Overcoming the water oxidative limit for ultra-high-workfunction hole-doped polymers

Abstract

It is widely thought that the water-oxidation reaction limits the maximum work function to about 5.25 eV for hole-doped semiconductors exposed to the ambient, constrained by the oxidation potential of air-saturated water. Here, we show that polymer organic semiconductors, when hole-doped, can show work functions up to 5.9 eV, and yet remain stable in the ambient. We further show that de-doping of the polymer is not determined by the oxidation of bulk water, as previously thought, due to its general absence, but by the counter-balancing anion and its ubiquitously hydrated complexes. The effective donor levels of these species, representing the edge of the ‘chemical’ density of states, can be depressed to about 6.0 eV below vacuum level. This can be achieved by raising the oxidation potential for hydronium generation, using large super-acid anions that are themselves also stable against oxidation. In this way, we demonstrate that poly(fluorene-alt-triarylamine) derivatives with tethered perfluoroalkyl-sulfonylimidosulfonyl anions can provide ambient solution-processability directly in the ultrahigh-workfunction hole-doped state to give films with good thermal stability. These results lay the path for design of soft materials for battery, bio-electronic and thermoelectric applications.

Double-type-I charge-injection heterostructure for quantum-dot light-emitting diodes

“…18 As the HIL for mTFF, we used hole-doped pTFF-C 2 F 5 SIS, 24,25 a solution-processed selfcompensated polymer with stable ultrahigh work function of 5.9 eV and immunity to 'dopant migration'. 26,27 It furnishes an Ohmic hole contact to mTFF, with a pinned effective work function of 5.7 eV. 28,29 For the ETL, we used PFOP and F8BT.…”

The Quest for Air Stability in Organic Semiconductors

“…The instability of p-doped OSCs in ambient air is thus largely dependent on the potential hole traps formed by their counteranion, spectator ions if any, and their hydrated complexes. The effects of water-aided dielectric screening of charge carriers or dipole formation, furthermore, limit the work function of p-doped OSCs under ambient conditions. ,, This dipole formation at the surface can be useful in conferring self-protection onto large WF p-doped OSCs that were otherwise expected to undergo WOR. Meanwhile, O 2 , a weak oxidant, may be reduced by n-doped OSCs, form charge transfer complexes that reversibly p-dope OSCs, or take part in irreversible photoinduced oxidation of OSCs. , …”

“…Water has been widely reported to be omnipresent, even in inert environments, by existing in small clusters within nanovoids of the OSCs. These trace amounts of water can be found in polyelectrolytes , or even hydrophobic polymers despite the materials being handled under inert conditions. The hole transport of a wide range of OSCs was investigated by Blom and co-workers in a hole-only diode configuration using SCLC measurements.…”

“…Vacuum level at 0 eV. Reprinted or adapted with permission under a Creative Commons CC-BY 4.0 license from ref . Copyright 2021 Springer Nature.…”

A polymer electrolyte design enables ultralow-work-function electrode for high-performance optoelectronics

“…Inverse photoemission spectroscopy (IPES) and ultraviolet photoemission spectroscopy (UPS) measurements were performed to characterize the WF and bandgap, during which a very low radiation energy and fast scanning were used to make sure that TPHP(LiClO 4 ) was not damaged (Supplementary Fig. 3) 35 . The results show that WF and bandgap of TPHP is reduced from 4.0 to 3.0 eV and from 4.4 to 3.6 eV, respectively, when introducing LiClO 4 with a mole ratio of Li + /TPHP of 2:1 (Fig.…”