A new schematic for poly(3-alkylthiophene) in an amorphous film studied using a novel structural index in infrared spectroscopy

Abstract

The molecular structure of poly(3-alkylthiophene-2,5-diyl) in an amorphous film reveals that the short axis of the thiophene ring is kept highly oriented parallel to the substrate, whereas the long axis along the polymer chain is largely disordered. This is unveiled by infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS), achieved by analyzing the orientation angles of three mutually orthogonal vibrational modes localized on the thiophene ring with the aid of a newly developed structural index. This new analytical technique is useful irrespective of the crystallinity of the thin film. As a result, the intrinsic chemical parameters controlling the molecular orientation are understood in a unified manner, and the reason that the hexyl group gives the best results for a photovoltaic cell is also revealed.

Comprehensive Understanding of Structure‐Controlling Factors of a Zinc Tetraphenylporphyrin Thin Film Using pMAIRS and GIXD Techniques

“…The CÀHo ut-of-plane deformation vibration (g(CÀH))b and of an aromatic ring appears in the low wavenumber region of 900-650 cm À1 ;w hile the CÀHi n-plane deformation vibration (d(CÀH)) band appears in 1600-950 cm À1 .T he g(CÀH) and d(CÀH) modes have transition moments perpendicular and parallel to the aromatic ring plane, respectively.I n particular, the g(CÀH) mode is fortunately highly localized on the ring with as trong absorption coefficient,w hich is quite useful for analyzing the molecular orientation of the ring. [23,29] As found in the Chl-SC film (Figure 1a), the pMAIRS-IP and -OP spectra have comparable band intensities in all the regions,w hich straightforwardly indicates that ZnTPP in the film has ar andom orientation. After annealing this film (denoted as "Chl-SC-Af ilm"; Figure 1b), the bands at 798 and 718 cm À1 , both of which are assigned to the g(CÀH) modes of porphyrin ring (g(CÀH) por ;F igure 2a), [40][41][42] develop significantly in the OP spectrum.…”

“…The p-polarized IR light was obtained with aH arrick Scientific (Pleasantville, NY,U SA) PWG-U1R wire-grid polarizer.T he transmitted light through the sample was detected by al iquid-N 2 cooled mercury-cadmium-telluride detector with am odulation frequency of 60 kHz. The angle of incidence was changed from 98 to 448 by 58 steps, [23,51] and the interferogram was accumulated 1000 times for each step.…”

“…[19][20][21][22] The molecular orientation and crystal structure are keyf actors for the thin-film structure, both of which significantly influence the device performance. For example, aP 3HT film is knownt oh aved ifferent molecular orientations, thiophenyl ring-parallel and ring-verticalo rientations to the substrate surface, [23] in which the films exhibit largely different carrier mobility. [24,25] As well as P3HT,e vaporated films of pentacene are also known to show different mobilitiesd epending on the ring orientation and crystal structure.…”

Fringe and Noise Reductions of pMAIRS Spectra Using Principal Component Analysis

“…For the structural analysis of P3HT, the two bands that appeared at approximately 1510 and 820 cm -1 are particularly useful; the bands are assigned to the anti-symmetric stretching vibration (denoted as ν(C=C)) and out-of-plane deformation vibration (γ(C-H)) modes of the thiophene ring, respectively. 24 As found in Fig. 1, the intensity ratio of the γ(C-H) band between IP and OP spectra changes depending on the spin speed: the SC-8000 film exhibits a stronger γ(C-H) band in the OP spectrum than that in the IP spectrum; while the IP and OP spectra of the SC-250 film yield comparable band intensities.…”

“…This indicates that the SC-8000 thin film is comprised of the face-on oriented P3HT molecules judging from the direction of the transition moment. 24 On the other hand, the SC-250 film comprises both face-on and edge-on orientations in the thin film. These results agree with a former study by Delongchamp et al 23 The difference of the molecular orientation can be attributed to the crystallinity of the film.…”