DARK CONDUCTIVITY IN THE X-POLYMORPHS AND ALPHA-POLYMORPHS OF LITHIUMPHTHALOCYANINE

This study focuses on the correlation existing between the structure a nd the continuous dark conductivity in single crystals, compacted powd ers and thin films of the lithium phthalocyanine radical (PcLi). The d c conductivity at room temperature measured in PcLi compacted powders is of about 2-4 . 10(-5) (Ohm cm)(-1) for the x structure and 1-2 . 10 (-4) (Ohm cm)(-1) for the alpha structure. The temperature dependence of the conductivity measured in the range 130-300 K hints at an activa ted-like behaviour with activation energies in the range 0.1-0.2 eV de pending on the structure. PcLi thin films grow either in the x or the alpha structure depending on the substrate temperature (T-s) during de position on the glass substrate. The conductivity shows a clear and st rong dependence on the substrate temperature T-s and increases between 1 . 10(-6) (Ohm cm)(-1) for T-s = 25 degrees C and 5 . 10(-4) (Ohm cm )(-1) for T-s = 200 degrees C. Annealing of the PcLi thin films obtain ed for T-s = 25 degrees C at 200 degrees C under vacuum results in a c lear increase of the conductivity by more than one order of magnitude. The conductivity is independent on the presence of oxygen in powdered samples and single crystals, indicating the intrinsic nature of the s emiconducting behaviour. Contrary to alpha-PcLi thin films (T-s simila r to 200 degrees C), the conductivity in PcLi x-form thin films (25 de grees C < T-s < 125 degrees C) is sensitive to the presence of ambient atmosphere (sigma(air)/sigma(vac) similar to 2.3 at 300 K for T-s = 2 4 degrees C). This weak variation of the conductivity cannot be attrib uted to extrinsic charge carrier generation as observed for other meta llophthalocyanines. (C) 1998 Elsevier Science S.A. All rights reserved .