Molecular p doping of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl and n doping of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene

Abstract

This report details p-type doping of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) and n-type doping of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) with decamethylcobaltocene (DMC). Both CBP and TPBi are widely used as charge transport layers in organic light-emitting diodes (OLEDs). We confirm efficient p-type F4-TCNQ doping of CBP by observing, as a function of increasing dopant concentration, a shift of the Fermi level (EF) toward the highest occupied molecular orbital (HOMO) in ultraviolet photoelectron spectroscopy (UPS) scans, a larger amount of fluorine in the film, a transition of the core C 1s electron levels to lower binding energies in X-ray photoelectron spectroscopy (XPS) measurements, and an increase of the current in vertical metal/organic semiconductor/metal devices, which in the context of the Gaussian Disorder Model, can be attributed to an injection barrier lowering and a charge carrier concentration enhancement. In the case of doping of TPBi with DMC, as the dopant concentration increases, a shift of EF away from the HOMO in UPS and a transition of the core C 1s electron levels to higher binding energies in XPS are observed, both of which are indicative of n-type doping. The presence of inherent degradation of TPBi films during UPS measurements is also established. These findings may pave the way for development of higher performance OLEDs based on doped transport layers.