1338 Phys. Chem. Chem. Phys., 2011, 13, 1338–1344 This journal is c the Owner Societies 2011 Synthesis, characterization and photovoltaic properties of poly(thiophenevinylene-alt-benzobisoxazole)swz Jared F. Mike, a Kanwar Nalwa, b Andrew J. Makowski, a Daniel Putnam, b Aime´e L. Tomlinson, c Sumit Chaudhary b and Malika Jeffries-EL* a Received 26th April 2010, Accepted 21st October 2010 DOI: 10.1039/c0cp00353k Herein we report the synthesis of two solution processible, conjugated polymers containing the benzobisoxazole moiety. The polymers were characterized using 1 H NMR, UV-Vis and fluorescence spectroscopy. Thermal gravimetric analysis shows that the polymers do not exhibit significant weight loss until approximately 300 1C under nitrogen. Cyclic voltammetry shows that the polymers have reversible reduction waves with estimated LUMO levels at 3.02 and 3.10 eV relative to vacuum and optical bandgaps of 2.04–2.17 eV. Devices based on blends of the copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) exhibited modest power conversion efficiencies. Theoretical models reveal that there is poor electron delocalization along the polymer backbone, leading to poor performance. However, the energy levels of these polymers indicate that the incorporation of benzobisoxazoles into the polymer backbone is a promising strategy for the synthesis of new materials. Introduction The direct conversion of sunlight into energy using photo- voltaic cells (PVC)s has been recognized as an essential component of future global energy production. As a result of their optical and electronic properties, conjugated organic materials are sought after to replace inorganic materials in PVCs. This is due to the many attractive features of organic materials, such as the ability to tune their electron properties for specific applications through chemical synthesis and the simplicity of processing using solution-based techniques. 1,2 One current challenge in the field is the development of conjugated polymers with high electron affinity and/or low band gaps for use in bulk heterojunction PVCs. 3 High electron affinity can reduce the energy loss during the transfer of electrons from the donor to [6,6]-phenyl C61 butyric acid methyl ester (PCBM), a widely used electron acceptor, increasing the output power of the PVC. 4–6 Similarly, low band gap allows for the absorption of photons at longer wavelengths, increasing the percentage of solar energy that can be harvested. 7,8 Currently, a popular design strategy for the manipulation of the energy levels of conjugated polymers is the synthesis of so-called D–A donating (D) and electron-accepting (A) moieties. 9 In these polymers, which are composed of alter- nating electron donating and electron accepting moieties, the hybridization of the LUMO from the accepting moiety and the HOMO from the donor moiety can be used to reduce the polymers band gap and/or vary its energy levels. 9–11 Poly(3-alkylthiophene)s are widely studied due to their excellent thermal and environmental stability, high hole mobility, and solution processibility. 12,13 Accordingly, electron- rich alkylthiophenes have been widely used as donor moieties in D–A polymer architectures. 11,14–16 Fully conjugated rigid-rod polybenzobisoxazoles (PBBO)s are multifunctional materials widely known for their excellent tensile strength, thermal stability, 17,18 efficient electron transport, 19,20 photoluminescence, 21–27 and high electron affinity. 21,28–31 Thus the incorporation of the benzobisoxazole (BBO) moiety into D–A polymer architectures is beneficial due to its high electron affinity. 16 Despite these advantageous properties, the use of PBBOs has been limited, largely due to their poor solubility, which requires PBBOs to be processed from acidic solutions. Furthermore, the harsh reaction conditions for the synthesis of PBBOs prevent their derivatization. 18,31–36 To realize the untapped potential of the BBO moiety for the development of novel conjugated polymers, we recently developed an alternative approach toward BBO synthesis using mild conditions. 37 As a result, we can now synthesize soluble PBBO by copolymerizing them with aryl monomers bearing flexible side-chains. 37,38 Herein we report the synthesis of two new polymers, namely poly[(3,4-didodecylthiopene vinylene)-alt-benzo[1,2-d;5,4-d 0 ]- bisoxazole]-2,6-diyl (PTVcBBO) and poly[(3,4-didodecylthiopene vinylene)-alt-benzo[1,2-d;4,5-d 0 ]bisoxazole]-2,6-diyl (PTVtBBO). The unique combination of the BBO, thiophene, and vinylene moieties greatly enhances the properties of the resultant poly- mer by: (1) incorporating vinylene linkages to minimize steric interactions between consecutive aromatic rings, reducing the band gap further; 39–41 (2) increasing rotational freedom of the polymer backbone, improving the polymer’s solubility; and (3) adding alkyl side chains along the polymer backbone to increase the solubility significantly. a Department of Chemistry, Iowa State University, Ames, IA 50011, USA. E-mail: malikaj@iastate.edu; Fax: +1 515-294-0105 b Department of Electrical Engineering, Iowa State University, Ames, IA 50011, USA c Department of Chemistry, North Georgia College & State University, GA 30597, USA w Electronic supplementary information (ESI) available: AFM images, Raman spectra, current/voltage characteristics and TGA curves. See DOI: 10.1039/c0cp00353k z The band diagram that is shown in the graphical abstract uses HOMO and LUMO values for PCBM from ref. 4. PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by Iowa State University on 20 December 2011 Published on 16 November 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00353K View Online / Journal Homepage / Table of Contents for this issue