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. 2021 Apr 21;14(9):2107.
doi: 10.3390/ma14092107.

Selective Extraction of Nonfullerene Acceptors from Bulk-Heterojunction Layer in Organic Solar Cells for Detailed Analysis of Microstructure

Masahiro Nakano  1 , Akira Takahara  1 , Kenji Genda  1 , Md Shahiduzzaman  2 , Makoto Karakawa  1   2   3 , Tetsuya Taima  1   2   3 , Kohshin Takahashi  1
Affiliations

Selective Extraction of Nonfullerene Acceptors from Bulk-Heterojunction Layer in Organic Solar Cells for Detailed Analysis of Microstructure

Masahiro Nakano et al. Materials (Basel). .

Abstract

Detailed analyses of the microstructures of bulk-heterojunction (BHJ) layers are important for the development of high-performance photovoltaic organic solar cells (OSCs). However, analytical methods for BHJ layer microstructures are limited because BHJ films are composed of a complex mixture of donor and acceptor materials. In our previous study on the microstructure of a BHJ film composed of donor polymers and fullerene-based acceptors, we analyzed donor polymer-only films after selectively extracting fullerene-based acceptors from the film by atomic force microscopy (AFM). Not only was AFM suitable for a clear analysis of the morphology of the donor polymers in the BHJ film, but it also allowed us to approximate the acceptor morphology by analyzing the pores in the extracted films. Herein we report a method for the selective extraction of nonfullerene acceptors (NFAs) from a BHJ layer in OSCs and provide a detailed analysis of the remaining BHJ films based upon AFM. We found that butyl glycidyl ether is an effective solvent to extract NFAs from BHJ films without damaging the donor polymer films. By using the selective extraction method, the morphologies of NFA-free BHJ films fabricated under various conditions were studied in detail. The results may be useful for the optimization of BHJ film structures composed of NFAs and donor polymers.

Keywords: bulk-heterojunction; morphology; nonfullerene acceptor; organic solar cell.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Typical structure of OPV device with BHJ layer (a), illustration of selective extraction method and subsequent analysis of donor/acceptor blend films (b), and chemical structures of NFAs (c) and donor polymers (d).
Figure 2
Figure 2
Absorption spectra of thin films of PTB7 and EH-IDTBR (a) and spectra obtained after extraction of EH-IDTBR from blend films (before: purple, after: black) using various organic solvents (b): n-hexane, (c): DMF, (d): THF, (e): DCM, (f): Et2O, (g): BGE, (h): toluene, (i): pyridine.
Figure 3
Figure 3
XPS spectra of BHJ films before and after extraction and of pure films of PTB7 and EH-IDTBR.
Figure 4
Figure 4
AFM images of blend films (2 μm × 2 μm) before (a,b) and after (c), (d) BGE washing. PTB7/EH-IDTBR films (a), (c) and PTB7/ITIC films (b,d).
Figure 5
Figure 5
J-V characteristics of OSC devices composed of PTB7:EH-IDTBR.
Figure 6
Figure 6
AFM images of BHJ films composed of PTB7:EH-IDTBR after BGE washing (2 μm × 2 μm). D/(D + A) ratios of PTB7:EH-IDTBR are as follows: (a) 0.20, (b) 0.33, (c) 0.40, (d) 0.50, (e) 0.60, (f) 0.33 with 1 vol% nitrobenzene, and (g) 0.33 with 2.5 vol% nitrobenzene.
Figure 7
Figure 7
Dependence on D/(D + A) ratio (D: donor, A: acceptor) of grain/pore size (averaged values) and number of grains/pores (pieces μm−2) (a,b). Dependence on additive amount of grain/pore size and number of grains/pores (c,d).
See this image and copyright information in PMC

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