Thickness Dependence of Carrier Mobility and the Interface Trap Free Energy Investigated by Impedance Spectroscopy in Organic Semiconductors

The authors report the hole mobilities of organic semiconductors (OSCs): N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine and N,N′-bis (3-methyl- phenyl)-N,N′-diphenylbenzidine in various thick films (50-800 nm) by impedance spectroscopy. The experimental results show that the mobility increases with the increase of thickness. After extrapolating the area of electric field by fitting the P-F equation, we find that the thickness ratio is the primary cause for the change of the carrier mobility. Based on this, after excluding the crystallization and morphology influence factors through XRD and AFM, the conception of interface trap free energy was proposed, and at last such phenomenon was ascribed to the interface trap free energy λ_Trap between electrode and the material, namely dG = λ_Trap·dA.