Intrinsic Charge Transport on the Surface of Organic Semiconductors

The air-gap field-effect technique enabled realization of the intrinsic (not limited by static disorder) polaronic transport on the surface of rubrene (${\mathrm{C}}_{42}{\mathrm{H}}_{28}$) crystals over a wide temperature range. The signatures of this intrinsic transport are the anisotropy of the carrier mobility, $\mu$, and the growth of $\mu$ with cooling. Anisotropy of $\mu$ vanishes in the activation regime at low temperatures, where the transport is dominated by shallow traps. The deep traps, introduced by x-ray radiation, increase the field-effect threshold without affecting $\mu$, an indication that the filled traps do not scatter polarons.

Figure 1

The surface conductivity, ${\sigma }_{\square }$, as a function of the gate voltage, $V_g$, measured with the 4-probe air-gap transistor stamp along the $b$ axis of rubrene (the source-drain voltage $V_{\mathrm{S}\mathrm{D}}=5\mathrm{V}$, the drain is grounded). The inset schematically shows the transistor stamp: $L=0.75\mathrm{m}\mathrm{m}$, $D=0.25\mathrm{m}\mathrm{m}$, $W=1.25\mathrm{m}\mathrm{m}$, $t=5\mu \mathrm{m}$; $C_i=0.2\mathrm{n}\mathrm{F}/{\mathrm{c}\mathrm{m}}^2$.

Figure 2

The temperature dependence of the field-effect mobility and the threshold voltage measured along the $a$ and $b$ axes of rubrene crystals. The inset schematically shows the electronic states near the HOMO energy.

Figure 3

The effect of the x-ray radiation on OFET characteristics. $I_{\mathrm{S}\mathrm{D}}(V_g)$ have been measured simultaneously along the $a$ and $b$ axes using a 2-probe transistor stamp with two perpendicular channels. After each x-ray exposure, the crystal was measured at 300 K in air and in the dark; $\Delta t$ indicates the cumulative exposure time. The mobility proportional to the slope of $I_{\mathrm{S}\mathrm{D}}(V_g)$ is not affected by x-ray irradiation, whereas $V_{\mathrm{t}\mathrm{h}}$ is noticeably increased.

Figure 4

The temperature dependence of the field-effect mobility $\mu (T)$ and the threshold voltage $V_{\mathrm{t}\mathrm{h}}(T)$ of the rubrene OFET measured by the 4-probe transistor stamp along the $b$ axis before (open circles) and after (solid circles) the x-ray exposure for $\Delta t=4\min $. The inset is the dependence $\mu (T)$ along the $b$ axis for another device (not exposed to x rays): the low-$T$ fit is the Arrhenius dependence with the activation energy $\sim 70\mathrm{m}\mathrm{e}\mathrm{V}$.