Enhancing Phase Separation and Photovoltaic Performance of All- Conjugated Donor−Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains Florian Lombeck, †,‡ Hartmut Komber, § Alessandro Sepe, ∥ Richard H. Friend, † and Michael Sommer* ,‡,⊥,# † Optoelectronics Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. ‡ Makromolekulare Chemie, Universitä t Freiburg, Stefan-Meier-Straße 31, 79104 Freiburg, Germany § Leibniz-Institut fü r Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany ∥ Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700, Fribourg, Switzerland ⊥ Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg, Germany # FIT, Freiburger Zentrum fü r interaktive Werkstoffe und bioinspirierte Technologien, Georges-Kö hler-Allee 105, 79110 Freiburg, Germany *S Supporting Information ABSTRACT: Phase separation of all-conjugated donor− acceptor block copolymers is more difficult to achieve compared to classical coil−coil systems owing the intrinsic similarity of the two blocks having both rigid conjugated backbones and alkyl side chains and their generally low degrees of polymerization. Here we demonstrate that side chain fluorination of a poly(carbazole-alt-dithienylbenzothia- diazole) segment (SF-PCDTBT), to be used as electron acceptor block in combination with poly(3-hexylthiophene) P3HT as donor block in all-conjugated donor−acceptor block copolymers of type SF-PCDTBT-b-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT leading to phase separation for already moderate molar masses. Key to the successful synthesis of a new TBT-monomer with semifluorinated side chains is a direct arylation step that elegantly bypasses classical cross-coupling reactions in which the semifluorinated side chain causes low yields. Suzuki polycondensation of the semifluorinated TBT monomer with a suitable carbazole comonomer and in situ termination by P3HT-Br is optimized extensively with respect to the yield of the end-capping efficiency and molar mass control of the PCDTBT segment. When the fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by n-hexyl chains (hex-PCDTBT), the tendency for phase separation with covalently connected P3HT is much reduced as shown by differential scanning calorimetry and grazing incidence small-angle scattering measurements on thin films. Favorably, of all the block copolymers made only SF-PCDTBT-b-P3HT is microphase separated, exhibits face-on orientation of P3HT domains, and additionally displays surface segregation of the SF- PCDTBT segment at the polymer/air interface. All of these properties are beneficial for single layer single component solar cells. SF-PCDTBT-b-P3HT exhibits the best solar cells performance with a high open-circuit voltage of 1.1 V and a power conversion efficiency of ∼1% which largely outperforms devices based on the analogous H-PCDTBT-b-P3HT and hex-PCDTBT-b-P3HT. ■ INTRODUCTION Conjugated polymers have attracted considerable attention within the last two decades due to their feasible integration into lightweight, flexible, and transparent electronic devices such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVs), owing to their good charge carrier mobilities, bright and tunable light emission, and broad absorption in the visible range of the electromagnetic spectrum. 1−5 A major advantage compared to inorganic semiconductors is the processability of organic semiconductors from solution enabling lower manufacturing costs and high throughput device fabrication via multiple printing techniques. To date, various p-type polymers are known while the majority of high performance OPV devices utilize soluble fullerene derivatives as n-type material in the active layer. 6 A notable andfor light harvesting devices important drawback of fullerenes is the weak light absorption in the visible and near IR regions. An alternative that bypasses the poor contribution of fullerenes to the photocurrent is the implementation of n-type polymers. Potential advantages Received: August 20, 2015 Revised: October 21, 2015 Published: October 29, 2015 Article pubs.acs.org/Macromolecules © 2015 American Chemical Society 7851 DOI: 10.1021/acs.macromol.5b01845 Macromolecules 2015, 48, 7851−7860