In-Crystal and Surface Charge Transport of Electric-Field-Induced Carriers in Organic Single-Crystal Semiconductors

Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity ${\sigma }_{\square }$ and Hall coefficient $R_H$. The Hall mobility ${\mu }_H$ ($\equiv {\sigma }_{\square }{R}_H$) reaches nearly $10{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ when relatively low-density carriers ($<{10}^{11}{\mathrm{cm}}^{-2}$) distribute into the crystal. ${\mu }_H$ rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.

Figure 1

(a) Transfer characteristics of a rubrene single-crystal field-effect transistor at 290 K. The red filled circles show the drain current and the blue filled circles show the four-probe sheet conductivity of the channel. The inset is a top view of the device. (b) Evolution of transverse voltage in the Hall-bar shaped rubrene single-crystal field-effect transistor (sample A) at 290 K for four different gate voltages from 25 to $-175\mathrm{V}$. Magnetic field is swept back and forth from $-10$ to 10 T perpendicular to the conduction channel, as shown in (c). (d) Inverse Hall coefficient (red filled circles) and sheet conductivity (blue filled circles) plotted as function of gate voltage at 290 K, for the Hall-bar shaped rubrene single-crystal field-effect transistor (sample A) illustrated in the inset. (e) Hall mobility as a function of gate voltage as a result of the simultaneous measurement of the Hall coefficient and the sheet conductivity of the same rubrene single-crystal field-effect transistor sample A (red filled circles) at 290 K. The blue solid line indicates a typical estimation of carrier-distribution depth with the unit of $c$-axis lattice constant of rubrene. The black dashed line shows the two-monolayer criterion for a guide.

Figure 2

(a) Four-probe sheet conductivity, (b) inverse Hall coefficient and (c) Hall mobility of the same rubrene single-crystal field-effect transistor (sample A) as shown in Fig. 3 at various temperatures. (d) Hall mobility of another similarly prepared rubrene single-crystal field-effect transistor (sample B) at various temperatures.