Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{–}\text{decyl}\text{–}7\text{-phenyl-}\left[1\right]\text{benzothieno}[3,2\text{–}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}\left[1\right]\text{benzothieno}[3,2\text{–}b]\text{naphtho}[2,3\text{–}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

Figure 1

(a)−(c) Molecular structures of (a) Ph-BTBT-C10, (b) Ph-BTNT-C10, and (c) parylene C. (d)−(f) Optical micrographs of (d) device A, (e) device B, and (f) device C. Scale bars show 100 μm. The thickness of parylene layers are measured to be 26 and 40 nm for (e) and (f), respectively.

Figure 2

Transfer characteristics of single-crystal transistors at 300 K. (a)−(c) $I_{\mathrm{d}}\text{−}{V}_{\mathrm{g}}$ characteristics of (a) device A, (b) device B, and (c) device C. (d)−(f) $V_{\mathrm{g}}$ dependence of device mobility of (d) device A, (e) device B, and (f) device C.

Figure 3

(a), (b) Output characteristics of (a) device A and (b) device A′. (c) Width-normalized access resistance of device A (black) and device A′ (green) as a function of $V_{\mathrm{d}}$ at $V_{\mathrm{g}}=-40\mathrm{V}$. Width-normalized channel resistance of device A at $V_{\mathrm{g}}=-40\mathrm{V}$ is shown by red broken line. (d) Temperature dependence of $R_{\mathrm{s}}W$ of device A measured at $V_{\mathrm{d}}=-1\mathrm{V}$.

Figure 4

Band-like carrier transport in bilayer-type OSCs. (a)−(c) Temperature dependence of $I_{\mathrm{d}}\text{−}{V}_{\mathrm{g}}$ characteristics of (a) device A, (b) device B, and (c) device C. (d)−(f) Temperature dependence of device mobility of (d) device A, (e) device B, and (f) device C. All of the solid curves in (d)−(f) show the relationship of $\mu \propto T^{-\alpha }$, where α was distributed within a range of $0.8<\alpha <1.4$.

Figure 5

(a) $I_{\mathrm{d}}\text{−}{V}_{\mathrm{g}}$ characteristics of device A′ at 300 K. (b) $\mu \text{−}{V}_{\mathrm{g}}$ relationship of device A′ at 300 K. (c) Temperature dependence of device mobility. Open and filled circles denote the mobility of device A and device A′, respectively.

Figure 6

Activated behavior at low-temperature region. (a) Temperature dependence of $I_{\mathrm{d}}\text{−}{V}_{\mathrm{g}}$ characteristics of device A below 140 K plotted in a semilogarithmic scale. Inset shows an Arrhenius plot of the same data. (b) Activation energies estimated from the Arrhenius plot. (c) Schematic of the relationship between $E_{\mathrm{A}}$ and $V_{\mathrm{g}}$.

Figure 7

Distribution of trap DoS extracted from $V_{\mathrm{g}}$-dependent activation energies. Solid curve shows the fitting curve for the results of device A. Literature data (broken curves) were obtained from pentacene [35], C10-DNBDT-NW [36], TMTSF [37], Rubrene [15].