3,6-Dialkylthieno[3,2-b]thiophene Moiety as a Soluble and Electron Donating Unit Preserving the Coplanarity of Photovoltaic Low Band Gap Copolymers Laure Biniek, 1,2 Christos L. Chochos, 1 Nicolas Leclerc, 1 Olivier Boyron, 3 Sadiara Fall, 2 Patrick L eve ` que, 2 Thomas Heiser 2 1 Laboratoire d’Ing enierie des Polyme ` res pour les Hautes Technologies, Universit e de Strasbourg, Ecole Europ eenne de Chimie, Polyme ` res et Mat eriaux, 25 rue Becquerel, 67087 Strasbourg, France 2 Institut d’Electronique du Solide et des Syste ` mes, Universit e de Strasbourg-CNRS, 23 rue du Loess, 67037 Strasbourg, France 3 Universit e de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polyme ` res et Proc ed es (C2P2), Bat 308F, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France Correspondence to: N. Leclerc (E-mail: leclercn@unistra.fr) Received 30 November 2011; accepted 18 January 2012; published online 10 February 2012 DOI: 10.1002/pola.25961 ABSTRACT: It has been shown recently, that the presence of alkyl side chains at the 3-positions on the thiophene rings placed next to 2,1,3-benzothiadiazole core in the backbone of several conju- gated polymers results in severe steric hindrance and prevents efficient planarity of the thiophene-2,1,3-benzothiadiazole-thio- phene (TBzT) segment. Both properties have a strong influence on the optoelectronic properties of the polymer and need to be considered when the polymer is to be used for organic electronics applications. In this work, we modified a previously synthesized oligothiophene copolymer, consisting of two 3,4 0 -dialkyl-2,2 0 - bithiophene units attached to a 2,1,3-benzothiadiazole unit (TBzT segment) and a thieno[3,2-b]thiophene unit, by optimizing the lat- eral alkyl side chains following a density functional theory inves- tigation. It is demonstrated that eliminating the alkyl side chains from the 3-positions of the TBzT segment and anchoring them onto the thieno[3,2-b]thiophene, using an efficient synthesis of the 3,6-dihexylthieno[3,2-b]thiophene unit, allows us to reduce the energy band gap. In addition, the chemical modification leads to a better charge transport and to an enhanced photovol- taic efficiency of polymer/fullerene blends. V C 2012 Wiley Periodi- cals, Inc. J Polym Sci Part A: Polym Chem 50: 1861–1868, 2012 KEYWORDS: bulk heterojunction; calculations; charge transport and conjugated polymers; conjugated polymers; 3,6-dihexylth- ieno[3,2-b]thiophene; density functional theory; low band gap copolymers; polymer solar cells; side chains INTRODUCTION In recent years, the development of novel photoactive materials for bulk donor (D)-acceptor (A) het- erojunction (BHJ) solar cells has allowed a significant improvement in the device power conversion efficiency (PCE). 1–8 Major efforts have been put into the design of new conjugated polymers that are used as electron donor when blended with a soluble fullerene derivative (the elec- tron acceptor, for instance [6,6]-phenyl C 61 butyric acid methyl ester or PCBM). Previous investigations have allowed identifying crucial polymer properties that limit the short circuit current density (J sc ) and open circuit volt- age (V oc ), and therefore the PCE. These properties include the charge carrier mobility 9,10 and the frontier orbital energy bands. Also, the electronic coupling between the polymer and the fullerene has a strong impact on the de- vice properties. In particular, the formation of a charge- transfer complex at the D/A interface has been observed in numerous polymer/fullerene blends. Its energy level scales with the difference between the polymer HOMO level and the fullerene LUMO level and has been shown to determine the V oc . 11 A fine adjustment of the polymer ionization potential and electron affinity is therefore necessary to pro- vide an optimal absorption band gap, an efficient exciton dissociation, and a maximized open circuit voltage. Syn- thetic strategies frequently used to tune the frontier energy levels of conjugated polymers are either the enhancement of the quinoid character of a polyaromatic conjugated chain 12–14 or the synthesis of donor-acceptor ‘‘D-A’’ copoly- mers containing alternating electron-rich and electron-defi- cient building blocks. 15 However, the understanding in how the molecular building blocks and more specifically differ- ent solubilizing side chains and positions along the conju- gated backbone, influence the HOMO and LUMO energy lev- els is still limited. 16–18 Also, the link between the polymer molecular structure and its solid-state self-organization in the presence of PCBM is still unclear. V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 1861–1868 1861 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE