11 research outputs found
Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells
The
effects of interface roughness between donor and acceptor in a bilayer
heterojunction solar cell were investigated on a polymerâpolymer
system based on polyÂ(3-hexylthiophene) (P3HT) and polyÂ(dioctylfluorene-<i>alt</i>-benzothiadiazole) (F8BT). Both polymers are known to
reorganize into semicrystalline structures when heated above their
glass-transition temperature. Here, the bilayers were thermally annealed
below glass transition of the bulk polymers (â140 °C)
at temperatures of 90, 100, and 110 °C for time periods from
2 min up to 250 min. No change of crystallinity could be observed
at those temperatures. However, X-ray reflectivity and device characteristics
reveal a coherent trend upon heat treatment. In X-ray reflectivity
investigations, an increasing interface roughness between the two
polymers is observed as a function of temperature and annealing time,
up to a value of 1 nm. Simultaneously, according bilayer devices show
an up to 80% increase of power conversion efficiency (PCE) for short
annealing periods at any of the mentioned temperatures. Together,
this is in agreement with the expectations for enlargement of the
interfacial area. However, for longer annealing times, a decrease
of PCE is observed, despite the ongoing increase of interface roughness.
The onset of decreasing PCE shifts to shorter durations the higher
the annealing temperature. Both, X-ray reflectivity and device characteristics
display a significant change at temperatures below the glass transition
temperatures of P3HT and F8BT
Irreversible Adsorption Erases the Free Surface Effect on the <i>T</i><sub>g</sub> of Supported Films of Poly(4-<i>tert</i>-butylstyrene)
When
cooled at constant rate, a 25 nm thin film of polyÂ(4-<i>tert</i>-butylstyrene) vitrifies 50 K lower than in bulk. This
record sets the largest depression in thermal glass transition temperature
(<i>T</i><sub>g</sub>) ever observed upon confinement at
the nanoscale level. Same as for other supported polymer layers, this
reduction in <i>T</i><sub>g</sub> has been attributed to
the presence of a free surface, the ensemble of molecules at the interface
with air remaining in the liquid state also at temperatures well below
bulk <i>T</i><sub>g</sub>. Here, we verify that such tremendous
shifts can be erased upon prolonged annealing in the liquid state,
hinting at a metastable nature of confinement effects. We demonstrate
that the recovery of bulk behavior and the manifestation of the free
surface are enslaved to the kinetics of irreversible adsorption of
chains on the supporting substrate
Taming the Strength of Interfacial Interactions via Nanoconfinement
The
interaction between two immiscible materials is related to
the number of contacts per unit area formed by the two materials.
For practical reasons, this information is often parametrized by the
interfacial free energy, which is commonly derived via rather cumbersome
approaches, where properties of the interface are described by combining
surface parameters of the single materials. These <i>combining
rules</i>, however, neglect any effect that geometry might have
on the strength of the interfacial interaction. In this Article, we
demonstrate that the number of contacts at the interface between a
thin polymer coating and its supporting substrate is altered upon
confinement at the nanoscale level. We show that explicitly considering
the effect of nanoconfinement on the interfacial potential allows
a quantitative prediction of how sample geometry affects the number
of contacts formed at the interface between two materials
Neutron Reflectometry Elucidates Density Profiles of Deuterated Proteins Adsorbed onto Surfaces Displaying Poly(ethylene glycol) Brushes: Evidence for Primary Adsorption
The
concentration profile of deuterated myoglobin (Mb) adsorbed
onto polystyrene substrates displaying polyÂ(ethylene glycol) (PEG)
brushes is characterized by neutron reflectometry (NR). The method
allows to directly distinguish among primary adsorption at the grafting
surface, ternary adsorption within the brush, and secondary adsorption
at the brush outer edge. It complements depth-insensitive standard
techniques, such as ellipsometry, radioactive labeling, and quartz
crystal microbalance. The study explores the effect of the PEG polymerization
degree, <i>N</i>, and the grafting density, Ï, on
Mb adsorption. In the studied systems there is no indication of secondary
or ternary adsorption, but there is evidence of primary adsorption
involving a dense inner layer at the polystyrene surface. For sparsely
grafted brushes the primary adsorption involves an additional dilute
outer protein layer on top of the inner layer. The amount of protein
adsorbed in the inner layer is independent of <i>N</i> but
varies with Ï, while for the outer layer it is correlated to
the amount of grafted PEG and is thus sensitive to both <i>N</i> and Ï. The use of deuterated proteins enhances the sensitivity
of NR and enables monitoring exchange between deuterated and hydrogenated
species
Ratio (upper part) of C16/C18 fatty acids and (lower part) of unsaturated/saturated fatty acid (UFA/SFA) when <i>Pichia pastoris</i> cells were grown in hydrogenated (H) and deuterated (D) media at 18 and 30°C.
a<p>: Total, total fatty acids; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PS, phosphatidylserine; PG, phosphatidylglycerol; CL, cardiolipine.</p
Fatty acid distribution of the main phospholipids produced by <i>Pichia pastoris</i> cells grown in a hydrogenated medium at 30°C (red) and at 18°C (green) and in a deuterated environment at 30°C (blue) and 18°C (cyan).
<p>In all individual phospholipids, the deuterated environment triggers the enrichment in C18:1 fatty acids PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PS, phosphatidylserine. Errors bars represent the standard deviation from three different phospholipids extractions and separations. Data represent mean values ± s.d (nâ=â3). In histograms, *<i>P</i><0.05 from Student's <i>t</i>-test, assuming equal variance. PC: for C16:0, the difference is significant between H30°C and H18°C, between H30°C and D30°C and between H30°C and D18°C. For C16:1, there is a significant difference between H30°C and D30°C and between H18°C and D18°C. For C18:0, there is a significant difference between H18°C and D18°C, between H18°C and H30°C and between H18°C and D30°C. For C18:1, the difference is significant between all the different temperatures and isotopic contents. For C18:2, there is a significant difference between D30°C and D18°C, between D30°C and H30°C and between D30°C and H18°C. For C18:3, the difference is significant between all the different temperatures and isotopic contents. PE: for C16:0, the difference is significant between H30°C and D30°C, between H30°C and H18°C and between H30°C and D18°C. For C18:1 and C18:3, the difference is significant between all the different temperatures and isotopic contents. PS and PI: for C16:0, there is a significant difference between H30° and D30°C and between H30° and D18°C. For C16:2 and C18:1, there is a significant difference between H30°C and D30°C, between H18°C and D18°C and between D18°C and D30°C. For C18:2 and C18:3, there is a significant difference between H30°C and D30°C and between H18°C and D18°C.</p
GC-MS analysis of ergosterol isotopomers from <i>Pichia pastoris</i> unsaponifiable extracts.
<p>A, TIC of an acetylated extract from cells grown in hydrogenated medium. The peak at RTâ=â36.10 min is ergosterol. B, TIC of an acetylated extract from cells grown in deuterated medium. The peak at RTâ=â35.26 min is deuterioergosterol. C, mass spectrum of ergosteryl acetate. Prominent ions and interpretation of the fragmentation pattern: M<sup>+</sup>(438), M<sup>+</sup>-acetate-H (378), M<sup>+</sup>-acetate-H-CH<sub>3</sub> (363), M+-acetate-H-side chain (253). D, mass spectrum of deuterioergosteryl acetate. Prominent ions and interpretation of the fragmentation pattern: M<sup>+</sup>(481), M<sup>+</sup>-acetate-D (420), M<sup>+</sup>-acetate-D-CD<sub>3</sub> (402), M<sup>+</sup>-acetate-D-side chain (278).</p
Total fatty acid distribution in <i>Pichia pastoris</i> cells grown in a hydrogenated environment at 30°C (red) and 18°C (green) and in a deuterated environment at 30°C (blue) and 18°C (cyan).
<p>Data represent mean values ± s.d (nâ=â3). In histograms, *<i>P</i><0.05 from Student's <i>t</i>-test, assuming equal variance. For C16:0, there is a significant difference between H 30°C and H 18°C, between H 30°C and D30°C, H 30°C and D18°C, between H 18°C and D 30°C but not between H18°C and D18°C. For C16:1, there is a significant difference for all 4 different temperatures and isotopic contents. For C16:2, there is a significant difference between D30°C and D18°C. For C18:1, there is a significant difference for all 4 different temperatures and isotopic contents. For C18:2, there is a significant difference between D30°C and H30°C, between D30°C and D18°C, between D30°C and H18°C. For C18:3, there is a significant difference for all 4 different temperatures and isotopic contents.</p
Substrate-Induced Phase of a [1]Benzothieno[3,2â<i>b</i>]benzothiophene Derivative and Phase Evolution by Aging and Solvent Vapor Annealing
Substrate-induced
phases (SIPs) are polymorphic phases that are
found in thin films of a material and are different from the single
crystal or âbulkâ structure of a material. In this work,
we investigate the presence of a SIP in the family of [1]ÂbenzothienoÂ[3,2-<i>b</i>]Âbenzothiophene (BTBT) organic semiconductors and the effect
of aging and solvent vapor annealing on the film structure. Through
extensive X-ray structural investigations of spin coated films, we
find a SIP with a significantly different structure to that found
in single crystals of the same material forms; the SIP has a herringbone
motif while single crystals display layered ÏâÏ
stacking. Over time, the structure of the film is found to slowly
convert to the single crystal structure. Solvent vapor annealing initiates
the same structural evolution process but at a greatly increased rate,
and near complete conversion can be achieved in a short period of
time. As properties such as charge transport capability are determined
by the molecular structure, this work highlights the importance of
understanding and controlling the structure of organic semiconductor
films and presents a simple method to control the film structure by
solvent vapor annealing
Xâray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2â<i>b</i>]benzothiophene Derivatives in Bulk and Thin Films
A detailed structural study of the
bulk and thin film phases observed
for two potential high-performance organic semiconductors has been
carried out. The molecules are based on [1]ÂbenzothienoÂ[3,2-<i>b</i>]Âbenzothiophene (BTBT) as conjugated core and octyl side
groups, which are anchored either symmetrically at both sides of the
BTBT core (C<sub>8</sub>âBTBTâC<sub>8</sub>) or nonsymmetrically
at one side only (C<sub>8</sub>âBTBT). Thin films of different
thickness (8â85 nm) have been prepared by spin-coating for
both systems and analyzed by combining specular and grazing incidence
X-ray diffraction. In the case of C<sub>8</sub>âBTBTâC<sub>8</sub>, the known crystal structure obtained from single-crystal
investigations is observed within all thin films, down to a film thickness
of 9 nm. In the case of C<sub>8</sub>âBTBT, the crystal structure
of the bulk phase has been determined from X-ray powder diffraction
data with a consistent matching of experimental and calculated X-ray
diffraction patterns (<i>R</i><sub>wp</sub> = 5.8%). The
packing arrangement of C<sub>8</sub>âBTBT is similar to that
of C<sub>8</sub>âBTBTâC<sub>8</sub>, that is, consisting
of a lamellar structure with molecules arranged in a âherringboneâ
fashion, yet with lamellae composed of two head-to-head (or tail-to-tail
as the structure is periodic) superimposed molecules instead of only
one molecule for C<sub>8</sub>âBTBTâC<sub>8</sub>. As
for C<sub>8</sub>âBTBTâC<sub>8</sub>, we demonstrate
that the same phase is observed in bulk and thin films for C<sub>8</sub>âBTBT whatever the film thickness investigated