Enhanced Performance of Fullerene n-Channel Field-Effect Transistors with Titanium Sub-Oxide Injection Layer

Enhanced performance of n-channel organic field-effect transistor (OFETs) is demonstrated by introducing a titanium sub-oxide (TiOx) injection layer. The n-channel OFETs utilize [6,6]-phenyl-C-61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C-71 butyric acid methyl ester (PC71BM) as the semiconductor in the channel. With the TiOx 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 R-c values for FETs with the TiOx layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiOx layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiOx. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiOx layer acts as a passivation layer that prevents penetration of O-2 and H2O; devices with the TiOx injection layer exhibit a significant improvement in lifetime when exposed to air.