Control of the Interchain π−π Interaction and Electron Density Distribution at the Surface of Conjugated Poly(3-hexylthiophene) Thin Films

Abstract

Interchain interaction, i.e., pi-pi stacking, can benefit the carrier transport in conjugated regio-regular poly(3-hexylthiophene) (P3HT) thin films. However, the existence of the insulating side hexyl chains in the surface region may be detrimental to the charge transfer between the polymer backbone and overlayer molecules. The control of the molecular orientation in the surface region is expected to alter the distribution of the pi electron density at the surface to solve such problems, which can be achieved by controlling the solvent removal rate during solidification. The evidence that the pi-electron density distribution at the outermost surface can be controlled is demonstrated by the investigation using the powerful combination of near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and the most surface-sensitive technique: Penning ionization electron spectroscopy. From the spectroscopic studies, it can be deduced that the slower removal rate of the solvent makes the polymer chains even at the surface have sufficient time to adopt a more nearly equilibrium structure with edge-on conformation. Thus, the side hexyl chains extend outside the surface, which buries the pi-electron density contributed from the polymer backbone. Contrarily, the quench of obtaining a thermo-equilibrium structure in the surface region due to the faster removal of the solvent residual can lead to the surface chain conformation without persisting to the strong bulk orientation preference. Therefore, the face-on conformation of the polymer chain at the surface of thin films coated with high spin coating speed facilitate the electron density of the polymer backbone exposed outside the surface. Finally, thickness dependence of the surface electronic structure of P3HT thin films is also discussed.

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

“…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.…”

Enhancement of field-effect mobility due to structural ordering in poly(3-hexylthiophene) films by the dip-coating technique

“…The most extensively investigated factor among others affecting the field-effect mobility is the molecular ordering of nanocrystallites in P3HT thin-film transistors (Sirringhaus et al, 1999;Cho et al, 2006;Hao et al, 2007;Joshi et al, 2008). An alternative way to enhance the charge carrier transport is the dip-coating technique, which can facilitate self-assembly into a nanofibrillar lamellar structure (Sirringhaus et al, 1999;Salleo et al, 2010).…”

“…Despite the effort made in recent years to apply the dipcoating technique to small molecules and oligomers (Hao et al, 2007;Sandberg et al, 2002;Wang et al, 2004), little is known about the mechanism of conjugated polymer reorientation with respect to the dipping direction and its relation to field-effect mobilities. Only a few studies have been specifically focused on these matters Hao et al, 2007;Sandberg et al, 2002;Wang et al, 2004) and no consensus has been established yet.…”

Morphology and orientation of thin poly(3-hexylthiophene) (P3HT) films on differently silanized silicon oxide

“…In particular, according to XRD investigations, the P3HT macromolecules organize themselves into a sequence of π-π interacting molecules with their backbone essentially edge on oriented with respect to the substrate as shown in Figure 1a. [6][7][8][9][10][11][12][13][14][15] thus giving rise to a filament morphology [6][7][8][9][10][11][12][13]. The model used to explain these morphological structures has been presented by Sirringhaus [6] and Salleo [16].…”

“…[11,14,15,[17][18][19][20][21]. Besides, an appropriate choice of the substrate could be fundamental in determining the edge-on face-on configuration.…”