TIPS pentacene unit cell in two different orientations, showing that the acene units in TIPS pentacene have a periodic columnar stacking. Left: A view of the unit cell down the  direction. Right: The unit cell down .
A schematic of the thin film solution casting system with controlled temperature and atmosphere.
The top-contact thin film transistor configuration used in this study. The schematic is not to scale and a detailed description of each layer is available in Sec. II.
An optical micrograph showing a TIPS pentacene film that had grain widths larger than . This TIPS pentacene film was deposited in the controlled solution casting system shown in Fig. 2, with a nitrogen gas flushing rate of 0 l/min and TIPS pentacene solution in toluene at . Figure 4 shows the long needle shape of large TIPS pentacene grains.
An AFM image showing a TIPS pentacene film that had a grain width smaller than . This TIPS pentacene film was deposited in the controlled solution casting system shown in Fig. 2, with a nitrogen gas flushing rate of 0.5 l/min and 0. TIPS pentacene solution in toluene at room temperature. The figure shows the rounded and isotropic shape of small TIPS pentacene grains in three dimensions. However, in contrast to the low voltage TEM image shown in Fig. 6, grain boundaries are not quite obvious in this AFM image. The bottom right corner of the AFM image is the edge of the evaporated gold electrode.
Right: A bright-field low voltage electron micrograph (Delong, LVEM-5 at 5 kV) illustrating that the grain boundaries in TIPS pentacene films with small grain sizes (in this case, on the order of hundreds of nanometers) adopted irregular but close-to-isotropic geometries. The average length of grain interfaces is around 20 nm in the pictures above. Left: An inverted bright-field image of the image on the right showing high contrast in grain-boundary curves of TIPS pentacene domains. The TIPS pentacene films shown here were deposited thermally with a vacuum evaporator at a deposition rate of about 10 nm/s.
Top: Typical (a) output and (b) transfer characteristics for a TIPS pentacene thin film transistor with ; Bottom: Typical (c) output and (d) transfer characteristics for a TIPS pentacene thin film transistor with .
Grain-size dependent hysteresis in TIPS pentacene thin film transistors . The average grain width was estimated from the channel width and number of crystals across the channel, according to the definition of grain width in Fig. 4. Transistors A and B had average grain widths of 10 and , respectively, with the average angles between the long axes of the TIPS pentacene crystals (or the  direction) and source-drain channel direction about 30º, so that the effect of crystal misorientation could be reduced significantly. Transistor A exhibited much higher hysteresis, although the drain current in transistor B was significantly larger. Gold electrodes (source and drain) are shown on both sides of the TIPS pentacene channels in the polarized light micrographs above. The color variation in the channel was mostly due to orientation change (and some thickness fluctuation) of the crystalline domains.
Grain-size dependent mobility of solution-processed TIPS pentacene thin film transistors ( and ) on a logarithmic scale. Solution-processed TIPS pentacene films with different grain widths were deposited with toluene solution, wafer substrate, and 0–0.5 l/min flushing rate at room temperature by using the controlled system in Fig. 2. Grain widths of were achieved in this study. The experimental data are represented by diamonds (with error bars), while the calculated values are in open circles and open squares. Calculated curve 1 was simulated with the needle model [Fig. 11(a)] for the large grain width region and a mixed microstructure (0.5%–3% needles with a majority of spheres) for the smaller grain width region, with more needles added toward the distinct morphological transition. Calculated urve 2 was based on the sphere-only [Fig. 11(b)] model for the smaller grain width region. In the small grain width region , the mixed-microstructure (calculated curve 1) seemed to be more accurate than the sphere-only model (calculated curve 2), suggesting that the small morphological anisotropy in these films (which was neglected in the sphere-only model) could play a significant role. Gold electrodes (source and drain) are shown with the TIPS pentacene channels in the inserted polarized light micrographs. The color variation in the channel was mostly due to orientation change (and some thickness fluctuation) of the crystalline domains. The abrupt change in saturation mobility from films with grain width smaller than to films with larger than was attributed to the grain-boundary geometry change from more equiaxed shapes to long needles (Fig. 11). Precise estimation of the angle was more difficult to obtain in TIPS pentacene domain of larger with only optical microscopy. Variations in measured mobilities with grain width could have several reasons: discrepancies between the morphology of semiconductor-insulator surface and that of the top view in micrographs, local variations in the crystal orientation (and thus angle), fluctuations in impurity and moisture levels, etc.
Grain-size dependent mobility of solution-processed TIPS pentacene thin film transistors ( and ) on a linear scale. The top figure shows the grain width dependent mobility in both small and large grain width ranges. The bottom graph shows the roughly linear relationship between and for large grain widths as described in Eq. (8). The calculated line is based on the calculated curve 1 in Fig. 9, whereas the difference between calculated curves 1 and 2 of Fig. 9 is negligible here on a linear scale. Solution-processed TIPS pentacene films with different grain widths were deposited with toluene solution, wafer substrate, and 0–0.5 l/min flushing rate at room temperature by using the controlled system in Fig. 2. The standard deviations of both and for each data point are available in Fig. 9.
Simplified geometry descriptions of TIPS pentacene films used in grain-size dependent mobility calculation (Figs. 9 and 10). is the grain width of the needle-shaped or equiaxed TIPS pentacene crystal. is the grain length projected along the channel direction. The angle is the average angle between the long axis of needle-shaped TIPS pentacene crystals (or the  direction) and the channel direction (from source to drain). “” stands for source electrode (gold) and “” represents drain electrode (gold). The abrupt change in saturation mobility from films with grain width smaller than to films with larger than (Fig. 9) was attributed to the grain-boundary geometry change from more equiaxed shapes [Fig. 11(b)] to long needles [Fig. 11(a)].
The effect of grain width on the threshold voltage and on/off ratio in solution-processed TIPS pentacene transistors.
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