Bithiopheneimide−Dithienosilole/Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure−Property−Device Performance Correlations and Comparison to Thieno[3,4‑c]pyrrole- 4,6-dione Analogues Xugang Guo, †,▽ Nanjia Zhou, ‡,▽ Sylvia J. Lou, † Jonathan W. Hennek, † Rocío Ponce Ortiz, †,§ Melanie R. Butler, † Pierre-Luc T. Boudreault, † Joseph Strzalka, ∥ Pierre-Olivier Morin, ¶ Mario Leclerc, ¶ Juan T. Ló pez Navarrete, § Mark A. Ratner,* ,† Lin X. Chen,* ,† Robert P. H. Chang,* ,‡ Antonio Facchetti,* ,†,‡,⊥ and Tobin J. Marks* ,†,‡ † Department of Chemistry and the Materials Research Center, the Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Department of Materials Science and Engineering and the Materials Research Center, the Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Physical Chemistry, University of Ma ́ laga, Campus de Teatinos s/n, Ma ́ laga 29071, Spain ∥ X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ¶ De ́ partement de Chimie, Universite ́ Laval, Quebec City, Quebec G1V 0A6, Canada ⊥ Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States * S Supporting Information ABSTRACT: Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor−acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOsessential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R 2 Si→R 2 Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π−π spacings vs linear N- alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure−property−device performance correlations are provided by a thieno[3,4- c]pyrrole-4,6-dione (TPD)−dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD- based polymers achieve greater subunit−subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π−π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC 71 BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V oc > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V oc with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure−property relationships in TPD, BTI, and related polymer semiconductors. ■ INTRODUCTION Polymer solar cells (PSCs) have recently received great attention as renewable energy sources because of their compatibility with fabricating large-area, flexible, and cost-effective devices via roll- to-roll (R2R) processing techniques. 1,2 Moreover, solar cell power conversion efficiencies (PCEs) have increased substan- tially over the past few years as a consequence of improved active layer materials, 3,4 film morphology optimization, 5 interface engineering, 6,7 and improved device architectures. 8−10 Impres- sive PCEs surpassing 8% 6c,9b,10,11 have been achieved with PSC active regions configured in a bulk-heterojunction (BHJ) network of intermingled π-conjugated polymer electron donors and high electron-affinity fullerene electron acceptors. 12−15 Received: August 16, 2012 Published: October 3, 2012 Article pubs.acs.org/JACS © 2012 American Chemical Society 18427 dx.doi.org/10.1021/ja3081583 | J. Am. Chem. Soc. 2012, 134, 18427−18439