Approaching the Trap-Free Limit in Organic Single Crystal Field-Effect Transistors

Crystalline organic semiconductors, bonded by weak van der Waals forces, exhibit macroscopic properties that are very similar to those of inorganic semiconductors. While there are many open questions concerning the microscopic nature of charge transport, minimizing the density of trap states (trap DOS) is crucial to elucidate the intrinsic transport mechanism. We explore the limits of state-of-the-art organic crystals by measuring single crystalline rubrene field-effect transistors that show textbook like transfer characteristics, indicating a very low trap DOS. Particularly, the high purity of the crystals and the very clean interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of 65 ${\rm mV / decade}$, remarkably close to the fundamental limit of $58.5 {\rm mV / decade}$. From the measured subthreshold behavior we have consistently quantified the trap DOS by two different methods, yielding an exceedingly low trap density of $D_{bulk} = 1 \times 10^{13}~{\rm cm^{-3}eV^{-1}}$ at an energy of $\sim0.62~{\rm eV}$. These numbers correspond to one trap per eV in $10^8$ rubrene molecules. The equivalent density of traps located at the interface is $D_{it} = 3 \times 10^{9}~{\rm cm^{-2}eV^{-1}}$ which puts them on par with the best crystalline ${\rm SiO_2/Si}$ field-effect transistors.