Effects of alkyl side chain and electron-withdrawing group on benzo[1,2,5]thiadiazole–thiophene-based small molecules in organic photovoltaic cells
- 497 Downloads
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 %.
KeywordsHigh Occupied Molecular Orbital Lower Unoccupied Molecular Orbital Power Conversion Efficiency Photovoltaic Property High Occupied Molecular Orbital Level
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. NRF-2015R1A1A3A04001498).
- 4.Lee J, Kim M, Kang B, Jo SB, Kim HG, Shin J, Cho K (2014) Side-chain engineering for fine-tuning of energy levels and nanoscale morphology in polymer solar cells. Adv Energy Mater 4:1400087Google Scholar
- 12.Lin Y, Ma L, Li Y, Liu Y, Zhu D, Zhan X (2014) Small-molecule solar cells with fill factors up to 0.75 via a layer-by-layer solution process. Adv Energy Mater 4:1300626Google Scholar
- 22.Love JA, Nagao I, Huang Y, Kuik M, Gupta V, Takacs CJ, Coughlin JE, Qi L, van der Poll TS, Kramer EJ, Heeger AJ, Nguyen TQ, Bazan GC (2014) Silaindacenodithiophene-based molecular donor: morphological features and use in the fabrication of compositionally tolerant, high-efficiency bulk heterojunction solar cells. J Am Chem Soc 136:3597–3606CrossRefGoogle Scholar