Improved Performance of Solution-Processed n-Type Organic Field-Effect Transistors by Regulating the Intermolecular Interactions and Crystalline Domains on Macroscopic Scale

The development of four new n-channel naphthalene diimide (NDI) and perylene diimide (PDI) copolymers (NDI-Ph, NDI-BT, PDI-Ph, and PDI-BT) and their solution processed thin film transistor (TFT) devices are reported. Remarkable enhancements in the electron transport behavior for all the four copolymers were achieved on improving the intermolecular interactions in their thin film structures. These solution processable n-type copolymers having NDI and PDI backbone were synthesized in high yields (83–86%) by palladium catalyzed Suzuki coupling reactions, and their excellent solubility in several organic solvents allowed their deposition in organic thin film transistor (OTFT) devices from solution directly. Since these copolymers possess crystalline domains, annealing their films induced crystalline phases in the thin film structures with a very high degree of enhancement in crystallinity that was more prominent for PDI copolymers as compared to NDI derivatives. This resulted in significant enhancement in the intermolecular interactions in the thin film state on the macro scale, facilitating improved and higher charge carrier transport in annealed devices as compared to the as-spun devices that have lesser crystalline phases. The transport measurements performed for these four copolymers helped us to understand the difference in transport mechanism between D–A and A–A moiety and confirmed that tuning the thin film structures and the electronic properties by modifying the copolymer backbone structures as well as annealing them at appropriate temperature has profound implications on the level of improvement in electron transport behavior. The enhancement in μ e values for all four copolymers is very large for any reported n-type copolymers. It is observed that the extended conjugation in the four copolymer structures, the efficient intermolecular interactions in the thin film state, and the formation of crystalline domains in the copolymers after annealing are, in principle, responsible for the enhanced device performance. These copolymers demonstrated electron mobility enhancement of several orders and are reported to be as high as 0.8 cm 2 V –1 s –1 and 0.2 cm 2 V –1 s –1 with I on / I off ratios 10 5 for NDI-Ph and NDI-BT, while those of PDI-Ph and PDI-BT are 0.04 cm 2 V –1 s –1 and 0.032 cm 2 V –1 s –1, respectively, with I on / I off ratios of 10 3 –10 4 .