Understanding the morphology of polymer‐based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low‐bandgap silole‐containing conjugated polymer, ...poly(4,4′‐bis(2‐ethylhexyl)dithieno3,2‐b;2′,3′‐dsilole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl (PSBTBT) with 6,6phenyl‐C61‐butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As‐Spun”, “Pre‐Annealed”, and “Post‐Annealed” films were studied by scanning force microscopy, contact angle measurements, X‐ray photoelectron spectroscopy, near‐edge X‐ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As‐Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing‐incidence X‐ray diffraction and small‐angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ∼10 nm in size, leading to an improvement in photovoltaic performance.
A detailed description of morphology is presented for blend films of a low‐bandgap silole‐containing conjugated polymer, poly(4,4′bis(2‐ethylhexyl)dithieno3,2‐b;2′ 3′‐dsilole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl (PSBTBT) with 6,6phenyl‐C61‐butyric acid methyl ester (PCBM). Versatile techniques were used to characterize the morphology of the “As‐Spun”, “Pre‐Annealed”, and “Post‐Annealed” thin films, mimicking the real solar cell devices, and to provide insight into the performance of these mixtures in actual devices.
We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO2) gas. A series of bilayers composed of bottom hydrogenated polystyrene ...(h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3R g (R g is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO2 and subsequent solvent leaching, we reveal the presence of the CO2-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vicinity of the solid substrate even in the presence of the effective plasticizer.
The structure of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C4mpyrNTf2) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, ...cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of C4mpyr+ at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of C4mpyr+ onto the gold electrode.
Swollen polymer brushes are found in various applications including biomedical coatings where the brush provides stability against harsh environmental conditions and mediates interactions with the ...surroundings. The surface height fluctuations of planar polystyrene brushes (0.04–0.61 chains/nm2) highly swollen in toluene vapor are so strongly slowed by the tethering of the chains that they are unobservable in the current experimental window of length and time. This is the case despite the fact that the segmental dynamics of the brush chains should be very fast due to the substantial plasticization by the solvent. With respect to thermally stimulated fluctuations, the surfaces of these swollen brushes are solidlike on time scales and length scales pertinent to many practical applications.
The effects of CO
2
annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates ...were investigated. By using
in situ
neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as
P
= 2.9 MPa and at
T
= 48 °C which is below the bulk melting temperature (
T
m
). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air
via
subsequent annealing at given temperatures below
T
m
. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO
2
-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO
2
annealing: the limited plasticization effect of CO
2
at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.
We demonstrate the roles of an irreversibly adsorbed polymer layer formed
via
CO
2
annealing in the crystallization/dewetting processes of supported PEO films.
The scaling of the thickness, h s, of a densely grafted polymer brush of chain length N and grafting density σ swollen in vapor agrees quantitatively with the scaling reported by Kuhl et al. for ...densely grafted brushes swollen in liquid. Deep in the brush, next to the substrate, the shape of the segment concentration profile is the same whether the brush is swollen by liquid or by vapor. Differences in the segment concentration profile are manifested primarily in the swollen brush interface with the surrounding fluid. The interface of the polymer brush swollen in vapor is much more abrupt than that of the same brush swollen in liquid. This has implications for the compressibility of the swollen brush surface and for fluctuations at that surface.
The surface height fluctuations of a layer of low molecular weight (2.2k) untethered perdeuterated polystyrene (dPS) chains adjacent to a densely grafted polystyrene brush are slowed dramatically. ...Due to the interpenetration of the brush with the layer of "untethered chains" a hydrodynamic continuum theory can only describe the fluctuations when the effective thickness of the film is taken to be that which remains above the swollen brush. The portion of the film of initially untethered chains that interpenetrates with the brush becomes so viscous as to effectively play the role of a rigid substrate. Since these hybrid samples containing a covalently tethered layer at the bottom do not readily dewet, and are more robust than thin layers of untethered short chains on rigid substrates, they provide a route for tailoring polymer layer surface properties such as wetting, adhesion and friction.
We utilize neutron reflectometry (NR) to probe the amount of residual solvent in thin polystyrene (PS) and poly(methyl methacrylate) (PMMA) films spin-coated from deuterated toluene solutions onto ...silicon substrates. The effect of thermal treatment at temperatures below and above the glass transition of PS and PMMA are examined to determine how these treatments affect the residual solvent. The parameters investigated include the total film thickness and molecular mass for PS and only thickness for PMMA. We find that the volume fraction of solvent in PMMA films is less than a few percentages, while no residual solvent was detected in PS films. The residual solvent content of the PMMA films had a slight dependence on thickness. These results are corroborated utilizing Fourier Transform infrared (FTIR) spectroscopy. We discuss these observations in the context of a large body of seemingly conflicting literature on residual solvent in polymer thin films.
Morphology control in bulk heterojunction (BHJ) solar cell is considered to be critical for the power conversion efficiency (PCE). In this paper, we present a novel approach that introduces ...polystyrene that organizes the poly(3-hexylthiophene) (P3HT) into columnar phases decorated by 6,6-phenyl C61-butyric acid methyl ester (PCBM) at the interface. This structure represents a realization of an idealized morphology of an organic solar cell, in which both exiciton dissociation and the carrier transport are optimized, leading to increased power conversion efficiency.
Interfacial evolution of deuterated polycarbonate (dPC) and poly(methyl methacrylate) (PMMA) bilayer films supported on silicon substrates was examined by neutron reflectometry. The dPC was in the ...glassy state and the PMMA was in the melt state when the samples were annealed at temperatures (from 400 to 415 K) between the bulk glass transition temperature (T g) of dPC (T g ∼ 418 K) and PMMA (T g ∼ 390 K). Asymmetric concentration profiles were observed as a result of composition dependence of the mobility. Once the slow moving dPC molecules leave the dPC-rich side (which was glassy) and enter the PMMA side, the dPC molecules become fast moving and diffuse freely in a viscous melt environment, while the fast moving PMMA molecules become slow as PMMA enter the glassy dPC matrix. Slight change in molecular mass of dPC gives rise to remarkable difference in concentration profile, indicating that mutual diffusion is very sensitive to molecular friction and entanglement of medium. With the penetration of PMMA into the dPC-rich side, the sluggish relaxation of dPC with high molecular mass results in the swelling of the dPC layer.
![Structure of [C4mpyr][NTf2]...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0743-7463/small "Structure of [C4mpyr][NTf2] Room-Temperature Ionic Liquid at Charged Gold Interfaces")
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