Field-effect mobility of polycrystalline tetrabenzoporphyrin thin-film transistors

A possible relation between a thin-film microstructure and an organic thin-film field-effect transistor (OFET) behavior is discussed in terms of nonlinearity in the extraction of the device electrical parameters. Staggered source and drain electrode OFETs were fabricated using a soluble precursor form of the organic small molecule semiconductor tetrabenzoporphyrin, and characterized using linear and nonlinear best-fit methods. Linear best-fit models overestimated the field-effect mobility and accumulation threshold voltage when compared to a nonlinear best-fit model that accounts for dispersive charge-carrier transport. The deviation between the methods is found to be consistently less than that for polymer OFETs, as indicated by smaller nonlinearity factors of γ=1.2 and 1.7 in the linear and saturation regimes, respectively. The nonlinear field-effect mobility exhibits a sublinear gate-bias dependence wherein the mobility increases at a slower rate in strong accumulation than near threshold. Furthermore, nonlinear curve fitting indicates lower trap characteristic temperatures as compared to polymer OFETs, and a relatively moderate density of grain-boundary trap states localized at the dielectric interface and in the bulk to be filled before accumulation-related conduction dominates.