Linear-type carbazoledioxazine-based organic semiconductors: the effect of backbone planarity on the molecular orientation and charge transport properties

Abstract

We report the synthesis of a linear-type dibromocarbazoledioxazine (CZ) derivative as a new precursor for semiconducting polymers. The chemical structures of the CZ unit and its polymers with thiophene or thienothiophene spacers (namely, PCZT and PCZTT) were fully characterized. PCZT and PCZTT possessed similar medium optical band gap (E ^opt _g) and electrochemical band gap (E ^cv _g) of around 1.70 eV estimated from the onset absorption and electrochemical redox potentials of the thin films, respectively. Computational density functional theory (DFT) calculations suggested that the backbone of the PCZT might be highly twisted, while that of PCZTT could be very planar. The effect of different backbone geometries on the charge-transport properties was studied by using thin film transistors (TFTs). The TFT device based on PCZTT showed a four times higher hole mobility as compared to that based on PCZT. The superior TFT performances of PCZTT were reasonably attributed to its edge-on backbone packing orientations toward the Si substrate revealed by the grazing-incidence wide-angle X-ray scattering (GIWAXS), which was favorable for in-plane charge transport in the TFT devices.

Chart 1. Chemical structures of (top) well-known dye/pigment units used for high-performance semiconducting polymers; (middle) angular and linear-type carbazoledioxazines; (bottom) 2,7-carbazole-, 3,6-carbazole-, and 1,8-carbazole-based copolymers.

Scheme 1. Synthesis of the linear-type dibromocarbazoledioxazine monomer and its Stille polycondensation to give the corresponding semiconducting oligomers.

Fig. 1. ¹H NMR spectra of (a) 4 and (b) CZ in CDCl₃.

Fig. 2. MALDI-TOF mass spectra of (a) PCZT and (b) PCZTT.

Fig. 3. (a) UV-vis-NIR absorption profiles of CZ, PCZT, and PCZTT in dilute 1,2-dichlorobenzene solution and thin film states spin-cast on a glass substrate; (b) cyclic voltammograms of the CZ, PCZT, and PCZTT films drop-cast on a glassy carbon electrode, measured in acetonitrile with 0.1 M (nC₄H₉)₄NClO₄ at the scan rate of 0.1 V s⁻¹; (c) calculated HOMO/LUMO orbitals and side-view geometry of PCZT; and (d) calculated HOMO/LUMO orbitals and side-view geometry of PCZTT (using DFT B3LYP/6-31G(d), long and branched alkyl chains are substituted by the methyl group).

Fig. 4. Transfer characteristics of TFTs based on the thin films of (a) PCZT without annealing; (b) PCZT annealed at the optimized temperature of 300 °C; (c) PCZTT without annealing; (d) PCZTT annealed at the optimized temperature of 300 °C (L = 100 μm and W = 1 mm. All the measurements were done under ambient conditions). (e) Effects of annealing temperatures on the hole mobilities of PCZT and PCZTT.

Fig. 5. GIWAXS patterns of the thin films at different annealing temperatures. PCZT: (a) RT and (b) 300 °C; PCZTT: (c) RT and (d) 300 °C.

Fig. 6. Tapping-mode AFM topography images of the thin films of (a to c) PCZT and (d to f) PCZTT, prepared by thermal annealing at different temperatures. Annealing temperatures (for 10 min) are depicted in each image. AFM size: 2 × 2 μm².