Morphology Of Polymer Thin Films Research Articles

Morphology Control in Films of Isoindigo Polymers by Side-Chain and Molecular Weight Effects.

The performance of devices relying on organic electronic materials, such as organic field-effect transistors (OFET) and organic photovoltaics (OPV), is strongly correlated to the morphology of the conjugated material in thin films. For instance, several factors such as polymer solubility, weak intermolecular forces between polymers and fullerene derivatives, and film drying time impact phase separation in the active layer of a bulk heterojunction OPV device. In an effort to probe the influence of polymer assembly on morphology of polymer thin films and phase separation with fullerene derivatives, five terthiophene-alt-isoindigo copolymers were synthesized with alkyl side-chains of varying lengths and branching on the terthiophene unit. These P[T3(R)-iI] polymers were designed to have similar optoelectronic properties but different solubilities in o-dichlorobenzene and were predicted to have different tendencies for crystallization. All polymers with linear alkyl chains exhibit similar thin film morphologies as investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). The main differences in electronic and morphological properties arise when P[T3(R)-iI] is substituted with branched 2-ethylhexyl (2EH) side-chains. The bulky 2EH substituents lead to a blue-shifted absorption, a lower ionization potential, and reduced ordering in polymer thin films. The five P[T3-iI] derivatives span hole mobilities from 1.5 × 10-3 to 2.8 × 10-2 cm2 V-1 s-1 in OFET devices. In OPV devices, the 2EH-substituted polymers yield open-circuit voltages of 0.88 V in BHJ devices yet low short-circuit currents of 0.8 mA cm-2, which is explained by the large phase separation observed by AFM in blends of P[T3(2EH)-iI] with PC71BM. In these P[T3(R)-iI] systems, the propensity for the polymers to self-assemble prior to aggregation of PC71BM molecules was key to achieving fine phase separation and increased short-circuit currents, eventually resulting in power conversion efficiencies of 5% in devices processed using a single solvent.

Enhancing Long-Range Ordering of P3HT by Incorporating Thermotropic Biphenyl Mesogens via ATRP

The methoxybiphenyl liquid crystalline mesogen 6-((4′-methoxy-[1,1′-biphenyl]-4-yl)oxy)hexyl methacrylate (MeOBP) was incorporated into block copolymers with regioregular poly(3-hexylthiophene) (P3HT) by a combination of Grignard metathesis (GRIM) and atom transfer radical polymerization (ATRP) techniques. Side chain engineering with a thermotropic liquid crystalline segment produced an improved morphology resulting in field-effect mobilities of ∼10–2 cm2/(V s) measured in organic thin film transistors (OTFTs). It was observed that improved π–π stacking distances between edge-on oriented polythiophene segments upon annealing facilitated charge transport which was accomplished with π–π stacked biphenyl mesogens with P3HT block. Variation of surface morphology of polymer thin films upon annealing was investigated by tapping mode atomic force microscopy (TMAFM). A combined study of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) was also performed to identify liquid crystalline...

Insights into the Influence of Solvent Polarity on the Crystallization of Poly(ethylene oxide) Spin-Coated Thin Films via in Situ Grazing Incidence Wide-Angle X-ray Scattering

Controlling polymer thin-film morphology and crystallinity is crucial for a wide range of applications, particularly in thin-film organic electronic devices. In this work, the crystallization behavior of a model polymer, poly(ethylene oxide) (PEO), during spin-coating is studied. PEO films were spun-cast from solvents possessing different polarities (chloroform, THF, and methanol) and probed via in situ grazing incidence wide-angle X-ray scattering. The crystallization behavior was found to follow the solvent polarity order (where chloroform chloroform > methanol). When spun-cast from nonpolar chloroform, crystallization largely followed Avrami kinetics, resulting in the formation of morphologies comprising large spherulites. PEO solutions cast from more polar solvents (THF and methanol) do not form well-defined highly crystalline morphologies and are largely amorphous with the presence of small crystalline regions. The difference in morphological development of PEO spun-cast from polar solvents is attributed to clustering phenomena that inhibit polymer crystallization. This work highlights the importance of considering individual components of polymer solubility, rather than simple total solubility, when designing processing routes for the generation of morphologies with optimum crystallinities or morphologies.

Mechanism for Conducting Filament Growth in Self‐Assembled Polymer Thin Films for Redox‐Based Atomic Switches

The switching mechanisms of atomic switches based on poly(ethylene oxide) (PEO) are systematically investigated. By using self-assembled PEO and Ag-PEO thin films, stack-structured devices exhibit stable bipolar switching behavior over 10(3) cycles. Direct observation of filament growth behavior in planar-structured devices reveals the effects of the polymer thin-film morphology, and the presence of electrochemically active electrodes, on the switching characteristics.

Constructional details of polystyrene-block-poly(4-vinylpyridine) ordered thin film morphology

One of the most important issues for spin-coated diblock copolymer thin films is to distinguish the blocks by common techniques, especially for the diblock copolymers containing one block could form active domain. Aiming different estimations on the composition of surface microdomains in polystyrene-block-poly (4-vinylpyridine) (PS-b-P4VP) atomic force microscope (AFM) phase images, AFM, transmission electron microscopy, and droplet shape analyzer were used to identify the constructional details of the polymer thin film morphology. It was confirmed that PS block is harder than P4VP block in symmetric PS-b-P4VP films and corresponds to brighter regions in AFM phase image. It is helpful to distinguish the nanodomains that originate from the different blocks in PS-b-P4VP thin films. The structure evolutions of PS-b-P4VP film annealed with different selective solvents were studied and discussed. The results indicated that the core-corona inversion process is due to the cores of micelles swell and coalesce together rather than local reorganization of the chains.

Influence of single wall carbon nanotubes and thermal treatment on the morphology of polymer thin films

Homogeneous and stable thin films of poly(butylene terephthalate) PBT and its nanocomposites based on single wall carbon nanotubes (SWCNTs) were prepared by spin coating. PBT thin films show crystalline structures for thicknesses above 40 nm, consisting of submicrometer size 2D-spherulites. In the case of nanocomposites, carbon nanotubes act as nucleating agents and provide a template for the crystallization of PBT. This gives rise to hybrid shish-kebab structures, even in the thinnest films (∼10 nm thick). Melting and recrystallization provoke the crystallization of PBT and its nanocomposites, and can be used to control morphology. For PBT thin films, the orientation of crystalline lamellae undergoes a transformation, changing from a disposition perpendicular to the substrate (“edge-on”) to a parallel arrangement (“flat-on”) after recrystallization. In the case of the nanocomposites, the CNT influence on the polymer crystallization morphology in thin films is less significant than in the bulk due to the effect of the substrate interactions. Using Raman microscopy it is possible to directly observe both, the degree of dispersion and the location of carbon nanotubes in the films. The results reveal that bigger agglomerates act as more effective nucleating points than isolated bundles of SWCNTs during crystallization of the polymer matrix.

Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency

We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of π electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at ∼630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our finding suggests that the thin-film preparation, and thus the formation of polymer morphology, is a much more complicated process than previously assumed.

Device performance and polymer morphology in polymer light emitting diodes: morphology dependent emission spectra

Polymer thin films for the light-emitting devices (LEDs) are usually obtained by spin-casting from slightly viscous solutions. The performance of polymer light-emitting diodes is strongly influenced by the thin film morphology. It is known that the polymer thin film morphology (or aggregation) is determined by the solvents used for obtaining polymer solutions. In this manuscript, we demonstrate that the polymer aggregation can also be controlled by the concentration of the polymer solution and the rotational speed used during spin-casting of the films. The correlation between the film morphology and emission spectra for polymer light-emitting diodes fabricated using poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) has been studied. Our findings indicate that there is a strong correlation between the “vibronic structure” observed in the emission spectra of the MEH-PPV devices and the aggregation of the polymer chains.