Alternative Face-on Thin Film Structure of Pentacene

Abstract

Pentacene attracts a great deal of attention as a basic material used in organic thin-film transistors for many years. Pentacene is known to form a highly ordered structure in a thin film, in which the molecular long axis aligns perpendicularly to the substrate surface, i.e., end-on orientation. On the other hand, the face-on oriented thin film, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO2. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal. In the present study, the face-on orientation grown on a SiO2 surface has first been identified by means of visible and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the multiple techniques readily reveals that the face-on phase is definitely realized as the dominant component. The face-on film is obtained when the film growth is kinetically restricted to be prevented from transforming into the thermodynamically stable structure, i.e., the end-on orientation. This concept is useful for controlling the molecular orientation in general organic semiconductor thin films.

Controlling the concentration gradient in sequentially deposited bilayer organic solar cells via rubbing and annealing

“…Molecular aggregation in very thin polymorph lms is generally analyzed by the grazing incident X-ray diffraction (GIXRD) technique but combining GIXRD and pMAIRS enables a more accurate morphological characterization of the thin lms. 32,33 In fact, molecular orientation of polymeric thin lms is oen overlooked by XRD-based techniques because of their poor crystallinity. 33,34 Unidirectional hole transport in highly ordered P3HT thin lms can be enhanced through crystallite reorientation.…”

“…In this sense, pMAIRS may be the adequate technique to probe unrubbed and rubbed P3HT-based samples as it provides information on molecular orientation for each chemical group irrespective of the thin lm crystallinity. 32,36 In fact, pMAIRS has been previously employed to reveal structural details that are oen missed by GIXRD. 37,38 The in-plane (IP) and out-of-plane (OP) infrared (IR) absorbance spectra of P3HT and P3HT/PCBM lms are shown in Fig.…”

Second Generation of Multiple-Angle Incidence Resolution Spectrometry

“…With respect to the molecular orientation analysis in a thin film, multiple-angle incidence resolution spectrometry (MAIRS) [8][9][10][11] represented by the p-polarized MAIRS (pMAIRS) 9 technique especially attracts keen attention for the uniquely high analytical accuracy, reproducibility and robustness even for a thin film having a surface roughness, which is not only for theoretical interest, but it is practically useful. [12][13][14][15][16][17] MAIRS yields two spectra simultaneously from an identical thin-film sample revealing the in-plane (IP) and out-of-plane (OP) components of each transition moment (Figure 1), which have exactly the same surface selection rules as those of the conventional transmission and reflectionabsorption (RA) spectrometry, respectively. 18 Therefore, the two spectra of MAIRS are named as MAIRS-IP and OP spectra.…”

“…When MAIRS is coupled with a Fourier transform infrared (FT-IR) spectrometer, 11 an additional benefit is obtained that the polymorph and crystallinity of analytes in a thin film can also be discussed on the accurate band position as related to the molecular orientation. 14,16,17 Here, we have a big problem that perfect subtraction of the many water-vapor related peaks in the fingerprint region is often difficult, [24][25][26] and the remained derivative-shaped peaks make key bands of the thin film invisible as shown in Figure 2a. In a similar manner, the interference fringes appear on several substrates such as silicon (Si) 27 and thin-glass plates 28 because of multiplereflections in the substrate, 29,30 which is further emphasized by the polarizer.…”

Morphology, Structure, and Dynamics of Pentacene Thin Films and Their Nanocomposites with [C2C1im][NTf2] and [C2C1im][OTF] Ionic Liquids

“…The extensive overlap of π molecular orbitals contributes to large intermolecular electronic couplings and efficient charge transport. [3][4][5][6] The rapid oxidation in the air, observed upon a concomitant presence of humidity, oxygen, and visible light, and the low solubility in most solvents are major problems of pentacene. [7,8] Hence, the thin film deposition employing conventional techniques using pentacene solutions is unfavorable, and the vapor deposition methods are very relevant for the thin film assembly of pentacene.…”