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 (∼10 ¹⁵ charges cm^-2) and metallic conductivities in poly(3-hexylthiophene) (P3HT) is reported. Films of P3HT are electro-statically 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 μSsq^-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.