4 research outputs found
Long-Term Stabilization of Organic Solar Cells Using Hindered Phenols as Additives
We
report on the improvement of long-term stability of organic solar
cells (OPV) using hindered phenol based antioxidants as stabilizing
additives. A set of seven commercially available hindered phenols
are investigated for use in bulk-heterojunction OPV. Polymer:fullerene
films based on polyÂ(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) are characterized with respect to the initial
power conversion efficiency and the long-term stability improvement
under illumination in ambient conditions. FTIR spectroscopy is used
to trace chemical degradation over time. OPV performance is recorded
under ISOS-3 conditions, and an improved long-term performance of
OPV devices, manifested in increased accumulated power generation
(APG), is found for octadecyl 3-(3,5-di-<i>tert</i>-butyl-4-hydroxyphenyl)Âpropionate.
Using this additive, APG is increased by a factor of 3 compared to
the reference. Observed differences in the stabilization of tested
additives are discussed in terms of energetic trap states formation
within the HOMO/LUMO gap of the photoactive material, morphological
changes, and chemical structure
In Situ X‑ray Scattering Studies of the Influence of an Additive on the Formation of a Low-Bandgap Bulk Heterojunction
The
evolution of the morphology of a high-efficiency, blade-coated,
organic-photovoltaic (OPV) active layer containing the low band gap
polymer polyÂ[(4,8-bisÂ[5-(2-ethylhexyl)Âthiophene-2-yl]ÂbenzoÂ[1,2-b:4,5-b′]Âdithiophene)-2,6-diyl-<i>alt</i>-(4-(2-ethylhexanoyl)-thienoÂ[3,4-<i>b</i>]Âthiophene))-2,6-diyl]
(PBDTTT-C-T) is examined by in situ X-ray scattering. In situ studies
enable real-time characterization of the effect of the processing
additive 1,8-diiodoocatane (DIO) on the active layer morphology. In
the presence of DIO, X-ray scattering indicates that domain structure
radically changes and increases in purity, while X-ray diffraction
reveals little change in crystallinity/local order. The solidification
behavior of this active layer differs dramatically from those that
strongly crystallize such as polyÂ(3-hexylthiophene) and small molecule
containing systems, exposing significant diversity in the solidification
routes relevant to high-efficiency polymer–fullerene OPV processing.
In the presence of DIO, we find quantitative agreement between the
evolution of the phase structure revealed by small-angle X-ray scattering
and the binodal phase structure of a simple Flory–Huggins model
In Situ Characterization of Polymer–Fullerene Bilayer Stability
A 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 polyÂ(3-hexylthiophene),
P3HT, and polyÂ[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-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]ÂPCBM 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 (<i>T</i><sub>g</sub>) is critical to the bilayer stability, with no significant
changes occurring below <i>T</i><sub>g</sub> of the high <i>T</i><sub>g</sub> PCDTBT. Above the polymer <i>T</i><sub>g</sub>, 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
Blade Coating Aligned, High-Performance, Semiconducting-Polymer Transistors
Recent demonstration
of mobilities in excess of 10 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> have energized research
in solution deposition of polymers for thin film transistor applications.
Due to the lamella motif of most soluble, semiconducting polymers,
the local mobility is intrinsically anisotropic. Therefore, fabrication
of aligned films is of interest for optimization of device performance.
Many techniques have been developed to control film alignment, including
solution deposition via directed flows and deposition on topologically
structured substrates. We report device and detailed structural analysis
(ultraviolet–visible absorption, IR absorption, near-edge X-ray
absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic
force microscopy) results from blade coating two high performing semiconducting
polymers on unpatterned and nanostructured substrates. Blade coating
exhibits two distinct operational regimes: the Landau–Levich
or horizontal dip coating regime and the evaporative regime. We find
that in the evaporative deposition regime, aligned films are produced
on unpatterned substrates with the polymer chain director perpendicular
to the coating direction. Both NEXAFS and device measurements indicate
the coating induced orientation is nucleated at the air interface.
Nanostructured substrates produce anisotropic bottom contact devices
with the polymer chain at the buried interface oriented along the
direction of the substrate grooves, independent of coating regime
and coating direction. Real time studies of film drying establish
that alignment occurs at extremely high polymer volume-fraction conditions,
suggesting mediation via a lyotropic phase. In all cases the final
films appear to exhibit high degrees of crystalline order. The independent
control of alignment at the air and substrate interfaces via coating
conditions and substrate treatment, respectively, enable detailed
assessment of structure–function relationships that suggest
the improved performance of the nanostructure aligned films arise
from alignment of the less ordered material in the crystallite interphase
regions