The Control of Electronic States Spreading Outside the Conjugated Polymer Surface
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0965-S13-02
The Control of Electronic States Spreading Outside the Conjugated Polymer Surface Xiao Tao Hao1, Takuya Hosokai2, Noritaka Mitsuo2, Satoshi Kera2, Kazuyuki Sakamoto1,2, Koji Kamiya Okudaira1,2, and Nobuo Ueno1,2 1 Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan 2 Graduate School of Science and Technology, Chiba University, Chiba, 263-8522, Japan
ABSTRACT The surface electronic structures of conjugated regio regular and regio random poly (3hexylthiophene) (rr-P3HT and rra-P3HT) thin films were studied by near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy and Penning ionization electron spectroscopy (PIES). The distribution of the surface electronic states was controlled on rr-P3HT and rra-P3HT thin films with different molecular ordering by varying the coating process and PIES was adopted to observe the electronic states existing outside the surface. INTRODUCTION Interest in π-conjugated polymers has recently soared due to the potential applications in opto-electronic devices. Studying surface induced electronic characteristics in these polymers is essential to understand the working mechanism and enhance their performance in organic devices, such as light-emitting diodes, thin film transistors, solar cells etc [1,2]. Polythiophene and its derivatives are the first conducting polymers that are stable toward oxygen and moisture, in which poly (3 hexyl-thiophene) (P3HT) is one of the most popular materials as an active layer used for devices. Conjugated P3HT can have different configurations [3], head-to-tail (HT), tailto-tail (TT), and head-to-head (HH). The regio-regular P3HT consists of only 3-hexyl thiophene with ordered structure in which the side groups are arranged head to tail, while regio-random P3HT consists of TT, HH and HT 3-hexylthiophene in a random structure. The difference in ordering and crystallinity between the two configurations would have effect on the surface electronic structures of polymer thin films. It has been recently discovered that films cast from rr-P3HT form two-dimensional (2D) lamellae perpendicular to the substrate, whereas rra-P3HT forms lamellae to a lesser extend [4]. The electronic states distributed outside the polymer surface is essential to the electronic wave functions overlap between the underlying polymer layer and an overlayer molecule to achieve high performance devices with layer-by-layer structures made by these polymers. The distribution of the electronic states spreading outside the polymer surface relies on the chain conformation greatly due to the existence of the side pendant groups. Surface sensitive techniques like near edge X-ray absorption fine structure spectroscopy (NEXAFS), ultra-violet photoelectron spectroscopy (UPS) and Penning ionization electron spectroscopy (PIES) can be used as powerful spectroscopic tools to characterize the surface of conjugated polymer thin films for understanding the electronic structure, energetic, thermochromism, molecular orderin
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