Research On High Performance C₆₀-Based N-type Organic Thin-film Transistors

Organic thin-film transistors have attracted more and more attention due to their inherent advantages, like light weight, mechanical flexibility, low-cost production, and low temperature processes. N-type OTFTs play a great part in Organic logic circuits. However, compare with the development of p-type OTFTs, the research of n-type OTFTs is relatively lagging behind.In this paper, C₆₀-based n-type organic thin film transistors were demonstrated, and were optimized by introducing different modification layer, optimizing the electrode/active layer interface and optimizing the insulation layer. Finally, the highest mobility of 30 cm2 / V·s was achieved.1. Employing the pentacene layer to improve the crystallinity and morphology of C₆₀ layer, high mobility pentacene/C₆₀ heterojection based ambipolar organic thin-film transistors were demonstrated. It was found that hole mobility increased monotonically, and the electron mobility increased at first and then decreased, with the increasing thickness of the pentacene layer. The morphologies of both pentacene and C₆₀ layers, which are investigated by atomic force microscopy, demonstrated the effect mechanism of the pentacene layer thickness on the device performance. The most optimized ambipolar OTFT was achieved with a 4.5-nm-thick pentacene layer, whose hole mobility is 0.39cm2/Vs, and the electron mobility is 6.3cm2/Vs. Furthermore, to improve the performance and reduce the working voltage, we try to reduce the thickness of the dielectric layer. We adopted ultrathin dielectric AlOX/P（MMA-GMA） hybrid layer as the gate dielectric whose thickness is 15 nm. The working voltage is within 3V.2. Introducing tetratetracontane as modified layer to effectively improve the interface morphology, the high-performance n-type organic thin film transistor was achieved. We found that the heat annealing operation on tetratetracontane make device performance greatly improved. AFM figures of tetratetracontane and C₆₀ interface show that interface morphologies was significantly optimized due to the heat annealing operation, and the mobility was obvious improved, which raised more than 10 times. During the experiment, we made use of Bphen as a buffer layer to enhance the injection of carriers. The metal Ag was introduced as the source/drain electrode, in order to reduce the barrier between the work function of metal and the LUMO energy level of organic material. Experiments show that, the mobility of OTFT modified by Bphen mobility of carriers increased significantly, which was 4.67 cm2/Vs. Furthermore, we focused on choosing a suitable insulation material to improve the device performance. We tried four different polymer insulation layers, which are PMMA, PVA, PS and P（VDF- CTFE- TrFE） respectively. When choosing the PVA polymer as the insulation layer, a high performance n-type organic thin film transistor was achieved; whose mobility is 30 cm2/Vs. Through the comparative analysis, PVA has the proper dielectric constant and molecular structure. Furthermore, the PVA surface has high tension and good quality interfacial morphology, which lead to the high device performance.