Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance

Lanqi Huang, Guangjun Zhang, Kai Zhang, Qiang Peng*, Ricky M S WONG

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)

Abstract

A new series of low band gap D-A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB-n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB-n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350-800 nm) of PfBB-n polymers can compensate for the relatively weak absorption of PC71BM, particularly in the 300-400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB-n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8-9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density (Jsc) of 16.6 mA cm-2 and open-circuit voltage (Voc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB-n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1-b:3,4-b′]dithiophene building block, BDT, is highly useful for the construction of low band gap D-A polymer for highly efficient PSCs.

Original languageEnglish
Pages (from-to)34355-34362
Number of pages8
JournalACS applied materials & interfaces
Volume10
Issue number40
DOIs
Publication statusPublished - 10 Oct 2018

Scopus Subject Areas

  • Materials Science(all)

User-Defined Keywords

  • aggregation
  • low band gap
  • organic solar cells
  • polymer donor
  • side-chain effect

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