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

Abstract

Vibrational spectroscopy is adopted to investigate the film structure 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)) 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 fieldeffect 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.

On the packing and the orientation of P(NDI2OD-T2) at low molecular weight

“…Indeed a detailed study on supramolecular packing compared with 2D simulation appeared recently [24], confirming different configurations of the repeating unit inside the macromolecules [11,21,56], i.e. T2 and NDI2OD moieties can assume S, SA, and A configurations (shown in Table 4 Ratios of neat areas of (1 0 0) (IP) and (2 0 0) (OP) in selected films from 2D images measured at different incident angles (a i Scheme 1).…”

“…T2 and NDI2OD moieties can assume S, SA, and A configurations (shown in Table 4 Ratios of neat areas of (1 0 0) (IP) and (2 0 0) (OP) in selected films from 2D images measured at different incident angles (a i Scheme 1). These were evaluated modifying two monomer models calculated by Fazzi [56] from fine ab initio calculations, using MATSTUDIO package [34]. Also the relative calculated energies were similar [21], hence the assumption of presence of all these configurations along with the backbone, especially for larger M n is reasonable, as well as the coexistence of different configurations along with the macromolecules.…”

High‐Mobility Naphthalene Diimide and Selenophene‐Vinylene‐Selenophene‐Based Conjugated Polymer: n‐Channel Organic Field‐Effect Transistors and Structure–Property Relationship

“…[31,67] The dihedral angles between the NDI and SVS units are 47.70° and 43.78° ( Figure S5b, Supporting Information), which are larger than those reported for NDI and TVT, and NDI and bithiophene units, being 33.44° and 32.82°, and 32.88° and 40.61°, respectively. [66,68,69] The difference between the dihedral angles of these copolymers suggests a clear departure from extensive and continuous planarity in the polymer films with the most distorted being the PNDI-SVS copolymer. Physical properties, including optical bandgap, and HOMO and LUMO energetic levels, are summarized in Table 1.…”

Experimental evidence that short-range intermolecular aggregation is sufficient for efficient charge transport in conjugated polymers

“…Indeed, a detailed characterization of this polymer reveals a rather unique solid-state microstructure displaying the simultaneous presence of disordered phases and highly interconnected, fiber-like ordered phases, where the polymer backbones lay preferentially faceon with respect to the substrate (12) and characterized by a dihedral angle between the NDI and thiophene units (13). Whereas a face-on orientation is observed in the bulk of the film, a more edge-on orientation is revealed at the film surface (14,15).…”