New D-A1-D-A2-Type Regular Terpolymers Containing Benzothiadiazole and Benzotrithiophene Acceptor Units for Photovoltaic Application

Mukhamed L. Keshtov*, Alexei R. Khokhlov, Serge A. Kuklin, Fang-Chung Chen, Emmanuel N. Koukaras, Ganesh D. Sharma

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

Two novel regular terpolymers that are of D-A1-D-A2 type and contain benzothiadiazole and 2,5-dibromo-8-dodecanoylbenzo[1,2-b:3,4-b′:5,6-d″]trithiophene (P1) or 2,8-dibromo-5-dodecanoylbenzene[1,2-b:3,4-b′:5,6-d″]trithiophene (P2) acceptor units with the same thiophene donor were synthesized through Stille coupling, and their optical and electrochemical properties were investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels of these terpolymers indicate that there is sufficient LUMO offset with PCBM for efficient exciton dissociation, and their deeper HOMO levels ensure the high open-circuit voltage for the resultant bulk heterojunction solar cells. Measurements on the solar cell devices also confirm that compared to those based on P2 the devices based on P1 possess a higher short-circuit photocurrent (Jsc) as well as a higher fill factor (FF), which is attributed to the lower bandgap and higher hole mobility for P1, whereas the Voc is higher for the devices that are based on P2, which may be a result of P2 having a lower HOMO energy level than P1. The optimized polymer solar cells fabricated using P1:PC71BM (DIO/CF) and P2:PC71BM (CF/DIO) for the active layers showed a PCE of 7.19% and 6.34%, respectively. Atomic force microscopy (AFM) images of P1:PC71BM blend films show that they exhibit more suitable morphology with favorable interpenetrating networks, which favors high Jsc and FF. Moreover, P1 exhibits a more crystalline nature than P2 that also favors the charge transport. This may be a result of better molecular packing, more distinct phase separation of the blended films, as well as a reduction of charge recombination.

Original languageEnglish
Pages (from-to)32998-33009
Number of pages12
JournalACS Applied Materials and Interfaces
Volume8
Issue number48
DOIs
StatePublished - 7 Dec 2016

Keywords

  • bulk heterojunction
  • D-A1-D-A2 terpolymer
  • polymer solar cells
  • power conversion efficiency
  • solvent additive

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