REVERSIBLE LIGHT-INDUCED FATIGUE OF 2-LAYER ORGANIC PHOTOCONDUCTORS STUDIED BY TIME-OF-FLIGHT PHOTOCONDUCTIVITY MEASUREMENTS

Reversible light-induced fatigue of two-layer organic photoconductors consisting of a charge generation layer (CGL) and a charge transport layer (CTL) was studied by time-of-flight (TOF) photoconductivity measurements. The CGL contains 50% X-metal-free-phthalocyanine in a polyester and the CTL is a solid solution of 33% p-diethylaminobenzaldehyde-diphenylhydrazone in a polyester. Prolonged light exposure under applied fields above 10(5) V cm-1 causes remarkable changes in electro-photographic properties and the TOF current pulse shape. After light exposure the TOF current increases and the drift velocity of holes through the CTL decreases compared with those before light exposure. From the experimental results, it is concluded that light-induced fatigue of the two-layer photoconductor is a consequence of the build-up of the negative space charge caused by the electron trapping in the CGL and the positive space charge caused by the hole trapping at the CGL-CTL interface. The TOF current waveform strongly reflects the internal potential profile modulated by these space charges. A rest in the dark reverses the TOF current waveforms. The recovery process of TOF waveforms shows that holes trapped at the interface are first released, and then electrons trapped in the CGL are released thermally.