Controlled p doping of the hole-transport molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine with tetrafluorotetracyanoquinodimethane

Abstract

We investigate $p$ -type doping of the hole-transport organic molecular material ${\mathrm{N,N}}^{\prime }-\mathrm{diphenyl}-{\mathrm{N,N}}^{\prime }-\mathrm{bis}\text{(1-}\mathrm{naphthyl}{\text{)-1,1}}^{\prime }-\mathrm{biphenyl}{\text{-4,4}}^{\prime }-\mathrm{diamine}$ (α-NPD) with tetrafluorotetracyanoquinodimethane $({\mathrm{F}}_4-\mathrm{TCNQ})$ using direct and inverse photoemission spectroscopy, contact potential difference measurements, and in situ current–voltage $\mathrm{(I–V)}$ measurements. The close match between the ionization energy of α-NPD and the electron affinity of ${\mathrm{F}}_4-\mathrm{TCNQ}$ leads to an efficient charge transfer between highest occupied molecular orbital of the host and lowest occupied molecular orbital of the dopant. The Fermi level moves down towards the valence states by 0.62 eV in the 0.5% doped film with respect to the undoped film, and a narrow space charge layer $\text{(∼60\hspace{0.17em}Å)}$ forms at the interface with Au. Hole injection in the doped devices increases by several orders of magnitude due to tunneling through the depletion region. The large relaxation energy of the ionized α-NPD molecule limits the movement of the Fermi level and, ultimately, the hole injection.