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Title: 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-oxide/ poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/poly (3-hexylthiophene):phenyl-C{sub 61}-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{sup −}Li{sup +} 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.
Authors:
; ;  [1] ; ;  [2] ;  [1] ;  [3] ;  [4]
  1. Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan)
  2. National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 (Japan)
  3. (JST), PRESTO, Kawaguchi, Saitama 322-0012 (Japan)
  4. (TIMS), University of Tsukuba, Tsukuba, Ibaraki 305-8571 (Japan)
Publication Date:
OSTI Identifier:
22300265
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 104; Journal Issue: 24; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ANIONS; ANNEALING; BUFFERS; DEPOSITION; DISSOCIATION; ELECTRODES; ELECTRON SPIN RESONANCE; ENERGY LEVELS; FULLERENES; INDIUM OXIDES; INTERFACES; LAYERS; LITHIUM FLUORIDES; MOLECULES; POLYMERS; SCATTERING; SEGREGATION; SOLAR CELLS; SURFACES; TIN ADDITIONS