Voltage-Induced Metal-Insulator Transition in Polythiophene Field-Effect Transistors

We have extensively studied the carrier transport in regio-regular polythiophene field-effect transistors (FETs) from room temperature to 4.2 K. At low temperatures, Zabrodskii plots ($d\ln \sigma /d\ln T$) demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition at a carrier density of $5\times {10}^{12}{\mathrm{cm}}^{-2}$. The carrier transport in the insulating regime is well described by phonon assisted hopping in a disordered Fermi glass with Coulomb interaction between the hopping charge carrier and the opposite charge left behind, as described by Efros and Shklovskii.

Figure 1

Conductivity vs inverse square-root temperature for a RR-P3HT FET with $\mu =0.07{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ (a) at $V_{sd}=-60\mathrm{V}$ and varying $V_g$. Top inset is a double logarithmic plot at $V_g=-40\mathrm{V}$ (blue), $V_g=-50\mathrm{V}$ (red), and $V_g=-150\mathrm{V}$ (black). Bottom inset shows the logarithmic derivative (Zabrodskii plot). The straight line is a power-law fit to the $V_g=-50\mathrm{V}$ data. (b) At $V_g=-150\mathrm{V}$ for varying $V_{sd}$. The straight lines are exponential fits to the data. Top inset is a double logarithmic plot at $V_{sd}=-1\mathrm{V}$ (blue), $V_{sd}=-24\mathrm{V}$ (red), and $V_{sd}=-60\mathrm{V}$ (black). Bottom inset shows the Zabrodskii plot. The fit to the $V_{sd}=-1\mathrm{V}$ data has slope $-1/2$ and the $V_{sd}=-24\mathrm{V}$ data are fitted to a power law.

Figure 2

Conductivity vs inverse square-root temperature for a device with $\mu =0.02{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at $V_{sd}=-60\mathrm{V}$ and varying $V_g$ ($-20\mathrm{V}$ steps) between $V_g=-10\mathrm{V}$ and $V_g=-150\mathrm{V}$. The straight line is an exponential fit to the $V_g=-90\mathrm{V}$ data. Inset shows the logarithmic derivative of the conductivity at $V_g=-150\mathrm{V}$. The straight line is a power-law fit to the data.

Figure 3

Conductivity vs inverse square-root electric field for a device with $\mu =0.08{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at 12.4 K with varying $V_g$ ($-10\mathrm{V}$ steps). The straight lines are exponential fits to the data. The electric field across the transistor channel is the measured four-probe voltage divided by the effective channel length. Inset shows $V_g$ vs $V_{sd}$ in the critical regime for the device shown in Fig. 1. The dashed line is a guide to the eye.