Controlling Microstructure-Dependent Mobility of Organic Thin Film Transistors by Various Crystalline Rates
Wei-Yang Chou1*, Sheng-Kuang Peng1, Cheng-Chang Lu1, Jrjeng Ruan2, Horng-Long Cheng1, Fu-Ching Tang3, Hwo-Shuenn Sheu4
1Department of Photonics, National Cheng Kung University, Tainan, Taiwan
2Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
3Department of Physics, National Cheng Kung University, Tainan, Taiwan
4National Synchrotron Radiation Research Center, Hsinchu, Taiwan
* presenting author:Wei-Yang Chou,
In the past few decades, organic thin film transistors have become high anticipation in polymeric semiconductors (PSCs) because of their unique advantages, such as low temperature manufacturing, solubility processing, and structural flexibility. Different growth mechanisms of PSCs produce various microstructures of thin films and change the performance of PSC-based devices. We proposed an effective method to enhance the microstructure-dependent electrical performance of PSC-based devices. Poly(3-hexylthiophene) (P3HT) thin film formed by the spin-coating process (denoted as normal P3HT) showed a near-amorphous structure with a large amount of small domains and grain boundaries. We used a way of growth guiding during the P3HT crystallization to conduct a better orientation of P3HT molecules. Hexamethylbenzene (HMB), an easily crystallized material, was mixed with P3HT to form a HMB-processed P3HT (H-P3HT). H-P3HT specimens were placed on a thermal gradient system to move from the high temperature point toward the low temperature point at various velocities (Vm). During the growth of the H-P3HT thin films, the HMB molecules separated out of H-P3HT surface and the P3HT crystallized along the motion direction of the specimens. Thus, the crystallized HMB molecules could guide the growth of the P3HT molecules. After this process, the HMB molecules were removed from the specimens under vacuum ambiance.
The microstructural features of P3HT thin films were examined through microscopy and spectroscopic methods. At low Vm range, the mobility of the H-P3HT thin films were enhanced with increasing Vm. However, at high Vm range, their mobility was decreased as Vm increased. The H-P3HT-based devices exhibited a better electrical performance than the normal P3HT. These improvements of the microstructures provided an ideal charge transport path to reduce the exhaustion of energy for carrier drifting within active layer. Thus, the output current of the H-P3HT devices was larger than that of the normal P3HT devices. Accordingly, we demonstrated a simple technique to improve the electrical performance of PSC-based devices. This technique can also be applied to enhance the performance of other organic electronic devices.

Keywords: polymer semiconductor, P3HT, organic thin-film transistor, thermal gradient, crystallized guiding