Electronic and photonic processes in organic LEDs
Prof. dr. Reinder Coehoorn
Philips Research Laboratories Eindhoven, Photonic Materials and Devices department, and
Eindhoven University of Technology, Molecular Materials and Nanosystems group, Applied Physics department.
Organic Light Emitting Diodes (OLEDs) are potentially the key components in a novel lighting technology, providing several advantages as compared to today's incandescent or fluorescent lamp technologies. Although OLED technology is less well developed than inorganic LED technology, based on III-V semiconductors, rapid progress in efficiency and lifetime is presently being realized.
I will first give an introduction on the structure, the basic functioning and the present technological progress. It is shown that the layer structure in optimal devices is extremely complicated, comprising stacks of sometimes more than 10-20 active organic layers, with nanometer scale thicknesses, each with their own specific function. It is shown that the microscopic physics of the electron and hole transport processes, exciton formation and radiative decay, are fundamentally different in these disordered materials, than in crystalline inorganic semiconductors. E.g., we have recently demonstrated that disorder leads to a strong carrier concentration dependence of the mobility, and to non-uniform (filamentary) current densities.
A rational development of these complex systems cannot be done without combining experimental studies with numerical modelling. It will be shown how using the recently developed microscopic transport model, mentioned above, the full current density-voltage-luminance curves have been obtained for blue emitting polymer LEDs. Use is also made of a light outcoupling model, within which the coupling between the local excitation and the optical cavity modes is calculated. Finally, an overview will be given of some intriguing but unresolved questions, including e.g. the ratio between singlet and triplet excitons, formed upon recombination in the polymer layer.