Optoelectronic Properties of Metal-Free Binary Porphyrin Nanostructures

Nanostructures fabricated from tetrapyridylphenyl porphyrin (TPyP) and tetrasulfonatophenyl porphyrin (TSPP) in a 1:1 ratio are conducting and also photoconducting.     I-V curves for the nanorods (in a vacuum environment) were collected in the dark and with (405, 473, and 633 nm) illumination. These wavelengths fall within the absorption spectrum of the porphyrin solid and allow determination of an action spectrum.  With illumination, the nanorods show a sharp increase in conductivity followed by a slow increase in conductivity over time until a saturation photocurrent is reached. Once the light is off, the current decays slowly over time.  This is referred to as persistent photoconductivity.  The TPyP:TSPP nanorods exhibit an increase in photo-conductivity with an applied positive gate voltage and a decrease with an applied negative gate voltage, consistent with the electrons acting as the primary mobile charge carrier.  Quantification of the persistent photoconductive behavior and a model for electron transport through the nanostructures will be presented.