Improving photovoltaic performance of the linear benzothienoindole-terminated molecules by tuning molecular framework and substituted position of terminals

- Five small molecules (SMs) with a D-A-D and a D′(A-D)2 framework were synthesized.
- Effect of the substituted position of benzo[4,5]thieno[2,3-b]indole (BTI)-terminal (BTI) were studied.
- A power conversion efficiency of 4.80% was obtained in BDT(DPP-8-BTI)2-based solar cells.

A novel type of the benzo[4,5]thieno[2,3-b]indole (BTI)-terminated small molecules (SMs) with a D′(A-D)2 framework were synthesized and characterized, i.e. BDT(DPP-9-BTI)2, BDT(DPP-8-BTI)2 and 2T(DPP-8-BTI)2, in which the central donor (D′) units are benzodithiene (BDT) and dithiophene (2T), the armed acceptor (A) unit is dithienyl-diketopyrrolopyrrole (DPP), and the terminated donor (D) unit is BTI with different substituted position. For comparison, the SMs with a D-A-D framework, DPP-9-2BTI and DPP-8-2BTI, were also synthesized. The photophysical, electro-chemical, thermal, and photovoltaic properties were preliminary investigated. It is found that the molecular framework and the substituted position of BTI have a significant influence on these properties. The D′(A-D)2 type SMs exhibited better photovoltaic per-formances than the D-A-D type SMs. Furthermore, the SMs suspending 8-sbstituted BTI displayed better photovoltaic performances than those corresponding SMs suspending 9-sbstituted BTI. The highest power conversion efficiency of 4.80% and a maximum short-circuit current density of 11.91 mA cm−2 were obtained in the solution-processed BDT(DPP-8-BTI)2-based solar cells using [6,6]-phenyl-C-71-butyric acid methyl ester as acceptor. It indicates that tuning the molecular framework and substituted position of the BTI terminals is an efficient strategy to improve photovoltaic performances for their small molecules.

A novel type of the benzo[4,5]thieno[2,3-b]indole (BTI)-terminated small molecules (SMs) with a D′(A-D)2 framework was synthesized and characterized. In compared to the SMs with a D-A-D framework, these D′(A-D)2 type SMs exhibited better photovoltaic performances. A significantly increasing power conversion efficiency of 4.80% was obtained for the SM with 8-sbstituted instead of 9-sbstituted BTI terminal in the PC71BM-based solar cells.117