| Organic thin-film transistors (OTFTs) are potential candidate for low cost electronics and can be fabricated via solutions on flexible substrates. Solution-processed organic electronic devices have attracted much attention due to their non-vacuum and environment-friendly processability. Compared with other solution-based fabrication processes, inkjet printing possesses merits of direct-patterning capability and non-contact approach, while dip-coating exhibits superiority as it enables directional growth.This dissertation discusses solution fabrication of the semiconductor layer of TIPS-pentacene OTFTs via inkjet printing or dip-coating technique. The research is focused on the effect of surface gradient involved in the solution processing and discussed in more details.(1) An inkjet printing method with Au-induced directional crystallization was proposed that employs difference in surface energy to induce film growth with higher crystallinity and orderliness. Inkjet printing of TIPS-pentacene, a small molecule semiconductor was performed as a single-line film with the starting point set on a patch of Au film, which had been surface-treated to have a slightly higher dispersive surface energy than the dielectric substrate. A larger dispersive surface energy on the metal resulted in advanced contact-line pinning thereof between the liquid and the substrate and induced film growth along the printing direction. With the influence of the differential surface energy, larger grain size as well as more convergent crystalline microstructure was obtained, which is beneficial to fabricate of organic thin-film transistors with improved electrical performance and device-to-device uniformity by controlling the overlapping length L. With increasing L from 150 μm to 350 μm, the field-effect mobility of devices on average enhanced from 0.03 cm2V-1s-1 to 0.077 cm2V-1s-1, yielding the maximum mobility as high as 0.151 cm2V-1s-1. The field-effect mobility obtained a more concentrated distribution as is evident for L= 250 μm.(2) Wetting/nonwetting surface patterning of the substrates is performed under the protection of a patterned photoresist layer in gas-phase 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (FDTS), after which one-step dip-coating and patterning of TIPS-pentacene with micron-scale patterns (400×500 μm2) is demonstrated. The mobility distribution as well as the film morphology apparently shows that the device performance is optimized at the optimal withdrawal speed, which is dependent on the vapor pressure of the solvent. The fabricated devices with patterned exhibited the field-effect mobility of 0.66 cm2V-1s-1 and an on/off ratio of 4×104 at an optimal withdrawal speed of 7 mm/min in chloroform. Crystal morphology and electrical mobility of the unpatterned film are optimized at the same withdrawal speed compared to the patterned films. The random crystal orientation of the patterned films causes a remarkable decrease in device performance and the film coverage of the dip-coated film is to be improved.(3) Further study on the influence of solvent composition, drawing speed, ink concentration, substrate temperature and the surface material is evaluated and optimized for higher film coverage. Primary factor affecting the film coverage and field-effect mobility of the device are investigated. The field-effect mobility (0.264 cm2V-1s-1) of devices as well as the film coverage (57.16%) is improved when the unpatterned film is dip-coated at optimized temperature of 30℃ from the solution in toluene. |
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