The charge injection efficiency of organic field-effect transistors (OFETs) is found to be a critical factor determining the subthreshold characteristics of these devices. OFETs fabricated using a wide band gap organic semiconductor and gold source/drain contacts display large threshold voltage and poor subthreshold characteristics. Insertion of a metal-oxide charge injection layer at the contact/semiconductor interface lower the injection barrier height, resulting in marked improvements in threshold voltage and subthreshold slope and strong suppression of the short-channel effect. The improved subthreshold characteristics are attributed to enhanced charge injection and the consequent promotion of charge accumulation.
Semiconductor channels of organic field-effect transistors have been directly self-organized from a solution phase. The alkyl-modified surface was locally patterned by using a phenyl self-assembled monolayer (SAM) for the channels. Drop-cast small organic molecules were selectively crystallized on the phenyl SAM region. The self-organized process allows the simultaneous formation of polycrystalline transistor arrays from the patterned channels. The phenyl SAM under the channel is critical for the improvement of device stability. Further optimization of the deposition process realized direct growth of a single crystal channel from solution between prefabricated electrodes, and the single-crystal transistors exhibited excellent performance.
The solubility and low processing temperatures of organic semiconductors enable fabrication of electronic devices using relatively simple printing technologies, and hold promise for realizing flexible plastic devices by environment‐friendly production methods at low cost. In particular, by effectively using the self‐assembling ability of molecules, production methods for organic semiconductor devices are expected to become more efficient in terms of energy and material consumption. We have developed two solution‐based methods for self‐organized formation of organic semiconductor crystals, including area selective nucleation of crystalline semiconductor films and direct formation of organic single crystals. These bottom‐up methods of device fabrication, wherein the intrinsic functionalities of molecules are utilized for spontaneous assembly, may become a core technology for future plastic electronics.
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