Influence of electrical stress voltage on cathode degradation of organic light-emitting devices

Our in situ experimental observations of the influence of electrical stress voltage on organic light-emitting device growth in dark spot areas are pre sented. We demonstrate the use of microsized silica particles to create uni formly sized defects on the protective layer. This is an efficient way to c ontrol the location and the number of dark spots. The growth in dark spot a rea was studied at different driving voltages from 0 up to 11 V. Dark field microscopy was used to monitor the dark spot size below the turn-on voltag e. The bright field was used at or above the turn-on voltage. Our observati ons indicate that dark spot growth was strongly affected by the electrical stress voltage. A linear growth rate with respect to the voltage was observ ed with a fitting parameter better than 99.7% when the device is driven abo ve the turn-on voltage. We interpret the dark spot growth in terms of the d iffusion of moisture and oxygen accompanied by cathode layer chemical and p hysical changes. (C) 2001 American Institute of Physics.