Improved photovoltaic performance of 2,7-pyrene based small molecules via the use of 3-carbazole as terminal unit

Small molecule of Py-2DTBTCz with an Ar(A–D)2 framework was designed and synthesized as photovoltaic donor materials, in which 2,7-pyrene (Py) and benzothiadiazole (BT) were respectively used as central aryl (Ar) and arm acceptor (A), while 3-carbazole (Cz) as terminal donor (D). For comparison, an A–Ar–A type SM of Py-2DTBT without Cz terminal unit was also prepared. The effects of Cz on optical, electrochemical, hole mobility and photovoltaic properties were investigated. In solution-processed solar cells, it was observed that the Py-2DTBTCz based devices show significant improvement in photovoltaic performance compared to those of Py-2DTBT. Power conversion efficiency (PCE) of Py-2DTBTCz could be as high as 2.89% with a short-circuit current density (Jsc) of 7.68 mAcm−2. The results indicate that appending an enlarged Cz π-system to the terminal of A–Ar–A-type small molecules for the construction of an Ar(A–D)2 framework can significantly improve the photovoltaic performance of SMs.