Well-Balanced Carrier Mobilities of Ambipolar Transistors Based on a Low Band Gap Small Molecule Semiconductor

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 structure consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The electrical properties of the resulting OTFTs were systematically investigated as a function of the thermal annealing conditions. The hole and electron mobilities of as-spun Si1TDPP-EE-COC6 were 3.3 × 10-4 and 1.7 × 10-4 cm2/(Vs), respectively. The carrier mobilities increased significantly upon thermal annealing at 150°C, yielding a hole mobility of 0.003 cm2/(Vs) and an electron mobility of 0.002 cm2/(Vs). The perfor mance 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, as revealed by grazing incidence X-ray diffraction and atomic force microscopy measurements. Importantly, the use of atomically thin CVD-grown single layer graphene (SLG) source/drain electrodes further increased the carrier mobilities. The SLG OTFTs prepared using the 150°C-annealed Si1TDPP-EE-COC6 exhibited a hole mobility of 0.011 cm2/(Vs) and an electron mobility of 0.015 cm2/(Vs), which are among the highest values yet reported for DPP-based small molecules. The improved electrical performances of the SLG OTFTs originated from the step-less flat surface and the wellaligned energy levels of the SLG electrodes in contact with the Si1 TDPP-EE-COC6 molecules. The use of CVD-grown SLG electrodes provided a facile method for improving the OTFT performance based on DPP-based small molecule semiconductors.