ma5021227_si_001.pdf (2.16 MB)
In Situ Characterization of Polymer–Fullerene Bilayer Stability
journal contribution
posted on 2015-01-27, 00:00 authored by Deborah Leman, Mary Allison Kelly, Stuart Ness, Sebastian Engmann, Andrew Herzing, Chad Snyder, Hyun Wook Ro, R. Joseph Kline, Dean M. DeLongchamp, Lee J. RichterA consensus
is emerging that mixed phases are present in bulk heterojunction
organic photovoltaic (OPV) devices. Significant insights into the
mixed phases have come from bilayer stability measurements, in which
an initial sample consisting of material pure layers of donor and
acceptor is thermally treated, resulting in swelling of one layer
by the other. We present a comparative study of the stability of polymer/fullerene
bilayers using two common OPV polymer donors poly(3-hexylthiophene),
P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)],
PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl
ester, phenyl-C71-butyric acid methyl ester, [60]PCBM bis-adduct,
and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry
to characterize the quasi-steady state behavior of the films, we find
that the polymer glass transition temperature (Tg) is critical to the bilayer stability, with no significant
changes occurring below Tg of the high Tg PCDTBT. Above the polymer Tg, we find the behavior is irreversible and most consistent
with swelling of the polymer by the fullerene, constrained by tie
chains in the polymer network and influenced by the rubbery dynamics
of the mixed region. The swelling varies significantly with the nature
of the fullerene and the polymer. Across the eight systems studied,
there is no clear relationship between swelling and OPV device performance.
The relationship between the observed swelling and the underlying
fullerene–polymer miscibility is explored via Flory–Rehner
theory.