Journal of Materials Science

, Volume 51, Issue 14, pp 6770–6780 | Cite as

Effects of alkyl side chain and electron-withdrawing group on benzo[1,2,5]thiadiazole–thiophene-based small molecules in organic photovoltaic cells

  • Ara Cho
  • Chang Eun Song
  • Sang Kyu Lee
  • Won Suk Shin
  • Eunhee Lim
Original Paper


A series of D–A–D type small molecules containing thiophene and benzo[1,2,5]thiadiazole (BT) as electron-donating and electron-accepting units, respectively, were synthesized using palladium-catalyzed Suzuki coupling reactions. The electron-accepting units of BT were modified with pyridyl nitrogen, a single fluorine atom, or two fluorine atoms to enhance their electron-withdrawing abilities, which resulted in DH5TPys, F-DH5TBs, or 2F-DH5TBs, respectively. The solubilizing hexyl groups were introduced at two different β-positions (3- and 4-positions) on both ends of the thiophene rings. The optical, electrochemical, and photovoltaic properties of the small molecules varied, depending on the introduction of the electron-withdrawing substituents as well as the alkyl variation. The highest occupied molecular orbital (HOMO) energy levels of the small molecules were decreased by the increasing number of fluorine substituents; 4-substitution resulted in deeper HOMO energy levels than 3-substitution. 2F-DH5TB-4 had the lowest HOMO energy level of −5.49 eV. In the organic photovoltaic cells with the configuration ITO/PEDOT:PSS/small molecule:PC71BM/LiF/Al, 2F-DH5TB-4 showed the highest power conversion efficiency (PCE) of 1.11 % owing to the lowest HOMO levels and thus increased the open-circuit voltage. However, two DH5TPys showed low PCEs of 0.20 % despite the deep HOMO energy levels and good UV absorption because of the protonation of pyridyl nitrogen with acidic PEDOT:PSS. In the inverted structure of ITO/ZnO/small molecule:PC71BM/MoO3/Ag, both DH5TPys showed improved PCE values of up to 1.16 %. The device performance of 2F-DH5TB-4 also improved in the inverted structure, showing the PCE of 1.27 %.


High Occupied Molecular Orbital Lower Unoccupied Molecular Orbital Power Conversion Efficiency Photovoltaic Property High Occupied Molecular Orbital Level 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. NRF-2015R1A1A3A04001498).


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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  • Ara Cho
    • 1
  • Chang Eun Song
    • 2
  • Sang Kyu Lee
    • 2
  • Won Suk Shin
    • 2
  • Eunhee Lim
    • 1
  1. 1.Department of ChemistryKyonggi UniversitySuwon-SiRepublic of Korea
  2. 2.Korea Research Institute of Chemical Technology (KRICT)DaejeonRepublic of Korea

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