Solvent Effects on Chain Orientation and Interchain π‐Interaction in Conjugated Polymer Thin Films: Direct Measurements of the Air and Substrate Interfaces by Near‐Edge X‐ray Absorption Spectroscopy

Abstract

The orientation and packing of semiconducting polymer chains at film interfaces crucially affects charge injection and transport at those interfaces. Although X-ray diffraction techniques have been used extensively as a structural probe, they are not particularly sensitive to these interfaces. Near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), on the other hand, is sensitive to surface orientation and has been applied to investigate polymer structures, for example, in polyimide, [1] polyfluorene, [2] and polythiophene.[3] Here we show that NEXAFS band shapes and intensities are in fact also sensitive to the intermolecular p-interaction in regioregular poly(3-hexylthiophene) (rreg-P3HT) films. NEXAFS spectra of the same material deposited from different solvents and processing conditions are substantially different. This indicates a far more extensive breakdown of the "building-block" picture than previously reported, [4,5] allowing p-interactions to be studied. The surface sensitivity has recently been established to be ca. 2.5 nm for the total electron yield in rreg-P3HT, [6] which is highly relevant to interfacial charge transport and surface-induced self-organization. Rreg-P3HT thin films were deposited from a wide range of solvent qualities, and both the air and substrate interfaces were studied. Delamination was used to expose the substrate interface. We found that: i) the thiophene (Th)-ring orientation at both these interfaces was disordered and showed only a weak preference for the out-of-plane (oop) stacking favorable for field-effect mobility (l FET ); ii) the hexyl side chains were also disordered; and iii) the interchain p-interaction was strongly dependent on the deposition solvent quality, and varied in opposite directions between the air and substrate interfaces, being generally better at the substrate than the air interface. Direct imaging by friction-mode atomic force microscopy (AFM) revealed the expected presence of a mosaic surface morphology with a finer length scale than surface topography. Transistor l FET measurements suggested that the high l FET of rreg-P3HT is still limited by heterogeneity, which we attribute to the distribution of interface domains of varying orientation and/or packing. This ability to directly measure chain segmental orientation and interaction at the interfaces will open the way to their systematic optimization for even higher device performance.The self-organization behavior of polymers at interfaces is of significant scientific and technological interest. Much of the structural knowledge of conjugated polymers such as rreg-P3HT, a prototype high-l FET polymer, has come from wideangle [7,8] and grazing-angle X-ray diffraction (GIXRD) [9,10] studies. For polythiophenes, the chains p-stack to give short interchain distances (3.8 Å in rreg-P3HT) in the lamellae plane, but long distances (17 Å in rreg-P3HT) between lamellae separated by the alkyl side chains. [7][8][9][10] These p stacks further show a preference for either oop stacking in which the plane...

Vertical confinement and interface effects on the microstructure and charge transport of P3HT thin films

“…39 Our model suggests that interfaces inhibit the formation of crystallites and that the polymer/dielectric interface is approximately 30% as crystalline as the bulk in films spun on OTS and 20% as crystalline in films on bare silicon oxide. As discussed above, the bulk crystallinity is found to be almost equal for the two surface treatments, in agreement with our hypothesis of a three-layer model in which the interfacial chemistry only affects crystallization in a thin layer in close proximity to that interface.…”

“…As discussed above, the bulk crystallinity is found to be almost equal for the two surface treatments, in agreement with our hypothesis of a three-layer model in which the interfacial chemistry only affects crystallization in a thin layer in close proximity to that interface. Although contradicting results have been reported as to the degree of structural order at the polymer/air interface relative to the bulk in thin films of P3HT, [39][40][41][42][43] it has been suggested that a thin disordered region exists at the polymer/air interface as well. 39 Ho et al 39 used near-edge X-ray absorption finestructure spectroscopy (NEXAFS) to demonstrate disorder in both the thiophene ring orientations and hexyl side chains in thin films of regioregular P3HT within the technique's $2.5 nm probing depth of the polymer/air interface.…”

“…Although contradicting results have been reported as to the degree of structural order at the polymer/air interface relative to the bulk in thin films of P3HT, [39][40][41][42][43] it has been suggested that a thin disordered region exists at the polymer/air interface as well. 39 Ho et al 39 used near-edge X-ray absorption finestructure spectroscopy (NEXAFS) to demonstrate disorder in both the thiophene ring orientations and hexyl side chains in thin films of regioregular P3HT within the technique's $2.5 nm probing depth of the polymer/air interface. Additionally, Porzio et al used depth dependent XRD to show that P3HT is less ordered near the air/polymer interface than in buried regions of the film.…”

Surface Orientation of Polystyrene Based Polymers: Steric Effects from Pendant Groups on the Phenyl Ring

“…38,39 In addition, techniques are continually emerging to probe organic material interfacial region, including second harmonic and sum frequency generation spectroscopy, 40,41 angle resolved ultraviolet photoelectron spectroscopy, 42 various X-ray scattering techniques, 43,44 metastable-atom electron spectroscopy, 45 and near edge X-ray absorption fine structure (NEXAFS). 46,47 NEXAFS provides two unique measurement capabilities that complement these other techniques including (1) a high intensity synchrotron source allowing for monochromatic incident excitation and enabling enhanced peak resolution, and (2) polarized excitation enabling investigation of bond orientation. NEXAFS has proven useful at monitoring organization of self-assembled monolayers, 48,49 interfacial composition in lithographic polymers, 8,50−52 chemistry in polymeric materials, 53,54 chemical modification of polymeric surfaces, 55 as well as the near surface composition and orientation of block copolymers.…”

Structural Characterization of Conjugated Polyelectrolyte Electron Transport Layers by NEXAFS Spectroscopy

“…The structural features of CPE surfaces are therefore of critical importance to better understand device function and to provide a basis for rational molecular design. Neutral conjugated polymers spontaneously organize from solution upon deposition [9,10] and the degree of order can be influenced by the deposition method, [11] solvent, [12,13] and substrate characteristics. [14] Critical parameters such as charge carrier mobility and emission efficiencies strongly depend on the resulting distance and orientation between the optoelectronic units.…”