Performance improvement mechanisms of organic thin-film transistors using MoO_x-doped pentacene as channel layer

Organic thin-film transistors (OTFTs) with various MoO_x-doped pentacene channel layers were fabricated and investigated. Compared the OTFTs with the 0.50 mol% MoO_x-doped pentacene to the conventional OTFTs without MoO_x dopant, the maximum output current was increased from -11.6 to -37.9 μA, the effective field-effect mobility was enhanced from 0.71 to 1.60 cm²/V-s, the threshold voltage was reduced from -21.2 to -14.8 V, and the on/off current ratio slightly decreased from 3.6 × 10⁶ to 1.2 × 10⁶. The performance improvement was attributed to the highest occupied molecular orbital (HOMO) of the MoO _x-doped pentacene gradually approached to the Au work function with increasing the doping percentage of MoO_x, which led to reduce the contact resistance and to enhance the p-type characteristics of the MoO _x-doped OTFTs by increasing the hole density and enhancing the hole-injection efficiency. However, the output current and the field-effect mobility decreased with an increase of the MoO_x doping percentage, if the doping mole percentage of MoO_x was higher than 0.50%. This behavior was attributed to the Fermi level pinning effect, gradual increase of hole concentration and significant degradation of crystallinity.