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 polymer–polymer system based on poly­(3-hexylthiophene) (P3HT) and poly­(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 (≈140 °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>_g of Supported Films of Poly(4-<i>tert</i>-butylstyrene)

When cooled at constant rate, a 25 nm thin film of poly­(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>_g) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in <i>T</i>_g 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>_g. 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 poly­(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, σ, 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 σ, while for the outer layer it is correlated to the amount of grafted PEG and is thus sensitive to both <i>N</i> and σ. 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 (n = 3). 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 RT = 36.10 min is ergosterol. B, TIC of an acetylated extract from cells grown in deuterated medium. The peak at RT = 35.26 min is deuterioergosterol. C, mass spectrum of ergosteryl acetate. Prominent ions and interpretation of the fragmentation pattern: M⁺(438), M⁺-acetate-H (378), M⁺-acetate-H-CH₃ (363), M+-acetate-H-side chain (253). D, mass spectrum of deuterioergosteryl acetate. Prominent ions and interpretation of the fragmentation pattern: M⁺(481), M⁺-acetate-D (420), M⁺-acetate-D-CD₃ (402), M⁺-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 (n = 3). 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[3,2‑<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 “bulk” structure of a material. In this work, we investigate the presence of a SIP in the family of [1]­benzothieno­[3,2-<i>b</i>]­benzothiophene (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 π–π 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

X‑ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2‑<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]­benzothieno­[3,2-<i>b</i>]­benzothiophene (BTBT) as conjugated core and octyl side groups, which are anchored either symmetrically at both sides of the BTBT core (C₈–BTBT–C₈) or nonsymmetrically at one side only (C₈–BTBT). Thin films of different thickness (8–85 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 C₈–BTBT–C₈, 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 C₈–BTBT, 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>_wp = 5.8%). The packing arrangement of C₈–BTBT is similar to that of C₈–BTBT–C₈, that is, consisting of a lamellar structure with molecules arranged in a “herringbone” 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 C₈–BTBT–C₈. As for C₈–BTBT–C₈, we demonstrate that the same phase is observed in bulk and thin films for C₈–BTBT whatever the film thickness investigated