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Frontiers of Chemical Science and Engineering

ISSN 2095-0179

ISSN 2095-0187(Online)

CN 11-5981/TQ

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2018 Impact Factor: 2.809

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Front. Chem. Sci. Eng.    2021, Vol. 15 Issue (1) : 127-137    https://doi.org/10.1007/s11705-020-1936-7
RESEARCH ARTICLE
Effects of BTA2 as the third component on the charge carrier generation and recombination behavior of PTB7:PC71BM photovoltaic system
Leijing Liu1, Hao Zhang1, Bo Xiao2, Yang Liu1, Bin Xu1, Chen Wang3, Shanpeng Wen3, Erjun Zhou2, Gang Chen4, Chan Im5, Wenjing Tian1()
1. State Key Laboratory of Supramolecular Structures and Materials, Jilin University, Changchun 130012, China
2. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
3. State Key Laboratory on Integrated Optoelectronics and College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
4. Key Laboratory of Physics and Technology for Advanced Batteries, Ministry of Education, Jilin University, Changchun 130012, China
5. Department of Chemistry, Konkuk University, Seoul 05029, Republic of Korea
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Abstract

Effects of a benzotriazole (BTA)-based small molecule, BTA2, as the third component on the charge carrier generation and recombination behavior of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) organic solar cells (OSCs) were investigated by optical simulation of a transfer matrix model (TMM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV) technique, atomic force microscope (AFM), and the Onsager–Braun model analysis. BTA2 is an A2-A1-D-A1-A2-type non-fullerene small molecule with thiazolidine-2,4-dione, BTA, and indacenodithiophene as the terminal acceptor (A2), bridge acceptor (A1), and central donor (D), respectively. The short-circuit current density of the OSCs with BTA2 can be enhanced significantly owing to a complementary absorption spectrum. The optical simulation of TMM shows that the ternary OSCs exhibit higher internal absorption than the traditional binary OSCs without BTA2, resulting in more photogenerated excitons in the ternary OSCs. The photo-CELIV investigation indicates that the ternary OSCs suffer higher charge trap-limited bimolecular recombination than the binary OSCs. AFM images show that BTA2 aggravates the phase separation between the donor and the acceptor, which is disadvantageous to charge carrier transport. The Onsager-Braun model analysis confirms that despite the charge collection efficiency of the ternary OSCs being lower than that of the binary OSCs, the optimized photon absorption and exciton generation processes of the ternary OSCs achieve an increase in photogenerated current and thus improve power conversion efficiency.
Keywords third component      organic solar cells      charge carrier generation      charge carrier recombination      bimolecular recombination     
Corresponding Author(s): Wenjing Tian   
Just Accepted Date: 09 May 2020   Online First Date: 30 June 2020    Issue Date: 12 January 2021
 Cite this article:   
Leijing Liu,Hao Zhang,Bo Xiao, et al. Effects of BTA2 as the third component on the charge carrier generation and recombination behavior of PTB7:PC71BM photovoltaic system[J]. Front. Chem. Sci. Eng., 2021, 15(1): 127-137.
 URL:  
https://academic.hep.com.cn/fcse/EN/10.1007/s11705-020-1936-7
https://academic.hep.com.cn/fcse/EN/Y2021/V15/I1/127
Fig.1  (a) Molecular structures of the third component (BTA2), PTB7, and PC71BM; (b) energy band diagram to represent the energy levels.
Fig.2  Spectral irradiance of the air mass 1.5 under different wavelengths and the absorption spectra of PTB7, PC71BM, and the third component, i.e., BTA2, in films prepared by spin-coating of the corresponding chlorobenzene solutions.
Fig.3  (a) J-V characteristics of PTB7:PC71BM OSCs with different mass concentration of BTA2; (b) J-V characteristics of the optimized ternary OSCs and the traditional binary OSCs without BTA2.
Devices Voc/V Jsc/(mA?cm?2) FF/% me/(cm2?V?1?s?1) mh/(cm2?V?1?s?1) PCE/%
Without BTA2 0.73 15.3 65.25 4.35×10?4 1.83×10?4 7.2
With 10% BTA2 0.74 16.7 64.14 2.63×10?4 9.8×10?5 8.0
Tab.1  Device performance of the traditional binary OSCs and the optimized ternary OSCs with 10% mass concentration of BTA2
Fig.4  Comparitive analysis between the optimized ternary OSCs with 10% mass concentration of BTA2 and the traditional binary OSCs without BTA2 corresponding to (a) incident photon-to-current efficiency spectra and (b) absorption spectra.
Fig.5  Q, EQE, and IQE spectra of (a) the traditional binary OSCs without BTA2 and (b) the optimized ternary OSCs with 10% mass concentration of BTA2; (c) Q and (d) IQE calculated and analyzed by the transfer matrix optical model.
Fig.6  Transients of photo-induced charge extraction by linearly increasing voltage technique versus delay time of PTB7:PC71BM solar cells (a) without the third component BTA2 and (b) with 10% BTA2; (c) Time-dependent charge carrier density of PTB7:PC71BM solar cells with and without the third component of 10% BTA2; here, the solid line is the fitting curve; (d) Trap-limited bimolecular recombination rate versus delay time of PTB7:PC71BM solar cells with and without the third component of 10% BTA2.
Devices n0/cm?3 t/ms a gbi/(cm3?s?1)
Without BTA2
With 10% BTA2		 8.22×1016
9.34×1016		 9.14
7.79		 0.897
0.821		 5.94×10?13
9.18×10?13
Tab.2  Parameters of the OSCs with and without the third component of 10% BTA2 from the fitting in Fig. 6(c)
Fig.7  Atomic force microscope phase (a1 and b1) and topography (a2 and b2) images of the active layer of PTB7:PC71BM with (a1) (a2) and without (b1) (b2) the third component of 10% BTA2.
Fig.8  Space-charge-limited current measurements for the (a) electron-only and (b) hole-only devices with and without the third component of 10% BTA2.
Fig.9  Experimental and fitted effective photocurrent density (Jph = Jillumination ? JDark) versus the effective applied voltage (V– V0) in the OSCs with the third component of 10% BTA2 and without BTA2.
Devices gbi/(cm3?s?1) G/(1025 m?3) hcc
With 10% BTA2 9.18×10?13 1.25 85%
Without BTA2 5.94×10?13 1.03 91%
Tab.3  Parameters of the OSCs with the third component of 10% BTA2 and without BTA2 from the fitting with the Onsager–Braun model
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