Surface and Bulk Structural Characterization of a High-Mobility Electron-Transporting Polymer

Abstract

This publication investigates the microstructure of the high electron mobility polymer poly([N,N 0 -bis(2-octyldodecyl)-11 naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5 0 -(2,2 0 -12 biothiopene)) (P(NDI2OD-T2)) using a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption fine structure (NEXAFS), and optical spectroscopies. GIXD measurements confirm extended in-plane lamellae ordering and out-of-plane π-π stacking. Angle-resolved surface-sensitive NEXAFS measurements, however, indicate a lack of preferred in-plane or out-of-plane orientation of either the conjugated backbone or side chains at the P(NDI2OD-T2) film surface. This lack of pronounced orientation effects is attributed to a significant proportion of amorphous content in the film, with NEXAFS measurements sensitive to both amorphous and crystalline content while GIXD is sensitive only to coherent ordering. Furthermore, it is found that increased crystallinity in annealed thin films is not beneficial to the performance of top-gate bottom-contact field-effect transistors. In fact, both highly crystalline and amorphous P(NDI2OD-T2) devices exhibit similar device performance with saturation mobilities around 0.04 and 0.02 cm 2 /(V s), respectively, almost 1 order of magnitude lower than the high-performance devices where the pristine morphology has not been subjected to significant heat treatment. Changes in the optical absorption and Raman spectra of P(NDI2OD-T2) thin films with annealing are also presented and discussed.

Charge Transport Anisotropy in Highly Oriented Thin Films of the Acceptor Polymer P(NDI2OD‐T2)

“…[13][14][15] Vertical charge transport measurements showed a signifi cant drop of the electron mobility upon meltannealing, in accordance with the observed texture changes, where a higher mobility was seen for the face-on orientation, suggestive of effi cient charge transport along the π-stacking direction. On the other hand, transistor performance appeared to be less affected by the structural changes.…”

“…While at fi rst it was assumed that P(NDI2OD-T2) forms mainly amorphous layers, [ 6 ] X-ray diffraction analysis and transmission electron microscopy (TEM) revealed the semicrystalline character of P(NDI2OD-T2) thin fi lms in recent years. [13][14][15][16][17] Rivnay et al were the fi rst to show a remarkable degree of in-plane order in as-cast fi lms with an unconventional face-on texture in the bulk. [ 13 ] A striking texture change was observed upon melt-annealing, when the polymer chains undergo a transition from mainly face-on to edge-on.…”

From Monolayer to Multilayer N‐Channel Polymeric Field‐Effect Transistors with Precise Conformational Order

“…[13] Furthermore, recent studies addressed detail of P(NDI2OD-T2) film microstructure, how it can be varied with thermal annealing, and how this affects FET performance. [14] However, fundamental questions remain about the nature of P(NDI2OD-T2) film microstructure in the very first few layers of the transistor channel.…”

“…This is also corroborated by recent observations on the texture of P(NDI2OD-T2) thin films after melt annealing, revealing an increased crystallinity along with edgeon lamellar conformation upon energy minimization. [14] In general, the molecular orientation with respect to the substrate in the mono-/multilayer films critically depends on the relative strengths of the molecule-substrate versus intermolecular interactions. In the present case, the out-of-plane molecular order is reached at the air/water interface during compression and thereby transferred to the substrate.…”

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

“…This particular morphology allows good electron transport in both organic diodes and field-effect transistors (OFETs), with high in-plane mobilities of ∼0.1-0.6 cm 2 ·V -1 ·s -1 even in the presence of a substantial degree of disorder (11). However, film morphologies with predominantly edge-on oriented polymer chains were observed to yield lower performance in both diodes and top-gated OFETs because of the reduced out-of-plane mobility and larger injection barrier (16)(17)(18)(19). In a recent publication, Neher and coworkers also showed that this polymer has a strong tendency to aggregate, which greatly affects the bulk optical and electrical properties (20,21).…”