A-D-A type conjugated oligomers based on benzothiadiazole and their photovoltaic applications

- Conjugated oligomers based on carbazole and benzothiadiazole.
- Thiophene ring as a π-extender.
- Photovoltaic properties of BHJ type based on oligomers and PC71BM.

To investigate the effect of combination between strong electron donor and strong electron acceptor on the diverse properties of conjugated small molecules based on carbazole (Cz) having 2,7-linkage as an electron donor and 2,1,3-benzothiadiazole (BT), 2,7-bis-[5-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-thiophen-2-yl]-9-(2-octyl-dodecyl)-9H-carbazole (Cz-BT) and 2,7-bis-[5′-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-[2,2′]bithiophenyl-5-yl]-9-(2-octyl-dodecyl)-9H-carbazole (CzT-BT), are synthesized by the Suzuki coupling reaction. In addition, we introduce thiophene (T) ring as the π-bridge to extend the effective conjugation length. Cz-BT and CzT-BT exhibit the band gap of 2.22 and 2.15 eV, respectively, due to intermolecular charge transfer between Cz and BT. As increasing the number of T rings, the band gap is lowered and the HOMO and LUMO energy is increased. Inverted-type organic solar cells (OSCs) with a configuration of ITO/ZnO/Cz-BT or CzT-BT:PC71BM/MoO3/Al are fabricated. Maximum power conversion efficiency (PCE) of the OSCs based on small molecules is exhibited up to 1.58%.

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