Dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing: Direct observation by electron spin resonance

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-oxid...

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Veröffentlicht in:Applied physics letters 2014-06, Vol.104 (24)
Hauptverfasser: Liu, Dong, Nagamori, Tatsuya, Yabusaki, Masaki, Yasuda, Takeshi, Han, Liyuan, Marumoto, Kazuhiro
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Sprache:eng
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Zusammenfassung: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.
ISSN:0003-6951
1077-3118
DOI:10.1063/1.4883858