The Impact of Local Work Function Variations on Fermi Level Pinning of Organic Semiconductors

This photoemission study shows that the work function (Φ) of indium–tin-oxide (ITO) can be increased from 4.2 up to 6.5 eV upon the deposition of the molecular electron acceptors tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and hexaazatriphenylene-hexacarbonitrile (HATCN). The evolution of sample Φ and the hole injection barrier upon subsequent deposition of the hole transport material N, N ′-bis(1-naphthyl)- N, N ′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) was studied for different acceptor precoverages of ITO, corresponding to different initial Φ values. When Φ of the acceptor covered substrate exceeds a critical value Φ crit, the highest occupied molecular level of multilayer α-NPD is found to be pinned 0.5 eV below the Fermi level (E F ). Noteworthy, Φ crit is found at 5.2 eV, which is 0.4 eV higher than expected for α-NPD (4.8 eV), and vacuum level alignment does not apply even before E F -pinning sets in. An electrostatic model that accounts for nonuniformity of the substrate at acceptor submonolayer coverages and the associated local work function changes explains the origin of “delayed” E F -pinning.