Organic Optoelectronic Devices
The Organic LED
An Organic LED (OLED) consists of several layers of organic materials sandwiched between a cathode and an anode, which typically consists in a transparent conductive oxide, such as indium tin oxide (ITO), zinc oxide, etc. Applying a positive voltage to the anode relative to the cathode causes electron and hole current injection from cathode and anode, respectively. On electron-hole recombination at the interface light emission occurs.
Devices made on plastic substrates can have flexible mechanical properties.
Recently, Organic Light-Emitting Diodes (OLEDs) have demonstrated efficiencies as high as several tens of lm/W, clearing the way for a application in display (such as AMOLED) and solid state illumination.
Our research activity includes the characterization and the reliability study of OLEDs by means of DC and pulsed stress tests, thermal stress and UV exposure. Characterization techniques includes: electro-optical characterization of the performance of the devices by DC, AC, and transient measurements (such as Impedance spectroscopy and DLTS), the analysis of the effect of temperature on the characteristics of the devices, the impedance analysis, the modeling and simulation of the devices.
Fig. 2 shows an example of OLED degradation under accelerated electrical and thermal stresses. The operating voltage shift and luminescence decrease are strongly correlated: The same traps producing the voltage shift also acts as exciton quencher or e-h recombination centers. State of the art OLEDs are stable during thermal storage if no bias is applied. If external bias is applied, temperature accelerate the degradation.