Collective orientation barrier in growth of organic thin films revealed by pMAIRS

Sae Nagai,Toshio Nishi,Yuta Inaba,Shigetaka Tomiya,Hajime Kobayashi

doi:10.1103/physrevmaterials.5.013801

Abstract

We used $p$-polarized multiple-angle incidence resolution spectrometry (pMAIRS) to investigate a collective orientation barrier (COB) in the growth of organic semiconductor (OSC) films. We demonstrate a temperature-dependent variation in the growth of pentacene (PEN) on $\mathrm{Si}{\mathrm{O}}_{2}$ films as a model system. The molecular orientation varied from lying to standing as the growth temperature increased. This change suggests that the formation of a standing orientation is thermally activated compared with the lying state. The nucleation of standing-oriented islands occurs by molecular self-assembly at sufficiently high temperatures. Conversely, molecules deposit in a flat-lying state at low temperatures owing to hindrance of the kinetic barrier to reorientation from lying to standing states. The COB is defined as a collective energy barrier for the reorientation processes throughout the growth. The COB is quantitatively estimated from an Arrhenius plot of the probability to form a standing orientation, derived from the dichroic ratio measured by pMAIRS. We found that the COB in the growth of PEN on the $\mathrm{Si}{\mathrm{O}}_{2}$ system was approximately 0.02 eV, which is a key parameter for determining the molecular orientation. A quantitative evaluation of the COB will be applicable to other systems and enable effective control over the molecular orientation of OSC films.

Highlights

Organic-semiconductor (OSC) thin films function as active layers in organic electronic devices such as organic thin film transistors, organic light emitting diodes, and organic photovoltaics [1,2,3,4,5,6]
We evaluate the collective orientation barrier (COB) with the use of polarized multiple-angle incidence resolution spectrometry (pMAIRS), which is a powerful method for quantitatively estimating the degree of molecular orientation in organic thin films. pMAIRS measurements yield both in-plane (IP) and out-of-plane (OP) vibrational spectra simultaneously, which give a dichroic ratio of IP to OP components of a vibrational mode
We have demonstrated the temperature-dependent variation of film structures for PEN on SiO2 at a fixed deposition rate to investigate COB as a collective energy barrier for reorientation processes during growth with the use of pMAIRS

Summary

Introduction

Organic-semiconductor (OSC) thin films function as active layers in organic electronic devices such as organic thin film transistors, organic light emitting diodes, and organic photovoltaics [1,2,3,4,5,6]. The performance of these devices strongly depends on the structure of the OSC films, including factors such as molecular orientation, morphology, and crystallinity [2,4,5,6]. The standing orientation with the molecular long-axis nearly vertical to the substrate is thermodynamically stable because of the lower

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Conclusion