Hole transport in porphyrin thin films

Abstract

Hole transport in p-type organic semiconductors is a key issue in the development of organic electronic devices. Here the diffusion of holes in porphyrin thin films is investigated. Smooth anatase (formula presented) films are coated with an amorphous thin film of zinc-tetra(4-carboxyphenyl) porphyrin (ZnTCPP) molecules acting as sensitizer. Optical excitation of the porphyrin stimulates the injection of electrons into the conduction band of (formula presented) The remaining holes migrate towards the back electrode where they are collected. Current-voltage and capacitance-voltage analysis reveal that the (formula presented) system can be regarded as an (formula presented) heterojunction, with a donor density of (formula presented) for (formula presented) and an acceptor density (formula presented) for ZnTCPP films. The acceptor density in porphyrin films increases to (formula presented) upon irradiation with 100-mW (formula presented) white light. Intensity-modulated photocurrent spectroscopy, in which ac-modulated irradiation is applied, is used to measure the transit times of the photogenerated holes through the films. A reverse voltage bias hardly affects the transit time, whereas a small forward bias yields a decrease of the transit time by two orders of magnitude. Application of background irradiation also reduces the transit time considerably. These observations are explained by the presence of energy fluctuation of the highest-occupied molecular orbital level in the porphyrin films due to a dispersed conformational state of the molecules in the amorphous films. This leads to energetically distributed hole traps. Under short circuit and reverse bias, photogenerated holes reside most of the time in deep traps and their diffusivity is only (formula presented) Deep traps are filled by application of a forward bias and by optical irradiation leading to reduction of the transit time and a concomitant increase of the diffusivity up to (formula presented). © 2001 The American Physical Society.