This journal is © The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 4099--4101 | 4099 Cite this: Chem. Commun., 2014, 50, 4099 Substantial photovoltaic response and morphology tuning in benzo[1,2-b:6,5-b 0 ]- dithiophene (bBDT) molecular donors† Tobias Harschneck, a Nanjia Zhou, a Eric F. Manley, ab Sylvia J. Lou, ab Xinge Yu, ab Melanie R. Butler, ab Amod Timalsina, ab Riccardo Turrisi, a Mark A. Ratner,* ab Lin X. Chen,* abc Robert P. H. Chang,* d Antonio Facchetti* abe and Tobin J. Marks* ab The influence of solubilizing substituents on the photovoltaic perfor- mance and thin-film blend morphology of new benzo[1,2-b:6,5-b 0 ]- dithiophene (bBDT) based small molecule donor semiconductors is investigated. Solar cells based on bBDT(TDPP) 2 -PC 71 BM with two different types of side chains exhibit high power conversion efficiencies, up to 5.53%. Small molecule semiconductors have attracted significant interest as electron donors for organic photovoltaic (OPV) cells, driven by several advantages vs. their polymeric counterparts, such as monodispersity, easier and more efficient purification, amenability to more characterization methods, and negligible batch-to-batch variations. 1,2 Furthermore, small molecule-based solar cells now exhibit power conversion efficiencies (PCEs) as high as 7.8%. 3–5 In this context, p-acceptor–donor–acceptor (A–D–A) molecular donors using diketopyrrolopyrrole (DPP) as the A unit and various thienoacenes such as benzo[1,2-b:4,5-b 0 ]dithiophene (BDT), 6,7 naphtho[2,3-b:6,7-b 0 ]dithiophene (NDT), 8 and naphtho- [1,2-b:5,6-b 0 ]dithiophene (zNDT) 9 as the D unit, have repeatedly proven successful (Fig. 1). However, to the best of our knowledge the benzo[1,2-b:6,5-b 0 ]dithiophene (bBDT) framework has never been used in molecular OPV donors, despite investigations in D–A OPV co-polymers 10–13 and organic field effect transistors (OFETs). 14,15 Nevertheless, DFT calculations on bBDT-centered molecules bearing two thiophene-capped DPP (TDPP) acceptor units reveal appealing frontier molecular orbital (FMO) energies and planar geometries (Fig. S1, ESI†), rendering the bBDT(TDPP) 2 (1) core a promising OPV donor candidate. Considering the importance of solubilizing side chains for the self-assembly and performance of OPV donors, 16 we investigated three bBDT(TDPP) 2 derivatives, with systematically varied substitution pattern on the bBDT core and on the TDPP units. We report here preliminary findings on the synthesis, charge transport, and OPV properties of these new molecules and their corresponding blends with fullerenes. The synthetic route to the donor materials 1a–c is summarized in the ESI † (Scheme S1). To characterize the optical properties of 1a–c, UV-Vis spectrometry was carried out on dilute CHCl 3 solutions as well as on thin films (Fig. 2A, Table S2, ESI†). The spectra of 1a–c are almost identical, suggesting that the solution electronic structure is essentially substituent-independent. On proceeding from the solution to the film state, the 1a–c optical spectra exhibit an additional shoulder at B670 nm while the principal optical onset undergoes a significant red-shift to B730 nm, corresponding to an optical band gap of B1.70 eV. Both effects can be ascribed to extensive molecular aggregation as well as enhanced core planarity in the solid state. 17 Cyclic voltammetry (CV) in CH 2 Cl 2 was used to estimate 1a–c FMO energies (Fig. 2B, Table S3, ESI†). CV plots Fig. 1 Acceptor (A)–donor (D)–acceptor (A) molecular donors based on thienoacenes and DPP. a Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA b Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208, USA c Chemical Science & Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA d Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA e Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc49620a Received 19th December 2013, Accepted 4th March 2014 DOI: 10.1039/c3cc49620a www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 04 March 2014. Downloaded by Northwestern University on 14/10/2014 00:06:44. View Article Online View Journal | View Issue