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

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. (C) 2017 Elsevier Ltd. All rights reserved.