Hole transport in porphyrin thin films

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 ${\mathrm{TiO}}_2$ 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 ${\mathrm{TiO}}_2.$ The remaining holes migrate towards the back electrode where they are collected. Current-voltage and capacitance-voltage analysis reveal that the ${\mathrm{TiO}}_2/\mathrm{Z}\mathrm{n}\mathrm{T}\mathrm{C}\mathrm{P}\mathrm{P}$ system can be regarded as an $n-p$ heterojunction, with a donor density of $N_D=2.0\mathrm{}\times {10}^{16}{\mathrm{cm}}^{\mathrm{-}3}$ for ${\mathrm{TiO}}_2$ and an acceptor density $N_A=4.0\mathrm{}\times {10}^{17}{\mathrm{cm}}^{\mathrm{-}3}$ for ZnTCPP films. The acceptor density in porphyrin films increases to $1.3\times {10}^{18}{\mathrm{cm}}^{\mathrm{-}3}$ upon irradiation with 100-mW ${\mathrm{cm}}^{\mathrm{-}2}$ white light. Intensity-modulated photocurrent spectroscopy, in which ac-modulated irradiation is applied, is used to measure the transit times of the photogenerated holes 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 $7\times {10}^{-11}{\mathrm{cm}}^2{\mathrm{}\mathrm{s}}^{\mathrm{-}1}.$ 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 $2\times {10}^{-7}{\mathrm{cm}}^2{\mathrm{}\mathrm{s}}^{\mathrm{-}1}.$