Polymer Light-Emitting diodes

A typical polymer light-emitting diode (PLED) consists of a thin layer of undoped conjugated polymer sandwiched between two electrodes on top of a glass substrate. Experimentally, attention has especially been focused on PLEDs that contain the conjugated polymer poly(phenylene vinylene) (PPV) or its derivatives which may have an external conversion efficiency larger than 1% photons/charge carrier. The PPV is spin-coated on top of a patterned indium-tin-oxide (ITO) bottom electrode which forms the anode. The cathode on top of the polymer consists of an evaporated metal layer for which Ca is used. Under forward bias electrons and holes are injected from the cathode and the anode, respectively, into the polymer. Driven by the applied electric field, the charge carriers move through the polymer over a certain distance until recombination takes place. The device operation of a PLED is thus determined by three processes: charge injection, charge transport, and recombination. The research group works on the development of a device model for polymer based LEDs which contains all the basic physical processes. The understanding of the device operation of PLEDs will indicate directions for further improvement of the performance.