High Carrier Density and Metallic Conductivity in Poly(3‐hexylthiophene) Achieved by Electrostatic Charge Injection

The use of electrostatic charge injection (i.e., the transverse field effect) to induce both very large two‐dimensional hole densities (～ 10¹⁵ charges cm^–2) and metallic conductivities in poly(3‐hexylthiophene) (P3HT) is reported. Films of P3HT are electrostatically gated by a solution‐deposited polymer‐electrolyte gate dielectric in a field‐effect‐transistor configuration. Exceptionally high hole field‐effect mobilities (up to 0.7 cm² V^–1 s^–1) are measured concurrently with large hole densities, resulting in an extremely large sheet conductance of 200 μS sq.^–1. The large room‐temperature conductivity of 1000 S cm^–1 together with the very low measured activation energies (0.7–4 meV) suggest that the metal–insulator transition in P3HT is achieved. A maximum in sheet conductance versus charge density is also observed, which may result from near‐filling of the valence band or from charge correlations that lower the carrier mobility. Importantly, the large hole densities in P3HT are achieved using capacitive coupling between the polymer‐electrolyte gate dielectric and P3HT (i.e., the field effect) and not via chemical or electrochemical doping. Electrostatic control of carrier density up to 10¹⁵ charges cm^–2 (～ 10²² charges cm^–3) opens opportunities to explore systematically the importance of charge‐correlation effects on transport in conjugated polymers without the structural rearrangement associated with chemical or electrochemical doping.