High-Performance Field-Effect Transistors Based on Polystyrene-b-Poly(3-hexylthiophene) Diblock Copolymers

X. Yu, Kai Xiao, Jihua Chen, N. V. Lavrik, K. Hong, B. G. Sumpter, and D. B. Geohegan

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory

PDF of article published in ACS Nano 5, 3559 (2011).

Idea: How can we direct the self-assembly of conducting polymers, and what are the optimal nanophase architectures for improved charge transport?

    •  PS-b-P3HT diblock copolymers with insulating PS blocks were used to induce phase separation for nanoscale morphologies with molecular order described by DFT calculations

    •  The P3HT block length was found to induce different morphologies and crystallinity which improved the transport properties of OFETs beyond that of neat P3HT.

(left) Molecular structure and TEM image of PS-b-P3HT block copolymer. (middle) AFMimages(500nm 500nm) of the lamella rstructure of the P2 film. Inset shows the schematic illustration of the lamellar nanostructures. Left images show the schematic illustrations of the top view and side view of the lamellar structures of the P2 film. (lower) Semiempirical AM1 Hamiltonian-optimized lamellar structure of the P2 film. (right) Transfer characteristics of the device.

 

Polystyrene-b-poly(3-hexylthiophene) (PS-b-P3HT) block copolymers with fixed PS block length were synthesized by combined atom transfer radical polymerization (ATRP) and Grignard metathesis (GRIM) polymerization. The self-assembled structures of these diblock copolymer thin films based on PS-b-P3HT have been studied by TEM, SAED, GIXD, AFM, and additionally by first principles modeling and simulation. These block copolymers undergo microphase separation and form nanostructured spheres, lamellae, nanofibers, or nanoribbons in the films dictated by the molecular weight of the P3HT block. Within the diblock copolymer thin film, PS blocks segregate to form amorphous domains, and the covalently bonded conjugated P3HT blocks exist as highly ordered crystalline domains through intermolecular packing with their alkyl side chains aligned normal to the substrate while the thiophene rings align parallel to the substrate through π␣π stacking. The conjugated PS-b-P3HT block copolymers exhibited significant improvements in organic field-effect transistor (OFET) performance and environmental stability as compared to P3HT homopolymers, with up to a factor of 2 increase in measured mobility (0.08 cm2/(V 3 s)) for the P4 (85 wt % P3HT). Overall, this work demonstrates that the high degree of molecular order induced by block copolymer phase separation can improve the transport properties and stability of conjugating polymers, which are critical for high-performance OFETs and other organic electronics.

Credit - This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.