Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Recently it has been reported that Nafion oligomers, i.e., 2-(2-sulfonatotetrafluoroethoxy)-2-trifluoromethyltrifluoroethoxyfunctionalized oligotetrafluoroethylenes, also called perfluorinated ionomers (PFIs), can be blended into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) films to increase their workfunctions beyond 5.2 eV. These PFI-modified films are useful for energy-level alignment studies, and have been proposed as hole-injection layers (HILs). It is shown here however that these HILs do not provide sufficiently fast hole transfer into adjacent polymer semiconductor layers with ionization potentials deeper than approximate to 5.2 eV. X-ray and ultraviolet photoemission spectroscopies reveal that these HILs exhibit a molecularly-thin PFI overlayer that sets up a surface dipole that provides the ultrahigh workfunction. This dipolar layer persists even when the subsequent organic semiconductor layer is deposited, as evidenced by measurements of the diode built-in potentials. As a consequence, the PFI-modified HILs produce a higher contact resistance, and a lower equilibrium density of holes at the semiconductor contact than might have been expected from simple thermodynamic considerations of the reduction in hole-injection barrier. Thus the use of insulating dipolar surface layers at the charge-injection contact to tune its workfunction to match the relevant transport level of the semiconductor is of limited utility to achieve ohmic contact in these devices.