Concepts of the HOMO and LUMO Traps from the Carrier Dynamics of Organic Semiconductor Isomers α-NPB and β-NPB

The carrier dynamics of two isomers, α-NPB and β-NPB, were investigated through impedance spectroscopy based on the PSO algorithm. Their structure differences are only the N atom substitution sites on the naphthalene group. For α-NPB, the N-substitution is on the α-site of naphthalene, and for β-NPB, it is on the β-site. The carrier mobility of α-NPB is much higher than that of β-NPB. But AFM morphology shows that the film of β-NPB is smoother than that of α-NPB, which is in contrast with the common idea that, for a similar molecular structure, smoother film means a larger carrier mobility. In addition, the electron mobility of α-NPB is negatively related with the electrical field, but the hole mobility of α-NPB and the electron and hole mobilities of β-NPB are all positively related with an electrical field. In a common viewpoint, the carriers should be accelerated by an electrical field since they are charged carriers. Such phenomena have to be ascribed to a trap, but only a geometric trap is not enough...