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

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 condi...

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Veröffentlicht in:The Journal of chemical physics 2020-10, Vol.153 (14), p.144704-144704
Hauptverfasser: Kato, Masahiro, Nakaya, Masato, Matoba, Yuki, Watanabe, Shinta, Okamoto, Koichi, Bucher, Jean-Pierre, Onoe, Jun
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Sprache:eng
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Zusammenfassung: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.
ISSN:0021-9606
1089-7690
DOI:10.1063/5.0022262