Anomalous Pressure Effect in Heteroacene Organic Field-Effect Transistors

Anomalous pressure dependent conductivity is revealed for heteroacene organic field-effect transistors of dinaphtho[2, 3-b:$2^{\prime }$, $3^{\prime }$-f]thieno[3, 2-b]thiophene single crystals in the direction of $a$ and $b$ crystallographic axes. In contrast to the normal characteristics of a monotonic increase in mobility $\mu$ with the application of external hydrostatic pressure $P$ in conductors, we found that the present organic semiconductor devices exhibit nonmonotonic and gigantic pressure dependence including an even negative pressure coefficient $d\mu /dP$. In combination with a structural analysis based on x-ray diffraction experiments under pressure, it is suggested that on-site molecular orientation and displacement peculiar in heteroacene molecules are responsible for the anomalous pressure effect.

Figure 1

A photograph of a single crystal platelet of DNTT. Angles marked by black (red) and gray (blue) symbols are 100° and 130°, respectively. Crystalline directions for the $a$ and $b$ axes are indicated by white arrows.

Figure 2

Pressure dependence of (a) latticeconstants for three axes and (b) $\beta$ as the intersection angle between the $a$ and $c$ axes. (c) Ratio of compressibility between the $a$ and $b$ axes as a function of applied pressure. (d) Two suggested molecular arrangements in cases of shrinkage along the $a$ axis (above) and $b$ axis (below), respectively. Curved arrows indicate the direction of molecular rotation. (e) Crystal structure viewed perpendicular to $ac$ plane. Dark and light molecular images indicate molecules located on the front and back sides, respectively.

Figure 3

(a) Transport and (b) output properties of DNTT-FET for the $a$-axis conducting direction. (c) Transport properties of DNTT-FET for the $b$-axis conducting direction. Pictures for each device are inserted in (a) and (c). (d) Four-terminal conductivity along the $a$ and $b$ axes at ambient condition. The estimated carrier mobility is $2.1{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ for the $a$ axis and $1.3{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ for the $b$ axis at $V_G=-100\mathrm{V}$, respectively.

Figure 4

Pressure effect on DNTT-FET for the $a$ and $b$ axes. Those for pentacene and rubrene devices are also plotted for comparison. Circles and squares indicate the data obtained while in the compression and decompression processes, respectively.