## 3 Results and Discussion  
The structural crystallinity of a solidified film after ultrasound irradiation of the P3 HT solution was remarkably enhanced from the grazing incidence X-ray diffraction (GIXD) and photoluminescence (PL) measurements. The GIXD data of the spin-coated P3 HT films with a distinct (100) diffraction peak in Figure 1a indicate, indeed, that \(\pi\)-\(\pi\) stack aggregation-induced molecular ordering is more prominent than that in the pristine P3 HT, which is consistent with a lower PL intensity in the sonicated P3 HT film, as seen Figure 1b [4]. Comparison of the PL intensity between the pristine and the sonicated P3 HT films shows that a higher PL intensity was observed in the pristine P3 HT, which is attributed to more enhanced interactions between the polymer chains in the sonicated P3 HT [4]. The origin of the enhanced crystallinity in the sonicated P3 HT film is consistent with the color change of the P3 HT solution from bright orange to dark brown, as seen in the inset of Figure 1a, which is a feature of aggregation of P3 HT polymer chains [1]. The structural difference will be discussed later. The enhanced molecular ordering in the sonicated P3 HT film led to a far larger hole mobility. Figure 2 compares the FET transfer characteristic curves of the pristine and sonicated P3 HT FETs, from which the FET hole mobilities were extracted. The hole FET mobility increased from \(2.2\times 10^{-4}\) cm\({}^{2}\)/Vs in the pristine P3 HT to \(3.2\times 10^{-3}\) cm\({}^{2}\)/Vs after ultrasonic irradiation. As reported in previous studies, indeed, sonicated P3 HT films prepared by spin-coating show enhanced crystallinity, thereby enhanced carrier mobility [5; 16]. For both P3 HT FETs, we observed gate scan direction-dependent drain current hysteresis, which is typically observed in disordered materials such as organic semiconductors. With a high density of electronic traps, induced carriers in the electrical channel by the gate electric field are easily trapped or de-trapped depending on the polarity of the gate electric field. To understand electrical channel formation depending on the aggregation-induced anisotropic ordering of the P3 HT chains, thereby enhancing carrier mobility due to increased crystallinity, we monitored the magnitude of the drain current as a function of time during the solidification process. We compared the temporal evolution of the source-drain current for the pristine and sonicated P3 HT with the in-situ probing structure, as shown in Figure 3a. On dropping a droplet of P3 HT solution onto the test device, we observed a dramatic current increase in both devices due to percolation of the P3 HT polymer chains in the solution state, as seen in Figure 3b,c. As the solvent evaporates, the local concentration of the P3 HT at the free surface remarkably increases, causing a higher electrical conductivity near the free surface after the current onset [10; 23; 24; 25]. This is consistent with our previous report in which the channel formation of the sonicated P3 HT in different solvents was investigated [10]. Figure 1: (**a**) grazing incidence X-ray diffraction (GIXD) data of the pristine and sonicated poly(3-hexylthiophene) (P3 HT) films prepared by spin-coating of a P3 HT solution (5 mg/mL, chloroform). The inset shows the image of the pristine (bright orange) and sonicated (dark brown) P3 HT solutions. (**b**) Comparison of the photoluminescence (PL) intensity of the pristine and sonicated P3 HT films. The spin-coated P3 HT film thickness is estimated to be 90 nm. Figure 2: Comparison of the I\({}_{\text{D}}\)-V\({}_{\text{G}}\) transfer characteristic plots of the P3 HT films prepared by spin-coating. The applied drain voltages were \(-8\) V. For the pristine and sonicated P3 HT films spin-coated from the P3 HT solution (10 mg/mL, chloroform), the FET hole mobilities were \(2.2\times 10^{-4}\) and \(3.2\times 10^{-3}\) cm\({}^{2}\)/Vs, respectively.