Interface Structures and Electronic States of Epitaxial Tetraazanaphthacene on Single-Crystal Pentacene.

Yuki Gunjo,Makoto Tadokoro,Kazuhiko Mase,Kana Takahashi,Hajime Kamebuchi,Riku Takeuchi,Kaname Kanai,Masaki Iwashita,Ryohei Tsuruta,Yasuo Nakayama,Tomoyuki Koganezawa

doi:10.3390/ma14051088

Abstract

The structural and electronic properties of interfaces composed of donor and acceptor molecules play important roles in the development of organic opto-electronic devices. Epitaxial growth of organic semiconductor molecules offers a possibility to control the interfacial structures and to explore precise properties at the intermolecular contacts. 5,6,11,12-tetraazanaphthacene (TANC) is an acceptor molecule with a molecular structure similar to that of pentacene, a representative donor material, and thus, good compatibility with pentacene is expected. In this study, the physicochemical properties of the molecular interface between TANC and pentacene single crystal (PnSC) substrates were analyzed by atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), and photoelectron spectroscopy. GIXD revealed that TANC molecules assemble into epitaxial overlayers of the oriented crystallites by aligning an axis where the side edges of the molecules face each other along the direction of the PnSC. No apparent interface dipole was found, and the energy level offset between the highest occupied molecular orbitals of TANC and the PnSC was determined to be 1.75 eV, which led to a charge transfer gap width of 0.7 eV at the interface.

Highlights

The structural and electronic properties of a heteroepitaxial molecular p–n heterojunction formed by stacking TANC on pentacene single crystal (PnSC) surfaces were investigated by non-contact mode atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and ultravioletvisible absorption spectroscopy (UV-vis)
The PnSC surface was exposed to air and ambient light after fabrication, and the surface was presumed to incorporate a few percent of oxidized species [20,22]
The crystallographic structure of the TANC/PnSC heterojunctions was determined by GIXD conducted at BL19B2 of SPring-8

Summary

Introduction

Molecules with nitrogen-containing heterocycles are attracting recent interest as non-fullerene acceptor molecules for the application of organic solar cells and as high mobility n-type organic semiconductors [12]. In this context, the target material of this work, 5,6,11,12tetraazanaphthacene (TANC), where four methine groups in naphthacene are replaced by four nitrogen atoms, is one prominent example known to exhibit the n-type semiconductor operation [7]. The structural and electronic properties of a heteroepitaxial molecular p–n heterojunction formed by stacking TANC on PnSC surfaces were investigated by non-contact mode atomic force microscopy (nc-AFM), grazing-incidence X-ray diffraction (GIXD), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and ultravioletvisible absorption spectroscopy (UV-vis). An energy level diagram at the TANC/PnSC heterojunction is determined based on the XPS and UPS results

Materials and Methods

3.3.Results

Electronic

Conclusions