Solution-processed core-extended naphthalene diimides toward organic n-type and ambipolar semiconductors

In this paper, we report four new core-extended NDI molecules 1, 2, 3 and 4. 1 and 2 entail two benzene rings and two naphthalene rings, respectively; 3 and 4 contain benzene and naphthalene rings as well as 2-(1,3-dithiol-2-ylidene) malonitrile groups. All of them can be easily synthesized with acceptable yields. HOMO/LUMO energies of 1, 2, 3 and 4 were -5.66 eV/-3.93 eV, -5.73 eV/-3.91 eV, -5.88 eV/-4.22 eV and -5.89 eV/-4.23 eV, respectively. OFETs with thin-films of 1, 2, 3 and 4 were successfully fabricated with conventional techniques. Thin-films of 1 and 2 exhibit ambipolar semiconducting properties under N₂ atmosphere with relatively balanced hole and electron mobilities, which can further increase after annealing; hole and electron mobilities reach 0.047 and 0.016 cm² V^-1 s^-1, respectively for the thin-film of 2 after annealing at 140 °C. Moreover, two identical ambipolar transistors with thin films of 2 were combined into an inverter circuit with a gain of 11. 3 and 4 behave as n-type semiconductors in air, and the electron mobility can reach 0.22 cm² V^-1 s^-1 for the thin-film of 3 with an I_on/off of 10 ⁶ after annealing at 120 °C. XRD and AFM studies are also presented for understanding the variation of carrier mobilities of thin-films of 1, 2, 3 and 4 after annealing at different temperatures.