Organic Light-emitting Device In The Voltage Distribution

This work is dedicated to the investigation of internal potential distribution in organic light emitting diodes (OLEDs).By means of inserting an interior aluminum electrode in an OLED, we have realized direct measurement of internal potential in the device. However, due to the low mobility and high resistivity of organic semiconductors, device's resistance is extremely high (1 G Ohm) at low bias. Therefore one cannot accurately measure the potential distribution by conventional voltmeters simply. This problem has long been ignored in previous works, so that their results are not reliable, and explanations therein are misleading sometimes.We have developed an accurate way to measure the potential distribution based on dc bridge. Errors induced by OLED's high resistance are thus eliminated and one can perform reliable measurement of the interval potential distribution of OLED in the bias range 2-40V. The validity of the new method is confirmed by separate measurements using a high-resistance voltmeter.Applying this new method, we have found that potential distribution of different organic layers perform differently with the bias applied. When the bias increases, the potential of the interval closer to the cathode increases first, and then decreases, while the interval closer to the anode performs in the opposite way. Both potentials become stable only when the bias is large enough. This behavior holds true even in the single-layer OLEDs. To our knowledge, this is the first observation of such a novel property. Our results also indicate that the change in carrier injection or space-charge limitation would influence the potential distributions significantly.