Hole transport in porphyrin thin films - art. no. 115323

Hole transport in p-type organic semiconductors is a key issue in the devel opment of organic electronic devices. Here the diffusion of holes in porphy rin thin films is investigated. Smooth anatase TiO2 films are coated with a n amorphous thin film of zinc-tetra(4-carboxyphenyl) porphyrin (ZnTCPP) mol ecules acting as sensitizer. Optical excitation of the porphyrin stimulates the injection of electrons into the conduction band of TiO2. The remaining holes migrate towards the back electrode where they are collected. Current -voltage and capacitance-voltage analysis reveal that the TiO2/ZnTCPP syste m can be regarded as an n-p heterojunction, with a donor density of N-D=2.0 x 10(16) cm(-3) for TiO2 and an acceptor density N-A = 4.0 x 10(17) cm(-3) for ZnTCPP films. The acceptor density in porphyrin films increases to 1.3 x 10(18) cm(-3) upon irradiation with 100-mW cm(-2) white light. Intensity -modulated photocurrent spectroscopy, in which ac-modulated irradiation is applied, is used to measure the transit times of the photogenerated holes t hrough the films. A reverse voltage bias hardly affects the transit time, w hereas a small forward bias yields a decrease of the transit time by two or ders of magnitude. Application of background irradiation also reduces the t ransit time considerably. These observations are explained by the presence of energy fluctuation of the highest-occupied molecular orbital level in th e 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 7 x 10(-11) cm(2) s(-1 ). Deep traps are filled by application of a forward bias and by optical ir radiation leading to reduction of the transit time and a concomitant increa se of the diffusivity up to 2 x 10(-7) cm(2) s(-1).