Role of ${\rm Fe}_{3}{\rm O}_{4}$ as a $p$ -Dopant in Improving the Hole Injection and Transport

Fe₃O₄ has been demonstrated to be an efficient p-dopant in improving the performance of organic light-emitting devices. This paper investigates in detail the role of Fe₃O₄ in improving the hole injection and the hole transport by the ultraviolet/visible/near-infrared absorption, x-ray, and ultraviolet photoelectron spectroscopy. The results demonstrated that Fe₃O₄ as a p-dopant has different effectiveness when it is doped into different host materials. The improved properties of the OLEDs with the p-doped N, N´-diphenyl-N,N´-bis (1,1´-biphenyl)-4,4´-diamine layer is mainly due to the enhanced hole injection through the lowering of the hole injection barrier, while the enhanced hole transport plays a more important role for the OLEDs with the p-doped 4,4´,4”-tris (3-methylphenylphenylamino) triphenylamine due to their higher ability in the formation of charge transfer complex.