Enhanced mobility for increasing on-current and switching ratio of vertical organic field-effect transistors by surface modification with phosphonic acid self-assembled monolayer

For typical organic transistors with a lateral source/drain contact geometry, self-assembled monolayer (SAM) has been extensively applied to modify the organic/dielectric interface state and to improve the in-plane charge transport. Here, we demonstrate that SAM modification can also be applied to improve the out-of-plane charge transport in vertical organic field-effect transistors (VOFETs), leading to a simultaneous increase in the oncurrent and ON/OFF ratio. We investigate a series of phosphonic acid (PA) SAMs with different alkyl chain length for modification of p-type dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) VOFETs and their corresponding lateral transistors for comparison. The perforated source electrode of VOFETs is fabricated using surfactant-assisted colloidal lithography, which provides excellent control of the perforations fill factor and allows the introduction of an insulating cap layer to eliminate the off-current. Overall, the VOFET modified with dodecyl-PA SAM shows the optimal performance, with a 6 times enhanced mobility (and on-current) and a larger ON/OFF ratio compared to the unmodified device. Similar performance enhancement is also observed in the lateral transistors with SAM modifications. From detailed analyses of DNTT film morphology, crystallinity, and molecular orientation, we demonstrate that SAM modification exerts a strong impact on film morphology not only near the interface but also in the bulk, which plays a major factor for enhancing the mobilities in two types of transistors. The highest mobility obtained in the dodecyl-PA SAM modified VOFET is primarily attributed to the formation of largest grains with a resultant decrease of boundary defect density, despite that the crystallinity and molecular packing are not much improved than the other devices.