Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer

Abstract

The molecular orientation and microstructure of films of the high-mobility semiconducting polymer poly(N,N-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) (P(NDI2OD-T2)) are probed using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. In particular a novel approach is used whereby the bulk molecular orientation and surface molecular orientation are simultaneously measured on the same sample using NEXAFS spectroscopy in an angle-resolved transmission experiment. Furthermore, the acquisition of bulk-sensitive NEXAFS data enables a direct comparison of the information provided by GIWAXS and NEXAFS. By comparison of the bulk-sensitive and surface-sensitive NEXAFS data, a distinctly different molecular orientation is observed at the surface of the film compared to the bulk. While a more "face-on" orientation of the conjugated backbone is observed in the bulk of the film, consistent with the lamella orientation observed by GIWAXS, a more "edge-on" orientation is observed at the surface of the film with surface-sensitive NEXAFS spectroscopy. This distinct edge-on surface orientation explains the high in-plane mobility that is achieved in top-gate P(NDI2OD-T2) field-effect transistors (FETs), while the bulk face-on texture explains the high out-of-plane mobilities that are observed in time-of-flight and diode measurements. These results also stress that GIWAXS lacks the surface sensitivity required to probe the microstructure of the accumulation layer that supports charge transport in organic FETs and hence may not necessarily be appropriate for correlating film microstructure and FET charge transport.

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

“…Such topographic features are usually observed in the parent polymer P(NDI2OD-T2) and they have been ascribed to selfassembling phenomena occurring during film deposition and solidification and just initiated by aggregates present in solution; [3] this correspondence with P(NDI2OD-T2) morphology is also reflected by GIWAXS, revealing a mostly face-on stacking in the bulk, and by NEXAFS, evidencing a pronounced edgeon molecular arrangement at the surface. [72] In the case of the as-cast film (Figure 4a) (R rms = 0.7 nm), topographic features are many tens of nm wide (up to 40 nm), while their length is inaccessible, with the elongated structures being mostly intertwined and interconnected to each other; however, segments up to 500 nm long with coherent directionality can be observed. From the section analysis of those nanostructured segments that are less woven and more clearly stacked (Figure 4b), thickness values of ≈2.3-2.9 nm were extracted, thus in good agreement with a mostly edge-on orientation.…”

“…When trying to establish such a nexus, we have first to take into account that, in FETs in full accumulation, charges probe only a few nm-thick semiconductor layer at the interface with the dielectric, and this critical region may display a different microstructure from the bulk. [72] Moreover, charge transport in organic semiconductors not only depends on features like crystallinity, but, especially in films with a consistent amorphous-like phase, important factors are also film interconnectivity [1,78] and degree of noncrystalline orientational order, [79] as recently evidenced for example for PNDI2OD-T2. [3,80] Therefore, typical bulk average structural characterization of the ordered phase, as obtained by GIWAXS, has to be completed with more specific, channel sensitive investigations.…”

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). 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).…”

MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum

“…To monitor the surface of the film while limiting the scattering from the bulk, the substrate was rotated such that the X-ray incidence angle was Θ = 0.071 • , well below both the glass critical angle Θ c,glass = 0.152 • and the BPhen critical angle Θ c,Bphen = 0.127 • . Although the intensity at this shallow grazing angle is relatively low, we can achieve significant depth sensitivity, with a minimum effective penetration depth 32 given by z 0 = 1 2 √ r e π ρ, where r e is the Thomson scattering length and ρ is the electron density of the compound. Assuming vacuum deposited Bphen is crystalline, with an orthorhombic crystal structure 33 (a = 7.253 Å, b = 10.810 Å, c = 21.14 Å), the penetration depth z 0 = 116 Å which is slightly larger than that reported for crystalline P3HT of 85 Å.…”

“…Assuming vacuum deposited Bphen is crystalline, with an orthorhombic crystal structure 33 (a = 7.253 Å, b = 10.810 Å, c = 21.14 Å), the penetration depth z 0 = 116 Å which is slightly larger than that reported for crystalline P3HT of 85 Å. 32 As a result, changes in the depth dependence of the crystallographic features with increasing film thickness can be monitored closely in films relevant for optoelectronic devices.…”