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 λTrap between electrode and the material, namely dG = λTrap·dA.

In this paper, four pyrene–fluoroene derivatives with conjugated and nonconjugated pyrene substitution were designed and synthesized. In PFP1 and PFP2, there are nonconjugated pyrene substitution on C9 and conjugated pyrene on C2 and/or C7 of the fluorene moiety, and in the control molecules BP1 and BP2, there is only the conjugated pyrene in the C2 and/or C7 of the fluorene core. There is a special π–π hyperconjugation effect between nonconjugated pyrene and the pyrene–fluorene conjugation in the system (PFP1 and PFP2), which means the electron cloud of such two isolated conjugation systems (nonconjugated pyrene group and pyrene–fluorene group) could be delocalized and transferred to each other. Because of delocalization of electron cloud, the molecule size of PFP1 and PFP2 might have been decreased and led to decreased phase transition temperature compared with that of BP1 and BP2. Also due to the electron transfer between the molecules, the intermolecular force between PFP1 and PFP2 has been improved, which is the reason that they are more amorphous than BP1 and BP2. The easy electron transfer also makes the PFP1 and PFP2 show improved hole injection and device performance compared with that of BP1 and BP2.