Novel naphtho[1,2-b:5,6-b′]dithiophene core linear donor-π-acceptor conjugated small molecules with thiophene-bridged bithiazole acceptor: design, synthesis, and their application in bulk heterojunction organic solar cellsElectronic supplementary information (ESI) available: Figures showing the NMR spectra, TGA curve, DSC thermogram, PL spectra of small molecules. See DOI: 10.1039/c2jm30934c

This study involves the development of new solution processable organic small molecules for photovoltaic applications. We have rationally designed and synthesized two novel, symmetrical and linear D-A-D-A-D-type π-conjugated organic small molecules bearing a rigidly fused naphtho[1,2- b :5,6- b ′]dithiophene core flanked by bithiazole ( M3 ) or triphenylamine-capped thiophene(3-decanyl)-bridged bithiazole ( M4 ) conjugated moieties through thiophene(3-decanyl) spacer. The resultant small molecules have been characterized by thermal analysis, UV-Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. Their applications in field effect transistors and solution processed bulk-heterojunction (BHJ) organic solar cells (OSCs) have also been explored. Due to the presence of an adequate number of 3-decanylthiophene moieties as short π-bridging units into the conjugated molecular backbone, both the small molecules have good solubility in common organic solvents and form highly ordered self-assembled π-π stacks in their solid states with long decyl chains organized by interdigitation. Additionally, they exhibit good thermal stability with decomposition temperatures exceeding 380 °C. Photophysical and electrochemical studies reveal that these molecular donors have comparable optical band gaps (∼1.99 to 2.02 eV) and nearly similar HOMO-LUMO energy levels, both of which are aligned with the PC 61 BM/PC 71 BM electron acceptors. The preliminary BHJ photovoltaic cells configured with the device structures of ITO/PEDOT:PSS/small molecule:PC 71 BM/Lif/Al were evaluated. The small molecule M3 was found to deliver the best power conversion efficiency of 1.09% when processing the active layer from chloroform solvent. In contrast, under identical device conditions M4 gave improved performance with a maximum efficiency of 1.62%. The morphological studies using atomic force microscopy showed that the PCE enhancement for M4 is mainly due to improvement in the nanoscale film morphology of the M4 -PC 71 BM blend. The synthesis and photovoltaic performances of new naphtho[1,2- b :5,6- b ′]dithiophene-containing donor-π-acceptor conjugated small molecules are described.