Molecular Level Investigation of the Film Structure of a High Electron Mobility Copolymer via Vibrational Spectroscopy

Vibrational spectroscopy is adopted to investigate the film structure of poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (P­(NDI2OD-T2)) at the molecular level. Both Raman and IR spectra are measured for P­(NDI2OD-T2) solutions and films. A good match with density functional theory (DFT) calculations at the B3LYP/6-311G** level is obtained, so that the main spectral features could be assigned. No significant spectral shifts are recorded when passing from very diluted solutions to the solid state, while clear variations in the relative intensity of specific spectral markers are observed. The comparison of the spectral patterns shown by IR spectra recorded with reflection–absorption IR spectroscopy (RAIRS) and in normal transmission experiments allows to derive a structural model of the polymer. In as-cast films, or in films subjected to mild thermal treatments, below the melting point, the backbone of the polymer chains lies preferentially in the substrate plane, with the T2 units lying flat parallel to the substrate and the NDI2OD unit featuring a dihedral angle θ with the T2 unit (θ ≈ 38°). This structure and polymer orientation is consistent with reported good bulk electron mobility in vertical diodes structures and high field-effect mobility in lateral field-effect transistors. Furthermore, we observe that upon a melt-annealing treatment, a clear modification of the RAIRS spectrum occurs suggesting either a loss of the preferential orientational order of the film or a flip of some domains featuring the polymer segments tilted out of the substrate

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{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-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

Photoactive Molecular Junctions Based on Self-Assembled Monolayers of Indoline Dyes

We demonstrate the feasibility of a photodetector based on an ensemble molecular junction, where a self-assembled monolayer of an organic donor–acceptor dye is directly sandwiched between two electrodes. In such a device, upon photoexcitation and generation of a charge-transfer state on the molecule, charges are dissociated and directly collected at the electrodes without the need of transport through a bulk phase, as in usual photodetectors. We show that the device can work in photovoltaic regime and the spectral response can be tuned by varying the light absorbing dye. Therefore, the electro-optical properties of the downscaled device can be unambiguously related to the physical–chemical properties of the molecules, a commonly difficult point to demonstrate in a molecular junction device, because of the uncertainties of the interplay between molecules and electrodes. The proposed device, which relies on a simple self-assembly process, has a strong potentiality for fast responding, downscaled detectors, ultimately limited by charge dissociation dynamics, and can be considered also as a useful tool to investigate fundamental electro-optical processes in molecular monolayers