www.MaterialsViews.com www.advenergymat.de FULL PAPER © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 10) 1301234 wileyonlinelibrary.com Adv. Energy Mater. 2014, 4, 1301234 Alkyl Chain Engineering of Solution-Processable Star-Shaped Molecules for High-Performance Organic Solar Cells Jie Min,* Yuriy N. Luponosov, Andreas Gerl, Marina S. Polinskaya, Svetlana M. Peregudova, Petr V. Dmitryakov, Artem V. Bakirov, Maxim A. Shcherbina, Sergei N. Chvalun, Souren Grigorian, Nina Kaush-Busies, Sergei A. Ponomarenko, Tayebeh Ameri, and Christoph J. Brabec 1. Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) based on small molecules as donor materials have attracted exten- sive attention due to their numerous advantages, which include well-defined molecular structure, easy purification, easy mass-scale production, high charge carrier mobility, and better batch-to-batch reproducibility. [1–4] Recently, the power conversion efficiency (PCE) of the solu- tion-processed small molecule (SM) OSCs has reached 7–8% via designing novel linear small molecules. [5,6] Most of small molecules as donor materials for pho- tovoltaic applications normally contains four key constituent components: donor/ acceptor (D/A) units, conjugated bridges, heteroatom substitutions, and side chains. The creative design and choice of donor or acceptor units, conjugated bridges, and heteroatom substitutions has been successfully conquered the shortcomings The impact of alkyl side-chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star-shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyano- vinyl units derivatized with various alkyl end-capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure-properties relationships. UV-vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)-lowest unoc- cupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influ- ences the photovoltaic properties of these small-molecule-based BHJ devices. Solution-processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current ( J sc ) and fill factor (FF), with an efficiency as high as 4.76% without any post-treat- ments; these are among the highest reported for solution-processed OSCs based on star-shaped molecules. DOI: 10.1002/aenm.201301234 J. Min, A. Gerl, T. Ameri, C. J. Brabec Institute of Materials for Electronics and Energy Technology (I-MEET) Friedrich-Alexander-University Erlangen-Nuremberg Martensstraße 7, 91058, Erlangen, Germany E-mail: Min.Jie@ww.uni-erlangen.de Y. N. Luponosov, M. S. Polinskaya, P. V. Dmitryakov, A. V. Bakirov, M. A. Shcherbina, S. N. Chvalun, S. A. Ponomarenko Nikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences Profsoyuznayast. 70, Moscow, 117393, Russia S. M. Peregudova Nesmeyanov Institute of Organoelement Compounds Russian Academy of Sciences Vavilova St. 28, Moscow, 119991, Russia S. Grigorian Institute of Physics University of Siegen Emmy-Noether-Campus Walter-Flex-Str. 3, D-57068, Siegen, Germany N. Kaush-Busies Heraeus Precious Metals GmbH & Co. KG Conductive Polymers Division (Clevios) Chempark Leverkusen Build. B202, D-51368, Leverkusen, Germany S. A. Ponomarenko Chemistry Department Moscow State University Leninskie Gory 1–3, Moscow, 119991, Russia C. J. Brabec Bavarian Center for Applied Energy Research (ZAE Bayern) Haberstraße 2a, 91058, Erlangen, Germany