The dependence of the surface photovoltage of NiPc and CuPc films on oxygen ambient has been measured, and the observed effects can be quantitatively accounted for with a theoretical model which involves the transfer of charge from the Pc ring to the adsorbed O2 and the formation of a Pcδ+–Oδ−2 species at the surface of the Pc film. Symmetry arguments based on a simple bonding model are used to explain the nature of these O2 electron acceptor surface states. Charge transfer complexes such as this are important both to the doping of these organic semiconductors, and to a fundamental understanding of the oxidations photocatalyzed by these porphyrinlike materials which are closely related to some of the processes which occur during photosynthesis.
The surface photovoltage signals and the associated relaxation times generated by a laser pulse at the surface depletion layers of anthracene (0.8 μV, 5.6 msec), tetracene (12. μV, 10.0 msec), and pentacene (17.5 μV, 20.0 msec) appear to vary with the increasing amount of electron delocalization. As expected, the photovoltage of these materials depends logarithmically on light intensity until a saturation value corresponding to the complete energy band flattening at the surface is achieved, and this energy band bending is larger for pentacene than it is for tetracene. The photovoltage signal is observed to decay exponentially following the laser pulse with a relaxation time that is independent both of the wavelength and intensity of the light. It is established that this is in agreement with theoretical predictions based on a simple model involving the recombination of the photoinjected charge. The photovoltage spectral dependence of all three polyarenes have maxima which correspond to maxima in the corresponding optical absorption spectra due to the allowed singlet–singlet transitions. In addition, the photovoltage spectrum of anthracene has maxima that correspond to the ’’forbidden’’ singlet–triplet transitions, which are comparable in size to the photovoltage arising from the allowed singlet–singlet transitions. This observation implies that the dissociation of excitons to form free carriers is independent of the distance of the exciton from the anthracene surface. The corresponding singlet–triplet transitions for tetracene and pentacene are outside the spectral region examined and thus were not observed.
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.
Films of phthalocyanine dyes are increasingly being utilized to photosensitize a variety of optoelectronic devices. One practical limitation to further application is the insulating nature of these films. In this paper, we demonstrate how permanently conductive (2 ft-1 cm-1) films of NiPc films can be prepared by heating in an iodine ambient at temperatures ranging from 140 to 200 °C. (Pleating in iodine at lower temperatures results in NiPc films which were only transiently conductive.) These changes in conductivity are accompanied by changes in the optical absorption, morphology, and X-ray diffraction. Before treatment in iodine, the NiPc films are blue and have a sharp absorption edge at 750 nm. In contrast, the iodized NiPc films are green and have a less pronounced absorption edge at 700 nm with an additional absorption peak at 978 nm. The iodine treatment also resulted in some cracking in the NiPc film. Although the blue and green NiPc films are both tetragonal, there are pronounced shifts in the X-ray diffraction peaks, and the green films are preferentially oriented along the (200) direction.
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