Morphological and optical properties of α- and β-phase zinc (∥) phthalocyanine thin films for application to organic photovoltaic cells

  • Masahiro Kato
    Department of Energy Science and Engineering, Nagoya University 1 , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
  • Masato Nakaya
    Department of Energy Science and Engineering, Nagoya University 1 , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
  • Yuki Matoba
    Department of Energy Science and Engineering, Nagoya University 1 , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
  • Shinta Watanabe
    Department of Energy Science and Engineering, Nagoya University 1 , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
  • Koichi Okamoto
    Department of Physics and Electronics, Osaka Prefecture University 2 , Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
  • Jean-Pierre Bucher
    Université de Strasbourg and Institutde Physiqueet Chimiedes Matériaux de Strasbourg (IPCMS) 3 , UMR 7504, 67034 Strasbourg, France
  • Jun Onoe
    Department of Energy Science and Engineering, Nagoya University 1 , Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

抄録

<jats:p>We have investigated the morphological and optical properties of α- and β-phase Zinc Phthalocyanine (ZnPc) thin films for application to organic photovoltaic cells (OPVs). It was found that the α-phase is completely converted to the β-phase by thermal annealing at 220 °C under ultrahigh vacuum conditions. When the α- to β-phase transition takes place, the surface roughness of the ZnPc film became flat uniformly with a nanometer order of unevenness by anisotropic growth of crystalline grains along a lateral direction to substrates. Correspondingly, the optical absorbance of the β-phase film became greater by 1.5–2 times than that of the α-phase one in an ultraviolet–visible–near infrared (UV–vis–NIR) wavelength range, which plays a role in increasing the number of photogenerated excitons. On the contrary, time-resolved photoluminescence measurements showed that the average lifetime of excitons for the β-phase film became shorter by 1/6–1/7 than that for the α-phase one, which plays a role in decreasing the number of excitons achieving the donor/acceptor interface where excitons are separated to carriers (holes and electrons). Both the increase in the number and the shortening in the average lifetime have a trade-off relationship with each other for contribution to the photoelectric conversion efficiency of OPVs. Then, we examined an external quantum efficiency (EQE) of OPVs using the α- and β-phase films as a donor and obtained that the former OPV (α-phase) exhibits a higher EQE by ∼2 times than the latter one (β-phase) in the wavelength range of 400 nm–800 nm.</jats:p>

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