Effect of fluorination of naphthalene diimide-benzothiadiazole copolymers on ambipolar behavior in field-effect transistors

Abstract

Two naphthalene diimide (NDI)-benzothiadiazole (BT) based conjugated polymers with high molecular weight, P1 and P2, were synthesized by introducing F atoms to modulate the electron-donating ability of the BT moiety. 3-Decyl-pentadecyl branched alkyl side chains were employed and expected to improve the molecular organization and device performance. Both polymers have excellent solubility in common organic solvents. UV-vis-NIR absorption and cyclic voltammetry indicate that the maximum absorption wavelength of P2 is blue-shifted and the HOMO energy level of P2 is decreased in comparison with P1. Two dimensional wide angle X-ray scattering of thin films revealed a similar organization of both polymers. A less balanced transport in field-effect transistors with increased electron mobility of 0.258 cm² V^-1 s^-1 and lowered hole transport of 2.4 × 10^-3 cm² V^-1 s^-1 was found for P2. Polymer devices of P1 exhibited a balanced ambipolar transport, with a hole mobility of 0.073 cm² V^-1 s^-1 and electron mobility of 0.086 cm² V^-1 s^-1.

Scheme 1. Synthetic route for polymers P1 and P2.

Fig. 1. UV-vis absorption spectra of P1 and P2 in chloroform solution (c = 10⁻⁶ M) and thin film.

Fig. 2. The reduction and oxidation curves of P1 and P2. The films were deposited from chloroform solutions.

Fig. 3. Energy level diagram of three monomers and the monomeric subunits of P1 and P2, the energy level values are obtained from DFT calculations (B3LYP, 6–31 g(d)).

Fig. 4. Tapping mode height AFM images of (a) P1 and (b) P2 thin films after annealing at 200 °C. Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns of (c) P1 and (d) P2 thin films after annealing at 200 °C.

Fig. 5. (a) and (b) Transfer, (c) and (d) output characteristics of P1 and P2 thin films after annealing at 200 °C.