Two π-conjugated acceptor-donor-acceptor-donor-acceptor-type (A-D-A-D-A) oligothiophenes, TT-(2T-DCV-Hex)2 and BT-(2T-DCV-Hex)2 were designed and synthesized with thienothiadiazole (TT) or benzothiadiazole (BT) as the core and dicyanovinyl (DCV) as the terminal acceptor groups for comprehensively investigating and understanding structure-property relationships. The resulting oligomers were first characterized by thermal analysis, UV-Vis spectroscopy, and cyclic voltammetry. By simply changing the BT to TT core in these two oligothiophenes, the highest occupied molecular orbital levels were varied from-5.55 eV for BT-(2T-DCV-Hex)2 to-5.11 eV for TT-(2T-DCV-Hex)2, and the optical band gaps were varied from 1.72 eV for BT-(2T-DCV-Hex)2 to 1.25 eV for TT-(2T-DCV-Hex)2, ascribed to the stronger electron accepting character of the TT core. However, the power conversion efficiency of bulk heterojunction organic solar cells (OSCs) with TT-(2T-DCV-Hex)2 as donor and [6,6]-phenyl C70-butyric acid methyl ester (PC71BM) as acceptor was measured to be 0.04% only, which is much lower than that of BT-(2T-DCV-Hex)2:PC71BM (1.54%). Compared to the TT-(2T-DCV-Hex)2 system, the BT-(2T-DCV-Hex)2 based device shows smoother film surface morphology, and superior charge generation and charge carrier mobilities. Therefore, the results clearly demonstrate that in addition to modifying the alkyl side chains and p-bridge lengths, the design of new small molecules for high-performance OSCs should also aim to choose suitable acceptor units.
Bibliographical notePublisher Copyright:
© 2015 Society of Photo-Optical Instrumentation Engineers.
- Donor-acceptor oligomer
- charge carrier mobility
- charge generation
- dicyanovinyl group
- organic solar cell
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Renewable Energy, Sustainability and the Environment