Electrical Properties of Organic Solids. II. Effects of Added Electron Acceptor on Metal-Free Phthalocyanine

Abstract

The addition of ortho‐chloranil to the surface of films of metal‐free phthalocyanine has been found (a) to increase the dark conductivity of such films by as much as 10⁷, (b) to increase the steady‐state photoconductivity by as much as 10⁵, and (c) to result in the formation of unpaired electrons whose concentration decreases reversibly as a result of illumination. These systems exhibit a light‐induced polarization, the phthalocyanine layer becoming more positive with respect to the ortho‐chloranil layer. Kinetic studies demonstrate that, upon illumination, a single process (time constant=60 sec) results in the increase in conductivity, the decrease in unpaired spins and the increase in polarization. The results are consistent with the following scheme. An electron transfer from phthalocyanine to ortho‐chloranil occurs in the dark at room temperature producing holes in the phthalocyanine layer and ortho‐chloranil negative ion radicals (high conductivity, ESR signal). Illumination results in the transfer of an electron from an excited phthalocyanine molecule to the ortho‐chloranil negative ion producing further phthalocyanine holes and ortho‐chloranil double negative ion (increase in conductivity, increase in polarization, decrease in ESR signal). By equating spin concentration with charge carrier concentration (phthalocyanine holes) it is possible to calculate a mobility of 10^‐4 cm²/v/sec for holes in the phthalocyanine layer. With this value, a quantum yield of unity is calculated for the production of charge carriers in doped phthalocyanine. The experiments indicate a quantum yield of less than 10^‐1 for undoped phthalocyanine. The over‐all results of adding a strong electron acceptor to a film of phthalocyanine are thus to (a) produce charge carriers in the dark, (b) increase the quantum yield for production of charge carriers by light and (c) increase charge carrier lifetime.