-EE-COC6 molecules and the SLG electrodes compared to the Au electrodes. This work suggests that careful chemical design is essential to enhance balanced ambipolar transistor performance based on small conjugated molecules, and the SLG is a good electrode material to promote the carrier mobilities.