Beyond the metal-insulator transition in polymer electrolyte gated polymer field-effect transistors

Abstract

We have studied the carrier transport in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) field-effect transistors (FETs) at very high field-induced carrier densities (10(15) cm(-2)) using a polymer electrolyte as gate and gate dielectric. At room temperature, we find high current densities, 2 x 10(6) A/cm(2), and high metallic conductivities, 10(4) S/cm, in the FET channel; at 4.2 K, the current density is sustained at 10(7) A/cm(2). Thus, metallic conductivity persists to low temperatures. The carrier mobility in these devices is approximately 3.5 cm(2).V(-1).s(-1) at 297 K, comparable with that found in fully crystalline organic devices.

Fig. 1.

Polymer electrolyte gated FET structure and room temperature device characteristics. (a) A schematic illustration of the four-probe device structure complete with the ionic polymer electrolyte layer (Bottom) with the molecular structure shown (Top). The molecular structure of pBTTT-C14 is also shown (Middle). (b and c) Typical current vs. V_g (b) and conductivity vs. V_g (c) data measured for a device in ambient air at room temperature and V_sd = −1 V. The V_g sweep rate was ≈60 mV/s.

Fig. 2.

Low-temperature device characteristics. (a) Conductivity vs. temperature at varying V_sd for a device cooled beforehand from room temperature to 4.2 K while holding the gate bias constant at V_g = −3 V. (Inset) The conductivity vs. V_g at 4.2 K. (b) Current (and conductivity) vs. source–drain electric field for the same device shown in a, at 4.2 K. (Inset) The channel (filled circles) and contact (open circles) resistance vs. V_sd at 4.2 K.

Fig. 3.

Conductivity vs. temperature for three polymer electrolyte gated (circles) devices at V_sd = −1 V. All devices were cooled beforehand to ≈260 K while holding the gate bias constant at V_g = −3 V. The data for a device without the electrolyte layer (squares), gated instead by using the bottom contact (SiO₂, 200-nm thick), are shown at fixed V_sd = −60 V and varying V_g.