Candidate:
Hyunwoo
Choi
Title:
Polymer
semiconductor
and
2D
materials
hybrid
nanocomposite
system
for
inkjet-printed
TFT
applications
Date:
October
7,
2022
Time:
3:00
PM
Place:
EIT
3142
Supervisor:
Wong,
William
Abstract:
Patterned using subtractive processes, conventional thin-film deposition techniques inevitably require a high-vacuum deposition and photolithography to define functional layers in a device structure. Inkjet printing technology has received considerable attention to realize low-cost and mass production of large-area electronics at low temperatures using an additive process approach. However, the materials used in the printing process is based on solution-based electronic inks formulated with organic electronic materials. Among them, the conjugated polymers are widely used as a semiconductor for thin-film transistor (TFT) applications, but they possess poor charge transport properties compared to other single or polycrystalline inorganic semiconductors. To overcome this limitation of the polymer semiconductor, a hybrid organic/inorganic semiconductor ink for inkjet printed TFTs was explored. The hybrid semiconductor ink was prepared by mixing two different materials, molybdenum disulfide (MoS2) nanosheets and solution-based poly(3-hexylthiopene-2,5-diyl) (P3HT), the former is a two-dimensional semiconductor and the latter a conjugated polymer. To enhance the level of exfoliation and stability of MoS2 nanosheets in P3HT, the surfactant trichloro(dodecyl)silane (DDTS), was used to functionalize the MoS2 surface. Printed TFTs using the nanosheet suspension were found to enhance the field-effect mobility by approximately 3× compared to TFTs without the suspension. The introduced single-crystalline MoS2 nanosheets in the P3HT matrix improved the electrical and structural properties of the inkjet-printed thin-film polymer. With this finding and understanding, the effect can be applied to second-generation polymer semiconductors known as donor-acceptor (D-A) co-polymers. These materials have been reported to have the highest mobilities among printable polymers with ambipolarity, which is beneficial for configuring complementary metal-oxide-semiconductor (CMOS) circuits. New nanocomposite semiconductor inks were also formulated to demonstrate the effect of 2D nanoparticles on the electronic properties of D-A copolymers, diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT). Printed TFTs using this new hybrid semiconductor showed that the field-effect mobility of the devices increased by 33 % and 140 % in both hole (p-type) and electron (n-type) transports, respectively. Atomic force microscopy (AFM) results of the printed hybrid thin film revealed that strongly aggregated polymer domains were observed in films containing the MoS2 nanosheets. In ultraviolet–visible–near infrared spectroscopy (UV-vis-NIR) measurement, increased intensity of 0-0 and 0-1 peaks from hybrid film indicates improved charge transport was due to enhanced intermolecular charge transfer in the microstructure of the polymer film. This work revealed that solution processed MoS2 nanosheets increase the crystallinity and electrical properties of polymeric thin films to improve performance parameters for inkjet-printed TFTs.