The Institute for Molecular Science, National Institute of Natural Sciences announced on March 3, 2011 that a research group led by Professor Masahiro Hiramoto has succeeded in converting conduction-type of fullerene from n- to p-type by molybdenum oxide (MoO3) doping. Details are published online in Applied Physics Letters on February 28, 2011*.
Although organic thin-film solar cells are promising devices because of the advantages of being light weight, flexible and low cost, the conduction-type of organic semiconductors has not been controlled by doping impurities as is done in silicon. Two species of organic semiconductors, n-type fullerene (C60) and p-type phthalocyanine (Pc), need to be used to form built-in fields in solar cells.
Researchers noticed that MoO3 is used to raise holes in organic electroluminescent materials. They have succeeded in converting conduction-type of C60 from n- to p-type by co-evaporation of MoO3 and C60. Energetic value of the Fermi level, 4.60eV, for nondoped C60 films measured by the Kelvin vibrating capacitor method was positively shifted to 5.88 eV by the co-evaporated doping of MoO3 at a concentration of 3300 ppm and approached the valence band of located at 6.4 eV. The upward bending of energy band in the Schottky junction formed at the interface between a metal (silver, Ag) and p-type C60 film formed by MoO3 doping was confirmed based on the photovoltaic properties. Organic solar cells could be fabricated by a single material - fullerene C60.
*Masayuki Kubo, Kai Iketaki, Toshihiko Kaji, and Masahiro Hiramoto, "Conduction-type control of fullerene films from n- to p-type by molybdenum oxide doping", Applied Physics Letters Vol.98, No. 7, p. 073311 (2011); doi:10.1063/1.3556312 (3 pages); published online 18 February 2011.
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