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Our in situ experimental observations of the influence of electrical stress voltage on organic light-emitting device growth in dark spot areas are presented. We demonstrate the use of microsized silica particles to create uniformly sized defects on the protective layer. This is an efficient way to control the location and the number of dark spots. The growth in dark spot area 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 voltage. The bright field was used at or above the turn-on voltage. Our observations indicate that dark spot growth was strongly affected by the electrical stress voltage. A linear growth rate with respect to the voltage was observed with a fitting parameter better than 99.7% when the device is driven above the turn-on voltage. We interpret the dark spot growth in terms of the diffusion of moisture and oxygen accompanied by cathode layer chemical and physical changes.

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