Structural Characterization of Highly Oriented Naphthalene-Diimide-Bithiophene Copolymer Films via Vibrational Spectroscopy

Epitaxially grown highly oriented crystalline films, named form I and form II, and spin-coated films of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, P(NDI2OD-T2), have been investigated through infrared vibrational spectroscopy techniques (infrared absorption in double transmission at normal incidence (IRA-TR) and reflection absorption infrared spectroscopy at grazing angle incidence (RAIRS)) to get access to polymer chain orientation and structure. An analytic model to correlate the experimental intensities of the IR bands with structural parameters has been developed and applied for the three film morphologies. While spin-coated and form I films show P(NDI2OD-T2) chains lying parallel to the substrate in the face-on arrangement, form II films feature a structure with chains tilted out from the surface. The combined experimental and theoretical methodology gives insights into the local molecular orientations of naphthalene diimide (NDI2OD) and bithiophene (T2) counits. This approach can be easily extended to a variety of organic polymer semiconductors, allowing one to directly correlate molecular structure to properties such as charge transport, which is of fundamental relevance for developing quantitative models for applications in organic electronics and photovoltaics

Structural Characterization of Highly Oriented Naphthalene-Diimide- Bithiophene Copolymer Films via Vibrational Spectroscopy

Epitaxially grown highly oriented crystalline films, named form I and form II, and spin-coated films of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, P(NDI2OD-T2), have been investigated through infrared vibrational spectroscopy techniques (infrared absorption in double transmission at normal incidence (IRA-TR) and reflection absorption infrared spectroscopy at grazing angle incidence (RAIRS)) to get access to polymer chain orientation and structure. An analytic model to correlate the experimental intensities of the IR bands with structural parameters has been developed and applied for the three film morphologies. While spin-coated and form I films show P(NDI2OD-T2) chains lying parallel to the substrate in the face-on arrangement, form II films feature a structure with chains tilted out from the surface. The combined experimental and theoretical methodology gives insights into the local molecular orientations of naphthalene diimide (NDI2OD) and bithiophene (T2) counits. This approach can be easily extended to a variety of organic polymer semiconductors, allowing one to directly correlate molecular structure to properties such as charge transport, which is of fundamental relevance for developing quantitative models for applications in organic electronics and photovoltaics

Influence of Processing Solvents on Optical Properties and Morphology of a Semicrystalline Low Bandgap Polymer in the Neutral and Charged States

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Influence of Processing Solvents on Optical Properties and Morphology of a Semicrystalline Low Bandgap Polymer in the Neutral and Charged States

We present a systematic study of the morphology and absorption properties of a typical donor–acceptor polymer (PCPDTBT) with semicrystalline behavior in solution and in thin films. In-situ spectroelectrochemical data give information about the evolution of the absorption spectra from neutral to charged species. The experimental data are supported by theoretical calculations in the framework of the density functional theory (DFT). Regarding thin film structures, we show that the choice of the solvent has significant influence on the morphology in thin films: whereas CS₂ and CHCl₃ give rather structureless (amorphous) morphologies, films from 1-CN exhibit a clear crystalline nanofiber morphology. Accompanying UV/vis/NIR spectra of films are highly dependent on the morphology and therefore on the choice of the processing solvent. The absorption of fiber morphologies is strongly red-shifted compared to the structureless films

Intertwined Lamello-Columnar Coassemblies in Liquid-Crystalline Side-Chain Pi-Conjugated Polymers: Toward a New Class of Nanostructured Supramolecular Organic Semiconductors:

A set of liquid-crystalline polymeric systems, associating at once the regioregular polythiophene backbone and pending mesogenic triphenylenes, is reported. Two series, namely regular homopolymers and alternating copolymers, were prepared by adapting a Grignard metathesis-based methodology, allowing some of the relevant structural parameters to be sequentially and independently modified. The thermal and self-organization behaviors of these uncommon macromolecular systems were investigated by polarized-light optical microscopy, differential scanning calorimetry and temperature-dependent small-angle X-ray scattering. Most polymers self-organize into mesophases possessing intertwined lamello-columnar morphologies, resulting from the simultaneous coexistence of lamellar and columnar sublattices. The successful preparation of oriented thin films of several of these polymeric homologues allowed further investigations by atomic force microscopy, transmission electron microscopy, electron diffraction, and grazing-incidence SAXS, which provided a deeper insight of the intricate supramolecular organizational modes, including the complete elucidation of the structure of the lamello-columnar mesophases. This simple and versatile strategy provides a route to elaborate polymeric materials incorporating two intercalated separate pathways toward charge carrier transport, of paramount importance for future electronic and optoelectronic applications

Impact of thermal annealing on the semicrystalline nanomorphology of spin-coated thin films of regioregular poly(3-alkylthiophene)s as observed by high-resolution transmission electron microscopy and grazing incidence X-ray diffraction

The impact of thermal annealing on the growth of crystalline nanodomains of poly(3-alkylthiophene) (P3AT) in thin films (15–20 nm thick) was investigated as a function of length of alkyl side chain by combined low-dose high-resolution transmission electron microscopy (HR-TEM) and grazing incidence X-ray diffraction (GIXD). Statistical analysis of the data yields the characteristic dimensions of the face-on oriented P3AT nanocrystals, i.e., average stem length l$_c$ along backbones and lateral dimension perpendicular to the stems l$_a$ along side chains. The following trends were identified: (i) in as-spin-coated films, the proportion of face-on oriented nanocrystals increases with the number of carbon atoms in the side chain, (ii) annealing favors the lateral in-plane growth of the nanocrystals along the side chain direction (a$_{P3AT}$ axis), (iii) for a given P3AT, the proportion of face-on oriented domains increases with annealing temperature, (iv) lateral growth along the a$_{P3AT}$ axis is most efficient for the longer octyl side chains, and (v) thermal annealing induces only modest lamellar thickening which is limited by the poor slide diffusion of $\pi$-stacked P3AT chains as opposed to lateral growth favored by weak van der Waals interactions between layers of $\eta$-alkyl side chains. The increase in the population of face-on oriented crystallites, observed when the length of the side chain increases, coincides with a corresponding decrease in the field effect mobility in annealed P3AT thin films