Influence of Mn as a redox-active central metal on the electrical conduction behaviour of phthalocyanine thin films

Thin films of phthalocyaninatomanganese (PcMn) in the thickness range of 10 0 nm have been prepared by vapour deposition on quartz glass substrates. Th e films were characterized in situ during film growth and following film de position by measurements of the electrical conductivity under DC applied el ectric fields parallel to the substrate surface. The dependence of the cond uctivity on the average film thickness was determined and the mechanism of film growth is discussed. Without breaking the vacuum, a temperature gradie nt was established again parallel to the substrate surface and a thermopowe r was detected. Its dependence on the size of the temperature gradient gave a Seebeck coefficient of -780 mu V K-1 at 460 K. From the negative sign it is evident that electrons are the majority carriers in PcMn in a freshly p repared thin film. The temperature dependence of the Seebeck coefficient ga ve the thermal activation of charge carrier generation as Delta E = 0.19 eV , which is discussed in terms of the position of the Fermi energy in the fi lms. The temperature dependence of the electrical conductivity gave an acti vation energy E-A = 0.38 eV considerably higher, indicating a thermal activ ation of charge carrier transport and hence supporting a hopping mechanism rather than delocalized transport. Under exposure to oxygen the conductivit y showed a fast small increase followed by a slow large decrease, which is discussed considering surface as well as bulk interactions. Hole conduction was measured for the completely oxidized films by a positive Seebeck coeff icient. Optical spectroscopy performed ex situ was used to allow further di scussion of redox interactions of the films. Evidence was found for the pre sence of Pc(-2)Mn(+3)O-2(-1) as well as Pc(-2)Mn(+3)O(-2)Mn(+3)Pc(-2) and P c(-2)Mn(+2) in freshly prepared films. Films exposed to air for as long as several months were completely oxidized to Pc(-2)Mn(+3)O-2(-1). Copyright ( C) 2000 John Wiley & Sons, Ltd.