Using electron spin resonance (ESR), we clarified the origin of the efficiency degradation of polymer solar cells containing a lithium-fluoride (LiF) buffer layer created by a thermal annealing process after the deposition of an Al electrode (post-annealing). The device structure was indium-tin-oxide/ poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/LiF/Al. Three samples consisting of quartz/P3HT:PCBM/LiF/Al, quartz/P3HT:PCBM/Al, and quartz/PCBM/LiF/Al were investigated and compared. A clear ESR signal from radical anions on the PCBM was observed after LiF/Al was deposited onto a P3HT:PCBM layer because of charge transfer at the interface between the PCBM and the LiF/Al, which indicated the formation of PCBM−Li+ complexes. The number of radical anions on the PCBM was enhanced remarkably by the post-annealing process; this enhancement was caused by the surface segregation of PCBM and by the dissociation of LiF at the Al interface by the post-annealing process. The formation of a greater number of anions enhanced the electron scattering, decreased the electron-transport properties of the PCBM molecules, and caused an energy-level shift at the interface. These effects led to degradation in the device performance.
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16 June 2014
Research Article|
June 17 2014
Dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing: Direct observation by electron spin resonance
Dong Liu;
Dong Liu
1Division of Materials Science,
University of Tsukuba
, Tsukuba, Ibaraki 305-8573, Japan
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Tatsuya Nagamori;
Tatsuya Nagamori
1Division of Materials Science,
University of Tsukuba
, Tsukuba, Ibaraki 305-8573, Japan
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Masaki Yabusaki;
Masaki Yabusaki
1Division of Materials Science,
University of Tsukuba
, Tsukuba, Ibaraki 305-8573, Japan
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Takeshi Yasuda;
Takeshi Yasuda
2
National Institute for Materials Science
, Tsukuba, Ibaraki 305-0047, Japan
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Liyuan Han;
Liyuan Han
2
National Institute for Materials Science
, Tsukuba, Ibaraki 305-0047, Japan
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Kazuhiro Marumoto
Kazuhiro Marumoto
a)
1Division of Materials Science,
University of Tsukuba
, Tsukuba, Ibaraki 305-8573, Japan
3Japan Science and Technology Agency (JST), PRESTO, Kawaguchi, Saitama 322-0012,
Japan
4
Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba
, Tsukuba, Ibaraki 305-8571, Japan
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a)
Electronic mail: marumoto@ims.tsukuba.ac.jp
Appl. Phys. Lett. 104, 243903 (2014)
Article history
Received:
January 28 2014
Accepted:
June 02 2014
Citation
Dong Liu, Tatsuya Nagamori, Masaki Yabusaki, Takeshi Yasuda, Liyuan Han, Kazuhiro Marumoto; Dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing: Direct observation by electron spin resonance. Appl. Phys. Lett. 16 June 2014; 104 (24): 243903. https://doi.org/10.1063/1.4883858
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