Self-Assembly, Molecular Packing, and Electron Transport in n-Type Polymer Semiconductor Nanobelts

Abstract

We have found that poly(benzobisimidazobenzophenanthroline) (BBL) nanobelts can be prepared by a simple high-yield, solution-phase process, which enables dispersions of the nanobelts in a large number of solvents including environmentally benign solvents such as methanol and water. Characterization of the nanobelts by transmission electron and atomic force microscopies, electron diffraction, and X-ray diffraction showed that the BBL polymer chains are oriented parallel to the long axis of each nanobelt. This unique packing motif is unlike the reported packing of polymer chains in other nanostructures, such as poly(3-hexylthiophene) nanowires, where the polymer backbone packs face-to-face along the nanowire direction. This unusual molecular packing in BBL nanobelts is explained by the rather strong intermolecular interactions, which are a result of the rigid and planar polymer chains. We investigated electron transport in single nanobelts and nanobelt networks via field-effect transistors and observed mobilities up to ∼7 × 10−3 cm2 V−1 s−1 and on/off current ratios of ∼1 ×104. The n-channel nanobelt transistors showed stability and repeatability in air for more than 6 months, which is the most stable among current n-channel polymer transistors. These results demonstrate that the BBL nanobelts are promising for organic electronics and nanoelectronics.

Solution-processed, high-performance n-channel organic microwire transistors

“…These unique materials combine the high-performance of singlecrystalline structures with solution-processability by dispersion (17,18). Several p-channel (hole-transporting) organic wires prepared by solution-processing have exhibited Ͼ 0.1 cm 2 /Vs for singlewire transistors (19)(20)(21)(22), whereas only a few solution-synthesized n-channel organic wires have been reported, typically with low performance ( Ϸ10 Ϫ3 Ϫ 10 Ϫ2 cm 2 /Vs) (23,24).Despite the intrinsic high mobility of single-crystalline wires, precise wire placement and wire-to-wire performance variation, due to the difference in the contact quality at the wire/insulator and wire/electrode interfaces, substantially hinder successful device integration (10, 25). Therefore, the technology to achieve network films of organic MWs deposited from a dispersion with controlled alignment and density is acutely desired.…”

“…These unique materials combine the high-performance of singlecrystalline structures with solution-processability by dispersion (17,18). Several p-channel (hole-transporting) organic wires prepared by solution-processing have exhibited Ͼ 0.1 cm 2 /Vs for singlewire transistors (19)(20)(21)(22), whereas only a few solution-synthesized n-channel organic wires have been reported, typically with low performance ( Ϸ10 Ϫ3 Ϫ 10 Ϫ2 cm 2 /Vs) (23,24).…”

Tuning Luminescence and Conductivity through Controlled Growth of Polymorphous Molecular Crystals

“…We conclude that the crystals of form D grew along the a axis, which coincides with the π-π stacking direction. [23,24] Adv. Electron.…”

Water-gated organic nanowire transistors

“…Depending on the material, and growth conditions, some variations on the NW morphology have also been observed, e.g. flat 'nanobelts' [12] or curved 'nanofibres' [13]. We will here refer to all of these related morphologies as 'nanowires' as generic term, except when discussing a specific sample.…”

“…As n type organic semiconductor, we have selected poly(benzimidazobenzophenanthroline) (BBL). BBL can act as n type material in organic TFTs with good mobility ( e = 0.1 cm 2 /Vs) [20], and Briseno et al [12] have reported on the growth of BBL 'nanobelts'. BBL possesses a rather deep 'lowest unoccupied molecular orbital' (LUMO) of 4.0 eV [21], which allows reasonable electron injection even from high work function metals, and may make BBL consistent with water gating, despite the potential trapping of electrons by water and/or oxygen: Nicolai et al [22] recently suggested from theoretical calculations that such trapping can be avoided when the LUMO is more than 3.6 eV below vacuum level.…”