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

Abstract

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 ≈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.

Ohmic Contact in 2D Semiconductors via the Formation of a Benzyl Viologen Interlayer

“…Doped polymeric materials that form Ohmic contacts have been studied recently in the context of conventional semiconductors . But, solution‐processed polymeric contacts have not been used in 2D semiconductors until now.…”

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors

“…Although the vacuum workfunctions ϕ of these polymers are high, ≈5.2 eV, they are still not high enough to furnish ohmic hole contacts to organic semiconductors (OSCs) with ionization potentials I p ≳5.4 eV, many of which are of great technological interest 4 5 . Although the insertion of an insulating dipolar surface layer with the correct polarity can tune ϕ 6 7 , the attendant increase in kinetic barrier for charge injection ultimately limits its usefulness 8 9 .…”

Pure Bromide‐Based Perovskite Nanoplatelets for Blue Light‐Emitting Diodes

“…They offer us another choice to seek multifunctional materials, which are effective both in blocking nonradiative current losses and facilitating charge injection. Perfluorinated ionomer (PFI) was reported to incorporate between hole‐transporting layer and perovskite emitter to increase the valence band maximum of HTL . Rogach's group employed a thin film of PFI (5 nm) into CsPbBr 3 green LEDs, and pulled down the hole injection barrier from 1.09 to 0.75 eV (Figure e,f) .…”