Photoconductivity in crystalline phthalocyanines

The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H₂Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H₂Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H₂Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H₂Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H₂Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.