Fluorescence lifetime imaging of polymer LEDs

We investigate the temporal properties of the fluorescence emitted by such devices after excitation with an ultrashort optical pulse using a novel wide-field method, fluorescence lifetime imaging (FLIM). This has not previously been applied to PLEDs and provides a new technique for studying the processes leading to black spot formation. The FLIM system developed at Imperial College uses an ultrafast time-gated image intensifier based on microchannel plate technology. This (coupled together with a CCD) can stroboscopically record time-gated images of the fluorescence emitted by a sample after pulsed excitation by changing the trigger delay between optical excitation and signal collection by the CCD. The images from the CCD are then analysed computationally pixel-by-pixel using, for instance, a single exponential decay model so that an image of the fluorescence lifetimes can be created on a false colour scale (a FLIM map). PLED defects have also been studied with FLIM, including bubble formation in the polymer layer. The application of fluorescence lifetime imaging provides a novel method to achieve contrast in microscopic studies of the properties of PLEDs and to probe the mechanisms behind defect formation.