2B03
The Photoconductivity of Cobalt Phthalocyanine Thin Films(Kyoto Univ., Inst. Chem. Res.)

Richard Murdey and Naoki SatoIntroduction
The phthalocyanine ligand can complex with a wide variety of central metal ions.
Many of the open shell metal-phthalocyanine complexes, such as cobalt phthalocyanine(CoPc), exhibit interesting spin properties. As far as the solid state photo-electrical propertiesof the first row transition metal phthalocyanine complexes are concerned the electron
configuration of the central metal ion does not generally appear to have a much influence.
There is rarely any mixing between the current-carrying electronic states and the atomic
orbitals of the central metal. The metal ion may, however, indirectly alter the electricalproperties of the material via steric effects which change the packing structure, and hence theintermolecular orbital overlaps. CoPc may therefore be considered a “generic” phthalocyanine
complex, and we examine the photoconductivity in the context of exploring the typicalbehavior of this class of organic semiconductor.
Our aim is to address one of the fundamental difficulties is discussing electricalmeasurements on organic thin films: the difficulty in assigning the observed properties tointrinsic or extrinsic effects. Organic films have amorphous and/or polycrystalline structure,with a high defect density and a large number of grain boundaries. The chemical purity is also
relatively low. Extrinsic effects (acceptor, donor, and trap sites) would normally be expected todominate the measured electrical properties, so the challenge is instead to observe andidentify intrinsic (trap free) behavior. We do this by carefully purifying the starting materials
and by using in situ measurement techniques under an ultrahigh vacuum environment.
Experimental
CoPc (Aldrich) was sublimed three times under 30 Pa N2 gas flow before use. Single
crystal sapphire [0001] substrates (Shinkosha) were first annealed at 1000 °C in air to expose
atomically flat terraces, over which 30 nm titanium electrodes spaced 0.1 mm apart werevacuum deposition through a metal mask. Once placed inside the measurement chamber, thesubstrates were degassed at 250 °C under UHV before use. Leakage currents (dark orphotoconductance) were below 10 fA at 105 V m1. Films were prepared by thermal deposition
from a resistively heated crucible. The substrate temperature was kept at 125 °C, and thedeposition rate was held constant at 1.5 ± 0.5 nm min1. After deposition, the film wasannealed at 200 °C for 12 h. Photocurrent action spectra were evaluated for wavelengths from400 nm – 1180 nm, using a Bunkokeiki SM-25 monochromatic light source and a Keithley
6487 picoammeter-sourcemeter. The incident photon flux was 1019 photons m–2 and theapplied bias was 105 V m1. Measurements were obtained at a repetition rate of
approximately 0.1 Hz, with dark current subtraction performed for each data point. The
optical absorbance was measured ex situ on a reference 10 nm CoPc film, prepared identicallyto the measurement sample but without the metal electrodes.

Figure 1. Photocurrent yield for a 50 nm thick CoPc film, and the reference absorption
spectrum.
Results and Discussion
If the absorptance of the film can be determined, photocurrent action spectra may
be expressed as “photocurrent yield” having units of electrons per absorbed photon. It can beshown that the photocurrent yield is directly proportional to the internal quantum yield[1],and is therefore the most useful photocurrent-derived quantity for the discussion of themechanistic pathways involved in the photo-generation of charge carriers. By way of example,
typical of the results, the action spectrum of a 50 nm CoPc film is shown in Figure 1. The
photocurrent yield increases at photon energies above 2 eV immediately following the firstsinglet absorption excitation “Q-band” - indeed overlapping the higher-order vibrational
excitations. Following the work of Geacintov [2] and Silinsh [3] on linear acenes we attribute
this rise to an increase in the intrinsic photo-generation yield at photon energies near thetransport gap.
References[1] A. Rose, Concepts in Photoconductivity and Allied Problems, Interscience, New York, (1963) 168.
[2] N. Geacintov, M. Pope, J. Chem. Phys, 50 (1969) 814.
[3] E.A. Silinsh, V.A. Kolesnikov, I.J. Muzikante, D.R. Balode, Phys. Status Solidi B, 113 (1982) 379.