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 cells

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 PC₆₁BM/PC₇₁BM electron acceptors. The preliminary BHJ photovoltaic cells configured with the device structures of ITO/PEDOT:PSS/small molecule:PC₇₁BM/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–PC₇₁BM blend.