Electrospinning Technique for Fabrication of Coaxial Nanofibers of Semiconductive Polymers

Abstract

In this work, the electrospinning technique is used to fabricate a polymer-polymer coaxial structure nanofiber from the p-type regioregular polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n-type conjugated ladder polymer poly(benzimidazobenzophenanthroline) (BBL) of orthogonal solvents. Generally, the fabrication of polymeric coaxial nanostructures tends to be troublesome. Using the electrospinning technique, P3HT was successfully used as the core, and the BBL as the shell, thus conceptually forming a p-n junction that is cylindrical in form with diameters in a range from 280 nm to 2.8 µm. The UV-VIS of P3HT/PS blend solution showed no evidence of separation or precipitation, while the combined solutions of P3HT/PS and BBL were heterogeneous. TEM images show a well-formed coaxial structure that is normally not expected due to rapid reaction and solidification when mixed in vials in response to orthogonal solubility. For this reason, extruding it by using electrostatic forces promoted a quick elongation of the polymers while forming a concise interface. Single nanofiber electrical characterization demonstrated the conductivity of the coaxial surface of ~1.4 × 10^-4 S/m. Furthermore, electrospinning has proven to be a viable method for the fabrication of pure semiconducting coaxial nanofibers that can lead to the desired fabrication of fiber-based electronic devices.

Keywords: BBL; P3HT; flexible electronics; organic semiconductors.

Figure 1

Coaxial nanofiber was fabricated by using P3HT (a) and BBL (b) as the semiconducting polymers. Because pure P3HT is unable to form fibers (due to its low molecular weight), PS (c) was added to improve mechanical support for fiber formation.

Figure 2

Coaxial nanofiber with p- and n-type semiconducting polymers. This nanofiber provides an opportunity to form p-n junctions that can be tethered for electroactive textiles and single-nanofiber devices.

Figure 3

Continuous fabrication of coaxial nanofiber p-n junctions, using the electrospinning technique. The BBL solution (a, blue) forms the shell/sheath, and the P3HT/PS solution (b, red) forms the core. An electric field overcomes the surface tension, stretching the solution and forming nanofibers. Inset: (c) The coaxial needle tip and magnification (d) after the Taylor cone formation, with the diode symbols representing the continuous heterojunction formed between the core and the shell.

Figure 4

Comparison of the UV/VIS spectra of pure PS (7 wt.%), pure P3HT (2 wt.% in CHCl₃), and the P3HT/PS (0.4 wt.%/7 wt.%) blend indicates that adding polystyrene does not change the physical or optical characteristics of P3HT. The BBL solution (0.39 wt.%) exhibits its characteristic broad absorption peak at approximately 540 nm.

Figure 5

TEM image of a coaxial nanofiber with a P3HT/PS (0.4 wt.%/7 wt.%) core and a BBL (0.39 wt.%) shell. This nanofiber was the smallest one examined. The scale bar is 100 nm.

Figure 6

TEM images of coaxial nanofibers (P3HT/PS core (white arrow) with BBL shell (black arrow)) created simultaneously within a single run of the electrospinning technique. Scale bars are 1 µm (a), 2 µm (b,c), and 5 µm (d).

Figure 7

SEM image of coaxial nanofiber electrically connected to the BBL shell (a), device structure with electrode separation of 40 µm (b), and current-voltage (I–V) characterization of a single fiber (c).