Acid-functionalized fullerenes used as interfacial layer materials in inverted polymer solar cells

Two types of carboxylic acid functionalized fullerence derivatives, 4-(2-ethylhexyloxy)-[6,6]-phenyl C-61-butyric acid (p-EHO-PCBA) and bis-4-(2-ethylhexyloxy)-[6,6]-phenyl C-61-butyric acid (bis-p-EHO-PCBA), were synthesized and investigated as an interfacial layer for inverted polymer solar cells (iPSCs). The -COOH groups on the PCBAs chemisorb to inorganic metal oxide (TiOX), generating fullerene-based self-assembled monolayers (FSAMs). The devices with the mono-and bis-FSAMs exhibited substantially lower series resistance (R-S) values of 2.10 Omega cm(2) and 1.46 Omega cm(2), compared to that (4.15 Omega cm(2)) of the unmodified device. The TiOX films modified with mono-and bis-FSAMs showed higher contact angles of 50 degrees and 91 degrees, respectively, than that of the pristine TiOX film (33 degrees). The increased contact angles were attributed to the enhanced hydrophobicity, improving the wetting properties with the organic photoactive layer. In addition, a comparison of device characteristics with electroactive FSAMs and non-electroactive benzoic acid SAMs clearly indicates that the FSAMs may suggest an additional pathway for photo-induced charge transfer and charge collection to ITO. After surface modification with FSAMs, the short-circuit current density (J(SC)) and fill factor (FF) values increased substantially. The iPSCs based on poly(5,6-bis(octyloxy)-4-(thiophen-2-l)benzo[c][1,2,5]thiadiazole) (PTBT) and [6,6]phenyl-C-61-butyric acid methyl ester (PCBM) as an active layer showed remarkably improved power conversion efficiency up to 5.13% through incorporation of the FSAMs-based interfacial layer. (C) 2013 Elsevier B.V. All rights reserved.This study was supported by the NRF Grant (2009-C1AAA001-2009-0093020, 2010-0026163, 2009-00605, 2010-0019408, 2013R1A1A1A05004475) and the World Class University program (R31-2008-000-20004-0, R31-2008-000-20012-0) funded by the Ministry of Education, Science and Technology, Korea. This work was also supported by New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) (20113010010030) and the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-2415)