The effect of the central ligand on the surface photovoltage of phthalocyanine films

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Abstract
The photovoltage at light intensity saturation, and the relaxation time associated with this photovoltage have been measured for a variety of phthalocyanine (Pc) thin films and the results are: H2Pc(10.60 mV, 25.3 msec), NiPc(10.99 mV, 43.2 msec), CuPc(5.24 mV, 47.2 msec), and ZnPc(0.01 mV, 18.4 msec). No observable photovoltage signal was obtained for either FePc or CoPc. The spectral dependence of the photovoltage signals for these phthalocyanine dyes is related to their optical absorption spectra. Efforts have been made in the literature to connect the semiconducting properties of these organic dyes to the differing luminescent, magnetic, and optical properties of the various phthalocyanines. This work indicates that the size of the photovoltage signal is unrelated to any of these parameters. The size of the photovoltage is, however, closely correlated with the alignment of the energies of the d orbitals on the central metal ligand with the valence band molecular orbital of the outer phthalocyanine ring, as determined from photoemission measurements.