Contact Engineering in 2D Materials via an nm -Thick Hydrosoluble Polymer Insertion

In 2D layered materials, optimizing the metal contacts is a critical challenge for thin-film transistor applications. Transition metal dichalcogenides (TMDCs) are well-studied materials with atomic-level flatness, high mobility, and large electron mass, making them promising candidates to surpass silicon-based transistors. However, the Fermi level pinning (FLP) effect at the metal contact remains a key barrier. As of now, the insertion of 2D materials such as hexagonal boron nitride and deposited oxides has been demonstrated to reduce FLP, but these methods are neither scalable nor mild procedures near room temperature, which is necessary to avoid defect formation on TMDCs. Here, we demonstrate a solution-processable approach for forming a thin organic polymer layer (nm scale) on the contacts to suppress the FLP effect in WSe₂ transistors. The organic polymer used was poly-l-lysine (PLL) in an aqueous solution, and the key surface phenomenon is the spontaneous formation of a very thin polymer layer on the contacts through simple casting and rinsing during an intermediate treatment in the device fabrication process. The temperature-dependent transport characteristics and Schottky barrier height estimations of WSe₂ transistors demonstrate that inserting a PLL layer effectively reduces the FLP. The pinning factor (S) is significantly modulated from S = −0.001 in non-PLL-treated devices to −0.17 in PLL-treated devices, highlighting the effectiveness of PLL in mitigating FLP and improving interfacial energy alignment. This work uses a scalable solution process that relies simply on the interaction between the PLL polymer and the surface of WSe₂. This approach offers a versatile strategy for improving device performance and can be extended to other 2D materials.