  Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely
used to build optoelectronic devices. However, as a hygroscopic water-based
acidic material, it brings major concerns for stability and degradation,
resulting in an intense effort to replace it in organic photovoltaic (OPV)
devices. In this work, we focus on the perfluorinated ionomer (PFI) polymeric
additive to PEDOT:PSS. We demonstrate that it can reduce the relative amplitude
of OPV device burn-in, and find two distinct regimes of influence. At low
concentrations there is a subtle effect on wetting and work function, for
instance, with a detrimental impact on the device characteristics, and above a
threshold it changes the electronic and device properties. The abrupt threshold
in the conducting polymer occurs for PFI concentrations greater than or equal
to the PSS concentration and was revealed by monitoring variations in
transmission, topography, work-function, wettability and OPV device
characteristics. Below this PFI concentration threshold, the power conversion
efficiency (PCE) of OPVs based on
poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester
(P3HT:PCBM) are impaired largely by low fill-factors due to poor charge
extraction. Above the PFI concentration threshold, we recover the PCE before it
is improved beyond the pristine PEDOT:PSS layer based OPV devices.
Supplementary to the performance enhancement, PFI improves OPV device stability
and lifetime. Our degradation study leads to the conclusion that PFI prevents
water from diffusing to and from the hygrosopic PEDOT:PSS layer, which slows
down the deterioration of the PEDOT:PSS layer and the aluminum electrode. These
findings reveal mechanisms and opportunities that should be taken into
consideration when developing components to inhibit OPV degradation.
