Interfacial Ring Orientation in Polythiophene Field-Effect Transistors on Functionalized Dielectrics

Abstract

The impact of surface chemistry on interfacial molecular orientation is studied for polymeric organic field-effect transistor (oFET) assemblies using vibrational sum frequency generation (VSFG) spectroscopy. The carbonÀ carbon vibrational modes on the backbone of side-chain deuterated poly(3hexylthiophene) (DP3HT) are demonstrated to be an excellent handle for molecular orientation. DP3HT is utilized to avoid overlap in this spectral region with alkyl CH bending vibrations. Raman and FTIR spectroscopies are used to characterize the vibrational spectra of the thin films, and band assignments are confirmed by DFT calculations. Organosilane self-assembled monolayers are used to prepare oFET dielectrics with a range of surface energies, as measured by their water contact angles. The surface chemistry is found to have a profound influence on the field-effect carrier mobilities with lower surface energies producing higher mobilities. Polarization selective VSFG spectroscopy is then used to determine the relative orientation of the CdC symmetric stretching mode with respect to the surface normal. High field-effect mobilities for DP3HT on low surface energy functionalized dielectrics are directly correlated with the relative orientations of this vibrational transition dipole moment as measured by VSFG. The connection between this nonlinear spectroscopic observable and molecular structure enables this approach to confirm that low surface energy dielectrics lead to edge-on orientation of DP3HT conjugated chains with consequently higher mobilities.

Molecular Ordering of Conjugated Polymers at Metallic Interfaces Probed by SFG Vibrational Spectroscopy

“…A molecular order investigation via SFG-VS on the interface between P3HT and different functionalized silica (SiO 2 ) surfaces concluded that surface chemistry directly influences the π-stacking arrangement, which, in turn, correlates with fieldeffect mobilities of polymeric transistors. 20 Similar conclusions were reached by an investigation that combined X-ray diffraction to probe polymer crystallinity and SFG-VS to evaluate the quality of surface functionalization. 21 On the basis of a quantitative analysis, a recent report 10 used SFG-VS to determine the molecular orientation of P3HT rings on SiO 2 and AlO x substrates, both for as-cast samples and after thermal annealing.…”

The isotopic effects of deuteration on optoelectronic properties of conducting polymers

“…Figure 2 and Table 2 present Raman spectroscopy measurements of different P3HT isotopes mixed with PCBM. Upon side-chain deuteration, the C-C inplane ring skeleton modes at 1,380 cm À 1 and the C-H symmetric stretching mode at 2,822 cm À 1 are shifted downwards by almost 20 cm À 1 because the heavier deuterium decreases the vibrational frequency 29 . The Raman shift of the C-H stretching mode is in agreement with the expected isotope shift based on the square root of the C-D/C-H mass ratio 3 , and the vibrational frequency of the C-S-C ring in-plane skeleton mode at 728 cm À 1 remains unchanged, suggesting that the vibrational modes of the thiophene backbone are not affected by side-chain deuteration.…”

“…Dry THF was used as the solvent through the synthesis. Side-chain-deuterated P3HT was obtained from adopted literature procedures by reacting of 3-bromothiophene and 1-bromohexane-d13 under refluxing 29 . 2,5-Dibromo-3-hexylthiophene-d13 was polymerized using the Kumada catalyst transfer polymerization method.…”

Probing Electronic Structures of Organic Semiconductors at Buried Interfaces by Electronic Sum Frequency Generation Spectroscopy

“…In the homodyne detection scenario, the signal I SFG is proportional to the square of x eff (2) , and the products of near IR ( I IR ) and 800nm ( I 800nm ) beam intensities. As we show below, by normalizing I SFG of the sample to a SFG signal from the reference sample, the dependence on I IR and I 800nm can be removed.…”

“…As we show below, by normalizing I SFG of the sample to a SFG signal from the reference sample, the dependence on I IR and I 800nm can be removed. As a result, we can directly determine x eff (2) , which contains x ijk ,n (Fig.1a),…”