Effects of Long Range Order, Temperature and Phase on the Photoconversion Properties of Liquid Crystal Porphyrin Films

Photocurrent and photovoltage measurements on thin films of a liquid crystal zinc porphyrin are described. The film order exerts a pronounced effect on the magnitude of the photocurrent in these quasi-1-dimensional semiconductors, and can be the predominant limitation to charge transfer in highly disordered films. The changes in photocurrent and photovoltage as the temperature is increased through the solid phase to the discotic liquid crystalline phase are measured and explained as resulting from changes in exciton diffusion length, charge carrier mobilities and the relative rates of exciton dissociation by electron and hole injection into the electrode. The greater disorder of the liquid crystalline phase relative to the solid phase is apparently responsible for the sudden decrease in photocurrent and photovoltage at the crystal to liquid crystal phase transition.