Doping-dependent charge injection and band alignment in organic field-effect transistors

We have studied metal/organic semiconductor charge injection in poly(3-hexylthiophene) (P3HT) field-effect transistors with Pt and Au electrodes as a function of annealing in a vacuum. At low impurity dopant densities, Au/P3HT contact resistances increase and become nonohmic. In contrast, Pt/P3HT contacts remain ohmic even at far lower doping. Ultraviolet photoemission spectroscopy (UPS) reveals that metal/P3HT band alignment shifts dramatically as samples are dedoped, leading to an increased injection barrier for holes, with a greater shift for Au/P3HT. These results demonstrate that doping can drastically alter band alignment and the charge injection process at metal/organic interfaces.

Figure 1

(a) Log-log plot of the transport characteristics of a Au/P3HT device with $L=10\mathrm{\mu }\mathrm{m}$ at $T=300\mathrm{K}$ and at a fixed $V_G=-60\mathrm{V}$ for different annealing as described in the text. (b) Similar plot for a Pt/P3HT device with identical geometry as (a).

Figure 2

(a) Gate voltage dependence of $R_c$ for different anneals for the Au device at room temperature. (b) $R_c∕R_{ch}$ as a function of $V_G$ for a Au device with $L=10\mathrm{\mu }\mathrm{m}$.

Figure 3

Linear plot of $I_D-V_D$ for injection from Au and Pt for a sample at $300\mathrm{K}$ and $V_G=-80\mathrm{V}$. This sample was dedoped such that bulk conductivity at $300\mathrm{K}$ was below our measurement threshold.

Figure 4

(a) UPS cutoff energy shift as a function of annealing time for both P3HT/Au and P3HT/Pt. Inset: UPS cutoff of a P3HT/Au sample for different annealing times at $350\mathrm{K}$. (b) Energy level diagram of band alignment, based on the results of the UPS data for Au/P3HT before and after the annealing process, showing the large change in the barrier for hole injection.