A method to determine the presence of oxygen in organic light-emitting diodes (OLEDs)
Organic light emitting diodes (OLEDs) have great potential to replace current commercial display offerings. However, most of the materials comprising the OLEDs suffer degradation effects from the presence of environmental oxygen and water, as both compounds can penetrate into the device. Much research and effort have been put into fabrication methods and proper device encapsulation to help mitigate these environmental effects.
A new approach for assessing the level of molecular oxygen (O2) in OLEDs is presented. As a first step, organic films that serve as the emitting layers in OLEDs were fabricated and exposed to known levels of gas-phase O2. By doping such layers with a photoluminescent, oxygen sensitive dye molecule, measurements of the oxygen level under different gas-phase O2 concentrations were made using a voltage pulsed LED as an excitation source and a photomultiplier tube that monitored the decaying photoluminescence (PL). The PL decay time is related to the O2 level in the organic layer.
Next, devices were made by similarly doping the emitting layers with the aforementioned oxygen sensitive molecule and the devices' electroluminescence (EL) decay following a voltage pulse was observed; the EL decay times were compared with those obtained from the PL experiments of the same emitting layers. By comparing these two sets of data, the oxygen contained within a specific layer of the device can be measured by simply applying a brief voltage pulse to the device, collecting and then analyzing the resultant EL decay curve. This method is inexpensive and easy to introduce to the fabrication process and could have repercussions as the quest for longer device lifetimes continues since the oxygen level in the emitting layer of the device can then be related to its long-term stability.