Resolving the contact voltage dilemma in organic field effect transistors

In spite of the great interest in organic field effect transistors, many of their aspects are still not well understood. In particular, efforts to uncover the origin of the contact resistance and the underlying physics have lead to apparently contradictory results. Here we show that all these features can be understood by a unified description that takes into account thermionic emission with diffusion-limited injection at the source contact and space-charge limited conduction near it. Moreover, the usual field effect transistors behavior at a certain distance from the source and the conduction in the depletion region emerge not as ad hoc assumptions but directly from the proposed mechanisms.

Figure 1

$I_{\text{ds}}\times V_c$ from a set of P3HT/Cr devices with $W=200\mu \text{m}$ and $V_g=60\text{V}$. The gate insulator is a ${\text{SiO}}_2$ layer of 200 nm. The data are from Fig. 7a of Ref. 24 and the continuous curves from the model. Inset: the barrier height as function of $T^{-1}$ with the line a fitting only for $T\ge 210\text{K}$.

Figure 2

As in Fig. 1 but for $T=210\text{K}$ and different $V_g$. The data are from Fig. 7b of Ref. 24. Inset: the resulting $\mu \left(T\right)$.

Figure 3

$I_{\text{ds}}\times V_{\text{ds}}$ from a P3HT/Au-Cr OFET, with $L=5\mu \text{m}$, $T=240\text{K}$, and the other features as in Fig. 1. The data are from Fig. 5 of Ref. 24. Inset: the straight lines $V_T\times V_g$ from the fittings of Fig. 2 (circles) and Fig. 3 (squares).