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 for that. The geometric trap should have a similar effect to both electron and hole carriers, and it also cannot explain the higher mobility of α-NPB than β-NPB. Thus, all the experimental data show that there are some new kinds of traps existing in the OSCs. Because their site in energy space is close to the HOMO and LUMO of OSCs, they are noted as HOMO and LUMO traps. With the help of HOMO and LUMO traps, the experimental data can be easy to explain.