Enhanced performance of fullerene n-channel field-effect transistors with titanium sub-oxide injection layer

Enhanced performance of n-channel organic field-effect transistors (OFETs) is demonstrated by introducing a titanium sub-oxide (TiO_x) injection layer. The n-channel OFETs utilize [6,6]-phenyl-C61 butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) as the semiconductor in the channel. With the TiO _x injection layer, the electron mobilities of PC₆₁BM and PC₇₁BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R_c) shows significantly decreased Rc values for FETs with the TiO_x layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO_x layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO_x. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO_x layer acts as a passivation layer that prevents penetration of O₂ and H ₂O; devices with the TiO_x injection layer exhibit a significant improvement in lifetime when exposed to air.