An electron-accepting molecular unit exhibiting an orientational preference favorable for organic photovoltaic applications Kouki Akaike a, ⁎ ,1 , Shinji Ando a,2 , Hideo Enozawa a , Atsuko Kosaka a,b , Takashi Kajitani a,b , Takanori Fukushima a,b, ⁎⁎ a RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan abstract article info Article history: Received 24 September 2014 Received in revised form 12 February 2015 Accepted 2 March 2015 Available online 28 March 2015 Keywords: Electron acceptor Indacenetetraone Thiophene Face-on orientation Electron transport Organic photovoltaics Control of molecular orientation of organic semiconductor is essential for efficient light absorption and charge- carrier transport in organic optoelectronic devices. We synthesized compound 1 as a fundamental electron- accepting building block for the design of n-type semiconductors and conducting polymers. We found that this molecule, upon evaporation onto a substrate such as SiO 2 and electron-donor films, spontaneously assembles with a face-on orientation relative to the substrate surface. This orientation is favorable for thin-film organic pho- tovoltaics. Despite relatively small π-conjugation, 1 showed strong absorption in visible-light region and an ap- propriate lowest unoccupied molecular orbital energy for electron transfer with electron donors including copper phthalocyanine and poly(3-hexylthiophene). Accordingly, thin-film devices, fabricated using 1 and elec- tron donors, exhibited a clear photovoltaic response. This suggests that compound 1 provides a promising build- ing block for the development of active materials in organic photovoltaics. © 2015 Elsevier B.V. All rights reserved. 1. Introduction For the development of high-performance organic electronic de- vices, controlling the orientation of organic semiconductors in bulk and at interfaces carries the same importance as the design of their elec- tronic properties [1,2]. For instance, thin-film organic photovoltaic (OPV) devices, a face-on molecular orientation is strongly required not only for maximizing light-absorption efficiency but also for ensuring carrier-transport pathways toward electrodes [3,4]. Despite this struc- tural requisite, semiconducting π-conjugated molecules generally pre- fer to adopt edge-on orientation. To achieve face-on molecular orientation, several approaches have been proposed, as exemplified by insertion of a buffer layer [5,6], rubbing technique [7,8], and interface doping [9]. Nonetheless, it would be ideal if semiconducting molecules, displaying spontaneous face-on orientation, could rationally be designed. Recently, we reported a hybrid molecule of tetrathiafulvalene and indacenetetraone that can serve as an electron-acceptor component for a polythiophene-based bulk-heterojunction OPV device [10]. In this context, we conceived that indacenetetraone could provide an electron-accepting building block for the design of new n-type semi- conductors and conducting polymers. For this purpose, we designed simple yet unknown compound 1 (Scheme 1) as a fundamental motif. Unexpectedly, in the course of the synthetic work of 1, we found that this compound, upon evaporation, spontaneously assembles on various substrates with a “face-on” orientation. Here we report the structural, electronic, and carrier-transport properties of evaporated films of 1. We also describe the performance of prototype OPV devices fabricated using a bilayer heterojunction film composed of 1 and copper phthalo- cyanine (CuPc) or poly(3-hexylthiophene) (P3HT). 2. Experimental details 2.1. General Unless otherwise noted, reagents and solvents were used as received from Sigma Aldrich [2-thiophenecarboxaldehyde, CuPc, P3HT, bathocuproine (BCP), and poly(3,4-ethylenedioxythiophene: poly(styrenesulfonate) (PEDOT:PSS) (Clevios P Al 4083)] and Kanto Chemicals (acetic acid and hexane). Compound 2 was prepared according Thin Solid Films 583 (2015) 34–39 ⁎ Corresponding author. Tel.: +49 30 2093 7553; fax: +49 30 2093 7443. ⁎⁎ Correspondence to: T. Fukushima, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. Tel.: +81 45 924 5220; fax: +81 45 924 5976. E-mail addresses: kakaike@physik.hu-berlin.de (K. Akaike), fukushima@res.titech.ac.jp (T. Fukushima). 1 Present address: Institut für Physik, Humboldt Universität zu Berlin, Brook-Taylor- Straße 6, Berlin 12489, Germany. 2 Present address: Kanagawa Academy of Science & Technology (KAST) and Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. http://dx.doi.org/10.1016/j.tsf.2015.03.002 0040-6090/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf